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The Ductile Iron News - Issue 3, 2001

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To Promote the production and application of ductile iron castings Issue 3, 2001

The Ductile Iron SocietyVisits Neenah Foundry at115th Technical andOperating Meeting

Photos of Meeting Attendees

More photos of Neenah Foundry

FEATURES

Cover Story - DIS Visits NeenahFoundry at 115th T&O Meeting DIS Operating Committee Meeting

Simulation of Microstructure andMechanical Properties in DuctileIron

Offsetting Macro-Shrinkage inDuctile Iron

Near Net Shape Ductile IronComponents - A Novel ApproachUsing Semi-Solid Forming

Shrinkage in Nodular Iron

Solving Casting Problems

Basic Metallurgy

FEF College Industry ConferenceDEPARTMENTS

Associate Member Profile SuperiorGraphite - Best Plant News Briefs Obituary Xarifa Sallume Bean,1909-2001

View Ductile Iron Related Publications

Located in Strongsville, Ohio, USA15400 Pearl Road, Suite 234; Strongsville,Ohio 44136 Billing Address: 2802 Fisher Road, Columbus, Ohio 43204 Phone (440) 665-3686; Fax (440) 878-0070email:[email protected]

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The Ductile Iron News - Neenah Pictures

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Cover, page 1, page 2, page 3

Before quoting your job, we carefully listen to yourpeople to be certain we completely understandyour design, application, and specifications. Everyquestion is asked, every detail is considered, everypart of the process is planned before your castingis quoted. Variables become constants prior toproduction of your castings.

Neenah's pattern shop provides industry leading useof synthetic materials and computer technology forproduction tooling.

Complex cores, beyond the range of most foundries,are routine in Neenah's daily production.

To ensure metallurgic uniformity, Neenah operatessome of the longest cooling lines in the industry

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Neenah combines advancedtechnology with traditionalcraftsmanship to produce castings thatmeet or exceed your expectations.

Neenah enhances Disamatictechnology with unique coresettingcapabilities that provide castings with acompetitive advantage.

Minimizing process variation and maximizing processefficiency receive critical attention at Neenah.

Customer needs define and drive our manufacturingprocess, resulting in comprehensive, full servicecapabilities.

At Neenah, excellence in both product reliability andcustomer service is the number one priority.

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Neenah Foundry...people, processes, and capabilities uniquelyqualified to meet your needs.

Neenah Foundry Assessment Information

Located 100 miles Northwest of Milwaukee, WisconsinProducing Gray (class 30 and 35) and Ductile Iron castings (D4018,D4512, D5506 and D7003)Annual shipping capacity in excell of 150,000 tonsFully facilitized pattern shopTransportation fleet

Melt Capability

Melt: Three 72" Refractory line water-cooled Hot Blast cupolas with O2 injection (60 ton/hour)

Duplex: Three 60 ton Vertical Channel furnaces

Pour: Three 10 ton and One 6 ton Jnker horizontal channel (pressurized) stopper rod furnaces with in-stream inoculation (Note: pouring systems have dual inoculators)Core

Two CB-30 horizontal isocure core machines

45" x 40" platen

350 lbs. maximum core weithg

Two 18 liter Peterle vertical isocure core machines

31.5" x 23.8" platen

100 lbx. maximum core weight

Two 12 liter Peterle vertical isocure core machines

27.5" x 23.8" platen

70 lbs. maximum core weight

Two 12 liter Peterle vertical shell core machines

19.6" x 17.9" platen

35 lbs. maximum core weight

Molding

One 2013 Mark 4 Disamatic molding machine

Mold size is 21" x 25.6"

100 lbs. maximum pour weight

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Up to 410 molds/hour

Two 2070 Type B Disamatic molding machines

Mold size is 31.5" x 37.4"


automatic pattern changing system

automatic core setting

up to 260 molds/hour per machine

One BMD air impulse molding line.

44" x 50" flasks with 10" cope and 16" drag heights


Other

Fully facilitized process control labs, paint line, complete casting engineering services.

2121 Brooks AvenueBox 729

Neenah, WE 54957Phone: 920 725-7000

Fax: 920 729-3682



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The Ductile Iron News - Operating Committee Meeting

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DIS Operating Committee MeetingWednesday, October 3, 2001

Oshkosh, WisconsinMinutes

The newly formed Operating Committee met for the first time. Thirty-six peoplerepresenting nine Foundry members and 17 Associate Members were inattendance. We opened with "round robin" introductions and with each person inattendance discussing, briefly, the markets and business environment from theirperspective. In general, the comments were that business was slow, however somemembers reported "pockets of prosperity". In general, slow times have resulted inbusinesses improving their performance and engineers looking for new solutions.This has resulted in an increase in quoting activity and requests for products andservices related to improved product performance or reduced manufacturing costs.

After introductions, DIS President, Denny Dotson introduced the new committeeformat and the structure that had been first discussed at the Waterloo, Ontariomeeting last spring. It was the goal of that steering committee to establish acommittee structure that more effectively "tapped" the talents of the DISmembership and one that would improve the effectiveness of the organization. Atan ad hoc steering committee meeting in Waterloo, four subcommittees weredefined to combine and replace the existing committee structure. They are asfollows:

-Marketing Committee / DIMG: Paul Gerhardt, Chmn.-Programs and Publications Committee: Jim Wood, Chmn.-Member Services Committee: Tim Brown, Chmn.-College & University Relations: John Keough, Chmn.

The DIS Board is dealing with any necessary modifications in DIS policy related tothese changes. Those policy issues are outside the scope of these newly formedcommittees.

Denny Dotson asked the Operating Committee members present to choose asubcommittee that best fits their skills. Then he charged each subcommittee withthe responsibility of establishing a Mission Statement and some measurablesrelated to the execution of that Mission. The Operating Committee then broke outinto the newly established committees. The organization and outcomes of thatsubcommittee work follow.

Ductile Iron Marketing Committee

Gene Muratore* Jerry Wurtsmith Rick GundlachJim Stevenson John Wagner Bob O'RourkeTerry Lusk John Hendrix Ron AufderheideJim Mullins

*Committee Chairman, Paul Gerhardt was absent. In his absence Gene Muratorefilled the role of acting Chairman.

Mission Statement: To disseminate information about the attributes of Ductile Ironto the metal forming industries by all appropriate means. This includes, but is notlimited to:

1. Paid and non-paid press releases and advertising of the DIMG technicalbooklets.

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2. Use of the DIS website.

3. "Cast It" and "Cast It in Ductile Iron" on video and compact disk.

4. Exhibiting at trade shows.

5. Seminars for design, materials and manufacturing engineers.

Goals: Increase awareness of the attributes of Ductile Iron (castings) amongstdesign engineers, material specifiers, and component purchasers/manufacturers, byexpanding on the efforts of the DIMG, through additional funding from the DIS.

Current Activities: The following describe the scope of the activities of the DIMG.

1. Ongoing press releases for DIMG literature

2. Preparation of a press release for the "Cast It" video conversion to CD. Thisrelease will go to trade magazines, not on the website, as the originalpressing may be insufficient vs. the volume of requests on the internet.

3. All DIMG literature is now uploaded to the DIS website and is fullydownloadable.

4. Negotiations are underway with AFS to share space at the SAE show inDetroit in March 2002 and at the ConAgg/ConExpo/IFPE/SAE Off Highwayshow in Las Vegas in March 2002.

Discussion items:

1. Wells Durabar has secured space at the Las Vegas show and has offeredthe DIMC an opportunity to share the space. Final details will be worked outwith Bob O'Rourke. Investigating the possibility of a "live" computer link inthe booth for access to the DIS website.

2. The DIMC will be responsible for programs/speakers at these venues.

3. Very few members of the committee have viewed the "Cast It" video. Onecopy of the video was circulated to all committee members from the RioTinto office with an address list of committee members for mailing.

4. Discussions were held on how to get more visitors to the website. Onepossible solution is to purchase a list of names and make a mass email tothe list. The title of the email must be attractive in order to avoidinstantaneous deletion of the email by wary engineers.

5. The AFS has announced that there will be no shared space at the SAEshow in Detroit. The committee must evaluate the benefit of spacespecifically for the DIMC. This would incur and additional expense that is notcurrently in the DIMG budget.

6. The feasibility of a DIMC Bulletin Board on the website was discussed. Itwas uncovered that the DIS website will unveil a Contact Us section within60 days that could serve the same purpose of a bulletin board. This can bea platform for queries from design engineers regarding Ductile Iron.

7. No discussion of the DIMG budget was held, although the numbers are asfollows:

Revenue:

DIMG $10,000RTIT $10,000

Total $20,000

Expense Budget:

02 SAE Showw/AFS $ 1,500



Postage etc. $ 2,500Advertising $15,000Video to CD $ 3,000

Total $22,000

Programs and Publications Committee

Jim Wood-Chmn. Gene Muratore David SparkmanSteve Sauer Al Alagarsamy Tony ThomaKathy Hayrynen Cory Ashburn

Mission Statement: To obtain the highest quality of speakers to communicate thenewest technology of Ductile Iron and related processes. The can be accomplishedthrough speakers at each T&O meeting and various publications of the Ductile IronSociety.

Goals and Objectives:

1. Establish speak guidelines (by July 2002)

2. Address the issue of commercialism (by June 2002)

3. Review presentations in advancea. Slides / PowerPointb. Full written reportc. 200 word abstracts

4. Producers subjects of interest (members to provide to Chmn. Oct 01)

5. Review speakers package for adequacy (by Oct 2002)

6. Any changes in the program chairmen (by July 2002)

7. Promote DIS Research Committee presentations (by Oct 2002)

8. Speaker gifts (by Oct 2002)

9. Keith Millis Symposium arrangements (Oct 2003)

10. ADI Conference (Fall 2002)

11. Articles and abstracts on DIS website (by June 2002)

12. $400 payment for speaker abstracts of presentations (Oct 2001)

Member Services Committee

Dennis Dotson* Alan Anderson Hugh Kind Barry Snyder

*Chairman Tim Brown was absent and Denny Dotson served as Acting Chairman

Mission Statement: To make DIS valuable to existing and potential customers.

Goals:

1. Establish a list of 50 qualified potential new members,

2. Bring three potential members to each meeting

3. Review participation at DIS events and in services (compared to past years).

4. Conduct a survey of membership on benefits of DIS

College and University Relations Committee

John Keough-Chmn. Christof Heisser George DiSylvestro



Mike Mroczek Kris Kitchen Julie FitzpatrickJames Mikoda Dan Korpi Jim Csonka

Mission Statement: To expose the maximum number of young adults possible toDuctile Iron technology as it relates to its:

Design

Application

Manufacture

Research

Career Opportunities

Action Items (Oct 2001 - May 2002)

1. Provide a hard copy of "Ductile Iron Data for Design Engineers" to allstudents attending the FEF CIC conference and those attending the AFSCast Expo.a. Keough to acquire the booksb. Mroczek to prepare the DIS stickersc. Fitzpatrick to draft a letter to accompany the books.

2. Invite local students to next DIS meeting.a. Csonka / Keough




The Ductile Iron News - Photos of Attendees

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Photos of Attendees at the 115th T&O Meeting - Neenah Foundry

Click on any photo to see enlargement

Andy Adams with GeneMuratore

John Keough with GeneMuratore

Ron Aufderheide withGene Muratore

Mark Eckert with GeneMuratore

Cristof Heisser with GeneMuratore

Chuck and Mary Jo Kurtti

Eli David with Gene Muratore Frank and BarbaraHeadington

John Andrews Bill Barrett

Denny Dotson Gene Muratore Laura Strohmayer

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The Ductile Iron News - Simulation of Microstructure and Mechanical Properties in Ductile Iron

file:///C|/WEBSHARE/062013/magazine/2001_3/micro.htm[6/19/2013 10:21:26 AM]


Simulation of Microstructure and MechanicalProperties in Ductile Iron

Abstract:Since the introduction of "Solidification Simulation" in the foundry industry, whichhappened almost 20 years ago, only a few of the available simulation tools havematured into true "Casting Process Simulation" tools. The specific solidificationbehavior of ductile iron is very complicated, hence, challenging to model. Thispaper will cover the mechanisms of the solidification and cooling of ductile iron thatare considered in one of the leading casting process simulation tools. One exampleshows the elimination of risers on an actual ductile iron casting to show thefinancial savings in the foundry and the difference between a simple solidificationsimulation and a highly sophisticated casting process and micromodeling tool. Acomparison of actual microstructure measured in test castings and simulatedmicrostructures are shown, as well.

Development of Casting Process Simulation for Iron Castings:

Fig. 1: Timeline of simulation tool development

Initially the term "Solidification Simulation" meant exactly that. Tools used ahomogeneous temperature distribution (one temperature) throughout the entirecasting as starting condition. Very often just single values for thermophysicalproperties, i.e. density, conductivity, specific heat capacity were considered by thecodes, not temperature dependent values. Some tools didn't even consider themold material surrounding the casting. Those tools were used to predict hotspots incastings. These tools considered neither the influence of the temperature loss ofthe melt during the filling process, nor material transport phenomena. The use ofthese tools lead to many over-risered castings especially in iron foundries. Thesolidification of gray and ductile iron is characterized by the interaction of multiplecomponents and their volume changes. One of the most important factors is thegraphite expansion versus the shrinkage of the metal matrix during the ironsolidification. Both are influenced tremendously by the metallurgy, i.e. thecomposition, inoculation, graphite precipitation, and the melt treatment. Not to beforgotten should be the influence of the mold material with regard to mold stability(mold wall movement) and moisture content.

The need to consider all of these factors leads to the necessity to micro-model thecreation of the microstructure during the solidification (Figure 1). Actually, it isbeneficial to consider certain effects, like fading of inoculants and pre-solidifyingsections of the casting, during the filling process. In many cases sound ironcastings can be produced without risers. But only highly sophisticated casting

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process simulation tools can be used to simulate these kind of castingsuccessfully.

The modeling of the microstructure during the solidification process allows the toolto continue simulating the cooling process of the casting all the way down to roomtemperature. Hence, a prediction of the microstructure at room temperature can bemade in conjunction with a prediction of mechanical properties. This functionalitybecomes more and more important for the cooperation between the iron foundrieswith casting designers, especially in combination with the prediction of residualstresses and distortion in castings.

Example 1: Ductile Iron Ring Casting

The foundry producing the 5600-lbs. ductile iron (Grade 80-60-03) ring casting hadproblems with under riser shrink. No matter how many risers they used, alwaysshrinkage porosity appeared below the risers (Figures 2 through 4).

Fig. 2: Ring casting with removed risers

Fig. 3: Detail view of broken off riser connection

Fig. 4: Picture of shrinkage under riser



The casting is poured into a very rigid chemically bonded mold, which would allowthe foundry to consider a riserless gating design. However, the use of a simple"Solidification Simulation" tool predicted a ring shaped shrink inside the casting(Figure 5).

Fig. 5: Ring-shaped shrinkage predicted by "Solidification Simulation"

At that time the yield of the casting was 77% and the scrape rate was 50%. It wasdecided to use a casting process simulation tool (MAGMASOFT) to reproduce thepresent under riser shrink and verify the appropriate process setup in the castingprocess simulation. The initial casting process simulation considering the fillingprocess, the metallurgy and melt treatment, as well as, the appropriate moldstability and properties reproduced the present under-riser shrink (Figure 6)

Fig. 6: Simulated under-riser shrink

Fig. 7: Simulated under-riser shrink

second run was conducted using a riserless design. The results show a castingwith only minor defects on the surface, but none inside the casting (Figures 8 and9).



Fig. 8: Minor surface shrink on riserless design

Fig. 9: No shrink inside casting with riserless design

After these simulation runs the foundry started producing defect-free castingswithout risers. The minor surface shrink, if present, is of no concern because it getsremoved by the machining. The financial impact of this change is significant (Figure10).

# of Castings lbs. $/lbs. $

Yield Savings 40 480 $0.35 $ 6,720

Cost of Sleeves 40 8 $8.00 $ 2,560

Riser Removal 40 8 $5.00 $ 1,600

ProductionSavings $10,880

Scrap Casting 20 5600 $0.65 $72,800

Annual Savings $83,680

Fig. 10: Savings of more than US$ 80,000.00 per year have been realized

Not only are the costs reduced for each new casting due to yield improvement,elimination of exothermic sleeves and riser removal costs, but the overall scrap ratehas been reduced to 4%, too. This eliminated the need to produce additional 20castings per year to deliver 40 sound castings. Using "Casting Process Simulation"instead of" Solidification Simulation saved more than US$ 80,000.00.

This example proves that it is essential to consider the entire casting process andmicro-model the creation of the ductile iron microstructure to get an accurateshrinkage prediction.

Example 2: Ductile Iron Ring Casting



A ductile iron test casting was poured as part of the Thin Wall Iron Group (TWIG)research program. The part included interconnected plates (stair step casting) andseparate plates with different wall thickness ranging from 2 to 6 mm in thickness(Figure 11).

Fig. 11: Test Casting

Image analysis was used to evaluate the microstructure in a center plane of thestepped area and the separate plates. A casting process simulation was conductedconsidering the entire casting process, the metallurgy and the cooling processincluding phase changes to predict the as-cast microstructure at room temperature.The comparison of the measured and the simulated values for nodule-count, ferriteand carbide distribution show very close matches (Figures 12 through 16). Besidesthe confirmation of the wall-thickness dependency of the microstructure it was alsoconfirmed how important it is to consider the temperature loss of the melt duringthe filling process and the resulting preconditioning of the sand mold due to thefilling. Differences in wall-thickness and the resulting differences in local coolingrates, alone cannot explain the measured distributions in the stepped area of thecasting.

Fig. 12: Comparison of nodule-count distribution in step plate



Fig. 13: Comparison of ferrite distribution in step plate

Fig. 14: Comparison of carbide distribution in step plate

Fig. 15: Comparison of nodule-count distribution in separate plate

Fig. 16: Comparison of ferrite distribution in separate plate

Example 3: Mechanical Property Prediction in Ductile Iron Crankshaft

In the frame of a casting engineering project the casting process of a ductile ironcrankshaft was evaluated with regard to casting defects, microstructure and as-castmechanical properties. After implementing the process conditions present in thisparticular foundry the simulation showed a close match to the microstructure andmechanical properties found in the castings (Figure 17). The final simulation lead toan optimized gating system, which reduced the filling time by 45% and eliminatedinclusion problems, found previously in the castings. The riser size was reduced,improving the yield of the casting. A significant cost reduction was achieved by theelimination of chills, after the simulation showed that a defect-free casting withsufficient mechanical properties could be produced without them.



Fig. 17: Mechanical property distribution in ductile iron crankshaft




The Ductile Iron News - Offsetting Macro-shrinkage in Ductile Iron

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Offsetting Macro-Shrinkage in Ductile IronWhat Thermal Analysis Shows

By David SparkmanMay 30, 2001

Last Revision November 7, 2001

AbstractThe natural shrinkage that occurs during the solidification of Ductile Iron can be offset by the expansioncaused by the formation of graphite. Though this has been known for some time, thermal analysis hassome interesting contributions to understanding exactly what is going on, and offers some opportunities forbetter control of late graphite expansion in moderate section sizes. Different modes of solidification areexamined and measured, and the early and late graphite content are calculated using thermal analysis.Carbon flotation is seen as a fourth form of solidification that is both hypereutectic and hypoeutectic.

Introduction to Macro-shrinkage and Expansion

Ductile Iron consists of primarily two materials: a steel matrix surrounding graphitic nodules. The steelmatrix can be ferritic, pearlitic or martensitic, or a combination of any two. The majority of ductile castingsare generally ferritic with less than 10% pearlite. A small amount of retained austenite is generally presentand in combination with micro carbides, retains about 20% of the carbon1. This carbon can then betransformed into graphite during heat-treating.

The steel matrix will typically shrink 1.2 % when cooling from 2000 degrees to room temperature. Offsettingthis is the transformation of dissolved carbon into nodules of graphite, which occupy 12% more volume asgraphite than as carbon.

One insidious form of shrinkage is a suck-in. It is caused by the same factors as shrinkage, but shows nointernal porosity as the volume loss is transferred to the surface of the casting. Suck-ins are caused by thecombination of a high shrinkage iron, and a thin or weak casting wall that cannot resist the internal pull.This could be due to a combination of a casting designed hot spot and/or hotter than normal iron. Eutecticand hypereutectic iron is more susceptible to this problem than hypoeutectic iron. Though these castingsmight not show internal shrinkage, they should be counted as having shrinkage nonetheless.

Two other forms of voids appear in iron: micro-shrinkage, and gas or blows. The micro-shrinkage appearsin the grain boundaries5 10 11 as the final solidification takes place, and is caused by micro-segregationwhere the grain boundaries become enriched in low melting elements and phases8. Gas is caused byNitrogen and Hydrogen being present in the iron9.

Figure 1. These are threeexamples of different levels ofmacro-shrinkage in thermalanalysis cups. Shrinkage occursat the point of the last metal tosolidify, so is located around thethermal couple for easy detection.Some suck-in occurred insamples 4B and 3B.

Literature Review

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Skaland and Grong1 suggest that up to 20% of the carbon in iron does not transform to graphite orpearlite, but is tied up as micro partials of carbides that only convert to graphite on heat treating. They basethis on the results of studies of heat-treating, which increases both the total graphite and the nodule count.This suggests that 20% of the carbon present must be discounted, as it will not form graphite duringsolidification.

Heine3 suggests that higher nodule counts lead to less shrinkage, but that above about 4.70, carbonfloatation sets in, and then the nodule counts will vary greatly from the depleted zone to the flotation zone.He also reported two Liquidus arrests in strongly hypereutectic irons4.

Stefanescu et al5 suggest that shrinkage be broken down into macro-shrinkage caused by feedingproblems, micro-shrinkage caused by contraction of the solid metal, and by micro-porosity caused by gasevolution within the iron. In this paper, we will use Stefanescu's definitions of shrinkage and examine whatcan be done to minimize macro-shrinkage.

Komkowski12 in a Master's thesis found that by deoxidizing iron, he could cause significant undercooling ofthe austenite arrest. This agrees with the early research by Alagarsamy13 on the oxygen effect on theLiquidus temperature. This research showed that the presence of oxygen raised the liquidus temperature.While Alagarsamy suggested oxygen raised the liquidus measurement, Komkowski suggests that theoxygen was simply the nucleant that prevented undercooling of the liquidus and that the oxidized state wasequivalant to the steady state. In current practice, one manufacturer of thermal analysis cups uses puretellurium metal in a capsule, one uses an exposed tellurium that will oxidize, one uses tellurium mixed withcalcium bearing bentonite, and one uses tellurium mixed with a small amount of iron oxide. The calcium-bearing cup has been seen to under cool during the liquidus.

This is an important consideration in the current practice of using an inoculant that contains up to 6%Calcium. Since calcium is the strongest deoxidant available for molten iron, it could be expected tosuppress the formation of dendrites in the casting, and lead to greater undercooling.

Graphite Growth in Solidifying Iron

Graphite is a hexagonal-closepack form of carbon that can grow in both the liquid and solid forms of iron.In theory, in irons above the eutectic composition of carbon, the graphite first nucleates in the liquid, andthen continues to grow in the solid. In irons below the eutectic composition, the graphite does not start togrow until the iron reaches eutectic temperature. As seen in a micro, the larger nodules are from growthinitiated in the liquid, and the smaller nodules are from growth that does not start until solidificationtemperatures are reached. During heat-treating, the existing nodules increase in size, and very smallnodules appear1.

The graphite nodules that form in the liquid in hypereutectic irons continue to grow as the iron cools, so theamount of growth that occurs in the liquid is smaller than what would be assumed by examining the micro.

The expansion from the graphite that grows in the liquid, generally pushes liquid back into the riser ordown sprue, and does not offset shrinkage. This is because hypereutectic irons do not form thick wallsbefore the eutectic temperature is reached, and of course, there are no dendrites to block this reversefeeding.

Late graphite is defined as graphite that grows during or after the eutectic solidification. This late graphitecan exert internal pressure to offset the shrinkage we would like to prevent.

So in order to minimize shrinkage, it is necessary to maximize the formation of late graphite without havingto reduce the actual amount of graphite. Understanding what happens in a non-steady state solidification ofDuctile Iron suggests a few ways that this can be done.

In a hypoeutectic mode of solidification, austenite forms as a solid with a lower than average carboncontent. This increases the carbon content of the remaining liquid until it reaches the eutectic composition.Likewise, in a hypereutectic mode of solidification, graphite nodules form in the liquid, removing carbonfrom the liquid until it is reduced to the eutectic composition



Figure 2. Phase diagram showing movement of carbon concentration in liquid metal as iron solidifies.

It would seem from figure 2 that the maximum amount of carbon that can be formed in late graphite isdetermined by the eutectic composition, and as long as the iron is at eutectic or above, the amount of lategraphite will be the same. But there are some methods that can actually increase the amount of lategraphite. The first is to reduce the silicon, the second is to reduce the pearlite, and the third is to runslightly hypereutectic and make use of magnesium's ability to suppress the formation of graphite. The firsttwo methods will also significantly change the properties of the iron, so they may not be possible toimplement. The third, which involves running a C.E. from 4.40 to 4.55, opens some possibilities.

Thermal Analysis shows how this third method works and how it actually decreases shrinkage. TA alsoshows the pitfalls of higher C.E.s and where adding more carbon may actually increase shrinkage.

Increasing Graphite to Avoid Shrinkage

Thermal analysis reveals that under dynamic conditions, the amount of late graphite can be increasedconsiderably by hitting a hypereutectic chemistry between 4.33 and 4.60 that solidifies without a graphiticliquidus. To actually benefit from this window, the C.E. should be slightly hypereutectic (4.4+) and safelyaway from a higher C.E. that would form a graphitic liquidus. Our research indicates that this point is about4.6+, though it may change with section size and magnesium level.

In qualifying curve types in thermal analysis, there are three basic shapes: One that shows an austeniticliquidus and a eutectic arrest, one that shows a graphitic liquidus and eutectic arrest, and one that onlyshows a eutectic arrest.

Surprisingly, the eutectic only mode is very common in iron used for small and medium size casings. Whentesting the chemistry for these eutectic only irons, it was found that the carbon equivalent varied from theeutectic composition of 4.33 all the way up to 4.58. The samples above 4.66 carbon equivalent generallyshow a graphite liquidus.

It is speculated that the magnesium is inhibiting the graphite liquidus up to about a 4.6 carbon equivalent.The level of magnesium in the iron may also have an effect on how much of a carbon equivalent can besuppressed. This means that an iron with a C.E. of 4.55 can behave as a eutectic iron but will add anadditional 22 points of carbon to counteract the shrinkage. But an iron with a C.E. of 4.65 will behave notmuch differently than one of 4.33 C.E. in suppressing shrinkage.

C.E. Silicon Carbon Graphite In Liquid Late Graphite Improvement Over Eutectic

C.E. Silicon Carbon GraphiteIn LiquidLate

GraphiteImprovement Over

Eutectic4.20 2.40 3.40 0.00 2.72 -3.5%4.25 2.40 3.45 0.00 2.76 -2.1%4.30 2.40 3.50 0.00 2.80 -1.1%4.33 2.40 3.53 0.00 2.82 Base Line4.35 2.40 3.55 0.00 2.84 0.7%4.40 2.40 3.60 0.00 2.88 2.1%4.45 2.40 3.65 0.00 2.92 3.5%4.50 2.40 3.70 0.00 2.96 5.0%4.55 2.40 3.75 0.00 3.00 6.3%



4.60 2.40 3.80 0.00 3.04 7.8%4.65 2.40 3.85 0.32 2.82 0%4.70 2.40 3.90 0.37 2.82 0%

Figure 3. Assumptions: 20% carbon retained in matrix, no graphitic liquidus forms till above 4.60 C.E.Above 4.70 C.E. there is a risk of carbon flotation.

This would account for the frequency that eutectic freezing modes are found. The Eutectic is no longer justa point, but a small range from 4.33 to about 4.60 due to the presence of magnesium. This can result in anincrease of 13% more carbon forming in the late solidification, or shrinkage being reduced by 1.6% of thetotal volume of the carbon. This suggests that the amount of shrinkage in castings can vary considerablyover a small carbon range.

Figure 4. Expanded region of eutectic zone due to magnesium suppression of graphite formation.

Once the carbon equivalent becomes higher than the suppressed value, then the effect will be lost, theextra carbon will be removed by graphite formed in the liquid, and macro-shrinkage will increase.

This goes against the idea of counteracting shrinkage by simply increasing the carbon content. It suggeststhat we, instead, should increase the carbon until the iron is slightly hypereutectic, but does not yet exhibita graphite liquidus.

Carbon Flotation in small castings

As the carbon content increases into the graphitic liquidus area, a stronger graphitic liquidus occurs thatmay not simply reduce the carbon content to eutectic, but may actually remove enough carbon to reducethe C.E. level below the eutectic. This results in an unusual thermal analysis curve that has both agraphitic liquidus and an austenite liquidus followed by the eutectic arrest. This then proves even furtherthat increasing the carbon beyond the graphitic liquidus may drastically increase shrinkage.

Heine and others have previously documented multiple arrests in their research, but these arrests were notidentified as anything other than graphitic arrests4. This is the first time that multiple liquidus arrests havebeen identified in a single sample.

The dynamics of inoculation, magnesium, carbon content, and other alloys make a system that needs to betightly controlled to supply the necessary amount of carbon and alloys and yet prevent a graphitic liquidusfrom increasing shrinkage and porosity.

Results

Samples were taken from many foundries in this research. Two are presented as demonstrating theinterrelationships of freezing mode, shrinkage, late graphite and nodule count. The results are from thethermal analysis instrument using the same calibration for both foundries. While the readings areapproximant, they are in agreement with the measurements of the foundries, i.e. the 77% nodularity wasrecorded as an 80%.

Table 1 and 2 show typical results from two different foundries having different chemistry aims andinoculation practices. The test data shows considerable interrelationship between shrinkage, and nodulecount in the hypoeutectic irons, and in table 1, the shrinkage seems to be related to both nodule count,and the double arrest.

The Hypo-hypereutectic arrest in table 1 greatly reduced the available late graphite and increased theshrinkage. The nodule count relates well to the nodularity. This foundry would do well to reduce theircarbon slightly and avoid hypereutectic freezing modes. Late graphite control would greatly benefitshrinkage in this foundry.



Mode Nodularity Nod Count Late Graphite Shrink Under-coolingEutectic 84 330 100 1 8Hypoeutectic 86 330 86 0 5Hypo-Hyper 93 380 69 12 9Eutectic 85 380 100 6 7Hyper 77 330 76 2 11Hyper 78 300 75 nm 9

Table 1 Generally hypereutectic iron (nm - not measured)

In table 2 there is a completely different chemistry practice with a slightly higher inoculation practice. Lategraphite comes out during about 93% of the solidification, but it is not enough to offset the lower carbonlevel and higher inoculation practice. This foundry would do well to decrease their inoculation down to the300 levels if possible. If chill problems prevent this, then they might consider raising the C.E. to produceeutectic mode solidification.

Mode Nodularity Nod Count Late Graphite Shrink Under-coolingHypoeutectic 84 470 93 Nm 1Hypoeutectic 88 470 87 19 1Hypoeutectic 89 470 92 18 1Hypoeutectic 87 450 96 17 0Hypoeutectic 91 470 95 22 0Hypoeutectic 90 370 92 9 1Hypoeutectic 94 320 95 1 0Hypoeutectic 86 370 92 9 1Hypoeutectic 94 320 91 2 0

Table 2 Generally hypoeutectic iron (nm - not measured)

Discussion

Shrinkage has many causes. The question is: Is shrinkage an intermittent problem or a consistentproblem? Consistent problems are problems that require a redesign of the gating and risering system,additions of chills, and even a redesign of the casting or change in the carbon equivalent of the iron. Anintermittent problem is generally where the foundry man is at a loss for a solution. While tramp elementsthat cause significant alloy segregation in the grain boundaries8 can cause small micro-shrinkage bylowering the grain boundary freezing temperature, this discussion is directed more toward graphite controlto offset normal macro-shrinkage.

There are four solidification modes that can occur in ductile iron: hypoeutectic, hypereutectic, eutectic, anda combination of hyper-hypoeutectic. These classifications are applied to the shape of the thermal analysiscurve, not the chemistry. These curves may differ from what can be expected from chemistry because ofthe speed of cooling and the suppression of graphite formation due to magnesium. Faster cooling will shiftthe mode from hypereutectic toward eutectic, and from eutectic toward hypoeutectic.

In the hypoeutectic mode there is an austenitic liquidus arrest, followed by a eutectic arrest. In thehypereutectic mode there is a graphitic liquidus arrest followed then by a eutectic arrest. In the eutecticmode there is only a eutectic arrest. In the hyper-hypoeutectic mode there is first a graphitic liquidus arrestfollowed by an austenitic liquidus arrest, and then finally, the eutectic arrest.

Hypereutectic Mode

In a hypereutectic mode iron, graphite nodules first form in the liquid. This is a moderately low energyreaction that may go on for some time. The heat generated from the graphite slows the cooling rate, andtherefore prolongs the length of the arrest. Since no solid metal is precipitated during this arrest, the wallsof the casting are thin to non-existent depending on the temperature gradient.

During this cooling time, the expansion due to the graphite may simply push iron back into the riser, or, if itis a riserless casting or the gating is frozen off, will cause some mold wall movement, if the wall is still thinor the liquid is still a large portion of the casting. Since hypereutectic irons will not form thick casting walls



before entering the eutectic arrest, they should be risered, or there will be mold wall movement! This goesagainst conventional thinking, but such previous thinking was probably based on hypereutectic chemistry,and a eutectic freezing mode where no graphite forms in the liquid.

The formation of graphite nodules in the liquid reduces the remaining carbon in the iron down to theeutectic level. Assuming a 3.9 carbon and a 2.4 silicon iron (C.E. of 4.7), this will lead to a carbon levelremaining in the liquid of 3.53% with the balance of 0.37% going to expansion in the liquid riser or moldwall movement.

4.33 C.E. - (2.4 Si / 3) = 3.53 C

Figure 4. Hypereutectic liquid iron is depleted of carbon down to the eutectic point by formation of graphite

Once the graphite liquidus is finished, the eutectic forms and the remaining carbon down to the capabilityof the austenite to hold carbon (2% C.E.) is rejected from the austenite in the form of graphite. Againassuming a 3.9 carbon and a 2.4 silicon iron, this will lead to the formation of about 2.7% graphite in theiron at eutectic.

2.0 C.E. - (2.4 Si / 3) = 1.2 % C in austenite3.9 C - 0.37 graphite - 1.2 C in austenite = 2.33% graphite formed at eutectic temperature3.9 C - 0.37 graphite in liquid - 0.78 retained carbon = 2.75 graphite for expansion.

Figure 5. Note the large area of the graphitic arrest in the Cooling Rate graphic. This represents aconsiderable amount of graphite coming out. The energy production of the graphitic liquidus is not as greatas an austenite liquidus. This iron would be subject to macro-shrink, but the micro-shrink is ok. Thegraphite shape is also poor with several clusters of fast growing graphite present.

The remainder of the carbon can transform into graphite as the iron cools further. The amount of retainedcarbon in the unheat-treated room temperature iron is about 20%1 plus whatever carbon is retained inpearlite or carbides. If we assume no pearlite, then the total expansion of the graphite that benefits fightingshrinkage would be 2.75%, and the wasted graphite expansion would be 0.36% or 13% of the totalexpansion of graphite.

Hypoeutectic Mode

In a hypoeutectic mode, an austenite liquidus forms, and dendrites grow into the liquid, increasing thecarbon content of the remaining liquid. This iron will develop a stronger casting wall to resist mold wallmovement, but will have less graphite formed to offset macro-shrinkage. For an iron with 3.4 carbon and2.1 Silicon (C.E. of 4.1), a little less than 10% of the casting will be solid before the eutectic is reached.

2x + (1-x)* 4.33 = 4.1 C.E. (lever rule)



x = 9.87%

At the eutectic, the graphite formed would be 2.1%

2.0 C.E. - (2.1 Si / 3) = 1.3 % C in austenite3.4 C - 1.3 C in austenite = 2.1% graphite formed at eutectic temperature

3.4 C - 0.68 retained carbon = 2.72 graphite for expansion.

Applying similar logic to the previous example, we would gain a total of 2.72% graphite to fight expansion.This is not much different than the hypereutectic mode result.

Figure 6. Hypoeutectic mode solidification: austenite liquidus and eutectic

Eutectic Mode

In the eutectic mode, there is no liquidus arrest. Due to the presence of magnesium, a single arrest(eutectic) mode can occur between 4.3 C.E. and as high as a 4.6 C.E. Assuming 2.4 silicon, this iron couldcontain from a 3.5 to a 3.8 carbon. At the eutectic, this would produce a range from 2.3 to 2.6% graphite: avariation of 13%.

2.0 C.E. - (2.4 Si / 3) = 1.2 % carbon in austenite3.5 C - 1.2 C in austenite = 2.30% graphite formed at eutectic temperature3.8 C - 1.2 C in austenite = 2.60% graphite formed at eutectic temperature3.5 C - 0.70 retained carbon = 2.80% graphite for expansion.3.8 C - 0.76 retained carbon = 3.04% graphite for expansion.

Applying similar logic to the previous examples, we would gain a total of between 2.80% and 3.04%graphite to fight expansion. There is no liquid expansion problem, and the 3.8% carbon example has 13%more beneficial graphite then the slightly higher 3.9% carbon hypereutectic iron.

Figure 7. Single arrest eutectic mode solidification

Hyper-Hypoeutectic Mode

This mode occurs more often than suspected. A large graphitic liquidus starts a reaction that removes somuch carbon from the liquid, (possibly through flotation) that the remaining liquid turns hypoeutectic, and anaustenite liquidus follows. This material has the worst aspects of a hypereutectic iron (mold wall movement,



no appreciable wall thickness, low graphite contribution to fight shrink) and has all the bad aspects of ahypoeutectic iron (even lower graphite contribution to fight shrink).

Figure 8. Expanded region of eutectic zone due to magnesium suppression of graphite formation.

Assuming a 3.9 carbon and a 2.4 silicon iron (C.E. of 4.7), and that the iron falls to a 4.25 C.E. this willlead to a carbon level remaining in the liquid of 3.45% with the balance of 0.45% going to expansion in theliquid riser or mold wall movement.

4.25 C.E. - (2.4 Si / 3) = 3.45 C

The eutectic forms, and the remaining carbon down to the capability of the austenite to hold carbon (2%C.E.) is rejected from the austenite in the form of graphite. Again assuming a 3.9 carbon and a 2.4 siliconiron, this will lead to the formation of about 2.6% graphite in the iron at eutectic.

2.0 C.E. - (2.4 Si / 3) = 1.2 % carbon in austenite3.9 C - 0.45 graphite in liquid - 1.2 C in austenite = 2.25% graphite formed at eutectic temperature3.9 C - 0.45 graphite in liquid - 0.78 retained carbon = 2.67 graphite for expansion.

The remainder of the carbon can transform into graphite as the iron cools further. The amount of retainedcarbon in the unheat-treated room temperature iron is about 20%1 plus whatever carbon is retained inpearlite or carbides. If we assume no pearlite, then the total expansion of the graphite that benefits fightingshrinkage will be 2.67%, and the wasted graphite expansion will be 0.45% or 17% of the total expansion ofgraphite.

Figure 9. The two liquidus arrests are followed by the eutectic arrest. The first liquidus arrest is large butnot energetic (graphitic). The second liquidus arrest is small but very energetic (austenite).

Conclusion

Macro-shrinkage is the result of the interaction of several complex influences in the iron. If the shrinkage isconstantly present from day to day, then the gating and risering vs. the iron chemistry needs to be revised.But if the problem comes and goes, and the chemistry seems to be consistent during these episodes ofshrinkage, then the problem is most likely in the control and timing of the graphitizing process.

Magnesium opens up the C.E. range of a eutectic iron by inhibiting the formation of a graphite liquidus.This opens up the possibility to have more carbon in the iron to offset shrinkage so long as no graphiticliquidus occurs. This phenomena needs to be studied more in terms of effective magnesium vs. carbonlevel vs. inoculation. Calcium further changes the nucleation of the iron by inhibiting the formation ofaustenite dendrites and promoting a single heavily under cooled eutectic arrest.



Small-localized carbon flotation may be far more common than previously thought, and can result in slowcooling sections anytime that the graphitic liquidus occurs in that section size. This can account for 15 to20% less graphite being available to counteract the macro-shrinkage. This can also occur in iron when thecarbon equivalent is on the high side of safe, and the effective magnesium is on the low side of the normaloperating range. Inoculation may also influence the appearance of the graphitic liquidus.

The eutectic mode of freezing with irons that are above the eutectic in chemistry will give the most "lategraphite" to counteract macro-shrinkage. There is as much as a 13% gain in late graphite possible with thismode of solidification. Likewise, irons of the same C.E. level that are lower in silicon will have moregraphite to counteract shrinkage.

Thermal analysis provides a unique picture of how all these factors combine together to produce differentmodes of freezing. It can identify irons susceptible to carbon flotation, as well as when the iron will have agraphitic liquidus.

Before and after in-stream inoculation

References

1. T. Skaland and O. Grong: "Nodule Distribution in Ductile Cast Iron," AFS Transactions 91-56, p 153-157 (1991).

2. Torbjorn Skaland: A Model for the Graphite Formation in Ductile Cast Iron, University of Thronheim,Norway. (1992)

3. R.W. Heine: "Nodule Count: The Benchmark of Ductile Iron Solidification," AFS Transactions 93-84,p 879 (1993)

4. R.W. Heine: "Carbon, Silicon, Carbon Equivalent, Solidification, and Thermal Analysis Relationshipsin Gray and Ductile Cast Irons," AFS Transactions 72-82, p 462 (1972)

5. D.M Stefanescu, H.Q. Qiu and C.H. Chen: "Effects of selected metal and mold variables on thedispersed shrinkage in spheroidal graphitic cast iron," AFS Transactions 95-057, p 189 (1995)

6. T.N. Blackman: "Graphite Flotation in Ductile Iron Castings," AFS Special Report (1988)7. A.G. Fuller, T.N. Blackman: "Effects of Composition and Foundry Process Variables on Graphite

Flotation in Hypereutectic Ductile Irons," AFS Special Report (1988)8. R. Boeri, F. Weinberg: "Microsegregation in Ductile Iron," AFS Transactions 89-106, p 179 (1989)9. Richard Fruehan: "Gases in Metals," ASM Handbook volume 15 Castings, p 82 (1992)

10. D.A. Sparkman, C.A. Bhaskaran: "Chill Measurement by Thermal Analysis," AFS Transactions 96-127, p 969 (1996)

11. David Sparkman: "Using Thermal Analysis Practically in Iron Casting," Modern Castings November1992, p 35

12. Carsten Komkowski: unpublished Masters thesis work on deoxidation of Iron, Kiel, Germany.13. A.Alagarsamy, F.Jacobs, G.Strong, R.Heine: "Carbon Equivalent vs. Austenite Liquidus: What is the



correct Relationship for Cast Irons" AFS Transactions 84-31, p 871 (1984)




The Ductile Iron News - Near Net Shape DI Components

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Near Net Shape Ductile Iron Components - A NovelApproach Using Semi-Solid Forming

by: P.H. Mani

Introduction: Nearly all metals and alloys of commercial importance solidifydendritically, either with a columnar or with an equiaxed dendritic structure.

When an alloy that normally solidifies dendritically is vigorously stirred duringsolidification, the dendritic structure can be broken up and replaced by the more orless spherical structure. The resulting semi-solid structure deforms homogeneouslyand can be formed into shapes by several methods.

Semi-solid forming is the generic term applied to a process in which a mixture ofsolid and liquid phase metal is introduced into a mold or a die for net shapeforming. One might think of the process as a hybrid between casting and forgingand because the equipment used more closely resembles the die casting process.Semi-solid manufacturing seems to have fallen into the domain of metal casters.

Semi-solid processing of non-ferrous alloys, especially Aluminum alloys are inproduction phase for many critical automotive components. They have taken a biteon the traditional market share of ferrous components, especially ductile iron.

However, the possibility of near net shaping of high melting point alloys in thesemi-solid state has already been demonstrated.

This paper presents the results of the possibilities of making semi-solid processedductile iron components and the potential applications for such components.

Semi-solid processing of 'as cast' ductile iron slugs:

It was decided to manufacture a series of ductile iron gears (cog wheels) using thesemi-solid forming technique. Fig 1 shows the drawing of the proposed gear.

Figure 1. Gear (cog wheel) Material: Ferritic Ductile Iron

FEATURES







Basic Metallurgy






The chemistry of the slugs is as follows:

Element % Element %

Carbon 3.65 Silicon 2.63

Manganese 0.29 Chromium 0.03

Aluminum 0.01 Sulfur 0.007

Phos 0.013 Copper 0.140

Nickel 0.02 Magnesium 0.045

Moly less than 0.01 Tin less than 0.01

Titanium less than 0.01

The slugs were machined to a dimension of 58mm diameter by 80mm height.

A graphite die was machined to the shape of the gear. A D2 Steel die or any othermetallic die would be used in production. The machined slugs were placed insidethe induction coil with the thermocouple inserted in position. See Figs 2,3,4 for theset up.

Figure 2. Experimental arrangement for heating trials of ductile iron slug.



Figure 3

Figure 4. Hot slug traveling towards die cavity



Nitrogen was used as an inert gas to provide the inert atmosphere inside the coil.By having the slugs in the center of the induction coil and having the top andbottom of the slugs insulated with insulating pads after the 'soaking' period theradial temperature difference was around 12-15oF (approximately 5oC) and theaxial temperature difference was around 25-30oF (12-15oC). This temperaturedifference appears to be sufficient to allow semi-solid forming of the ductile ironslugs within the processing window. The as-cast slug was first heated to atemperature of 2120oF (1160oC) for about 170 seconds and then further heated toa temperature of 2138oF(1170oC) and held at this temperature for another 90seconds.

The dwell time when the forging load was applied was set to 30 seconds.

The slug was injected into the die in this condition to produce the gears.

Fig 5&6 shows the gears made by this process.

Figures 5 and 6. S.S.M. (Thixoformed)ductile iron cog wheels (gears)

The presence of carbides in the microstructure due to rapid cooling of the liquidfraction of the slug during the semi solid forming is eliminated by 'synchronized'annealing above the critical temperature.



Semi solid formed components can be austempered in 'tandem' with this process toobtain high strength, high toughness properties.

In addition to ductile iron, compacted graphite iron can be semi-solid formed to nearnet shapes.




The Ductile Iron News - Shrinkage in Nodular Iron

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Shrinkage in Nodular IronEli David Senior Manager Technical Services, Globe Metallurgical

With increasing complexity in casting geometry and continued stringentrequirements for completely sound castings, understanding and predicting theshrinkage behavior of ductile cast iron parts is all the more crucial for successfulfoundry operations.

Four distinct regions can be isolated when observing ductile iron solidify.

A. Liquid contraction from the superheat temperature to the liquidus. Thiscontraction is very predictable since it is dependent on the coefficient ofexpansion of the alloy (generally around 1.5% by volume per 100oC).

B. Liquid shrinkage through the liquidus temperature. A phase change takesplace at this juncture. A portion of the liquid iron transforms to solidaustenite. Occasionally for highly hypereutectic irons graphite precipitates atthe liquidus instead of austenite, resulting in expansion rather thancontraction.

C. Eutectic expansion follows the liquidus. The remaining liquid transforms intoaustenite and graphite. Expansion always occurs during the eutectictransformation and it is very significant. This is because all of the carbon inthe liquid iron minus the carbon dissolved in the austenite precipitates asgraphite during the eutectic. The volume fraction of graphite (in the eutectic)that precipitates can be calculated using the lever rule. For an iron with atypical 3.65% carbon (Co =3.65%) the fraction percent of graphite in theeutectic is as follows:G/G+g = Co-Cg/CG-Cg = (3.65-1.90)/(100-1.90) = 1.78%The eutectic consists of 98.22% austenite and 1.78% graphite by weight.The amount of carbon dissolved in the austenite is roughly 1.90%. Thereforeof the 3.65% compositional carbon, 1.87% is dissolved in the austenite and1.78% precipitates, hopefully, as graphite.Graphite has a much higher specific volume compared to iron causing theexpansion that is observed. The density of graphite is 2.2 g/cc compared to7 g/cc for that of iron.

D. Solid contraction is also dependent on the expansion coefficient.

These changes are depicted schematically in Fig.1 for three different irons.

The following should be noted:

a. All three irons undergo a net expansion during solidification. The volume ofthe solidified iron at the end of solidification (before solid contraction) isgreater than the volume of the liquid poured into the mold!

FEATURES







Basic Metallurgy






b. Hypereutectic ductile irons have been measured to exhibit volumetricexpansion as high as 4%.

c. For the same carbon equivalent ductile will expand more than gray.d. Feed metal must be supplied by risers and/or the gating system for all cast

irons in zone A. Additional feed metal must be provided in zone B forhypoeutectic irons.

e. The reason eutectic expansion cannot be effectively utilized to compensatefor earlier contraction and shrinkage is that green sand mold walls dilate(move outward) when subject to the enormous expansion forces. Note (inFig. 2) that at the end of solidification when the metal contracts the moldwall stays at its maximum dilated position.

Solidification Mechanisms: Cast iron solidification is very different from that of apure metal. Pure metals solidify with a solidification front that is very well definedand a clearly delineated solid liquid interface. Ductile cast iron solidification, on theother hand, is characterized by a very thin solidified skin and if conditions are notoptimal a large mushy zone. The outer skin formed during gray cast ironsolidification is much heavier than that of ductile. Flake graphite is a betterconductor of heat compared to nodular. The heavier skin prevents the transmissionof the eutectic expansion forces to the mold walls. This is the reason why grayirons need less risering than ductile even though ductile iron solidification results ina larger net expansion.

The width of the mushy zone and the aspect ratio of the austenite dendrites havebeen linked to the feeding capability of the riser. Generally short stubby dendrites ina narrower mushy zone will produce better feeding characteristics. Narrower mushyzones are obtained when nodular iron solidifies as a eutectic with very littleseparation between the liquidus and eutectic temperatures. Austenite thatprecipitates during the liquidus tends to grow much larger in size. Finer eutecticaustenite is also believed to improve feeding capability and to be associated withhigher nodule counts. Most foundry engineers have to rely on experience or guessat how far a particular riser will feed. Even though research has produced testpatterns that can evaluate feeding distances, very few foundries take the time toevaluate this key variable. The problem is compounded particularly since themushy zone changes from tap to tap depending on the metallurgy and quality ofthe iron. Therefore the feeding distance itself is a function of the metallurgicalintegrity of the iron.

Comparative Solidification Schematic - Fig. 3



For the purposes of this paper shrinkagewill be divided into four categories:

1. Pull downs or suckins.2. Macro shrink larger than 5 mm3. Micro shrink or shrinkage porosity

less than 3 mm4. Microscopic grain boundary

shrinkage. Generally only visibleunder a microscope at amagnification greater than 100X.

Fig. 4

The current paper will focus on the first three types only. These defects occur atvery different and distinct times during solidification as depicted in Fig.5.



Thermal analysis is probably the strongest tool available in the foundry man'sarsenal to understand and combat shrinkage defects.

For example a high value for the area S1 is associated with a lot of primaryaustenite and a large mushy zone and therefore with an iron that is more likely toproduce pull down and macro shrinkage upon solidification.

In fact large variations in S1 have been observed from treatment batch to batch(before post inoculation) in the same foundry on the very same day. Base ironholding time appears to be the single most dominant variable contributing to thisdeviation. Strong post inoculation appears to mitigate the variance in S1.

Pull downs or suckins are produced very early in solidification. The skin formedat the top cope surface is extremely thin. If feed metal is not provided thencontraction will cause a negative pressure just below the skin. The atmosphericpressure then pushes the wall inward producing the "pull down" or "outer sunk"defect

Macro shrink generally appears a little later. The skin formed is thick enough andwill not cave in. The negative pressure consequently produces rather large shrinkholes. If this defect appears at the riser contact or inside the casting cavityrelatively close to the riser (as it generally does) proper risering technique can andshould be utilized to solve the problem.

The first observation when trouble-shooting macro shrink should be "Did the riserpipe?" The remedies applied are very different depending on whether the riserpiped or not.

If the riser piped properly then possible solutions are:

1. Increase riser size2. Check carbon equivalent. It may be too low 3. Lower pouring temperature

However, if the riser did not pipe then the analysis is not as straight forward andthe following are recommended:

1. Reduce riser contact modulus. The contact modulus may be too largekeeping the contact open during the casting eutectic expansion leading toback feeding.

2. Reduce the modulus of the ingate feeding the riser. If the ingate stays opentoo long initial feed metal will be delivered to the casting cavity from thegating system rather than the riser. The top of the riser will then freeze off



preventing proper piping. Conical risers are particularly vulnerable to thisphenomenon.

3. Check carbon equivalent. It may be too high4. There may be too many risers present5. Pouring temperature may be too cold

If macro shrink appears infrequently and intermittently (comes and goes) and stillwithin the known limit of the risers feeding capability, then variations in metallurgicalintegrity (larger mushy zone and S1 inhibiting feeding) or poor sand compactionwith soft molds are more than likely the culprits particularly if the chemistry checksout OK. From a chemistry point of view, hypoeutectic irons (both gray and ductile)are far more susceptible to macro shrink and outer sunks. A large separationbetween liquidus and eutectic (as would be expected with hypoeutectic irons)produces a lot more primary austenite thereby reducing the riser's ability to feed. Inductile irons, which tend to be hypereutectic except when pouring very heavysections, it is desirable for the casting to freeze as a eutectic alloy i.e. with theliquidus arrest as close as possible to the eutectic. Generally when the liquidusappears at a much higher temperature from that of the eutectic, primary austenite isprecipitating from the melt even though the chemical composition is hypereutectic.In ductile irons this happens because of the strong undercooling effects of elementssuch as magnesium and rare earths. Furthermore, highly oxidizing conditions in themelt coupled with high melting temperatures and long holding times reduce thecarbon activity causing a chemically hypereutectic iron to solidify as if it werehypoeutectic.

Micro shrinkage porosity appears very late in solidification. At this stage feedpaths are well closed. This type of shrink commonly appears on isolated bosses oroutside the riser's ability to feed. The only possibility to obtain sound castings is torely on late eutectic graphite precipitation, with its inherent expansion, to "fill in" theshrinkage voids. Eutectic solidification patterns where most of the graphite comesout early are undesirable.A uniform precipitation pattern is preferred. A good thermal analysis program canhelp measure such variables.

Since it is helpful to have graphite come out late then, by definition, amicrostructure with varying nodule sizes (nodule bifurcation) or a bi- modal nodulesize distribution will be less likely to produce micro shrink. Graphite that comes outearly in the eutectic will grow to a larger size when compared to that of graphitethat precipitates toward the end of the eutectic, since the late graphite will not havesufficient time for growth. Care must be taken when evaluating structures since one is viewing a three-dimension picture in 2D. The size of any given nodule will not only depend on thenodule size but also where the nodule happened to be sectioned. Furthermore,great care should be taken, when making such analysis, that the bimodaldistribution is not due to pre-eutectic graphite precipitation. Pre-eutectic arrestsassociated with exceedingly hypereutectic irons can also exhibit a bi-modaldistribution. Graphite that precipitates during the liquidus generally ends up muchlarger in size than the eutectic graphite. This is generally an undesirable outcome.Therefore thermal analysis curves should be viewed concurrently with themicrostructure. Furthermore, several late solidification phenomena can also beevaluated from the cooling curves. These will not be discussed in this paper otherthan to add that they are invaluable in determining the amount of graphite thatprecipitates late in the eutectic and therefore the susceptibility of the iron to microshrinkage defects.

General Foundry Practice: There can be no substitute for good common sensefoundry practice. Avoid super heating, long holding times, oxidized charge materialsand poorly compacted soft molds. Keep carbon as high as possible, siliconmaintained at the lower end of normal operational ranges, appears to reduce shrinkdefects. Residual magnesium should be maintained at levels to ensure propernodularity and no higher. Rare Earth elements should be optimized depending onthe level of tramp elements such as sulfur, oxygen and bismuth (if added).Inoculant addition should be precisely controlled and the type and quantity shouldbe optimized. Clamping cope and drag molds will help reduce shrink defects. Forflask less molding ensure that mold weighting is sufficient.

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The Ductile Iron News - Solving Casting Problems with New Sleeve Technology

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Solving Casting Problems with New Sleeve Technology

Ronald C. Aufderheide Ralph E. Showman Foundry Products Division AshlandSpecialty Chemical Company Division of Ashland Inc.

Abstract

Foundrymen are constantly being confronted with challenges to improve theiroperations and lower costs while at the same time producing higher qualitycastings. One of the ways to achieve lower costs and improve the soundness of acasting is to incorporate the use of exothermic riser sleeves. This can lead toimproved yield while solving shrinkage problems. However, at the same time, theuse of exothermic riser sleeves can create other problems. This paper will discusstwo defects that, under certain conditions, can be created by the use of exothermicriser sleeves in ductile iron.

The first defect is a surface "fish-eye" defect that is caused by the buildup ofexothermic sleeve material in the molding sand. This defect doesn't occur on acasting just because it is made using exothermic sleeves; rather, it occurs on theductile iron castings that are made with sand that has been contaminated withexothermic sleeve materials. Tests showed that the presence of fluorine in theexothermic sleeve formulations contributed to the formation of fish-eye defects. Anew exothermic sleeve was developed that did not contain fluorine and thateliminated the fish-eye defect.

The second defect is a degradation of the graphite nodules in ductile iron castings.Testing showed that the amount of flake graphite is related to the type andcomposition of the exothermic sleeve. The degradation was highest when sand-based exothermic sleeves were used. Fiber-based exothermic sleeves producedslightly less degradation, and the new-technology cold box-based fiber-free andfluorine-free exothermic sleeves produced the least amount of degradation whenexothermic sleeves were used. Insulating riser sleeves did not show anydegradation tendencies. Depending upon the type of exothermic sleeves used,special considerations need to be made with respect to the placement, size, andquantity of sleeves used so that no contaminated metal gets into the casting itself.

Introduction

Ductile iron castings have unique riser requirements compared to the feeding ofother metals. The volume changes in the casting are not a simple contraction asthe metal cools. For example, when the graphite nodules are formed, the metalactually expands, which can push metal back into the riser and gating system ifthese are not properly designed. This, along with the subsequent contraction of themetal as it cools, creates a strong demand on the feeding capabilities of the riser.

Now, more than ever, foundries are trying to find ways to reduce their overall costto produce a casting. One way to reduce costs is to incorporate the use ofexothermic sleeves around the risers. This allows the use of smaller risers thatimprove yield and reduce the contact surface area of the riser to the casting, whichcosts money to grind off.

There are a variety of sizes, shapes, and formulations available in the exothermiccategory of riser sleeves. Traditionally the sleeves were made of fibrous refractorycombined with a blend of materials that produce an exothermic reaction morecommonly known as a thermite reaction. The most common fuel material isaluminum. When mixed with an oxidizer and an initiator/fluxing material andexposed to extreme heat, the aluminum is oxidized, giving off heat as the reactionproceeds.

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The Ductile Iron News - Solving Casting Problems with New Sleeve Technology

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2Al + Fe203 ----> Al203 + 2Fe + Heat (Fluoride initiator / Flux + Heat)

In addition to fiber-based exothermic sleeves, sand-based exothermic sleeves hadbeen gaining favor with many ductile iron foundries. Sand-based, high-densitysleeves are formulated to contain more aluminum fuel and to generate a greateramount of heat. This heat is first required to raise the temperature of the sand-based sleeve, before favorably influencing the temperature of the metal in the riser.

Finally, in 1997 Fiber-free New Technology Sleeves were introduced, providinganother exothermic sleeve alternative. The refractory material is a round aluminasilicate material bonded by cold box resin technology. During the development ofthis technology, it became apparent that the requirements for an exothermic sleevefor ductile iron applications are different from those for an exothermic sleeve usedto make steel castings. This is especially true for cold risers in ductile iron.

Cold risers are those risers that are filled with metal after the metal has movedthrough the casting, as opposed to hot risers that are filled by the gating systembefore the metal goes into the casting. This makes the metal in the cold risercolder and closer to solidifying. In order to get an exothermic reaction to start, themetal needs to give up some of its energy to the riser sleeve. However, if too muchenergy is given up, the surface of the riser begins to skin. Once the skin hasformed, the exothermic reaction of the riser sleeve is not enough to remelt it, andthe riser becomes less efficient in its feeding capabilities. To solve this, a special,fast-igniting exothermic sleeve is needed so that the energy taken out of the metalin the cold riser is minimized. It has been found that cold ductile iron risers exhibitimproved performance when their formulation has been optimized so that theyignite at lower temperature and energy levels, have a faster ignition time, and burnat higher temperature

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