Topic : Design & Methoding Of Steel Castings

Sixth National Foundry Conclave

01-02 March 2013 : Hotel Le Meridien : Coimbatore

Presented By : Mr. V.S. Jayabal , M.Tech., Managing Director , JRE Valves & Pumps (P) Ltd.,

Designing & Methoding of Steel Castings

The Birth of the casting

By V.S.J

The Story

My IIT Programme , in the Foundry Technology , M.Tech , Started with this story , as told by my respected Professor in the year of Aug 1970.

By V.S.J

Fundamentals of Casting

Casting, one of the oldest manufacturing processes, dates back to 4000 B.C. when copper arrowheads were made.

Casting processes basically involve the introduction of a molten metal into a mold cavity, where upon solidification, the metal takes on the shape of the mold cavity.

Simple and complicated shapes can be made from any metal that can be melted. Example of cast parts: frames, structural parts, machine components, engine blocks, valves, pipes, statues, ornamental artifacts…..

By V.S.J

Geometric simplicity:– Although casting can be used to produce complex part

geometries, simplifying the part design usually improves castability

– Avoiding unnecessary complexities:Simplifies mold‑makingReduces the need for coresImproves the strength of the casting

Corners on the casting:– Sharp corners and angles should be avoided, since they are

sources of stress concentrations and may cause hot tearing and cracks

– Generous fillets should be designed on inside corners and sharp edges should be blended

Product Design Considerations

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Minor changes in part design can reduce need for coring

Design change to eliminate the need for using a core: (a) original design, and (b) redesign.

Product Design Considerations

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Product Design Considerations

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Gating System

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Pattern design

1. Dimensional consistency

2. Padding & Feeder Locations .

3. Reduce the no. Of cores as much as possible.

4. The Fettling shop work , must be minimized as much as possible.

5. Preferable to go for a aluminum Patterns .

6. Wherever not possible , wood / Aluminum construction can be used.

7. Ex. Ribs , bosses , changeable flanges can be made with Aluminum.

8. Multiple patterns depending up – on the quantum requirements.

The Following Points Shall be considered , before selecting the parting line

By V.S.J

1. It’s Preferable to match plate the patterns with runners , risers incl. filters & Integral test bars , wherever reqd.

2. It’s always a good idea to have the integral test bars for the castings weighing ex. 700 Kg’s & above.

3. Chill Pattern need to be made for each & every application.

4. Suitable Colour coding is Preferable.

5. Method Drgs must be referred every time.

Pattern Shop

By V.S.J

1. Design suitability for the foundry purpose.

2. The Application of the casting

3. Working Medium.

Methoding

Primary Considerations

By V.S.J

Preparing a mold cavity of the desired shape with proper allowance for shrinkage.

Melting the metal with acceptable quality and temp.

Pouring the metal into the cavity and providing means for the escape of air or gases.

Solidification process, must be properly designed and controlled to avoid defects.

Mold removal. Finishing, cleaning and inspection operations.

Casting Processes

By V.S.J

Most widely used casting process, accounting for a significant majority of total tonnage cast

Nearly all alloys can be sand casted, including metals with high melting temperatures, such as steel, nickel, and titanium

Castings range in size from small to very large Production quantities from one to millions

Over View of Sand Casting

During pouring, buoyancy of the molten metal tends to displace the core, which can cause casting to be defective

Force tending to lift core = weight of displaced liquid less the weight of core itself

Fb = Wm ‑ Wc

where Fb = buoyancy force;

Wm = weight of molten metal displaced; and

Wc = weight of core

Buoyancy in Sand Casting Operation

This Buoyancy effect can be avoided , with the proper design of the Core Print ; In worst cases by Chaplet.

Fluidity :The capability of a molten metal to fill mold cavities

Viscosity :Higher viscosity decreases fluidity

Surface tension :Decreases fluidity; often caused by oxide film

Inclusions :Insoluble particles can increase viscosity, reducing fluidity

Solidification pattern :Fluidity is inversely proportional to the freezing temperature range

Fluidity of Molten Metal

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Mold design :The design and size of the sprue, runners, and risers affect fluidity

Mold material and surface :Thermal conductivity and roughness decrease fluidity

Superheating :The temperature increment above the melting point increases fluidity

Pouring :Lower pouring rates decrease fluidity because of faster cooling

Heat transfer :Affects the viscosity of the metal

Increasing Fluidity : The fluidity of the Steel can be increased by Micro level

additions of Cerium Mishmetal.

Fluidity of Molten Metal

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Types Of Solidification

1. Mushy Formation

2. Directional Solidification

By V.S.J

Solidification contraction for various cast metals

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Metal or alloy Volumetric solidification

contraction %

Metal or alloy Volumetric solidification contraction %

Aluminum 6.6 70% Cu-30% Zn 4.5

Al-4.5% Cu 6.3 90% Cu-10% Al 4

Al-12% Si 3.8 Gray Iron Expansion to 2.5

Carbon steel 2.5-3.0 Magnesium 4.2

1% Carbon steel 4 White iron 4.0-5.5

Copper 4.9 Zinc 6.5

Pure metals solidify at a constant temperature;

Pure Metals / Alloys

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Alloys solidify within a temperature range

Pure Metals / Alloys

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Directional Solidification Pattern of Quartz Growth of Dendrites

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Directional Solidification Pattern of Quartz Growth of Dendrites

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Directional Solidification Pattern of Quartz Growth of Dendrites

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Directional Solidification Pattern of Quartz Growth of Dendrites

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Directional Solidification Pattern of Quartz Growth of Dendrites

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Directional Solidification Pattern of Quartz Growth of Dendrites

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Directional Solidification Pattern of Quartz Growth of Dendrites

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The solidification time is a function of the volume of a casting and its surface area.

Solidification time = C ( volume / surface area)2, (1)

Where C is a constant that reflects mold material, metal properties and temperature. Thus large sphere solidifies and cools to ambient temperature at a much slower rate than dose a smaller sphere.

Solidification Time

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Sphere: V = (4/3) лr3, r = (3/4 л)1/3, And A = 4 лr2 =4 л (3/4 л)2/3 = 4.84;

Cube: V = a3, a = 1, A = 6a2 = 6;

Cylinder: V = лr2b = 2 лr3, r = (1/2 л)1/3, and A = 2 лr2 + 2 лrb = 6 лr2 = 6 л(1/2 л)2/3 = 5.54

Example:

Solidification times for different shapes:Three pieces being cast have the same volume but different shapes. One is a sphere; one is a cube, and the other a cylinder with a height equal to its diameter. Which piece will solidify the fastest and which one the slowest? The volume is unity, so we have from equation (1):

Solidification time = 1 / surface areaThe respective surface areas are: Thus the respective solidification time’s t are

tsphere = 1 / (4.84 2) 0.043 C,

tcube = 1 / (62 ) 0.028 C,

t cylinder = 1 / ( 5.542) 0.033 C.

Hence , For Feeding We are using Cylindrical Feeders , instead of Sphere feeders ; Because Sphere is having a Slow Solidification Time.

Solidification Time

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1. Liquid to Liquid shrinkage Risers

2. Liquid to solid Shrinkage Risers

3. Solid Shrinkage Pattern contraction

allowance

Types Of Shrinkages

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Shrinkage - Feed Metal requirements :

1. The Super Heated steel, more than what is reqd can

cause a severe problem.

2. The Most scientific form of Calculating the

solidification time – Modulus method.

Shrinkage

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1. Identify the Locations – Normally the heavy

junctions.

2. Try to reduce the no. of locations by connecting

them with the pads.

3. The Pads can be either metallic or ins / exo pads.

4. Calculate the solidification time .

5. This is simplified by the Modulus method.

6. Modulus = Volume / Surface Area

Calculation of the Feeder Dimensions

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7. To achieve the directional solidification , the modulus

need to be increased progressively to the neck & to the

feeder.

8. Ex. Casting Modulus is Mc = 1

Neck – 10% More Mn = 1.1

Feeder - another 10% Mf = 1.2

9. Application of Ins/ Exo Sleeves.

10. The Efficiency of the sleeves is governed by the Modulus Extension Factor ( MEF ).

Calculation of the Feeder Dimensions

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11. Ex. If MEF is 1.2 then , Mf is = 1.2 / 1.2 = 1

1.3 1.2 / 1.3 = 0.92

1.4 1.2 / 1.4 = 0.85

1.5 1.2 / 1.5 = 0.80

The feeder Dimensions can be calculated based on the

– modulus.

Further , depending up – on the contact area available.

The ratio between the diameter & height can be fixed

Thus , smaller & smaller feeders are reqd with efficient

sleeves.

Calculation of the Feeder Dimensions

By V.S.J

12. The No . Of Feeders can also be further reduced

by the effective utilization of the external Chills.

13. The Chill thickness shall be Min 80 % of the

contact area.

14. In the case of the deep pockets in the casting ,

Sand can be superheated & may cause defects.

15.To avoid this Zircon / Chromite sand can be

used , because of their higher density & thermal

conductivity , the problem is minimized.

Calculation of the Feeder Dimensions

By V.S.J

1. Inspite of the best Practices in the melting & moulding system , the metal

steel picks up inclusions , as it flows through the pouring cup & sprues.

2. There are different types of ceramic filters are available.

3. The size of the filters reqd , for particular size of casting is recommended

by the filter manufacturers.

4. These filters have to be installed in the running system .

5. Our experience shows , remarkable improvement in the defects caused by inclusions.

6. This is more advantage , if used for expensive stainless steels .

Use Of Metal Filters

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Typical Methoding

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Typical Methoding

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A. Metallic Projections

B. Cavities

C. Discontinuities

D. Defective surface

E. Incomplete Casting

F. Incorrect dimensions or shape

G. Inclusions

Casting defects

By V.S.J

By V.S.Jayabal , M.Tech.,Managing Director , JRE Valves & Pumps (P) Ltd.,