Austenitic stainless steels

Austenitic stainless steels have high ductility, low yield stress and relatively high ultimatetensile strength, when compare to a typical carbon steel.

A carbon steel on cooling transforms from Austenite to a mixture of ferrite andcementite. With austenitic stainless steel, the high chrome and nickel content suppressthis transformation keeping the material fully austenite on cooling (The Nickel maintainsthe austenite phase on cooling and the Chrome slows the transformation down so that afully austenitic structure can be achieved with only 8% Nickel).

Heat treatment and the thermal cycle caused by welding, have little influence onmechanical properties. However strength and hardness can be increased by coldworking, which will also reduce ductility. A full solution anneal (heating to around1045°C followed by quenching or rapid cooling) will restore the material to its originalcondition, removing alloy segregation, sensitisation, sigma phase and restoring ductilityafter cold working. Unfortunately the rapid cooling will re-introduce residual stresses,which could be as high as the yield point. Distortion can also occur if the object is notproperly supported during the annealing process.

Austenitic steels are not susceptible to hydrogen cracking, therefore pre-heating is seldomrequired, except to reduce the risk of shrinkage stresses in thick sections. Post weld heattreatment is seldom required as this material as a high resistance to brittle fracture;occasionally stress relief is carried out to reduce the risk of stress corrosion cracking,however this is likely to cause sensitisation unless a stabilised grade is used (limitedstress relief can be achieved with a low temperature of around 450°C ).

Austenitic steels have a F.C.C atomic structure which provides more planes for the flowof dislocations, combined with the low level of interstitial elements (elements that lockthe dislocation chain), gives this material its good ductility. This also explains why thismaterial has no clearly defined yield point, which is why its yield stress is alwaysexpressed as a proof stress. Austenitic steels have excellent toughness down to trueabsolute (-273°C), with no steep ductile to brittle transition.

This material has good corrosion resistance, but quite severe corrosion can occur incertain environments. The right choice of welding consumable and welding technique canbe crucial as the weld metal can corrode more than the parent material.

Probably the biggest cause of failure in pressure plant made of stainless steel is stresscorrosion cracking (S.C.C). This type of corrosion forms deep cracks in the material andis caused by the presence of chlorides in the process fluid or heating water/steam (Goodwater treatment is essential ), at a temperature above 50°C, when the material is subjectedto a tensile stress (this stress includes residual stress, which could be up to yield point inmagnitude). Significant increases in Nickel and also Molybdenum will reduce the risk.

Stainless steel has a very thin and stable oxide film rich in chrome. This film reformsrapidly by reaction with the atmosphere if damaged. If stainless steel is not adequatelyprotected from the atmosphere during welding or is subject to very heavy grindingoperations, a very thick oxide layer will form. This thick oxide layer, distinguished by itsblue tint, will have a chrome depleted layer under it, which will impair corrosionresistance. Both the oxide film and depleted layer must be removed, either mechanically(grinding with a fine grit is recommended, wire brushing and shot blasting will have lesseffect), or chemically (acid pickle with a mixture of nitric and hydrofluoric acid). Oncecleaned, the surface can be chemically passivated to enhance corrosion resistance,(passivation reduces the anodic reaction involved in the corrosion process).

Carbon steel tools, also supports or even sparks from grinding carbon steel, can embedfragments into the surface of the stainless steel. These fragments can then rust ifmoistened. Therefore it is recommended that stainless steel fabrication be carried out in aseparate designated area and special stainless steel tools used where possible.

If any part of stainless-steel is heated in the range 500 degrees to 800 degrees for anyreasonable time there is a risk that the chrome will form chrome carbides (a compoundformed with carbon) with any carbon present in the steel. This reduces the chromeavailable to provide the passive film and leads to preferential corrosion, which can besevere. This is often referred to as sensitisation. Therefore it is advisable when weldingstainless steel to use low heat input and restrict the maximum interpass temperature toaround 175°, although sensitisation of modern low carbon grades is unlikely unlessheated for prolonged periods. Small quantities of either titanium (321) or niobium (347)added to stabilise the material will inhibit the formation of chrome carbides.

To resist oxidation and creep high carbon grades such as 304H or 316H are often used.Their improved creep resistance relates to the presence of carbides and the slightlycoarser grain size associated with higher annealing temperatures. Because the highercarbon content inevitably leads to sensitisation, there may be a risk of corrosion duringplant shut downs, for this reason stabilised grades may be preferred such as 347H.

The solidification strength of austenitic stainless steel can be seriously impaired by smalladditions of impurities such as sulphur and phosphorous, this coupled with the materialshigh coefficient of expansion can cause serious solidification cracking problems. Most304 type alloys are designed to solidify initially as delta ferrite, which has a highsolubility for sulphur, transforming to austenite upon further cooling. This creates anaustenitic material containing tiny patches of residual delta ferrite, therefore not a trueaustenitic in the strict sense of the word. Filler metal often contains further additions ofdelta ferrite to ensure crack free welds.

The delta ferrite can transform to a very brittle phase called sigma, if heated above 550°Cfor very prolonged periods (Could take several thousand hours, depending on chromelevel. A duplex stainless steel can form sigma phase after only a few minutes at thistemperature)

The very high coefficient of expansion associated with this material means that weldingdistortion can be quite savage. I have seen thick ring flanges on pressure vessel twistafter welding to such an extent that a fluid seal is impossible. Thermal stress is anothermajor problem associated with stainless steel; premature failure can occur on pressureplant heated by a jacket or coils attached to a cold veesel. This material has poor thermalconductivity, therefore lower welding current is required (typically 25% less than carbonsteel) and narrower joint preparations can be tolerated. All common welding processescan be used successfully, however high deposition rates associated with SAW couldcause solidification cracking and possibly sensitisation, unless adequate precautions aretaken.

To ensure good corrosion resistance of the weld root it must be protected from theatmosphere by an inert gas shield during welding and subsequent cooling. The gas shieldshould be contained around the root of the weld by a suitable dam, which must permit acontinuous gas flow through the area. Welding should not commence until sufficienttime has elapsed to allow the volume of purging gas flowing through the dam to equal atleast the 6 times the volume contained in the dam (EN1011 Part 3 Recommends 10).Once purging is complete the purge flow rate should be reduced so that it only exerts asmall positive pressure, sufficient to exclude air. If good corrosion resistance of the rootis required the oxygen level in the dam should not exceed 0.1%(1000 ppm); for extremecorrosion resistance this should be reduced to 0.015% (150 ppm). Backing gasses aretypically argon or helium; Nitrogen Is often used as an economic alternative wherecorrosion resistance is not critical, Nitrogrn + 10% Helium is better. A wide variety ofproprietary pastes and backing materials are available than can be use to protect the rootinstead of a gas shield. In some applications where corrosion and oxide coking of theweld root is not important, such as large stainless steel ducting, no gas backing is used.

A pdf guide to weld purgingHuntingdon Fusion Techniques Limited

Carbon content:304 L grade Low Carbon, typically 0.03% Max304 grade Medium Carbon, typically 0.08% Max304H grade High Carbon, typically Up to 0.1%

The higher the carbon content the greater the yield strength. (Hence the stengthadvantage in using stabilised grades)

Typical Alloy Content

304316316 Ti320321347308309

(18-20Cr, 8-12Ni)(16-18Cr, 10-14Ni + 2-3Mo)(316 with Titanium Added)(Same as 316Ti)(17-19Cr, 9-12Ni + Titanium)(17-19Cr, 9-13Ni + Niobium)(19-22Cr, 9-11Ni)(22-24Cr, 12-15Ni)

304 + Molybdenum304 + Moly + Titanium-304 + Titanium304 + Niobium304 + Extra 2%Cr304 + Extra 4%Cr + 4% Ni

All the above stainless steel grades are basic variations of a 304. All are readily weldableand all have matching consumables, except for a 304 which is welded with a 308 or 316,321 is welded with a 347 (Titanium is not easily transferred across the arc) and a 316Ti isnormally welded with a 318.

Molybdenum has the same effect on the microstructure as chrome, except that it givesbetter resistance to pitting corrosion. Therefore a 316 needs less chrome than a 304.

310 (24-26Cr,19-22Ni) True Austenitic. This material does not transform to ferriteon cooling and therefore does not contain delta ferrite. Itwill not suffer sigma phase embrittlement but can be trickyto weld.

904L (20Cr,25Ni,4.5Mo) Super Austenitic Or Nickel alloy. Superior corrosionresistance providing they are welded carefully with lowheat input (less than 1 kJ/mm recommended) and fasttravel speeds with no weaving. Each run of weld shouldnot be started until the metal temperature falls below100°C. It is unlikely that a uniform distribution of alloywill be achieved throughout the weld (segregation),therefore this material should either be welded with an

over-alloyed consumable such as a 625 or solutionannealed after welding, if maximum corrosion resistance isrequired.

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Carbon Steel To Austenitic Steel

When a weld is made using a filler wire or consumable, there is a mixture in the weldconsisting of approximately 20% parent metal and 80% filler metal alloy ( percentagedepends on welding process, type of joint and welding parameters).

Any reduction in alloy content of 304 / 316 type austenitics is likely to cause the formationof matensite on cooling. This could lead to cracking problems and poor ductility. To avoidthis problem an overalloyed filler metal is used, such as a 309, which should still formaustenite on cooling providing dilution is not excessive.

The Shaeffler diagram can be used to determine the type of microstructure that can beexpected when a filler metal and parent metal of differing compositions are mixed togetherin a weld.

The Shaeffler Diagram

The Nickel and other elements that form Austenite, are plotted against Chrome and otherelements that form ferrite, using the following formula:-

Nickel Equivalent = %Ni + 30%C + 0.5%Mn

Chrome Equivalent = %Cr + Mo + 1.5%Si + 0.5%Nb

Example, a typical 304L = 18.2%Cr, 10.1%Ni, 1.2%Mn, 0.4%Si, 0.02%C

Ni Equiv = 10.1 + 30 x 0.02 + 0.5 x 1.2 = 11.3Cr Equiv = 18.2 + 0 + 1.5 x 0.4 + 0 = 18.8

A typical 309L welding consumable Ni Equiv = 14.35, Cr Equiv = 24.9

The main disadvantage with this diagram is that it does not represent Nitrogen, which is avery strong Austenite former.

Ferrite NumberThe ferrite number uses magnetic attraction as a means of measuring the proportion of deltaferrite present. The ferrite number is plotted on a modified Shaeffler diagram, the DelongDiagram. The Chrome and Nickel equivalent is the same as that used for the Shaefflerdiagram, except that the Nickel equivalent includes the addition of 30 times the Nitrogencontent.

Examples

The Shaeffler diagram above illustrates a carbon steel C.S , welded with 304L filler. Point Arepresents the anticipated composition of the weld metal, if it consists of a mixture of fillermetal and 25% parent metal. This diluted weld, according to the diagram, will containmartensite. This problem can be overcome if a higher alloyed filler is used, such as a 309L,which has a higher nickel and chrome equivalent that will tend to pull point A into theaustenite region.

If the welds molten pool spans two different metals the process becomes more complicated.First plot both parent metals on the shaeffler diagram and connect them with a line. If bothparent metals are diluted by the same amount, plot a false point B on the diagram midwaybetween them. (Point B represents the microstructure of the weld if no filler metal wasapplied.)

Next, plot the consumable on the diagram, which for this example is a 309L. Draw a linefrom this point to false point B and mark a point A along its length equivalent to the totalweld dilution. This point will give the approximate microstructure of the weld metal. Thediagram below illustrates 25% total weld dilution at point A, which predicts a goodmicrostructure of Austenite with a little ferrite.

The presence of martensite can be detected by subjecting a macro section to a hardness

survey, high hardness levels indicate martensite. Alternatively the weld can be subjected to abend test ( a side bend is required by the ASME code for corrosion resistant overlays), anymartensite present will tend to cause the test piece to break rather than bend.

However the presence of martensite is unlikely to cause hydrogen cracking, as any hydrogenevolved during the welding process will be absorbed by the austenitic filler metal.

Evaluating Dilution

Causes Of High Dilution

• High Travel Speed. Too much heat applied to parent metal instead of on fillermetal.

• High welding Current. High current welding processes, such as Submerged ArcWelding can cause high dilution.

• Thin Material. Thin sheet TIG welded can give rise to high dilution levels.• Joint Preparation. Square preps generate very high dilution. This can be reduced

by carefully buttering the joint face with high alloy filler metal.http://www.avestapolarit.com/upload/steel_properties/Schaeffler_large.jpg

Large Schaeffler/Delong Diagram (Outokumpu.com)

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Last Modified 19 Jan 2004

Duplex stainless steels

Typically twice the yield of austenitic stainless steels. Minimum Specified UTS typically680 to 750N/mm2 (98.6 to 108ksi). Elongation typically > 25%.

Superior corrosion resistance than a 316. Good Resistance to stress corrosion cracking in achloride environment.

Duplex materials have improved over the last decade; further additions of Nitrogen havebeen made improving weldability.

Because of the complex nature of this material it is important that it is sourced from goodquality steel mills and is properly solution annealed. Castings and possibly thick sectionsmay not cool fast when annealed causing sigma and other deleterious phases to form.

The material work hardens if cold formed; even the strain produced from welding can workharden the material particularly in multi pass welding. Therefore a full solution anneal isadvantageous, particularly if low service temperatures are foreseen.

The high strength of this material can make joint fit up difficult.

Usable temperature range restricted to, -50 to 280°C

Used in Oil & Natural Gas production, chemical plants etc.

Standard DuplexS31803 22Cr 5Ni 2.8Mo 0.15N PREn = 32-33

Super Duplex: Stronger and more corrosion resistant than standard duplex.S32760(Zeron 100) 25Cr 7.5Ni 3.5Mo 0.23N PREn = 40

Micro Of Standard Duplex

Dark Areas:- Ferrite

Light Areas:- Austenite

Duplex solidifies initially as ferrite, then transforms on further cooling to a matrix of ferriteand austenite. In modern raw material the balance should be 50/50 for optimum corrosionresistance, particularly resistance to stress corrosion cracking. However the materialsstrength is not significantly effected by the ferrite / austenite phase balance.

The main problem with Duplex is that it very easily forms brittle intermetalic phases, such asSigma, Chi and Alpha Prime. These phases can form rapidly, typically 100 seconds at900°C. However shorter exposure has been known to cause a drop in toughness, this hasbeen attribute to the formation of sigma on a microscopic scale.Prolonged heating in the range 350 to 550°C can cause 475°C temper embrittlement.For this reason the maximum recommended service temperature for duplex is about 280°C.

Sigma (55Fe 45Cr) can be a major problem when welding thin walled small bore pipe madeof super duplex, although it can occur in thicker sections. It tends to be found in the bulk ofthe material rather than at the surface, therefore it probably has more effect on toughness thancorrosion resistance. Sigma can also occur in thick sections, such as castings that have notbeen properly solution annealed (Not cooled fast enough).

However most standards accept that deleterious phases, such as sigma, chi and laves, may betolerated if the strength and corrosion resistance are satisfactory.

Nitrogen is a strong austenite former and largely responsible for the balance between ferriteand austenite phases and the materials superior corrosion resistance. Nitrogen can’t be addedto filler metal, as it does not transfer across the arc. It can also be lost from molten parentmetal during welding. Its loss can lead to high ferrite and reduced corrosion resistance.Nitrogen can be added to the shielding gas and backing gas, Up to about 10%; however thismakes welding difficult as it can cause porosity and contamination of the Tungsten electrode

unless the correct welding technique is used. Too much Nitrogen will form a layer ofAustenite on the weld surface. In my experience most duplex and super duplex are TIGwelded using pure argon.

Backing / purge gas should contain less than 25ppm Oxygen for optimum corrosionresistance.

Fast cooling from molten will promote the formation of ferrite, slow cooling will promoteaustenite. During welding fast cooling is most likely, therefore welding consumables usuallycontain up to 2 - 4% extra Nickel to promote austenite formation in the weld. Duplex shouldnever be welded without filler metal, as this will promote excessive ferrite, unless the weldedcomponent is solution annealed. Acceptable phase balance is usually 30 – 70% Ferrite

Duplex welding consumables are suitable for joining duplex to austenitic stainless steel orcarbon steel; they can also be used for corrosion resistant overlays. Nickel based weldingconsumables can be used but the weld strength will not be as good as the parent metal,particularly on super duplex.

• Low levels of austenite: - Poor toughness and general corrosion resistance.

• High levels of austenite: - Some Reduction in strength and reduced resistance to stresscorrosion cracking.

Good impact test results are a good indication that the material has been successfullywelded. The parent metal usually exceeds 200J. The ductile to brittle transition temperatureis about –50°C. The transition is not as steep as that of carbon steel and depends on thewelding process used. Flux protected processes, such as MMA; tend to have a steepertransition curve and lower toughness. Multi run welds tend to promote austenite and thusexhibit higher toughness

Tight controls and the use of arc monitors are recommended during welding and automatic ormechanised welding is preferred. Repair welding can seriously affect corrosion resistanceand toughness; therefore any repairs should follow specially developed procedures. SeeBS4515 Part 2 for details.

Production control test plates are recommended for all critical poduction welds.

Welding procedures should be supplemented by additional tests, depending on theapplication and the requirements of any application code:-

• A ferrite count using a Ferro scope is probably the most popular. For best accuracythe ferrite count should be performed manually and include a check for deleteriousphases.

• Good impact test results are also a good indication of a successful welding procedure

and are mandatory in BS4515 Part 2.

• A corrosion test, such as the G48 test, is highly recommended. The test may notmodel the exact service corrosion environment, but gives a good qualative assessmentof the welds general corrosion resistance; this gives a good indication that the weldingmethod is satisfactory. G48 test temperature for standard duplex is typically 22°C, forsuper duplex 35°C

Typical Welding Procedure For Zeron 100 (Super Duplex)

Pipe 60mm Od x 4mm Thick Position 6G

Maximum Interpass 100°C Temperature at the end of welding < 250°C

1.6mm Filler Wire 85 amps 2 weld runs (Root and Cap)

Arc energy 1 to 1,5 KJ/mm Travel speed 0.75 to 1 mm/sec

Recommended Testing

1. Ferric Chloride Pitting Test To ASTM G48 : Method A2. Chemical analysis of root

3. Ferrite count

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Last Modified 19 March 2002