Welding Carbon Steel to Austenitic Steel

8/3/2019 Welding Carbon Steel to Austenitic Steel

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Carbon Steel To Austenitic

Steel


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Carbon Steel To Austenitic Steel

When a weld is made using a fil ler w ireor consumable, there is a mix ture in theweld consisting of approximately 20%

parent metal and 80% filler metalalloy,

( percentage depends on w elding

process, type of joint and welding

parameters).


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Any reduction in alloy content of 304 /316 type austenitics is l ikely to causethe formation of matensite on

cooling. This could lead to crackingproblems and poor ductil ity. To avoidthis problem an overalloyed fil ler metal

is used, such as a 309, which should st illform austenite on cooling providing

dilution is not excessive.


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The Shaeffler diagram can be used todetermine the type of microstructurethat can be expected when a fil ler metal

and parent metal of differingcompositions are mixed together in aweld.


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DiagramShaefflerThe


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The Nickel and other elements that formAustenite, are plotted against Chromeand other elements that form

ferrite, using the follow ing formula:-

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

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


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Carbon Steel To Austenitic SteelExample,

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 = 11.3 Cr Equiv = 18.2 + 0 + 1.5 x 0.4 + 0 = 18.8

A typical 309L w elding consumable NiEquiv = 14.35, Cr Equiv = 24.9 The main disadvantage w ith this diagram

is that it does not represent Nitrogen,which is a very strong Austenite former.


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Ferrite Number

The ferrite number uses magneticattraction as a means of measuring the

proportion of delta ferrite present. Theferrite number is plotted on a modifiedShaeffler diagram, the Delong Diagram.The Chrome and Nickel equivalent is thesame as that used for the Shaefflerdiagram, except that the Nickel equivalentincludes the addition of 30 times theNitrogen content.


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Carbon Steel To Austenit ic Steel

The Shaeffler diagram above illustrates acarbon steel C.S , welded w ith 304L fil ler.

Point A represents the anticipatedcomposition of the weld metal, if itconsists of a mix ture of fil ler metal and

25% parent metal. This diluted weld,according to the diagram, w ill containmartensite. This problem can be overcome

if a higher alloyed filler is used, such as a309L, which has a higher nickel andchrome equivalent that w ill tend to pullpoint A into the austenite region.


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I f the welds molten pool spans twodifferent metals the process becomesmore complicated. First plot bothparent metals on the shaeffler diagram

and connect them w ith a line. If bothparent metals are diluted by the sameamount, plot a false point B on thediagram midway between them. (PointB represents the microstructure of theweld if no fil ler metal was applied.)


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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 apointAalong its length equivalent to thetotal weld dilution. This point will give theapproximate microstructure of the weldmetal. The diagram below illustrates 25%

total weld dilution at pointA, which predicts agood microstructure of Austenite with a littleferrite


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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 a bendtest ( a side bend is required by the ASME code

for corrosion resistant overlays), any martensitepresent w ill tend to cause the test piece to breakrather than bend.

However the presence of martensite is unlikely tocause hydrogen cracking, as any hydrogenevolved during the welding process w ill beabsorbed by the austenitic fil ler metal.


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Evaluating Dilution


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Causes Of High Dilution High Travel Speed. Too much heat applied to

parent metal instead of on fil ler metal. High welding Current. High current welding

processes, such as Submerged Arc Welding cancause 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 carefullybuttering the joint face w ith high alloy fi l lermetal.

Welding Carbon Steel to Austenitic Steel

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