Selecting and using stainless steel filler metals
Meet the columnist
Peter Stones
IEng MWeldI IWE / EWE As part of the ESAB Specialty Alloys Group , Peter is technical support for stainless and nickel alloy filler metals . Peter is actively involved with TWI and is a non-executive director of The Welding Institute . Peter worked for Sandvik for 10 years and was Global Product Manager for Sandvik Welding up to 2018 , when ESAB purchased the filler metals business . Email : Peter . stones @ esab . com
Q : Do you have any tips for selecting the correct grade of stainless filler metal ? There seem to be so many different options for every grade , and why is ferrite level so important ?
A : The process of choosing a stainless filler metal involves matching it to the base materials , selecting a shielding gas and often calculating ferrite content and total heat input . With many possible combinations of materials , the best choice might not be apparent , especially for those not familiar with the metallurgical nuances . For example , joining 304L to 316L ( a common application ) presents several filler metal options :
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316L ( 19 % Cr , 12 % Ni , 3 % Mo , 0.4 % Si ). A perfectly good choice , but the weld pool will be comparatively sluggish , making it more difficult to weld .
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316LSi , which has the same composition as above , but with 0.9 % silicon . The small , additional silicon content improves wetting and makes the weld pool more fluid and easier to manipulate .
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309LSi ( 24 % Cr and 13 % Ni ) can enhance corrosion resistance by “ over-matching ” chromium content compared to the other options .
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Generally , the higher alloyed of the two base metals should be used . In this example , 308L ( 19 % Cr , 9 % Ni ) or 308LSi ( 19 % Cr , 9 % Ni , 0.9 % Si ) could be used , but it is not an optimum choice .
Selecting the best filler metal also requires understanding service conditions such as corrosive environment ,
Figure 1 . TIG welding stainless exhaust grade 400 ferritic steel . temperature and required mechanical properties . For example , in nitric acid , a 310L filler ( which has no molybdenum ) will perform better than a 316L or 310LMo due to selective attack of the molybdenum by this specific acid . Cost also influences filler metal selection . Fabricators use 309L to join 400 Series ferritic alloys , but the high alloy content of 309L ( 24 % Cr and 13 % Ni ) adds cost . A modified 307 filler , with 8 % Ni and 7 % Mn , might be a lower-cost alternative because manganese costs less and acts as an austenitizing agent to create a suitable crack resistant microstructure in the weld metal .
Shielding gas Whether to simplify cylinder management or reduce cost , fabricators often try to use the same shielding gas as they do for MIG or Flux-Cored welding of carbon steel . It does not work with stainless grades because shielding gases with a carbon dioxide ( CO 2 ) content of 5 % or more add too much carbon to the weld metal , degrading its corrosion resistance . Recommendations for short circuit MIG welding stainless include “ tri-mix ” gases that contain a blend of 85 to 90 % He , up to 10 % Ar and 2 – 5 % CO 2 ; alternatively Ar + 1-2 % CO 2 gives good results . A common choice for spray transfer MIG is Ar + 1-2 % O 2 . Options include Ar + ( 1-2 % CO 2 ); Ar + 30 % He + 1-2 % O 2 ; and Ar + 30 % He + 1-2 % CO 2 . Adding O 2 gives the weld pool better fluidity and provides arc stabilization ; helium gives
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