Stainless Steel Technical Guide

What Are They?

In transition joints (welding two members of dissimilar composition) or in welding like members of a weldment with non-matching filler metals, differences in properties of the weld metal, heat-affected zone, and base metal together with the consequences of postweld heat treatment and end use should be duly considered. Weldments produced with dissimilar filler metal are often put into service as welded. The following comments reflect some of the best practices currently followed.

• Where preheating or postheating of martensitic grades is impractical, austenitic filler metals such as 309(L) or 312 can be used with less chance of cracking — the increased ductility of the filler metal taking up the shrinkage stresses of the cooling weld. (If the service temperature is above 800°F, Type 312 should be avoided due to potential embrittlement from sigma phase). Note that the statement above refers to less chance of cracking with austenitic filler metal. In highly restrained joints this approach would be risky since the heat-affected zones of the weldment would be prone to hydrogen-related cracking.
• Never use 308 to join austenitic stainless steel to plain carbon or low alloy steel. Sometimes it works, but if the resultant dilution produces a weld composition with less than about 17Cr/7Ni cracking may result. Type 310 is often used successfully for this application since it takes care of a lot of dilution and has a lower coefficient of expansion than 308; however, type 310 has no ferrite and is therefore sensitive to microcracking. Never use 310 on some weathering steels such as Cor-Ten A, Cor-Ten High-Temp or Mayari R which may contain 0.07-0.15% P.
• The two leading austenitic stainless steel filler metal contenders for dissimilar metal welding are 309(L) and 312. The “magic” companies that do an excellent job in maintenance and repair welding all favor a version of Type 312 for most applications. This grade takes care of dilution and contains a level of ferrite in excess of 25 FN which provides crack resistance and high strength. Type 309(L) is also widely used, particularly where service temperatures exceed 800°F; however, with a lower ferrite level (perhaps 7-12 FN) it is not as crack resistant as 312. Types 309Mo and 310 have also been used successfully. For submerged arc applications (which result in relatively high penetration, hence greater dilution) 312 is preferred over 309(L).
• Because of thermal expansion mismatch — causing thermal fatigue at high temperatures — austenitic stainless steel filler metal should not be used to join ferritic low alloy steels like 2.25 Cr, 1 Mo to austenitic stainless steel where the service temperature exceeds 800°F (e.g. power plants). Some success in applications of this type has been attained using nickel base ERNiCr-3 filler metal which has a coefficient of thermal expansion between these two base metals.
• In highly stressed applications use of a buttering technique — providing a transition deposit of 309(L), 310 or 312 weld metal on the mild steel or low alloy steel member of a weldment prior to welding to the stainless member — has been successful where other procedures have failed. In some cases a successful joint can even be made with 308 instead of 309(L) or 312 since dilution on the buttered side would be less of a problem.
• A general rule in welding dissimilar grades of austenitic stainless steels is to use filler metal of the lower alloy composition. For example, use 309(L) filler metal to join Type 310 to Type 304 base metals.
• Welding of 17-4 PH to 15-5 PH precipitation hardening stainless steels can be successfully accomplished with 17-4 PH filler metal (AWS E630 or ER630) since the two grades have the same general welding and heat treating characteristics. For welding 17-4 PH to a plain carbon or low alloy steel, Type 309(L) filler metal is suggested.
• Since there are no 17-7 covered electrodes produced (because of the problem of loss of aluminum across the arc) welding of 17-7 PH steel is sometimes done with 17-4 PH covered electrodes. The welding technique should involve a weaving motion that includes a short dwell on the 17-7 PH base metal. This ensures enough dilution to adequately enrich the weld metal such that it may be capable of at least limited response to the double treatment of the 17-7 PH base metal. If high strength is not required at the 17-7 PH weld joint, Type 309L or 309Cb covered electrodes may be considered.
• For joining cast iron to steel, use AWS ENiFe-CL.
• For joining ENiFe-CI cast iron stainless steel, first butter the stainless side with Cast-Alloy 60 (using relatively low currents and overlapping of weld beads to minimize dilution), then join the cast iron to the buttered stainless steel with the same type of electrode.
• For hardsurfacing cast iron, For joining ENiFe-CI cast iron stainless steel (low currents and overlapping of beads) then follow with one or two layers of Hobart Hardalloy 148, 140 or 155.
• For joining cast iron to austenitic manganese steels, use ENiFe-Cl. Buttering of the cast iron with ENiFe-Cl may be necessary before making the joint. A more cautious approach would be to butter both the cast iron and the austenitic manganese before making the weld with the nickel-iron ENiFe-Cl electrodes.