Welding of CS to SS is done every day in some industries, an is not an uncommon occurance in most. The welding part is usually the easy one. Corrosion, galvanic action, stresses due to different coefficient of expansion are often the tough issues. Whether it is the right choice for your application, is for the designer to say, because only he/she has all the right info. Should you get alarmed whenever you see this type of weld? Certainly not! Also, most codes do not require what seems to be the excessive qualification required at your last place of work.
AWS addresses CS and SS in different codes, but for ASME section IX, in a lot of cases, would not require additional welder qualification. (for instance, a welding welding 316 to 316 using a 316 electrode would be qualified welding 316 to CS using a 309 electrode assuming all other variables remained the same)
Galv corrosion has be considered during material selection by process, it has noting to do with type 309 (its almost same either you weld with 308 or 309)
In joining austenitic stainless steel to carbon or low-alloy steel for service applications involving exposure to temperatures
not exceeding 370 °C (700 °F), it is good practice to use a stainless steel filler metal with a total alloy content high
enough to prevent the formation of martensite in the weld after dilution by the base metal and to preserve residual
amounts of ferrite to minimize the possibility of hot cracking resulting from welding under severe restraint. Dilution is the
change in chemical composition of a welding filler metal caused by the admixture of the base metal or previously
deposited weld metal in the deposited weld bead. It is normally measured by the percentage of base metal or previously
deposited weld metal in the weld bead.
The Schaeffler diagram is particularly useful when examining what filler metal is suitable for joining dissimilar metals.
The example in Fig. below shows the joining of a carbon steel (point A) to a type 304 austenitic stainless steel (point B) using
type 309 as the filler metal (point D). Point C shows what the composition of the weld metal would be if these items were
joined without a filler metal. If we assume that each base metal is fused to the same extent, point C will lie halfway
between A and B. Because the welding is done using type 309 filler metal, the composition of the weld metal will lie
along the line CD, depending on the degree of dilution. At point E a suitable weld metal composition is obtained, that is,
an austenitic structure with 8 to 9% ferrite (Ferrite Number). This weld metal will be crack resistant, in contrast to that
obtained at point C, which is very sensitive to cracking because of martensite formation. Types 309 and 309L (25Cr-
12Ni) filler metals are most widely used for joining carbon or low-alloy steel to austenitic stainless steel; they normally
contains about 8 to 15 FN. Types 304Cb, 309Mo, 309MoL, and 312 (29Cr-9Ni) are progressively more strongly ferritic.
Satisfactory welds are also obtained with nickel-chromium-iron filler metal and these filler metals allow the service
temperature to exceed 370 °C (700 °F) and minimize some stress relieving problems.
Galv corrosion has be considered during material selection by process, it has noting to do with type 309 (its almost same either you weld with 308 or 309)
In joining austenitic stainless steel to carbon or low-alloy steel for service applications involving exposure to temperatures
not exceeding 370 °C (700 °F), it is good practice to use a stainless steel filler metal with a total alloy content high
enough to prevent the formation of martensite in the weld after dilution by the base metal and to preserve residual
amounts of ferrite to minimize the possibility of hot cracking resulting from welding under severe restraint. Dilution is the
change in chemical composition of a welding filler metal caused by the admixture of the base metal or previously
deposited weld metal in the deposited weld bead. It is normally measured by the percentage of base metal or previously
deposited weld metal in the weld bead.
The Schaeffler diagram is particularly useful when examining what filler metal is suitable for joining dissimilar metals.
The example in Fig. below shows the joining of a carbon steel (point A) to a type 304 austenitic stainless steel (point B) using
type 309 as the filler metal (point D). Point C shows what the composition of the weld metal would be if these items were
joined without a filler metal. If we assume that each base metal is fused to the same extent, point C will lie halfway
between A and B. Because the welding is done using type 309 filler metal, the composition of the weld metal will lie
along the line CD, depending on the degree of dilution. At point E a suitable weld metal composition is obtained, that is,
an austenitic structure with 8 to 9% ferrite (Ferrite Number). This weld metal will be crack resistant, in contrast to that
obtained at point C, which is very sensitive to cracking because of martensite formation. Types 309 and 309L (25Cr-
12Ni) filler metals are most widely used for joining carbon or low-alloy steel to austenitic stainless steel; they normally
contains about 8 to 15 FN. Types 304Cb, 309Mo, 309MoL, and 312 (29Cr-9Ni) are progressively more strongly ferritic.
Satisfactory welds are also obtained with nickel-chromium-iron filler metal and these filler metals allow the service
temperature to exceed 370 °C (700 °F) and minimize some stress relieving problems.
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