Dissimilar metal welding, SS to CS

[James Scrufy]

As I have been engaged to issue PQR for welding austenitic stainless steel weld neck flanges (SS 304, SS 316) to carbon steel nozzles and since it is the first time for me that I need to qualify such a dissimilar material combination, I am seeking for some instructions that should be followed for the execution of this welding.

[wgadmin]

This welding can be done using the most common arc welding methods like GTAW, SMAW, GMAW, SAW depending on the geometry and the dimensions of weld profile using high alloyed welding consumables (SS 309) to avoid cracking that could possibly occurs using for example SS 308 filler metal since because of lower ferrite level in SS 308, there is higher possibility for creation of martensitic formation (very hard form of steel) during cooling and thus the weldmend is prone to cracking. Moreover since carbon steel is more vulnerable to hydrogen cracking than austenitic stainless steel, careful drying of welding consumables is mandatory.

In case of elevated temperatures, nickel based consumable like AWS 5.14, ErNiCr-3 is advisable since it produces welds that have a coefficient of thermal expansion between the carbon steel and the austenitic stainless steel (in this regard you could have a look to an other topic of this forum: Dissimilar metal weld cracking in high temperature service).

Relating to the preheating requirement, as a general guideline I could say that carbon steels with carbon levels over 0.30% and joint thickness greater than 25 mm require preheat at 80 deg C. As far as the requirement for PWHT is concerned, this basically depends on the construction code that is applicable to your equipment.