Hi Ali

Generally you do not need to perform any post weld heat treatment, (PWHT) because the Nb added to the base metal and filler will tend to stabilize the weld and HAZ. As with all Austenitic alloys, you just need to weld it relatively cold, with a relatively low heat input.

Generally the H grades are used where high temperature strength properties are required. You will therefore find that often these alloys operate within the sensitization range in any case, so a PWHT will not deliver any lasting benefits, even if it did reduce the weld sensitization.

This sort of information is not really given in "standards", but I may refer to ASME 8 Div 1, that shows that PWHT is not a requirement for any of the P8 materials. (347H is a P8 Gr1 material)

I hope this helps.

Regards
Niekie Jooste
Fabristruct Solutions

Hi Ali

I am concerned that you intended to use a standard E347 type electrode on 347H base material! The standard electrodes are now formulated with low C levels and consequently lower Nb is required; this will degrade the creep properties of the weld. Additionally the ferrite level ~ 6FN will be prone to sigma formation at the typical operating temperature that the 347H base material will operate at.

This is a classic case of not understanding the operating environment and utilising a corrosion resistant material for a high temperature application.

You should be using a 347 electrode that is formulated specifically for high temperature applications i.e. C ~ 0.05 and ferrite controlled at the low end ~ 4FN.

There is also history of HAZ cracking in >12.5mm thick joints due the lower ductility of the modified electrode. In this case the E16.8.2 type should be used which was developed to combat this problem.

I guess the heater is in a hydrocracker or hydrotreater unit. A thermal stabilization treatment at 899 -900 deg C for two hours is specified for increasing the corrosion resistance to intergranular corrosion and polythionic acid stress corrosion cracking. The ASTM standard for A-376 and A 312 gives this treatment for the stabilzed grade austenitic stainless steels as a supplementary requirement without specifiying the temperature and time. Chevron and UOP specifications generally specify such requirements for both the parent pipe and welds.

C.Rajaram
Gujarat Refinery
India