etching austenitic stainless steel after PWHT 3

[siamak60]

Hi
One of our subcontractor send us a deviation which describes that a carbon steel Pressure vessel which some of it's nozzles are made from stainless steel shall be PWHT at temperature about 620 degrees of centigrade, and as it is forbidden in our specification to do PWHT on s.s materials; he insist on doing furnace PWHT and then etching s.s nozzles to save time. I wanted to know that if anyone knows any reference about such a thing to remove carbides from surface and also how we can be sure that these action was done well and a new Passive layered established on surface of those nozzles ?
thanks for your consideration

 

[Guest102023]

More information is needed here. I will assume by stainless steel you mean something like 304 or 316. What I can tell you is that carbide precipitation affects the entire part, not just the surface, and that passivation is no protection against SCC in sensitized material.

There are situations where SS is heat treated at that temperature. Potential corrosion will depend on the service environment.

p.s., it is 'celsius', not 'centigrade'

 

[metengr]

he insist on doing furnace PWHT and then etching s.s nozzles to save time. I wanted to know that if anyone knows any reference about such a thing to remove carbides from surface and also how we can be sure that these action was done well and a new Passive layered established on surface of those nozzles ?

I would not accept the above as corrective action if the nozzles are austenitic stainless steel. The reason for prohibiting exposure of austenitic stainless steel nozzles to PWHT is the stainless steel nozzles can become sensitized and have reduced corrosion properties. This is not a surface effect, it is throughout the material. So, either decide if you can accept sensitized material for vessel process conditions or replace the nozzles with new ones and use a buttering technique. What this means and what should have been done originally by the contractor is that the carbon steel weld prep accepting the stainless steel nozzles should have been buttered using either 309 or Inconel weld metal, and subjected to PWHT because of the requirements for the carbon steel substrate. Once this operation was completed, the new stainless steel nozzles could be welded directly to the butter layer with no subsequent PWHT. Obviously, this step was missed.

 

[siamak60]

Dear Friends
Thanks for your Consideration
Nozzle Material is 316L and it's carbon content is about 0.03% and the equipment is solvent absorber Column Designed Based on PD 5500 with Design Temperature of -46/+85 degrees of celsius and design pressure is about 82 bar and operating temperature is +36/+49 degrees of celsius and fluid service is aqueous 45.5%wt MDEA solution which deemed lethal and sour.3mm of 316L applied as clad on upper parts of column that is some how pressurized by sour vapor.also some of nozzles are made from 316L.
thanks