[MW:15324] Re: Hardness Control for HP Steam Separator Tie-ins

I would give you my oppinion as below.

 

1. May production hardness test be avoided if it is in table 2 in NACE SP 0472?

 --> Yes, it is. NACE says the industrial experience. I think there is enough support mention in NACE so that Hardness will be below the criteria.

--> However, cooling rate and temper bead you mentioned is for wet H2S, not caustic . Please see the caustic cracking just below it.

--> For caustic service, entire weldment shall be checked.

 

 2. Industrial Experience

--> As for caustic service, the damage mechanism is based on tensile strength from caustic so there will be possible crack on welding area which is suceptible to crack for CS.

--> To prevent this, generally PWHT shall be applied. If the area of caustic belongs to Area B in caustic soda chart, PWHT shall be applied.

--> For welding area, it is rapid cooling area by welding heat. That means liquid metal becomes to garmma austenite(FCC STRUCTURE) and garmma austenite becomes alpha ferrite with FE3C(BCC STRUCTURE) during welding.

--> When FCC structure becomes to BCC, there is volume change. Atomic Packing factor is from 74%(FCC) to 68%(BCC). If cooling process is rapid such as welding, there is accumulated stress.

--> As it were, welding and HAZ area is suceptible area to caustic.

--> So, PWHT shall be applied when caustic concentration belong to area B in caustic soda chart technically.

--> How to meause the PWHT is effective ? It is to check hardness.

--> Conclusively, you had better check caustic concentration and temperature whether PWHT shall be applied or not. Also, hardness test in Table 2 can be exceptional case if it does not belong to area B in caustic soda chart.

 

2012년 9월 16일 일요일 오후 2시 59분 43초 UTC+9, akshat 님의 말:

Dear Welding Experts, 

For the weld tie-ins to High Pressure Steam Separator, contractor wants an exception for hardness check for qualifying WPS - they want to use old WPS, qualified based on Rockwell B hardness tests. 

My view is that to prevent Caustic SCC in HP Steam system, hardness control particularly at fusion line (HAZ) is essential; hence hardness testing shall be carried out using a permitted methods i.e, Vickers 10 scale as per NACE SP0472 for WPS Qualification and cooling rates and temper bead thermal methods for hardness control shall also be applied.  Production hardness tests may be avoided based on the exception given in Table 2 (for the right combination of welding process and filler materials) if preproduction hardness test is carried out and filler materials is recorded.

Appreciate your comments about my understanding as above. What is the practice at your plant? Many materials / welding engineers still consider steam service as clean service and relax any special welding precautions/ requirements.

Thanks,

Akshat Agrawal 

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