625 overlay on 8630 yield stress reduction. 1

[VictorMeldrew]

We often use a 625 weld overlay on 8630 components that need to comply with API 6a and NACE MR 01-75 and we are finding a serious reduction (10% to 20%) in the yield stress of the underlying material. Does anyone know of a way to recover this whilst maintaining the NACE compliance?

 

[metengr]

What is the process you are using, and are your welding requirements for preheat and interpass temperature for the weld overlay? What is the heat treatment condition of the 8630 material?

 

[VictorMeldrew]

Thanks for the quick response metengr.
Weld process is GTAW with hot wire filler, preheat to 150C interpass 150C to 300C postweld heat treat 660C. The 8630 starts off quenched and tempered with a mimimun yield of 85ksi.

 

[metengr]

The preheat temperature looks fine. The hot wire GTAW process is similar in deposition rate to a GMAW (MIG) process so you are adding significant heat input to the weld region and surrounding base metal. I would tighten the interpass temperature to 200 deg C max. Regarding PWHT, I don't have an 8630 tempering curve at the moment, but you need to remain about 10 deg C below the original tempering temperature of the 8630 to assure minimal effect of PWHT temperature on bulk properties, and to assure adequate tempering of the base metal HAZ.

 

[VictorMeldrew]

metengr,

Thanks looks like PWHT might be too hot, the orginal temper was at 660C.