Inquirer:-

I am a quality professional, I want to know that for austenitic stainless steel what should be the PWHT temperature and cooling and heating rate for the stellite hardfacing. I have used ASME SEC-II part C- SFA 5.21 -ERCoCr-A on CF8M material. Overlay thickness is 7mm on 22 mm thick CF8M.

Reply:-

Dear Sir,

CF8M is the cast equivalent of Type 316 while, CF8 is the equivalent of Type 304. CF8M contains 2-3%wt Mo to improve pitting corrosion resistance in chloride environment.
This grade is non-hardenable, but it is given solution treatment in the temperature range of 1038°C – 1149°C, to dissolve any chromium carbides and other precipitates that may have formed during the casting or welding process, which can improve the material’s corrosion resistance and toughness.
Typical heating rate for 40 mm thickness is between 56°C to 111°C per hour-slow and uniform to avoid any thermal shock or distortion.
Typical soaking time is depending of 1 hr /inch thumb rule min 1.5 -2 hr.

The cooling rate should be rapid enough to prevent the reformation of precipitates in the material but slow enough to avoid any thermal shock or distortion. A typical cooling rate for this thickness is between 83°C to 111°C per hour down to 649°C, followed by water quenching.

Consumable ERCoCr-A is a cobalt based alloy, thickness of layer is 7mm so overall for 47 mm , 2 hr soaking time is sufficient. But make sure , while cooling at the rate of 80°C- 100°C per hour down to 649°C, you quench it immediately in water to avoid reprecipitation of carbides that may form in weld deposits and base metal during welding process.
Temperature range between 470°C- 870°C, causes formation of chromium carbide which makes loss of corrosion resistance.
I advice you to run a procedure qualification test, to check the hardness, microstructure and corrosion resistance requirements requested by your client.

Hope, This suggestion might help you.

Thanks

References:-

  • American Welding Society (AWS) D10.8: Recommended Practices for Welding Austenitic Chromium-Nickel Stainless Steel Piping and Tubing
  • AWS A5.21: Specification for Bare Stainless Steel Welding Electrodes and Rods
  • AWS A5.13: Specification for Surfacing Electrodes for Shielded Metal Arc Welding