PWHT of Low Carbon Steel above 1330F; how detrimental?

[tetris]

To prevent stress corrosion cracks in a weld we do a post-weld heat treatment. The material is a simple low carbon (approx 0.2% C) and low alloy steel. Due to the big size and complicated shape of the object we can't heat the weld itself. To make sure that the weld itself sees a temperature high enough to make sure any residual stress is relieved we have to heat up some areas of the object to a temperature very close to the Ac1 temperature (1330F/723C). There is a risk that in some spots we might exceed the temperature.

Question: If we exceed the temperaure, besides that the material will loose some of it's strength, are there any other detrimental effects to be considered or expected?

Note: We'll cool down the material slow enough to make sure we have no hardening efect through the austenit>ferrit conversion.

Thanks a lot everybody for any help or tips.

 

[PAN]

In my opinion, the required properties such as fatigue strength may be decreased.

 

[arunmrao]

Portable induction heaters or electrical heaters are available which can be utilised for carrying out localised post weld heat treatment operations. This is commonly used in boilers where localised repairs are carried out. Hope this helps.

 

[gdodd]

Suggest you lower your stress relieving temperature to 550 degrees C (1020 degrees F) for post weld heat treatment.
This is sufficient for a welded low carbon/alloy steel combination and will reduce the possibility of transformation above AC1.
If alloy steel has a low carbon content, post weld heat treatment may not be necessary.

Good Luck

 

[nousakmj]

You're not going to do much to the base material at 1330 deg F -- certainly not for a 1020 material which will consist primarily of ferrite and pearlite. You shouldn't have to worry about austenite to ferrite transformations -- at the temperature you're talking about you won't have enough austenite around worth considering. You also shouldn't affect the properties of the base material by any significant degree -- the yield strength might drop a little at worst but not enough to consider significant. Localized heating is more likely to result in distortion or residual stresses than any other problem. Try using resistance heating pads for localized PWHT.

 

[tetris]

We'll use a combination of resistance heaters and induction heaters, but still we can't heat the weld itself, cos access is very cramped.

Thanks for the tips everybody!