Microhardness & Cooling Rate

[ebuntario]

I've welded an IN-792 with an IN-625. I tried 3 type of preheating before welding. 1st preheated to 200 celcius, 2nd preheated to 400 celcius and 3rd preheated to 400 celcius. After done welding it, I did a vickers microhardness test on the fusion line/HAZ. The results was a decrease of hardness value with increasing preheating temperature. How is this possible? Am I doing it right? I'm trying to understand it & getting pretty frustrated. Thx guys

 

[metengr]

First off, preheating is not recommended for the above alloys or combination of alloys. What may be happening is that you are adding to the heat input of the weld region resulting in over-aging of beneficial precipitates in the base metal HAZ. Remove the preheat requirement. What is the welding process, joint configuration and filler metal are you using or is this an autogenous weld?

 

[ebuntario]

Why is preheating no recommended? I thought preheating should be done to prevent hot cracking on HAZ.

 

[metengr]

No, not for Ni-base alloys. These alloys are either solid solution-strengthened or strengthened by precipitates. There have been cases where a post weld stress relief is performed because of joint thickness/restraint issues for certain Ni-base alloys (like IN-617).

 

[ebuntario]

I see. Here's another one, which one is correct? If all cooling process is done by air (room temperature), will higher preheating temperature cause the cooling rate to increase or not?

 

[metengr]

It will cause the cooling rate to decrease. For ferritic materials preheat is beneficial for welding to avoid harmful phases and higher residual stresses that can form as a result of higher cooling rates.