4140 to cast steel preheat

[WelderCWI]

I have a gear that is made up of a cast steel center hub and a 4140(annealed) ring that the teeth wil be cut in post weld. (Flame hardened after machining) This is approximately 24" dia. and we will be welding a 1/4" fillet weld all around both sides with either FCAW (E71T) or GMAW (ER70S-6). The question is what should the preheat, interpass and postheat temperatures be? After postheat it will cool overnight to ambient and customer will pick up in the morning. My guess would be approximately 400 degree preheat. 600+ degree postheat (to diffuse Hydrogen) and cool as slowly as possible. I tacked the piece in place (at ambient) with GTAW and ER70S-2 and had cracking issues. Any suggestions/ corrections you could offer would be very helpful.

 

[metengr]

WelderCWI;
Your stated preheat and interpass temperatures are acceptable considering the sizes of the attachment welds. I would have the hub and rting preheated to 400 deg F and held at this temperature for 15 minutes before welding.

 

[blacksmith37]

A higher preheat would not hurt. I had to go to 500F on 4140 castings (2" + thickness) to prevent cracking.

 

[Guest102023]

'Donut' shaped welds are the worst case for shrinkage stresses. Use the higher preheat and heat the ENTIRE assembly. Follow extreme moisture control practice for the filler and the part.

You are using the correct filler metal. Did the shop foreman (in his infinite engineering wisdom) not insist on a 'matching' filler metal?