Welding 4140: Preheat High? 3

[ballinbuck]

I am trying to find out how much preheat to do when welding 2.50" thick 4140 bar to 1026 tube. I used Lincoln's calculator and got a preheat of 525 degrees F. I'm not very experienced with welding but this seems a bit high to me.
Here are the 4140 chemical properties I took into account:
C=0.43
Mn=1.00
Si=.35
Cr=1.10
Mo=.35

Thanks in advance,
DAH

 

[metengr]

The CE(IIW) for this material is 0.89. I would use a preheat of 450 deg F, and hold the preheat for 15 minutes before welding. Use low hydrogen electrodes for welding.

 

[gtaw]

I agree with "metengr". The high preheat is required and make sure you use a low hydrogen electrode that has been baked (not simply held in an electrode oven at 250 degrees F)or use a welding process such as GTAW or GMAW that uses bare filler metal to eliminate or minimize the potential of introducing hydrogen into the weld puddle.

I would suggest a stress relief operation immediately after welding. Don't let the weldment cool to room temperature and do the stress relief the next day. It will be too late.


Best regards - Al

 

[unclesyd]

Not mentioned and dependent on the weld configuration is that once welding is started don't stop until finished.

If practical one method we used to handle a few 4140 weldments was to immediately cover the part with vermiculite insulation and allow to cool. Dependent on the weld constraints we either used as welded or if deemed necessary do a stress relief.

 

[ballinbuck]

Thanks all! I'd like to investigate methods of post weld heat treating and stress releiving for 4140. Is wrapping the area with an insulated blanket for slow cooling sufficient? What types of blankets are available to use? I know unclesyd mentioned vermiculite insulation, any others?

 

[unclesyd]

Vermiculite is the cheapest though you can use about anything to retard the heat loss.
Any type high temperature insulation cloth.

Any type of fibrous insulation used for high temperature. Some types of fiber glass will smoke pretty bad unless you cycle the through an oven, not mama's.

Any other granular insulation. You can even use warm dry sand, Playpen sand.


Old timers used Slacked Lime or an asbestos cloth.

 

[gtaw]

While the methods mentioned by "unclesyd" will slow the cooling rate, it will not reduce the residual stress. Residual stress is a contributing factor in delayed hydrogen cracking.

Good luck.

Best regards - Al