Post-weld heat treat for part to be hardened after weld,

[Coctyle]

My company makes specialty hand tools. When possible, we weld together parts of standard tools. Most of the time, the material is 4140 and we heat treat it to somewhere between 40 and 50 HRC after welding. In the past, post-weld subcritical annealing has been done, probably due to a generic suggestion from the heat treater or welder. All heat treatment is done at a different facility than the welding. Therefore the parts cool to room temperature (or below) before the anneal. After anneal, the parts cool and then are heated, quenched, and tempered.

Is there any reason for the sub-critical anneal before Q+T heat treatment? I don't see any benefit, unless it could be done before the part fully cools after weld, which is not the case.

 

[metengr]

This practice may have been done to ensure removal of hydrogen and ductility to prevent cracking before a Q&T heat treatment performed at a later time and in a different facility. I agree with you if care is taken during welding to avoid hydrogen cracking.

 

[weldstan]

It is good practice to perform a dehydrogenation heat treatment in hardenable alloys like 4140 to avoid delayed hydrogen cracking prior to Q&T when appreciable delays in final heat treatment are incurred. It could well be deleted depending on tight control of welding parameters.

 

[Coctyle]

What about normalizing to refine grain structure in the weld zone? Is that suggested or necessary to achieve a consistent response from the quench and temper heat treatment?

 

[metengr]

Not necessary. By austenitizing for quench and temper for final heat treatment you are achieving the same affect.

 

[Maui]

Is any machining performed over an area that includes the weld zone before hardening and tempering? If this is the case, then an anneal would certainly make sense.

Maui

 

[Coctyle]

MAui: Why would you say that an anneal would make sense if the part is going to be austenitized? Wouldn't that relieve residual stresses from machining? Or is there some other reason?

 

[Maui]

The hardness inside the weld zone would likely be substantially higher than the hardness outside of the weld zone. If you need to machine through this region after welding and before heat treating the welded part, then the milling cutter may have a very difficult time cutting this harder material inside the weld zone. By annealing the material the hardness would become much lower and more uniform, making it much easier to machine.

Make sense?

Maui

 

[mfgenggear]

it also stress relieves & soften the parts the parts prior to re-hardening and is specified in many AMS2759/1 and 2 specifications. this is to ensure parts do not crack if it is above 40 HRc hardness. it's old school and it works.

 

[Coctyle]

Thanks Maui. I had misinterpreted your earlier comment. I thought you meant after machining, to relieve stresses that might be induced by machining for instance.

 

[Coctyle]

mfgenggear: Thanks for the comment. Would the anneal have to be done before the part cools to room temperature to avoid cracking of hardened material? Does the AMS spec that you referenced specify that the anneal be done before the part cools below some specific temperature?

 

[mfgenggear]

yes see AMS2759/2 para 3.4 all inclusive