how to weld two case hardened parts

[yshgao]

Hi there,

I have one induction hardened part (I call it as part 1). Its material is 4140. Now, I have to weld another part (Part 2) on part 1. The thickness of part 2 is 1.0 mm. If I pick 4140 as the material of part 2 and go for the case hardening. Can we weld two hardened parts together?

Or should we weld them together and then go for the hardening?

Please advise.

 

[yshgao]

Hi, again,

After talking with my design engineer, these two parts have to be surface hardened before the welding due to the assembly request.

Please advise whether we can weld them? Also, I think the projection welding should be the only way we have to go.

 

[metengr]

yshgao;
Go with the second option of welding followed by hardening. You do not want to weld on parts that have been surface treated to increase hardness because of concerns with cracking from welding (residual stresses and affects on surface mechanical properties).

 

[yshgao]

Hi Metengr,

Thanks for your time.

The welding must be the last operation. The cracks may not be the big concern due to the small force on this part.

My concern is the weldability between two AISI 4140 pre-hardened parts.

 

[IRstuff]

Doesn't seem to make much sense to me; wouldn't the welding potentially damage the hardening?

TTFN

FAQ731-376

 

[yshgao]

Hi IRstuff,

There is no much worry about the hardness in the welding area. My question is whether we can weld them as easy as the normal welding process. Is any influence of the hardened surface on the weldability?

Hope I can make my question understandable.

 

[CoryPad]

Your question is understandable.

The proposal to weld after hardening of 4140 steel is bad. The steels are marginally weldable in the as-received condition, but they are very susceptible to cracking (not small cracks, but complete separation of the two parts at the weld line) after surface hardening.

 

[TVP]

yshgao,

Any type of welding (arc, laser, resistance, etc.) is a bad idea on parts that have been previously hardened. Cracking and delayed fracture are very real possibilities. Tell your design engineer to determine another method for joining the parts, or to design a way that the two parts can be joined on a surface that is not previously hardened.

 

[DABwilldo]

There are coatings that a good heat treatment shop can apply to a part to block the carburization process. Conceivably, you could shield the areas to be welded with one of these coatings, case harden the parts, and then weld them together afterwards.

Without knowing anything more about the parts in question, it's impossible to say whether or not this is practical.

I'll add my voice to the chorus that state welding case hardened steel is a bad idea.

 

[yshgao]

Thanks DABwilldo and everyone,

I will keep your idea in mind. I will check with my designer whether we can change the design.

But, at least we knew, these two parts are weldable if we do not care about the cracking. If the force or load is not the big concern, this welding operation may work.

 

[SnTMan]

Braze 'em?

 

[metengr]

yshgao;
After reviewing your responses to the above posts I am amazed that you would still attempt to weld these components. By stating that welding is a bad idea is not a green light to weld these components.

If you don’t care about load-carrying capacity or cracks, I would suggest you use a structural adhesive and be done with it.

 

[jsokal]

We (major aerospace engine manufacturer) often weld previously hardened material when necessary. It is critical to include a tempering (stress relieve) heat treatment afterwards.
That being said, even though it is normal practise to weld hardened parts as described above, we do not weld on the case hardened surfaces.
For example we have a shaft that is made of two details, a 4340 fully heat treated,induction hardened gear teeth part and a 9310 carburized, hardened,cold treated and tempered part where the teeth and bearing surfaces are carburized, The ends of these two parts that are joined by EB welding are only core hardened never ever case/induction hardened.
You then have to choose a tempering temperature that will not be high enough to affect the case hardness of either part,..usually a bit of a compromise.

John