Welding 4140

[ImpracticalM89]

Hi, my first post here on this great forum. I have a question about welding 4140 steel and hardening. I am working with a 1.5" long X 0.625" diameter 4140 cold rolled rod. I milled a 3/8" flat on the long side of the rod, which I want to TIG weld a 0.5x0.375"x0.30" block of 4140 (for a particular application). My goal is to heat treat the 4140 to achieve a hardness of 40 HRC. So my question is, should I weld the two parts together before the heat treatment or after? Thanks in advance! M89

 

[Northman Ind.]

if ya gots a print there should be a specific procedure...

Though, if it were me, I'd harden after welding, maybe normalize first, cause welding... but hardening involves going up to and past normalizing/annealing temps anyway so?
If you weld it after hardening you run the risk of cracks, or losing said hardness in some spots

 

[ImpracticalM89]

Thank you for the quick response. I’m just a home hobbyist so no procedure here . I have hardened lots of 4140 so good there. What you suggests make sense. I also have another small 4140 part that I need to weld to unknown hardened steel, maybe 8620? I have to hardened that 4140 first then weld to the unkown. So I’ll do both the same way. Thanks again. Now I am off to the welder…

 

[Orange Vise]

Harden after welding.

Welding will create a temperature gradient where some areas will reach full hardness (HRC55+), whereas cooler areas will be effectively tempered down in to the HRC 20s and lower.

I also have another small 4140 part that I need to weld to unknown hardened steel, maybe 8620?

8620 is a case hardening steel, so it uses a completely different procedure than 4140.

 

[ImpracticalM89]

The priority is to hardened the small parts which will take up most of the weld temp. The rod and unknown part have a lot of thermal mass. I’ll try weld first, then profile the resultant hardness and let you now how it turns out.

 

[SND]

I've tig welded a bit of 4140, O1 and HSS tool bits for custom cutting tools. I normally use 308 stainless filler, they say 309 is a bit better but I ain't buying a tube for a few inches of weld on something that's "in shop" use, if it breaks I'll add more weld.
If you preheat the pieces a bit they're less likely to crack at the weld, and if you can get them up to HT temp right after welding that's likely even better.

 

[implmex]

Hi ImpracticalM89:
Some things to consider if you intend your welds to survive any significant load or cyclical stresses.
There is a specific procedure for welding this material involving a preheat, maintaining the heat during welding and a slow cool to prevent post weld cracking.

The rods you use are also important, and vary depending on the application...4130 is one choice 7018 is another, I've heard of 308 stainless, and I'm sure there are more.

It is NOT recommended to weld the material in the hardened condition.

Welding engineers have a whole recipe for handling this material simply because it is far more challenging that welding something like mild steel, and it's way less tolerant to deviations from proper procedure if you are going to stress the assembly.

Probably the biggest requirement is to preheat, maintain the heat during welding and slow cool.
The other is to avoid hydrogen embrittlement (that's why 7018 rod is a common recommendation if you're going to stick weld it)
Obviously the rods need to be kept very dry.

So a lot depends on what you intend to do with your assembly.
If it's going to go in a critical system (like for example a car suspension) you cannot just go at it with home shop methods.

Anyway, to answer your original question, heat treat after welding.

Cheers

Marcus
www.implant-mechanix.com
www.vancouverwireedm.com

 

[ImpracticalM89]

Thanks for all the great advice! I have one of the three rod parts welded and the welds look robust. I am going to clean the welds up tomorrow and profile the hardness due to the welding process before any heat treatment is done. Best M89

 

[metalmaster1]

I just welded a 4140 assembly using 80s d2 rod, the part was preheated to 450 degrees.
the weld cracked immediately, swithed to 312 stainless and no cracks

 

[ImpracticalM89]

I ground and polished the welds this morning and everything looks solid. The block I welded to the rod is between 40-45HRC. The rod near the weld seam is the same and further away and less important is below 40. The small part is what takes the wear and tear. So now what? Should I just temper the part with a target of 40HRC OR should this be run through a complete heat treatment? I look forward to hearing your feedback. M89

 

[EnginCycles]

I just welded a 4140 assembly using 80s d2 rod, the part was preheated to 450 degrees.
the weld cracked immediately, swithed to 312 stainless and no cracks

100%! 312 is the best of the stainless welding rods IMO. Welding 4140 is so prone to cracking. 4130 is so much better for welding.

 

[Maintenance guy]

For our offroad parts we weld we use 4130 (I understand they're different grades) We preheat material roughly 200 degrees for large assemblies or just let whatever heat gets added during the tacking process to smaller stuff. Post weld we wrap them up in a big welding blanket to maintain a looong cooling process to limit distortion and prevents cracking as well. Once its fully cooled then we heat treat. Filler we use is ER70S-6 never had an issue with cracking. Just my $.02

 

[ImpracticalM89]

ER70S-6 is exactly what I successfully used on my parts. I love how the rod flows.

 

[G-ManBart]

Thanks for all the great advice! I have one of the three rod parts welded and the welds look robust. I am going to clean the welds up tomorrow and profile the hardness due to the welding process before any heat treatment is done. Best M89

Looks don't really mean much and you're talking an alloy that's on the challenging side to weld properly. 4140 gets brittle when things aren't just right, but it can still look fine.

The only really true way to know if the welds are good is to make a test piece or two with the same thickness/joint configuration, cut and etch the weld and do some sort of strength test to see where it fails.