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Proper pre & post heat for welding 4140 annealed

Proper pre & post heat for welding 4140 annealed

Proper pre & post heat for welding 4140 annealed

I am having a hard time finding out the proper technique of welding 4140 in the annealed state to a A500B structural tube. The 4140 shaft (2.375" dia.) runs through the tubing center (4.5" outer dia-with a 2.5" center dia.) the 4140 shaft sticks out either side of the tubing,  a 5/8" weld on either end bonds the two together. The process laid out by our previous engineer was a pre-heat (no set temp. on his print) and weld. The guys in the shop would just heat up with a torch for a moment  and run there  passes and let it air cool. Now, this process seemed to be fine until recently the 4140 shaft snapped off right on the edge of the weld bead. This sub-assembly goes into a larger assembly which was designed in SolidWorks with full FEA testing. The FEA testing showed the shaft to have a 3 to 1 safety factor. The problem with SolidWorks, it doesn't take into consideration what happens to material when it is welded. Not having a welding background, I am having a hard time finding out information on welding this material. The 2.375" shaft needs to have great tensile properties, high impact properties, and a high core strength. After many calls and internet searches, I'm very confused. Some people recommend only pre-heat (200F or 600F?)& weld, some say pre-heat (600F), weld, post-heat (800F), and wrap and slowly cool, and someone mentioned inter-pass temps? If someone could lead me in the right direction, it would greatly be appreciated.

RE: Proper pre & post heat for welding 4140 annealed

For a 4140 annealed shaft material, I would recommend an elevated preheat of 350 deg F, and hold this temperature for 15 minutes before welding. Verify the preheat temperature using a Tempstik, not as before.

You didn't ask about filler metal so I would assume you have a filler metal that has proven itself. Regarding PWHT, for 4140 using only a 5/8" fillet weld, I would not perform PWHT. I would recommend low hydrogen electrodes for welding and some post-weld nondestructive testing (like Liquid penetrant or magnetic particle inspection).

RE: Proper pre & post heat for welding 4140 annealed

Filler wire per our welding supplier is metalcore met71 when bonding A500B to 4140.

PWHT, are you refering to post heating?


RE: Proper pre & post heat for welding 4140 annealed

Yes, PWHT stands for post weld heat treatment.

RE: Proper pre & post heat for welding 4140 annealed


PWHT is different than the term you are using, post-heating.

Post-heating is a hydrogen bake out step typically performed at 600-800 F for 2 hrs.  Also known as DHT (dehydrogenation heat treatment).

PWHT as metengr is talking about is at a higher temperature usually in the 1100-1300 F range which stress relieves the weld and surrounding area.

The DHT step can be omitted with the use of low hydrogen electrodes as metengr has suggested.  Ultra low diffusible hydrogen, H4 is best if available.

RE: Proper pre & post heat for welding 4140 annealed

metengr & DVWE,

I have my welding supplier looking to see if our weld wire meets the H4 specifications.

With the use of a "low hydrogen electrode" the DHT can be omitted, how about the PWHT?


RE: Proper pre & post heat for welding 4140 annealed

Yes regarding PWHT.

RE: Proper pre & post heat for welding 4140 annealed

What probably happened in this instance is that the weld was allowed to cool too quickly.  Doing this will form martensite in the Heat Affected Zone (HAZ - edge of weld bead) which is very hard and brittle.

If preheat is applied properly as metengr has outlined, and allowed to cool slowly, you will not form martensite in the HAZ, and therefore, you will not need PWHT.

Another thing to be conscientious about is the P and S content of the metal.  Do you know what the levels were in the piece that broke?

RE: Proper pre & post heat for welding 4140 annealed

Have you considered using an electrode like Allstate 275 from ESAB. This electrode is the workhorse in our fab shop for repair work especially on alloy steels.


I'm going to get in trouble with a lot of people for saying this, but here goes. We use Allstate 275 as a universal welding electrode for any unknown alloys or dissimilar metals. We had process components made from 1035, 4140-4150, AH5, D2, H11 that sometimes required expedient weld repair to prevent unscheduled outages and 275 was the electrode of choice.  We also used it for routine repair of the above components.  

Allstate 275 is essentially 312 SS, with an excellent flux, and it is like several other maintenance electrodes on the market.  275 was one of the first on the market so it was picked up and kept after trying several other late comers.

There is no problem with 7018 for welding the low alloys, we have always preferred 275 when there was any question of the welding conditions or parameters. Though 275 is Austenitic it's work hardening properties are at times a benefit.

The welding procedure is essentially the same as posted above, 300F-400F preheat, weld and allowed to slow cool. We wrap our parts in glass or ceramic cloth or wool.


RE: Proper pre & post heat for welding 4140 annealed


The material spec sheet for 4140 has P (max .035) and S (max .040).


RE: Proper pre & post heat for welding 4140 annealed

I just got the specs of the welding wire we use for this assembly:

METALLOY 71, has a H4 specification.

Thanks for all the help,

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