Stainless filler to TIG 4140?

The problem you're up against is quenching of the welded area due to the heat-sink effect of the adjoining material. One source [1] says for 0.20--0.40% carbon steels, no preheating is needed unless plate is over 0.500" thick. But over 0.40% needs preheat, regardless of thickness. I would pre-heat to 350--400 oF,

You may not need post heat. Unless you're machining the welded area, it probably isn't required.

If you do post heat, remember that the purpose is to draw a temper in the hardest part of the weld So I think you'll need a much higher temperature... Look at the graph below, in the lower right quadrant [2]. Using Rockwell C of 30 (commercial heat treat) as a reference, that is Brinell BHN of 286. I would use the full hard line because that's the worst-case scenario around the weld. In either case, you'll need 800 oF or more to make much of a difference. HOWEVER it probably is not needed unless you are going to machine those welds. If you post heat, you're only tempering a very small part of the weld, so it might be OK to heat the weldment and let it cool in still air. The mass of the adjoining parts should slow the cooling in the weld metal. Personally, I would turn the oven off and leave it to cool within. If I couldn't fit the weldment in the oven, I would put it on round bar to minimize contact with the welding table, heat with MAPP gas, and then throw a welding blanket over the whole works.


heat_treat.jpeg

1. Metals and How to Weld Them by T B Jefferson, G Woods, pub James F Lincoln Arc Welding Foundation
2. Machinery's Handbook 24
 
As I understand it, when welding on 4140 or other steels containing carbon, the post heat treat is the help the weld maintain the strength of the base metal. Pre heating the 4140 is to keep cracking to a minimum from the 'quicker' cooling causing the steels to pull away from each other at the junction of lower and higher carbon containing metals.

I think I'm going to play it safe and use the 309ss for filler and TIG it after heating. 309 is marketed for dissimilar metals specifically carbon steel to non carbon steel when pre and post heat isn't an option.
durableoreo said:
The problem you're up against is quenching of the welded area due to the heat-sink effect of the adjoining material. One source [1] says for 0.20--0.40% carbon steels, no preheating is needed unless plate is over 0.500" thick. But over 0.40% needs preheat, regardless of thickness. I would pre-heat to 350--400 oF,

You may not need post heat. Unless you're machining the welded area, it probably isn't required.
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In this case the 4140 ring will have much more mass than the supports under it.
 
Pretty sure you're wrong about the logic there. Post heat does not "maintain the strength of the base metal". The quenching effect of cooler surrounding metal result in hardening around the weld, which increases local strength (and brittleness). Post heat draws a temper.

HOWEVER, preheat and 309 will definitely work. Good luck with the project. Post a picture so we can see how it turns out.
 
If your primary concern is cracking after the weld, then 309 is a great solution. Nickel does wonders in that regard.
 
durableoreo said:
Pretty sure you're wrong about the logic there. Post heat does not "maintain the strength of the base metal". The quenching effect of cooler surrounding metal result in hardening around the weld, which increases local strength (and brittleness). Post heat draws a temper.

HOWEVER, preheat and 309 will definitely work. Good luck with the project. Post a picture so we can see how it turns out.
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You're right, I wasn't clear. Post heat treatment is used when welding and you need to prevent weak spots around the weld and heat affected zone. After welding heating everything equally and cooling slowly helps maintain strength throughout the base material and weld preventing the weld or HAZ being significantly different strength. That post heat treat temp is usually 1000* or more.

What I'm going to do by heating the assembly to 350 before welding and reheating to 350 after welding and letting it slowly cool will do nothing for the strength but will hopefully eliminate cracking at the weld.
 
It seems like a nit but as far as I know, there are no spots that become weak upon welding. There is a dilution effect of the filler metal and the crystal structure that develops as the weld cools. If the weld cools slowly, the iron molecules have time to grow large randomly oriented crystals and the carbon is relegated to the odd clumps and C-Fe complexes (ferrite and pearlite). If the weld cools quickly, the carbon ends up in different complexes with the iron (austenite and ferrite), the grains are smaller, and the who structure is harder. Within a quickly cooled weld, there is a hard brittle line but it is not weak in the sense of material science.

There are some situations where the shrinkage of the cooling weld can cause a crack immediately. Socket joints come to mind. The hardness within a weld is a con when put into service where flexing and vibration is high. In that case, I suspect that fatigue would be sooner than for a uniform material. There's lots of information about stress risers in the Handbook but I don't see anything that applies directly. So I guess you're right in some circumstances. But for a steady rest, it will probably be fine with or without a 350-oF post heat.

Well, I think I've had enough to say in this thread...

I'll leave you with this link for your viewing pleasure, a little tip for reading this far: http://www.steadyrest.net/index.php/rip-series-100
 
MrWhoopee said:
Another vote for 309. I've gotten to where I use stainless filler on all steel (and even CI) unless it needs black oxide.
It just flows nicely and doesn't crack.
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Hmm, I'll have to try it out.
Oh, I have some :)
 
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