Screw auger shaft and flights

1. It won't hurt to preheat the Mn steel to weld.
2. The Mn steel really works best when it is pre-work-hardened.
This is often done by either heavy peening or explosive surface hardening.

What weld filler are you using?
Have you thought of trying Mn steel weld overlay?
These high Mn alloys are common in railroad work and there is a lot of information available on how to form and weld them.

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Plymouth Tube

First, why not consider making the auger shaft out of the same Hadfield steel bar stock provided the strength requirements of the auger shaft are met using this material? This takes the element of working with dissimilar metals out of the equation.

Second, if you are going to use the Hadfield steel in combination with the 8620 shaft material, local preheat will be needed for the 8620 steel, regardless. What is good is that the carbon content is on the low (0.20 mass percent) end, which will improve weld ability.
As for filler metal, I would suggest using 309 stainless steel filler metal to join the Hadfield (high Mn-austenitic steel) to the low alloy steel. You could also look into a filler metal SFA 5.13 molybdenum-manganese steel EFeMn-B. However, this is more applicable for hardfacing/repair application on Hadfield substrate material or for welding over carbon or low alloy steel substrate materials.
Either way, I would strongly suggest you qualify a welding procedure using ASME Section IX as a guide to ensure adequate strength and ductility of the weld region.

@ ED stainless, what do you mean by "It won't hurt to preheat the Mn steel to weld" ? 14% Mn-steel starts to precipitate at 250 - 300 degrees celsius, so any welding needs to be done as "cold" as possible. Preheating will only make it worse. Or am I mistaking?

I'd go for a ductile butter layer (307-16 or similar) on the manganese steel, before welding it onto the shaft.

beware, if you preheat the shaft, it will shrink afterwards when it cools down. as the flight shouldn't be preheated (!!!), it will remain the same dimension. Regard both actions together, and you'll see a rather large shrinking stress which will aid cracking in the welded zone.

I wouldn't trust this without further investigation.

That is correct, there should be no preheat for the Hadfield (high-Mn) steel.

I started off presuming that your 14Mn had been annealed and quenched.
It will be fine for re-heat to 275C (500F) for even very long hold times.

If you choose to weld with a low alloy (softer) filler you may need to watch for wear of the welds.
The welds may need to be overlayed to prevent them from being the weak link.

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Plymouth Tube

I agree, base material is 14Mn, annealed and quenched.
I disagree, that temperature of 275°C won't hurt the casting. Carbide precipitation happens rather fast in base material when welding. interpass temp should be kept as low as possible ( < 200°C).
Preheating only promotes higher material temp, so precipitation will happen faster and more easily.

Please do elaborate your point of view, maybe I can learn something as I'm currently having problems with repair welding of large Hadfield castings.
Problems we're encountering are of a somewhat different nature (casting porosities which crack through our butter layer) but carbide precipitation also causes loss of material (chips, crumblings upto several cm's large, depending on crack length). Gouging and/or grinding away defects can be enough to initiate crack growth, let alone welding.

The issues that I have seen with castings often have to do with inhomogeneity of the material.
If there are areas where the Mn is a bit low and C a bit high then carbide precip will happen 10x as fast.
Most of my experience is with wrought material, we used some preheat, only about 200F, and our sections were not very thick.

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Plymouth Tube