Welding 4140 HT

[JacobP12345]

We are working with a heat treated torsion bar of .5'' diameter and 36'' long (HRC 43). We have been welding it to standard A36 we had plasma cut on both ends. One for a tab, and one for a mounting point. So far even with 140 ksi of subjected stress the weld has held with no heat treatment (preheat heat or PWHT) despite repeated cycles. In fact normally the tab or bar itself fails first. After reading about the risk of cracking with 4140 welds, especially PHT 4140, I was curious if fatigue life could be improved by adding in a preheat to 400 degrees F and slow cooling via an insulated blanket without risk of compromising the tensile strength of the heat treated bar. Does anyone have experience with this?

 

1 – 0.5" diameter is smaller than the range where 4140 is usually specified. Lower alloy steels or even 1045 can meet your hardness spec in small diameters. However 4140 is what you have.

2 – weldability or 4140 is reasonable when precautions are taken. The dissimilar combination poses no problems; I suggest you match the lower strength base metal, so a 70ksi class filler metal. 400°F is adequate; you actually want to take care that the interpass temperature does not run away in this very small configuration. Small weld beads and take your time between passes.

3 – plasma or any other method of cutting is not a factor, because you will want to grind or machine the surface to bright metal before welding.

4 – the only cracking you need to be concerned with is cold aka delayed cracking due to moisture getting into the weld zone. The best way to avoid this is to use a 'dry' welding process like GTAW, preheat and post-weld soak to facilitate the dissipation of any hydrogen that may be present. On such a small heat sink a thermal blanket will not achieve much; you will need to add supplementary heating to maintain 400°F for one hour – a furnace works best.

5 – fatigue strength is basically a direct function of hardness/tensile strength. However, when it comes to fatigue, defects (including shaft misalignment) and stress raisers trump all other considerations. In any event, you cannot do better than the strength of the weaker base metal.

6 – I think your biggest challenge will be maintaining straightness across the joint.


"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[JacobP12345]

All these things are fantastic to know. After extensive testing and calculation the 4140 is needed because the torsion bar is subjected to a required 90 degree twist and needs to provide enough torque to open a 32 inch long 150 lb lid. This ends up subjecting it to roughly 140ksi of stress.
The A36 steel is used to create a lever arm that provides the torque to the round 4140 bar as well as the mount on the other end attaching the bar to the door. We are experimenting with using half inch thick 4140 for this lever are since the lever arm itself was yielding pretty significantly. What do you mean by straightness of the join? Thank you for the help.

 

[EdStainless]

Any defection in the mounting bars or the bars being out of square with the tube will result in bending as you twist. This can significantly shorten fatigue life.
A pre-heat is a very good idea. Your biggest risk is getting hydrogen cracking of the HAZ even before these go into service. A bar with preexisting cracks will fail quickly in use.

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P.E. Metallurgy, Plymouth Tube