stress relieving 4130 tube

[SuedePflow]

I've scowered the web and read quite a bit on this topic. But I still have questions that I want to run past you guys.

It seems that a properly triangulated structure made of 4130 tube could sometimes benefit from a post-weld heating of the weldment to stress relieve and to reduce brittleness of the HAZ. I read several sources that stated to use a neutral flame of an oxy-acetylene torch to evenly heat to around 1100 degrees and then let it slowly air cool.

1) Could I use a MAP gas torch to heat it instead? My oxy-acetylene torch is packed up in storage and not easily accessible. And it seems that MAP gas would have less potential for hot spots anyway.

2) If there any downfall to the stress relieving process? I just want to make sure there's no way to screw it up and consequently substantially weaken the material.

Any advice would be greatly appreciated.

[SuedePflow]

I got curious about it and did a little stress project in the garage last night. I welded up two identical 12" triangle structures using 1.25" x .065" 4130 tube. I stress relieved one and not the other and then I made a cut through both and measured deflection of each.

I made a .110" wide cut. The stress relieved structure closed the gap to about .095". The non-sress relieved structure closed the gap to about .050". So, it's clear that the process did indeed help relieve residual stresses. At this point, I just want to make sure the process isn't harmful to the strength and integrity of the material.

In these pics, the stress relieved piece is on the left and the other on the right:

[SuedePflow]

An exerp from Carrol Smith's book "Engineered to Win":

[SuedePflow]

Anyone?

[dave powelson]

I believe a more appropriate term is normalizing.
Gradual even heat application, spreading out and around from the joint.
Do not allow base metal to heat to the point of looking 'wet'--IOW--ready
for surface fusion.
Using temperature crayons is a good idea.
8-900˚F, pick your preference.
IA/A&P I do a/c frame repairs under--insists on this.
We have the advantage of inspecting the repairs,
at annual inspections.

Makoman should chime in here.

[It appears to me that normalizing chrome moly structures
after repair or full build, is not understood/or necessarily done
by many of the latest generation fabricators. (Then there's the issue of mixing
full hard tubing with normalized in certain funny car frames--with dramatic,
catastrophic results--aka John Force, etal.)

However, what I noticed, decades ago--when tiggin' in,
a new tube into a joint for a repair of hard impact---lettle bits of
parent tubing in the old joint area, can fracture, come spitting and
flying out...IOW as that Shelby article mentions---the air cooling
of new tube, produces a thru hardened condition that has much less ductility
than the parent tube in the HAZ of the tube joint.
A 'good' failure in the structure will show stretching and bending,
not shear tearing, ripping or fracturing.]

[WookieWelding]

by post weld heat treating it you are taking any martensite you have created in the haz and turning it into Tempered martensite .....Martensite is bad so preheating before welding decreases the formation of martensite then post heat treat tempers any martensite that forms in the haz .. so in that the less martensite that forms the more ductile and stronger the joint is ...and by tempering the martensite with post weld heat treat it becomes more ductile and stronger then if you did not post weld heat treat ... i could go further into the metallurgy but i think i got my point across preheat 300-400 F post weld heat to 1100 F for 30 mins then air cool

[duffman1278]

You can get out of the martensite phase by heating up the steel to just above 1600 degrees F and allowing a SLOW cooling of the part. The cheap way of doing slow cooling at home would be to heat it up to the temperature I mentioned and dump the part in sand so it cools down slowly. Letting it air cool won't do you as good either, slow cooling would be best. You do not want to quench it immediately or you'll risk cracking and even if you don't you'll have a very brittle part.

[assassin_works]

ok i think i got what yall are talking about but what if your doing a whole chassis or tube frame would you need a big oven to do the whole thing in at once ?

[makoman1860]

Mapp, OA, natural gas, etc all will work fine. The time required to "soak" depends on the material thickness. For most tubing sections it is only a couple minutes. Air cooling in still air is fine as there is enough residual energy to ensure a slow enough cooling rate.
Check out the newest aircraft mechanics handbook from the FAA.

http://www.faa.gov/library/manuals/aircraft/

FAA-H-8083-31 Volume 1, Chapter 5 on welding.

[SuedePflow]

Thanks for the replies!

Originally Posted by dave powelson

I believe a more appropriate term is normalizing.
Gradual even heat application, spreading out and around from the joint.
Do not allow base metal to heat to the point of looking 'wet'--IOW--ready
for surface fusion.
Using temperature crayons is a good idea.
8-900˚F, pick your preference.
IA/A&P I do a/c frame repairs under--insists on this.
We have the advantage of inspecting the repairs,
at annual inspections.

Correct. A more proper term for stress relieving would be normalizing.

I really just want to relieve some residual stresses, and reduce brittleness and bring back some ductility to the HAZ. I do have a temp stick (1050* F.). I guess I'll give it a shot. I'll heat the weldments with a MAP gas torch and allow them to cool in still air.

[makoman1860]

Originally Posted by SuedePflow

Thanks for the replies!


Correct. A more proper term for stress relieving would be normalizing.

I really just want to relieve some residual stresses, and reduce brittleness and bring back some ductility to the HAZ. I do have a temp stick (1050* F.). I guess I'll give it a shot. I'll heat the weldments with a MAP gas torch and allow them to cool in still air.

Actually I will correct you as well, Normalizing is not correct either. Tempering is the technical term. The "stress relieving" part of it comes from the material entering the plastic state, but yet having less contraction durring cooling relative to the original weld. So you ARE relieving" stress" from a structure, but not microstructure standpoint, call it thermal relaxation. In either case, it does the job.

[SuedePflow]

Originally Posted by WeldingWookie

by post weld heat treating it you are taking any martensite you have created in the haz and turning it into Tempered martensite .....Martensite is bad so preheating before welding decreases the formation of martensite then post heat treat tempers any martensite that forms in the haz .. so in that the less martensite that forms the more ductile and stronger the joint is ...and by tempering the martensite with post weld heat treat it becomes more ductile and stronger then if you did not post weld heat treat ... i could go further into the metallurgy but i think i got my point across preheat 300-400 F post weld heat to 1100 F for 30 mins then air cool

For 30 minutes? Everything I'm reading is recommending a short period of heat for thin stuff.

Also, we did pre-heat the work, so I'm more-so interested in this for extra insurance even though I feel it's not critical for this particular project.

Originally Posted by duffman1278

You can get out of the martensite phase by heating up the steel to just above 1600 degrees F and allowing a SLOW cooling of the part.

What I'm attempting to do is more-so tempering, not heat treating. 1600* F sounds like heat-treating heat. From what I've read, tempering is around 1000-1100* F.

As I understand it, the base material is normalized to begin with. And once TIG welded, the HAZ becomes quenched and forms martensite. And to help this area to become less brittle, I would want to temper at around 1000-1100*F for a short period of time and let it slowly air cool in still air. Does that sound about right?

[SuedePflow]

Originally Posted by makoman1860

Actually I will correct you as well, Normalizing is not correct either. Tempering is the technical term. The "stress relieving" part of it comes from the material entering the plastic state, but yet having less contraction durring cooling relative to the original weld. So you ARE relieving" stress" from a structure, but not microstructure standpoint, call it thermal relaxation. In either case, it does the job.

Excellent. I did some reading on tempering after my last post (#10), so this makes perfect sense to me. Does my above post regarding tempering sound correct?

[makoman1860]

Originally Posted by SuedePflow

Excellent. I did some reading on tempering after my last post (#10), so this makes perfect sense to me. Does my above post regarding tempering sound correct?

You got it!