stress relieving 4130 tube

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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."There are two types of people in America - those who try to classify everyone into two types and those who don't."
Reply: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:"There are two types of people in America - those who try to classify everyone into two types and those who don't."
Reply:An exerp from Carrol Smith's book "Engineered to Win": Attached Images"There are two types of people in America - those who try to classify everyone into two types and those who don't."
Reply:Anyone?"There are two types of people in America - those who try to classify everyone into two types and those who don't."
Reply: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--readyfor 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 structuresafter repair or full build, is not understood/or necessarily doneby 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 ofparent tubing in the old joint area, can fracture, come spitting andflying out...IOW as that Shelby article mentions---the air coolingof new tube, produces a thru hardened condition that has much less ductilitythan 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.]Blackbird
Reply: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 coolLast edited by WeldorWes; 03-22-2012 at 12:21 AM.Miller Xmt 350Lincoln Ln-25Ahp 200xSmith Gas Mixer AR/HTig is my Kung FuThrowing down dimes and weaving aboutInstagram http://instagram.com/[email protected]
Reply: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.
Reply: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 ?Lincoln Power Arc 4000 Thermal Arc Fabricator 252 iThermal arc 186Thermal Arc 26 tigTweeko 200 amp spool gunHobart AirForce 400WP-17V-12R
Reply: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.
Reply:Thanks for the replies! Originally Posted by dave powelsonI 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--readyfor 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.
Reply:Originally Posted by SuedePflowThanks 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.
Reply:Originally Posted by WeldingWookieby 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
Reply:Originally Posted by makoman1860Actually 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.
Reply:Originally Posted by SuedePflowExcellent.  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?