Stress relieve - 1200F, will oxide scale form? 1

[twistedneck]

I found that 750F is not a great enough temperature to relieve all the residual stresses in 4130 steel sheet. I'd like to go up in temperatiure to 1200F (that is what ASME recommends).. but 1200F is so hot I'm worried about decarburization, oxide scales (hematite, etc.).

What is the highest temp i can stress relieve at w/o worries of decarburization and scale build up?

Thanks, twistedneck.

 

[metengr]

twistedneck;
Decarburization is not a concern below 1600 deg F for this steel. At 1200 deg F, slight surface oxidation will occur in air. Can you deal with this? If not you can mask the steel surface with a high temperature masking compound, but it will require mechanical cleaning after stress relieving.

 

[twistedneck]

Of course slight decarburizatoin is ok, but i'm working with 1mm sheet 4130 and i cant afford to loose much harness off the surface.. How much do you think i'll loose?

Is there a point where minimal scale will form kind of like the thinner oxide layers at lower tempering temps?

Thanks for the reply.

 

[metengr]

twistedneck;
What exactly is your application? You mentioned ASME required PWHT. Are you sure about this for only a 1mm sheet thickness?

Oxide formation is primarily driven by metal temperature in air. So, lowering the metal temperature, results in less oxidation. This analogy is similar to the formation of temper colors on steel where the steel is heated thru various temperature ranges, and different colors appear on the surface.

 

[twistedneck]

This is not a PWHT, this is a post stamping stress relieve to reduce internal stresses (its got spot welded components) prior to normal heat treat. W/o any pre-heat treat stress relieve, the sheet metal structure becomes a pretzel during quench.

 

[metengr]

twistedneck;
Ok. I don't believe adding a stress relief heat treatment step prior to your normal heat treatment after fabrication, as you stated, is going to help.

The distortion occurs because of the geometry and thickness of the sheet during subsequent heating and quenching for normal heat treatment. You need to provide some support or fixturing during the heating process for normal heat treatment. By the way for normal heat treatment you will need to use a protective atmosphere furnace to avoid decarburization.

 

[twistedneck]

metenger, w/o the 2hr pre-soak at 750F, the parts turn into bananas. I was hoping a higher temp soak at 1200F would help even more. There is a full argon atmosphere - but its not perfect.

Experiments with fixturing only caused peralite to form in areas contacting these.. due to slow quench times related to large heat mass of fixtures.

What i'm gathering from you is going to a 1200F pre-soak wont do much if anything better than the 750F pre-soak.

 

[metengr]

twistedneck;
Working with only 1mm sheet thickness for a quench and temper heat treatment is very difficult, at best. I am going to ask the obvious - why do you need to heat treat this sheet at this thickness range? I would think that the material could be used "as-is" after forming. What are you expecting to gain from heat treatment after forming?

 

[twistedneck]

metengr, many of the components see repeated impact, yielding (mushrooming), wear, and fracture failure modes. Parts need to be 42Rc.

I hear you on the why bother part, to answer, these parts routenely yield in service when left in the as formed state.

 

[CoryPad]

Thin sheet metal parts that need hardening routinely are austempered to prevent excessive distortion. You might try Applied Process for more information:

http://www.appliedprocess.com/index.html

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[metengr]

Yes, I would investigate CoryPad's austempering suggestion, and drop the stress relief step.

 

[twistedneck]

I already have a full PID salt pot for Austempering and have successfully Austempered these, I've also done upquench Austempering. I blew out the salt pot burners and SSR's because i didn't have a proper overflow (my salt absorbed too much moisture and boiled over).

Salt quench and Austemper reduced warp significantly, but did not eliminate it. I can't afford to fix the burners right away so i've tried many types of quench and brine works the best apart from molten salt.

I think you guys answered my question though, stress relieve can only go so far. I'll up the temp on stress relieve to 800F for 2hrs.. I doubt it will help. I'll also agitate my quench bath (seems i'm getting most warping where two sheets are spot welded together).

 

[metengr]

twistedneck;

I can't afford to fix the burners right away so i've tried many types of quench and brine works the best apart from molten salt.

As a famous actor was once quoted as saying in a movie, " A man has got to know his limitations" and apparently you reached your limit. Get the burner's fixed or outsource the austempering operation.

 

[twistedneck]

metengr, that's the story of my life! lol.. i never know my limitations.

However, that doesn't change facts, Austempering did not solve the problem, fixturing did not solve it, quench medium, etc.. I'm focusing on the last two factors - agitation and pre-stress relieving.

 

[swall]

Is this the same project we discussed in thread 330-166421?

 

[twistedneck]

Swall, yes its the same project. however, i was trying to find out if I could go higher on my pre-heat treat stress relieve step w/o scale or decarb issues.

I got the answers i ask for, i can go higher with diminished returns, and there will be scale build up.. but decarb wont be an issue.. (more stuff like pitting).

Thanks, Jeff

 

[swall]

Twistedneck--it may not be necessary to go all the way to 1200F. If you look at the stress relieving section in the ASM Metals Handbook you will find a graph showing stress relief equivalencies for different times and temperatures. You will also find the Larsen-Miller equation, which will allow you to calculate these. So, how will you know when you have removed all of the residual stresses? I would suggest you take your "stress relieved" parts and then heat them to your normal austenitizing temp, for the normal time and then slow cool them. Then observe if they warp. You may have to support the test part(s) so they don't sag under their own weight. You could use your fixture for this. Once you get "stress free" parts,try the austempering again and see where you're at. If austempering still won't do the trick, you may have to resort to a Q&T but temper straighten in fixtures such as the one you made. We make some Bellevilles and temper straightening is the only way we can get them to the proper shape.

 

[SMF1964]

If you're looking at stress-relieving, what about vibratory stress-relief?

 

[bklauba]

Can you describe the overall size and weight of your part? Post a dwg or pic?

BK

 

[twistedneck]

swall, that is an awsome response! now i'll go and learn about Larsen-Miller.. thanks very much. I've resorted to straightening, but of course that's for one off's only.

SMF1964, vibratory? holy crap.. way over my head on that one. Can you elaborate?

bklauba, its a sheet metal box 10" log 1.5" square. Its got some weldments on each side, slightly thicker 1.5mm spot welded on - the problem area!