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Heat treatment of 4130 channels

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sealmat

Materials
Aug 3, 2009
2
We are having problems related to flatness after heat treatment on 4130 rectangular channels which a about 7" long, stamped from 0.06" sheet. The rectangular 'legs' are about 0.3" tall. We are performing a prehardening stress-releif per AMS 2759/1, followed by hardnening, quench and temper to get a final hardness of 43-47 HRC. After heat treatment, the parts are bent (concave) slightly. We want to maintain a flatness of 0.010". Austenize is being done at 1575F for 1.5 hrs. We have tried oil quench and martemper. After oil quench and subsequent temper at 575F (for 3hrs), the parts are out of flatness by 0.15-0.50" while after marquench@ 350F and 575F (for 3 hrs) temper, the parts are consistantly out of flatness by about 0.04". I am wondering if someone can recommend what we can do to improve the flatness of these channels.Are the heat treatment times and temperatures adequate? Thanks for your help
 
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Based on the part dimensions, you should consider austempering the parts to produce a bainitic structure with your desired hardness without the distortion inherent in martensitic transformation.
 
How are you quenching the parts? You will get less distortion if you fixture them vertically (long slender parts will bow away from the quench). For tempering, you can clamp them flat so they will stress relieve themselves in a flat condition. My guess is that just clamping them flat for tempering will get you where you want.

rp
 
Thanks for your inputs CoryPad and redpicker. We are quenching the parts vertically. I was thinking on the same lines redpicker - about fixturing the parts during tempering. I am wondering if we should do a stress relief after tempering and also if the time between quench and temper should be limited to less than 2 hours. Do the rest of the heat treatment parameters seem where they should be?
 
I agree that austempering is a good option for long, thin parts like this, but if you must have a martensitic microstructure, then keep the marquench and fixture the parts during tempering. This does not need to be an elaborate device, just something to keep them restrained. Stress relief after tempering is not needed. I don't think that there will be any substantive benefit for flatness by limiting the time between quench and temper, but it is always a good practice to limit this time. The austenitizing temp is in the correct range, so no problem there. 1.5 hours should be more than enough time to homogenize a load of thin parts like this, so there should not be any problems with nonuniform temperature prior to quenching.
 
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