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Heat treatment

Heat treatment

Heat treatment

(OP)
1. How important are the following steps in the heat treat process of steel(6150)?  What if these steps of the heat treat are missed?:

Annealing, Subcritical Annealing, Pre-hardening stress relieving, and post tempering stress relieving.

2, If the material was hardened too high during the temper can the temper be reperformed to a higher temp in order to get a lower hardness without ill effects.  

Thanks  

RE: Heat treatment

I have to say this looks like homework to me.

How about some more info on what shape and size the part is.

Nick
I love materials science!

RE: Heat treatment

(OP)
The part is actually a landing gear leg and is a 1" dia rod approximately 30" long that has a 112 degree bend in it.  Material is 6150 with a desired hardness of 43-46 HRC.  It was first tempered to 775 F with a hardness of 48 and then retempered to 825 F back in the acceptable hardness range.  

RE: Heat treatment

I presume that this part was extenstivly tested to qualify it.  The heat treatment should have been fixed at that time and any deviation is unacceptable.
While re-tempering is a minor sin, there are a lot of ways with different heat treatments to reach similar hardness that result is significantly different fatigue resistance and toughness.

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RE: Heat treatment

rb88,

I agree with EdStainless that this part likely involves significant testing for qualification, which can make process changes difficult to accept/implement.  Having said that, my answers to your questions are as follows:

1. Annealing and sub-critical annealing are performed in order to increase the formability of the steel and to decrease the forces needed for deformation.  A low-ductility grade like 6150 is difficult to deform without cracks, etc. unless it is annealed.  The sub-critical anneal is a recrystallization step after some intermediate forming that is needed in order to complete the final shape.  Pre-hardening stress-relieving is done in order to reduce the tensile stresses that are present on the surface and to reduce the amount of distortion that will occur after hardening.  Post-tempering stress relieving is likely performed after chromium plating or some similar process.  It will be difficult to eliminate any of these without a thorough evaluation of the entire manufacturing process.

2. This is unlikely to be a significant problem, provided that the microstructure is good (100% martensite transformation, no decarburization, etc.).  

RE: Heat treatment

(OP)
Thanks for all the responses.

RE: Heat treatment

I apologize, I remember a question like that off a test in school. Other than that, what they said.

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