Heat Treat Steel, Rockwell C - Variability in Complex Machined Shapes

qu1nn

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#1
Newb question:

If you have a 1.5" x 2"x 6" block of S7 that has been heat treated to approximately Rockwell C50. Knowing that the block (prior to heat treat) is machined into a complex shape and lets say that the thinnest member is 0.25"

Rockwell C testing at various locations gives results from 44-55.
It seems like when testing at the thickest section you will get the lowest reading, whereas testing at the thinnest section (0.25") you will get the highest reading.

for the purpose of this post lets assume that the rockwell hardness tester is perfect.

I understand the following question is vague , and there seems to be a multitude of reasons for hardness changes, but

Could the variance seen (44-55 RHC) possibly be attributed due to the thickness of the material? or is it more reasonable to believe that the variation could it be due to how the piece is heat treated?


Thanks for your time
qu1nn
 
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Miner

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#2
What type of steel and what type of heat treatment?

If the material is case hardened, you may machine through the case into softer material.
 

qu1nn

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#3
material = S7
Vacuum Heat treat to roughly rockwell C50

The question is not about machining, it is about consistency of rockwell hardness across different depths of material after heat treat.

Thanks
Qu1nn
 

Miner

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Staff member
Admin
#4
I missed your comment that the HT was post-machining.

HT is always a time vs temperature process. Thicker sections require longer times to change temperature during heating and when cooling.

If the process is correctly established, you will have better consistency between sections of different thickness. If the process is poorly established, you can get marked differences between sections.
 

qu1nn

Involved In Discussions
#5
Thank you for the responses.

what could better consistency mean?

so let me change up my question a bit....
do you think it is even possible to for the material / dimensions (rough, above) to obtain a Rockwell throughout the entire part within a couple of counts (ie C49-C51) even though there are sections of different thicknesses ...

what would be reasonable to expect?

(again errors due to Rockwell measurement tester are not intended to be discussed)

qu1nn
 

Big Jim

Trusted Information Resource
#6
Think of it somewhat as cooking a steak. The thinner portions of the steak cook faster and will be more done than the center portions.

Hardening from heat treating is only a surface hardening. The depth of the hardening will vary from several factors including the thickness of the material, the composition of the material, the oven temperature, the length of time in the oven, and how it is quenched.

Usually, there are two problems with heat treating before machining. First, it makes it harder to machine. Second, the machining removes some of the effects of the heat treatment as it removes surface metal. Machining before heat treatment sometimes causes problems too, in that sometimes the metal warps or distorts. That is why in some critical cases a part may be rough machined, heat treated, then finished machined.
 
I

iamtroll

#7
Not only would you expect to get natural variation in the hardness of the material, but if you have a .250" cross section that is unsupported over any length, the part may flex under load which I would expect to give you a false hardness reading lower rather than higher as the section gives way. Although you say you are seeing the opposite.
 
D

dbulak

#8
I would also consider changing the rockwell scale. Try using a 30N or even a 15N scale on the thiner areas of the part. The readings could be converted back to the C scale.
 
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