MTR: Stress relieving following tempering (following quenching)

[bekks]

Could someone explain to me why "stress relieving" is sometimes performed after quench and tempering by the manufacturer? The material in question is 4130. I've noticed that sometimes the MTRs state a stress relief at slightly lower temperature, but similar time to the temper temperature and time. I also noticed it stated that no weld repair was completed, so that doesn't appear to be the reason. Is it because hardness values were slightly too high?

 

[EdStainless]

What strength levels are you working with?
At higher strength especially a double temper or T + SR will tend to give you more uniform properties
This should minimize movement when machining.

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P.E. Metallurgy, consulting work welcomed

 

[bekks]

Thanks Ed,
I looked through about 10 MTRs that I had handy and noticed the second heat treatment on two. The one I'm interested in is 84/107 ksi (yield/UTS) actual values. I also noticed it had a relatively high vanadium value of 0.012%, which I would expect to require a longer temper.

 

[EdStainless]

There are some handy tables in AMS 2759-1 that show adjustments to tempering temperatures for differences in as-quenched hardness.
At that low of a strength level you must be at 1200-1300 tempering.
I don't know why they would SR.

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P.E. Metallurgy, consulting work welcomed

 

[Tmoose]

"Could someone explain to me why "stress relieving" is sometimes performed after quench and tempering by the manufacturer?"

Straightening ( cold bending) the finished part often requires a new stress relief. Naturally the SR temp must be below the tempering temp to maintain part hardness.

From ASTM A962 -

S51. Stress Relieving
S51.1 A stress relieving operation shall follow straightening after heat treatment. The minimum stress relieving temperature
shall be 100 °F [55 °C] below the tempering temperature. Tests for mechanical properties shall be performed after stress relieving.
​

 

[bekks]

Thanks for the thoughts.
Tempering temperature was around 1300F, as you assumed Ed.

The finished material was 6" diameter bar stock. When I said manufacturer I should have maybe said "mill".

 

[EdStainless]

The section noted by TM is a Supplemental requirement in A962, so it is only invoked when the purchaser requires it.
But it is an excellent guide as to when, why, and how you would SR.
The straightening of long products (rod and bar) rarely involves high enough strain to require SR.
Material that will be machined (especially if heavy or asymmetric cutting) is usually dual tempered or SR.


= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Dhurjati Sen]

Vanadium causes secondary hardening while tempering. Subsequently, a stress relief is done to ensure the mechanical properties are uniform and within limits.

DHURJATI SEN
Kolkata, India