Does stress relieving reduce hardness 1

[gearman1234]

We have 4140 forging. Approximate dia 12" and ID 8". This forging is at 28/32 Rc. Would the stress relieving reduce its hardness? How much temperature and what soaking time is applicable for such a section? I would appreciate information on this .

 

[Carburize]

A typical heat treatment to produce what you have would be
Normalize
Austenitize
Oil Quench
Temper around 1050 degrees F holding for 1 hour per inch of controlling section thickness
Stress relieve 50 - 100 degrees F below the tempering temperature for 1/2 hour per inch of controlling section thickness.

The stress relief cycle should produce no detectable effect on the hardness measured after tempering.

 

[gearman1234]

I digged a little bit in to books on Metallurgy. As per the information I got from the books, the Fe-Fe3C phase transformation diagram shows transformation line at 727 Deg C( 1340 Deg F). I beleive the hardness would only change if we enter into austenite phase which would happen only if we exceed the 727 deg C temp. I would assume that if we stress relieve at around 650 deg C then also the transformation would not occur and hardness would not change.

Does it look OK or am I missing something?

 

[unclesyd]

The information from "Carburize" is quite correct. I think your parts were probably quenched from the forging temperature, then drawn to your hardness range, this is the normal hardness for preheatreated 4140. If you have the thermal history of the part you may get by with only his recommendation for stress relief, as based on your hardness values the part would have been drawn around 1100°F. Heat treatment of a part that size is expensive. What is the stress relief for?

 

[gearman1234]

The H/T history showed that the part after hardening and quenching was tempered at 650 Deg C and that replaces separate stress relieving operation.

The stress relieving is required as the part is going to be nitrided at around the same temperature after finish gear shaping. If the stress are not relieved before the finish shaping they would get relieved during nitriding and that would cause the part to distort.

To avoid this, we have to stress relieve the part at the nitriding temperature.

 

[Metalguy]

Looks like you're really going to perform a double temper, w/both done at 650 C. You might lose a point or 2 of hardness, but you also need to realize that your hardness numbers are probably lower within the gear vs. the surface.

4140 isn't a real deep hardening steel, depending on the actual comp. One heat will harden much deeper than another one.

If you can post the actual comp. we can figure it out for you.

 

[gearman1234]

Infact we were not required to a double tempering. The tempering was done at 650 deg C and that took care of the stress relieving without any loss of hardness.

 

[Metalguy]

Sounds like you don't have a problem.

 

[gearman1234]

After carrying out the actaul experiment it was found to be OK.

 

[unclesyd]

I still would double temper 4140 material

 

[gearman1234]

Is there any particular technical reason for double tempering? Our heat treater has said that double tempering is a waste and the same puprpose i.e. of of stress relieving is achieved by single tempering after quenching.

 

[Metalguy]

Double tempering is performed for a different reason than simply relieving/reducing residual tensile stresses. There is a *potential* problem with some steel alloys-that of "retained austenite" following the 1st temper. RA is soft and can cause crack initiation, but the more severe problem is when it reverts to untempered martensite. Cryo-treatment before the 2nd temper is a very wise thing to do for those alloys where RA is a problem, but I am not aware of 4140 having much of a problem with it. The problem steels usually have some Ni.

 

[unclesyd]

This is a practice we used based on years of experience in using AISI 4140 for mechanical components. We found over the years that this material was much more notch sensitive than most books allowed. This was one method that we used to alleviate some of the problem.
We also made it a practice to never single temper any material. Even with advent of Cryogenic treatment we still required a double temper.

 

[gearman1234]

I agree with Unclesyd. Usually the double tempering is recommended when the part in question is designed for shock loading and has notch design. In other standard applications, it adds to time and cost.

 

[fritzcoinc]

A properely engineered stress relieve will not reduce hardness. The temp. of the stress relieve must be 50 to 100 degrees F. below the tempering temprature used during the quench and temper. If a higher temprature is used the hardness will be reudced. Soak time should be 1 hour per inch of maximum crosection. If this is a critical component, check surface hardness before and after stress relieve to verify no change occured.

 

[gearman1234]

Thanks for the info, Fitzcoinc.

 

[BTIGUY]

Gearman1234

If you want to stress relieve to assure machine stability, shape stabilization, or to reduce premature fatigue then I suggest that you could use sub-harmonic stress relief on your 4140 tubes. The nice part about this process is that it does not change "hardness" at all nor does it cause treatment distortion.

A program that occurred a few years ago involved 4135 forged tubes about 7"OD, 6"ID x 72". Tolerances for the inside full cut bore was extremely tight. Before sub-harmonic stress relief was used the scrap rate was 65%. After sub-harmonic was used 0% scrap and remained at 0% for four years!

My suggestion - get your hardness where you need it, then apply sub-harmonic stress relief between rough and finish machining.

BTI Guy

 

[metman]

gearman1234,
In response to your question about hardness change below the lower critical temperature (phase change). Quenched steel does not require a phase change to change hardness. As you have already esperienced, tempering is done below the lower critical temperature and this reduces hardness.

Jesus is THE life,
Leonard