Re-Heat Treating 4140 Q&T Steel 2

[sls2]

Ref:thread330-234493

Have reviewed existing threads on this subject. For 4140 rough machined parts (1/8" stock per side) around 1/2" to 1" thick wall we quench & temper to achieve say 140ksi ultimate minimum. In-process tensile tests on a prolongation or accompanying sample sometimes come in low and we need to re-heat treat the part.

My question is
1) what is the proper technique to re-heat treat the part to reach the required properties. Subcritical anneal followed by repeat Q&T with slightly lower tempering temperature?
2) what negative effects might we encounter from repeated attempts. Is there a limit on how many times this can be done.

Thx

 

[metengr]

sls2;
Can you provide specifics on your heat treatment process steps? Quenching medium?

 

[sls2]

Thx Metengr

1. Heat uniformly to 1525/1575F
2. Hold one hour per inch of thickness
3. Quench in agitated oil at 140/170F
4. Temper at 950/1200F as required to achieve specified hardness (4 ranges to pick from)

sls2

 

[salmon2]

Why quencht so warm? Seems like you have retained matensite, which translates to ferrite and pearlite later. That is why it is low, otherwise 140ksi UTS should be easy for 4140.

 

[redpicker]

There is no reason to sub-critical anneal prior to the re-Q&T. The original temper will be adequate to stress relieve the material before re-austenitizing.

Your austenitization temperature seems a bit low, but it should be OK. With some highly annealed starting microstructures, however, 1550 may not be high enough, so I think you would bet better consistency if you stay towards the higher end (highly annealed 4140 can be a bit sluggish when it comes to austenitization).

If in doubt, you should check with your oil supplier for the correct oil temperature. This range (140/170) seems about right to me, though.

Theoretically, you can re-heat treat an infinite number of times. On a practical basis, however, you get some distortion on every quench and after 4 or 5 quenches, the distortion can become a real issue. Most specifications I have seen that limit the number of re-quenching will put the limit at 3 (although very few specifications actually have a limit). Of course, with 4140 you will have to perform an appropriate NDE after heat treatment.

rp