Re-Heat Treating 4140 Q&T Steel
Re-Heat Treating 4140 Q&T Steel
(OP)
Ref:thread330-234493: need some help with heat treating theory and practice
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
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





RE: Re-Heat Treating 4140 Q&T Steel
Can you provide specifics on your heat treatment process steps? Quenching medium?
RE: Re-Heat Treating 4140 Q&T Steel
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
RE: Re-Heat Treating 4140 Q&T Steel
RE: Re-Heat Treating 4140 Q&T Steel
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