intercritical step in heat treatment of 4330 mod

[pwtomlin]

Does anyone have experience in the heat treatment of 4330 mod? Right now we're developing a heat treat procedure for a new casting and this is the first we've worked with this material. We're trying to get test bars to pass yield strength (min 130) and Charpy V-notch (min 20 ft-lbs at -40 F) tests before we heat treat the castings themselves.

Currently the closest we've come is by using a 4-step heat treat.

1. 2 hours at 1750 F, air cool.
2. 2 hours at 1650 F, water quench.
3. 2 hours at 1050 F, water quench.
4. 2 hours at 1050 F, water quench.

We've played around with the double temper and the tempering temperature some. We're considering adding an intercrical step in between 2 and 3, roughly 1400 F for 2 hours and water quench. Is there any reason to think this would do more harm than good?

 

[TVP]

What is the exact chemical composition, including nitrogen, aluminum, etc.? If the grain size is too large, you will never achieve the impact properties.

 

[pwtomlin]

C 0.29
Mn 0.79
Si 0.42
P 0.010
S 0.009
Cr 0.82
Ni 2.4
Mo .43
Cu 0.006
Al .05
V .01
Ti .06
N .013

 

[TVP]

I am not an expert on steel castings, but I think the nitrogen content is too high. Wrought alloys that are used for low temperature applications would restrict the nitrogen content to less than 0.007% by mass. Have you measured the grain size?