Heat treatment of 4330V 4

[salmon2]

Experts,

Used to work with small diameter (<10'' OD) low alloy carbon steel tubing and casing and now deal with big diameter bar stocks. Got a question on a MTR of 4330V I received today:

The heat treatment process of this heat of material, 11'' OD:
Hardened 1600F 7.0 Hours Oil Quenched
Tempered 1100F 11.0 Hours Air Cooled

I am surprised by those durations! I know it is 11.0 in. OD, but still wondering if they are too long. 7 hours for austenitizing and 11 hours for tempering? I assume they are the holding times, not including heating time. Will 7 hours Austenitizing blow up grain size like crazy? BTW, the final yield strength is 150ksi. Thanks for all coments in advance.

 

[redpicker]

No, those are about normal for conventional heat treatment. They would be based on furnace thermocouple reaching set-point. I have seen several "rules" for setting furnace times (one hour per inch, one-half hour per inch, 4 hours plus 15 minutes per inch, etc...). Often, it is best to leave it to the heat treater to determine the times since he knows how his equipment responds best.

As far as grain growth, with 4330V, at 1600F you don't have much to worry about. You will find you get about a 6 or 7 regardless of holding times.

With such large OD material, though, you should be sure the location of the test samples (from the OD surface) is representative of where you will need the specified properties in the final part. While 4330V can be considered (at least by some) to be deep hardening, the properties will decrease with increasing depth. This means that a test sample taken at, say, 1.5" below the surface is not going to represent the properties at mid-radius. Also, with a large bar such as this, you can see a significant end-effect, so you need to be sure your test samples are far enough away from the end (or otherwise make sure the end effect is accounted for). And, at 150KSI minimum yield, impact strength is going to be important to avoid brittle failure.

rp

 

[metengr]

Agree with redpicker especially the tempering time because with the V alloy addition and the nature of the carbide types there is resistance to tempering at 1100 deg F.

 

[salmon2]

Thanks a lot, that is helpful.

 

[arunmrao]

Agree about tempering as others. Just feel that the longer holding times,might cause scaling,if the furnace does not have protective atmosphere.

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[salmon2]

Thank you, arunmrao.

 

[Maui]

Also be aware of the bar temperature and the amount of time that the bars should sit before they are loaded back into the furnace for the temper cycle. In a large diameter bar like this you will usually have a temperature gradient across the diameter after it comes out of the quenching oil. Use of a hand held optical pyrometer will usually detect a "bounce back" temperature rise on the surface of the bar a few minutes after it comes out of the oil. You should verify that the entire cross-section of the bar is below the aim temperature before you charge it back into the furnace for the temper cycle.

 

[salmon2]

Thank you Maui. Good info.