4140 NQT Long Cycle Times

[jmec87]

Regarding 1.22in (31mm) diameter 4140 bar with the following heat treatment:
Normalize 4 hours at 1580°F (860°C)
Austenitize 3.5 hours at 1562°F (850°C)
Quench in Oil - 82 to 88°F (28 to 31°C) bath temperature
Temper 6.5 hours at 1247°F (675°C)

The requirements for this material are 80ksi (552MPa) min yield strength, 100ksi (689MPa) min ultimate strength, 207 - 237 HBW hardness, and 15ft-lbs (20J) longitudinal charpy min average at -46°C. The MTR claims that it meets all these requirements. Reported results are 82ksi (565MPa) yield strength, 104ksi (714MPa) ultimate strength, 217 - 223 HBW hardness, and charpy results of 31, 33, 35 ft-lbs (42, 45, 47J).

Questions:
1. Considering the long heat treatment times, are the reported results plausible?
2. If the mechanical property requirements are met, would the long heat treatment times (or anything else about this heat treatment process) cause concerns about the material?

 

[bobjustbob]

The times seem a bit long, I would think 3 hours at temp would suffice. That said, I don't know what other constraints they have in their shop. It may be more efficient for them to run the parts at those times due to combining loads.

You may get some grain growth but the normalize and austenitize temperature are pretty low so I would not worry about that. 6.5hrs in temper, while it seems inefficient on the face of it I don't think it hurts anything. The quench oil temperature looks cold but I don't know what oil they are using.

 

[EdStainless]

I agree with Bob, the times all look long.
I usually use 1600/1550, but your temps obviously work.
I would use 2.5 hr holds in my furnace, but that could be part of the story (equipment) that we don't know.
I would also double temper rather than use on long one.
You could lower the temper temperature a little, maybe 1200F.
You are real close to the min strength and a slight shift in C would put you under.

I am amazed by cases like this, and we have a lot of customers that do the same thing.
Buy a high strength steel, Q&T it, and use it is a very low strength condition.
There are so many other ways (without the cost of Q&T) to get these properties.

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P.E. Metallurgy, Plymouth Tube