How big are your components? What is the batch size? Can the components be traced from the original stock by their heat numbers . What is the post heat treatment operation? Any grinding or straightening operation. Depth of decarb expected by the heat treater ? Microstructural constituents  and retained austenite. Hardness at the surface and at the core.

These need to be discussed with the heattreater along with the designer before releasing the sheet.

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Austenitize (controlled atmosphere) at 1550F, oil quench, temper at 600F. A reputable heat treater should understand this if you just give him the hardness range you require for the finished parts. He also needs to review the configuration of the parts in order to avoid quench cracking and distortion issues.

Thank you for the quick reply, I am going to speak to the h.t. today,
FYI, part is a complex configuration with several deep (.3 in) pockets and webs, there are generous radii in both the floor and sides of the pockets, and yes, I am concerned about cracking.

Another question, what is the maximum hardness / toughness area for this material?

Am I in the right range?

The component will see severe duty in high speed automotive applications.

Thanks

You may want to specify marquench as opposed to conventional oil. I routinely achieve 43-47 HRC after tempering at 800F with this oil on bar stock 2.5-3" dia. This will also minimize distortion and reduce propensity for cracking. Follow up with mag particle inspection, especially if you have to do any mechanical straightening.

Yes, marquench would be an option to avoid quench cracking. Another would be to quench in 1000F salt and hold just long enough to equalize the temperature, then quench into normal quench oil. Additionally, a stress relief at 1200F for an hour (before the austenitize,quench and temper cycle) might be usefull to minimize heat treat distortion.

Make sure that the heat trater makes a proper quench, the temper at about 250-300C to get to the hardenss you need.
Tempering at 450C will give you about 40 HRC.

I am definately out of my league when it comes to metalurgy...and I do not want to step on anyones toes...but from where I am sitting, it looks like your minimum hardness requirement is barely achievable with a perfect process with 4340.

According to AMS-H-6875, 4340 can reach 200-220 KSI (43-46C) with an 850F tempering.  

If you really have to have those hardness numbers, it might be a safer bet to switch to a different alloy material that can reach those numbers nominally.  

If I were in the decision loop, I wouldn't hesitate recommending Hy-Tuf Vacuum Melt. This material will easily achieve 220-240 KSI, which is what you are asking for.

That is just my too cents.

Rob

Ebug,

Upon quenching, 4340 can have a hardness of 60 HRC.  You reheat to a certain time-temperature window to "temper" the material to a lower hardness (that also brings out higher toughness/ductility).

Regards,

Cory

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MIL-H-6875H has a gap between the 200-220 KSI and the 260-280 ksi heat treatments. I am no metallurgist but I believe it is connected to some heat treatment embrittllment in the in the range of 220ksi and 260ksi.

Thanks to all for the help, I have talked to the h.t. and we are going with a salt bath for tempering.

Should be able to get to 48 without cracking.

machinemanvf,
Please come back with your results. Good Luck!

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