Heat treatment of SAE 440B steel

[PeterCharles]

We've had a few problems with some pins in 440B steel failing due to corrosion.
According to a consultants report --

.. evidence of grossly intercrystalline corrosion pits and some associated intercrystalline branched cracks that appeared characteristic of stress corrosion cracking.

.. significant grain boundary carbide precipitation was evident and most likely originated from the heat treatment process due to tempering within the 425-565 deg C range. Tempering within the range 150-370 deg C prevents reduced impact values and poor corrosion resistance.

The presence of the grain boundary carbides has significantly reduced the corrosion resistance of the material making the pins more susceptable to corrosion in service.


If the pins were re heat treated and tempered at the lower temperature (say 200 deg C) would the carbides be eliminated leading to improved corrosion resistance.

Any comments would be appreciated.

 

[swall]

What are the details of the hardening process? What is the pin diameter?

 

[EdStainless]

Re-heat treat some samples and make sure that the grain boundary carbides are dissolved. Then temper at 360-370C.
This should be as good as you can do.

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Plymouth Tube

 

[PeterCharles]

Swall,
The pins are typically 25mm dia and traditionally the heat treatment was oil quench from 1050 deg C, followed by tempering at 100-150 deg C as per manufacturers data sheet.
Due to some confusion a new subcontractor has been tempering at a higher temperature to give Rc57, I believe in the 400-450 deg C range.
The manufacturers tempering data (from some years ago) suggests-
100 C - 730 Hv
200 C - 650 Hv
300 C - 610 Hv
400 C - 650 Hv
500 C - 650 Hv
550 C - 520 Hv
600 C - 400 Hv
There was no comment about any adverse problems with high tempering temperatures.

EdStainless,
We are currently getting some samples re-treated to see if we can get rid of the grain boundary carbides.


I just thought I'd canvass other opinions on the problem. Also it's being said that 420 steel would not have this problem due to the lower carbon content, true??

 

[TVP]

No, it is not true that Type 420 would be immune to this problem. It is well known that martensitic stainless steels (Types 410, 416, 420, 431, 440) cannot be tempered in the range of 370-565 due to grain boundary carbide precipitation. Type 440B should be austenitized at 1010-1065 C, oil quenched, and then tempered below 370 C in order to attain hardness of 53-59. Subzero treatment after oil quenching (- 75 + 10 C) will minimize retained austenite and provide maximum dimensional stability. ASM HANDBOOK Volume 4 Heat Treating describes all of this on pages 777-786, last revised by Joseph Douthett (Armco) in 1991. Mr. Douthett may be retired now, but he is one of the foremost experts on stainless steel metallurgy.

 

[PeterCharles]

TVP
Many thanks for your comments on 420. I've just increased my knowledge significantly.

 

[EdStainless]

TVP hit this one on the head.

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Plymouth Tube

 

[PeterCharles]

Just one final point on this, would higher carbon content steels (440) produce more carbide precipitation than lower carbon content steels (420)under similar tempering conditions ??

 

[CoryPad]

Yes

 

[TVP]

CoryPad is correct, higher C content will produce more carbide precipitation.

 

[PeterCharles]

Thanks!