Decarborisation in heat treated 440C stainless products 1

[Craigt12]

Hi,

I have a problem with a 20mm decorborised layer being formed on heat treated 440C. The components are heat treated in a vacuum furnace at a temperature of 1050 degC and oil quenched.
It was initially though that the cause of this layer was due to air being drawn through the vacuum chamber because of a leak - the vacuum pump continually runs, drawing a small amount of nitrogen through the furnace to maintain the vacuum. the furnace has been checked by 'experts' who could find no evidence of leaks.
As a non-metallurgist I am not entirely sure that this is not a naturally ocurring layer, created when heat treating to this temperature. If anybody can offer an insight it would be greatly appreciated.
Further to this, what are the consequences of having a de-carborised layer on the component - other than a softer 20micron skin around the part. Can it induce corrosion, or allow stress cracks to occur?

With thanks for your replies, and apologies if this question has appeared more than once - the original questions did not seem to get posted, although as a first time user I may simply have been impatient!

 

[NickE]

I know for certain that the de-carb layer can significantly reduce the performace of a part in bending fatigue.

Also every specification I've seen for heat treated steels specifies that no-decarb shall be allowed. There are probably many reasons for that.

Your first question is at the bottom of the thread about how to measure a difference of 0.05mm. It shoudl eventually get cleaned off when admin gets to it.



Nick
I love materials science!

 

[TVP]

Craigt12,

Presumably you are using Type 440C stainless in an application that requires wear resistance or has high normal forces applied to the surface (bearing, slider, etc.). If this is the case, then decarburization will significantly decrease wear resistance, decrease resistance to scuffing/scoring/pitting, as well as reduce fatigue strength. Do you know the complete manufacturing history of the parts? Is it possible for the decarb to already be present prior to vacuum heat treating? Are the parts adequately cleaned prior to vacuum heat treating? Residues on the surface can react with the carbon in the steel even though the bulk atmosphere is neutral. Are the parts ground after HT? Any exposure to elevated temperature in the presence of oxygen can result in decarburization.

 

[arunmrao]

20 mm depth of decarb is unusually a large amount. How long were the parts kept in the furnace at 1050C.

 

[swall]

Yes, arunmrao, I wondered about that myself. He first says 20mm, but then at the end of the post, he is talking about 20 microns. So, Craigt12, is the decarb 20 microns or 20 mm?

 

[redpicker]

My guess would be that he means 20 micro-meters instead of 20 millimeters. That would be about 0.0008 inches, excessive in my book. At 20 millimeters, or 0.8 inch, it is just unbelieveable.

Yes, it would generally be considered to be detrimental to the part. Exactly how detrimental would depend on exactly what it was being used for. If the heat treatment was specified as being a vacuum heat treat, I would suspect the designer (or whoever specified the heat treatment) did not want any decarburization.

With that depth of decarburization, if it was processed in a vacuum furnace with "a small amount of nitrogen", I would suspect the decarbruization existed prior to the heat treatment.

rp

 

[Craigt12]

Firstly, thank you to everybody who has offered a reply so far.

The decarburization (correct spelling noted!) is approximately 20 microns not mm. The parts are cleaned in a vapour degreaser prior to being heat treated. The vapour degreaser is using a Trychloroethylene equivalent product. However, the only way we have determined that the parts are correctly cleaned is by visual inspection.