Very hard surface layer 1

[spigor]

I have come accross an item (a spiral bevel gear set), which has been reportedly made of 17NiCrMo6 (1.6566) carburizing steel and the surface hardness is 68 HRc, while a typical would be around 58-62 HRc. Does anybody know how did they achieve 68 HRc on this steel?

Here's more information:
The items were checked with two different Rockwell Hardness Testers and the result was always 68 HRc. Then the gears were sent out to a laboratory for a material analyzis. They have reported a microhardness HV 0.8N (80G) at distance from the surface .05-2.0 mm as follows:

The chemical composition:

The surface layer structure:

The surface layer:

Based on these results they have indentified the material as 17NiCrMo6 after carburizing. A 1 mm thickness of the layer with minimal hardness of 550HV has been reported and that the maximum hardness of 940 uHV equals to approx. 68 HRc.

 

[CoryPad]

Untempered martensite can achieve that hardness level, perhaps these parts missed the tempering step of the process.

 

[spigor]

CoryPad
Below there's a tempering curve for the 17NiCrMo6, showing that the surface hardness can reach 64 HRc max.
Is there a special process required to reach 68 HRc, e.g. higher quenching temperature?
If so, is this special process common?
Thanks in advance.

 

[CoryPad]

It would be unusual to reach 68 HRC, usually that is for extremely fast quenching rates. For example, using brine as a quenchant. But this seems unlikely since that can produce distortion and cracking for case carburized gears. Can you have the lab use SEM+EDS on the sectioned specimen and look for composition near the surface?

 

[Lyrl]

I would suspect carbonitriding. If you are able to get a sample to SEM/EDS to test for nitrogen near the surface that could confirm this.

 

[gearcutter]

Lyrl - Is it really possible for the carbonitriding process to develop the quoted hardness at depths in excess of 0.50mm?

 

[FennLane]

Once you have reached the quenching rate needed to produce a complete transformation to martensite it is unlikely that faster cooling rates will have any impact on hardness.

The thermal capacity of the part will impact on the cooling rate needed for complete transformation as will the influence of the Austenite Grain Size.

Austenitic carbonitriding can typically develop case depths of up to 1mm (0.7 is commonplace) and produces harder layers than carburising.

Ferritic Nitrocarburising also known as Tuftriding or Tenifer treatment is a much lower temperature process and case depths of 0.05mm would be more common.

 

[spigor]

Unfortunately, the original gears were returned to the customer, no further tests are possible.