Total Case Depth, 2

[MiguelD38]

I had an issue with total case depth which to my undertanding and based on SAE J423 is the depth of hardness where the hardened layer reaches the same hardness and properities as the base or core material. I was familiar with effective case depth, which we normally test by microhardness test method to a specific hardness point, ususally 50HRC, but could be different depending upon the carbon content. I came accross a test sample with carbonitriding case, to measure effective case at 50HRC. surface hardnes in the low 60s. However the total case which I had not done before threw me up. The sample shows a large transition after the effective case, and levels up to the base metal hardness almost .050" after the effective case depth. See attachement. So by definition of total case per SAE J423 this does not apply. Further testing on the same sample by a different lab reports .0105" for effective case and .012" for total case depth ??? samples shows at .012" depth, HRC of 48.5. Hardness at the core and base metal is 29.5 to 34 HRC. Is carbonitriding process contributing to this gradual decrease in hardness, if so, where should we call out total casedepth? need to modify my procedure to include total case but not sure how should I state to measure it to avoid confusion

 

[TVP]

Can you post the hardness information as a graphics file (.tif, .jpg, etc.) instead of .xlsx? I don't have the latest version of Microsoft Office yet...

In general, carbonitriding produces shallow cases on the order of what you are reporting. Did the other lab provide a graph of hardness vs. depth, or did they simply state effective case depth = 0.015" and total case depth = 0.012"?

 

[MiguelD38]

They just reported and return samples back with indentations. I contacted them to let me knwo how they determine the total case, they said they used the SAE J423 and their own procedure, asked for further details but I am not sure they will disclose that info. Let me know if you can oppen the file is a word doc. 97-2003 format.

 

[mfgenggear]

I converted the file to PDF

MfgEngGear

 

[mfgenggear]

Miguel

To my experience, it is normal practice to call out total case depth for these reasons.

#1) the core of the parent material is as hard or harder than 50 HRc. " the reason for using visual instead of superficial or microhardness test".

#2) very shallow case where it is impractical to use effective case depth. the reason for using visual.

if the lab your using refuses to work with you by explaining their procedure. then I would suggest to contact
an other lab that will.

some customer proprietary specification will use 60 HRc as the cut off instead of 50 HRc.

I prefer the visual "microscopic" method of verifying total case depth. I do not like using superficial hardness test
also depending on the type of parent material & type of case hardening, there will be diffrent results with effective vs total case depth.
I have done test on AISI 9310, 4340M, nitralloy 135, H-11, UT-18 & so on.

also ask a Heat Treater for suggestions. do sample test to verify heat treat results.

 

[TVP]

I reviewed the hardness information that you provided, and my thoughts are as follows:

1. The total case depth of your sample would be ~ 0.06 inches (1.5 mm) using the hardness measurements and the definition of SAE J423. It would be interesting to etch this sample and microscopically evaluate the total case depth to see if they are similar.

2. Total case depth is not a universally used measurement. For example, ISO 2639 Steels — Determination and verification of the depth of carburized and hardened cases has only one definition and it is the following:

case-hardened depth (of a carburized and hardened case):
perpendicular distance between the surface and the layer having a hardness of 550 HV 1 in accordance with
ISO 6507-1 or equivalent Knoop hardness in accordance with ISO 4545

As you can see, it is essentially the same as the definition of Effective Case Depth from SAE J423. There is no ISO definition of total case depth.

 

[MiguelD38]

See attachement. It is a picture of the sample I checked and shows different depth, the visual examination shows a less deep casing base on the structure and or apperance. Sorry for the pic. my camera is not a high resolution one. But you can tell that the casing ends some where around the 50 HRC point and at the high 40s. which is the last indetation approx. .002" further down, and not .050" down as hardness suggested per SAE J423. Now I am wondering if this gradual change in hardness is due to the affect of the casing process. My experience is limitted in this field. Thanks

 

[mfgenggear]

Miguel

If the product is able to achieve effective case depth then is not necessary to also specify total case depth.

like I said in my previous post the only reason to specify total case depth if it's not possible to obtain effective case depth measurements.

 

[TVP]

Miguel,

I agree with mfgenggear. However, if you need to specify a total case depth for some reason, just specify it with a large tolerance, such as TCD = 0.012" minimum or TCD = 0.012 + 0.05".

 

[tbuelna]

MiguelD38,

I would also agree with mfgenggear. The Rockwell C scale is probably not suitable for the thin case produced by nitriding. A superficial hardness scale like Rockwell 15N would be better.

As for controlling case hardness and depth, what is really most important is that the finished (ground or honed) case depth is adequate for the loads/cycles the part will experience. While putting controls on the case depth and hardness after nitriding are important, you should also consider controlling stock removal during finish operations. With a thin nitride case and some heat treat distortions, it is easy to end up with a finish ground part that has a case that is thinner and softer than you would like. With your particular part, a finish grind of .010" per side would reduce your case hardness by Rc 9 points.

Hope that was helpful.
Terry