Nitrided 4140 2

[WeldGk]

I am nitriding a threaded shaft of 4140 (approx 2" OD). I am looking for a case hardness of at least 50HRc after a nitriding treatment at 1075F for 90 minutes (repeated once). I want a case depth of 0.002" and a return to HRC 33 max at a depth of 0.006". I have not had lkuck attaining these results, any suggestions?

Also, utilising the same procedure for 17-4 is not producing the desired results either. I am not that familiar with nitriding. I am wondering if anyone has any suggestions? I am trying to achieve the same results as was described for 4140. Thus far, I am achieving around 40HRc for the 17-4. Is the time or temperature for nitriding insufficient? Or, is the expectation of 50HRc unreasonable?

 

[unclesyd]

Are you trying to nitride the threaded area?
Are you using a gas nitriding procedure?

We routinely gas nitride 4140 and get a hardness and case depth considerably better than your requirements.

We do not gas nitride 17/4 but use liquid nitriding, mainly Kolene's QPQ process. We have never achieved consistent results trying to gas nitride 17/4

 

[WeldGk]

The threaded area is being nitrided using the QPQ process for both the 17-4 and the 4140. We have tried 3 different nitriders and have had consistent results from all three for both materials. I am wondering if the time or the temperature needs to be tweaked to achieve better results?

Is there a way using the QPQ process to bring the results up for the 17-4 material?

 

[unclesyd]

I would get in touch with Kolene through the Nitromet Division concerning your problem.

Can you give a little more detail about the thermal history of the 17/4 prior to the QPQ processing?

We used this process for both the aged 17/4 and used the QPQ process to to carry out the precipitation HT process. As you state the process is time dependent, as we used 4 hrs when we used the process for the precipitation treatment. When we aged 17/4 with the process I know we got around 70 Rc as the surface hardness. You have me wandering what our time were for the aged materials. I tried to get some of our numbers but are unable at this time to find anyone familiar with them.

http://www.nitromet.com/A55B6F/nitromet/nitromet.nsf/b0da3a2f63208410852571930048a8da/Home

 

[swall]

How are you checking hardness? Normally a file hardness check is used for nitride case surface hardness.The QPQ process should be capable of producing a surface hardness of Rc 60 min.

 

[WeldGk]

The 17-4 material that was used was solution annealed at 1900F (air cool), double age-hardened (1150F, 4h, air cooled). Based on your comment, I should try a longer nitriding treatment for the 17-4.

The testing that is being done is Knoop Microhardness with a conversion to HRc. (a case depth profile)

 

[swall]

Knoop microhardness, even at very low loads, is not suitable for determining surface hardness of a salt bath nitrided case. You need to get a set of SAE J864 test files. These can be obtained from larger mill supply houses. Use the microhardness traverse to get your case depth measurement and use the files for surface hardness. So, before you go any further with process changes, get the files and find out your actual surface hardness.

 

[WeldGk]

Just one more question. I am having difficulty finding an explanation for why the Knoop Microhardness test is not great for nitrided coupons. Is there any way someone could enlighten me a bit? Thanks.

 

[swall]

With microhardness testing on a cross section, it is difficult to get much closer than within .001" inches of the surface, even using a light load such as 100g. With a .002" case, you don't have much leeway in placing the indentation. If I had to try it on a .002" nitride case, I would probably use a 50g load. You also need a metallographic prep process that will give you a flat sample with no edge rounding or chipping of the case.

 

[TVP]

KimberleyS,

As swall mentioned, the difficulty is obtaining a good cross-section with no edge rounding and then making the indentation close enough to the surface without falling outside of the guidelines for a valid microhardness measurement. I certainly would not use a load higher than 50 g for a small case thickness, and maybe would use 25 g. Struers has some good information on metallographic preparation of nitrided components:

http://www.struers.com/resources/elements/12/68238/Application Notes Nitrided coatings English.pdf

http://www.struers.com/resources/elements/12/2475/35art3.pdf

Another article from the Struers website shows microhardness results on nitrided stainless steels that were obtained using a very low load of 6.25 g:

http://www.struers.com/resources/elements/12/41780/e-Structure 9_EN.pdf