Nitrided 4140
Nitrided 4140
(OP)
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?
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?





RE: Nitrided 4140
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
RE: Nitrided 4140
Is there a way using the QPQ process to bring the results up for the 17-4 material?
RE: Nitrided 4140
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.
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RE: Nitrided 4140
RE: Nitrided 4140
The testing that is being done is Knoop Microhardness with a conversion to HRc. (a case depth profile)
RE: Nitrided 4140
RE: Nitrided 4140
RE: Nitrided 4140
RE: Nitrided 4140
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:
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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:
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