Facing issues (black spots) in core microstructure of case carburized steel

[Sumeet59]

Hi All,
This is my first post so sorry if I have made any mistakes. I am facing issues during case carburizing of universal joint crosses of 20MnCr5 raw material. I am observing black spots in the core microstructure after carburizing. These spots are visible are etching and polishing and also visible under a microscope. Photographs are attached (

https://files.engineering.com/getfi...le=After_carburizing_micro(black_spots).pdf).

These spots go away if I re-quench and temper the part. Photographs of the re-quenched and tempered parts are attached (

https://files.engineering.com/getfi...e=After_re-quench_micro(no_black_spots).pdf).

I have taken trials with different lots from the steel mill and the results are the same. I have also tried to measure the hardness (with micro-Vickers tester) at that particular location and there is no change from the surrounding areas. I have also taken special care while preparing the cut sample to see that the spots are not a result of sample preparation. Also since these are not observed after re-quench and temper, I do not think it is due to improper sample preparation. The furnace and quench oil are the same for the case carburizing operation and the re-quenching operation. All other aspects of the micro-structure, the surface and core hardness, and the case depth are found to be OK (within spec). I am not finding these issues in other components we do on the same furnace. Please help me figure out what these black spots are? and why are they occurring. Details of the process are below:

Case carburizing process:
Furnace: Seal Quench Furnace of 600 kg capacity
Quench Oil: Hi-quench MF (Hardcastle)
Boost cycle: Time=4.5 hrs, CP=1.15%, Methanol: 2L, Temperature: 940 C
Diffuse cycle: Time= 2.5 hrs, CP=0.75%, Temperature: 940 C
Hardening cycle: Time=1.5 hrs, CP=0.75%, Temperature: 860 C
Case depth: 1.2 to 1.5 mm, Surface hardness: above 63 HRC (as quenched), Core hardness: 320-330 HV

Re-quenching cycle:
Furnace: Seal Quench Furnace of 600 kg capacity -same furnace
Quench Oil: Hi-quench MF (Hardcastle) - same oil
Hardening cycle: Temperature: 870 C, Time= 2.5 hrs soaking (after attaining the temperature)

Thanks,

 

[mfgenggear]

I suggest taking a sample of the raw stock and one of the parts , and send it to an independent lab for failure analysis. have them do an analysis with a scanning electron microscope, :" SEM ", and spectral analysis as well, to verify if the steel is contaminated, I also recommend to use a Ishikawa fish bone chart and list all the possibilities of what is causing the problem. also discuss with the sales engineer of the manufacture of the carburize ovens for the carburize procedure and the quench procedure.

 

[Dhurjati Sen]

The black spots are Carbon (graphite) nodules. The aluminium deoxidation practice of the original steel could be behind this typical graphitization. Check with the manufacturer / MTC of the original stock regarding the deoxidation practice

DHURJATI SEN
Kolkata, India

 

[Sumeet59]

Hi mfgenggear thanks for the feedback. I will submit the samples to a lab so they can check under an electron microscope.

Hi Dhurjati, we will get in touch with the steel manufacturer. However I would like to get your views as to why the graphite nodules dissolve / disappear after reprocessing.

Thanks

 

[Dhurjati Sen]

During reprocessing (i.e. after re-quench and temper), the core reaches a temperature at least between AC1 & AC3, enough to dissolve the graphite nodules.

Is it a standard practice in your shop to check the microstructure after carburizing?

DHURJATI SEN
Kolkata, India

 

[Sumeet59]

Hi Dhurjati,
Thanks for your response. We check the microstructure after processing each batch. During the case carburizing process, we heat the part to above Ac1 temperature for a long time. However after quenching we are observing these spots. Please find carburizing cycle below:
Case carburizing process:
Furnace: Seal Quench Furnace of 600 kg capacity
Quench Oil: Hi-quench MF (Hardcastle)
Boost cycle: Time=4.5 hrs, CP=1.15%, Methanol: 2L, Temperature: 940 C
Diffuse cycle: Time= 2.5 hrs, CP=0.75%, Temperature: 940 C
Hardening cycle: Time=1.5 hrs, CP=0.75%, Temperature: 860 C
Quenching in Hi-qurnch MF

Case depth: 1.2 to 1.5 mm, Surface hardness: above 63 HRC (as quenched), Core hardness: 320-330 HV