Grinding Cracks?

[will123]

I have a component made from a case hardening steel called Hi-Tuf. The part contains a numbner of fine cracks on a flat ground surface (the surface had subsequently been shot peened). They look like grinding cracks. However on sectioning the part there is no sign of overheating or surface modification. There is no abrupt hardness change near the surface, which, I would have thought rules out grinding cracks. Before this problem was spotted it had been in surface for a little time. I have inspected other parts they do not exhibit the same problem.

Your thoughts would be much appreciated?

 

[metengr]

Several questions that might help to provide an answer;

1. Have you confirmed via nondestructive examination (i.e., Liquid Penetrant (PT) testing) that these are indeed cracks?

2. If these are actual surface indications as confirmed by 1) above, the next step would be removal of a sample containing the surface cracks and having a metallurgical analysis to determine root cause. There could be one or more reasons for the cause of these cracks.

 

[will123]

Thank you for the reply.

In answer to your questions.

1. Mag Particle did not resolve cracks, I was they surprised as they can clearly be seen under a stereo microscope. However the part has been sectioned across crack and the surface prep. They can be seen under the microscope. The crack depth is approximately 0.15mm.

2. I have examined prepped surface after etching with Nital 2 and fuond no obvious problems. As I mentioned above they do look like grinding cracks, however there is no sign of overheating of the material in the area.

 

[metengr]

Follow-up to your answers;

1. Are the indications located on a surface that was shaped or formed into a bend or some other type of geometry?

2. You mention observation of the surface indications with a microscope, was this conducted with only a low power stereomicroscope ? If so, I would have a cross-section containing the surface indications prepared for metallographic examination (you might have to send this out to a lab). This examination method would allow you to evaluate the surface cracks and microstructure at much higher magnification (400X). If necessary, you can also have microhardness testing performed on this same sample to confirm hardness and case depth.

 

[Carburize]

If you etch the surface using the procedure in ISO 14104 "Surface temper Etch Inspection after grinding" you should be able to tell if the defects were caused by abusive grinding.

 

[will123]

Thank you both for replying I appreciate your help.

Metengr, in answer to your questions:-

1. The component was machined from solid and the surface in question was ground.

2. The section was examined using a metallurgical microscope not just the stereomicroscope. We have conducted microhardness. The surface hardness is slightly above specification and effective case depth is correct. There is no obvious loss or increase in hardness at the surface, which does appear to rule out abusive grinding. However, I cannot determine another cause for the cracks.

Carburise,

I do not currently have assess to ISO 14104, although I will look into it. However, we have etched the cross-section with Nital 2, this showed no sign of overheating.

 

[Carburize]

Did you etch the ground surface?

 

[TVP]

What was the exact sequence used, including heat treating? Perhaps these are cracks that formed from existing seam defects in the bar, or they could be quench cracks that formed after quenching.

 

[metengr]

Adding to what TVP mentioned above, the 0.006" (0.15mm) deep indications could be intergranular attack from oxidation during heat treatment. Evaluate the indications in the unetched and etched condition for comparison.

 

[will123]

Thank you for your replies and apologies for no getting back to you until now.

The manufacturing sequence is:
1. Machine.
2. Carburise.
3. grind.
4. shot peening.

We have manged to open up one of the cracks, the fracture surface is very crystaline under a stereomicroscope with the area near the surface darker than the rest. Therefore, it is starting to look like quench cracking, which initiated at machining mark that were subsequently ground away.

 

[Carburize]

Quench cracks are typically black but they are also typically smooth - so if you are seeing a distinct crystalline appearance to the fracture surface they may not be quench cracks.

 

[TVP]

Are you tempering after carburizing? What is the geometry of the part? Quenchant used? Carburizing temperature? Perhaps you should investigate a less severe quench or improve the surface finish prior to carburizing. If you are not tempering the martensite, then grinding will certainly exacerbate any residual tensile stresses and cause the cracks to worsen.