Identifying Quench Cracks

[metalman8357]

Hi all,

I just received parts from a supplier and some of them appear to have cracking. I want to determine if the cracking is a result of machining, or if they came about from the quenching process. The material is S2 tool steel, and it was held at 1600F for 1 hour and then water quenched. My question is, is there a way to tell if the cracking was from the quenching process by mounting transverse sections of the part and looking at the microstructure? If so, what should I be looking for?

Thanks

 

[redpicker]

It depends on your heat treating process. Was the material austenitized in a proctecive atmosphere? If it was austenitized without a controlled atmosphere, and the cracks existed prior to quenching, there should be decarburization (a loss of carbon) along the sides of the crack. This should be easily seen when polished and etched with Nital.

If it was heat treated in a controlled atmosphere, it will be harder to tell. You will have a more difficult investigation.
What is the shape of the parts? Where are the cracks found? How are the parts made (machined from billet? forged? castings?).

rp

 

[metalman8357]

The part is a broached hex in the end of a cylindrical shaft. The OD is 0.34" and the hex dimension is .188" flat to flat. The hex recess has a depth of 0.30". The crack is 0.5" in length and runs along the axis of the shaft. The crack appears to have started at a superficial scratch on the OD. My guess is that either the crack was initiated during the broaching process (more specifically the clamping mechanism), or during heat treatment. The part is through hardened to 55HRC.