Need Help Analyzing Micrographs.

[mxracer17788]

Hi All,

We had two lots of fasteners come in and we did a maximum bend angle on them as usual. One lot (Sample 2012/5/16) was capable of bending the 30 deg. requirement and well beyond in some cases. The other lot (Sample 2012/4/25) was consistently cracking prematurely. Both have a core of 42HRC and a case of 50HRC @ 0.002". The samples were etched with Nital, and sample 2012/4/25 shows about a 0.0008" decarb. on the surface. I'm not a materials engineer so I would appreciate any help in analyzing the attached micrographs. I have a few questions...what are the large spherical black dots in the core and shank micrographs of sample 2012/5/16 and why are these not present in the other sample? I'm assuming that the cracking sample had a heavily decarburized surface prior to heat treatment, and this is why it did not quench fully, but I dont understand how this could lead to premature cracking. One thing i'm confused about is that sample 5/16 has a carbon enriched surface of 0.005" while the cracking sample has a carbon enriched surface of 0.010" but it was supposedly heavily decarburized beforehand? Any ideas?

 

[mxracer17788]

Also need to mention this is 1021 steel.

 

[mxracer17788]

figured it out nvm.

 

[Guest102023]

Don't really understand the question, but I have one of my own: how do you achieve 42 HRC core hardness in 1021 steel?

 

[mxracer17788]

I'm sorry it's 1022 steel. And we achieve a core hardness of 42 HRC because the fasteners have a small diameter (0.120") and they are carburized. Essentially they are through hardened but the case is made even harder than the core.