Low Alloy Steel Striations After Being Coated w/ Zinc Phosphate

[kingsley47]

Basically, we have some fully machined bolting inserts (7+'' O.D.)that are made from 4340 and were normalized, quenched and tempered, prior to machining. We sent these inserts out to get phosphated.

When the inserts were returned, there were light gray bands along the body, of varying diameter and varying shades of gray. I have attached a photo to this post.

The coater calls it heat treat marks and says he has seen it before in low alloy steels. However, he has no idea what causes it.

Has anyone else seen this phenomenon before and if they have do they know what causes it?

 

[swall]

I suspect you are seeing burn from overly aggressive machining. Just like what you would see with a hydrochloric acid/nital etch for grinding burn.

 

[redpicker]

I agree with swall this is an etching effect and that it ie likely caused by machining. Was the OD ground? If not, I am not sure I would call it "burn" since, to me anyway, that suggests creating temperatures exeeding the transformation temperatures and forming martensite, which could cause cracking. Hardness tests in the light vs. dark areas would settle that question. If there is a large difference in hardness, then I'd be concerned.

If there was not much machined from the OD after heat treatment (say, less than 1/32" on the diameter), you could be seeing effects from processing the original bar that remain after machining and heat treatment.

rp

 

[kingsley47]

Checking the hardness along the body was actually the first thing we did. We didn't see any variance outside of what you would expect to find.

These parts get sandblasted prior to coating. Is that not enough to remove any sort of etching defect that we would see on the surface?

 

[swall]

Actually, sandblasting is part of the surface prep for a hydrochloric/nital inspection procedure. And to clarify, by "burn" I was referring to both re-hardening burn and over temper burn where he machining only locally heated the surface to cause a band of over tempered material. Both of these show up on the HCl/nital inspection. These conditions tend to be shallow and it is unlikely you would find them even with a superfical hardness test (would need to section and do a microhardness exam).

 

[metengr]

I concur with swall. Having seen similar problems with post heat treatment surface grinding marks on wind turbine gear teeth, these are very consistent in appearance.