Black Oxide Coating and Embrittlement

[metalman8357]

Hi all,

Does anyone have experience or know if the black oxide coating process can cause embrittlement, i.e. hydrogen embrittlement? I have parts made of A2 tool steel that are hardened to 54 HRC and subsequently coated with black oxide. We are noticing that lately they have been failing below the proof load. The parts are only 0.040" at the thickest section. I'm not sure what process our vendors uses for the black oxide coating process so I'm not sure if their is a pickling process, or what temperatures the caustic bath is kept at. Also, they could be electroplating for all I know. I'm just wondering, with such a high strength steel, is it necessary to stress relieve these machined parts after coating?

 

[Screwman1]

I have not heard of a black oxide process causing a hydrogen failure; it's a pretty porous material, so it should allow any atomic hydrogen to dissipate. You mention electroplate; zinc and black chromate is a completely different process and it would not be within any black oxide specification. If your coater is doing some type of an electroplate process, you will have hydrogen problems and you also have a coating vendor who doesn't follow your specs.

 

[metengr]

metalman8357;
To remove hydrogen, the term is called bake out and is not a stress relief. The reason is that the bake out is performed at 450 deg F to 550 deg F to allow nacent hydrogen to diffuse from the substrate. The bake out temperature is well below the temperature required for stress relief.

 

[Metaljon]

You may need to apply a baking treatment as recommended by Metengr. Have you checked the hardness to ensure that it is within specification?