Hydrogen Embrittlement Problems

Q. Sorry in advance for my not perfect English: We have a problem with hydrogen embitterment. We are buying screws according to customers specs #6 diameter, stnl.stl. AISI 410. The manufactured parts then go through the following steps: remove the copper, heat treatment to 170000-190000 PSI, passivation according to QQ-P-35 Type II. After all this, the parts come to the final test. The result is hydrogen embrittlement which now means that the parts are scrap. What can be the reason of this problem and is there a change to retreat the parts? S.S.

 

A. I think there could be several potential sources of embrittlement, but without further information, I can only give you the steps to go through to determine for yourself the source.

1. You briefly mention a step to remove copper from the part. What is the source of the copper and why is there copper on the screws? Is this some sort of acidic stripping process to remove copper plating from the parts? If so, the acid would be the likely source of hydrogen for the embrittlement. You could minimize the degree of pickling the screws see but that may only be of minimal help.
2. You also mention a heat treatment step. Most heat treatments are performed in a specific environment in order to achieve specific metallurgical properties. Some stainless heat treatments involve a “bright anneal” step that may involve a hydrogen atmosphere. Is that the case here?
3. Are there any other upstream manufacturing processes performed that may be causing the embrittlement?

In any of the cases above, you may not be able to significantly change the manufacturing process in any significant way to avoid the hydrogen embrittlement. If this is the case, I would recommend a baking step to drive off the residual hydrogen. A soak at 200–240°C (390–465°F) for a few hours would be effective to eliminate the hydrogen embrittlement.

The last thing you may want to consider is your heat treatment step again, but this time focus on temperature. Ferritic stainless steels can also suffer from a situation known as 475°C embrittlement. As the name suggests, the ferritic stainless steels becomes hard and brittle when soaked or slow cooled through the temperature range of about 600°C to about 400°C. This is due to the precipitation of a chromium-rich phase in this temperature region. So the higher the chromium content in your ferritic, the quicker this can occur. If your heat treat process is causing this, you should eliminate any soak or slow cooling in this temperature region.