Hydrogen Embrittlement And Heat Treating

We recently had some parts fail during an endurance test. The mill that produced the material is claiming that we induced hydrogen into the part during our heat treatment. The material is two and one half inches in diameter, 4350. We heat the parts for two and one half hours in an endothermic atmosphere, quench in mild oil. then double temper, two hours at 375 F. The evidence of hydrogen damage is visible approximately one eight of an inch below the surface at the site of large inculsions, aluminum oxide. We have ran this operation for 40 years and I have never seen this occur. I was under the impression that only atomic hydrogen acts this way in steel, and that atomic hydrogen would not be present at 1550 F. At what temperature does molecular hydrogen become atomic hydrogen. Thanks in advance for any help.