Hydrogen Embrittlement In Heat Treatment

We are heat treating 4350 material using endothermic gas as a protective atmosphere. The parts are failing at the site of large aluminum inclusions located approximately one eight of an inch below the surface. The mill that produced the material claims we induced hydrogen into the material during our heat treating, and the hydrogen migrated to the inclusion, which caused internal cracking. I have never heard of this happening in our operation. The parts are heated to 1550 F for two and one half hours, quenched in oil, then double tempered. I have heard that only atomic hydrogen can diffuse into material and cause this type of damage. At what temperature does molecular hydrogen become atomic hydrogen, and has anyone else ever had this problem. Thanks in advance.

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Neil

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Hydrogen inside steel is present in atomic form and diffuses interstitially. Molecular hydrogen gas outside steel can react and dissolve in atomic form. Given a clean oxide free surface this can happen even at room temperature.
If the hydrogen content is high enough, then on rapid cooling it can produce internal flaws called flakes. Back before vacuum degassing flaking was a big problem in the heads of railroad rails and larger wrought products.
In the petroleum refining industry hydrogen gas at high enough pressure and temperature can produce internal decarburization called hydrogen attack. Carbon from the carbides reacts with the hydrogen to produce internal methane bubbles. Service limits are given in Nelson curves.
Pittsburgh Pete