A5 Embrittlement (Hydrogen and Tempering)

Topic

According to Sieverts' law, the partial pressure of hydrogen to which the metal is exposed—whether during processing or in service—determines the solubility of dissolved hydrogen within the metal itself: the higher this partial pressure, the more hydrogen can dissolve into the metal's lattice before saturation is reached. Consequently, a higher partial pressure of hydrogen results in greater solubility of dissolved hydrogen within the metal.

It is when the dissolved hydrogen within the metal exceeds a specific threshold that the material becomes embrittled; above this threshold, the dissolved hydrogen weakens metallic bonds at the tip of any existing crack, thereby reducing the metal's effective fracture toughness below the level established by another model in this set (which applies in the absence of dissolved hydrogen). A higher concentration of dissolved hydrogen above that threshold results in a correspondingly lower effective fracture toughness.

Independently of the dissolved hydrogen, the thermal gradient generated within the metal during quenching—caused by the surface cooling much more rapidly than the core—creates a residual quenching stress, as detailed in another model within this set: the already rigid surface restricts the subsequent contraction of the still-hot core, subjecting the core to a residual quenching stress capable of causing cracking, even in the absence of dissolved hydrogen. A more pronounced thermal gradient during quenching results in a higher residual quenching stress and increases the likelihood that the gradient itself will cause quenching cracks. Thus, in this model, the combined action of dissolved hydrogen and the residual quenching stress—which initiated the crack—results in a hydrogen-assisted crack growth rate that neither factor would produce in isolation: the residual quenching stress keeps the crack tip under tension, attracting dissolved hydrogen to it; having diffused to that point (governed by the hydrogen diffusion coefficient established in another model within this set), the hydrogen continues to locally embrittle the crack tip, allowing it to advance. Consequently, a higher residual quenching stress or a higher concentration of dissolved hydrogen diffusing to the crack tip leads to a higher hydrogen-assisted crack growth rate.

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