Stress Relieving and Hydrogen De-embrittlement

13 April 2017

Robust metals are often judged on hardness. It’s a term that’s closely related to the stiffness of an alloy, which is all well and good, but what about ductility? What of plastic deformation and tensile strength? A material that’s solely engineered to be stiff is likely to hold its shape, but without elastic deformation, it becomes brittle, prone to fracturing. Now, with that undesirable material state in mind, let’s talk about stress relieving and hydrogen de-embrittlement.

What is Hydrogen Embrittlement? 

As the most common element in the universe, hydrogen is known to exist everywhere, from the cold vacuum of space to the hearts of the hottest stars. Somehow, that same element can undermine the elasticity factor we so carefully add to our toughest steel alloys. Titanium and aluminium alloys also suffer when diffused hydrogen atoms are introduced. The metal parts retain their formidable stiffness properties, but that diffused element corrupts the ductility of the part so that it becomes dangerously brittle. Released to a client, a brittle alloy could crack, fracture, and fail when stress is applied.

Stress Relief as an Embrittlement Countermeasure 

That brittle state cannot be allowed, not when stress is part-and-parcel of structural engineering. Consider stress relieving and hydrogen de-embrittlement an absolute necessity, then, a process that releases the hydrogen by “baking” the part until it’s free of the gas. In essence, the components are placed in an oven for several hours, perhaps longer, until the diffused hydrogen has been ousted from the material. Interestingly, there are several hydrogen de-embrittlement verification tests in use in the heat treatment industry, including destructive torque tests and nondestructive X-Ray Diffraction Residual Stress tests.

Knowing the Enemy 

Hydrogen embrittlement can be assessed by analysing crystal lattices in a metal part. There are bending tests that measure ductility and the elastic modulus. Still, the best solution has to be the proactive technique, the stress relieving methodology that eliminates the atomic remnant by baking the part. Better still, avoid processes that introduce hydrogen, the finishing and pre-processing techniques that electroplate or acid pickle the component. If those procedures are mandated, though, the de-embrittlement phase must be followed so that the element is removed from the alloy.

What has been diffused by one industrial process, can be ejected by another process. The heating solution counteracts gaseous diffusion. Finally, as high-grade steel, titanium, and aluminium alloys are vulnerable to this condition, the engine and aeronautics industry are mentioned in the same breath as hydrogen embrittlement. A diligent heat treatment facility is well aware of this fact and always prepared to counteract the presence of hydrogen molecules by applying a rigorously tested hydrogen de-embrittlement solution.

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