Failure Analysis

Hydrogen Embrittlement – Part 3

Posted on 13. Dec, 2010 by Rob in Failure Analysis

Hydrogen Embrittlement – Part 3
High Strength Steels Achilles Heel

Susceptible Materials

While some stainless steel grades are susceptible, high strength steels with tensile strengths and hardness above 130,000 psi and Rockwell C35, respectively, are the most prone to hydrogen embrittlement. Steels below these tensile and hardness levels are generally immune. Why?

Increasing hardness, most commonly by heat treating, is accompanied by a corresponding decrease in ductility. In simple terms, ductility is the ability to deform under stress rather than crack or fracture. When hydrogen atoms combine into molecules in a steel that exceeds the tensile strength and hardness threshold, the steel cracks under the pressure increase. But if the tensile and hardness levels are below the critical threshold, the higher degree of ductility allows the steel to deform, absorbing and redistributing the pressure increase, rather than cracking.

Susceptibility to hydrogen embrittlement increases in alloy steels with heat treatment to higher strength. The strength/susceptibility relationship, in fact, approaches exponential levels. In other words, doubling the heat treated strength, quadruples the steel’s susceptibility to hydrogen embrittlement.

Identifying and sorting embrittled parts from good components before they fail is virtually impossible. The detection limit of chemical analysis for hydrogen is generally well above the 5 to 10 ppm level at which embrittlement has been shown to occur. Even if such detection capability was available, hydrogen tends to concentrate at isolated locations within the part. This leaves the majority of the part at “low” or undetectable hydrogen levels. Chemical analysis of parts after failure, to determine if hydrogen embrittlement is the cause, is also not viable since hydrogen diffuses from the part after fracture.

Since most hydrogen embrittlement results from hydrogen absorbed during the manufacturing process, parts which are “batch” processed are usually either all embrittled or all “good”. The failure of one part from a “batch”, therefore, is usually a good indication that others from the same batch will fail over a similar time period.

Prevention

The two keys to avoiding hydrogen embrittlement are at the design stage and during the manufacturing process. Designers with little metallurgical training may not realize the implications of the materials and manufacturing processes they call for in the drawing specifications. As with other failure modalities, hydrogen absorption can inadvertently be “designed into” a part.

On the manufacturing side, avoiding reducing acids where possible removes an abundant source of hydrogen from potential exposure to the part. Where electro- plating is required, minimizing plating time and maximizing current density reduce the volume of absorbed hydrogen. Consideration of electroless plating or vapor deposition as alternatives eliminates the possibility for hydrogen absorption from the coating process altogether.

Protecting furnace charges or components that will be welded from corrosion, or cleaning them prior to use will avoid hydrogen introduction by these routes. This applies equally to parts that will be heat treated. Any processing that will elevate the temperature of the part, and most do, will also raise hydrogen to a higher level of mobility, increasing the potential for absorption.

Hydrogen Management

Despite the most stringent precautions, processing requirements will sometimes introduce hydrogen to parts that exceed the hydrogen embrittlement tensile and hardness threshold. Fortunately, there is a procedure that will effectively remove absorbed hydrogen. This oven heating process, referred to as “baking”, is performed as follows:

1. Parts must be baked within 4 hours of hydrogen exposure. Less is better.
2. Parts must be baked at 400º F.
3. Parts must be held at 400º F for a minimum of 4 hours. Longer may be required depending on part size, processing, etc.

To be effective, the time and temperature parameters must be strictly followed. Short-cuts or delays on any step will dramatically reduce the effectiveness of the entire process. For example, twice as much hydrogen will be baked out at 400º F versus 350ºF, and doubling the bake time doubles the amount of hydrogen that is baked out.

The sooner baking begins after exposure to hydrogen, the better. The 4 hour “window” is a maximum. Note that baking must be performed after each hydrogen exposure if 4 hours will elapse between multiple exposures. No amount of baking will salvage embrittled parts if these time and temperature parameters have not been met.

A final word of caution. A 30 year analysis of hydrogen embrittlement failures in the aircraft industry found that over 70% resulted from improper baking procedures.

Closing

There are competing theories on the mechanism by which hydrogen embrittlement occurs. That presented here appears to be the most widely accepted by the scientific community.

The subject of this article has been limited to hydrogen embrittlement in low alloy steels. Other alloys are subject to hydrogen embrittlement, thought the mechanisms discussed are the same. However, other hydrogen driven failure mechanisms have been observed, though they appear to be less common. These include hydrogen induced blistering, internal hydrogen precipitation and hydride formation in nonferrous metals.

Part 4 will briefly discuss the analysis of hydrogen embrittlement failures and illustrate several “infamous” and unusual failures.

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