HYDROGEN EMBRITTLEMENT – High Strength Steels Achilles Heel – Part 5

A LITTLE HISTORY

Hydrogen embrittlement is a relatively recent phenomenon. With a few exceptions, failures by this mode did not occur prior to the middle of the last century. In a sense, the genesis of hydrogen embrittlement was the jet engine.

In the late 1940’s a revolution was underway in aviation. Jet propulsion was rapidly replacing the old piston engine driven propeller technology and aircraft performance began to exceed levels that had been considered physically impossible just ten years earlier. The dramatic increase in power provided by jet propulsion demanded airframes that could withstand the resulting higher loading. That increase in performance, and aviations never-ending quest for weight reduction, only added to the demands placed on existing materials. The result was a push for higher strength alloys from which stronger and lighter components could be made.

Low alloy steels such as 4130 had been used in aviation in the past. However, these materials were typically used in the normalized heat treated condition, at tensile strengths in the 90,000 to 120,000 psi range – well below levels susceptible to hydrogen embrittlement. In response to demands for more strength, “radical” heat treatments resulting in tensile strengths approaching 200,000 psi were applied to 4130 and other “anemic” low alloy steels. Some of the first hydrogen embrittlement failures appeared in this material, though the cause was not initially recognized.

Enhanced low alloy steels, such as 4140 and 4340 were used in response to these failures, and the cycle was repeated, with the demand for more performance from smaller components resulting in processing to ever higher tensile strength levels.

One of the unfortunate consequences of increasing the strength of low alloy steels is a corresponding reduction in corrosion resistance. To combat increased corrosion in service, a variety of electroplated coatings, such as chromium, nickel and cadmium, were applied.

With a potent source of hydrogen now available from the plating baths used to protect the new high strength alloys, a dramatic increase in hydrogen embrittlement failures occurred in both the aerospace industry and other industries to which the new materials technology had filtered down. Once hydrogen had been identified as the Achilles heel of these materials, the prevention strategies described in part four of this series were developed.