Hydrogen embrittlement is a metal’s loss of ductility and reduction of load bearing capability due to the absorption of hydrogen atoms or molecules by the metal. The result of hydrogen embrittlement is that components crack and fracture at stresses less than the yield strength of the metal.
Causes leading to hydrogen absorption
Hydrogen can enter and diffuse through steel even at room temperature. This can occur during various manufacturing and assembly operations or operational use – anywhere that the metal comes into contact with atomic or molecular hydrogen.
In our standard process for which there is a possibility of absorption of hydrogen is electroplating. During electroplating, hydrogen is produced at the surface of the metal being coated. Electroplating is used to deposit zinc or cadmium on the socket set screws for corrosion protection of the steel.
Hydrogen absorption can also occur when a socket set screw is in service if the steel is exposed to acids or if corrosion of the steel occurs.
Requirements for failure due to hydrogen embrittlement
There are three requirements for failure due to hydrogen embrittlement:
- A susceptible material.
- Exposure to an environment that contains hydrogen.
- The presence of tensile stress on the component.
High-strength steels with a hardness of R/C 32 or greater are the alloys most vulnerable to hydrogen embrittlement. Although electroplated socket set screws in alloy steel fall in that category, they are designed to be used in a compression application deeming tensile stress absent, thereby making socket set screws an exception to the rule.
Prevention of hydrogen embrittlement
Baking socket set screws after electroplating at 375° +/- 25°F within 4 hours after the electroplating process for a period of 23 hours to allow the hydrogen to diffuse out of the metal.
For applications where socket set screws are used in a tensile situation, special attention to the hardness must be taken into consideration to avoid fracture due to hydrogen embrittlement. Alloy socket set screws have a standard hardness of R/C 45-53. This level of hardness does not allow any ductility in a tensile situation. It’s just too hard. By lowering the hardness will allow the part to be more ductile. That being said, our electroplated socket set screws are always baked within four hours after they are plated for a minimum of 23 hours.
Finally, there is no test for hydrogen embrittlement for socket set screws. The reason is that socket set screws are designed for a compression application making hydrogen embrittlement not applicable. The reason for this is when the socket set screws are put in compression the hydrogen molecules (if present) will not be activated. The hydrogen will only be activated in a tensile application and socket set screws are not designed to be put in a tensile application because of the high hardness.