What is hydrogen embrittlement?Hydrogen embrittlement is the process by which metals become brittle and fracture due to the introduction and subsequent diffusion of hydrogen into the metal.
PROCESS• Hydrogen is introduced to the surface of a metal and individual
hydrogen atoms diffuse through the metal.• Solubility of hydrogen increases at higher temperatures.• These individual hydrogen atoms within the metal gradually
recombine to form hydrogen molecules, creating pressure from within the metal leads to Hydrogen Induced Cracking (HIC)
HOW RELATED TO WELDING?• Source of hydrogen In the coating of welding electrodes. Grease or oil plates
• Temperature ranging -100 to 2000 C• Stress(Residual)• Moisture in shielding gas• Hydrocarbons in base material
• Basic coated low hydrogen electrodes like ultra low, low, medium low.• Low hydrogen welding process• Pre heating• Post heating
Problem: Loss of ductility.
Solutions:• Use of lower strength (hardness) or high resistance alloys• Careful selection of materials of construction and plating systems• Heat treatment (bakeout) to remove absorbed hydrogen.
Problem: High temperature hydrogen attack.
Solutions: • Selection of material (for steels, use of low and high alloy Cr-Mo
steels, selected Cu alloys, nonferrous alloys)• Limit temperature and partial pressure H2.
Problem: Internal cracking or blistering.
Solutions: • Use of steel with low levels of impurities (i.e., sulphur and
phosphorus)• Modifying the environment to reduce hydrogen charging• Use of surface coatings and effective inhibitors
TESTING• There are two ASTM standards for testing embrittlement due to
hydrogen gas.• The Standard Test Method for Determination of the Susceptibility of
Metallic Materials to Hydrogen Gas Embrittlement (HGE) Test• Uses a cylindrical tensile specimen tested into an enclosure
pressurized with hydrogen• Another ASTM standard exists for quantitatively testing for the
Hydrogen Embrittlement threshold stress for the onset of HIC due to platings and coatings from IHE and EHE.