Hydrogen-induced cracking

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Hydrogen-induced cracking, the English name is Hydrogen-induced cracking (HIC). Hydrogen-induced cracking usually occurs in aqueous solutions, as hydrogen diffuses into the matrix of the steel, causing it to become brittle and crack. Hydrogen-induced cracking often occurs because accidental factors during shaping or finishing cause hydrogen to enter the matrix of the steel. It is usually influenced by three factors: the properties of the material, environmental factors, and stress factors. During World War II, a Spitfire fighter of the Royal Air Force crashed due to a mechanical failure, killing the pilot on the spot. The authorities attach great importance to this matter; all parts of the crashed aircraft were collected, and a special investigation team was established to determine the cause of the crash. The airplane crash was caused by a break in the main shaft; many tiny cracks were found inside the broken shaft, which were referred to at the time as hairline cracks. Around 1940, after graduating with a doctorate, Dr. Li Xun, the founder of the Institute of Metal Research at the Chinese Academy of Sciences, began this research at the University of Sheffield in the UK. The prerequisite for solving this problem was how to quantitatively measure and analyze the hydrogen content in steel. Subsequently, Mr. Lee Hun invented a hydrogen determinator for measuring the hydrogen content in steel. It turned out that hydrogen was the cause of the fracture in the aircraft’s main shaft. Mr. Lee Hun also became a pioneer in the field of hydrogen-induced cracking. High-strength steels containing chromium and nickel are sensitive to hydrogen; steels with a higher carbon content are more prone to hydrogen-induced cracking. Low-carbon steels are less susceptible to hydrogen-induced cracking, and forgings with a dense microstructure are more prone to it than castings with a loose microstructure. Once hydrogen atoms penetrate into the steel, they reduce the atomic bonding forces between the grains, thereby decreasing the toughness of the steel. The fracture surface caused by hydrogen-induced cracking is similar to that of other brittle fractures; high-strength materials tend to exhibit grain-edge fractures. In low-carbon steel, fine and underdeveloped ductile pits are prone to appear on the grain-boundary facets; these are sometimes referred to as “chicken claw patterns”. Hydrogen-induced cracking is characterized by latency, whereas in welded components, its occurrence is sudden and can pose a serious threat to human life and property, thus requiring close attention. In the case of explosions, how to remove hydrogen from metals is the main concern for everyone. Certain steels or components used under special conditions must undergo dehydrogenation treatment; for example, galvanized parts used in aircraft must be dehydrogenated. Dehydrogenation is also required for galvanizing elastic parts and high-strength steel. Hydrogen removal is accomplished by using a heating treatment to drive hydrogen out of the interior of the parts. The dehydrogenation effect is related to the dehydrogenation temperature and holding time. The higher the temperature and the longer the time, the more thorough the dehydrogenation. Typically, the component from which hydrogen is to be removed can be placed in a vacuum oven and treated at a temperature of 200–250°C for 2–3 hours. Dehydrogenation in hot oil can achieve the same results as dehydrogenation in an oven; the heating is uniform, and the requirements for equipment are simpler.