Since nearly all fasteners are heat treated, it is useful to understand the types of heat treatments used in the fastener industry. It is important to note that the heat treatment of steel and alloy fasteners is different than for aluminum fasteners. The heating temperature, time and quench process vary depending on the fastener material, the desired material properties and the production rate. Fastener heat treatment can be divided into the three categories of steel, steel surface treatment and aluminum. Heat treatment of steel will be covered here. Steel surface heat treatment and aluminum heat treatment will be covered in Part 2.
Heat Treatment of Steel Fasteners
Subcritical annealing of steel (i.e., stress-relief annealing) is performed on the raw material to reduce stress by allowing it to recrystallize. It relieves the stresses generated during cold working, increases ductility and toughness, and produces the desired microstructure. This process is run below the lower critical temperature (Ac1) of the material, typically at 540-650°C (1000-1200°F). The process consists of heating to the desired temperature, soaking at the temperature for a specified time and then cooling, typically to room temperature.
Normalizing is also typically performed on raw material (e.g., wire, rod, bar) and is done at approximately 19-38°C (50-100°F) above the upper critical temperature (Ac3 or Acm) of the steel. Similar to annealing, normalizing consists of heating to the desired temperature, soaking at the temperature for a specified time and then cooling to room temperature.
Tempering is performed after quench hardening. It releases the internal stresses induced during the hardening process. Tempering transforms the structure into tempered martensite, which is both hard and ductile but not brittle. Some hardness is lost and the toughness is improved, achieving a significant gain in properties. Tempering also increases the shock resistance and reduces the tensile strength to the desired level. Tempering of fasteners is done at different temperatures (Table 1), depending on the type of steel and the desired strength and hardness.
Hydrogen Embrittlement Relief
Hydrogen embrittlement occurs due to electrochemical surface treatments such as plating. Hydrogen in atomic form enters the molecular lattice of the steel and causes microcracks to form (Fig. 1), which causes mechanical failure. It is prevented by heating the material at temperatures of 177-221°C (350-430°F) for a period of 8-24 hours. It is important to perform the baking process as soon as possible after plating. Otherwise, the hydrogen will become fully absorbed into the steel lattice, preventing it from being baked out. It is common practice to begin the baking process within three hours of plating, and many manufacturers specify a maximum of one hour. Due to the characteristics of the hydrogen and the steel lattice, higher-strength steels are more susceptible to hydrogen embrittlement.