Heat treatment of seamless steel tubes is a process for improving the properties of metal materials and their products. The material and its workpiece are heated to a suitable temperature for different purposes, insulated, and then cooled in different ways to change the internal structure to achieve the desired properties. The heat treatment of seamless steel tubes is usually divided into annealing, normalizing, quenching, tempering, failure, and chemical heat treatment and vacuum heat treatment.
By heat treatment, the use efficiency or life of the steel can be improved, and in some cases, the cheaper general metal material can be replaced by the more expensive special material. The basic methods of heat treatment of seamless steel tubes mainly include annealing, normalizing, quenching, tempering and combinations thereof.
Annealing is a process in which a seamless steel pipe is heated to a slightly higher or lower critical temperature for a period of time (ie, heat preservation) and then slowly cooled. Common annealing processes are classified into diffusion annealing, complete annealing, isothermal annealing, spheroidizing annealing, incomplete annealing, recrystallization annealing, and stress relief annealing.
Normalizing, also known as "normalization", is to heat the seamless steel pipe to a temperature above 40~60 °C or higher above the upper critical point (Ac3 or Acm) to achieve complete austenitization and homogenization of the structure. And then the furnace is released and the process is evenly cooled in the naturally circulating air. Compared with the annealing process, the normalizing process has the characteristics of fast cooling speed, fine material structure, improved mechanical properties, short production cycle, high equipment utilization rate and low cost.
Quenching is a process in which a seamless steel tube is heated to a suitable temperature, insulated, and then rapidly cooled (usually cooled in water, oil or air) to convert supercooled austenite into martensite structure. Generally used to improve the hardness and strength of parts, or to change their physical and chemical properties (such as conductivity, magnetic properties, corrosion resistance, etc.). Generally, the quenching methods include single-liquid quenching, two-liquid quenching, fractional quenching, and isothermal quenching.
Tempering is a heat treatment process in which the seamless steel pipe after quenching is heated to a temperature below the critical point Ac1, and then cooled to room temperature after being kept warm. It is a process that must be carried out after the quenching of the seamless steel pipe, and is also the latter process of the heat treatment.
The purpose of tempering is to improve the ductility and toughness of hardened steel, reduce and eliminate quenching stress, and stabilize the size and shape of the structure and workpiece. Air tempering is generally used after tempering, but rapid cooling is applied to alloy seamless steel pipes exhibiting high temperature temper brittleness. The tempering process can be divided into low temperature tempering, medium temperature tempering and high temperature tempering.