Introduction to Basic Concepts of Heat Treatment
1. “Heat Treatment”
`The heat treatment of steel is to apply different heating, heat preservation and cooling to the steel in a solid state to change its internal
`Organization structure, a processing technology to obtain the required performance. In most heat treatment processes, steel is heated
`The purpose is mainly to obtain austenitic structure, so it is customary to heat the steel from room temperature to austenitic
The process of state is called austenitization of steel. The most basic type depends on the heating and cooling methods, which can be
`Divided into four types: annealing, normalizing, quenching and tempering.
1. Annealing concept:
The so-called annealing is to heat the metal or alloy to an appropriate temperature, keep it for a certain period of time, and then
The heat treatment process in which the furnace is slowly cooled is essentially to transform the steel into pearlite after heating and austenitizing.
2. Annealing purpose:
(1). Reduce the hardness of steel and increase plasticity to facilitate cutting and cold deformation processing;
(2) Refine the grains, eliminate the structural defects caused by forging, welding, etc., uniform the structural composition of the steel, improve the performance of the steel or prepare for the subsequent heat treatment;
(3). Eliminate internal stress in steel to prevent deformation or cracking.
3. Annealing method:
Commonly used annealing methods include complete annealing, spheroidizing annealing, stress relief annealing, recrystallization annealing, diffusion annealing and isothermal annealing.
(1). Complete annealing: Also called recrystallization annealing, it is an annealing process in which the iron-carbon alloy is completely austenitized and then slowly cooled to obtain a structure close to the equilibrium state. Suitable for medium carbon steel and medium carbon alloy steel with a carbon content of 0.3%” 0.6%.
(2). Spheroidizing annealing: annealing process to spheroidize carbides in steel. Commonly used spheroidizing annealing includes ordinary spheroidizing annealing and isothermal spheroidizing annealing. This process is mainly used for molds, measuring tools and cutting tool steels of eutectoid steel and over-eutectoid steel.
(3). Stress-relief annealing: an annealing process to remove residual stress caused by plastic deformation processing, forging, welding, etc. and the forgings.
(4). Recrystallization annealing: also known as intermediate annealing, it means that the cold deformed metal is heated to above the recrystallization temperature and kept for a proper time to recrystallize the deformed grains into uniform equiaxed grains to eliminate deformation strengthening And residual stress heat treatment process.
(5). Isothermal annealing: the steel or blank is heated to a temperature higher than Ac3 (or Ac1), after keeping it for an appropriate time, it is cooled to a certain temperature in the pearlite temperature range and kept isothermally so that A heat treatment process that transforms the tensite into pearlite and then cools in the air. This annealing method is mainly used for alloy steels with relatively stable undercooled austenite.
1. Normalizing concept:
Normalizing is a heat treatment process in which steel or steel parts are heated to 30″50°C above Ac3 (or Accm), kept for a proper time, and then cooled in still air.
2. The purpose of normalizing:
(1). It can eliminate the network carbides in the hypereutectoid steel and improve the cutting performance of the steel;
(2) Refine the grains of overheated castings and forgings and eliminate internal stress;
(3) For medium and low carbon steels with a C content of <0.4%, normalizing can be used instead of annealing for pre-heat treatment;
(4). Less important workpieces with a C content of 0.4″0.7% can be used under normalizing conditions.
1. Quenching concept:
Quenching is a heat treatment process in which steel is heated to a temperature above Ac3 or Ac1 point, kept for a certain period of time, and then cooled at an appropriate speed to obtain martensite and (or) bainite structure.
2. The purpose of quenching:
The purpose of quenching is to transform supercooled austenite into martensite (or bainite) to obtain a martensite (or bainite) structure, and then temper with different temperatures to obtain the required mechanical properties.
(Note: Quenched workpieces are not allowed to be used directly on site, usually 1″ 2 times or more tempering processing must be performed immediately after that to adjust the structure and stress, etc.!)
3. Cryogenic treatment:
Cryogenic treatment is a heat treatment process in which steel parts are quenched and cooled to room temperature and then cooled in a medium below 0°C, also called cold treatment, which is the continuation of the quenching process.
Purpose of cryogenic treatment:
In order to minimize the retained austenite (unstable structure at room temperature, easy to cause size change, etc.), to further improve the hardness of the workpiece after quenching and prevent the residual austenite during use