Heat treatment “four fires”

1. Normalizing

The word “normalization” does not characterize the nature of the process. More precisely, it is a homogenization or grain refinement process designed to make the composition consistent throughout the part. From a thermal point of view, normalizing is a process of cooling in stillness or breeze after the austenitizing heating section. Typically, the workpiece is heated to about 55°C above the critical point on the Fe-Fe3C phase diagram. This process must be heated to obtain a homogeneous austenite phase. The actual temperature used depends on the composition of the steel, but is usually around 870°C. Due to the inherent properties of cast steel, normalizing is usually performed prior to ingot machining and prior to hardening of steel castings and forgings. Air quench hardened steels are not classified as normalized steels because they do not acquire the pearlitic microstructure typical of normalized steels.

2. Annealing

The word annealing represents a class that refers to a treatment method of heating and holding at an appropriate temperature and then cooling at an appropriate rate, mainly to soften the metal while producing other desired properties or microstructural changes. Reasons for annealing include improved machinability, ease of cold working, improved mechanical or electrical properties, and increased dimensional stability, among others. In iron-based alloys, annealing is usually performed above the upper critical temperature, but the time-temperature combination varies widely in temperature range and cooling rate, depending on the steel composition, state and desired results. When the word annealing is used without a qualifier, the default is full annealing. When stress relief is the sole purpose, the process is referred to as stress relief or stress relief annealing. During full annealing, the steel is heated to 90~180°C above A3 (hypoeutectoid steel) or A1 (hypereutectoid steel), and then cooled slowly to make the material easy to cut or bend. When fully annealed, the cooling rate must be very slow to produce coarse pearlite. In the annealing process, slow cooling is not necessary, because any cooling rate below A1 will obtain the same microstructure and hardness.

3. Quenching

Quenching is the rapid cooling of steel parts from the austenitizing or solutionizing temperature, typically from the range of 815 to 870°C. Stainless steel and high-alloy steel can be quenched to reduce the carbide existing in the grain boundary or to improve the distribution of ferrite, but for most steels, including carbon steel, low-alloy steel and tool steel, quenching is for microscopic A controlled amount of martensite is obtained in the tissue. The goal is to obtain the desired microstructure, hardness, strength or toughness with as little potential for residual stress, deformation and cracking as possible. The ability of a quenching agent to harden steel depends on the cooling properties of the quenching medium. The quenching effect depends on the composition of the steel, the type of quenching agent and the conditions of use of the quenching agent. The design and maintenance of the quenching system is also the key to the success of quenching.

4. Tempering

In this treatment, previously hardened or normalized steel is usually heated to a temperature below the lower critical point and cooled at a moderate rate, mainly to increase plasticity and toughness, but also to increase the matrix grain size. Tempering of steel is reheating after hardening to obtain a certain value of mechanical properties and release quenching stress to ensure dimensional stability. Tempering is usually followed by quenching from the upper critical temperature.