Main types of heat treatment for steel
Date:2025-06-04View:97Tags:Boier tube,Alloy steel pipe,ASTM A106 steel pipe
1. Annealing
Purpose: Reduce hardness, improve plasticity and toughness; eliminate internal stress; refine grains; improve machinability; prepare for subsequent heat treatment (such as quenching).
Process: Heat the steel to an appropriate temperature at or below the critical point (above Ac1 or Ac3), keep it warm for a sufficient time, and then cool it slowly (usually with the furnace).
Main types:
Full annealing: Heat to 30-50°C above Ac3, keep it warm and cool it with the furnace. Mainly used for hypoeutectoid steel to completely recrystallize the structure, refine the grains, and soften it fully.
Spheroidizing annealing: Heat to around Ac1 (slightly above or below), keep it warm for a long time or cool it slowly to spheroidize the carbides in the steel. Mainly used for hypereutectoid steel (such as tool steel, bearing steel), reduce hardness, improve toughness, and improve machinability.
Stress relief annealing: Heating to below Ac1 (usually 500-650°C), keeping warm and then slowly cooling. The main purpose is to eliminate residual internal stress in castings, forgings, welds or cold-worked parts, stabilize the size, and prevent deformation and cracking.
Recrystallization annealing: Heating to above the recrystallization temperature (usually 650-700°C), keeping warm and then air cooling or furnace cooling. It is used for steel after cold deformation to eliminate work hardening and restore plasticity and toughness.
Organization change: Obtain a structure close to equilibrium (such as pearlite, ferrite + spheroidal cementite).
2. Normalizing
Purpose: Refine the grains and make the structure uniform; improve the comprehensive mechanical properties (strength, toughness); eliminate the network carbides in hypereutectoid steel to prepare for spheroidizing annealing or quenching; improve the machinability of low-carbon steel; sometimes it can be used as the final heat treatment.
Process: Heat the steel to 30-50°C above Ac3 (hypoeutectoid steel) or Accm (hypereutectoid steel), keep it warm for an appropriate time, and then cool it in still or slightly flowing air.
Features: Cooling rate is faster than annealing and slower than quenching. The obtained structure is finer than annealing (sorbite), and the strength and hardness are slightly higher than annealing.
Application: Widely used in structural steel, low and medium carbon steel forgings, castings, as a preliminary heat treatment or final heat treatment.
3. Quenching
Purpose: Obtain martensite (or bainite) structure, greatly improve the hardness and strength of steel.
Process: Heat the steel to a temperature above the critical point (Ac3 or Ac1) (hypoeutectoid steel: Ac3+30-50°C; hypereutectoid steel: Ac1+30-50°C), keep it warm to homogenize the austenite, and then cool it rapidly at a rate greater than the critical cooling rate (usually in water, oil or salt bath).
Key points:
Cooling rate: must be fast enough to avoid austenite decomposing into pearlite or bainite during cooling.
Hardenability: The ability of steel to obtain martensite depth during quenching depends on its alloying element content.
Quenching stress and deformation cracking: Rapid cooling will produce huge thermal stress and organizational stress, which may cause deformation or even cracking of the workpiece. Strict process control and selection of appropriate cooling medium (water, oil, polymer solution, graded/isothermal quenching) are required.
Organization: mainly martensite, a supersaturated solid solution of carbon in α-Fe, with extremely high hardness and strength, but very brittle and high internal stress.
4. Tempering
Purpose: must be carried out immediately after quenching! Eliminate or reduce the internal stress caused by quenching; reduce brittleness; improve plasticity and toughness; obtain the required strength, hardness and toughness (plasticity) combination; stabilize organization and size.
Process: Reheat the quenched steel to a predetermined temperature (low, medium or high) below Ac1, keep it warm for a sufficient time, and then cool it at an appropriate speed (usually air cooling).
Main types (by temperature):
Low temperature tempering (150-250°C): Mainly eliminates some internal stress and brittleness, and the hardness does not decrease much. The structure is tempered martensite. Used for tools, gauges, bearings, carburized parts, etc. that require high hardness and high wear resistance.
Medium temperature tempering (350-500°C): Significantly reduce internal stress and brittleness, and improve elasticity and yield strength. The structure is tempered troostite. Used for springs, forging dies, impact tools, etc.
High temperature tempering (500-650°C): Obtain comprehensive mechanical properties with good combination of strength, plasticity and toughness (good strength and toughness). The structure is tempered troostite. Usually "quenching + high temperature tempering" is collectively referred to as quenching and tempering treatment, which is widely used in important structural parts (such as shafts, connecting rods, bolts, gears, etc.).
Core function: Transform the unstable quenched martensite into a more stable structure and precipitate carbides.
