4 Types of Heat Treatment of Steel

Steel heat treatment is a process that involves heating, holding, and cooling steel in the solid state to alter its internal microstructure and achieve desired properties. Heat treatment can eliminate defects in component blanks such as castings and forgings, improve the steel’s process properties such as machinability, formability, and enhance the mechanical properties of steel. By leveraging steel’s potential, heat treatment helps save production costs, improve component service performance, and extend product lifespan.

I. Quenching

Quenching of steel is a heat treatment process where steel is heated to a temperature above the critical temperature Ac3 for hypoeutectoid steel or Ac1 for hypereutectoid steel, held for a period to achieve full or partial austenitization, and then rapidly cooled at a rate greater than the critical cooling rate down to below the Ms temperature or isothermally held near Ms to promote martensite transformation.

The term quenching is also commonly used to refer to the solution treatment or any heat treatment involving rapid cooling of other materials, such as aluminum alloys, copper alloys and titanium alloys.

Purpose of Quenching

• Enhance the mechanical properties of semi-finished metal products or parts. For example, to increase the hardness and wear resistance of tools and bearings, improve the elastic limit of springs, and boost the overall mechanical properties of shaft components.
• Improve the material or chemical properties of certain special steels. Examples include increasing the corrosion resistance of stainless steel or enhancing the permanent magnetism of magnetic steel.

During quench cooling, in addition to selecting an appropriate quenching medium, the correct quenching method must be employed. Common quenching methods include: single-bath quenching, double-bath quenching, marquenching, austempering, and local quenching.

Characteristics of Steel After Quenching

• The formation of unstable or non-equilibrium microstructures such as martensite, bainite, and retained austenite.
• The presence of high internal stresses or residual stresses.
• The resulting mechanical properties are typically inadequate for use. Therefore, steel workpieces generally require subsequent tempering after quenching.

II. Tempering

Tempering is a heat treatment process that involves heating a quenched metal product or part to a specific temperature, holding it for a certain period, and then cooling it at a controlled rate. Tempering is performed immediately after quenching and is usually the final step of a heat treatment. The combined process of quenching and tempering is often referred to as the final treatment or quench-and-temper process.

Purposes of Tempering

• Reduce internal stresses and lower brittleness. Quenched parts possess high stress and brittleness, and without timely tempering, they are prone to deformation or even cracking.
• Adjust the mechanical properties of the workpiece. After quenching, a part has high hardness and high brittleness. Tempering is used to adjust the combination of hardness, strength, ductility (plasticity), and toughness to meet the diverse performance requirements of various components.
• Stabilize the workpiece dimensions. Tempering promotes the stabilization of the microstructure, ensuring that no further dimensional changes occur during subsequent use.
• Improve the machinability of certain alloy steels.
• Enhance microstructural stability, preventing microstructural changes during the service life of the workpiece and thus maintaining stable geometry and performance.
• Eliminate internal stresses, which improve the service performance and stabilize the part’s geometry.
• Adjust the mechanical properties of steel to meet service requirements.

These effects occur because higher temperatures increase atomic mobility, allowing atoms of iron, carbon, and other alloying elements to diffuse more rapidly. This facilitates the rearrangement and recombination of atoms, gradually transforming the unstable, non-equilibrium microstructure into a stable, equilibrium one. The elimination of internal stress is also related to the decrease in metal strength at elevated temperatures.

Generally, tempering steel leads to a decrease in hardness and strength and an increase in ductility. The higher the tempering temperature, the greater the change in these mechanical properties. However, some high-alloy steels, when tempered within a certain temperature range, precipitate fine metallic compounds, causing an increase in strength and hardness. This phenomenon is known as secondary hardening.

Tempering requirements: Workpieces with different applications must be tempered at different temperatures to satisfy their service requirements.

• Low-temperature tempering (150-250°C): It is used for tools, bearings, carburized and quenched parts, and surface-hardened parts. Low-temperature tempering results in little change in hardness, reduced internal stresses, and a slight increase in toughness.
• Medium-temperature tempering (350-500°C): It is used for springs to achieve high elasticity and necessary toughness.
• High-temperature tempering (500-600°C): It is applied to parts made of medium-carbon structural steel to achieve a favorable combination of strength and toughness.

When steel is tempered around 300°C, its brittleness often increases. This phenomenon is known as temper brittleness type I. Tempering in this temperature range should generally be avoided. Some medium-carbon alloy structural steels are also prone to becoming brittle if cooled slowly to room temperature after high-temperature tempering. This is called temper brittleness type I.

Adding molybdenum to the steel, or cooling rapidly in oil or water during tempering, can prevent temper brittleness type II. Reheating steel susceptible to temper brittleness type II to the original high tempering temperature can eliminate this brittleness.

In manufacturing, tempering is often categorized based on the heating temperature, according to the required properties:

• Low-temperature tempering (150-250°C): Microstructure is tempered martensite. Reduces internal stress and brittleness, improves ductility and toughness, and retains high hardness and wear resistance. Used for gauges, cutting tools, and rolling bearings.
• Medium-temperature tempering (350-500°C): Microstructure is troostite. Achieves high elasticity with certain ductility and hardness. Used for springs and forging dies.
• High-temperature tempering (500-650°C): Microstructure is sorbite. Achieves excellent overall mechanical properties. Used for gears and crankshafts.

The combination of quenching followed by high-temperature tempering is called quenching and tempering, which yields high strength combined with good ductility and toughness.

III. Normalizing

Normalizing is a heat treatment designed to improve the toughness of steel. It involves heating a steel component to a temperature 30 to 50°C above the Ac3 temperature, holding it for a period, and then cooling it in still air outside the furnace. The main characteristic is that the cooling rate is faster than annealing but slower than quenching. Normalizing utilizes this slightly faster cooling to refine the steel’s grain structure. This not only achieves satisfactory strength but also significantly improves toughness (measured by the AKV value) and reduces the component’s susceptibility to cracking. The overall mechanical properties of some low-alloy hot-rolled steel plates, low-alloy steel forgings, and castings can be greatly improved after normalizing, and this also enhances their machinability.

Applications of Normalizing

Hypoeutectoid steel: Normalizing is used to eliminate coarse, overheated grain structures and Widmanstätten structures in castings, forgings, and welds, and to remove banding in rolled products. It refines the grains and can serve as a preliminary heat treatment before quenching.

Hypereutectoid steel: Normalizing can eliminate the network or grain boundary film of secondary cementite and refine the pearlite, which not only improves mechanical properties but also facilitates subsequent spheroidizing annealing.

Low-carbon steel: Normalizing yields a higher amount of fine lamellar pearlite, increasing the hardness to HB 140-190. This prevents “sticking” during cutting and improves machinability. For medium-carbon steel, normalizing is often more economical and convenient when both normalizing and annealing are viable options.

Medium-carbon structural steel: Where mechanical property requirements are not high, normalizing can substitute for quenching and high-temperature tempering. This simplifies the operation and stabilizes the steel’s microstructure and dimensions.

High-temperature normalizing (150 to 200°C above Ac3): Due to the high diffusion rate at elevated temperatures, this process can reduce component segregation in castings and forgings. The resulting coarse grains from high-temperature normalizing can be refined by a subsequent second normalizing step at a lower temperature.

Certain low- and medium-carbon alloy steels used in steam turbines and boilers, normalizing is often used to obtain a bainitic structure, followed by high-temperature tempering, which provides good creep resistance at 400 to 550°C.

In addition to steel components and materials, normalizing is widely used in the heat treatment of ductile (nodular) cast iron to obtain a pearlitic matrix, thereby increasing its strength.

Since air cooling is characteristic of normalizing, the ambient air temperature, stacking method, air flow, and workpiece size all influence the resulting microstructure and properties. The as-normalized microstructure can also be used as a classification method for alloy steels. Alloy steels are typically classified into pearlitic, bainitic, martensitic, and austenitic steels based on the microstructure obtained when a 25 millimeter diameter sample is heated to 900°C and air-cooled.

IV. Annealing

Annealing is a metal heat treatment process that involves slowly heating the metal to a specific temperature, holding it for a sufficient time, and then cooling it at an appropriate rate. Annealing heat treatments are categorized into full annealing, incomplete annealing, and stress-relief annealing. The mechanical properties of annealed materials can be checked using tensile tests or hardness tests. Many steels are supplied in the annealed condition. Hardness testing for steel can employ the Rockwell hardness tester, and for thinner sheets, strips, and thin-walled pipes, the superficial Rockwell hardness tester can be used.

Purposes of Annealing

• Improve or eliminate various microstructural defects and residual stresses caused by casting, forging, rolling, and welding, thereby preventing workpiece deformation and cracking.
• Soften the workpiece for easier machining.
• Refine the grain structure and improve the microstructure to enhance the workpiece’s mechanical properties.
• Prepare the microstructure for the final heat treatment.

Common Annealing Processes

Full Annealing or Homogenizing Annealing: It is used to refine the coarse, overheated microstructure with poor mechanical properties that results from casting, forging, and welding of medium- and low-carbon steels. The workpiece is heated to 30 to 50°C above the temperature where all ferrite transforms into austenite, held for a time, and then slowly cooled in the furnace. During cooling, the austenite re-transforms, resulting in a refined steel microstructure.

Spheroidizing annealing: It is used to reduce the high hardness of tool steel and bearing steel after forging. The workpiece is heated to 20 to 40°C above the temperature where the steel begins to form austenite, held, and then slowly cooled. During cooling, the lamellar cementite in pearlite transforms into globular or spherical particles, reducing hardness.

Isothermal annealing: It is used to reduce the high hardness of certain high-nickel and high-chromium alloy structural steels for machining purposes. The parts are typically cooled relatively quickly to the temperature at which austenite is most unstable, held for a suitable time, and then the austenite transforms into troostite or sorbite, which reduces hardness.

Recrystallization annealing: It is used to eliminate the work hardening effect (increased hardness, reduced ductility) in metal wires and sheets caused by cold drawing and cold rolling. The heating temperature is generally 50 to 150°C below the temperature at which steel begins to form austenite. This range is necessary to eliminate the work-hardening effect and soften the metal.

Graphitizing annealing: It is used to transform cast iron containing a large amount of cementite into ductile or malleable cast iron with good plasticity. The process involves heating the casting to about 950°C, holding it for a certain time, and then cooling appropriately, causing the cementite to decompose and form flocculent graphite.

Diffusion annealing or homogenization: It is used to homogenize the chemical composition of alloy castings and improve their service performance. The method involves heating the casting to the highest possible temperature without causing melting and holding it for a prolonged time. This allows the various elements in the alloy to diffuse toward a uniform distribution before the casting is slowly cooled.

Stress-relief annealing: It is used to eliminate internal stresses in steel castings and welded components. For steel products, heating to 100 to 200°C below the temperature where austenite begins to form, holding, and then cooling in air, removes the internal stresses.