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Heat treatment is a key technology in the field of metal processing, which significantly changes the microstructure and physical properties of metal materials through the precise control of heating and cooling processes. Especially in the steel industry, heat treatment can greatly improve the mechanical properties of materials, such as hardness, toughness and wear resistance. From basic annealing to complex chemical heat treatments, each heat treatment has its own unique purpose and effect.
This article will delve into the heat treatment process of steel, covering the heating and cooling transitions of the material, and how different heat treatment types affect the properties of the final product. With an understanding of these critical processes, engineers and technicians can better select the right heat treatment method to meet specific application needs.
01 One
Definition and purpose of heat treatment
1. Definitions:
By heating, insulating, and cooling a solid metal or alloy to change its internal structure, it achieves the desired performance.
2 Purpose:
2.1 Improve the process performance of the material and ensure the smooth progress of subsequent processing, this heat treatment is called pre-heat treatment.
2.2 To improve the performance of materials and prolong the service life of parts, this heat treatment is called final heat treatment.
3. Classification of heat treatment
3.1 Ordinary heat treatment: including annealing, normalizing, quenching, tempering (four fires).
3.2 Surface heat treatment: surface quenching, chemical heat treatment.
3.3 Other heat treatment: vacuum heat treatment, deformation heat treatment, etc.
02 Two
Tissue transformation upon heating
In the heat treatment process, the microstructure transformation when heated is a very critical step, especially the conversion of pearlite to austenite. This transition typically consists of four main phases, so let's briefly understand the characteristics of each phase:
1. Austenite nucleation: At this stage, as the temperature increases, the pearlite begins to decompose, and a new phase, austenite, begins to form small "nuclei" at the original grain boundaries or other defects. These "nuclei" are the starting point for the growth of new phases.
2. Austenite growth: The formed austenite "nucleus" gradually grows, gradually replacing the original pearlite. In this process, the austenite grains continue to expand until they fill almost the entire microstructure of the metal.
3. Dissolution of remaining Fe3C: Fe3C (iron carbide) is the harder part of pearlite. During the formation and growth of austenite, the remaining Fe3C is gradually absorbed and dissolved by austenite. This process facilitates subsequent homogenization and ensures a more homogeneous internal composition of the metal.
4. Austenite homogenization: Finally, the austenite throughout the structure will reach a homogeneous state where its composition and temperature are more consistent at the microscopic level. This homogenized austenite provides an ideal starting point for the next cooling and transformation process, which enables better mechanical properties to be obtained.
Through the above four stages, the microstructure and properties of the metal will be significantly changed, laying the foundation for better material properties.
03 Three
Tissue transformation on cooling
1. Cooling transition of austenite
The cooling transition of austenite is an extremely critical step in the heat treatment process, which directly affects the final properties and microstructure of the steel. Let's take a layman's look at the process:
When steel is heated above the critical point A1, it is mainly present in a stable phase – austenite. Austenite is a phase in which carbon and iron atoms are uniformly mixed in steel at high temperatures, and has high toughness and ductility. However, when cooling begins, this stable austenite phase becomes unstable and must be transformed into other types of microstructures.
The beginning of this shift is when the temperature drops below the A1 point. At this point, the austenite ceases to be the most stable phase and begins to transform into other microstructures such as pearlite, bainite or martensite. The process and end result of this transformation is highly dependent on the rate and manner of cooling.
In the case of fast cooling rate, martensitic with high hardness may be formed; Slow cooling, on the other hand, can lead to the formation of pearlite or bainite, which have different performance characteristics such as strength, toughness, and hardness. Therefore, even for the same steel, different cooling methods can lead to large differences in properties after heat treatment under the same heating temperature and holding time.
45 steel heated to 840°C, mechanical properties after cooling under different cooling conditions
| Cooling method | σb/Mpa | σs/Mpa | d/% | ψ/% | HRC |
| Cool with the furnace | 519 | 272 | 32.5 | 49 | 15~18 |
| Air cooling | 657~706 | 333 | 15~18 | 45~50 | 18~24 |
| Cooling in oil | 882 | 608 | 18~20 | 48 | 40~50 |
| Submersible cooling | 1078 | 706 | 7~8 | 12~14 | 52~60 |
2. Establishment of isothermal transition curve of supercooled austenite of eutectic steel (metallographic hardness method)
也称“TTT曲线”（Time-Temperature-Transformation Curve），因形状类似“C”，常称“C曲线”。
With the help of the "C-curve", it is possible to understand the structure of the austenite under different cooling conditions and the properties of the transformation products, which provides a theoretical basis for the correct formulation and selection of heat treatment processes.
3. Eutectoid steel C curve and transformation products
3.1 Pearlite shape transition (also known as high temperature transition)
Transition temperature: A1~550°C; Transformation product: pearlite
A1~6500℃：珠光体片层较粗，P（珠光体-pearlite ）
6500℃~6000℃：珠光体层片较细，S（索氏体-sorbite ）
6000℃~5500℃：珠光体层片极细，T (屈氏体－troolstite）
The thickness of the ferrite and cementite layers of pearlite is related to the transition temperature. The lower the temperature, the finer the layers of the pearlite. The layers become thinner, the strength and hardness increase, and the plastic toughness increases.
3.2 Bainite Shape Transition (also known as Mid-Temperature Transition)
Transition Temperature: 550-ms (230°C)
Transformation product: bainite B (bainite) – a mixture of supersaturated F and cementite.
550~350°C:上贝氏体(upper bainite )(B上)羽毛状组织,强度与塑性都较低,脆性很高。
350℃~ Ms：下贝氏体（lower bainite ）（B下）针片状组织，综合性能好。
3.3 Martensite transition (also known as cryogenic transition)
转变温度：Ms（230℃）~Mf
转变产物：马氏体（martensite ）+A′(residual austenite )
Martensite: A supersaturated solid solution formed by carbon in α-Fe, denoted by M.
Classify:
低碳马氏体（low carbon martensite ）：呈板条状，具有较高的强度和塑韧性。 也称板条M（lath martensite ）。
High carbon martensite: lenticular, flaky with ridges in the middle. Its strength is very high, but the plasticity is poor and brittle.
3.4. C-curve of sub-eutectic steel
C-curve of a eutectic steel
过冷奥氏体连续转变冷却曲线（CCT曲线）(Continuous Cooling Transformation)
04 Four
Specific heat treatment processes
1. Extinguishing:
Definition: Heating a metal to a certain temperature, holding it for a sufficient amount of time, and then cooling it at a suitable rate
Objective:
- refinement of grains;
- Reduce hardness and improve the forming and cutting properties of steel;
- Relieves internal stress.
Classification: According to the purpose and process characteristics of annealing, it can be divided into complete annealing, incomplete annealing, isothermal annealing, spheroidization annealing, stress relief annealing, etc.
1.1完全退火（full annealing）
- Scope of application: sub-eutectic steel
- 加热温度：Ac3+30～50℃
- Objective: To refine the structure, reduce the hardness, improve the machinability, and eliminate the internal stress
- Tissue at room temperature: F+P
1.2球化退火（spheroidizing annealing ）
- Scope of application: eutectic steel and eutectic steel
- 加热温度：Ac1+20~30℃
- Objective: To spheroidize reticulated or flaky Fe3CII
- Tissue: globular pearlite
1.3等温退火(isothermal annealing )
- Process: Heating to Ac1+30~50 °C or Ac3+30~50 °C, after heat preservation, quickly cool to a certain temperature below Ar1, wait for A to become P class structure, and air cooling out of the furnace.
- Organization: Class P
- Advantages: short annealing time, uniform structure
1.4去应力退火（relief annealing ）
- Purpose: To remove residual stresses
- Heating temperature: T heating < AC1 (500~600°C)
- Application: Eliminate residual internal stress in castings, forgings, weldments, etc.
1.5 Homogenization annealing (diffusion annealing)
- Objective: To eliminate segregation; Homogeneous composition, tissue
- Heating temperature: AC3+150~250°C
- Structure: P+F for sub-eutectic steel.
- Application: Mainly used for alloy steel ingots, castings and forgings with high quality requirements.
1.6再结晶退火(recrystallization annealing)
- Process: Heat to 50-150 °C below Ac1, or T +30-50 °C, keep warm, slow cooling.
- Purpose: To eliminate work hardening and restore the plastic toughness of steel.
- Application: Elimination of work hardening of cold-worked workpieces. Such as annealing in the middle of the wire drawing process.
2. Shofire
Definition: A heat treatment process in which the workpiece is heated to 30~50°C above Ac3 or Accm, and then taken out of the furnace to cool in air after holding warm.
Objective: Low carbon steel: improve hardness and facilitate cutting.
Eutectoid steel: eliminate reticulated secondary cementite, which is conducive to P-spheroidization.
Medium carbon steel and medium carbon low alloy steel: the force is not large, and the performance requirements are not high, which can be used as the final heat treatment.
3. Flame
Purpose: To obtain M or B microstructure and improve the hardness and wear resistance of steel.
Selection of quenching temperature:
- 亚共析钢：AC3+30～50℃；
- 共析钢及过共析钢：AC1+30～50℃。
Quenching cooling is the key to determining the quality of quenching, and the ideal cooling rate should be the speed shown in the figure.
Above 650°C, slow, reduce thermal stress
650-400 °C, fast, avoid C-curve
Below 400 °C, slow, reduce phase change stress
常用的淬火介质(quenching medium)
At present, the commonly used cooling media in production are oil, water and brine, and their cooling capacity increases sequentially.
- Water: It has strong quenching ability, but there are soft spots on the surface of the workpiece, which is easy to deform and crack.
- Salt water: stronger quenching ability, smooth and clean surface of the workpiece, no soft spots, but more prone to deformation and cracking;
- Oil: The quenching ability is weak, but the workpiece is not easy to deform and crack
常见的淬火冷却方法(quench cooling method)
4. Reigning
The main purpose of tempering:
- Relieves internal stress and reduces brittleness
- Stabilize the tissue and workpiece dimensions
- Reduce hardness and improve plasticity
Changes in the structure and properties of tempering:
The microstructure transformation of quenched steel during tempering mainly occurs during the heating phase. With the increase of heating temperature, the microstructure of quenched steel changes in four stages.
4.1. Decomposition of martensite
- Tempering stage: when tempering at < 100°C, there is no change in the structure; When heated at 100~200°C, martensite will decompose.
- Tissue obtained: tempered martensite M back (supersaturated α solid solution).
- Performance change: the internal stress gradually decreases, and the performance is basically unchanged.
4.2 Residual austenite decomposition
- Tempering stage: 200-300°C. A′ decomposes, transforming into B.
- 获得组织：M回（Tempered Martensite）表示
- Performance changes: Stress is further reduced, strength and hardness are slightly reduced.
4.3. Martensite decomposition completion and cementite formation
- Tempering stage: 300-400°C. ε carbide is converted into a stable cementite.
- Tissue obtained: tempered troostite, denoted by Tempered troostite.
- Performance change: the internal stress is basically eliminated, the hardness is reduced, and the plastic toughness is increased.
4.4.Fe3C聚集长大和α固溶体的回复与再结晶
- Tempering stage: above 400°C. The α phase begins to recover, and recrystallization occurs above 500°C;
- Tissue obtained: tempered sorbite, denoted by Tempered Sorbite.
- Performance Change: Good overall performance is obtained.
Microstructure and mechanical properties of steel after tempering:
The general trend of mechanical properties change during tempering: with the increase of tempering temperature, the strength and hardness of steel decrease, and the plasticity and toughness increase.
| craft | Turning temperature (℃) | Tissue after tempering | Tempered hardness (HRC) | Performance characteristics: | use |
| Low-temperature incitation | 150～250 | M times | 58～64 | High hardness and high wear resistance; Brittleness, reduced internal stress | tool steel Rolling bearings, carburized parts, etc |
| Medium-temperature reigning | 250～500 | T times | 35～50 | The higher elastic limit and yield limit have a certain plasticity and toughness | spring steel Hot work molds |
| High-temperature incition | 500～600 | S times | 25～35 | Good overall performance | Important structural parts |
05 Five
Surface heat treatment
Surface heat treatment: A heat treatment process in which only the surface layer of the workpiece is heat treated to change its structure and properties.
Classification: Surface quenching and chemical heat treatment.
In production, there are many parts that require different properties on the surface and core, generally with high surface hardness, high wear resistance and fatigue strength; The heart requires good plasticity and toughness.
In this case, the selection of materials alone or the use of ordinary heat treatment methods can not meet the requirements. The solution to this problem is surface heat treatment.
1.表面淬火(surface quenching )
- Definition: A heat treatment process in which only the surface of a workpiece is quenched (+ tempered).
- Purpose: To make the workpiece table hard and tough.
- Surface hardening steel: medium carbon structural steel (0.4%-0.5% carbon)
- Method: Induction heating case quenching and flame heating case quenching.
2.感应加热表面淬火(induction surface quenching)
Basic principle: The induction coil is fed with alternating current→ forming eddy currents (skin effect), → surface layer gets A→ water cools to get M.
Classify:
- 高频感应加热：200～300kHz，0.5～2.5mm；
- 中频感应加热：0.5～10kHz，2～10mm；
- 工频感应加热：50Hz，10～20mm。
Rule: The higher the current frequency, the shallower the depth of the hardened layer.
3.火焰加热表面淬火(flame heating surface quenching)
- Definition: Flame heating surface quenching is the use of oxygen-acetylene (or other combustible gas) flame to heat the surface of the part, and then quickly cool the quenching, the depth of the hardened layer is generally 2~6mm.
- Application: Suitable for single-piece, low-volume production.
4.钢的化学热处理（chemical heat treatment）
- Definition: A heat treatment process in which steel parts are kept warm in an active medium at a certain temperature, so that one or several elements penetrate into its surface layer to change its chemical composition, structure and properties.
- Classification: According to the different elements infiltrated, chemical heat treatment can be divided into carburizing, nitriding, carbonitriding, boroning, aluminization, etc.
Basic process:
- Decomposition: make the chemical medium decompose the active atoms that infiltrate the elements during the heating and heat preservation process;
- Absorption: The active atoms are adsorbed by the surface of the workpiece to form solid solutions or special compounds;
- Diffusion: The infiltrated atoms diffuse inward from the surface of the workpiece to form a diffusion layer with a certain depth, that is, the permeable layer
5.钢的渗碳（Carburize of steel）
- Purpose: To improve the hardness and wear resistance of the surface of the workpiece
- Carburizing steel: low carbon steel or low carbon alloy steel
- Medium: the most commonly used gas (kerosene, benzene, etc.) with active carbon atoms.
- Temperature: 900-950°C in the austenitic zone
- Time: It takes about 10 hours depending on the depth of the seepage.
6. Other chemical heat treatment methods
6.1 Nitriding: a heat treatment process that allows reactive nitrogen atoms to penetrate into the surface of the workpiece at a certain temperature. Improve the surface hardness, wear resistance, fatigue strength, hot hardness and corrosion resistance of parts.
6.2 Carbonitriding: carbon and nitrogen penetrate into the surface of the workpiece at the same time. Improves surface hardness, fatigue and abrasion resistance, and combines the advantages of carburizing and nitriding.
6.3 Chromizing: It has good corrosion resistance and excellent oxidation resistance, hardness and wear resistance, and can replace stainless steel and heat-resistant steel for tool manufacturing.
6.4 boronizing: very excellent wear resistance, corrosion wear resistance and mud wear ability, wear resistance is significantly better than nitriding, carbon and carbonitriding layer, but not resistant to atmospheric and water corrosion. It is mainly used for mud pump parts, hot work molds and workpiece fixtures.