Lei Tingquan, an academician from the Chinese Academy of Sciences, once pointed out clearly that medium carbon structural steel was tempered at a high temperature of 500 ~ 700℃ after normal quenching, also known as tempering treatment (code: 515), and the obtained tissue was tempered Soxhlet, with good comprehensive mechanical properties. Therefore, quenching and tempering treatment is suitable for work under large dynamic loads, especially under composite stresses (tensile, compression, bending, torsion, impact, fatigue). They usually require a good match between strength and toughness, that is, high strength and toughness.
With the construction of a resource-saving society and the development of high-strength lightweight products, it is an important guarantee to continuously improve the quality of tempering treatment to classify the traditional tempering treatment and tempered steel. Using low carbon martensite steel instead of tempered steel is an important way to develop heat treatment of structural steel.
A. Classification of modulation quality
The purpose of quenching and tempering treatment is to obtain good comprehensive mechanical properties of the workpiece, not just hardness. Therefore, tempering treatment should be divided into three categories according to comprehensive mechanical properties: high toughness, strong toughness, and high strength.
(1) High toughness tempering of steel
High toughness quenching is the workpiece after quenching 650 ~ 700℃ ultra-high temperature tempering, also known as toughening treatment. Not only can the workpiece obtain a certain degree of strength and plasticity, but also can obtain quite high toughness. The mechanical properties after quenching and tempering are listed in Table 1. It is very suitable for the dimming and spheroidizing of steel used in the cold heading and cold extrusion, the dimming of fasteners, forming parts and bending parts, as well as the dimming of shaft and bar parts with high toughness.
Table 1 Mechanical properties of steel after quenched and tempered with high toughness
Strength of extension Rm／MPa | Yield strength Rel／MPa | Percentage elongation after fracture A（％） | Percentage reduction of area Z（％） | Impact absorbing energy Akv（0℃）/J | Hardness HBW |
| 500-650 | 350-500 | 22-32 | 60-75 | 80-250 | 160-230 |
(2) Strong toughness and tempering of steel
Strength and toughness tempering is the traditional tempering treatment. After quenching, the workpiece is usually tempered at 550 ~ 650℃, with the purpose to improve the toughness of steel, make the strength and toughness match, and obtain higher comprehensive mechanical properties. The application range of strong toughness quench is very wide, about 80% medium carbon structure steel workpiece adopt this kind of quench. The mechanical properties of tempered steel are listed in Table 2.
Table 2 Range of mechanical properties after quenching and tempering treatment of strength and toughness
Strength of extension Rm／MPa | Yield strength Rel／MPa | Percentage elongation after fracture A（％） | Percentage reduction of area Z（％） | Impact absorbing energy Akv（0℃）/J | Hardness HBW |
| 650-900 | 500-700 | 15-25 | 45-65 | 50-150 | 240-300 |
(3) High strength tempering of steel
High strength tempering is also called hard tempering. After quenching, the workpiece is tempered at a high temperature of 500 ~ 600℃, which makes the workpiece obtain higher plasticity and toughness while significantly improving hardness and strength. This process is very suitable for reducing weight, lightweight workpieces, such as high strength fasteners, high strength steel tie rod, anchor rod, various shaft parts and other workpiece requirements of high fatigue performance, high life. The mechanical properties of tempered steel are listed in Table 3.
Table 3 Range of mechanical properties after high strength quenching and tempering treatment
Strength of extension Rm／MPa | Yield strength Rel／MPa | Percentage elongation after fracture A（％） | Percentage reduction of area Z（％） | Impact absorbing energy Akv（0℃）/J | Hardness HBW |
| 900-1200 | 700-1000 | 8-18 | 40-55 | 35-80 | 300-380 |
B.Classification of tempered steel
It is emphasized in the literature that the hardenability of steel is the most important factor affecting the quality of tempering treatment. For steels with different hardenability, the depth of the quench penetration layer obtained after quenching will be different, and the microstructure will be different, so will the mechanical properties of the distribution along the section. The higher the mechanical performance requirements of the workpiece, especially when the section size of the workpiece is very large, the impact of hardenability is also greater. Therefore, according to the hardenability of steel, tempered steel can be divided into four types: tempered steel with the lowest hardenability, tempered steel with low hardenability, tempered steel with high hardenability and tempered steel with the highest hardenability.
(1) Tempered steel with the lowest hardenability
Tempered steel with the lowest hardenability refers to medium carbon unalloyed steel (medium carbon steel) with a critical diameter of 15 ~ 23mm when quenched by water at 20℃.Such as 35 steel, 40 steel, 45 steel, 50 steel.Among them, 45 steel (code U20452) is the most commonly used and used tempered steel in China.Because of the lowest hardenability, the quenching treatment of 45 steel is only suitable for small workpieces with a section size less than 25mm.
But over the years, many enterprises have just ignored the quenching and tempering treatment of 45 steel, the most important quality characteristic, not only the section size of 60 ~ 100mm workpiece, but even the diameter of 300 ~ 500mm large shaft type pre-heat treatment also uses tempering treatment. Due to the effect of section size, the larger the workpiece size, the lower the hardness after quenching, the shallower the hardening layer, the lower the comprehensive mechanical properties of the workpiece. Especially because of the coarse car allowance (generally 3 ~ 5mm), 45 steel workpiece after quenching and tempering will be most of the workpiece or all the hardened layer processing. When the diameter of the workpiece is larger than 60mm, the performance after quenching after tempering is similar to that after normalizing; when the size is larger than 100mm, the surface cannot even get martensite structure. Therefore, the workpiece with a large section size of 45 steel should be used under normalizing or surface quenching conditions.
(2) Tempered steel with low hardenability
Tempered steel with low hardenability refers to medium carbon low alloy steel with a critical diameter of 30 ~ 54mm and oil quenching critical diameter of 19 ~ 40mm when quenched with water at 20℃ (mass fraction is generally no more than 2.5%). There are mainly 35Cr, 40Cr, 45Cr, 35Mn2, 40Mn2, 45Mn2, etc. Among them, 40Cr steel (code A20402) is the alloy tempered steel with the largest usage in China. It is usually used for tempering parts with medium cross section size and higher mechanical properties than unalloyed steel (carbon steel).
(3) Tempered steel with high hardenability
Tempered steel with high hardenability refers to medium carbon low alloy steel quenched with water at 20℃, the critical diameter of 42 ~ 85mm, the critical diameter of oil quenching is 30 ~ 6mm.There are mainly 30CrMnSi, 35CrMnSi, 35CrMo, 42CrMo, 40CrNi steel, etc.35CrMo (code A30352) and 42CrMo (code A30422) steel are the most commonly used medium carbon and low alloy tempered steel. They are mainly characterized by high strength, good toughness, higher hardenability than 40Cr steel, high creep strength and lasting strength at high temperature, and can work for a long time at 500℃.
(4) Tempered steel with the highest hardenability
Tempered steel with the highest hardenability refers to medium carbon low alloy steel and medium carbon medium alloy steel quenched with water at 20℃, critical diameter of 60 ~ 126mm, critical diameter of oil quenching of 46 ~ 114mm.The main types of steel are 40CrNiMoA (code A50403), agtCrMnMo (code A34402), 37CrNi3 (code A42372) and so on.It is often used to manufacture high strength parts with large section and bearing impact load, such as eccentric shaft of horizontal forging machine, crankshaft of forging machine, high strength tie rod and anchor rod, etc.
The above four types of tempered steel only cover the tempered steel used for ordinary machinery. Due to the different chemical compositions, different section size,s and different heat treatment processes of the steel, there is no strict boundary between each type.
C.Low-carbon Martensite steel replaces tempered steel
Academician Lei Tingquan stressed in particular that tempering is a process to improve plasticity and toughness at the expense of strength, which is detrimental to the potential of materials. The strong quenching of low carbon steel and low carbon alloy steel into low carbon martensite is increasingly widely used, which has become an important way to play the strength and toughness potential of steel and extend the life of machine parts. Therefore, in industrial production, the strong quenching of low carbon steel can replace part of the medium carbon steel quenching and tempering treatment.
(1) 20Cr steel is strongly quenched with low-carbon martensite instead of tempered with 40Cr steel
After being tempered at 350℃, 20Cr steel has higher strength, hardness, plasticity and impact toughness than tempered 40Cr steel. Using 20Cr steel instead of 40Cr steel to make drill pipe lock joint can reduce or even avoid the failure of slip buckle. Due to its high plasticity, fracture toughness, multi-impact resistance and low cold and brittle conversion temperature (& LT;-70℃), can significantly improve the service life of drill pipe lock. The low carbon martensite has excellent machining performance, simplifies the process and reduces the cost. The mechanical properties of these two kinds of steels after quenching and tempering are shown in Table 4.
Table 4 Comparison of mechanical properties of 20Cr steel and 40Cr steel after heat treatment
| Grade of steel | Heat treatment process | HRC | ReH/MPa | Rm/MPa | A (%） | Z (%） | Akv/J |
| 20Cr | At 920℃, 10% NaCl solution was used for quenching and 350℃ for tempering | 37 | 1000 | 1213 | 13 | 64 | 64 |
| 40Cr | Oil quenching at 850℃, tempering at 500℃ | 36 | 830 | 980 | 9 | 45 | 39 |
(2) The low-carbon Martensite intensive quenching of 20 steel replaces the quenched and tempered treatment of 45 steel
The square sleeve of the chain-plate conveyor is made of 20 steel tubes instead of 45 tempered steel.20 steel pipe through 900℃X7min quenching people 10%NaCl aqueous solution, 160℃×30min tempering, the hardness of 42 ~ 46HRC, strength than tempered 45 steel increased 40%, simplified the process, assembly does not occur rupture, small distortion, improve the product quality production efficiency increased more than 1 times.
Make boom joint pin with 20 steel instead of 45 steel quench. The lifting arm connection pin was originally tempered with 45 steel. Part of the ends of the 6mm hole crack.20 steel, 1000℃×4.5 ~ 5min, was quenched into 5% ~ 15% (mass fraction) lye water, no crack.
(3) quenching hardening of 20CrMnTi steel replaces tempering of 40CrMnTi steel
After heat treatment, the mechanical properties of 20CrMnTi steel and 40CrMnTi steel are listed in Table 5.
Table 5 Comparison of mechanical properties of 20CrMnTi steel and 40CrMnTi steel after heat treatment
| Grade of steel | Heat treatment process | HRC | ReH/MPa | Rm/MPa | A (%） | Z (%） | Akv/J |
| 20Cr | At 920℃, 10% NaCl solution was used for quenching and 350℃ for tempering | 47 | 1208 | 1502 | 10 | 56 | 52 |
| 40Cr | Oil quenching at 850℃, tempering at 500℃ | 35 | 1209 | 1325 | 9 | 45 | 47 |
D.Conclusion
(1) The main purpose of tempering is to obtain good comprehensive mechanical properties of the workpiece. According to the classification of mechanical properties, quenchants can be divided into three types: high toughness quenchants, strong toughness quenchants, and high strength quenchants.
(2) Hardenability is the most important quality characteristic that affects the quality of tempering treatment. According to the hardenability tempered steel can be divided into 4 categories: lowest, lower, higher, and highest hardenability.
(3) Low-carbon Martensite steel is an important way to develop heat treatment of structural steel instead of tempered steel.