Quenched and tempered steel
After normal quenching, medium carbon structural steel undergoes high temperature tempering at 500-700℃, which can also be called quenching and tempering treatment (code 515). The obtained structure is tempered sorbite, which has good comprehensive mechanical properties. Therefore, the quenching and tempering treatment is suitable for large dynamic loads, especially the workpieces working under compound stress (tension, compression, bending, torsion, impact, fatigue). They usually require a good match of 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, the refined classification of traditional quenched and tempered steel and quenched and tempered steel is an important guarantee for continuously improving the quality of quenched and tempered treatment. The use of low-carbon martensitic steel instead of quenched and tempered steel is an important way for the development of structural steel heat treatment.
1. Classification of tempering
The purpose of the quenching and tempering treatment is to obtain good comprehensive mechanical properties of the workpiece, not just the hardness. Therefore, the quenching and tempering treatment should be divided into three categories: high toughness quenching and tempering, strong toughness quenching and tempering and high strength quenching and tempering according to the comprehensive mechanical properties.
(1) High toughness quenched and tempered
High toughness quenching and tempering is the process of quenching and tempering the workpiece at 650～700℃, also known as toughening treatment. Not only can the workpiece obtain high strength and plasticity, but also high toughness. The mechanical properties after quenching and tempering are listed in Table 1. High toughness quenching and tempering is very suitable for quenching and tempering spheroidization of steel for cold heading and cold extrusion, quenching and tempering of fasteners, formed parts, and bending parts, and quenching and tempering of high toughness shafts and rods.
Table 1 Mechanical properties of high toughness steel after quenching and tempering
|
Tensile strength MPa |
Yield Strength MPa |
Elongation (%) |
Rate of reduction in area (%) |
Shock absorption work J |
Hardness HBW |
| 500～650 | 350～500 | 22～32 | 60~75 | 80～250 | 160～230 |
(2) Strengthening and tempering
Strength and toughness quenching and tempering is a traditional quenching and tempering treatment. After quenching, the workpiece is often tempered at a high temperature of 550-650℃. The purpose is to improve the toughness of the steel, match the strength and toughness, and obtain higher comprehensive mechanical properties. The application range of toughness quenching and tempering is very wide, about 80% of medium carbon structural steel workpieces adopt this kind of quenching and tempering. The mechanical properties of the steel after quenching and tempering are listed in Table 2.
Table 2 Range of mechanical properties after toughness quenching and tempering treatment
|
Tensile strength MPa |
Yield Strength MPa |
Elongation (%) |
Rate of reduction in area (%) |
Shock absorption work J |
Hardness HBW |
| 650～900 | 500～700 | 15～25 | 45～65 | 50～150 | 240～300 |
(3) High intensity quenching and tempering
High-strength quenching and tempering is also called hard quenching and tempering. After quenching, the workpiece is tempered at a high temperature of 500 to 600 ℃, which enables the workpiece to obtain higher plasticity and toughness while significantly improving the hardness and strength. This process is very suitable for weight reduction and lightweight workpieces, such as high-strength fasteners, high-strength steel tie rods, anchor rods, various shaft parts and other workpieces that require high fatigue performance and long life. The mechanical properties of the steel after quenching and tempering are listed in Table 3.
Table 3 Range of mechanical properties after high-strength quenching and tempering treatment
2, the classification of quenched and tempered steel
The literature emphasizes that the most important factor affecting the quality of quenching and tempering treatment is the hardenability of steel. For steels with different hardenability, the depth of the hardenable layer obtained after quenching is different, and the microstructure is also different, so the mechanical properties distributed along the section are also different. The higher the requirements of the mechanical properties of the workpiece, especially when the cross-sectional size of the workpiece is large, the greater the influence of hardenability. Therefore, according to the level of hardenability of steel, quenched and tempered steels are divided into four types: the lowest hardenability quenched and tempered steel, lower hardenability quenched and tempered steel, higher hardenability quenched and tempered steel and the highest hardenability quenched and tempered steel.
(1) Quenched and tempered steel with the lowest hardenability
The quenched and tempered steel with the lowest hardenability refers to the medium carbon non-alloy steel (medium carbon steel) with a critical diameter of 15 to 23 mm after water quenching at 20°C. Such as 35 steel, 40 steel, 45 steel, 50 steel. Among them, 45 steel (code U20452) is the most commonly used and most used quenched and tempered steel in China. Due to the lowest hardenability, the quenching and tempering treatment of 45 steel is only suitable for small workpieces with a cross-sectional dimension less than 25mm.
However, for many years, many companies have just ignored the most important quality characteristic of hardenability in the quenching and tempering treatment of 45 steel. Not only the cross-sectional size of 60-100mm workpieces, but also the pre-heat treatment of large-scale shafts with diameters of 300-500mm are also adjusted Quality treatment. Due to the effect of section size, the larger the size of the workpiece, the lower the hardness after quenching, the shallower the hardened layer, and the lower the comprehensive mechanical properties of the workpiece. Especially due to the rough turning allowance (generally 3～5mm), the finishing turning of the 45 steel workpiece after quenching and tempering will process most or even all of the hardened layer of the workpiece. Because when the diameter of the workpiece is greater than 60mm, the performance after quenching and tempering is similar to the performance after normalizing. When the size is greater than 100mm, the martensite structure cannot even be obtained on the surface. Therefore, the workpiece with large cross-section size of 45 steel should be normalized or surface quenched. use.
(2) Quenched and tempered steel with low hardenability
Quenched and tempered steel with low hardenability refers to medium-carbon low-alloy steel (mass fraction generally not greater than 2.5%) with a critical diameter of 30 to 54 mm and an oil quenched critical diameter of 19 to 40 mm, quenched by water at 20°C. There are mainly 35Cr, 40Cr, 45Cr, 35Mn2, 40Mn2, 45Mn2 steel, etc. Among them, 40Cr steel (code A20402) is the most used alloy quenched and tempered steel in my country. It is often used for quenched and tempered parts with medium section size and higher mechanical performance requirements than non-alloy steel (carbon steel).
(3) Quenched and tempered steel with high hardenability
Quenched and tempered steels with high hardenability refer to medium-carbon low-alloy steels with a critical diameter of 42-85mm and an oil-quenched critical diameter of 30-6mm with water quenching at 20°C. 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 low-alloy quenched and tempered steels. The main features are high strength, good toughness, higher hardenability than 40Cr steel, and high creep at high temperatures. Strength and lasting strength, it can work for a long time at 500℃.
(4) Quenched and tempered steel with the highest hardenability
The quenched and tempered steel with the highest hardenability refers to the medium-carbon low-alloy steel and medium-carbon medium-alloy steel with a critical diameter of 60-126mm and an oil-quenched critical diameter of 46-114mm that are quenched with water at 20°C. The main steel grades are 40CrNiMoA (code A50403), agtCrMnMo (code A34402), 37CrNi3 (code A42372) steel and so on. It is often used to manufacture high-strength parts with large cross-sections and impact loads, such as the transmission eccentric shaft of a horizontal forging machine, the crankshaft of a forging press, high-strength tie rods, and anchor rods.
The above-mentioned four types of quenched and tempered steels only cover quenched and tempered steels for general machinery. Due to the different chemical compositions of the steels, different cross-sectional dimensions, and different heat treatment processes, there is no strict boundary between the types.
3. Low carbon martensitic steel instead of quenched and tempered steel
Academician Lei Tingquan particularly emphasized that quenching and tempering is a process that improves plasticity and toughness at the expense of strength, which is very 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 exert the potential of steel strength and toughness and extend the life of machine parts. Therefore, strong quenching of low-carbon steel can be substituted in industrial production. Part of medium carbon steel is quenched and tempered.
(1) 20Cr steel low carbon martensite is strongly quenched instead of 40Cr steel quenching and tempering treatment
After tempering at 350℃, 20Cr steel has higher strength, hardness, plasticity and impact toughness than quenched and tempered 40Cr steel. 20Cr steel can replace 40Cr steel to make drill pipe lock joints, which can reduce or even avoid sliding buckle failure. Due to its high plasticity, fracture toughness, multi-impact resistance and low cold brittle transition temperature (<-70℃), the service life of the drill rod lock head can be significantly increased. After the low carbon martensite is strengthened, it has excellent processing performance, and simplifies the process and reduces the cost. The mechanical properties of the two 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
(2) 20 steel low carbon martensite is strongly quenched instead of 45 steel quenched and tempered
Use 20 steel pipes instead of 45 steel quenched and tempered to manufacture the square sleeve of the chain conveyor. The 20 steel pipe is quenched into 10% NaCl aqueous solution at 900℃X7min, tempered at 160℃×30min, the hardness is 42～46HRC, the strength is 40% higher than quenched and tempered 45 steel, which simplifies the process, does not crack during assembly, and the distortion is small and improved Improved product quality and production efficiency more than doubled.
Use 20 steel instead of 45 steel for quenching and tempering to manufacture lifting arm connecting pins. The connecting pin of the lifting arm was originally made of 45 steel quenched and tempered. Cracks occurred in φ6mm holes at both ends of some parts. Using 20 steel, 1000℃×4.5～5min, quenched into 5%～15% (mass fraction) alkaline water, no cracks.
(3) 20CrMnTi steel quenched and strengthened instead of 40CrMnTi steel quenched and tempered
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
Fourth, the conclusion
(1) The main purpose of quenching and tempering is to make the workpiece obtain good comprehensive mechanical properties. According to the classification of mechanical properties, quenching and tempering is divided into three types: high toughness quenching and tempering, strong toughness quenching and tempering, and high strength quenching and tempering.
(2) Hardenability is the most important quality characteristic that affects the quality of quenching and tempering treatment. According to the hardenability of quenched and tempered steel, it is divided into 4 categories: lowest, lower, higher and highest hardenability.
(3) Low-carbon martensitic steel instead of quenched and tempered steel is an important way for the development of heat treatment of structural steel.
