1. Purpose
Alloy quenched and tempered steel is widely used to manufacture various important parts of automobiles, tractors, machine tools and other machines, such as gears, shafts, connecting rods, bolts, etc.
2. Performance requirements
Most of the tempered parts bear a variety of working loads, the stress conditions are relatively complex, and require high comprehensive mechanical properties, that is, they have high strength, good plasticity and toughness. Alloy quenched and tempered steel also requires good hardenability. However, different parts have different stress conditions and different requirements for hardenability.
3. Ingredient characteristics
(1) Medium carbon: The carbon content is generally between 0.25% and 0.50%, with the majority being 0.4%;
(2) Add elements such as Cr, Mn, Ni, Si, etc. to improve hardenability: In addition to improving hardenability, these alloying elements can also form alloy ferrite and increase the strength of steel. For example, the performance of 40Cr steel after quenching and tempering treatment is much higher than that of 45 steel;
(3) Add elements to prevent the second type of temper brittleness: Alloy quenched and tempered steel containing Ni, Cr, Mn is prone to produce the second type of temper brittleness when tempered at high temperatures and slowly cooled. Adding Mo and W to steel can prevent the second type of temper brittleness. The suitable content is about 0.15% to 0.30% Mo or 0.8% to 1.2% W.
Comparison of properties after quenching and tempering between 45 steel and 40Cr steel
Steel grade and heat treatment status Sectional size/ mm sb/ MPa ss/MPa d5/ % y/% ak/kJ/m2
45 steel 850℃ water quenching, 550℃ tempering f50 700 500 15 45 700
40Cr steel 850℃ oil quenching, 570℃ tempering f50 (center) 850 670 16 58 1000
4. Steel types and grades (1) 40Cr low-hardenability quenched and tempered steel: The critical diameter of oil quenching of this type of steel is 30mm ~ 40mm, and it is used to manufacture important parts of general sizes. (2) 35CrMo medium-hardenability alloy quenched and tempered steel: The critical diameter of oil quenching of this type of steel is 40mm ~ 60mm. Adding molybdenum can not only improve the hardenability, but also prevent the second type of temper brittleness. (3) 40CrNiMo high-hardenability alloy quenched and tempered steel: The critical diameter of oil quenching of this type of steel is 60mm-100mm, most of which are chromium-nickel steel. Adding appropriate molybdenum to chromium-nickel steel not only has good hardenability, but also eliminates the second type of temper brittleness.
5. Heat treatment and structural properties The final heat treatment of alloy quenched and tempered steel is quenching and high-temperature tempering (quenching and tempering treatment). Alloy quenched and tempered steel has high hardenability, and oil is generally used. When the hardenability is particularly high, it can even be air-cooled, which can reduce heat treatment defects. The final properties of alloy quenched and tempered steel are determined by the tempering temperature. Generally, tempering at 500℃-650℃ is used. By selecting the tempering temperature, the required properties can be achieved. In order to prevent the second type of temper brittleness, rapid cooling (water cooling or oil cooling) after tempering is beneficial to improving toughness. The structure of alloy quenched and tempered steel after conventional heat treatment is tempered sorbite. For parts that require wear-resistant surfaces (such as gears and spindles), induction heating surface quenching and low-temperature tempering are performed, and the surface structure is tempered martensite. The surface hardness can reach 55HRC~58HRC. The yield strength of alloy quenched and tempered steel after quenching and tempering is about 800MPa, and the impact toughness at the core hardness of 800kJ/m2 can reach 22HRC~25HRC. If the cross-section size is large and not hardened, the performance will be significantly reduced.
