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45# is a steel grade of carbon structural steel, corresponding to Japanese standard S45C, American standard: 1045, and German standard C45. Its characteristic is that it has higher strength and resistance to deformation than ordinary A3 steel.

Its chemical composition element ratio (%): Carbon C: 0.42～0.50; Chromium Cr: ≤0.25; Manganese Mn: 0.50～0.80; Nickel Ni: ≤0.25; Phosphorus P: ≤0.035; Sulfur S: ≤0.035; Silicon Si: 0.17～0.37

To

Mechanical behavior

Mpa: ≥600

Mpa: ≥355

%: ≥16

%: ≥40

Hardness HB: ≤197

The recommended heat treatment system for 45 steel specified in the GB/T699-1999 standard is normalizing at 850°C, quenching at 840°C, and tempering at 600°C, and the achieved performance is yield strength ≥355MPa

The GB/T699-1999 standard stipulates that the tensile strength of 45 steel is 600MPa, the yield strength is 355MPa, the elongation rate is 16%, and the reduction of area is 40%. Modulation treatment hardness specification size

Heat treatment process of 45# (number) steel and 40Cr steel quenched and tempered

Quenching and tempering is a double heat treatment of quenching and high temperature tempering, and its purpose is to make the workpiece have good comprehensive mechanical properties.

Quenched and tempered steel has two categories: carbon quenched and tempered steel and alloy quenched and tempered steel. Regardless of whether it is carbon steel or alloy steel, its carbon content is controlled strictly. If the carbon content is too high, the strength of the workpiece after quenching and tempering is high, but the toughness is not enough. If the carbon content is too low, the toughness will increase and the strength will be insufficient. In order to obtain good overall performance of quenched and tempered parts, the carbon content is generally controlled at 0.30~0.50%.

During quenching and tempering, the entire cross-section of the workpiece is required to be quenched, so that the workpiece has a microstructure dominated by fine needle-shaped quenched martensite. Through high temperature tempering, a microstructure dominated by uniform tempered sorbite is obtained. It is impossible for small factories to conduct metallographic analysis for each furnace, and generally only perform hardness testing. This means that the hardness after quenching must reach the quenching hardness of the material, and the hardness after tempering is checked according to the requirements of the drawing.

The operation of the quenching and tempering treatment of the workpiece must be carried out strictly in accordance with the process documents. We only provide some views on how to implement the process during the operation.

1. Quenching and tempering of No. 45 steel

No. 45 steel is a medium-carbon structural steel with good cold and hot workability, good mechanical properties, low price and wide sources, so it is widely used. Its weakness is that the hardenability is low, the cross-sectional size is large, and the workpieces with higher requirements are not suitable for use.

The quenching temperature of No. 45 steel is A3+(30~50) ℃. In actual operation, the upper limit is generally taken. A higher quenching temperature can speed up the heating of the workpiece, reduce surface oxidation, and improve work efficiency. In order to homogenize the austenite of the workpiece, sufficient holding time is required. If the actual amount of furnace installed is large, the holding time needs to be extended appropriately. Otherwise, there may be insufficient hardness due to uneven heating. However, if the holding time is too long, coarse grains and serious oxidative decarburization will also occur, which will affect the quenching quality. We believe that if the installed furnace volume is greater than the provisions of the process documents, the heating and holding time needs to be extended by 1/5.

Because of the low hardenability of No. 45 steel, a 10% saline solution with a high cooling rate should be used. After the workpiece enters the water, it should be quenched, but not cold through. If the workpiece is cooled in salt water, it may crack the workpiece. This is caused by the rapid transformation of austenite into martensite when the workpiece is cooled to about 180°C. Caused by excessive tissue stress. Therefore, when the quenched workpiece is rapidly cooled to this temperature region, a slow cooling method should be adopted. Since the temperature of the outlet water is difficult to control, it must be operated based on experience. When the shaking of the workpiece in the water stops, the outlet can be air-cooled (oil cooling is better). In addition, the workpiece should be moved but not static when entering water, and should be moved regularly according to the geometric shape of the workpiece. The static cooling medium and the static workpiece result in uneven hardness, uneven stress and large deformation or even cracking of the workpiece.

The hardness of the 45# steel quenched and tempered part after quenching should reach HRC56~59. The possibility of a large cross section is lower, but it cannot be lower than HRC48. Otherwise, it means that the workpiece has not been completely quenched, and sorbite or even ferrite may appear in the structure. Body tissue, this kind of tissue is still retained in the matrix after tempering, and cannot achieve the purpose of quenching and tempering.

For high temperature tempering after quenching of No. 45 steel, the heating temperature is usually 560~600℃, and the hardness requirement is HRC22~34. Because the purpose of quenching and tempering is to obtain comprehensive mechanical properties, the hardness range is relatively wide. However, if the drawings have hardness requirements, the tempering temperature must be adjusted according to the drawings to ensure the hardness. For example, if some shaft parts require high strength, the hardness requirements are high; while for some gears and shaft parts with keyways, milling and inserting are required after quenching and tempering, so the hardness requirements are lower. Regarding the tempering and holding time, it depends on the hardness requirements and the size of the workpiece. We believe that the hardness after tempering depends on the tempering temperature, and has little to do with the tempering time, but it must be back through. Generally, the tempering and holding time of the workpiece is always More than one hour.

2. Quenching and tempering treatment of 40Cr steel

Cr can increase the hardenability of steel, improve the strength and tempering stability of steel, and has excellent mechanical properties. Cr steel should be used for hardened and tempered workpieces with large cross-sectional dimensions or important. But Cr steel has the second type of temper brittleness.