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Strengthening quenching heat treatment process of thin gear

Thin gears are widely used, such as instrumentation, aerospace, and other fields. Because this field requires high precision of thin gears, it is necessary to ensure the processing accuracy and strictly control the deformation in the processing and production. This article uses 3Cr13 steel as the matrix to introduce a new strengthening quenching heat treatment process.

Process characteristics and process test

3Cr13 is a common stainless steel with good hardenability, hardenability, oxidation resistance and corrosion resistance. It can be quenched by oil, nitrate and air. Table.1 shows the chemical composition and critical temperature value of 3Cr13 steel. Table 2 shows the effect of tempering time on hardness.
Table 1 Chemical composition (mass fraction, %) and critical temperature of 3Cr13 steel

C

Si

Mn

Cr

Ac1/

Ac3/

0.25~0.34

0.60

0.60

12~14

800

1100

Table 2 The effect of tempering time on hardness (HRC)

Tempering temperature/

Tempering time/min

30

50

120

240

530~540

48

46

39~40

35~36

550~560

46

42~43

34.5

33

570~580

40~41

35~36

32

30~30.5

590~600

33.5

32

29.5

28

Figure 1 shows that as the quenching temperature increases, the hardness and strength increase, but the elongation σ, the reduction of area ψ and the impact toughness α decrease, almost in inverse proportion. When the quenching temperature is too low, less than or equal to 950℃, the main carbides and alloying elements are not fully dissolved into the austenite, the matrix structure contains more carbon and alloying elements, and the austenite alloying is insufficient, and there are more excess carbide, residual network carbide and “α” ferrite phase. When the undissolved carbides are distributed unevenly in a chain shape, the continuity of the matrix is destroyed, and a dual-phase structure appears after quenching, resulting in a sharp decrease in strength, toughness, and corrosion resistance. When the quenching temperature is too high, greater than 1000 ℃, coarse grains will appear, the retained austenite will increase, and the deformation will increase, resulting in a decrease in hardness, strength, plasticity and toughness. When the quenching temperature is greater than 1200℃, the “δ” ferrite eutectoid transformation occurs, and the martensite plus δ phase and dual phase structure appears after quenching. In short, the higher the quenching temperature, the more dissolved carbon and alloying elements in the austenite, the greater the degree of austenite alloying, the coarser the grains, and the supersaturated coarse martensite will be obtained after quenching, which will be tempered at medium and high temperatures. The more carbides are precipitated and the more intense it is. In addition, the high temperature quenching produces too much retained austenite and the appearance of “δ” iron element. The quenched structure is a dual-phase or multi-phase structure, which is easy to form positive and negative micro-batteries and cause corrosion. At the same time, coarse grains will reduce the overall mechanical properties. When the quenching temperature is moderate. That is, it can dissolve carbides and alloying elements, homogenize austenite, and have fine grains. After quenching, a single uniform low-carbon martensite can be obtained. Plus choose the best tempering temperature. It can obtain both good comprehensive mechanical properties and high corrosion resistance.

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Figure.1 The relationship between quenching temperature and mechanical properties (sample tempered at 600°C for 2h)
Figure 2 shows that as the quenching temperature increases and the corrosion resistance decreases when tempering above 400℃, it drops to the maximum peak when tempering at about 500℃. The higher the quenching temperature, the more the corrosion resistance decreases; but when tempering When the temperature is lower than 400℃, as the quenching temperature increases, the corrosion resistance increases. Because only a small amount of dispersed carbides are precipitated from martensite during low temperature tempering, the Cr content is basically still higher than 11%, so the corrosion resistance is high. Observation by electron microscopy revealed that the quenched martensite has a brittle phase like S, P and other inclusions and Cr precipitates precipitate along the grain boundary during the slow cooling process after tempering at a higher temperature. The precipitates are easily detached by electrolytic corrosion, showing clear crystals. However, it is not easy to find the rapid cooling of tempering, indicating that there are obvious characteristics of reversible tempering brittleness of the second type.
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Fig.2 The influence of tempering temperature on the corrosion resistance of 3Cr13 steel
Figure 3 shows that when the quenching temperature is constant, different tempering temperatures affect the mechanical properties. The tempering temperature is less than or equal to 500°C, with high hardness and strength, but low plastic toughness. As the tempering temperature gradually increases, the plastic toughness gradually increases, and the hardness and strength gradually decrease, almost inversely proportional to the change.
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Fig.3 Effect of tempering temperature on the mechanical properties of 3Cr13

Hardening process

Process flow: rough machining→stress relief annealing→finish machining→micro-deformation quenching→cold treatment→tempering→finished product.
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Fig.4 hardening process of 3Cr13 sheet gear

Clamping and preheating

The gear is clamped on the mandrel with annular groove, the groove depth is 5mm, the groove width is 5.3 ~ 5.5mm, and the gap between grooves is 3 ~ 4mm. The clamping function is to prevent deformation caused by violent swing during quenching heating and cooling. After clamping, it is preheated in the electric furnace which has been heated to 500 ℃ with support frame, and then it is put into the salt bath furnace which has been heated to about 860 ℃ for secondary preheating. After the gear is put in, cut off the power supply to prevent the deformation caused by electromagnetic salt bath overturning.

Quenching heating

After the gear is insulated by the second medium temperature salt furnace, it is gently taken out and transferred into the high temperature salt bath furnace which has been heated to 1030 ℃. After the power supply is cut off, the gear is put in smoothly. Cutting off the power supply is equal to cutting off the electromagnetic effect, so as to prevent deformation caused by salt bath overturning. It takes 3-5 minutes to let it cool down to 1000 ℃. It is just the holding time, and then take out the air cooling, take advantage of the high temperature, put a piece of gear on the plate press to flatten, after flattening, put it on another platform iron plate to cool to room temperature. The results show that the quenching hardness is more than 52hrc, and the quenched microstructure is low carbon martensite and a small amount of retained austenite.

Cold treatment

There is about 10% retained austenite after quenching in 3Cr13 steel. Even if a small amount of retained austenite forms a dual phase structure, the corrosion resistance will be reduced. The retained austenite will induce martensitic transformation and produce slight deformation during gear operation, which is also not allowed for precision gears. Cold treatment can transform retained austenite into martensite, improve corrosion resistance, hardness, wear resistance, fatigue resistance, stabilize microstructure and reduce deformation. The cold treatment was carried out in a dry ice and alcohol container. After the temperature was kept at – 60 ℃, it was taken out and air cooled to room temperature.

Mould tempering

After flattening and quenching, 99% of the flake gear has reached micro deformation, only a small amount of out of tolerance, and the die is assembled and tempered together. The mould mandrel is similar to the stud bolt, the outer diameter is smaller than the inner circle of the gear, and the gasket at both ends is 10 ~ 20mm larger than the outer diameter of the gear. After the gear plate is installed on the spindle, both ends are tightened with gaskets and nuts. After tempering for 1 h in the electric furnace, take out and tighten the nut again, and take out the oil cooling after repeated 3 times to prevent the occurrence of the second temper brittleness. After cold penetration, check the deformation, hardness and take samples for mechanical property test. The results show that the qualified rate of deformation, hardness and mechanical properties are all 100%, and the corrosion resistance is more than two times higher than that of the original process.

Conclusion

  • (1) Quenching heating, power-off and heat preservation can avoid deformation caused by salt bath overturning due to electromagnetic effect and superplastic tempering caused by clamping fixture, so as to achieve micro deformation of sheet gear, and the qualified rate is 100%.
  • (2) The low carbon lath martensite and a small amount of retained austenite were obtained by short-time heat preservation and air cooling quenching at 1000 ℃. After cold treatment, the retained austenite was transformed into martensite and almost single low-carbon martensite was obtained. The practice shows that the single martensite of 3Cr13 steel has the best corrosion resistance. After tempering at a lower temperature, only a small amount of dispersed carbides are precipitated, which keeps good corrosion resistance, strength and toughness.

Source: China China Pipe Fittings Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@epowermetals.com

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strengthening quenching heat treatment process of thin gear - Strengthening quenching heat treatment process of thin gear
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Strengthening quenching heat treatment process of thin gear
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This article uses 3Cr13 steel as the matrix to introduce a new strengthening quenching heat treatment process of thin gear.
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www.epowermetals.com
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