Low-temperature deformation heat treatment is also called metastable austenite deformation quenching. The process is as follows: after the steel is heated to the austenite state, it is rapidly cooled to below Ac1, above an intermediate temperature above the Ms point, forging or rolling, and then quenching to obtain martensite structure (see figure 1). In order to obtain a good fit of strength and toughness, it is generally undesirable to produce a non-martensitic structure during the deformation of metastable austenite and subsequent quenching, and thus, the supercooled austenite needs to have sufficient stability. Therefore, low-temperature deformation quenching should select steel with high hardenability.
â–² Figure 1 Schematic diagram of low temperature deformation quenching
1 Low temperature deformation heat treatment process
The process effect of low temperature deformation heat treatment depends on the selection of various process parameters in the deformation heat treatment process. These process parameters are: austenitizing temperature, deformation temperature, residence time before and after deformation and reheating, deformation, deformation mode, deformation speed, and cooling after deformation.
1.1 austenitizing temperature
The effect of austenitizing temperature on the low temperature deformation quenching effect is closely related to the chemical composition of the steel. The general rule is that the lower the austenitizing temperature, the higher the tensile strength after deformation quenching, the larger the area shrinkage, and the elongation remains basically unchanged. See Figure 2 and Figure 3.
â–²Fig. 2 Austenitizing temperature of 0.3% C-3%Cr-1.5Ni steel for low temperature deformation
(91%) Effect of tensile properties after quenching and tempering (100 °C tempering)
â–²Fig.3 Effect of austenitizing temperature of 40CrNiMo steel on tensile strength of low temperature deformation quenching
â—‹-1300°C pre-solution treatment â—-No pre-solution treatment
1.2 Deformation temperature
Figure 4 shows the variation of strength and plasticity index of 18CrNiW steel with temperature. It can be seen that the tensile strength decreases as the deformation temperature increases, and the elongation increases slowly.
â–²Fig. 4 Effect of deformation temperature of 18CrNiW steel on tensile properties after deformation and quenching
(shape variable 60%, tempering temperature 100 ° C)
Figure 5 shows the effect of the deformation temperature of H11 steel (0.35% C, 1.5% Mo, 5.0% Cr, 0.4% V) on the mechanical properties after deformation quenching and tempering.
â–²Fig. 5 Effect of deformation temperature of H11 steel on mechanical properties after deformation quenching and tempering
Shape variable 1-94%, 2-75%, 3-50%, 4-30%
(General tensile strength 2170MPa, yield strength 1680MPa)
It can be seen that as the deformation temperature increases, the tensile strength tends to decrease. The plasticity index elongation and the area shrinkage rate have a depressed area between 400 ° C and 500 ° C. This phenomenon is related to blue brittleness.
Figure 6 shows the effect of deformation temperature on the mechanical properties of 30CrNiMo steel.
â–²Fig.6 Effect of deformation temperature on mechanical properties of 30CrNiMo steel
(austenification 1150 ° C, 50% deformation,
After quenching and quenching, 200 ° C X 4h tempering)
1.3 Retention before and after deformation and reheating after deformation
If the austenite has high stability and the steel is cooled to the deformation temperature after austenitizing and remains austenite for a period of time, the retention before deformation has no effect on the properties after low temperature deformation quenching.
In order to obtain the desired strengthening effect, the deformation of the low-temperature deformation quenching should reach more than 60%. Under the general low temperature deformation condition, it is difficult to obtain such a large deformation variable at a time. However, many studies have found that there is almost no difference in the cumulative effect of multiple deformations and the effect of one deformation to the required amount. The experimental results are shown in Table 1 and Table 2.
â–¼Table 1 Effect of Intermediate Heating on Low Temperature Deformation Quenching Mechanical Properties of 30CrMnNiA Steel
â–¼Table 2 The effect of intermediate heating on the mechanical properties of H11 steel after low temperature deformation quenching and tempering
It is not necessary to quench immediately after low temperature deformation. In fact, staying for a period of time after deformation will not affect the quenching effect. Even after the deformation, the steel is heated to a temperature slightly higher than the deformation temperature and kept at this temperature, which can further improve the strength and plasticity of the steel. See Table 3 for details. This is due to the heating and heat preservation after deformation, which causes the austenite to have a grain multi-angle effect. This is the effect of polygonization.
â–¼Table 3 Effect of multi-angle treatment temperature and time on the properties of several steels after low temperature deformation quenching
1.4 shape variable
In the low temperature deformation quenching process, the deformation is an important process parameter. In general, the larger the deformation, the better the strengthening effect on the metal.
Figure 7 shows the effect of the deformation on the tensile properties of 0.3% C-3.0% Cr-1.5Ni steel. It can be seen from the figure that the tensile strength and the yield point rise linearly with the shape variable, and have little effect on the elongation, while the section shrinkage rate slightly decreases after reaching a certain shape variable. For AISI 4340 steel, the yield point rises by 5 MPa for every 1% increase in the deformation.
â–²Fig. 7 Effect of shape variable on tensile properties of 0.3% C-3.0%Cr-1.5%Ni steel
(austenification temperature 930 ° C, deformation temperature 540 ° C, tempering 330 ° C)
1.5 deformation mode
Deformation methods include rolling, extrusion, spinning, hammer forging, explosive forming, and deep drawing.
Generally, bars, strips and plates are rolled and deformed, and bars can also be extruded. Pipes with a diameter of <250mm can be used for spinning, various forgings are forged by hammer forging or press, pipes with a diameter of <76mm can be exploded, and pipes with a diameter of <305mm can be formed by deep drawing.
The results show that the difference of the low-temperature deformation strengthening effect is only related to the deformation temperature and the deformation variable, but has nothing to do with the deformation mode. The difference in the strengthening effect of different deformation modes is caused by the internal temperature change of the material (ie, the deformation temperature change) caused by the deformation speed. See Figure 8.
â–² Figure 8 Press piston speed of movement for low temperature quenching
Effect of tensile properties of VascoMA steel
(The starting extrusion temperature is 593 ° C, 649 ° C,
Shape variable 70%, tempering temperature 552 ° C)
1.6 Deformation speed
There is no consistent law of the effect of deformation speed on the strengthening effect. Sometimes the performance decreases with the increase of the deformation speed, and sometimes it is opposite. When the workpiece with a large cross section is deformed, the mechanical energy is converted into heat energy, and the core temperature increases rapidly with the increase of the deformation speed, and the strengthening effect is lowered. When the cross-section of the workpiece is not large, the deformation speed increases the temperature rise of the workpiece is not high, so that the deformation temperature is performed at a constant temperature, thereby exhibiting a strengthening effect.
1.7 Cooling after deformation
Whether quenching is required immediately after deformation depends on the degree of stability of the supercooled austenite. If the supercooled austenite is sufficiently stable and does not produce a non-martensitic structure, the heat preservation and heating after deformation have little effect on the strengthening effect, and sometimes even have a good effect (described above). When the supercooled austenite is deformed or deformed into a pearlite structure, the strengthening effect is significantly reduced, and when it is decomposed into bainite, the strengthening effect is small. Figure 9 shows the effect of non-martensitic structure content on tensile properties after low temperature deformation.
â–²Fig.9 Effect of non-martensite structure on tensile properties of H11 steel after low temperature deformation quenching
â—-539 °C rolling, shape variable 75% â—‹-482 °C rolling, shape variable 75%
■-539 °C rolling, shape variable 25% □-482 °C rolling, shape variable 25%
2 Low-temperature deformation heat treatment of tissue changes
2.1 Refinement of low temperature deformed martensite structure
Low temperature deformation can refine the martensite structure. At a certain austenitizing temperature, the larger the deformation, the finer the martensite, and the higher the yield strength of the steel. Figure 10 shows the relationship between the martensite size and the yield strength.
â–²Fig.10 Low temperature deformation of 0.32%C-3.0%Cr-1.5%Ni steel
Relationship between yield strength after quenching and martensitic sheet size
(austenification temperature: 1-930 ° C 2-1040 ° C 3-1150 ° C)
However, it cannot be said that the refinement of martensite structure is the only reason for obtaining a strengthening effect after low-temperature deformation quenching of steel. In practice, there is also the fact that martensite of the same grain size can be obtained at different deformation variables and different austenitizing temperatures, but the yield strength is different.
2.2 There are a large number of crystal defects in the low temperature deformation quenched steel
There are a large number of dislocations in the martensite refined at low temperature, and fine dispersed carbides are precipitated on the dislocation lines, and a finer subgrain structure is also present in the martensite flakes. The sub-crystal block is composed of dislocations and is a place where a large number of dislocations gather. The yield strength of steel is inversely proportional to the size of this sub-block.
The microstructure of low temperature deformation quenched martensite is inherited from deformed austenite. In the deformed austenite, there is a higher dislocation density and fine dispersed carbides precipitated in the deformation, indicating that the deformed austenite is initially in a work hardening state, and then the phase transformation becomes fine martensite and is in place. There is a dispersion of dispersed carbides in the wrong aggregates, and the two functions show a double superposition strengthening.
2.3 Precipitation of carbides in deformed austenite
When the steel is quenched at low temperature, the strength of metastable austenite increases with the increase of the deformation variable. When the deformation ratio exceeds 40%, the strength rises faster. This phenomenon cannot be explained by the increase of dislocation density. Due to the precipitated carbide pinning dislocations, the pinning dislocations cause a large increase in new dislocations, so the strengthening effect rises sharply. In addition, such reinforcement also has a high temper resistance.
In short, the martensite formed by low-temperature deformation quenching contains high-density dislocations and fine-dispersed carbides and a low solid solution carbon content, which are high-strength, low-temperature-deformed hardened steels with high strength and high toughness and plasticity. the reason.
3 Mechanical properties of steel after low temperature deformation heat treatment
3.1 Effect of chemical composition of steel on mechanical properties after deformation quenching
The chemical composition of steel is different, and the effect of low temperature deformation quenching is also different. The element that affects the strengthening effect is carbon. C in 0.3% -0.6% (wt) range, the strength after cryogenic deformation quenching with carbon content increases linearly rise, not decreased elongation, reduction of area decreased slightly faster. See Figure 11.
â–²Figure 11 Effect of carbon content on tensile properties of 3%Cr-1.5Ni steel
(Austenitizing 900 ℃, strain 540 ℃, deformation 91% 330 ℃ tempering)
â—-Low temperature deformation quenching â—‹-Ordinary quenching
Yield strength after hardening steel with increased deformation of the deformation and decreases with increasing carbon content, more significant strengthening effect. See FIG. 12, FIG 13, FIG 14.
â–² Figure 12 Shape variable pair of low temperature deformation heat treatment
Different carbon steels Yield Strength
——3%Cr steel ......SAE4340 ----410 stainless steel
â–²Figure 13 Carbon content is 1.86%Cr-2.33%Ni-1.05%Mn-1.03%Si
Effects of low temperature tensile strain hardening -1.03% W-0.47% Mo steel
(austenification 1000 ° C, deformation 550 ° C, 90% deformation, tempering 100 ° C)
â–²Figure 14 H11 steel with carbide forming elements
And low temperature of Fe-Ni-C alloy without carbide forming elements
Rate of increase in deformation quenching yield strength
Carbide forming elements can significantly improve the degree of work hardening of Fe-Ni-C alloy, with the greatest influence of Mo, followed by V, again Cr, and similar results on Fe-Mn-C austenitic alloy. Therefore, the carbide forming element can significantly increase the strength of the low temperature deformation quenched martensite. The experimental results of the metastable austenite deformation strengthening of the alloys listed in Table 4 are shown in Table 5.
â–¼Table 4 Chemical composition of experimental alloy (wt)
â–¼Table 5 Effect of carbide forming elements on mechanical properties of Fe-Ni-C alloy
The non-carbide forming element Si can improve the tempering resistance of steel. In 0.4% C (wt) Fe-Ni-Mo steel, 1.5% Si (wt) was added, and after deformation quenching and tempering at 200-300 ° C, the tensile strength can reach 2670 MPa. The yield strength reached 2350 MPa, while only 0.3% Si (wt), only 2200 MPa and 1960 MPa, respectively. Mn does not contribute to the improvement of the steel quenching strength, but the price is low, which can be used to replace the Ni to improve the stability of the metastable austenite, and to facilitate the low temperature deformation quenching.
Low-temperature deformation quenching can improve the tempering resistance of steel, that is, the steel member subjected to low-temperature deformation quenching can maintain the deformation strengthening effect when heated to a higher temperature. Figure 15 is a graph showing the hardness-tempering temperature of 45CrMnSi steel after austenitizing at 950 °C, compression deformation at 535 °C of 30%, and then quenching. It can be seen from the figure that the shape-quenched steel can maintain a high hardness when heated to a higher temperature.
▼图15 45CrMnSi steel low temperature deformation quenching and general
Hardness-tempering temperature curve of pass quenched samples
1-low temperature deformation quenching 2-normal quenching
Some steels have secondary hardening after tempering after ordinary quenching, and the phenomenon of low temperature deformation quenching will disappear, which also indicates higher tempering resistance after low temperature deformation quenching.
3.2 Mechanical properties of low temperature deformation hardened steel
a. Tensile properties. Under normal circumstances, low temperature deformation quenching can increase the strength by 300-700 MPa compared with ordinary quenching.
Low temperature deformation quenching can not only improve the mechanical properties of steel at room temperature, but also improve its high temperature performance. See Figure 16.
â–² Figure 16 Vasco MA steel low temperature deformation quenching
And high temperature instantaneous tensile strength of ordinary quenching
(â—-91% deformation quenching, 550 °C â—‹-normal quenching, 580 °C tempering)
b. Impact toughness. At present, there is no consensus on the influence of low temperature deformation quenching on the impact toughness of steel. Some experimental results show that low temperature deformation quenching can improve the impact toughness of some steels. Some experimental results show that there is no effect, and some experimental results are just the opposite. In the range of ≤0.4% C (wt), the low-temperature deformation quenching impact value of steel is generally lower than that of ordinary quenching.
c. Fatigue performance. In general, the fatigue performance limit of steel decreases as the static tensile strength of steel increases. When the tensile strength is less than 1000 MPa, the ratio of fatigue limit to tensile strength is between 0.5 and 0.6. When the tensile strength reaches 1500 MPa, the ratio of fatigue limit to tensile strength is reduced to 0.3-0.4. When the tensile strength is in the ultra-high strength state of 2000 MPa, the ratio of the fatigue limit to the tensile strength is only 0.3. It is generally considered that the relationship between the fatigue limit σ-1 and the tensile strength σb and the reduction ratio of the section is σ-1 = ψσb, and the low-temperature deformation quenching can increase the strength of the steel while making the plasticity index substantially unchanged. Therefore, the final performance is that the fatigue strength has been improved accordingly.
d. Delay the tendency to fracture. For high-strength steels with a strength of 1200 MPa or more, the stress caused by the static load in the medium containing H2 is below the yield strength, and suddenly breaks off after a certain loading time. This is the delayed brittle fracture phenomenon. Low temperature deformation quenching can significantly improve the delayed brittle fracture performance of steel.
e. Fracture toughness. The effect of low temperature deformation quenching on fracture toughness is rather disorderly. It is difficult to make a consistent conclusion because of the difference.
f. Anisotropy. The mechanical properties of low-temperature deformation hardened steel are directional, especially the plasticity toughness index is obvious, and the transverse direction is lower than the longitudinal direction.
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