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Transformation of hardened carbon steel during vacuum tempering

Vacuum quenching is a very important process in the processing of metal materials and is widely used in modern machinery manufacturing industry. Martensite and retained austenite are the two major components of the steel after vacuum quenching. Martensite and retained austenite, which have the tendency to spontaneously transform into ferrite and cementite equilibrium structure, are not stable, while quenching The vacuum tempering of steel is a method to promote the transformation of martensite and retained austenite into a balanced structure of ferrite and cementite. We call this transformation tempering transformation.

Martensite and austenite in quenched steel are the structures with the largest and smallest specific volume, respectively. The volume change will accompany the entire transformation process, and from this volume change, the phase transition during vacuum tempering can be understood. For example, when the martensite transforms, the volume of the steel will decrease; when the retained austenite transforms, the volume of the steel will increase. Therefore, when there is an obvious volume change, it must be caused by the transformation.

When the temperature is less than or equal to 100 ℃ vacuum tempering, the volume of the steel does not change. It is proved by X-ray analysis that only the segregation of carbon atoms in the martensite occurs at this time, and the decomposition does not start, which proves that there is no obvious transformation in the quenched steel. When vacuum tempering at 100℃～200℃, the first transformation of vacuum tempering (the first stage of tempering) occurs. During this phase, the volume of the steel shrinks. In this temperature range, it can be seen from the X-ray analysis that the martensite begins to decompose due to the reduction of squareness, and the supersaturated carbon atoms dissolved in the martensite are desolubilized and precipitated to precipitate ε carbides (the crystal structure is Orthorhombic lattice, molecular formula Fe2.4C), this ε carbide maintains a coherent relationship with martensite. ε carbide is not an equilibrium phase before the transformation to Fe3C, but a transition phase. At the same time, the carbon in the martensite is not fully precipitated due to the low temperature, so that they still contain saturated carbon. Therefore, after the first transformation of tempering, the structure of the steel is now composed of two parts, which are supersaturated α solid solution and ε carbide connected with the parent phase lattice. This carbide maintains a coherent relationship with martensite, and tempered martensite refers to the structure of the steel after the first transformation of vacuum tempering. At this time, the carbides coherently connected with the parent phase of the supersaturated solid solution are extremely small, the number of carbides precipitated by them is large, and the effect of dispersion hardening is large, so the hardness of the steel will not be reduced after vacuum tempering, and it is not suitable for The hardness of eutectoid and hypereutectoid carbon steels also increased slightly.

After the first transformation, the volume of the steel expands again when it is heated to 200°C to 300°C. Mainly because the retained austenite with the smallest specific volume is decomposed in the steel structure. The second transformation of tempering or the second stage of tempering refers to the decomposition of the residual austenite in the vacuum quenched carbon steel from 200 ° C to 300 ° C and the decomposition is completed, and generally transformed into lower bainite. At the end of the second transformation of tempering (about 300 ° C), the α solid solution still contains about 0.15% to 0.20% C. In the second stage of tempering, the continued decomposition of martensite will reduce the hardness of the steel, but the simultaneous decomposition of soft retained austenite into harder lower bainite can compensate for the reduced hardness, so the reduction in hardness of the steel does not Significantly.

If the supersaturated carbon continues to precipitate from the α solid solution, and the ε carbide is gradually transformed into Fe3C, it needs to continue to be heated until it ends at 400 °C, resulting in the volume of the steel shrinking again. This transformation is called the third transformation or tempered third stage. Obviously, because the supersaturated carbon is precipitated from the α solid solution, and a stable Fe3C phase that has no lattice connection with the parent phase appears, the internal stress is largely eliminated. After the third transformation, the steel is composed of ferrite and cementite. Continue to increase the temperature, and the fourth transformation of tempering occurs, at this time, the cementite particles will aggregate to obtain a coarser structure. In the fourth stage of tempering (temperature over 400 °C), the carbon content of the α solid solution has dropped to the equilibrium concentration. At this time, the body-centered cubic lattice changes from the body-centered cubic lattice to the body-centered cubic lattice, and the internal substructure undergoes recovery and recrystallization. , the resulting solid solution strengthening has completely disappeared. The hardness and strength of steel are positively related to the size and dispersion of cementite particles. The higher the vacuum tempering temperature, the smaller the dispersion, the lower the hardness and strength of the steel.

In summary, the transformation of quenched carbon steel during vacuum tempering roughly includes four stages, namely: martensite decomposition; retained austenite transformation; carbide aggregation and growth; α solid solution recovery and recrystallization.

The structure formed in each stage of carbon steel vacuum tempering is roughly as follows:

(1) Tempered martensite

When the high carbon quenched steel is tempered in a low temperature vacuum at 150 to 250 °C, the mixed structure of tempered martensite, retained austenite and lower bainite is obtained due to the precipitation of ε carbide and the partial decomposition of retained austenite. Mainly tempered martensite. Observed under an electron microscope, it can be seen that the tempered martensite maintains a flaky shape with fine ε carbides distributed on it.

After vacuum quenching and low-temperature vacuum tempering of medium carbon steel, a mixed structure of lath-like and flaky martensite is obtained; after vacuum quenching of low-carbon steel, a low-carbon lath-like martensite structure is obtained, no matter from vacuum tempering or low temperature. Vacuum tempering, its shape remains unchanged, because only the segregation of carbon atoms and no carbide precipitation.

(2) Tempered troostite

The structure obtained by vacuum tempering in the temperature range of 350℃～500℃ is tempered troostite, and its cementite is granular. Microstructure of steel containing 0.90% C after vacuum quenching at 815℃ and then vacuum tempering at 425℃. Vacuum tempered at 425°C for 1 minute, there are also longer pieces of cementite. After vacuum tempering at 425°C for 1 hour, the cementite has been aggregated and spheroidized into particles.

(3) Tempered sorbite

The microstructure obtained by vacuum tempering in the temperature range of 500℃～600℃ is tempered sorbite, and its cementite particles are thicker and less dispersed than tempered troostite.

Compared with the general structure, the vacuum tempered structure has better performance. If the hardness is the same, tempered troostite and tempered sorbite have higher strength, plasticity and toughness than general troostite (oil quenched) and sorbite (normalized). This is mainly due to the difference in tissue morphology.

Selection of vacuum tempering equipment: In addition to good process design, the selection of vacuum heat treatment equipment is also an important factor in completing the process. The RVT vacuum tempering furnace produced by SIMUWU is an excellent choice for this type of process. Its process performance can fully meet the needs of such thermal processing, with good temperature control accuracy, temperature uniformity and tempering uniformity. High process repeatability, stable production, quality output can be guaranteed.