Improvement of heat treatment process of 42CrMo large ring forgings

In this paper, the problem of cracks and coarse crystals in large ring forgings of Φ9m 42CrMo steel after heat treatment was theoretically analyzed, and the heat treatment process was optimized and improved, and the organization and properties of the ring forgings treated by the improved process were analyzed. The results show that the microstructure of 42CrMo steel ring forgings can be refined to a certain extent by normalizing at 860℃, water quenching at 840℃ and tempering at 610℃, and the comprehensive mechanical properties are good, and the heat treatment quality is stable, which meets the technical conditions.

Large ring forgings are not only used for large wind turbine bearings and wind tower flanges, but also have a wide range of applications in petrochemical pressure vessels, port machinery crane slewing bearings, heavy gas turbine ring parts, nuclear power unit ring parts, large carrier rocket body, etc. The application field of large ring forgings determines its complex working service conditions, which requires forgings with good internal quality and performance. Figure 1 shows the schematic diagram of Φ9m ring forgings. This part is made of 42CrMo steel ingot with high hardenability, and the ingot is smelted by “converter (electric furnace) + LF refining + vacuum degassing” process to reduce the composition deviation and the content of gas and inclusions as much as possible.

Figure.1 Φ9m ring forging parts schematic diagram

Table.1 Chemical composition of 42CrMo steel (mass fraction, %)

Φ9m ring forging original heat treatment process problems and analysis

The original heat treatment process of Φ9m ring forgings is quenched after forging, and the process curve is shown in Figure 2. The surface of the ring forgings treated by this process shows cracks with a depth of about 10mm, and the internal organization is coarse and uneven. Figure 3 shows the surface cracks and internal microstructure of the ring forgings after the original heat treatment process. Table 2 shows the mechanical properties of Φ9m ring forgings after heat treatment by the original process, where the sampling position of “3/4 place” is 3/4 of the radial thickness (from outside to inside) on the cross section of the forging, and the sampling position of “heart” is the intersection of radial thickness and height on the cross section of the forging. Each 1/2 intersection.

Ring forgings internal organization is coarse, will reduce the mechanical properties of the material, can not meet the technical requirements, and once the large ring crack is difficult to repair, resulting in parts scrap. Because of the larger size of the ring forgings, so the larger the original ingot in the casting process crystallization process is slow, resulting in the casting organization is unusually coarse; and because it is difficult to be forged through the process, the internal organization still retains the coarse cast organization, and large ring forgings forging cycle is long, heating more times, the distribution of deformation is not uniform, in the high-temperature region of the insulation time is longer, the heart of the organization is still relatively coarse after heat treatment. After the raw material is rolled by hot pressing billet and diameter and axial ring rolling, the metal fibers are distributed in circumferential direction (i.e. longitudinal), and the longitudinal strength is higher than the transverse direction due to the influence of rolling force, grain size and metal flow direction, resulting in uneven distribution of properties. Due to the small cross-sectional size of the part, the quenching of the entire part of the organization from austenite to martensite transformation, resulting in a large tissue stress, and parts in the heating, cooling due to the large temperature difference and generate a large thermal stress, when the internal stress exceeds the tensile strength of the material, will produce quenching cracking phenomenon. Moreover, the large ring forgings are not treated with hydrogen removal after forging, which will lead to the existence of hydrogen embrittlement white spots inside the forgings and intensify the tendency of material cracking.

Figure.2 Original heat treatment process curve of Φ9m ring forgings

Figure.3 Surface cracks (a) and internal microstructure (b) of ring forgings after heat treatment of the original process

Table.2 Mechanical properties of different parts of 9m ring forgings

Heat treatment process improvement of Φ9m ring forgings

Post-forging heat treatment of Φ9m ring forgings

The post-forging heat treatment of the workpiece can also be called preparatory heat treatment, which is a very important process for large forgings before heat treatment. Large ring forgings after forging to prevent the generation of white spots, the need for hydrogen treatment, and post-forging heat treatment can achieve this purpose, and can eliminate forging stress, adjust and improve the large forgings in the forging process formed by overheating and coarse organization, reduce the internal chemical composition of the ring forgings and metallographic organization of the inhomogeneity, for the final heat treatment of ring forgings for organizational preparation.
Fe-C phase diagram can be obtained: 42CrMo steel Ac₃ critical point of 780 ℃, taking into account the role of Cr, Mo elements in steel, 42CrMo steel austenitization temperature is set to 860 ℃, as long as the forging inside and outside temperature uniformity, to obtain all austenitic organization can be.

Figure 4 for 42CrMo steel TTT diagram (Time Temperature Transformation), from TTT diagram can be seen, the ring forgings cooled to about 400 ℃ when holding a certain amount of time, due to 400 ℃ higher than the MS point, when the austenite will not be transformed into brittle martensite, but a good toughness of bainite. As bainite is a ferrite based organization, and bainite in a large number of dislocations, interfaces and other defects, conducive to the diffusion of hydrogen. At the same time, increase the temperature can accelerate the diffusion of hydrogen, that is, the ingot is heated to A₁ slightly below, that is, about 700 ℃ isothermal, holding a certain amount of time, with the furnace slow cooling, cooling to 150-200 ℃ after the furnace air cooling. Slow cooling can not only reduce the hydrogen content, to avoid the generation of white spots, but also can eliminate the tissue stress and thermal stress, improve the organization. According to the above analysis, the optimized post-forging heat treatment process curve of 9m ring forgings is designed as shown in Figure 5.

Figure.4 TTT diagram of 42CrMo steel

Figure.5 Improved post-forging heat treatment process for 9m ring forgings

Quenching treatment of Φ9m ring forgings

Figure 6 shows the tempering process curve of the improved Φ9m ring forgings. From Fig. 6, it can be seen that the heating process of ring forging increases several different temperature homogenization processes, which can reduce the temperature difference between the surface and the heart of the ring forging, thus reducing the thermal stress inside the material and reducing the chance of quenching and cracking of the ring forging, and can shorten the holding time of the material in the high temperature range, which has a certain promotion effect on reducing the grain size of the material. From the Fe-C phase diagram, it can be seen that the 42CrMo steel in the temperature range of 730-780 ℃, its organization began to gradually transform into austenite. Some research shows that 42CrMo steel in 750 ℃ uniform heat insulation, in the phase transformation of spherical austenite generation and growth is dominant, with the extension of time, the number of spherical austenite increased and stable growth. But in the early phase transformation, there are also a small amount of needle austenite generation, but with the extension of the insulation time needle austenite gradually by the stable growth of spherical austenite swallowed, and finally completely disappeared, and finally in the completion of the phase transformation, the coarse original austenite grains disappeared, the steel organization has been significantly refined. And in 650 ℃ after the heating rate than the original process is large, because at this time the material is already in a plastic state, try to improve the heating rate of the material, you can increase the material in the heating transformation when the superheat degree, in order to obtain a certain degree of tissue refinement effect.

Figure.6 Improved quenching process of 9m ring forgings
In the improved process, the workpiece was pre-cooled for a period of time before quenching and cooling. Because the quenching heating temperature of 840 ℃ is much higher than the austenitizing temperature of 780 ℃, pre-cooling can properly reduce the surface austenitizing temperature, which reduces the rate of martensite formation, reducing the martensite transformation process of tissue stress, which can reduce the chances of quenching and cracking, while pre-cooling can reduce the heat storage of the workpiece, increasing the depth of the hardened layer of the workpiece.
Ring forgings after quenching internal stress is very large, need to tempering treatment. At the same time, because the heart of the quenching is still not fully transformed after the austenite, the workpiece needs to be transferred to the tempering furnace and at low temperatures to maintain a certain time to continue cooling the heart, so large forgings should be tempered immediately after quenching, the intermediate stay is generally not more than 1h. To avoid the workpiece surface and heart temperature difference is too large, the heating rate of tempering treatment should not exceed 100 ℃ / h. Because 42CrMo steel is a material with Since 42CrMo steel is a material with high-temperature tempering brittleness, water cooling must be used when the tempering temperature is in the range of 275-500℃, and air cooling can be used when the temperature is lower than 275℃.

The organization and performance of Φ9m ring forgings after process improvement

Microstructure analysis

The microstructure specimens are firstly ground and polished, and the internal organization is observed after corrosion by etching solution; then the specimens are put into saturated aqueous solution of picric acid plus detergent, and the grain size is observed under metallographic microscope after constant corrosion at 60℃ for a long time.

Figure 7 shows the microstructure of 42CrMo steel ring forgings after heat treatment. From Figure 7, it can be seen that the surface hardened layer of 42CrMoA steel after tempering is tempered sothenite (Figure 7(a)) which maintains the martensitic orientation, and the organization of bainite characteristics appears in 3/4 (Figure 7(b)), the organization is sothenite + bainite and a small amount of pearlite organization, the heart is coarse tempered sothenite + upper The core is coarse tempered sothite + bainite and a small amount of ferrite (Fig. 7(c)).

Figure.7 Microstructure of 9m ring forgings after improved process (a) surface; (b) 3/4 place; (c) core

Mechanical properties test

The specimens were intercepted from the normalized + tempered 42CrMo steel ring forgings by flame cutting method for tensile test and impact test, and the hardness test was carried out directly on the ring forgings, the results are shown in Table 3. -2007 “Charpy pendulum impact test method for metallic materials”, GB/T231-2009 “Brinell hardness test method for metallic materials”. The impact specimen is a V-notch specimen with an angle of 45°, a depth of 2mm and a radius of curvature of 0.25mm at the bottom. Hardness tester model 320HBS-3000 digital display Brinell hardness tester, its steel ball indenter diameter of 10mm, loading load weight of 3000kg, holding time of 10-15s. Table 3 data show that, after the improved heat treatment process of 42CrMo steel ring forgings, the mechanical properties of the surface of the workpiece to the heart is decreasing trend, of which the yield strength of the most significant decay.

Table.3 Mechanical properties of different parts of Φ9m ring forgings

Conclusion

A plant production of 42CrMo steel large ring forgings, after 860 ℃ forging normalized, 840 ℃ water quenching and 610 ℃ tempering process improvements, to obtain a good overall mechanical properties and ideal metallographic organization, and the material by normalizing and quenching two austenitization, after several times recrystallization, also make the ring forgings of the organization and grain refinement to a certain extent, the heat treatment quality is stable and reliable. Meet the technical requirements.
Author: Dai Yutong