1 Introduction Recently, the main shaft parts of several types of shearer in our factory have cracks after carburizing or carburizing and quenching, which is almost scrapped in batches, which seriously affects the production schedule. The shaft gear processing process is: forging normalizing machine processing carburizing high temperature tempering quenching and tempering. The technical requirement is that the depth of the layer is 1.51.8mm and the surface hardness is 5862HRC.
2 Physical and chemical testing 2.1 Chemical composition analysis Chemical composition analysis of cracked shaft gears, the results (mass score) see, in line with the technical requirements of GB/T3077-1999.
2.2 Macroscopic inspection Each cracked shaft gear has a longitudinal through crack. The shape of the cross-section crack is a wedge-shaped split. When the crack propagates from the surface in the longitudinal direction, it expands toward the inside of the section along the direction of the vertical surface, and forms a crack of the outer wide tip, and finally terminates near the center of the section. The fracture is granular, has no plastic deformation, and is partially layered fracture, which is a typical brittle fracture. From the longitudinal gold 2.3 microscopic test, from the anatomical test results of multiple cracked shaft gears, no mesh and strip carbides were found in the carburized layer. The depth of the carburized layer was 1.61.8 mm. There was a carburized layer in the longitudinal section. A large number of strips of retained austenite with distributed banded and granular carbides. There is a distinct longitudinal distribution of banded structure (and sulfide inclusions at 1/4 diameter. The banded structure is rated 3 according to GB/T13299-1991B series standard, and non-metallic inclusions are according to GB/T10561-1989 series. The ASTM standard is rated at 225. Samples are taken from the 1/4 diameter of the raw material. The results are as follows. The banded structure is rated 4 according to the above criteria. This indicates that the microstructure of the material, especially the lateral structure, is seriously deficient, especially carburizing. After the more obvious.
3 Results Analysis and Discussion According to the above test results, the shaft gear crack is a typical quenching crack.
The quenching longitudinal crack is the most sensitive with slender rods. The single deep crack and the wedge-shaped crack are the most characteristic basic forms of the longitudinal crack, and the hardening is a necessary condition for the formation of the longitudinal crack. The longitudinal crack is always formed from a weak link somewhere on the outer surface, and then expands in the longitudinal direction into a relatively straight single deep crack.
The reason is determined by the fact that the shear stress value of the tissue stress is large and the transverse mechanical properties of the shaft gear are relatively low.
(1) When the steel material is hot-rolled or forged, there is an enrichment zone and a depletion zone due to uneven distribution of alloying elements due to dendrite segregation, and it extends in the deformation direction during the deformation process. Since the diffusion and homogenization of alloying elements are much slower than that of carbon atoms, the banding segregation distribution of such alloying elements is not easily eliminated, and the decomposition of austenite and depleted austenite between alloying elements is decomposed. There are considerable differences in the laws of learning. In the case of carbon steel, the austenite with less alloying elements causes the carbon atoms in the region to be displaced to the rich alloying element region at the higher temperature due to the first decomposition of part of the ferrite at a higher temperature, thereby continuing to cool. After that, the alloy element-rich region forms a pearlite-based band structure, and the alloy element depletion region forms a ferrite-based band structure. If it is alloy steel, especially 18Cr2Ni4WA high alloy steel, the diffusion energy of chromium, nickel and other elements is very large, the diffusion coefficient is small in austenite, the homogenization is difficult, and the elements such as chromium and tungsten are strong carbide elements. It has a great affinity with carbon and it is more difficult to diffuse in austenite. Due to the increase of carbon concentration after carburizing, the stability of austenite in the alloy element-rich region is much higher than that in the depleted region. Whether it is carburizing or quenching, a large amount of band-like residue is generated in the layer. Austenite.
(2) The difference in composition of the carburized workpiece on the cross section changes the phase change sequence during quenching, and the heat generation stress generation process is more complicated. Due to the carbon concentration gradient in the carburized layer, the Ms point increases with the decrease of the carbon content in the layer. When quenching, the martensite starts at the inner layer with higher Ms point, and the surface temperature drops below the Ms point. The martensite transformation occurs in the surface layer, and the inner martensite transformation has ended.
When the surface high carbon martensitic transformation is accompanied by a larger volume expansion but is hindered by the inner layer, a large pressure is generated on the surface layer, and the core is a residual stress distribution of the tensile force. It is because of the above-mentioned residual stress distribution characteristics that carburizing and quenching parts are generally not susceptible to various types of cracks.
However, in fact, the distribution and size of the residual stress of the carburized quenching parts are sensitive to various conditions, such as the layered carbide and the non-martensitic structure, especially the banded retained austenite. Due to the large amount of retained austenite in the layer after carburizing and quenching of alloy steel, the volume expansion of martensite transformation is greatly reduced, so that compressive stress cannot be generated in the surface layer to offset or reduce the transformation caused by the transformation of the inner layer martensite. The tensile stress changes the stress distribution state of the surface layer, so that the surface is in a tensile stress state, and the hardenability of the alloy steel is larger, and the lower the Ms point, the greater the degree of the stress distribution state of the surface layer is changed.
(3) It is precisely because of the existence of surface band-shaped retained austenite that the strength of the shaft gear surface is weakened, and more importantly, the isochronism and uniformity of the structural transformation and the favorable stress distribution on the surface are changed. Or during quenching, due to the unequality of martensite transformation in the inner and outer layers of the shaft gear, the surface layer is firstly cracked at the weak point of the surface under the expansion caused by the transformation of the martensite in the inner layer, and along the belt. The longitudinal direction of the sulphide and sulfide non-metallic inclusions is cracked to the heart.
4 Conclusions (1) The shaft gear hot working process is reasonable and the carburizing quality is good.
(2) Due to insufficient hot rolling and forging of the material, dendritic segregation of the alloying elements is severe, resulting in obvious axial banding of the shaft gear and serious non-metallic inclusions.
(3) Due to the presence of the band-shaped residual austenite in the surface layer, not only the strength is weakened, but also the tensile stress on the surface of the gear is increased. Therefore, firstly, the surface of the gear is cracked, and the band structure of the core and the non-metallic inclusion are accelerated. The expansion of the crack finally leads to the quenching of the shaft gear.
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