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Analysis of several common causes of cracks in diamond saw blade

At present, the steel for the diamond saw blade base is mainly medium and high carbon spring steel. The non-metallic inclusions, uneven structure, microcracks and surface defects in the medium and high carbon spring steel materials can become the possible source areas of fatigue cracks, and promote the saw blade base. Early failure occurred. Non-metallic inclusions, surface defects and ribbon segregation in medium-high carbon spring steel are the main factors affecting the service life of the saw blade substrate. The surface of the diamond saw blade has pits, defects or sub-surfaces with densely distributed carbide particles or Banded segregation, which acts as a source of fatigue to promote fatigue failure of the saw blade substrate. In addition, due to the low fracture toughness of the material, this causes the saw blade matrix to undergo fatigue fracture prematurely.

1.1 Non-metallic inclusions

The non-metallic inclusions in the steel for the diamond saw blade base are mainly inclusions such as Al2O3 and TiN produced during the smelting process. Their influence on fatigue performance depends on the type, quantity, size, shape and distribution of inclusions on the one hand; on the other hand, the large-sized brittle inclusions and spheres are weakly bound by the microstructure and properties of the saw blade steel. Non-deformed inclusions are the most harmful. Moreover, the higher the strength level of the medium and high carbon spring steel, the more harmful the inclusions have on the fatigue limit.

1.2 Surface defects

There are three main types of surface quality problems: one is obvious rolling defects, folding and ear defects, and some scratches and bumps, mainly due to outdated rolling equipment, backward finishing facilities and inadequate hole design adjustment. In addition, the surface of the blank is improperly ground, and sharp edges and pits are scratched, and folding defects are easily formed after rolling. Second, surface cracks are longitudinally continuous or intermittently distributed on the surface of the steel, mainly due to the blank. Caused by residual cracks and subcutaneous defects, surface stress cracks may occur due to improper stress and cooling of the emulsion; thirdly, surface scratches and buckling, which are related to tooling conditions and improper operation, and may also cause scratches during packaging and transportation. . Their existence must be the origin of the material failure, which is easy to directly lead to material fracture. However, for the small defects such as pits, scratches, suede and pitting, people generally do not pay much attention to them. Although their existence is allowed by the standard, they will not become the main cause of failure, but their existence is certain. It is the weak part of the material, and they will also become the breakthrough point of cracking when the overall plasticity of the material is not good. This factor is often overlooked in failure analysis because small defects have been destroyed during failure or when specific locations have not been verified during sampling.

1.3 band segregation

In the production of medium-high carbon spring steel by continuous casting, a negative segregation zone (white bright band) sometimes occurs, and negative segregation is mainly caused by improper electromagnetic stirring or unstable cooling process. Selective crystallization is inevitable during solidification. The segregation coefficient K of carbon in iron is 0.13, and it can be calculated that the carbon content Cs of the dendritic dry on the molten steel when the carbon content is about 0.55% starts to be 0.0715%; under the action of electromagnetic stirring, the branches The crystal will be cut, a part of the cut dendrites will be dissolved, and the remaining part of the dendrites will be gradually swirled to the center to form a crystal nucleus, so that part of the molten steel will solidify first to form negative segregation; on the other hand, in the continuous casting process In the middle, the center looseness and segregation cannot be completely eliminated, because there is always a periodic dendritic bridge phenomenon in the solidification process, and negative segregation is formed at the bridge due to preferential crystallization.

Negative segregation, due to its low carbon content, is a bright white line on the sulphur and acid immersion low-power test strips and may remain after thermal processing. Due to the deformation, after hot working, it appears as a white strip with a narrow width on the cross section. In the spring steel with band segregation, in the austenite phase region, the surrounding carbon atoms of the band ferrite will diffuse to the inside during quenching heating, but the Si and S segregation to carbon atoms due to the short heating and holding time The obstruction effect, the concentration of carbon atoms in the original negative segregation band ferrite is still lower than the carbon content of the surrounding tissue. After quenching, the banded segregation structure forms a martensite structure with a carbon content lower than that of the surrounding, and after tempering, a white bright band-shaped tempered sorbite having a carbon content lower than that of the surrounding structure is formed.

The presence of banded segregation in the raw material can seriously affect the fatigue life of the steel for the diamond saw blade base. This is because under the cyclic load, the free band structure in the steel for the diamond saw blade base is easy to generate a fatigue crack source due to the relatively low strength. When the crack spreads to a certain extent under the external force, the saw blade base will Invalid.

On the other hand, the band segregation causes the in-band crystal grains with low alloy content to be easily roughened during the heating process to form coarse crystal grains; a large amount of fine alloy carbides are present in the belt with high alloy content, which hinders the grain. Grow up to get fine grains. When quenching and heating, the carbon atoms around the band-like ferrite diffuse into the band, but the heating temperature is not too high, the heating time is short, and the Si source is blocked, the austenite of the original ferrite segregation zone Failure to achieve uniformity with the surrounding carbon atoms, resulting in a martensite transformation after quenching, resulting in large tissue stresses between the bands.

It is found that the microstructure stress in the martensitic transformation is closely related to the grain size. The coarse grains increase the tendency of quenching cracks. At the same time, during the quenching process, the non-metallic inclusions are segregated along the grain boundaries. Band-like distribution, such inclusions are soft and have poor bonding strength with the matrix, which destroys the continuity of the saw blade matrix, resulting in weaker grain boundary and poor coordination of large grains, resulting in concentration of grain boundary stress. Therefore, under the action of stress, the brittle phase and the band-like inclusions become the source of cracks, promote the formation of quenching cracks and rapidly expand. When extending to the vicinity of the longitudinal centerline, the deformation is constrained, thereby releasing stress in the form of cracks and inducing quenching. The generation of cracks.

2 cracks caused by unreasonable heat treatment process

In the production of the saw blade base, the unreasonable heat treatment process is the main cause of cracks in the diamond saw blade matrix. Inappropriate heat treatment processes are mainly characterized by severe oxidation and decarburization, large quenching deformation, overheating and over-burning, and temper brittleness. The heat treatment crack is mainly caused by the high heating temperature of the medium and high carbon spring steel and the long holding time, which makes the austenite grains coarsened in the microstructure, and the martensite needle is coarse after quenching, which leads to the increase of internal stress and brittleness of the spring steel. Thus, a heat treatment crack is formed, and the steel crack defect of the diamond saw blade base body caused by the heat treatment is generally characterized by an intergranular fracture under a metallographic microscope.

2.1 Oxidation and decarbonization

When the diamond saw blade substrate is heated in an oxidizing atmosphere such as air, the surface is easily oxidized. Oxidation increases the surface roughness of the saw blade substrate, and the precision decreases, and the scale on the surface of the substrate tends to be the source of quenching soft spots and quench cracking, which lowers the strength of the substrate. Therefore, under the premise of ensuring the transformation of the medium and high carbon spring steel during heating, the heating temperature should be as low as possible, and the holding time should be as short as possible.

The surface oxidation of medium-high carbon spring steel is generally accompanied by surface decarburization. Decarburization is a common surface defect of diamond saw blade matrix, which has a significant effect on the performance of the substrate. Decarburization of the surface of the spring steel by 0.1 mm will significantly reduce the fatigue limit. Moreover, as the depth of the decarburized layer on the surface of the substrate increases, the fatigue life decreases remarkably. In particular, the presence of ferrite in the decarburized layer on the surface of the saw blade base reduces the fatigue limit by 50%. Due to decarburization, the surface hardness of the substrate decreases, and cracks are easily generated under the action of alternating stress, which causes the diamond saw blade matrix to fail prematurely.

The higher the carbon content, the higher the heating temperature and the longer the holding time, the greater the decarburization tendency of the matrix, and the microstructure of the saw carbon matrix with different carbon content is different after decarburization with steel. When the diamond saw blade base is oxidized and decarburized with the steel surface, the hardness and strength of the surface layer are lowered, and cracks and cracks are easily generated. In addition, when the different parts of the surface layer are quenched, the expansion coefficient is different, causing stress concentration, resulting in micro-cracks in the transition zone between the fully decarburized layer and the partially decarburized layer of the saw blade, and these visible or invisible micro-cracks become stress concentration. The zone, and as the origin of the continued development of the crack, causes failure or fracture of the matrix. Therefore, the depth of the decarburization layer of the diamond saw blade substrate should be limited.

2.2 Quenching deformation

For diamond saw blades with medium and high carbon spring steel as the base material, the elastic limit and hardness index of the matrix should be achieved, and the heat treatment method should adopt quenching + medium temperature tempering. However, the quenching of the diamond saw blade base has a very prominent feature, that is, the quenching deformation amount and the cracking tendency are large.

The reasons for the deformation are: heating, whether it is air or salt bath, the convection of heat will more or less impact on the substrate and cause heating deformation. The temperature difference caused by the different cooling speeds of the various sections of the section causes the volume shrinkage of the steel to be uneven, which leads to the formation of thermal stress. In addition, during the quenching, the supercooled austenite transformation to the martensite is accompanied by a ratio The phase change deformation caused by the change of the volume caused by the change of the volume. Since the substrate is a thin piece, the area is large and the thickness is thin, and its diameter is from Φ300 to 2200 mm, and the thickness is generally only 2 to 10 mm. The heat treatment deformation and cracking of such a large sheet piece during quenching is a difficult point of heat treatment of the substrate. How to reduce the quenching deformation and cracking control to meet the flatness and hardness of the substrate is the key to the heat treatment of the diamond saw blade substrate.

2.3 Overheating and overburning

When the diamond saw blade substrate is heated by the fire, the defect that the austenite grains are coarse due to the excessive heating temperature or the long holding time is called overheating. The superheated saw blade base body obtains a coarse and thick needle-shaped martensite structure after quenching, which is a quenched superheated structure and is liable to cause quenching cracks. Therefore, the strength of the quenched and overheated saw blade base is lowered, especially the impact toughness and plasticity are remarkably lowered, and brittle fracture is apt to occur. A slight overheating can be remedied by extending the tempering time. Severe overheating can be done by full annealing or normalizing to refine the grain.

If the quenching heating temperature is too high, the phenomenon of local melting of the austenite grain boundary or grain boundary oxidation is called over-burning. Over-burning seriously deteriorates the performance of the diamond saw blade matrix, and it is easy to produce heat treatment cracks. Therefore, over-burning is an impermissible heat treatment defect. Once it has been burned, it cannot be remedied and has to be scrapped.

2.4 temper brittleness

Heat treatment (mainly tempering) brittleness is another major cause of cracking in medium and high carbon spring steels. The temper brittleness of medium-high carbon spring steel usually refers to the brittleness exhibited by heating or slow cooling in the critical temperature range of 350 ~ 550 °C. At this time, the impact transition temperature tends to be higher, and the brittleness is medium and high carbon spring steel. After tempering in the temperature range of 350~550 °C, the microstructure along the original austenite grain boundary and the segregation of impurities at the grain boundary reduce the impact value and KIC value at room temperature. The sensitivity of the medium carbon spring steel to temper brittleness is affected by the residence time, alloy composition and impurity level in the critical temperature range.

Medium to high carbon spring steels are sensitive to brittleness when heated for a long time in the temperature range of 350 to 550 ° C or slowly cooled in this temperature range. Because the medium-high carbon spring steel adds about 1%~2% of Mn, it has the tendency of overheating sensitivity and temper brittleness, and its tempering temperature range is just in the first kind of temper brittleness and the second kind of temper brittleness combination. At the same time, if it is not possible to temper, heat and cool in time, the spring steel will be brittle.

3 cracks caused by improper selection of quenching medium

When the spring steel has a carbon content of <0.20%, it generally forms lath martensite, which has good plasticity and strength. When the carbon content is 0.60%, acicular martensite is formed, which is hard and brittle. When the carbon content is between 0.20% and 0.60%, a mixed structure of two martensites is formed. With the increase of carbon content, medium-high carbon spring steel has more and more acicular martensite and less and less lath martensite. Obviously, after the carbon content of the carbon spring steel is increased, due to the poor plasticity of the acicular martensite, the microstructure stress generated during the martensite transformation will lead to quenching cracks, so slow cooling is required below the martensite transformation temperature. .

From the supercooled austenite transformation curve and the end quenching curve of various spring steels, the martensite formation temperature of the medium and high carbon spring steel is generally around 300 °C. Therefore, a suitable cooling medium should have a maximum cooling rate at the tip of the nose and a suitable cooling rate below 300 ° C so that the supercooled austenite does not form a pearlite type or a bainitic type of structure.

If the quenching medium is cooled too fast below the martensite start transition temperature, fine acicular martensite begins to appear and the quenching deformation increases sharply. Due to the different time of surface and core tissue transformation, martensite transformation occurs inside when the surface has formed hard martensite. Due to the large specific volume of martensite and volume expansion, the surface layer is greatly affected. Tensile stress, when the tensile stress exceeds the yield limit of steel, produces a longitudinal crack from the front to the inside. The curved crack appears in the groove of the workpiece, at the hole. The crack at this time is mainly caused by the structural stress accompanied by thermal stress and deformation (shear) stress.

Due to the long residence time on the cooling bed and the too fast cooling rate, many retained austenites in the steel are transformed into martensite below the Ms point. Due to the low thermodynamic conditions, the transformation speed is extremely slow, and the martensitic transformation belongs to In the non-diffusion type phase transition, the carbon atoms in the austenite are unable to diffuse, forming a supersaturated solid solution state, causing lattice distortion. When the tissue stress is greater than the yield strength of the spring steel, cracking occurs.

The quality of medium-high carbon spring steel is largely determined by the smelting process, including the chemical composition of the steel, the cleanliness of the molten steel (gas, harmful elements, inclusions) and the quality of the slab (component segregation, decarburization and its surface condition). These aspects are the key control points for smelting operations. In addition, spring steel also requires sufficient hardenability to ensure uniform microstructure and mechanical properties throughout the spring section.

4 Conclusion

(1) Non-metallic inclusions, surface defects and ribbon segregation in medium-high carbon spring steel are the main factors affecting the service life of the saw blade base, and also the main crack source during the use of the diamond saw blade base.

(2) Severe oxidation and decarburization, large quenching deformation, overheating and over-burning, temper brittleness, etc. are the main causes of cracks in the diamond saw blade matrix.

(3) According to the material of the diamond saw blade base and the original organization and technical requirements, combined with the condition of the equipment and other factors, the quenching medium and the quenching method are correctly selected, and the intrinsic quality of the diamond saw blade base is regularly monitored and adjusted. Subsequent processing is critical.