Band segregation will affect the fatigue life of steel used for diamond saw blade substrate
In the production of high carbon spring steel by continuous casting, sometimes negative segregation band (white bright band) appears, which is mainly caused by improper electromagnetic stirring or unstable cooling process. Separation crystallization is inevitable during solidification. The segregation coefficient K value of carbon in iron is 0.13, so it can be calculated that the carbon content CS on the dendrite is 0.0715% when the liquid steel with carbon content of about 0.55% begins to crystallize; under the effect of electromagnetic stirring, the dendrite will be cut off, part of the cut dendrite will be dissolved, and the rest of the dendrite will be stirred and swirled to form crystal nucleus near the center, so that part of the liquid steel will solidify first Negative segregation is formed; on the other hand, in the continuous casting process, it is impossible to completely eliminate the center looseness and segregation, because there is always a periodic dendrite bridging phenomenon in the solidification process of the continuous casting slab, and the negative segregation is also formed at the bridging place due to the preferential crystallization.
Due to the low carbon content in the negative segregation part, it is white and bright lines on the test pieces of sulfur imprint and acid dip, which may still remain after hot processing. Due to the deformation, the hot-working section appears as a narrow white strip. When the spring steel with banded segregation is quenched and heated, the carbon atoms around the banded ferrite will diffuse to the interior in the austenite phase. However, due to the short heating and holding time, and the blocking effect of Si and s segregation on the carbon atoms, the carbon concentration in the original negative segregation banded ferrite is still lower than the carbon content in the surrounding structure. After quenching, the carbon content of banded segregation is lower than that of surrounding martensite, and after tempering, the white bright banded tempered sorbite with lower carbon content is formed.
The existence of banded segregation in the raw material will seriously affect the fatigue life of the steel for diamond saw blade. This is because under cyclic load, the free band structure in the steel for diamond saw blade matrix is easy to produce fatigue crack source due to its relatively low strength. When the crack expands to a certain extent under the action of external force, the saw blade matrix will fail.