In the process of heating and heat preservation of the steel used in the forging, the surface of the steel is lost by the surrounding oxidizing atmosphere, and all or part of the carbon is lost. This phenomenon that the carbon content on the surface of the steel becomes low is called decarburization. The area that causes the carbon content of the steel surface to decrease is called the decarburization layer, and the depth of the surface layer carbon content is called the depth of the decarburization layer.
The decarburization layer is further divided into a full decarburization layer and a partial decarburization layer, and the total depth of the decarburization layer is the sum of the total decarburization layer and the partial decarburization layer.
The decarburization of the steel used in the forging directly affects the properties of the material, so that the surface hardness is lowered, the wear resistance is deteriorated, and the surface of the forging and the core structure are also different, resulting in defects of quenching cracks.
The decarburization depth test of the steel used for forgings is evaluated by the "Decarburization Depth Determination Method for Steel" and "Decarburization Depth Determination Method for Steel".
The standard specifies the depth of the decarburization layer of the steel used for the forgings using the metallographic method, the hardness and the carbon content. The choice of method and its accuracy depend on the degree of decarburization, microstructure, carbon content, and forging shape.
Forgings are not allowed to intercept high-magnification metallographic samples for inspection. Non-destructive metallographic examination can be carried out, that is, directly selected on the workpiece according to the technical conditions, followed by rough grinding, fine grinding, polishing, etc. The above-described tissue display method shows the tissue, and a large-scale workpiece metallographic microscope is placed on the forging to inspect the polished surface.
The shell-like fracture is characterized by a smooth strip that is unequal in length, dark gray or bright gray, and separate in the longitudinal direction of the steel ingot parallel to the axial direction of the ingot. Flat, some are curved structures, similar to shells. The small shell-like fracture resembles a beef-like shape, so some people call it a "beef-like" fracture.
Microscopically, there are ductile fractures or mixed intergranular fractures along the columnar grain boundary, and there are fine inclusions, sulfides and element segregation at the crystal interface along the fracture of the crystal pit. The metallographic phase of the forgings showed that the corrosion products of the aluminum nitride precipitation phase were intermittently distributed along the primary grain boundary.
Generally, the position of the shell-like fracture is consistent with the position of the columnar crystal, but other parts of the ingot sometimes appear. The main reason for this is that during the solidification process, many inconspicuous inclusions are deposited along the primary columnar crystal grain boundary and the element segregation is heavier at the columnar crystal interface. It reduces the high temperature toughness of the steel ingot, weakens the connection of the grain boundaries, is prone to intergranular cracks, and forms a shell-like fracture.
Non-metallic inclusions are common inclusions in metallic materials. They are non-metallic compounds present in metallic materials and can be classified into endogenous inclusions and external slag inclusions depending on their source.
Endogenous non-metallic inclusions are inclusions formed during the physical and chemical reaction of steel during smelting and casting, and inclusions formed during the solidification process due to changes in the balance between components, and are generally referred to as deoxidation products. Common sulfides, oxides, nitrides, silicates, and the like. The amount and composition of these inclusions in the steel is related to the composition of the steel, the quality of the smelting, the casting process and the degassing process.
Endogenous non-metallic inclusions are small in size, relatively dispersed, and are microscopic defects. However, in special cases, a densely integrated stack or a broadband distribution constitutes a macroscopic defect. When the ingot is forged and deformed, the plastic inclusions are distributed in a spindle-like or strip-like shape along the direction of metal deformation, and the brittle inclusions are easily broken. The particles are distributed along the direction of deformation of the steel, and some inclusions are present. Distributed along the grain boundaries in a continuous or intermittent form.