Raw material defects mainly include surface cracks, surface folding, surface decarburization, surface coarse crystal rings, residual shrinkage cavities and inclusion defects, etc.
There are cracks on the surface of the raw material. If the cracks are not removed after the fastener is processed or the crack expands during the molding process, it will cause cracks on the surface of the fastener;
Longitudinal cracks in rod
Fastener raw material bar, due to improper rolling process in steel mills, causes longitudinal cracks extending along the rolling direction on the surface of the bar. This type of crack is generally linear, and the direction of the crack is basically consistent with the main deformation direction of rolling. There are many reasons for longitudinal cracks in raw materials. For example, there are scratch defects on the surface of the rolling blank. Due to stress concentration, cracks along the scratches occur during cooling. Another example is that the internal defects of the raw material are deformed along the direction of rolling or streamline and exposed on the surface of the bar, and crack under the action of stress.
Transverse cracks in rods
Fasteners are made of raw material rods. Due to improper cutting process in steel mills or improper extrusion process parameters, transverse cracks appear on the surface of the bar head at the cutting point. Transverse cracks on the surface are more harmful than longitudinal cracks. big.
internal cracks
Internal cracks in raw materials are also a common defect. Generally, the cracks are tortuous, tapered at the end, and partially bifurcated. If there is decarburization on both sides of the crack and there is an oxide layer in the crack, it means that the crack already existed before heat treatment.
In most cases, internal cracks are related to internal defects of the material, such as looseness, inclusions, etc. Especially in the process of extruding bars, since the deformation of the center line is theoretically zero, the original defects inside the material are due to the extrusion of the outer surface. The stress is much greater than that at the core, resulting in uncoordinated deformation and further cracking.
Raw material surface folding
Fasteners are made of alloy rods. Due to improper production processes in steel mills, folds or even cracks occur on the surface of the rods. For fasteners, the main reason for folding is that during the rolling process of the metal blank, the groove on the roll is not accurately sized, or the burrs produced on the worn surface of the groove are involved, forming a certain difference with the material surface. Angle of folding.
Surface decarburization of raw materials
The surface decarburization of the raw material is mainly caused by improper annealing of the raw material during the drawing process in the steel plant. The decarburization layer on the surface of the raw material is distributed on the surface of the fastener material, and there is no decarburization on the machined end surfaces at both ends. During use, decarburization not only causes the overall strength of the fastener to decrease, especially because the decarburization layer on the surface has low strength, it is easy to produce surface cracks in the decarburization layer, causing early fracture failure of the fastener. Especially for threads, surface decarburization leads to a significant reduction in thread strength, and failure phenomena such as tooth loss and tripping are prone to occur.
Coarse crystal ring on the surface of the raw material
Coarse grain rings on the surface of raw materials are defects that often occur in aluminum alloy and magnesium alloy extruded rods. They also appear in steel and other materials. The thickness of the coarse-grained ring shows a gradual increase from the initial end to the final end during extrusion. If the lubrication conditions during extrusion are good, the presence of coarse grain rings can be reduced or avoided after heat treatment. When the coarse-grained ring on the surface of the raw material causes the coarse-grained ring on the surface of the fastener, the performance of the fastener will be significantly reduced. Especially at the interface between the coarse-grained ring and the fine-grained ring, the difference in strength will lead to uncoordinated deformation and cracks. .
When there are coarse grains on the surface of the raw material, the surface roughness of the fastener will become worse during cold extrusion or moulding. In severe cases, it will appear like an orange peel, which is very important in the fastening of low-strength materials such as aluminum alloy or magnesium alloy. The most obvious part is that orange peel is essentially a network of micro-cracks along the grain boundary on the surface. During use, the micro-cracks expand and become macro-cracks. The main reason for the formation of coarse grain rings is improper cold drawing deformation of raw materials. For example, in the case of high-temperature alloy cold-drawn bars, the last cold-drawn deformation before the finished product is improperly controlled, causing the deformation of the surface layer of the material to fall within the critical deformation zone. After solid solution heat treatment, the grains in the critical deformation zone grow to form coarse grains. crystal ring.
Residual shrinkage of raw materials
The residual shrinkage in the raw material is mainly caused by the steel mill not removing the cap of the steel ingot cleanly, which remains inside the raw material during opening and rolling. In areas with dense residual shrinkage cavities, defects such as inclusions, porosity, and segregation will generally appear. After deformation or processing of fasteners in raw materials with residual shrinkage cavities, holes and irregular internal cracks will appear in the core of the fastener from beginning to end. The expansion of cracks can easily cause the failure of the fastener TB3 titanium alloy The screws broke several times during assembly. Analysis showed that the screw fracture was due to the fact that the residual shrinkage holes in the raw material were not completely removed and were not welded during the forging process. Due to the multiple forging deformations, the screw cracked along the original β grain boundary at the defective location;
Raw material inclusion defects
Impurity elements in raw materials mainly come from the raw materials used in the smelting process, pollution from the atmosphere, furnace walls or crucible materials during the smelting process, alloy ingots or master alloys, and pollution from the parts casting process, etc. When inclusions are aggregated and distributed or distributed in a chain along the deformation direction during the forming process, it is easy to cause the fastener to crack along the inclusions during processing and use.
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