Common defects of fastener raw materials

InFastenersIn the manufacturing process, due to unreasonable design, improper material selection, improper forming process, heat treatment and surface treatment process, fasteners produce surface or internal defects, which cannot meet the quality requirements. Then, let's learn about the common defects of fastener raw materials!

Common defects caused by the quality of fastener raw materials:

The defects of 1. raw materials mainly include surface crack, surface folding, surface decarburization, surface coarse crystalline ring, residual shrinkage cavity, inclusion defects and so on.

1. Surface crack defects of raw materials

If there are cracks on the surface of the raw material, the cracks cannot be eliminated after the fastener is processed, or cracks appear during the molding process, cracks will appear on the surface of the fastener.

1) Longitudinal crack of bar

Longitudinal cracks extending along the rolling direction will occur on the surface of fastener blank bars due to improper rolling process in steel mills. This crack is generally linear, and the direction of the crack is basically the same as the main deformation direction of rolling. Rolling raw material surface scars and other defects, due to stress concentration, cooling along the scar rupture, raw material longitudinal cracks for many reasons. In addition, the defects inside the raw material are deformed in the direction of rolling, that is, in the streamline direction, exposed on the surface of the bar, and cracked due to stress. The following figure shows the longitudinal crack morphology on the surface of the superalloy bar raw material.

2) Transverse crack of bar

Due to improper head cutting process or improper extrusion process parameters in steel mill, surface transverse cracks appear on the surface of bar head cutting. The following figure shows the pattern of the surface transverse etc. of the superalloy bar. Transverse cracks on the surface are more harmful than longitudinal cracks.

3) Internal division

Internal cracks in raw materials are also common defects. Generally, there are cracks and snakes, with sharp tails and local bifurcations. If there is decarburization on both sides of the crack and the crack has an oxide layer, it means that there is a crack before heat treatment. The following figure shows the transverse and longitudinal section metallographic morphology of the internal crack of the bolt blank.

Internal cracks are mostly related to internal defects of the material, such as porous, inclusions, etc. Especially in the process of extruding bars, the deformation of the center line is theoretically zero. Therefore, the original defects in the material are deformed due to the extrusion stress on the outer surface. It is much larger than the core, resulting in mismatch of deformation and further cracks.

2. Material surface broken

Due to the unreasonable production process technology of alloy bars for fasteners, the surface of the bars will break and crack. In the case of fasteners, the main reason for the formation of the fold is that the metal blank is inaccurate in the sizing of the die groove on the rolling roll, or the burr generated on the wear surface of the die groove is involved, forming a fold with a certain angle to the surface of the material.

3. The decarburization of the raw material surface is mainly due to the improper annealing of the raw material during the drawing process of the steel plant. The decarburization layer of the raw material surface is distributed on the surface of the fastener material, and the machined end faces at both ends are not decarburized. In use, decarburization will not only lead to the overall strength of the fastener, especially the low strength of the surface decarburized layer, so the decarburized layer is easy to produce surface cracks, causing early fracture failure of the fastener. Especially for screws, surface decarburization will cause the strength of the screw to be significantly reduced, and failures such as tooth loss and button loss are prone to occur. The decarburized structure of the thread surface is shown in the figure below. Coarse crystalline rings on the surface of raw materials

4. The coarse crystalline ring on the surface of raw materials is a frequent defect in magnesium-aluminum alloy and magnesium alloy extruded bars, and also appears in steel and other materials. The thickness of the coarse crystalline ring has a characteristic of gradually increasing from the beginning to the end at the time of extrusion. If the lubrication conditions during extrusion are good, the presence of coarse crystalline rings can be reduced or avoided after heat treatment. When the coarse crystalline ring on the surface of the raw material causes the coarse crystalline ring on the surface of the fastener, the performance of the fastener is obviously reduced, especially on the interface between the coarse crystalline ring and the fine crystalline ring, the deformation mismatch caused by the different strength causes the crack. As can be seen from the figure below, the coarse crystalline ring on the surface of a bolt shaft, the interface between the coarse crystalline ring and the normal tissue area is very obvious, and cracks are very easy to occur at this interface.

When the surface of the raw material has coarse crystallization, the surface roughness of the fastener during cold extrusion or span molding is also poor, and the surface roughness is orange peel-like in severe cases. This is more obvious in the fasteners of low strength materials such as magnesium aluminum alloy and magnesium alloy. Orange peel is essentially a mesh microcrack along the grain boundary on the surface, and the microcrack will develop into a macro crack when used. The main reason for the formation of coarse crystalline rings is the improper cold stretching deformation of the raw material. For example, the cold drawing deformation of the cold drawing bar of the high-temperature alloy is not properly controlled at the end of the cold drawing before completion, the deformation of the surface of the material enters the critical deformation area, and the grain growth in the critical deformation area after the solid solution heat treatment forms a coarse crystalline ring.

Defects in 2. raw materials

The impurity elements in the raw materials mainly come from the raw materials used in the material smelting, the atmosphere in the smelting, the pollution of the furnace wall or the crucible material, the alloy ingot or the master alloy, and the pollution in the casting of the parts. If the foreign matter is distributed in aggregate, or distributed in a chain shape along the deformation direction during forming, it is easy to break along the foreign matter during processing and use of the fastener. The following figure shows the longitudinal crack of GH2036 high-temperature alloy nut after foreign body processing. When observing the crack fracture surface, as shown in the following figure, a large number of foreign body particles are found in the fracture surface.