If we observe a stainless steel bolt that shows no visible quality issues with the naked eye under a magnifying glass, we will notice obvious porosity on the bolt surface. In severe cases, there may even be air-filled bubbles and white spots. These are direct causes of substandard strength in stainless steel bolts, which we refer to as porosity and segregation defects in the material structure.
In the industrial sector, the usage rate of stainless steel bolts has reached approximately 30% of all fastener products and continues to rise. To ensure the rational use and stable quality of stainless steel bolts, it is necessary to adopt correct methods to identify easily overlooked non-routine quality issues. The segregation defects mentioned above are one such issue.
Before production, the material for stainless steel bolts undergoes a series of processes such as cold rolling, pickling, and polishing. Some materials, after rough grinding, require fine sand grinding to remove surface burrs, ensuring the smoothness of the stainless steel bolts. The porosity mentioned here refers to insufficient compactness of the material, mostly caused by excessive local accumulation of impurities. Over time, corrosion will gradually lead to the formation of bubbles. If this phenomenon occurs on the bolt's cross-section, the impact is relatively minor; if it appears in the longitudinal area of the bolt shank, it indicates that porosity may have developed in the bolt's core. In such cases, professional testing instruments are needed to assess the extent of strength damage.
So, what is segregation? Simply put, it is the uneven distribution of various elements in the material. Observed under a microscope, the internal structure of the material appears as chaotic, branch-like formations, while other forms of uneven distribution may present as squares, dots, etc. Thanks to advancements in China's material technology, dot-like segregation has become rare. The common dendritic distribution is mostly caused by temperature changes during material formation: when temperature differences occur, the material structure leaves traces of varying thickness; as temperatures change again, new lines extend from the initial segregation lines, eventually forming a dendritic pattern. Although segregation is common and cannot be completely eliminated, excessive segregation will affect bolt strength.
Bubbles and residual small pores are tiny cavities formed during material solidification. A few individual bubbles are normal, but honeycomb-like bubbles constitute a quality problem. Continuous temperature changes cause slight differences in the solidification time of various crystal grains in the material. If inclusions are present in the material, they will form white spots or bubbles. In addition to temperature changes, the formation of bubbles is largely related to inclusions in the material. Bolt materials are made by smelting various alloys at high temperatures. Different alloys have different melting points; during smelting, alloys with higher melting points tend to leave unmelted residues, which in turn cause bubbles. Therefore, the smelting environment for bolt materials is crucial. One should not ignore adverse factors in the environment just because smelting is often done in open settings-large dust particles and metal particles in the environment are significant causes of quality issues.
In severe cases, the bolt surface may not only have white spots but also obvious cracks. Cracks have the most severe impact on the mechanical properties of bolts and are absolutely prohibited in material smelting. To ensure no cracks on the bolt surface, professional ultrasonic instruments should be used to detect abnormalities in the bolt's internal structure-this is an extremely important testing step.
In summary, segregation, white spots, and cracks are all unacceptable quality issues for stainless steel bolts. These quality issues must be controlled through strict inspection systems and high-standard quality judgment mechanisms when materials enter the factory and finished products leave the factory. Only by prioritizing quality can enterprises achieve sound development.
