If we use a magnifying glass to observe a stainless steel bolt with quality issues that cannot be seen with the naked eye, we can see that the surface of the bolt will have a noticeable feeling of looseness. In severe cases, there may also be some air filled bubbles and white spots, which are the direct reasons for the substandard strength of the stainless steel bolt. We refer to this situation as material tissue porosity and segregation defect.
In the industrial field, the usage rate of stainless steel bolts has reached about 30% of the entire fastener product, and it continues to rise. In order to better use stainless steel bolts reasonably and ensure the stable quality of stainless steel bolts, a correct way and method are needed to judge some unconventional quality problems of stainless steel bolts. What we mean by unconventional quality issues here is not that they are difficult to encounter, but rather some quality issues that we easily overlook in our daily lives. The segregation defect mentioned in the first paragraph above is one of them.
Before production, the material of stainless steel bolts needs to go through a series of processes such as cold rolling, acid vapor treatment, and polishing. Some materials also need to use some fine sand grinding technology after rough grinding to eliminate some burrs on the surface of the material, in order to ensure the smoothness of stainless steel bolts. The problem of material looseness that occurs at this time refers to a situation where the density of the material itself is not high enough. The reason for this situation is that too many impurities are concentrated in this area, and after a long period of corrosion, bubbles gradually appear. If this phenomenon occurs on the cross-section of the bolt, its impact will not be so significant. If it occurs on the straight line of the bolt rod, it indicates that the central part of the bolt may have developed looseness. At this time, it is necessary to focus on facing this problem, and professional testing instruments can be used to test the strength of the bolt to what extent it has been damaged.
What is segregation? In simple terms, segregation refers to the uneven distribution of various elements in the material. When observed under a microscope, it can be found that the internal structure of the material is like a tree branch, and some uneven distribution is manifested in various other shapes, such as square and slightly dotted. Of course, with the continuous improvement of material technology in China, such point like segregation has become less common. The common distribution of tree branches is caused by temperature changes during the formation process of the material. When there is a temperature difference, the distribution of the material will show coarse and fine marks. When the temperature changes again, the second and third segregation lines will extend from the first segregation line, forming the final distribution shape of the tree branches. Although segregation is quite common and cannot be completely eliminated, if the segregation state is too severe, it will affect the strength of the bolt.
Bubbles and small pore residues are small voids formed during the condensation process of a material. Individual bubbles are actually in a normal state, and if honeycomb like bubbles appear, it is a quality issue. Due to the continuous changes in temperature, there is a slight difference in the time it takes for each molecular structure of the material to slowly condense. When there is a stain in the material, it will form white spots or bubbles. The formation of all bubbles, apart from the influence of temperature changes, is often caused by the collapse of inclusions in the material. The material of bolts is synthesized from various alloy materials after high-temperature smelting, and different alloy materials have different melting temperatures. During the melting synthesis process, alloy materials with high melting points are prone to leaving some residues, which can cause the formation of bubbles. The smelting environment of the bolt material is also very important. Do not assume that it works in an open environment and can ignore a large number of adverse factors in the environment. On the contrary, large particles of dust and metal particles in the environment are important factors that cause quality problems.
When the surface of the bolt is severe, not only will white spots appear, but there may even be obvious cracks. The impact of such cracks is the most serious of all quality problems, and it has a significant impact on the mechanical properties of bolts. It is a problem that must not occur in material smelting. In order to ensure that there are no cracks on the surface of bolts, we should use professional ultrasonic instruments to inspect whether there are any abnormalities in the internal structure of bolts. This is a very important testing step.
In short, for stainless steel bolts, segregation, white spots, and cracks are all unacceptable quality phenomena. The several quality problems mentioned above require strict testing systems and high standard quality judgment mechanisms when materials enter the factory and finished products leave the factory. Only by placing quality issues in a top priority can enterprises develop healthily.
