Performance Characteristics Of Stainless Steel Bolts

Stainless steel bolts are a general term. In this article, stainless steel bolts include most stainless steel fasteners such as hexagon socket head bolts, hexagon head bolts, stud bolts, and nuts. After production, stainless steel fasteners do not require post-heat treatment to modify their mechanical properties (unlike carbon steel bolts). In most cases, they can be used after only surface cleaning (additional passivation treatment can be applied if further corrosion resistance enhancement is needed). Therefore, their performance characteristics are essentially the performance characteristics of the materials used.

Compared with ordinary carbon steel bolts, stainless steel bolts have a wider temperature application range, but their surface Rockwell hardness (HRC) is usually lower than that of carbon steel bolts. The core performance of stainless steel bolts is corrosion resistance-they can remain free from oxidation in exposed air environments for decades. Even at relatively high temperatures, they can work normally without significant changes in strength or torque parameters. If passivation treatment is performed on stainless steel bolts after production, their high-temperature resistance and corrosion resistance will be further improved.

Stainless steel bolts have a relatively high physical property called resistivity. While carbon steel bolts also have resistivity, the resistivity of stainless steel bolts of the same specification is more than five times that of carbon steel bolts. Resistivity is closely related to the coefficient of thermal expansion of bolts: under normal circumstances, the higher the ambient temperature, the larger the coefficient of thermal expansion of the part. For carbon steel bolts with low resistivity, as the temperature rises, their coefficient of thermal expansion increases to a certain extent, making them unusable due to dimensional changes beyond the applicable range. In contrast, stainless steel bolts-with resistivity five times that of ordinary carbon steel bolts-have a smaller change in thermal expansion coefficient with temperature and can maintain relatively stable dimensions at higher temperatures, which is one of the important reasons for their excellent high-temperature resistance.

The mechanical properties of stainless steel bolts are relatively moderate. Although they cannot match high-strength bolts of grade 10.9 or above, they are not inferior to bolts of grade 8.8 or below. Except for special working conditions, stainless steel bolts can basically meet most application requirements. There are also stainless steel materials with higher strength (such as duplex stainless steel), but using such materials to produce bolts leads to excessively high costs and significantly reduced cost-effectiveness. With the development of technology, the strength of commonly used stainless steel bolts is expected to gradually increase.

Customers often ask about the strength grade of stainless steel bolts. Strictly speaking, stainless steel bolts follow the performance grades specified in GB/T 3098.6 Fasteners - Mechanical Properties - Stainless Steel Bolts, Screws and Studs (e.g., A2-70, A4-80), rather than the "XX grade" classification system used for carbon steel bolts. For a rough comparison with carbon steel bolt grades: 304 stainless steel bolts (corresponding to performance grade A2-70) have mechanical properties close to grade 6.8, and 316 stainless steel bolts (corresponding to performance grade A4-80) are close to grade 8.8. However, this is only a rough reference-their mechanical properties must be tested by professional equipment in accordance with standards, and cannot be determined solely based on this comparison.

The significant differences in mechanical properties between bolts of different materials are mainly due to the varying content and composition of alloying elements in the materials. When various metal elements are combined in specific proportions, they endow the material with unique properties. Taking carbon as an example, it is a basic element in metal materials, and its content has a significant impact on performance: generally, the higher the carbon content, the higher the bolt strength; the lower the carbon content, the relatively lower the strength. The reason why stainless steel bolts have lower strength than high-strength bolts is mainly their low carbon content. The addition of alloying elements in materials is not arbitrary, but a result of comprehensive balance: the rust resistance of stainless steel bolts (which carbon steel bolts do not have) is closely related to their low carbon content; if carbon content is increased blindly, strength may improve, but rust resistance will be significantly reduced.

Silicon in the material can strengthen ferrite, improving the bolt's strength and hardness, but it slightly reduces the material's plasticity. A balance must be struck between performance and processability to ensure good formability during production. Manganese can combine with sulfur in the material to form manganese sulfide (MnS). Sulfur itself is insoluble in iron; if it combines with iron, it forms iron sulfide (FeS), which easily causes hot brittleness. However, MnS has a high melting point and good stability, which can effectively reduce the adverse effects of sulfur on the toughness and strength of bolts. It is evident that each element plays a specific role in the material. In modern materials science, the basic properties of a material cannot be changed by simply increasing or decreasing a single element; instead, the impact of each element must be comprehensively evaluated to ultimately develop a material formula with balanced performance.