Standards For High-Temperature Resistant Bolts And Applications Of Duplex Stainless Steel Bolts

Among currently commonly used high-temperature resistant bolts, products made of duplex stainless steel 2205 are widely applied. Duplex stainless steel 2205 bolts have excellent adaptability to high and low temperatures: their continuous stable operating temperature is usually 250-300℃ (can withstand around 400℃ for short periods), and they can tolerate low temperatures of -60℃, making them suitable for scenarios with sudden temperature changes. For example, 42CrMo chromium-molybdenum alloy steel bolts can work stably at around 360℃ for short periods, but their mechanical properties tend to fluctuate when the ambient temperature drops sharply or returns to normal; in contrast, duplex stainless steel 2205 bolts maintain stable performance in such alternating high and low temperature environments.

How to distinguish stainless steel bolts of different materials? Common 304 stainless steel bolts are marked "A2-70" on the head, 316 stainless steel bolts are marked "A4-70" or "A4-80", and duplex stainless steel 2205 bolts are mostly directly marked "2205" on the head (some may also be marked with corresponding standard codes, such as ASTM A193 Gr. S32205), allowing users to quickly identify them.

Differences Between High-Temperature Resistant Bolts and Ordinary Bolts

Many people mistakenly believe that high-temperature resistant bolts must have higher tensile strength and torque parameters than ordinary high-strength bolts, but this is not the case. The core difference between them lies in material properties rather than basic strength: domestically, 42CrMo chromium-molybdenum alloy steel is commonly used to produce high-temperature resistant bolts, whose strength indicators are similar to standard 12.9-grade high-strength bolts. However, 42CrMo material remains stable at high temperatures (around 360℃) without torque reduction or hardness decrease; ordinary bolts (such as carbon steel or conventional stainless steel bolts) can usually only work normally below 200℃, beyond which they fail to meet the designed mechanical performance requirements. Therefore, high-temperature resistant bolts made of 42CrMo are widely used in high-temperature working conditions such as pressure vessels and boilers.

In addition, under high-temperature environments, the anti-vibration performance of conventional high-strength bolts is far inferior to that of high-temperature resistant bolts: under the same specifications and temperature conditions, ordinary alloy steel high-strength bolts can only withstand about 80 vibrations before obvious loosening, while 42CrMo high-temperature resistant bolts can withstand approximately 200 vibrations before loosening. This characteristic of high-temperature resistant bolts cannot be replaced by ordinary bolts; arbitrary replacement may easily cause equipment failures due to bolt loosening, and such potential risks cannot be completely avoided in advance through emergency strategies.

High-temperature resistant bolts also have special requirements for surface treatment: taking 42CrMo as an example, its hardness is similar to that of 12.9-grade bolts. After production, it needs heat treatment and black oxide coating; galvanizing or nickel plating is usually unnecessary, because in high-temperature working conditions, the coating not only fails to provide protection but also affects the performance of the bolts due to high-temperature failure.

When accepting high-temperature resistant bolts, material confirmation is the top priority, and it is also necessary to check whether their stress data and mechanical properties meet the design requirements-both are indispensable. If the material is substandard, even with high hardness, it cannot achieve the performance of 42CrMo or 2205 duplex steel in high-temperature environments. Therefore, during acceptance, it is essential to require suppliers to provide material batch numbers and material analysis reports; for different installation environments, relevant personnel must receive professional training to master the usage precautions and corresponding standard requirements of high-temperature resistant bolts.

Characteristics and Applications of Duplex Stainless Steel Bolts (Taking 2205 as an Example)
 

Duplex stainless steel bolts have already played an important role during the construction of the Hong Kong-Zhuhai-Macau Bridge. On October 23, 2018, the Hong Kong-Zhuhai-Macau Bridge was opened to traffic. This 55-kilometer super large cross-sea bridge connecting Hong Kong, Zhuhai, and Macau has realized a 30-minute living circle among the three places, significantly reducing logistics costs and promoting coordinated development of the three regions. In this project, the usage of domestically developed and produced duplex stainless steel bolts reached 1,200 tons, completely breaking the previous dependence on imported duplex stainless steel bolts for bridge construction.

Previously, duplex stainless steel bolts used in domestic bridge projects mostly needed to be imported; however, enterprises participating in the construction of the Hong Kong-Zhuhai-Macau Bridge, through basic research on duplex stainless steel materials and control of finished product performance, produced bolts with quality equivalent to imported products. This not only set a benchmark for domestic counterparts but also enhanced their own technical strength, laying a foundation for the enterprises' development under the "Belt and Road Initiative".

Duplex stainless steel bolts (such as 2205) can meet high-requirement scenarios like bridges, mainly due to three core characteristics:

Excellent corrosion resistance: Its corrosion resistance is far superior to that of 304 stainless steel bolts. Bridges are in marine environments, where the corrosion intensity of seawater and high-salt mist air is much higher than in daily scenarios. 304 stainless steel bolts cannot meet long-term corrosion resistance requirements, while 2205 duplex steel, containing approximately 22% chromium and 3% molybdenum, can form a stable passive film, effectively resisting corrosion in marine environments.

High strength performance: The tensile strength of 2205 duplex stainless steel bolts is about 800MPa, and the yield strength is about 550MPa-1.5-2 times that of conventional 304 stainless steel bolts (tensile strength about 515MPa, yield strength about 205MPa). Its actual tested strength has exceeded that of carbon steel 8.8-grade bolts (tensile strength ≥800MPa, yield strength ≥640MPa), providing reliable strength guarantee for bridge connections. The high strength of duplex stainless steel comes from its austenitic-ferritic duplex structure, not high carbon content-in fact, 2205 duplex steel has extremely low carbon content (≤0.03%), lower than 304 stainless steel (≤0.08%), avoiding the risk of intergranular corrosion caused by high carbon.

Good welding compatibility: In bridge construction, some connected components need to be fixed by welding. Duplex stainless steel bolts can be compatible with welded areas without performance degradation due to welding heat influence; ordinary stainless steel bolts, if close to welded areas, are prone to structural transformation due to heat influence, resulting in decreased strength and corrosion resistance, and failing to meet connection firmness requirements. This advantage stems from the reasonable proportion of alloying elements (such as nitrogen) in 2205 duplex steel, giving it good thermal stability.

In addition to bridges, 2205 duplex stainless steel bolts are also widely used in petroleum facilities, biofuel refining, food packaging equipment, etc.: in petroleum drilling, their good galvanic compatibility can meet the needs of drill pipe connections; in the biofuel and food industries, their excellent corrosion resistance and low precipitation characteristics can meet sanitary requirements and avoid contamination of products.

Supplementary Notes

Clarification on material operating temperature: The previous statement that "duplex stainless steel 2205 can withstand 600℃ or 800℃" is incorrect. In fact, the continuous service temperature of 2205 duplex steel should not exceed 300℃, and the maximum short-term service temperature is about 400℃. Exceeding this range will cause imbalance of the duplex structure, significantly reducing corrosion resistance and strength.

Correction on bolt welding: In bridge construction, bolts themselves are usually not directly welded, but achieve reliable connection by adapting to welded connected components, avoiding damage to the bolts' own performance due to high welding temperatures.

Correction on carbon content: One of the core advantages of duplex stainless steel (such as 2205) is its low carbon content, not "relatively high". Low carbon content is the key to its excellent resistance to intergranular corrosion, which needs to be distinguished from the carbon content range of conventional stainless steel (such as 304).