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Physical properties: There are significant differences in the physical properties of stainless steel and carbon steel. The density of carbon steel is slightly higher than that of ferritic and martensitic stainless steels, but slightly lower than that of austenitic stainless steels； The resistivity increases in the order of carbon steel, ferritic stainless steel, martensitic stainless steel, and austenitic stainless steel ； The order of linear expansion coefficients is similar as well, with austenitic stainless steels having the highest value and carbon steels the lowest ； Carbon steel, ferritic stainless steels, and martensitic stainless steels are magnetic, while austenitic stainless steels are non-magnetic. However, they can become magnetic when cold working induces a martensitic phase transformation; this magnetism can be eliminated by heat treatment to remove the martensitic structure. Compared to carbon steel, austenitic stainless steel has the following characteristics: 1) High electrical resistivity, about 5 times that of carbon steel. 2) It has a high linear expansion coefficient, 40% higher than that of carbon steel, and this value increases as the temperature rises. 3) Low thermal conductivity, about 1/3 that of carbon steel. Due to these special physical properties of austenitic stainless steels, significant welding deformation occurs during the welding process. Especially when welding dissimilar metals (referring to those other than carbon steel and low-alloy steel), significant differences in the thermal conductivity and linear expansion coefficients of these two materials result in large welding residual stresses, which are one of the main causes of cracks in the welded joint. Mechanical properties: Whether it is stainless steel sheet or heat-resistant steel sheet, austenitic stainless steel sheets offer the best overall performance – they possess sufficient strength along with excellent plasticity, and their hardness is not high either; this is one of the reasons why they are widely used. Like the vast majority of other metal materials, austenitic stainless steel sheets exhibit an increase in tensile strength, yield strength, and hardness as the temperature decreases ； Plasticity decreases as the temperature drops. Its tensile strength increases rapidly in the range of 15–80°C, changes slowly as the temperature drops further, whereas the increase in yield strength is more uniform. More importantly, as the temperature decreases, its impact toughness declines slowly, with no brittle transition occurring. Therefore, type 18-8 stainless steel can maintain sufficient plasticity and toughness at low temperatures; at a temperature of -196°C, its impact absorption energy can reach 392 J ； It still maintains sufficient impact toughness values even at liquid helium temperatures (-270°C) ； What is even more valuable is its ability to prevent brittle fracture at stress concentration sites at liquid hydrogen temperatures (-273°C). Therefore, this type of steel plate is widely used in the manufacture of cryogenic equipment; it is an indispensable material for low-temperature engineering. However, to prevent hot cracking in 18-8 stainless steel, some ferrite-forming elements are often added to the weld metal, and the formation of ferrite reduces its low-temperature impact toughness. Therefore, when welding such low-temperature materials, sufficient attention must be paid. 18-8 stainless steel plates not only possess good mechanical properties at low temperatures, but also high heat strength at high temperatures. It maintains its chemical stability in both oxidizing media at 900°C and reducing media at 700°C, which is why this type of steel plate is also a commonly used heat-resistant material. Heat resistance: Heat resistance refers to the ability to resist oxidation or corrosion by gaseous media at high temperatures, that is, thermal stability, as well as having sufficient strength, that is, mechanical properties at high temperatures (heat strength). 1. Thermal stability: To ensure that stainless steel maintains stable resistance to oxidation and corrosion by gaseous media at high temperatures, it is necessary to prevent the formation of weak or easily fractured oxide films on the surface of the steel, such as FeO, Fe3O4, and Fe2O3. To prevent the formation of an oxide film, it is advisable to add alloying elements such as Cr, Al, and Si to steel, so that a dense oxide film with a strong bond to the steel surface is formed on its surface—alloy oxide films such as Cr2O3 and Al2O3. The oxide film of this alloy provides good protection, thereby extending the service life of the steel or allowing it to operate at higher temperatures. If the mass fraction of chromium reaches or exceeds 12%, adding alloying elements such as aluminum and silicon can enhance its antioxidant properties further. In stainless steel, when the oxide film exists mainly in the form of Cr2O3, it possesses the best resistance to sudden changes in ambient temperature. 2. High-temperature strength Factors to be considered for heat-resistant steel products during high-temperature service include: first, high-temperature strength. The indicators of high-temperature strength comprise creep strength, stress-rupture strength, and short-term high-temperature strength. When steel is subjected to a constant stress at high temperatures, due to the combined effect of stress and temperature, it can undergo slow plastic deformation at stresses below its yield strength; this phenomenon is known as creep. The rate of creep increases with rising temperature and increasing stress. Creep strength refers to the characteristic of a material, at a certain specified temperature, to resist embrittlement and high-temperature fatigue under long-term high-temperature operation when it reaches a certain critical creep rate (or a certain critical strain after a specified time). If the high-temperature strength of heat-resistant steel products does not meet the requirements specified in the design drawings, serious accidents can occur during operation ； Secondly, heat-resistant steel products are in operation for extended periods of time, and fatigue is an unavoidable issue during use; fatigue can lead to the formation of fatigue cracks. Fatigue cracks generally originate from surface layers or certain subsurface defects. Under the action of alternating stresses, these cracks gradually propagate until the remaining cross-section can no longer withstand the final cyclic stress, resulting in a sudden fracture. At lower temperatures, fatigue cracks are transgranular, whereas at higher temperatures, they develop along the grain boundaries leading to fracture.