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Stainless Steel is short for rust-resistant and acid-resistant steel; steel types that are resistant to weak corrosive agents such as air, steam, and water, or that possess rust resistance, are referred to as stainless steel. “The term “stainless steel” does not refer to just one type of stainless steel; rather, it denotes over a hundred types of industrial stainless steels, each of which exhibits good performance in its respective field of application. They all contain 17–22% chromium; better grades of steel also contain nickel, and the addition of molybdenum further improves resistance to atmospheric corrosion, especially in chloride-containing atmospheres. I. Classification of stainless steel Figure 1: Stainless steel pressure vessels 1. What are stainless steel and acid-resistant steel? Answer: Stainless steel is short for rust-resistant and acid-resistant steel; steel grades that are resistant to weak corrosive agents such as air, steam, and water, or that possess rust resistance, are referred to as stainless steel ； Steel grades that are resistant to corrosion by chemically aggressive substances such as acids, bases, and salts are known as acid-resistant steel. Their corrosion resistance varies due to differences in their chemical composition; ordinary stainless steel is generally not resistant to corrosion by chemical agents, whereas acid-resistant steel usually possesses rust resistance. 2. How is stainless steel classified? Answer: Based on their microstructural state, they are classified into martensitic steel, ferritic steel, austenitic steel, austenite-ferrite (duplex) stainless steel, and precipitation-hardening stainless steel, among others. (1) Martensitic steel: high strength, but poor ductility and weldability. Common grades of martensitic stainless steel include 1Cr13 and 3Cr13, etc. Due to their higher carbon content, they possess high strength, hardness, and wear resistance; however, their corrosion resistance is somewhat lower. They are used in components where high mechanical properties are required but only average corrosion resistance is needed, such as springs, turbine blades, and hydraulic press valves. This type of steel is used after quenching and tempering, and requires annealing after forging or stamping. (2) Ferritic steel: contains 15%–30% chromium. Its corrosion resistance, toughness, and weldability increase as the chromium content rises, and its resistance to chloride stress corrosion is superior to that of other types of stainless steels. Examples of metals in this category include Crl7, Cr17Mo2Ti, Cr25, Cr25Mo3Ti, Cr28, etc. Due to its high chromium content, it exhibits good corrosion resistance and oxidation resistance, but poor mechanical and processability; it is therefore mainly used in acid-resistant structures that are not subject to heavy stresses, as well as as an oxidation-resistant steel. This type of steel can resist corrosion caused by the atmosphere, nitric acid, and saline solutions. It also features good high-temperature oxidation resistance and a low coefficient of thermal expansion. It is used in equipment for nitric acid and food processing plants; moreover, it can be used to manufacture components that operate at high temperatures, such as parts for gas turbines. (3) Austenitic steel: It contains more than 18% chromium, as well as about 8% nickel and small amounts of elements such as molybdenum, titanium, and nitrogen. It has good comprehensive performance and can resist corrosion by various media. Solution treatment is generally used, which involves heating the steel to 1050–1150°C and then cooling it rapidly with water or air in order to obtain a single-phase austenite structure. (4) Austenite-ferrite (duplex) stainless steel: It combines the advantages of austenitic and ferritic stainless steels, as well as superplasticity. Stainless steel with approximately half austenite and half ferrite structure. At low carbon levels, the Cr content ranges from 18% to 28%, while the Ni content ranges from 3% to 10%. Some steels also contain alloying elements such as Mo, Cu, Si, Nb, Ti, and N. This type of steel possesses the characteristics of both austenitic and ferritic stainless steels. Compared to ferritic steels, it offers higher plasticity and toughness, lacks room-temperature brittleness, and exhibits significantly improved resistance to intergranular corrosion as well as better weldability. At the same time, it retains the brittleness at 475°C and the high thermal conductivity associated with ferritic stainless steels, as well as superplasticity. Compared to austenitic stainless steels, it has higher strength, as well as significantly improved resistance to intergranular corrosion and chloride stress corrosion. Duplex stainless steel exhibits excellent pitting resistance and is also a nickel-saving stainless steel. (5) Precipitation-hardening stainless steels: The matrix is of austenitic or martensitic structure; common grades of precipitation-hardening stainless steels include 04Cr13Ni8Mo2Al, etc. It is a stainless steel that can be hardened through precipitation hardening (also known as aging hardening). Based on their composition, they are classified into chromium stainless steel, chromium-nickel stainless steel, chromium-manganese-nitrogen stainless steel, etc. (1) Chromium stainless steel possesses certain corrosion resistance (against oxidizing acids, organic acids, and cavitation), as well as heat and wear resistance; it is generally used as a material for equipment in power plants, the chemical industry, the petroleum sector, and elsewhere. However, its weldability is poor, so attention should be paid to welding techniques, heat treatment conditions, etc. (2) During welding, chromium-nickel stainless steel is subjected to repeated heating, which causes carbide precipitation and reduces its corrosion resistance and mechanical properties. (3) Chromium-manganese stainless steel exhibits good strength, ductility, toughness, formability, weldability, wear resistance, and corrosion resistance. II. Difficulties in stainless steel welding and an introduction to the use of materials and equipment 1. Why is welding stainless steel somewhat challenging from a technical perspective? Answer: (1) Stainless steel is highly sensitive to heat; if it remains in the temperature range of 450–850°C for an extended period, the corrosion resistance of the welds and heat-affected zones will decline significantly ； (2) Prone to hot cracking ； (3) Poor protection, severe high-temperature oxidation ； (4) It has a high linear expansion coefficient, resulting in significant welding deformation. 2. What effective process measures can be taken for welding austenitic stainless steel? Answer: (1) Select welding materials strictly based on the chemical composition of the base material ； (2) Fast welding with low current, reducing heat input through low wire energy ； (3) Fine-diameter welding wires and electrodes, no oscillation, multi-layer multi-pass welding ； (4) Forced cooling of the weld and heat-affected zone to reduce the dwell time at 450–850°C ； (5) Argon shielding on the back side of the TIG weld ； (6) Weld the joint in contact with the corrosive medium last ； (7) Passivation of welds and heat-affected zones. 3. Why are 25–13 series wires and electrodes selected for welding austenitic stainless steels to carbon steel and low-alloy steel (different-metal welding)? Answer: When welding dissimilar steel joints that consist of austenitic stainless steel combined with carbon steel or low-alloy steel, the weld metal must be produced using wires from the 25–13 series (309, 309L) as well as electrodes such as Ö312 and Ö307. If other stainless steel welding materials are used, martensitic structures will form along the fusion line on the carbon steel and low-alloy steel side, resulting in cold cracks. 4. Why is a shielding gas of 98% Ar + 2% O2 used for solid stainless steel welding wires? Answer: When using solid stainless steel welding wire in MIG welding with pure argon gas shielding, the surface tension of the molten pool is high, resulting in poor weld formation and a \"humpbacked\" shape for the weld. Adding 1–2% oxygen can reduce the surface tension of the molten pool, resulting in a smooth and aesthetically pleasing weld shape. 5. Why does the surface of MIG welds made with solid stainless steel wire turn black? How to solve this problem? Answer: Solid stainless steel welding wire allows for a faster MIG welding speed (30–60 cm/min). By the time the shielding gas nozzle reaches the front part of the weld pool, the weld is still at a red-hot temperature, making it susceptible to oxidation by air; oxides are formed on the surface, causing the weld to turn black. The pickling and passivation method can remove the black scale and restore the original surface color of stainless steel. 6. Why does solid stainless steel welding wire require a pulsed power supply in order to achieve a jet transition and spatter-free welding? Answer: When using solid stainless steel wire for MIG welding, with a wire diameter of φ1.2, a current level of I≥260～280A is required to achieve jet transition ； When the value is below this, the droplet transition is of the short-circuit type, resulting in significant spattering; therefore, it is generally not recommended for use. Only by using a MIG power supply with pulses can a transition from pulse droplet formation at low amperages to that at high amperages be achieved (the minimum or maximum value is selected based on the wire diameter), resulting in spatter-free welding. 7. Why is CO2 gas shielding used for cored stainless steel welding wires, rather than a pulsed power supply? Answer: For the commonly used core-filled stainless steel welding wires (such as 308, 309, etc.), the flux formula in these wires is formulated to facilitate chemical-metallurgical reactions during welding under CO2 gas shielding. Therefore, in most cases, a pulsed arc welding power source is not necessary (pulsed power sources generally require the use of a mixed gas). If it is desired to enter the droplet transfer stage earlier, a pulsed power source or a conventional gas-shielded welding machine equipped with a mixed gas can be used for welding.