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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, but rather to over a hundred different industrial grades of stainless steel, each of which exhibits excellent performance in its specific area 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 1. What are stainless steel and acid-resistant steel? Answer: 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 ； Steel grades that are resistant to corrosion by chemically aggressive media such as acids, bases, and salts are known as acid-resistant steel. Their corrosion resistance differs due to variations 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 to classify stainless steel? 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 moderate corrosion resistance is sufficient, 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 improve with increasing chromium content. Its resistance to chloride stress corrosion is superior to that of other types of stainless steels. Examples of stainless steels in this category include Cr17, Cr17Mo2Ti, Cr25, Cr25Mo3Ti, and Cr28. 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 from the atmosphere, nitric acid, and saline solutions. It also features good oxidation resistance at high temperatures and a low coefficient of thermal expansion. It is used in equipment for nitric acid and food processing industries, and can also 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) Austenitic-ferritic (duplex) stainless steel: Combines the advantages of both austenitic and ferritic stainless steels, and exhibits 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 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 possesses 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 (to oxidizing acids, organic acids, and cavitation), heat resistance, and wear resistance; it is commonly used as a material for equipment in power plants, chemical industries, petroleum industries, etc. However, its weldability is poor; care should be taken regarding welding procedures, heat treatment conditions, etc. (2) During welding, chromium-nickel stainless steel undergoes repeated heating that leads to the precipitation of carbides, which 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 thermal 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 adopted 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 weaving, multi-layer multi-pass welding ； (4) Forceful cooling of the weld and heat-affected zone to reduce the residence time at 450–850°C ； (5) Argon shielding on the back side of the TIG weld ； (6) Weld the seam in contact with the corrosive medium last ； (7) Passivation treatment 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 where austenitic stainless steel is joined to 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, martensite structure will form at 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 \"humped\" 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 weld without spatter? Answer: When using solid stainless steel welding 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 spatter; 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 cored stainless steel welding wires at present (such as 308, 309, etc.), the flux formula in these wires is formulated to facilitate welding electro-chemical reactions under CO2 gas shielding; therefore, a pulsed arc welding power source is generally not required (pulsed power sources mostly need to use mixed gases). If it is desired to enter the droplet transfer stage earlier, a pulsed power source or a conventional gas-shielded welding machine equipped with mixed gases can be used for welding.