Stainless steel is the abbreviation of stainless acid-resistant steel. Steels that are resistant to weak corrosive media such as air, steam, and water or have stainless properties are called stainless steel. The term “stainless steel” does not simply refer to one type of stainless steel, but refers to more than one hundred types of industrial stainless steel, each of which has good performance in its specific application field. They all contain 17-22% chromium, and better steel types also contain nickel. Adding molybdenum can further improve atmospheric corrosion resistance, especially resistance to chloride-containing atmospheres.
First, the classification of stainless steel pipes
1. What are stainless steel and acid-resistant steel pipes?
Answer: Stainless steel is the abbreviation of stainless acid-resistant steel. Steels that are resistant to weak corrosive media such as air, steam, and water or have stainless properties are called stainless steel; and steels that are resistant to chemical corrosive media (acids, alkalis, salts, etc.) are called acid-resistant steel. Due to the difference in chemical composition between the two, their corrosion resistance is different. Ordinary stainless steel is generally not resistant to chemical media corrosion, while acid-resistant steel is generally rust-resistant.
2. How to classify stainless steel?
Answer: According to the organizational state, it can be divided into martensitic steel, ferritic steel, austenitic steel, austenitic-ferritic (duplex) stainless steel and precipitation hardening stainless steel.
(1) Martensitic steel: high strength, but poor plasticity and weldability. Common grades of martensitic stainless steel include 1Cr13, 3Cr13, etc. Due to the high carbon content, it has high strength, hardness, and wear resistance, but slightly poor corrosion resistance. It is used for some parts with high mechanical properties requirements and general corrosion resistance requirements, such as springs, turbine blades, hydraulic press valves, etc. This type of steel is used after quenching and tempering treatment, and annealing is required after forging and stamping.
(2) Ferritic steel: contains 15% to 30% chromium. Its corrosion resistance, toughness, and weldability increase with the increase of chromium content, and its resistance to chloride stress corrosion is better than other types of stainless steel. This category includes Crl7, Cr17Mo2Ti, Cr25, Cr25Mo3Ti, Cr28, etc. Due to its high chromium content, it has good corrosion resistance and oxidation resistance, but poor mechanical and process properties. It is mostly used for acid-resistant structures with low stress and as oxidation-resistant steel. This type of steel can resist corrosion from the atmosphere, nitric acid, and brine solutions, and has good high-temperature oxidation resistance and a small thermal expansion coefficient. It is used in nitric acid and food factory equipment, and can also be used to make parts that work at high temperatures, such as gas turbine parts.
(3) Austenitic steel: contains more than 18% chromium, about 8% nickel, and a small amount of molybdenum, titanium, nitrogen, and other elements. It has good comprehensive performance and can resist corrosion from a variety of media. Generally, solid solution treatment is used, that is, the steel is heated to 1050-1150℃, and then water-cooled or air-cooled to obtain a single-phase austenite structure.
(4) Austenitic-ferritic (duplex) stainless steel: It has the advantages of both austenitic and ferritic stainless steel and has superplasticity. Austenitic and ferritic structures each account for about half of the stainless steel. In the case of low C content, the Cr content is 18% to 28%, and the Ni content is 3% to 10%. Some steels also contain alloying elements such as Mo, Cu, Si, Nb, Ti, and N. This type of steel has the characteristics of both austenitic and ferritic stainless steel. Compared with ferritic stainless steel, it has higher plasticity and toughness, no room temperature brittleness, and significantly improved intergranular corrosion resistance and welding performance. At the same time, it also maintains the 475°C brittleness and high thermal conductivity of ferritic stainless steel and has the characteristics of superplasticity. Compared with austenitic stainless steel, it has high strength and significantly improved resistance to intergranular corrosion and chloride stress corrosion. Duplex stainless steel has excellent pitting resistance and is also a nickel-saving stainless steel.
(5) Precipitation hardening stainless steel: The matrix is austenite or martensite. Common grades of precipitation-hardening stainless steel include 04Cr13Ni8Mo2Al, etc. It is stainless steel that can be hardened (strengthened) by precipitation hardening (also known as aging hardening).
According to the composition, it is divided into chromium stainless steel, chromium-nickel stainless steel, and chromium-manganese-nitrogen stainless steel.
(1) Chromium stainless steel has certain corrosion resistance (oxidizing acid, organic acid, cavitation), heat resistance, and wear resistance, and is generally used as equipment materials for power stations, the chemical industry, petroleum, etc. However, its weldability is poor, and attention should be paid to the welding process, heat treatment conditions, etc.
(2) When chromium-nickel stainless steel is welded, it will precipitate carbides due to repeated heating, which will reduce corrosion resistance and mechanical properties.
(3) Chromium-manganese stainless steel has good strength, ductility, toughness, formability, weldability, wear resistance, and corrosion resistance.
Second, difficult problems in stainless steel pipe welding and introduction to the use of materials and equipment
1. Why is there a certain process difficulty in welding stainless steel pipes?
Answer:
(1) Stainless steel pipes are highly thermally sensitive. If they stay in the temperature range of 450-850℃ for a long time, the corrosion resistance of the weld and heat-affected zone will be seriously reduced;
(2) Thermal cracks are prone to occur;
(3) Poor protection and severe high-temperature oxidation;
(4) The linear expansion coefficient is large, which is prone to large welding deformation.
2. What effective process measures can be taken for welding austenitic stainless steel pipes?
Answer:
(1) Strictly select welding materials based on the chemical composition of stainless steel pipes;
(2) Fast welding with low current and low wire energy to reduce heat input;
(3) Thin diameter welding wire and electrode, no swing, multi-layer, and multi-pass welding;
(4) Forced cooling of weld and heat-affected zone to reduce the dwell time of 450-850℃;
(5) Argon protection on the back of TIG weld;
(6) The weld in contact with the corrosive medium is welded last;
(7) Passivation treatment of weld and heat-affected zone.
3. Why should 25-13 series welding wire and electrode be used for welding austenitic stainless steel pipes with carbon steel and low alloy steel (welding of dissimilar steels)?
Answer: For welding dissimilar steel joints connecting austenitic stainless steel pipes with carbon steel and low alloy steel, the weld deposited metal must use 25-13 series welding wire (309, 309L) and electrode (Austenitic 312, Austenitic 307, etc.). If other stainless steel welding materials are used, martensitic structure will be produced on the fusion line of carbon steel and low alloy steel, and cold cracks will appear.
4. Why does solid stainless steel welding wire use 98%Ar+2%O2 shielding gas?
Answer: When solid stainless steel welding wire MIG welding, if pure argon gas protection is used, the surface tension of the molten pool is large, the weld is poorly formed, and the weld is in the shape of a “hunchback” weld. Adding 1-2% oxygen can reduce the surface tension of the molten pool, and the weld is smooth and beautiful.
5. Why does the surface of the MIG weld of solid stainless steel welding wire turn black? How to solve this problem?
Answer: The MIG welding speed of solid stainless steel welding wire is relatively fast (30-60cm/min). When the shielding gas nozzle has run to the front molten pool area, the weld is still in a red-hot high-temperature state, which is easily oxidized by air, and oxides are generated on the surface, and the weld is black. The pickling passivation method can remove the black skin and restore the original surface color of the stainless steel.
6. Why does solid stainless steel welding wire need a pulsed power supply to achieve jet transition and spatter-free welding?
Answer: When MIG welding solid stainless steel welding wire, φ1.2 welding wire, when the current I≥260~280A, the jet transition can be achieved; less than this value, the molten droplet is short-circuited transition, the spatter is large, and it is generally not recommended. Only by using a pulsed MIG power supply can the pulsed droplet transition from small specifications to large specifications be achieved (select the minimum or maximum value according to the wire diameter) and spatter-free welding be achieved.
7. Why is CO2 gas protection used for flux-cored stainless steel welding wire instead of a pulsed power supply?
Answer: The flux formula in the commonly used flux-cored stainless steel welding wire (such as 308, 309, etc.) is developed according to the welding chemical metallurgical reaction under CO2 gas protection, so generally, a pulsed arc welding power supply is not required (pulsed power supplies basically require mixed gas). If you want to enter the droplet transition in advance, you can also use a pulsed power supply or a conventional gas-shielded welding machine with mixed gas welding.
Post time: Apr-27-2025