Introduction to common weld Stainless steel
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Welded Stainless steel is a technology that uses heat or pressure to join stainless steel materials together, usually used to make pipes, containers, structures, etc. Welding can be performed using a variety of methods, including arc welding, gas shielded welding (TIG, MIG/MAG), plasma arc welding, etc. Selecting appropriate welding methods and parameters is crucial to welding quality. The quality of weld Stainless steel is affected by factors such as welding process, material selection and operating technology, so experienced operators are required to operate to ensure welding quality and workpiece performance.
1. 9 major problems of weld Stainless steel
(1) What are stainless steel and stainless acid-resistant steel?
Answer: The main element “chromium” content in metal materials (nickel, molybdenum and other elements also need to be added)
Steel that can make steel in a passivated state and have stainless properties. Acid-resistant steel refers to steel that is resistant to corrosion in strong corrosive media such as acids, alkalis, and salts.
(2) What is austenitic stainless steel? What are the commonly used brands?
Answer: Austenitic stainless steel is the most widely used and has the most varieties. like:
〈1〉18-8 series: 0Cr19Ni9 (304) 0Cr18Ni8 (308)
〈2〉18-12 series: 00Cr18Ni12Mo2Ti (316L)
〈3〉25-13 series: 0Cr25Ni13 (309)
〈4〉25-20 series: 0Cr25Ni20 etc.
(3) Why is there a certain process difficulty in welding stainless steel?
Answer: The main process difficulties are:
<1> Stainless steel materials have strong thermal sensitivity. If they stay in the 450–850℃ temperature zone for a slightly longer time, the corrosion resistance of the welds and heat-affected zones will be seriously reduced.
<2> Hot cracking is prone to occur.
<3> Poor protection and serious oxidation at high temperatures. <4> The linear expansion coefficient is large, resulting in large welding deformation.
(4) Why should effective process measures be taken when welding austenitic stainless steel?
Answer: General process measures include:
<1> Welding materials must be strictly selected based on the chemical composition of the base metal.
<2> Small current, fast welding; small line energy, reduce heat input.
<3> Fine diameter welding wire and electrode, no swing, multi-layer and multi-pass welding.
<4> Forced cooling of welds and heat-affected zones to reduce the residence time at 450-850°C.
<5>Argon gas protection on the back of TIG weld.
<6> Welds in contact with corrosive media are welded last.
<7> Passivation treatment of welds and heat-affected zones.
(5) Why should 25-13 series welding wires and electrodes be used when welding austenitic stainless steel to carbon steel and low alloy steel (dissimilar steel welding)?
Answer: When welding dissimilar steel welding joints connecting austenitic stainless steel to carbon steel and low alloy steel, the weld deposited metal must use 25-13 series welding wires (309, 309L) and welding rods (Austrian 312, Austrian 307, etc.). If other stainless steel welding materials are used, martensite structure will be generated on one side of the fusion line of carbon steel or low alloy steel, which will cause cold cracks.
(6) Why does solid stainless steel welding wire need to use 98% Ar+2% O2 shielding gas?
Answer: When MIG welding of solid stainless steel wire, if pure argon gas protection is used, the surface tension of the molten pool will be large, and the weld will be poorly formed and will have a “hunchback” weld shape. Add 1-2% oxygen to reduce the surface tension of the molten pool and make the weld smooth and beautiful.
(7) Why does the surface of MIG welding seam of solid stainless steel welding wire turn black?
Answer: The MIG welding speed of solid stainless steel welding wire is relatively fast (30-60cm/min). The protective gas nozzle has moved to the front molten pool area. The weld is still in a red-hot and high-temperature state. It is oxidized by the air, oxides are generated on the surface, and the weld becomes hairy. black. Pickling and passivation methods can remove black skin and restore the original surface color of stainless steel.
(8) Why does solid stainless steel welding wire require a pulsed power supply to achieve jet transfer and spatter-free welding?
Answer: When MIG welding of solid stainless steel wire, φ1.2 welding wire can achieve jet transfer when the current I ≥ 260-280A; if the droplet is less than this value, it will be a short-circuit transfer and the spatter will be large, so it generally cannot be used. Only use pulsed
Only with a MIG power supply with a pulse current greater than 300A can we achieve pulse droplet transition under 80-260A welding current without spatter welding.
(9) Why are flux-cored stainless steel welding wires protected by CO2 gas? Don't need a pulsed power supply?
Answer: Currently, the flux-cored stainless steel welding wire (such as 308, 309, etc.) commonly used, the flux formula in the welding wire is developed based on the chemical metallurgical reaction of welding under CO2 gas protection, so it cannot be used for MAG or MIG welding; it cannot be used with tape. Pulsed arc welding power source.
2. Common welding techniques for weld Stainless steel
The most commonly used welding method for stainless steel is manual welding (MMA), followed by metal arc welding (MIG/MAG) and tungsten inert gas welding (TIG).
(1) Manual welding (MMA)
Hand welding is a very common and easy-to-use welding method. The length of the arc is adjusted by human hands, and it is determined by the size of the seam between the welding rod and the workpiece. At the same time, when used as an arc carrier, the welding rod is also a weld filler material. This welding method is very simple and can be used to weld almost all materials. For outdoor use, it has good adaptability and is no problem even when used underwater. Most welders can TIG weld. In electrode welding, the arc length is determined by the human hand: when you change the gap between the electrode and the workpiece, you also change the arc length. In most cases, welding uses direct current, and the electrode serves as both an arc carrier and a weld filler material. The electrode is composed of an alloy or non-alloy metal core wire and a welding rod coating. This coating protects the weld from air damage and stabilizes the arc. It also causes the formation of a slag layer, which protects the weld and gives it shape. The welding rod can be a titanium welding rod or a sealing rod, which depends on the thickness and composition of the coating. Titanium welding rod is easy to weld, and the weld seam is flat and beautiful. In addition, welding slag is easy to remove. If the electrode is stored for a long time, it must be rebaked. Because moisture from the air can quickly accumulate in the welding rod.
(2) Metal arc gas shielded welding (MIG/MAG)
This is an automatic gas shielded arc welding method. In this method, an arc is burned between the current carrier wire and the workpiece under a protective gas shield. The metal wire fed by the machine serves as a welding rod and melts under its own arc. Due to the versatility and specificity of the MIG/MAG welding method, it is still the most widely used welding method in the world. It is used on steel, non-alloy steel, low alloy steel and high alloy based materials. This makes it an ideal welding method for production and repair. When welding steel, MAG can meet the requirements of thin gauge steel plates only 0.6mm thick. The protective gas used here is reactive gas, such as carbon dioxide or mixed gas. The only limitation is that when welding outdoors, the workpiece must be protected from moisture to maintain the effects of the gas.
(3) Tungsten inert gas welding (TIG)
The arc is generated between the refractory tungsten welding wire and the workpiece. The shielding gas used here is pure argon, and the welding wire fed is uncharged. Welding wire can be fed by hand or mechanically. There are also some specific uses that do not require feeding of welding wire. The material being welded determines whether direct current or alternating current is used. When using direct current, the tungsten welding wire is set to the negative electrode because it has deep welding penetration and is suitable for different types of steel, but it does not have any “cleaning effect” on the weld pool.
The main advantage of TIG welding method is that it can weld a wide range of materials, including workpieces with a thickness of 0.6mm and above. The materials include alloy steel, aluminum, magnesium, copper and its alloys, gray cast iron, ordinary dry, various bronzes, Nickel, silver, titanium and lead. The main areas of application are the welding of thin and medium-thick workpieces and the use as a root pass on thicker sections.
3. Eight precautions for weld Stainless steel
(1) Chromium stainless steel has certain corrosion resistance (oxidizing acids, organic acids, cavitation), heat resistance and wear resistance. Usually used in power stations, chemical industry, petroleum and other equipment materials. Chromium stainless steel has poor weldability, so attention should be paid to the welding process and heat treatment conditions.
(2) Chromium 13 stainless steel has greater hardenability after welding and is prone to cracks. If the same type of chromium stainless steel electrode (G202, G207) is used for welding, preheating above 300°C and slow cooling at about 700°C after welding must be carried out. If the weldment cannot be subjected to post-weld heat treatment, chromium-nickel stainless steel electrodes (A107, A207) should be used.
(3) Chromium 17 stainless steel, in order to improve corrosion resistance and weldability, appropriate amounts of stabilizing elements Ti, Nb, Mo, etc. are added. The weldability is better than that of chromium 13 stainless steel. When using the same type of chromium stainless steel welding rod (G302, G307), preheating above 200℃ and tempering treatment at about 800℃ after welding should be carried out. If the weldment cannot be heat treated, chromium-nickel stainless steel electrodes (A107, A207) should be used.
(4) When welding chromium-nickel stainless steel, carbides will precipitate due to repeated heating, reducing corrosion resistance and mechanical properties.
- Chromium-nickel stainless steel welding rods have good corrosion resistance and oxidation resistance and are widely used in chemical industry, fertilizer, petroleum, and medical machinery manufacturing.
(6) Chromium-nickel stainless steel coatings are available in titanium-calcium type and low-hydrogen type. Calcium titanium type can be used for AC and DC, but the penetration depth is shallow during AC welding and it is prone to redness, so DC power supply should be used as much as possible. Diameters of 4.0 and below can be used for all-position welding parts, and diameters of 5.0 and above can be used for flat welding and flat fillet welding.
(7) The welding rod should be kept dry when used. The titanium-calcium type should be dried at 150°C for 1 hour, and the low-hydrogen type should be dried at 200-250°C for 1 hour (drying cannot be repeated multiple times, otherwise the coating will easily crack and peel off) to prevent the electrode from cracking. The coating will stick to oil and other dirt, so as not to increase the carbon content of the weld and affect the quality of the weldment.
- In order to prevent inter-eye corrosion due to heating, the welding current should not be too large, which is about 20% less than that of carbon steel electrodes. The arc should not be too long, and the interlayers will cool quickly. A narrow weld bead is preferred.
As a key manufacturing process, weld Stainless steel plays an important role in joining stainless steel materials and plays an irreplaceable role in various industrial and construction applications. It is widely used in manufacturing pipes, containers, structures and other fields, providing a solid foundation for modern engineering and construction.
However, with the continuous development of technology and changes in market demand, weld Stainless steel technology is also constantly evolving. In the future, we can expect more innovations and improvements to address new challenges and needs. From more efficient welding methods to more environmentally friendly materials, stainless steel welding technology will continue to seek breakthroughs and progress.
In this process, it will be crucial to strictly control welding quality, strengthen personnel training, and promote industry cooperation. Through unremitting efforts, we can ensure that weld Stainless steel will continue to play its key role in the future and contribute to the development and progress of various industries. Let us work together to meet future challenges and promote stainless steel welding technology to new heights!
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