I believe that many welding technicians have encountered this headache problem. During the welding process of 316 stainless steel pipes, stress corrosion cracking of the welded joints often occurs. This phenomenon is mainly due to the stainless steel pipe. The welding process is affected by factors such as the residual tensile stress in the welding area, the crystal structure of the welding seam, and the precipitation of carbides in the welding area. Due to the corrosion resistance and advantages of stainless steel, it has a wide range of industrial applications. Mastering the essentials of skilled stainless steel welding technology has become a required course for current welding technicians.

Reasonable design of welded joints is the key

The main measures to prevent stress corrosion cracking of 316 stainless steel pipes are to reasonably design welded joints to avoid the accumulation of corrosive media at the joints and reduce or eliminate the stress concentration in the joints. . Reduce or eliminate welding residual stress, and arrange the welding pass sequence reasonably in the process, such as using segmented step-back welding, etc. Minimize the restraint of the welded joint and perform stress-relieving annealing after welding. When it is difficult to implement heat treatment, change the surface state of the weldment, perform shot peening on the sensitized side surface to generate residual compressive stress in this area, or polish, electroplat or spray the sensitized surface to reasonably select the base metal and welding materials. Super-alloyed welding materials are usually used, that is, the content of corrosion-resistant alloy elements (Cr, Mo, Ni, etc.) in the weld metal is higher than that of the base metal.

Correctly master the welding method

Use reasonable welding technology, select welding methods with concentrated heat sources, small line energy, rapid cooling and other measures to reduce carbide precipitation and avoid overheating of the joint structure. Ensure that the welded joints are smooth and clean. Welding spatter, arc scratches, etc. are often where stress corrosion begins. Therefore, the appearance of the welded joint is also crucial. ?Due to defects such as cracks in the hot-rolled tube blank or the fractures that occur during use after the high-precision cold-drawn tube is made into a cylinder, almost no plastic deformation occurs and are generally brittle fractures. Brittle fracture is caused by a variety of reasons.

When designing, materials with a suitable cold-brittle transition temperature should also be selected based on the working temperature of the component. The deformation of the cold-drawn large seamless steel pipe is too small, and it cannot meet the requirements of surface finish and dimensional accuracy, nor can it meet the strength index of the component;

The deformation is too large, and the plasticity and toughness of the 316 stainless steel pipe are reduced too much. Moreover, the grains are stretched too thin, forming a fibrous structure, and the metal will have obvious anisotropy. The axial direction of the cold-drawn seamless steel pipe is parallel to the elongation direction of the grains, and the strength increases; the radial direction of the 316 stainless steel pipe is perpendicular to the elongation direction of the grains, and the strength decreases, while the maximum stress of the hydraulic cylinder exists in the radial direction of the seamless steel pipe, so too much deformation is detrimental to the full performance of the cold drawn pipe. For the fiber structure and crystal defects such as dislocations and vacancies that appear in steel, heat treatment methods such as annealing or normalizing are generally used to eliminate them. The purpose of annealing is to refine the grains, eliminate structural defects, reduce hardness, improve plasticity, and facilitate cold working. Precautions should be taken for surface treatment. Stainless steel plates should be stored on special storage racks. The storage racks should be wooden or painted carbon steel brackets or padded with rubber pads to isolate them from other metal materials such as carbon steel.

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What are the welding methods of stainless steel

1. Plasma arc welding

Plasma arc welding is the most suitable method for welding stainless steel. Because the plasma arc is a compression arc, the arc cross-section is compressed less, the energy is concentrated, and the temperature is high, it can fully penetrate within a certain thickness range. However, because its equipment is very expensive and the nozzle life is short, its use is not widespread.

2. Methods and principles of gas metal welding

Gas metal welding with inert gas is the most common method for welding stainless steel. Gas metal welding is divided into two categories: Extremely inert gas maintenance welding, molten polar oxidizing mixed gas maintenance welding, CO2 gas maintenance welding and flux cored wire gas maintenance welding.

Generally, inert gases such as argon, helium or their mixed gases are used as the protective gas in the welding area. Because there is no coating layer on the surface of the welding wire, the current can be greatly increased, so the base metal has a large penetration depth and the welding wire melts quickly. , high deposition rate, greatly improving production efficiency.

Principle: Melting arc arc welding uses the arc between the meltable welding wire and the workpiece to be welded as a heat source to melt the welding wire and base metal, and delivers protective gas to the welding area to make the arc and melt The welding wire, molten pool and nearby base metal are protected from damage by the surrounding air. The continuously fed welding wire metal continuously melts and transitions to the molten pool, and blends with the molten base metal to form the weld metal, thereby connecting the workpieces to each other.

3. Technical arc welding

Technical arc welding is a welding method that uses manually operated arc electrodes for welding. During manual arc welding, an arc is generated between the welding rod and the workpiece to partially heat the welding rod and the workpiece to a molten state. The melted droplets at the end of the welding rod and the molten base material fuse together to form a molten pool. As the arc moves forward, the molten The liquid metal in the pool gradually cools and crystallizes to form a weld.

Technical arc welding of stainless steel is the most widely used and can be used for welding all kinds of stainless steel. Its characteristics are that the heat affected zone of technical arc welding is small, it is easy to ensure quality, the equipment is simple, the operation is flexible, and it is adaptable to the technical requirements of various welding positions and different plate thicknesses. Nowadays, stainless steel welding rods can basically meet the welding needs of various stainless steels, and there are almost no restrictions on the selection of welding rods. The disadvantages are low production efficiency; poor working conditions; high demand for welders. In many cases, welders must have appropriate qualifications; the welding deposited metal of some materials does not meet the application requirements, such as ultra-high purity stainless steel; workpieces Thin plates with a thickness generally below 1 mm are not suitable for manual arc welding.

4. Submerged arc automatic welding is a welding method that covers the welding arc with a layer of granular meltable flux.

The arc light is not exposed. Currently it is mainly used for the welding of austenitic stainless steel medium and thick plates. It is characterized by large welding current, large penetration, and smaller grooves of the workpiece; fast welding speed and high production power; the weld metal solidifies slowly and the liquid metal There is more time for metallurgical reaction with the molten flux, which reduces the possibility of pores in the weld; the weld is beautifully formed, the working environment is good, the operation is simple, and the requirements for the welder are relatively simple. The disadvantage is that the welding heat input is large, the heat-affected zone is vast, and the welding seam structure is thick; when selecting materials, special consideration must be given to the cooperation of the welding wire and the flux; the welding direction can only be the flat welding direction; the relative position of the arc and the groove cannot be directly observed. Active tracking devices are required.

5. Tungsten inert gas shielded welding

Tungsten inert gas shielded welding (referred to as tig welding in English) can be divided into three types: manual welding, semi-automatic welding and automatic welding. Tungsten arc welding in tig welding is quite widely used in stainless steel. It is used to weld in all directions, usually without spatter, and the welds are beautifully formed. It is particularly accustomed to welding thin parts. When welding many thick parts, gig priming is often used to avoid the defects of manual arc welding, which is prone to cracks and difficult to remove slag. Inert gas can effectively block air. It is insoluble in metal and does not react well with metal. It can ensure the chemical composition requirements of stainless steel. The disadvantages are shallow melting depth, low deposition speed, low production efficiency and high production cost.

6. Laser welding-stainless steel laser welding machine

Laser welding can realize spot welding, butt welding, stack welding, seal welding, etc. on thin-walled materials and precision parts

. The laser power is high, the weld seam has a high aspect ratio, the heat affected zone is small, the deformation is small, and the welding speed is fast.

. The welds are of high quality, smooth and beautiful, with no pores, and the toughness of the post-weld material is at least equal to that of the parent material.

. Humanized design, LCD screen display and centralized button operation make it easier to operate www .haileilaser.com

. The four-dimensional ball screw worktable adopts imported servo control system and optional rotating worktable, which can realize automatic welding such as spot welding, linear welding and circumferential welding.

. The current waveform can be adjusted arbitrarily, and different waveforms can be set according to different welding materials to match the welding parameters and welding requirements to achieve the best welding effect.

Used for laser welding of carbon steel, ordinary alloy steel, stainless steel, different steels, stainless steel - low carbon steel, 416 stainless steel - 310 stainless steel, 347 stainless steel - HASTALLY nickel alloy, nickel electrode - cold Forged steel, bimetal strips with different nickel contents, titanium, nickel, tin, copper, aluminum, chromium, niobium, gold, silver and other metals and their alloys, and the same material of steel, Kovar and other alloys welding between. Welding between copper-nickel, nickel-titanium, copper-titanium, titanium-molybdenum, brass-copper, low carbon steel-copper and other dissimilar metals. Mobile phone batteries, jewelry, electronic components, sensors, watches, precision machinery, communications, handicrafts and other industries.

Generally speaking, in addition to the above, master welders should not forget that there is another very important factor, which is the selection of welding rods, because the selection of welding rods directly affects the welding effect. At present, the welding rod brands with more sales and better reputation on the market are Atlantic and Jinqiao. The former is slightly more expensive than the latter. Users can choose the brand that suits them according to their own characteristics. However, special attention should be paid to the fact that the material of the welding rod contains a large amount of iron powder, so there are requirements for the storage environment and storage time. Therefore, when using welding rods, you should carefully check whether the welding rods are damp and corroded. If there is any welding rods, it is likely to indicate that the welding rods have seriously deteriorated and the chemical properties have been seriously lost. This batch of welding rods must be scrapped.

Welding between alloys, steel, Kovar and other alloys of the same material. Welding between copper-nickel, nickel-titanium, copper-titanium, titanium-molybdenum, brass-copper, low carbon steel-copper and other dissimilar metals. Mobile phone batteries, jewelry, electronic components, sensors, watches, precision machinery, communications, handicrafts and other industries.

The above is what the editor introduced about stainless steel 316l welding. I hope it can help you!

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