A pressure vessel made in a factory is made of SUS316L stainless steel with a plate thickness of 16mm. Since it is the first time to use this thickness of SUS316L stainless steel material, a large number of relevant materials have been consulted, and a reasonable welding process has been developed through the welding process evaluation test.
1. Welding analysis of 316L stainless steel
316L stainless steel is austenitic stainless steel with good weldability, and special process measures are generally not required for welding. However, when submerged arc welding is used, the welding heat input is large. Due to the small thermal conductivity of the stainless steel and the large coefficient of linear expansion, under the local heating and cooling conditions, the welded joint may form a large tensile stress during the cooling process; Austenitic stainless steel is easy to form a columnar crystal welded joint structure with strong directionality, which is favorable for the segregation of impurities such as S and P, and promotes the formation of intercrystalline liquid interlayer, which causes thermal cracking. Therefore, low-power welding parameters and faster welding speed should be selected for submerged arc welding to avoid overheating and improve crack resistance.
2. 316L stainless steel submerged arc welding features
(1) Characteristics of 316L stainless steel Due to the inherent physical properties of 316L stainless steel, the melting speed of stainless steel wire and the melting rate of base metal are much larger than that of carbon steel under the same welding parameters, only on the basis of welding parameters of welded carbon steel. Proper adjustment to achieve the desired welded joint.
(2) Thermal crack in the central area of ​​the weld 316L The physical properties of the stainless steel are small in thermal conductivity. Therefore, under the condition of local heating and cooling of the weld, the high temperature residence time of the welded joint is longer, and the center area of ​​the weld is more exposed at high temperature. High tensile stress and strain, which is one of the basic conditions for generating hot cracks. The 316L stainless steel weld is usually easy to form a large columnar crystal structure with strong directionality, which is the second basic condition for generating hot cracks.
(3) The main reason for the decrease in corrosion resistance of the heat-affected zone is that the precipitation of chromium carbide in the austenite grain boundary causes the grain boundary to be depleted of chromium. Because 316L stainless steel has high specific resistance value, small thermal conductivity, and large heat input of submerged arc welding, which affects the cooling rate of the heat affected zone of the welded joint, it is easy to produce such defects.
3. Selection of welding materials
(1) The selection of welding wire mainly considers the basic principle that the welded joint is equivalent to the strength of the base metal and the composition is equivalent. At the same time, considering the factors such as resistance to intergranular corrosion, plasticity of welded joints, burning of alloying elements during welding, etc., after comprehensive consideration, selected The welding wire is ultra low carbon austenitic type H00Cr19Ni12Mo2. The composition of the wire alloy is basically the same as that of the base material. Generally, when the wcr is 15% to 25% and the wNi is 5% to 6%, the weld metal can be completely austenitic structure with high toughness. The content of chromium in the welding wire is slightly increased compared with the base metal, mainly considering that chromium is an important element of the base metal and the welding consumable. Increasing the chromium content can compensate for the burning of chromium during the welding process. Φ4mm welding wire is selected according to the thickness of the base metal and the approximate range of the welding current.
(2) Flux selection At present, there are two kinds of flux for submerged arc welding in China: smelting type and sintering type. The stainless steel flux with the largest amount of smelting flux is HJ260, and the stainless steel flux with the largest amount of sintered flux is SJ601. Since the smelting flux is prepared by melting and pulverizing the raw material in the electric arc furnace, it is difficult to add the deoxidizing agent and the alloy component to the flux, and it is difficult to adjust the ferrite content in the weld metal; at the same time, its molten pool is deep, It is difficult to remove slag on both sides of the weld and it is difficult to remove slag. Therefore, it is difficult to weld stainless steel with smelting flux. The sintered flux is obtained by adding a binder to the raw material of the powder and sintering it at a high temperature without melting the raw material. It is easy to add deoxidizer and alloy components to the flux, so that the flux can be used to adjust the amount of ferrite in the weld metal, so that the weld seam forms an A+5%F dual phase structure, effectively eliminating the directionality of the single phase A structure. Make it finer and reduce the generation of hot cracks.
When welding, SJ601 can transfer some elements Cr, Ti, Nb, etc., which are beneficial to prevent intergranular corrosion, to the weld metal. Cr can reduce the intergranular depletion of Cr, and the role of Ti and Nb is to preferentially combine with carbon to prevent C and Cr from binding, thereby avoiding the occurrence of Cr-depleted phenomenon.
SJ601 is an alkaline flux. Due to its weak oxidizing properties, the alloying elements are relatively low in burning during welding and can partially remove harmful impurities, thereby improving the weld metal properties. On the contrary, HJ260 flux is a smelting flux, which has the disadvantages of incomplete deoxidation and inability to infiltrate a large amount of alloy. It is not ideal for solving the intergranular corrosion problem of stainless steel submerged arc welding. Therefore, SJ601 flux is the ideal flux for solving intergranular corrosion problems in austenitic stainless steel submerged arc welding.
Based on the above analysis, the flux selected in this process is SJ601, and the SJ601 flux should be dried according to the specifications before use. The drying temperature is 350 °C and the temperature is kept for 2 hours.
4. Welding process
(1) The groove groove form is as shown in the drawing. In the 50mm range of the pre-weld groove and its two sides, the surface impurities are removed by electric stainless steel wire brush to expose the metallic luster, and white chalk powder is applied on both sides of the groove. To prevent welding spatter.
Groove form
(2) Welding parameters: welding current 550~570A, arc voltage 36~38V, welding speed 46~48cm/min; back welding current 600~650A, arc voltage 38~40V, welding speed 48~50cm/min.
5. Precautions
(1) Do not randomly arc on the weldment. In order to avoid damage to the surface of the weldment, affecting corrosion.
(2) In order to prevent burn-through and incomplete penetration, the first layer is electrode-arc welded. The welding rod grade is A022 (φ4.0mm). When welding the arc welding of the electrode, it should be kept as short as possible, and the straight moving strip should be pulled forward, because excessive lateral swing is easy to cause thermal cracking and insufficient protection.
(3) The amount of flux dispersal is based on the principle of no arc, and it should be placed as little as possible. Too much will make the bead forming poor, and it will easily produce surface defects such as pitting.
(4) Operate in strict accordance with the welding procedure specifications, and use rapid welding as much as possible to make the temperature between each layer of the weld bead <100 °C to reduce overheating and improve crack resistance.
(5) During circumferential seam welding, the inner ring joint is up-slope welded, and the outer ring joint is down-slope welded, so that the molten pool solidifies when the weldment is turned to the center line position, avoiding liquid metal overflow and deformation of the weld bead shape. .
(6) In order to prevent intergranular corrosion due to overheating, welds in contact with corrosive media shall be welded last.
(7) The welding fan can be used to weld the weld seam during the welding process to speed up the cooling.
(8) After welding, a stainless steel brush should be used to clean the slag and splash around the weld.
(9) The thickness and chemical composition of the arc-striking plate and the arc-receiving plate at both ends of the weldment shall be consistent with the parts to be welded, and there shall be no gap between the arc-striking plate, the arc-receiving plate and the weldment.
(10) When stopping and ending at the end of the welding process, fill the crater as much as possible to prevent solidification cracks at the crater.
(11) When cleaning or reworking the weld with carbon arc gouging, the welding defects such as incomplete penetration, pores and inclusions should be shaved off, and the width and depth of the planer after gouging are required to be uniform. The optomechanical machine removes the carburizing, copper infiltration, oxide layer, etc. inside and outside the planer, and carefully cleans the planer with a wire brush. When there is a defect or the local planer is too deep, it is necessary to use the welding arc welding (the material is the same) to achieve the same groove depth, and then apply the welding by submerged arc welding.
(12) During the welding process, the center of the weld should always be aligned and the offset should be adjusted in time.
(13) During the welding process, it is found that the arc is unstable in the flux burning, that is, when the phenomenon of “pulling†occurs, there will be defects such as burning through or causing stomata and slag inclusion. The reason should be stopped in time to find out the cause, and then repaired. Welding.
(14) In order to make the surface of the weld be beautiful, it is recommended that the width of the weld in front of the cover is not more than 20 mm and the depth is not less than 1 to 2 mm.
6. Conclusion
After the welding of the product is completed, local ray inspection is required according to the requirements of JB4730.2-2005 "Non-destructive testing of pressure equipment". The detection ratio is 20% of the length of each welded joint (100% of the joint weld of the head), grade III qualified. The A and B welding seam ratios of the welding products using this submerged arc welding process are above 98%, and the welding deformation is small. Practice has proved that this process is feasible.
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