Alloy 20 iron-nickel based alloy seamless steel pipe used in the process pipeline of the refinery’s alkylation unit was cracked at the weld seam during the operation of the equipment, which nearly led to serious consequences. The relevant department decided to commission Shanghai HY Industry Co., Ltd to replace the steel pipe.
Alloy20 (NS1403/N08020), an imported austenitic iron-nickel based corrosion resistant alloy, has good resistance to oxidizing and moderate reducing corrosion, excellent stress corrosion cracking resistance and good local corrosion resistance. It has satisfactory corrosion resistance in many chemical process media, including highly aggressive inorganic acid solutions, chlorine gas and various media containing chlorides, dry chlorine gas, formic and acetic acid, acid anhydride, seawater and brine, etc., making it an ideal material for media transport.
Material welding performance analysis
Alloy 20 iron-nickel based alloy (NS143/N08020) welding has similar problems as austenitic stainless steel, namely a high thermal sensitivity, high porosity generation rate, easy to produce defects such as unfused, not welded through. In addition, iron-nickel alloyed steel also has a high tendency to weld thermal cracking, weld area to produce intergranular corrosion tendency. Due to the high nickel content of iron-nickel alloys, liquid nickel mobility is poor, easy to produce unfused when welding.
Chemical composition and mechanical properties of the base material are as follows:
Alloy 20 Mechanical Properties:
Pearlite steel alloy element content is far less than austenitic steel, the redox reaction in the melt pool will cause the burning of alloying elements; welding pool edge, the liquid metal temperature is low, poor mobility, molten base metal on the edge of the melt pool and filler metal can not achieve a good fusion, so that the formation of a narrow area near the pearlite base material and the weld metal composition is different, the width of 0.2 ~ 0.6mm transition The closer to the fusion line, the stronger the dilution effect of pearlite steel, especially the dilution of Cr, Ni, Mo and other alloy elements. Consider the use of welding electrodes with high chromium-nickel content to improve the quality of the fusion zone of dissimilar steels, there will be no phase change process, the joint in all areas of the tissue stress is small.
The formation of carbon diffusion layer in the fusion zone is due to the higher carbon content of pearlite steel and less alloying elements, while the opposite is true for austenitic steel. In the process of high temperature heating, carbon migration occurs at the interface of pearlite steel and austenite steel, decarburization layer is formed on the side of pearlite steel, and carbon increase layer is formed on the side of austenite steel, resulting in a large difference in mechanical properties between the two sides and causing stress concentration. In order to hinder the formation of carbide, at the same time can shorten the weld high temperature residence time, can be used to increase the austenitic weld in the nickel content of the method.
The residual stress in the weld is caused by the large difference between the linear expansion coefficient of pearlite steel and austenitic steel (the ratio of expansion coefficient of pearlite steel to austenitic steel is 14:17), and the poor thermal conductivity of austenitic steel is only 50% of pearlite steel, so the weld produces a large residual stress in the weld and near the fusion line after welding. In the alternating temperature conditions, there may be thermal fatigue cracks on the side of pearlite steel in the fusion zone, making the joint fracture prematurely. If the preference is given to nickel-based materials with similar coefficient of linear expansion of pearlite steel and better plasticity as the filler metal, this will make the welding stress concentrated in the weld and plastic deformation ability of austenitic steel side, at the same time, the welding layer temperature should be strictly controlled, control the cooling rate to avoid drastic changes in temperature.
The bevel is suitable for cold processing manufacturing. If the plasma and other thermal cutting should be used to remove surface oxides and slag before welding, and Alloy20 surface adhesion of such impurities than stainless steel, should use fine grain abrasive belt and fine grain grinding wheel for grinding. Nickel alloy liquid metal fluidity ( especially wettability ) is poor, welding is easy to produce unfused, melt pool penetration depth is generally only about 50% of low carbon steel, austenitic steel of about 60%. By increasing the welding current to increase metal fluidity, the effect is not very effective, and will also cause the adverse effects of overheating. To ensure a good fusion and a certain depth of fusion, the bevel angle and the radius of the root arc should be slightly larger.
Welding method and welding material selection
Alloy 20 iron-nickel based alloy has the common characteristics of nickel alloys, welding generally does not produce cold cracking, but more likely to produce hot cracking. Because nickel and sulfur, phosphorus, etc. can form low melting point eutectic, nickel alloy solidification often forms coarse dendritic austenite crystals, low melting point impurities are more likely to concentrate in the grain boundaries, in the grain solidification shrinkage stress and the role of welding stress, not fully solidified grain boundaries low melting point material is easy to be pulled to form thermal cracks, so the welded material should be strictly controlled sulfur, phosphorus content.
Welding process parameters
To prevent overheating of the welded joint, a small heat input should be used, and the use of multi-layer multi-channel welding, each layer of heat input is not more than 20kJ/cm, the inter-layer temperature control at 100 ℃ or less, in order to prevent the weld and heat-affected zone grain growth, resulting in a decline in the toughness of the welded joint.
Welding of products
Cleaning before welding
Carefully clean the bevel before welding, remove moisture, dust, grease, paint, etc. from the surface of the welded parts, especially the debris containing sulfur and lead, to prevent Alloy20 from being embrittlement by sulfur and lead, resulting in cracking and cracks along the grain boundary of the weld.
Preheating before welding
Nickel alloy welding generally does not require preheating, because preheating will slow down the cooling rate of the weld, and too slow a cooling rate will make the weld hardened; but when the base material temperature is lower than 15 ℃, the joint should be heated to 15 ~ 20 ℃ on both sides of the 250 ~ 300 mm wide area to avoid moisture condensation leading to weld porosity.
Selection of welding method
In order to avoid thermal cracking during the welding of Alloy20, the welding process with relatively concentrated and small heat input and control of the interlayer temperature is selected, so manual tungsten arc welding is used. In order to ensure that the tungsten arc welding to obtain a good quality of the weld, the back of the weld needs to be protected with argon gas and welded with the “left welding method”. When welding with electrode arc welding, it is appropriate to short arc welding, small heat input, electrode does not swing; interlayer temperature should be low, usually controlled at ≤ 100 ℃. Skilled welders should be selected, the level of welding directly affects the construction quality of the weld. The transport bar should be smooth, the weld seam should be full, the transition should be smooth, the arc start and arc close should be filled to prevent the appearance of biting, porosity, arc pit crack.
Welding specimen inspection
100% RT and PT testing of the test piece weld, in line with JB/T4730.2-2005 “non-destructive testing of pressure-bearing equipment” level II and I qualified standards.
Through the AlloyX and Alloy20 weldability analysis, determine the appropriate welding materials and welding parameters, strict control of welding heat input, effectively prevent the generation of welding cracks, to ensure the quality of welding. The process pipeline after more than half a year of use, stable operation, fully meet the requirements of use.
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