Welding of 00Cr14Ni14Si4 austenitic stainless steel
Problems and precautions in the welding process C4 steel is a high-silicon austenitic stainless steel, which is mainly suitable for use in medium and low temperature% concentrated nitric acid medium. Its chemical composition and mechanical function test values are shown in Table 1. According to relevant information It is shown that in 98% concentrated nitric acid, the main element for passivation and dissemination is Si instead of C, so it can be seen from Table 1 that in order to improve the resistance of the steel to concentrated nitric acid corrosion, it is compared with 304L steel, C4 steel It reduces the Cr content in the steel, increases the Ni content, and significantly increases the Si content. In concentrated nitric acid, its corrosion resistance is more than ten times higher than that of 304L. However, because Si is prone to segregate in steels with a high Ni content, it forms low-melting eutectic interlayers with alloying elements such as Ni, S, and P, and thermal cracks are likely to occur under welding tensile stress. The higher the content of Si and Ni, the greater the tendency of weld cracking. Therefore, C4 steel is prone to cracks in the welding process, and the weldability is poor.
According to the above clarification and on-site welding test, the key point of C4 steel welding is to try to prevent the occurrence of hot cracks. Therefore, in order to effectively prevent thermal cracks, it is necessary to adopt the necessary methods of restraint in terms of material and process.
2C4 steel welding process 2.1 Welding methods Fair selection of welding methods is very important for the welding of C4 steel. We recycle the manual tungsten electrode argon arc welding back cover, manual arc welding filling, and the welding method of the cover Surface
. The purity of argon is 99.99%, and the welding machine model is Li 7-315.
The chemical composition and mechanical function of steel 2.2 Welding materials are required to guarantee the mechanical function and corrosion resistance of the weld metal. When welding C4 steel, select the special HC4 welding wire and AC4 electrode that match C4 steel, and must guarantee ferrite The body content is 6%12%, and its chemical composition and mechanical function are shown in Table 2. As the AC4 electrode eliminates the amount of carbonate intervention in the formulation to reduce the carbon content of the weld metal, it causes hydrogen holes in the weld The tendency is increased, so in actual operation, short arc welding should be applied.
Table 2 Chemical composition and mechanical properties of welding materials Chemical identity (%) Mechanical function 2.3 Welding process parameters When welding C4 steel, its process parameters have a crucial influence on the thermal cracking resistance of the weld and heat affected zone. When the welding heat input is too large, the weld zone and the heat affected zone stay at high temperature for a long time and the cooling rate is small, which will cause the structure of the weld zone and the heat affected zone to be thick, and the tendency of the weld to increase thermal cracking. Therefore, it is only necessary to limit the welding heat input, and it is necessary to recover the small current and fast welding. At the same time, the temperature between the layers of the weld bead is strictly controlled below 50, as a result of the refinement of the grain after the operation of the weld bead to deal with the reheating of the previous weld bead. To deal with welds with huge stresses such as head joints, adopt symmetrical segmentation and other methods to weld, and try to eliminate the stresses discussed.
Welding process parameters Welding sequence Welding material grade Welding material diameter Power polarity Welding current Arc voltage/V Back cover welding DC forward connection filling welding DC reverse connection cover welding DC reverse connection 2.4 Welding process evaluation in accordance with "Pressure Vessel Safety Skills Supervision Regulations" According to the requirements of JB4708 "Welding Process Evaluation of Steel Pressure Vessels", the assessment of the butt joint of C4 steel with a thickness of 7mm is held. For the discussion format, see the welding process parameters. See Table 3. Temperature 22*C. The evaluation test plate is 100% RT flaw detection, the evaluation is rated as class I, and the mechanical function test is carried out for the discussion. The tensile strength is 658MPa. The plastic fracture is in the heat affected zone, and the lateral Surface
is bent back (3a, 90* no cracks. In order to verify the resistance of C4 steel and its discussion Corrosion performance, according to the requirements of drawing skills GB4334. After passing the welding process assessment, it is necessary to strengthen the construction site management, such as the welding process card, strict implementation of the process type, strengthen quality review and monitoring. Other welders must have a strong sense of responsibility , Must hold the "Boiler and Pressure Vessel Welder Qualification Certificate" announced by the labor department to be able to work.
Adopting the above methods, a total of more than 600 meters of welding seam was welded on site. After 100% non-destructive testing, more than 2,000 X-rays were taken. The weld pass rate was 96.5%. The practice proved that the welding process and process adopted Laws such as regularity and quality management are effective.