API 5A J55 is a commonly used casing material. The tubing body is threaded to the coupling and must be welded to reinforce the strength of the threaded connection. A harsh working environment requires high quality for the pipe body and welding quality. We analyze its weldability by calculating carbon equivalent. The chemical composition of J55 casing is shown in the following table:
The weldability of the material is poor when the carbon equivalent exceeds 0.4, high preheating temperature and strict process are needed to obtain qualified welding quality. The carbon content of 0.34%~0.39% makes the transition curve of supercooled Austenite shift to the right, and the stability of supercooled Austenite increases. The addition of alloying elements, such as Cr, Mn, Ni and Cu, makes the transition curve of supercooled Aaustenite shift to the right, and enhances its stability and MS point (the beginning point of Mmartensite formation). All these effects increase the quenching tendency of J55, and it is easy to crack during welding.
The cold crack tendency of J55 casing is mainly due to the large quenching embrittlement crack. The highest hardness value of welding heat affected zone is high and the rapid cooling is easy to form martensite because of the high strength. In order to reduce the cooling rate, extend the cooling time of the welded joint from 800 ℃ to 500℃, improve the microstructure of the weld metal and reduce the maximum hardness of the heat-affected zone, preheating before welding and tempering after welding is required. J55 casing has a small hot crack tendency because it does not contain strong carbide and has low thermal conductivity, which is difficult to generate low fusion eutectic. The tensile strength of J55 is greater than or equal to 517 MPa, and the yield strength is 379-522MPa. we should use welding wire ER55-G which has similar strength. The welding wire has high welding Ni content, strong cold cracking resistance, and excellent comprehensive mechanical properties of the deposited metal. Our engineers make the following two plans:
Welding method 1: 80%Ar+20%CO2 gas welding. ER55-G welding wire with a diameter of 3.2mm. Welding parameters: current 250~320A, voltage 26 ~30V; Welding speed 35~50cm/min; The preheating temperature is 100℃, and the inter-layer temperature is not lower than the preheating temperature, but it is not allowed to be higher than the preheating temperature of 30℃. Post-welding treatment: air cooling without any heat treatment.
Welding method 2: The same welding materials and welding parameters as method one, only the change of post-welding heat treatment: tempering treatment, temperature 600±20℃, holding time for 4h; Heating rate 50℃/h, cooling rate 50℃/h.
The results of the two welding tests are as follows:
The tensile test of the first scheme is qualified. The impact values of the three samples in the heat-affected zone are 26,47,23, which are unqualified. The four lateral bending samples had cracks of 3.75mm, 4mm, 1.38mm and 0.89mm, respectively, which were unqualified. The test shows that this welding scheme is not reasonable.
The second scheme is qualified by tensile test; The impact values of the three samples in the heat-affected zone are 51,40,40, which are qualified. All the four side bending samples are intact and qualified; The experiment proves that this welding scheme is reasonable. Post-welding heat treatment can improve the welding microstructure and properties, which is one of the important factors for obtaining the welded joints that meet the technical requirements of J55 casing welding.