In order to improve the strength of the threaded connection of oil casing, the coupling and the threaded connection of the pipe body need weldling treatment. The harsh environment of oil casing requires not only high quality of the pipe itself, but also of the welding. API 5CT J55 is a commonly used oil casing pipe material. This artcle aims to analyze its weldability by analyzing alloy elements and carbon equivalent of J55 piping.
Firstly, let’s us learn to know the chemical composition of J55 casing pipe is showed below:
And then calculate its carbon equivalent according to the international institute of welding:
We know that the materials with carbon equivalent of more than 0.4 have poor weldability, requiring higher preheating temperature and strict technological measures to obtain qualified welding quality.
The higher carbon content of J55 (0.34%~0.39%) makes the transformation curve of supercooled austenite shift to the right and increases the stability of supercooled austenite. The addition of alloy elements such as Cr, Mn, Ni, Cu and so on makes the curve move further to the right and again enhances the stability of supercooled austenite, and also increases the Ms point (the starting point of martensite formation). All these factors increase the hardening tendency of J55 and make it easy to crack during welding.
J55 offers a great tendency of cold cracking, especially quenching brittle crack. Because of its high strength, the highest hardness in the heat affected zone of welding can be quickly cooled to form the sructure of Martensite easily. At this time, large line energy should be selected for welding and welding current should be appropriately increased, and the welding speed should not be excessively reduced. In order to reduce the cooling speed, extend the cooling time of welded joint from temperature range 800~500℃, improve the microstructure of weld metal and heat-affected zone, and reduce the maximum hardness of heat-affected zone. Preheat before welding and temper after welding are required.
J55 has a small tendency of thermal cracking because its low thermal conductivity is not easy to generate low eutectic and does not contain strong carbides.J55’s tensile strength is greater than or equal to 517 Mpa, yield strength is 379~522MPa, the welding wire ER55-G with similar strength should be selected, which has excellent welding process performance, more Ni, strong cold cracking resistance, and its deposited metal has excellent comprehensive mechanical properties.
In conclusion,the strength and internal stress of J55 and its welding material are large. J55 requires a large amount of heat input for welding, and it needs to hammer the welding seam while welding. After welding, the internal stress needs to be eliminated by heat treatment to avoid cracking after welding due to excessive stress.Post-weld heat treatment can also improve the microstructure and properties of welding.
Welding method: 80%Ar+20%CO2 gas shielded welding.
Choose ER55 - G welding materials, welding wire, Φ 3.2 mm in diameter.
Welding parameters: current 250~320A, voltage 26 ~30V, welding speed 35~50cm/min, preheating temperature is 100℃, to ensure that the temperature between layers is higher than the preheating temperature and not allowed to be higher than the preheating temperature of 30℃.
Post-welding treatment: tempering treatment: 600±20℃, insulation time: 4h; Heating rate 50℃/h, cooling rate 50℃/h.
The tensile test results obtained by the above methods: qualified. The impact values of the three samples in the heat-affected zone are 51, 40 and 40 respectively, which are all qualified, which proves that the process plan is reasonable. Post-welding heat treatment can effectively improve the welding microstructure and it is one of the important factors whether J55 welding can meet the requirements of welding joints.