Key points for welding low-temperature carbon steel

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Welding of low-temperature steel: The temperature range of -10 to -196°C is generally referred to as “low temperature” (in China, this range starts at -20°C), while temperatures below -196°C are considered “ultra-low temperature”. Low-temperature steel is a type of specialized steel that has been developed rapidly to meet the needs of industries such as energy and petrochemicals. Low-temperature steel is required to possess sufficient strength, plasticity, and toughness under low-temperature operating conditions, as well as good workability. It is mainly used to manufacture welded structures that operate at temperatures ranging from -20 to -253°C, such as containers for storing and transporting various liquefied gases. 1 Selection of welding methods and heat input. Common welding methods include shielded metal arc welding, submerged arc automatic welding, tungsten inert gas welding, and gas metal arc welding. When welding low-alloy steels for low-temperature applications, it is necessary to avoid swinging as much as possible in order to prevent the formation of coarse microstructures in the weld metal and the area surrounding it. Narrow weld beads and multiple layers of welding should be used; the welding current should not be too high. Rapid multi-pass welding is advisable to reduce overheating of the weld beads, and the reheating effect resulting from multiple layers of welding helps to refine the grain structure. During multi-pass welding, it is necessary to control the temperature between passes; therefore, a low heat input should be used, keeping it below 20 KJ/cm. If preheating is required, the preheating temperature and the inter-pass temperature during multi-layer, multi-pass welding must be strictly controlled. Welding line energy, also known as welding heat input, is the amount of heat from the welding arc per unit length of weld. The formula is E=U•I/v (joules/cm), where U is the arc voltage (volts), I is the welding current (amperes), and v is the welding speed (cm/min). Welding line energy is an important factor affecting the mechanical properties of the weld. When the welding current and arc voltage increase, the welding line energy increases; when the welding speed decreases, the welding line energy also increases. For low-temperature steels, an excessive welding heat input leads to a more severe decline in joint toughness, making pressure vessels prone to sudden failure when operating at low temperatures. Therefore, during welding, it is necessary to strictly control the welding current, arc voltage, and welding speed in order to ensure the various performance parameters of the welded joint. 2 Welding characteristics of low-temperature steel and its process measures: Due to their low carbon content, low-temperature steels have a low tendency to harden and to develop cold cracks, thus possessing good weldability. However, excessive welding heat input can cause coarse grain structures to form in the weld and heat-affected zone, thereby significantly reducing low-temperature toughness. Structural changes and forced alignment during manufacturing can generate high stresses in certain areas of the structure, increasing the risk of brittle failure of the equipment at low temperatures. To this end, the following points should be observed during the welding process: (1) Use a low welding heat input to minimize overheating and prevent the formation of coarse microstructures in the weld joint. Soldering arc welding often uses 12–15 KJ/cm, while submerged arc welding typically uses 20 KJ/cm. For this reason, φ5 electrodes should be avoided as much as possible in shielded metal arc welding, while φ3.2 wires are commonly used in submerged arc automatic welding; the thickness of each layer is about 2 mm in shielded metal arc welding and about 2.5 mm in submerged arc automatic welding. ⑵Use straight weld passes with multiple rapid pressure welds. The purpose is to reduce overheating and the tempering effect of the subsequent weld pass on the previous one, thereby refining the grain structure. ⑶Avoid forceful alignment to prevent local stress concentration in the structure. ⑷Minimize the inter-pass temperature as much as possible, avoid keeping the weld passes at high temperatures for long periods, and strive to carry out welding in a discontinuous manner. ⑸Ultra-low hydrogen electrodes and fluxes are usually selected; therefore, strict drying procedures must be followed before welding. If electrodes that have been issued are not used up within 4 hours, they should be returned to the secondary storage area for re-drying before being used again. Additionally, for low-temperature steel electrodes, it is necessary to conduct a retest on the diffused hydrogen content in the welded metal in accordance with relevant standards before use; the mercury method is typically employed, following GB/T3965-2012, and the results must meet the design requirements. ⑹ For construction in winter and for structures with large thicknesses, appropriate preheating should be carried out, with the temperature raised to at least 15°C. For the welding of thick plates, the preheating temperature is generally 50°C, while the inter-pass temperature is kept between 50 and 150°C. (7) Arc initiation must be carried out using an arc starting plate or within the groove; it is not allowed to initiate an arc at non-welding areas. (8) Post-weld stress-relief heat treatment of low-temperature steel can reduce the risk of brittle fracture in welded low-alloy steel products. Selection of welding materials for low-temperature steels commonly used: 16MnDR: J507GR, J507RH, ——E5015-G or E5016-G; H10Mn2, SJ101 ——F5P4-H10Mn2. 09MnNiDR: W707Ni, W807Ni ——E5515-N5, E7015-C1L, E7016-C1L; F7P7-ENi2-Ni2, H07MnNDR, SJ603W ——F5P7-H07MnNiDR. For 08Ni3DR: when the temperature is below -100°C, imported welding materials are generally chosen, such as E7015-C2L, E7016-C2L, F7P15-ENi3-Ni3, SA-203 Gr.D. This is an ASME material that requires impact resistance at -101°C; suitable materials include E7016-C2L and F7P15-ENi3-Ni3. Welding inspection for low-temperature pressure vessels: 1. Requirements per codes: For welding joints in low-alloy steel low-temperature pressure vessels with a design temperature below -40°C, 100% MT testing is required, with compliance at level I according to JB/T4730.4-2005. 2. If HG20584 is followed for the design and execution, for the welding of low-temperature steel materials that require ultrasonic testing, 100% MT inspection shall be carried out on the groove surface as well as on the groove surface after root cleaning, and it shall meet the requirements of Grade I as specified in JB/T4730.5-2005. 3. Stress concentration caused by defects at low temperatures increases the tendency of the structure to suffer brittle fracture at low temperatures. Care should be taken to avoid defects such as cratering, lack of penetration, and poor weld shape. Under normal circumstances, the weld bead height should be kept as low as possible; in some cases, it is required that the weld be smoothed after welding. Fillet joints must be smooth and must not protrude outward; welds with poor smoothness or poor shape must be polished. There should be no undercuts at the edges of the welds. 4. Steel stamp markings shall not be used on the surface of low-temperature containers.