Usually, the temperature range of -10~-196℃ is called "low temperature" (counting from -40℃ in our country), and the temperature range below -196℃ is called "ultra-low temperature". Low-temperature steel is a special steel that has been rapidly developed to meet the needs of energy, petrochemical and other industrial sectors. Low-temperature steel is required to have sufficient strength, plasticity and toughness under low-temperature working conditions, and should have good processing properties. It is mainly used to manufacture welded structures that work at low temperatures of -20~-253℃, such as storing and transporting various liquefied gases. containers, etc.
1. Selection of welding method and heat input
Commonly used welding methods include electrode arc welding, submerged arc automatic welding, tungsten arc welding, and gas metal arc welding.
When welding low-alloy low-temperature steel, in order to avoid the formation of coarse structures in the weld metal and the near-seam area and try not to swing, use narrow welding passes, multi-pass multi-layer welding, the welding current should not be too large, and fast multi-pass welding should be used to reduce the risk of welding. The weld bead is overheated and the grains are refined by the reheating action of multi-layer welding. When multi-pass welding, the inter-pass temperature must be controlled, and a small heat input should be used for welding, controlled below 20KJ/cm. If preheating is required, the preheating temperature and the inter-pass temperature during multi-layer and multi-pass welding should be strictly controlled.
Welding line energy is also called welding heat input, which is the welding arc heat obtained per unit length of weld.
Formula E=U·I/v (Joule/cm) where U: arc voltage (volt), I: welding current
(Ampere), v: welding speed (cm/min).
Welding line energy is an important factor affecting the performance of the welding mechanical head. When the welding current and arc voltage increase, the welding line energy increases, and when the welding speed decreases, the welding line energy increases.
For low-temperature steel, the welding line energy is too large, and the joint toughness decreases more seriously, making the pressure vessel prone to instantaneous damage when operating at low temperatures. Therefore, when welding, the welding current, arc voltage, and welding speed must be strictly controlled to ensure various performance indicators of the welded joint.
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2. Welding characteristics of low-temperature steel and its process measures
Due to its low carbon content, low-temperature steel has little hardening tendency and cold cracking tendency, and has good weldability. However, excessive welding line energy will form a coarse-grained structure in the weld and the heat-affected zone, which will greatly reduce the low-temperature toughness. The sudden change of the structure and the strong combination during manufacturing will cause high stress in the local part of the structure, thus increasing the Brittle failure of equipment at low temperatures. For this reason, the following points should be done during the welding process:
⑴Use small welding line energy to minimize overheating and prevent the appearance of coarse structures on the welded joints. Electrode arc welding usually uses 12-15KJ/cm, and submerged arc welding usually uses 20KJ/cm. For this reason, welding rod arc welding should not use φ5 welding rods as much as possible. Submerged arc automatic welding often uses φ3.2 welding wires. Each layer of welding rod arc welding is about 2mm, and submerged arc automatic welding is about 2.5mm.
⑵Adopt straight welding pass and multi-pass rapid pressure welding. The purpose is to reduce overheating and the tempering effect of the subsequent welding pass on the previous welding pass to refine the grains.
⑶ Avoid strong combination to prevent local stress concentration in the structure.
⑷ Reduce the interlayer temperature between welding beads as much as possible, avoid the welding beads being in a high temperature state for a long time, and try to achieve discontinuous welding.
⑸Ultra-low hydrogen welding rods and fluxes are usually used, so the drying process must be strictly followed before welding. If the used welding rod is not used up for more than 4 hours, it should be returned to the secondary warehouse to be dried again before use. In addition, for low-temperature steel electrodes, the deposited metal diffusion hydrogen retest must be carried out according to relevant standards before use. The mercury method is usually used and implemented in accordance with GB/T3965-2012. The compliance indicators meet the design requirements.
⑹ During winter construction and thicker structures, preheating should be carried out appropriately, at least to above 15℃. For large and thick plate welding, the preheating temperature is generally 50°C, and the inter-pass temperature is controlled between 50 and 150°C.
(7) The arc must be struck using an arc striking plate or within the groove. The arc must not be struck in non-welding parts.
(8) Stress-relieving heat treatment of low-temperature steel after welding can reduce the risk of brittle fracture of low-alloy steel welded products.
Selection of commonly used low temperature steel welding materials
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 08Ni3DR: For temperatures below -100℃, imported welding materials E7015-C2L, E7016-C2L F7P15-ENi3-Ni3 are generally selected
SA-203 Gr.D is ASME material, requiring -101℃ impact E7016-C2L F7P15-ENi3-Ni3
Welding inspection of cryogenic pressure vessels
1. Solid regulation requirements: For low alloy steel with design temperature lower than -40℃
Welded joints on cryogenic pressure vessels must undergo 100% MT testing and pass JB/T4730.4-2005 Level I.
2. If the design implements HG20584, for those with ultrasonic testing requirements
For welding of low-temperature steel, the groove surface and the groove surface after root cleaning should be subject to 100% MT inspection and qualified according to JB/T4730.5-2005 Level I.
3. The stress concentration caused by defects at low temperatures will increase the structure's low
Warm brittle slope breaking tendency. Care should be taken to avoid defects such as arc craters, incomplete penetration, and poor weld formation. Under normal circumstances, the weld reinforcement is minimized, and sometimes it is required to be ground smooth after welding; the corner joints should be smooth and not allowed to bulge outwards. Welds with poor roundness or poor shape must be polished; there must be no bite on the edge of the weld. side.
4. The surface of cryogenic containers shall not be marked with steel stamps.
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