Welding Of Low Temperature Tubes

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 even 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.

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:

• Use 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.
• Adopt 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.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

• Avoid strong combination to prevent local stress concentration in the structure.
• Reduce the interlayer temperature between welding beads as much as possible to avoid prolonged welding

When the welding is in a high temperature state, try to do welding discontinuously.

• Ultra-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.
• 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.
• The arc must be struck using an arc striking plate or within the groove. The arc must not be struck in non-welding parts.
• Stress-relieving heat treatment of low-temperature steel after welding can reduce the risk of brittle fracture of low-alloy steel welded products.