Pressure vessels and heat exchangers are characterized by a large-thickness steel structure, obtained by bending steel sheets of thickness 20 – 250 mm. The production of such equipment does not require high productivity but needs to meet the high-precision tolerances defined by stricter international standards such as ASME.
The key procedure of the plate rolling machine in the process of producing the composite steel plate pressure vessel is pre-bending and rolling the plate to finally obtain the required tolerance and facilitate welding into a tube.
Welding requirements for clad steel plates
The clad steel plate is a bimetallic plate made of stainless steel, nickel-based alloy, copper-based alloy, or titanium plate as the clad layer, and pearlitic steel as the base layer, which is made by explosive welding, clad rolling, surfacing, and other methods. The base layer of the composite steel plate should meet the requirements of joint strength and stiffness, and the cladding layer should meet the requirements of corrosion resistance. In order to ensure that the composite steel plate does not lose its original comprehensive properties, the base layer and the cladding layer must be welded separately.
The welding at the junction of the base layer and the cladding belongs to dissimilar steel welding, and its weldability mainly depends on the physical properties, chemical composition, joint form, and filler metal composition of the base layer and the cladding. At present, the clad steel plate with austenitic stainless steel as the clad layer and pearlite as the base layer is the most widely used, followed by the clad steel plate with ferritic steel as the clad layer and pearlite as the base layer.
Welding method of composite steel plate
Usually, there are electrode arc welding, submerged arc welding, tungsten argon arc welding, CO2 gas shielded welding, and plasma arc welding. At present, the cladding is usually welded by argon tungsten arc welding or electrode arc welding, and the base layer is welded by submerged arc welding or electrode arc welding.
Groove form of clad steel plate welding
The butt joint groove form can be V-shaped, X-shaped, V, and U-combined grooves. The backside of the joint can also be machined at short intervals to remove the cladding metal to ensure that the base weld bead is not welded to the cladding when welding the base bead. Generally, X-shaped groove double-sided welding is used as much as possible, first welding the base layer, then welding the transition layer, and finally welding the cladding layer. In order to ensure that the welded joint has good corrosion resistance.
At the same time, considering the welding characteristics of the transition layer, the welding workload on one side of the cladding layer should be minimized. Whether the cladding is on the inside or the outside, the base layer is welded first. When the cladding is on the inner side, the base welding root shall be cleaned from the inner side before welding the cladding. When the cladding is on the outside, the last weld bead of the base layer shall be polished. When welding the cladding, first weld the transition layer, and then weld the cladding.
Filler Metal Selection for Welding of Clad Steel Plates
In most cases, a suitable intermediate filler metal is selected as the transition layer of the steel, so as to control the iron content of the final weld bead of the cladding metal, avoid embrittlement and cracks in the weld bead at the cladding and the base layer, and ensure the cladding welding. Corrosion resistance, wear resistance, and other special properties.
4 welding process sequences of clad steel plates
- Usually, the base layer is welded first, and the weld of the first base layer (carbon steel, low alloy steel) should not penetrate into the cladding metal to prevent embrittlement or cracking of the weld metal.
- When welding (surfacing) the clad side, the effect of dilution must be considered. Therefore, when welding the cladding of the clad plate, a suitable filler metal should be selected to weld one or more transition layers first, and then weld the cladding. The filler metal of the transition layer must be able to tolerate the dilution of the base steel.
- The roots can be cleaned by carbon arc gouging, shoveling, or grinding. Before surfacing the transition layer, any residues in the root bevel must be removed.
- Post-weld heat treatment is required to eliminate welding residual stress. When selecting the heat treatment temperature, consideration should be given to the difference in the heat treatment specifications of the base layer and the clad layer; the influence on the corrosion resistance of the clad layer; whether the diffusion of elements at the interface between the base layer and the clad layer will produce brittle phases. , resulting in the deterioration of the performance of the steel plate; due to the difference in the physical properties of the base layer and the cladding layer, the cladding layer has stress corrosion cracking and so on.