Cooling form of large diameter steel pipe:
Large-diameter steel pipes are steel pipes with an outer diameter of 1000MM or more. Large-diameter seamless steel pipes are made of steel ingots or solid tube blanks that are perforated into capillaries, and then hot-rolled, cold-rolled, or cold-drawn.
1. Chain cooling bed. In the past, the chain-type cooling bed with a simple structure was mostly used, which has a simple structure and low cost. However, because it is easy to cause chain dislocation to bend large-diameter steel pipes, and the large-diameter steel pipes cannot be collected freely from the input roller table to the cooling bed entrance, it is rarely used now.
2. Walking cooling bed. This cooling bed is composed of walking beams and fixed beams. The cooled large-diameter steel pipe is held up by the walking beam, moved forward for a distance, and then placed in the tooth groove of the fixed beam. Properly adjusting the stroke of the rack can make the large-diameter steel pipe roll twice every step, to achieve the effect of straightening the large-diameter steel pipe. At present, almost all newly built pipe rolling units adopt stepping rack cooling beds.
3. Spiral cooling bed. This kind of cooling bed is cooled by pushing the large-diameter steel pipe on the cooling bed to move forward by the helical wire on the screw rod. With the rotation of the screw rod, in addition to the forward thrust, the large-diameter steel pipe is also subjected to a side thrust. Therefore, it moves laterally while advancing, and only moves forward.
When the cooling rate is faster, the carbon content and alloy composition are higher, the uneven plastic deformation produced under the action of thermal stress during the cooling process is greater, and the residual stress formed later is greater. On the other hand, during the heat treatment process of steel, due to the change of the structure, that is, when the austenite transforms into martensite, the increase of the specific volume will be accompanied by the expansion of the volume of the workpiece, and the phase transformation of each part of the workpiece will cause the volume to grow inconsistently and produce the structure. stress. The final result of tissue stress changes is that the surface is subjected to tensile stress and the heart is subjected to compressive stress, which is just the opposite of thermal stress.
Construction preparations before welding of large-diameter steel pipes:
1. Organize relevant operators, familiarize themselves with construction drawings and specifications, formulate welding procedures, and prepare welding operation instructions.
2. Welders should be trained for corresponding items and have operation qualification certificates for corresponding items.
3. Check whether the roundness and misalignment of the incoming steel pipes meet the relevant technical requirements.
4. Check whether there are scars, cracks, serious corrosion, and other defects on the surface of metal pipe joints.
5. Before the installation of large-diameter steel pipes, the pipe joints should be measured and numbered one by one, and it is advisable to use pipe joints with small diameter differences for butt welding.
6. Welding in windy, rainy, and snowy weather There is a windproof, rainproof, and snowproof shed.
7. Before the welder performs welding, the welder shall clarify the requirements and welding measures.