In the process of heating and cooling of elbow, due to the inconsistent cooling speed and time between the surface and the heart, the temperature difference will be formed, which will lead to the uneven expansion and contraction of the volume and produce stress, that is, thermal stress. Under the action of thermal stress, since the initial temperature of the surface layer is lower than that of the heart, the contraction is larger than that of the heart, so that the heart is strained. When the cooling is over, the surface layer is compressed and the heart is strained because the contraction of the final cooling volume of the heart cannot proceed freely. That is, under the action of the thermal stress of the carbon steel elbow, the surface layer of the workpiece is finally compressed and the core is tensile. This phenomenon is influenced by cooling rate, material composition and heat treatment process. When the cooling speed is faster, the carbon content and alloy composition is higher, the carbon steel elbow, during the cooling process under the action of thermal stress produced by the non-uniform plastic deformation, the final formation of residual stress is larger.
On the other hand, in the process of heat treatment of carbon steel elbow steel, due to the change of structure, namely austenite to martensite transformation, due to the increase of specific volume will be accompanied by the expansion of the workpiece volume, each part of the workpiece has phase transformation, resulting in inconsistent volume growth and resulting in structural stress. The final result of the change of tissue stress is the surface tensile stress and the core compressive stress, which is exactly opposite to the thermal stress. The size of tissue stress is related to the cooling rate, shape and chemical composition of the workpiece in the martensitic transformation zone.
The preheating of carbon steel elbow is helpful to reduce the maximum hardness of heat affected zone of medium carbon steel and prevent cold cracking. This is the main process of welding medium carbon steel. Preheating can also improve the plasticity of welded joints and reduce the residual stress after welding. Usually 35, 45 steel preheating temperature is 150 ~ 250℃, due to the thickness and stiffness of the relationship, if the carbon content is higher or crack tendency is larger, the preheating temperature can be increased to 250 ~ 400℃. If the weldment is too large and the overall preheating is difficult, local preheating can be carried out. The heating range of local preheating on both sides of the weld is 150~200 mm. When conditions permit, it is best to use alkaline electrodes. If post-welding stress relieving heat treatment cannot be performed, post-welding heat treatment should be performed immediately. In terms of design stress, carbon steel elbow has good compression performance.
However, due to the influence of various factors, the actual wall thickness of the floor heating pipeline is usually the same. Under this wall thickness, all kinds of pipelines can meet the requirements of floor heating, so the pressure resistance advantage of the national carbon steel elbow can not meet the requirements of floor heating. Reflection type; Thermal conductivity: Pipes used for floor heating require good thermal conductivity, low temperature thermal shock resistance and low temperature impact resistance. For the better. Carbon steel needs to be taken care in the welding process, used in the appropriate environment, to continuously meet the important performance, welding in the important environment, to ensure quality. In the groove form, the welded parts shall be welded with U groove as far as possible. In case of casting defects, the groove shape dug out with a shovel should be smooth. Its purpose is to reduce the proportion of base metal melted into weld metal, thereby reducing the carbon content in the weld and preventing cracks. Since the proportion of base metal melting to the first layer of weld metal is as high as 30%, low current and low welding speed should be used to reduce the base metal penetration depth. Post-welding heat treatment is the best method to eliminate stress immediately after welding, especially for weldments with large thickness, high stiffness and poor working conditions (dynamic load or impact load). The tempering temperature to eliminate stress is 600 ~ 650℃.