Classification control of welding stress and deformation
Welding stress and deformation often degrade the quality of the welded product and may even have to be scrapped because it cannot be remedied. The generation and development of welding cracks are also closely related to welding stress and deformation. In general, welding deformation is unfavorable for welding quality, but if you master the mechanism and law of deformation, you can use it and control it. For example, the deformation is used to correct the deformation.
1. The concept of welding stress and deformation During the welding process, the internal stress and deformation caused by the uneven heating of the arc heat is temporary. When the workpiece is cooled, the internal stress and deformation that remain inside the workpiece are called residual stress and residual deformation. We refer to the welding stress and deformation as the residual stress of the weld and the residual deformation of the weld.
1.1 Classification of welding stress
1.1.1 According to the basic causes of stress can be divided into:
Thermal stress - stress due to uneven temperature distribution during welding.
Tissue stress - due to temperature changes, causes stresses caused by tissue changes.
1.1.2 The time according to the stress can be divided into:
Instantaneous stress—The stress that exists in a given chamber under certain temperature and stiffness conditions.
Residual stress - generally refers to the internal stress that is still present after complete cooling after welding.
1.1.3 According to the effect of stress can be divided into:
Longitudinal stress - its direction is parallel to the weld axis.
Transverse stress - its direction is perpendicular to the weld axis.
1.1.4 According to the direction of stress in space, it can be divided into:
Unidirectional stress - exists in one direction in the weldment.
Two-direction stress—stress acts in different directions in a plane, also known as plane stress.
Three-way stress—stress exists in all directions along the space, also known as volumetric stress.
1.2 Classification of welding deformation
Due to the form of the welded joint, the thickness and shape of the workpiece, the length of the weld and its position, various forms of deformation occur during welding. Generally, it can be divided into: longitudinal deformation, lateral deformation, bending deformation, angular deformation, wave deformation, distortion and the like.
2. Welding deformation and stress formation Welding deformation and stress are caused by many factors colleagues. The most important factors are: uneven temperature distribution on the weld; shrinkage of the deposited metal; metal structure transformation of the welded joint and rigid constraint of the workpiece.
2.1 Uneven temperature distribution on the weldment Due to the action of the arc, the weldment is locally heated to the melting temperature, and a large temperature gradient is formed between the weld and the base metal. According to the principle of thermal expansion and contraction, the object is heated to be elongated, and the elongation at different temperatures is different. The high temperature region of the joint requires a large amount of elongation and is hindered to form a compressive stress; and the elongation at a lower temperature region is small. The part forms a tensile stress due to resistance to elongation in the high temperature region.
During the cooling process, the volume of the molten metal shrinks, and the base metal other than the joint restricts its shrinkage to form a tensile stress in the weld zone, and the base material is subjected to compressive stress adjacent to the weld zone.
The weld seam and the adjacent weld zone almost lose the yield strength at high temperature, and plastic deformation occurs under the action of stress. After cooling, residual stress and residual deformation are formed in the weldment.
The shrinkage weld metal of the deposited metal shrinks in volume during solidification and subsequent cooling. Deformation and stress are caused in the weldment, and the magnitude of the deformation and stress depends on the amount of shrinkage of the deposited metal pair, and the amount of shrinkage of the deposited metal depends on the amount of molten metal. For example, the angular deformation of the V-shaped groove is due to the large amount of deposited metal in the upper part of the weld, the large amount of shrinkage, and the small section of the lower part of the weld is small, the amount of deposited metal is small, the amount of shrinkage is small, and the up and down shrinkage is inconsistent. Caused.
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Ordinary mullite porcelain is made of aluminosilicate natural minerals as a main raw material, and is formed by a reaction sintering method in which a eucalyptus is synthesized in a sintering process or a mullite is first synthesized and then sintered. Due to the low purity of raw materials and high impurity content, in addition to Al2O3 and SiO2, the components also contain impurities such as TiO2, Fe2O3, CaO, MgO, Na2O, K2O, etc., so that there are a considerable amount of glass phase in the product, resulting in mechanical and thermal properties. Poor, the excellent performance of mullite ceramics at high temperatures cannot be fully utilized. Therefore, ordinary mullite ceramics in the industry can only be used as a general refractory material in the case where the temperature and high temperature strength are not high.
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