How is the air hole generated in the pressure vessel welding? How to tell?
The pores of the weld metal in the pressure vessel are one of the common weld defects. It can strongly reduce the tightness of the weld. The influence of the mechanical properties of the metal is mainly to reduce the plasticity. In general, the pores can reduce the plasticity of the weld by 40% to 50%, and the structure of the work under dynamic load is more serious. The pores have little effect on the strength, but too many pores will weaken too much due to the working section of the weld, and the strength will still decrease. The pores are caused by the bubbles generated in the molten pool that did not come out during crystallization. There are two types of gases that can generate pores when a pressure vessel is welded, that is, a metallurgical reaction to produce a gas that does not melt in a metal, such as co, h2o, etc.; a gas that dissolves in a metal at a high temperature and a sudden decrease in solubility during crystallization, and is supersaturated, such as nitrogen. And hydrogen. Some pores can be found on the surface of the weld, called penetrating pores, because of the contact with the air, the surface of the hole is oxidized. The external pores can be dense or dotted. Some of the pores are hidden inside the weld and must be discovered by perspective. From the cross section of the weld, it is distributed along the columnar grain boundaries and is in the form of a bark. Sometimes, individual dot or elliptical small pores can be seen in the root and middle of the weld. The pores inside the pressure vessel are not in contact with the air, so the pores are bright. Whether or not the pores are formed and exposed depends on the relationship between the speed at which the bubbles float and the crystallization rate of the molten pool. The crystallization rate is fast, or the bubble is small and the floating speed is slow, then the internal gas is formed.
