After the weld zone and the filler metal is melted metal joints, faster speed again after cooling and solidification to form. Weld cast from liquid metal crystals organization, coarse grains, segregation, not dense tissue. However, due to the small weld pool, fast cooling, strict control of the chemical composition, carbon, sulfur, phosphorus is relatively low, also adjust the chemical composition of the weld by alloying to contain a certain amount of alloying elements, and therefore, the performance of weld metal the problem is not to meet performance requirements, particularly strength easy to achieve. Fusion zone melting transition and non-melting regions. The chemical composition of the fusion zone uneven, thick tissue is often thick tissue overheating or coarse hardened tissue. Its performance is often the worst of welded joints. Fusion zone and heat affected zone of the overheated zone (or quench zone) is the worst of the mechanical properties of welded joints of the weak parts, will seriously affect the quality of welded joints. High temperature heat affected zone is heated weld zone microstructure and properties caused by changes in the region. Low carbon steel heat affected zone can be divided into hot zones, normalizing area and part of the phase transition region. (1) The maximum heating temperature hot zone area above 1100 ℃, coarse grains, and even overheating organization, called the hot zone. Ductility and toughness decreased significantly overheated zone, the heat-affected zone is the worst part of the mechanical properties. (2) The maximum heating temperature normalizing zone from zone Ac3 to 1100 ℃, the air cooling after welding to obtain finer grain size of normalizing organization called normalizing area. Better mechanical properties of normalizing area. (3) part of the phase transition zone to the maximum heating temperature from Ac1 Ac3 region, the only part of the organizational phase transition, called the partial phase transition zone. This area grain uneven performance is also poor. During the installation welding, welding welding methods more often. Welded joint is a high temperature heat source to the base metal is heated locally while irregularities and molten filler metal is melted and solidified form. Depending on the microstructure and properties of the various parts, welded joints can be divided into three parts. Figure 2-l, the welded joints in the area known as welding melts solidified weld, which consists of molten base metal and filler metal. The impact of the base metal is heated during welding (but not melted) and the microstructure and mechanical properties of the area called the heat-affected zone occurs. Fusion zone is at the junction of the weld metal and welded joints in the heat-affected zone, the fusion zone a very narrow enough, a width of 0.1 ~ 0.4mm. Iron machining, sheet metal processing Changzhou, residential fence processing www.pjxdbjc.com