What is the characteristic of the composition of low carbon steel welded joints and the microstructure and properties of metals in various regions during electrode arc welding?
(1) The welded joint consists of weld metal and heat affected zone
1) Weld metal: When welding, the temperature at the weld is above the liquidus, and the base metal and the filler metal form a common molten pool, which becomes an as-cast structure after condensation. During the cooling process, the liquid metal crystallizes from the fusion zone toward the center of the weld to form a columnar crystal structure. Since the electrode core and the coating have alloying effect during the welding process, the chemical composition of the weld metal is often superior to that of the base metal. As long as the welding rod and welding process parameters are selected reasonably, the strength of the weld metal is generally not lower than the strength of the base metal.
2) Heat-affected zone: During the welding process, the metal on both sides of the weld is affected by the heat of welding to produce a region where the structure and properties change.
(2) The thermal effects of low carbon steel are divided into fusion zone, superheat zone, normalized zone and partial phase change zone.
1) The fusion zone is located between the weld and the base metal, and part of the metal-baked portion is not melted, also called the semi-melting zone. The heating temperature is about 1 490 to 1 530 ° C. The composition and structure of this zone are extremely uneven, the strength is decreased, and the plasticity is very poor. It is the birthplace of cracks and local brittle failure.
2) Overheating zone Close to the fusion zone, the heating temperature is about 1 100~1 490 °C. Since the temperature greatly exceeds Ac3, the austenite grains grow sharply, forming an overheated structure, which greatly reduces the plasticity, and the impact toughness value decreases by about 25% to 75%.
3) Normalizing zone The heating temperature is about 850~1 100 °C, which is a normal normalizing heating temperature range. After cooling, a uniform fine ferrite and pearlite structure is obtained, and its mechanical properties are superior to those of the base material.
4) Partial phase change zone The heating temperature is about 727 ~ 850 °C. Only part of the organization changes, the tissue is not uniform after cooling, and the mechanical properties are poor.