Knowledge of welding methods.

What is brazing? How is brazing classified? What are the characteristics of brazed joints?

Brazing is to use metal with lower melting point than the base metal as brazing filler metal. After heating, the brazing filler metal melts and the weldment does not melt. The liquid brazing filler metal wets the base metal, fills the joint gap and diffuses with the base metal, thus firmly connecting the weldments together.

According to the different melting points of solders, brazing can be divided into soft soldering and hard soldering.

(1) soldering: the melting point of soldering solder is lower than 450°C, and the joint strength is lower (less than 70 MPa).

(2) brazing: the melting point of brazing filler metal is higher than 450°C, and the joint strength is higher (more than 200 MPa).

The bearing capacity of brazed joint is related to the size of joint surface. Therefore, lap joints and kits are generally used for brazing to make up for the lack of brazing strength.

The fusion welding method using electric arc as heat source is called arc welding. It can be divided into manual arc welding, submerged arc welding and gas shielded welding. Automatic welding has the advantages of simple equipment, flexible and convenient application and wide application, and can weld various welding positions, straight seams, circumferential seams and various curved welds. It is especially suitable for the welding of short welds and occasions with unchanged operation. Submerged arc welding has the characteristics of high productivity, good weld quality and good working conditions. Gas shielded welding has the characteristics of good protection effect, stable arc and concentrated heat.

submerged-arc welding

What are the characteristics of the composition of low carbon steel welded joints, the microstructure and properties of metals in various regions during arc welding in covered electrode?

(1) The welded joint consists of weld metal and heat affected zone.

1) weld metal: when welding is heated, the temperature at the weld is above the liquidus, and the base metal and filler metal form a common molten pool, which becomes as-cast structure after condensation. During the cooling process, the liquid metal crystallizes from the fusion zone to the center of the weld, forming columnar crystal structure. As the covered electrode core and coating have alloying effect in the welding process, the chemical composition of the weld metal is often superior to that of the base metal. As long as the covered electrode and welding process parameters are selected reasonably, the strength of the weld metal is generally not lower than that of the base metal.

2) Heat affected zone (HAZ): the zone where the structure and properties of the metals on both sides of the weld are changed due to the welding heat during the welding process.

(2) The heat affected zone of low carbon steel is divided into fusion zone, overheated zone, normalized zone and partial phase transformation zone.

1) the fusion zone is located between the weld and the base metal, and some metals are baked and some are not melted, which is also called semi-melting zone. The heating temperature is about 1 490 ~ 1 530 C. The composition and microstructure in this area are extremely uneven, the strength decreases and the plasticity is very poor, which is the birthplace of cracks and local brittle failure.

2) The overheated zone is close to the fusion zone, and the heating temperature is about 1 100 ~ 1 490 C. As the temperature greatly exceeds Ac3, austenite grains grow rapidly and form overheated structure, which greatly reduces plasticity and impact toughness by about 25% ~ 75%.

3) The heating temperature in the normalizing zone is about 850～1 100°C, which belongs to the normal normalizing heating temperature range. After cooling, uniform and fine ferrite and pearlite structures are obtained, and their mechanical properties are superior to those of the base metal.

4) The heating temperature of some phase change areas is about 727 ~ 850 C. Only part of the microstructure is transformed, and after cooling, the microstructure is uneven and the mechanical properties are poor.

electroslag welding

Electroslag welding uses the resistance heat generated by current passing through slag as a heat source, melts the filler metal and the base metal, and forms a firm connection between metal atoms after solidification. At the beginning of welding, the welding wire is short-circuited with the starting welding groove, and a small amount of solid flux is continuously added, which is melted by the heat of the arc to form liquid slag. When the slag reaches a certain depth, the feeding speed of the welding wire is increased, and the voltage is reduced, so that the welding wire is inserted into the slag pool, and the arc is extinguished, thus turning to the electroslag welding process.

Electroslag welding mainly includes nozzle electroslag welding, non-nozzle electroslag welding, wire electrode electroslag welding and plate electrode electroslag welding, etc.

Its disadvantages are that the input heat is large, the joint stays at high temperature for a long time, the weld is easy to overheat, the weld metal is in coarse crystalline as-cast structure, and the impact toughness is low, so the weldment generally needs to be normalized and tempered after welding.