Does the temperature change in the welding process directly affect the welding quality?

The welded joint consists of three parts: weld, fusion zone and heat affected zone

Under the action of thermal cycle, the microstructure and properties of weld base metal change obviously, which is called welding heat affected zone.

Microstructure distribution of hard quenched steel

Features: it is not easy to form martensite under welding air cooling conditions, such as low carbon steel, 16Mn, 15MnV and 15mnti.

According to the heating temperature and microstructure characteristics, it can be divided into four areas: superheated area, normalized area, partial normalized area and recrystallization area.

Superheat zone (coarse grain zone)

The temperature is between solidus and 1100 ℃ and the width is about 1 ~ 3 mm. During welding, the grains in this area are coarse. After cooling, the overheated structure with coarse grains is obtained, and the toughness and plasticity are reduced.

Phase transformation recrystallization zone (normalizing zone or fine grain zone)

The temperature is between 1100 ~ AC3 and the width is about 1.2 ~ 4.0 mm. After welding, air cooling makes the metal equivalent to normalizing treatment. Therefore, the structure is uniform and fine, with ferrite and pearlite, and the mechanical properties are better than the base metal.

Incomplete recrystallization zone (partial normalizing zone)

The heating temperature is between AC3 and AC1. During welding, part of it turns into austenite, and ferrite and pearlite are obtained after cooling. The rest are still original microstructure and poor mechanical properties.

Recrystallization zone

The base metal is deformed by cold working before welding, and there is a recrystallization zone at the temperature of ac1-450. The mechanical properties of the metal in this zone change little, but the plasticity is improved. In the heat affected zone, there is no recrystallization zone without cold plastic deformation before welding.

The welding heat affected zone is different from the weld. The performance of the weld can be ensured through the adjustment of chemical composition and welding process, but not in the heat affected zone. The microstructure distribution is uneven under the action of thermal cycle. The welding structure mainly considers the hardening, toughening, embrittlement, softening, comprehensive mechanical properties, fatigue properties and corrosion resistance of the heat affected zone, It shall be determined according to the specific requirements of the welded structure.

The hardening of the heat affected zone depends on the cooling conditions and chemical composition of the steel. Its essence is the properties of different metallographic structures. The hardness test is convenient. The highest hardness Hmax of the heat affected zone is often used to judge, which can indirectly predict the brittleness, toughness and crack resistance of the heat affected zone. It should be noted that even if it is the same structure, the hardness is different, which is different from the carbon content Alloy composition is related to cooling conditions.