About the welding technology of steel pipes

Welding heat affected zone

Welded joint is composed offusion zone, weld and heat affected zone.

Welding heat affected zone:HAZ (heat affected zone for short) is the area where themicrostructure and properties of solid base metal on both sides ofthe weld change significantly under the action of welding thermalcycle, which is called welding heat affected zone.

Microstructure distribution ofhard quenched steel

Features: martensite is noteasy to form under welding air cooling conditions. Such as 16Mn,15MnV, 15mnti, low carbon steel, etc.

According to themicrostructure characteristics and heating temperature, it can bedivided into four areas: normalizing area, overheating area, partialnormalizing area and recrystallization area.

Superheat zone (coarse grainzone)

The width is about 1 ~ 3 mmand the temperature is between 1100℃of the solidus. Duringwelding, the austenite grains in this area become larger and largerseriously. The superheated structure obtained after cooling iscoarse, and the toughness and plasticity are significantly reduced.

Phase transformationrecrystallization zone

Phase transformationrecrystallization zone (normalizing zone or fine grain zone)

The width is about 1.2 ~ 4.0mm and the temperature is between 1100 ~ AC3. After welding, themicrostructure is uniform and fine ferrite and pearlite. Air coolingmakes the metal in this area equivalent to normalizing treatment, sothe mechanical properties are better than the base metal.

Incomplete recrystallizationzone

Incomplete recrystallizationzone (also known as partial normalizing zone)

The heating temperature isbetween AC1 and AC3. During welding, only part of the structure istransformed into austenite; After cooling, fine ferrite and pearliteare obtained, and the rest are still original structure, so themechanical properties are poor and the grain size is uneven.

Recrystallization zone

If the base metal is deformedby cold working before welding and the temperature is between ac1-450℃, there is a recrystallization zone. The mechanical properties ofthe metal in this area do not change much, but the plasticity isimproved. If there is no cold plastic deformation before welding,there will be no recrystallization zone in the heat affected zone.