Understanding the heat-affected zone can improve welding quality

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The welding heat-affected zone: A weld joint is composed of three parts – the weld seam, the fusion zone, and the heat-affected zone – during welding. Welding heat-affected zone: abbreviated as HAZ (Heat Affect Zone), it is the region on either side of the weld where the solid base material undergoes significant changes in structure and properties as a result of the welding heat cycle; this region is known as the welding heat-affected zone. Microstructural distribution of hard-to-quench steels: Characteristic – martensite does not form under conditions of welding followed by air cooling. Such as low-carbon steel, 16Mn, 15MnV, and 15MnTi, etc. Based on the heating temperature and microstructural characteristics, it can be divided into four zones: the superheated zone, the normalizing zone, the partial normalizing zone, and the recrystallization zone. The overheated zone (coarse-grain zone) has a temperature ranging from the solidus line to 1100°C, and a width of about 1–3 mm. During welding, the austenite grains in this region grow significantly, and after cooling, a hyperheated structure with coarse grains is formed, resulting in a marked decrease in plasticity and toughness. Phase transformation recrystallization zone: The phase transformation recrystallization zone (normalizing zone or fine-grain zone) has a temperature range of 1100°C to Ac3, and a width of approximately 1.2 to 4.0 mm. Air cooling after welding treats the metal in this area as if it had undergone normalizing, resulting in a uniform and fine structure of ferrite and pearlite, and mechanical properties that are superior to those of the base material. Zone of incomplete recrystallization: The zone of incomplete recrystallization (also known as the partial normalizing zone) has a heating temperature between Ac3 and Ac1. During welding, only a portion of the structure transforms into austenite ； After cooling, fine ferrite and pearlite are formed, while the remaining portion retains its original structure; as a result, the grain size is uneven and the mechanical properties are poor. If the base metal has been deformed by cold working prior to welding, and the temperature is between Ac1 and 450°C, a recrystallization zone exists as well. The mechanical properties of the metal in this region change little, with only an increase in plasticity. If no cold plastic deformation occurs prior to welding, then recrystallization does not take place in the heat-affected zone