Reasons for differences in the welding heat-affected zone

1. Due to high-temperature burnout of alloying elements, the chemical composition does not fully meet the requirements of the material standards. 2. The metal grains at the joint with the base material are coarse.

Does submerged arc welding of 20 steel result in poor toughness in the heat-affected zone?

It is caused by the enlargement of grains due to repeated heating during welding.

Repost: Microstructural distribution in the heat-affected zone (1) Fully quenched zone: During welding, the heat-affected zone is in a region above AC3; due to the high hardening tendency of such steels, a quenched microstructure (martensite) is obtained after welding. Near the weld area (corresponding to the overheated zone of low-carbon steel), severe grain growth occurs, resulting in coarse martensite, whereas the area corresponding to the normalized zone yields fine martensite. Depending on the cooling rate and wire energy, bainite may also form, resulting in a mixed microstructure coexisting with martensite. This zone belongs to the same type in terms of its microstructural characteristics (martensite), differing only in thickness; therefore, it is collectively referred to as the fully quenched zone. (2) Partially quenched zone: The heat-affected zone where the base material is heated to a temperature between AC1 and AC3; under rapid heating conditions, little ferrite dissolves into austenite, while pearlite, bainite, sorbite, etc. transform into austenite. During the subsequent rapid cooling, austenite transforms into martensite. The original ferrite remains unchanged and grows to varying degrees, eventually giving rise to a martensite-ferrite structure; hence it is referred to as the incomplete quenching zone. Sorbite and bainite may also appear when the carbon content and alloy element content are low, or when the cooling rate is slow. If the base metal is in a quenched and tempered state prior to welding, then in addition to the fully quenched and partially quenched zones mentioned above, the microstructure in the heat-affected zone may also undergo varying degrees of tempering, which is referred to as the tempering zone (the area below AC1). In summary, under the action of the welding heat cycle, the microstructural distribution in the heat-affected zone of metals is uneven. Severe grain coarsening occurred in the fusion zone and the overheated zone, which are the weak areas of the entire welded joint. For steel grades with high carbon content, numerous alloying elements, and a strong tendency to harden, martensite forms in the quenched structure, which reduces plasticity and toughness and thus makes cracks more likely to occur.

It is mainly caused by welding processes with high wire energy or multiple heating cycles; the solution generally involves reducing the heat input during welding, for example by using welding processes with lower wire energy.