The part of the base metal that did not melt during welding but whose microstructure and properties were changed by the welding heat. Also called the HAZ; hardening, softening, grain coarsening and sensitization can occur there.
Japanese name: 溶接熱影響部
Heat-affected zone appears in 4 practice questions across 1 topic, including Materials processing and joining.
A joint made by fusion welding such as arc welding is divided into the weld metal, which melted and solidified; the heat-affected zone (HAZ) outside it, which did not melt but received heat; and the base metal, which is hardly affected by heat. Within the HAZ, the peak temperature is higher closer to the boundary with the weld metal (the fusion line) and lower farther away. According to the peak temperature, the HAZ of steel is divided into bands such as a coarse-grained zone where grains have grown large, a fine-grained zone refined by transformation, an intercritical zone where only part transformed, and a tempered zone that does not transform, each with different hardness and toughness.
The part of the HAZ of steel heated into the austenite range is quenched as the surrounding cold base metal draws off its heat, so in steels of high hardenability it becomes hard martensite. When a hard microstructure, diffusible hydrogen from the weld metal and tensile stress from restraint coincide, cold cracking occurs some time after welding. The guide to hardenability is the carbon equivalent; in steels where it is high, preheating or postheating slows the cooling and lets hydrogen escape. Increasing the heat input also slows the cooling, but the grains in the coarse-grained zone can grow too large and the toughness can fall.
The problems that arise in the HAZ depend on the material. In austenitic stainless steels, Cr carbides precipitate at grain boundaries in the band exposed to about 500 to 800°C, causing sensitization and intergranular corrosion. In precipitation-hardened aluminum alloys and in materials strengthened by cold working, precipitates coarsen or the material recrystallizes, softening the HAZ so that the joint is weaker than the base metal. Even in high-strength steels strengthened by quenching and tempering, parts heated above the tempering temperature can soften. Joint design must allow for such strength loss in the HAZ.
Answer: A. It slows the cooling of the weld so that hard martensite is less likely to form, and gives diffusible hydrogen time to escape
Cold cracking occurs when three things coincide: diffusible hydrogen that has entered the weld metal, a hard, low-toughness microstructure (martensite), and tensile stress from restraint of the weld. Preheating lengthens the cooling time through the range of 800 to 500°C, suppressing hardening, and also lengthens the time held at high temperature, making it easier for hydrogen to diffuse out. Using low-hydrogen electrodes after drying them and postheating right after welding are countermeasures with the same purpose. Hot cracking (solidification cracking) is the tearing of liquid films enriched in segregated elements such as P and S at the final stage of solidification, and both its mechanism and its countermeasures differ. Rapid quenching to harden the weld actually makes cold cracking more likely.
Answer: C. In zones heated to about 500 to 800°C, Cr carbides precipitate at grain boundaries, leaving Cr-depleted zones. Use SUS304L or Ti- or Nb-added steels
In the heat-affected zone a short distance from the weld line, there is a band exposed for some time to temperatures of about 500 to 800°C. There, Cr carbides such as Cr₂₃C₆ precipitate at grain boundaries, and the Cr concentration right next to the boundaries falls to a level that cannot sustain passivity, so corrosion proceeds along the grain boundaries. This is called sensitization. Countermeasures are to use a low-carbon steel (SUS304L) with less of the carbon that forms the carbides, to use stabilized steels (SUS321 or SUS347) in which carbon is tied up as Ti or Nb carbides, and to give a solution heat treatment after welding to dissolve the carbides again. Austenitic stainless steels do not become martensite on quenching, and Cr evaporation is not the main cause either.
It varies greatly with the welding process and heat input. In arc welding it is often a few mm, while in laser welding and electron beam welding, which have high energy density, the heat input is small and the zone is much narrower. The larger the heat input, and the thinner the plate so that heat escapes less easily, the wider the heat-affected zone.
To some extent. In steel, postweld heat treatment (stress-relief annealing) lowers residual stresses and tempers the hardened regions to restore toughness. Sensitization of stainless steel can be reversed by a solution heat treatment that dissolves the carbides again. In large structures, however, heat-treating the whole is difficult, so the basic approach is to prevent problems through the choice of material and welding conditions.
Yes. The material does not melt, but it is heated by friction, so outside the stir zone there form a thermo-mechanically affected zone, which received both heat and deformation, and a heat-affected zone, which received heat only. In precipitation-hardening aluminum alloys, precipitates coarsen and soften the material in this region, and it can be the weakest part of the joint.
In the heat-affected zone a short distance from the weld line, there is a band exposed for some time to temperatures of about 500 to 800°C.
When austenitic stainless steels such as SUS304 are welded,…The higher the Ceq of a steel, the more readily the weld heat-affected zone is quenched into hard martensite and the greater the risk of cold cracking.
The carbon equivalent used as a guide to the weldability of…A larger heat input slows the cooling of the weld, making the heat-affected zone less likely to harden, but it lengthens the time at high temperature, so grains can coarsen and toughness can fall.
Arc welding is carried out at a voltage of 25 V, a current …Martensite Stainless steel Passivity Sensitization Carbon equivalent Hardenability Extrusion Hot isostatic pressing Investment casting Shrinkage cavity