Why is it necessary to conduct impact testing on the heat-affected zone during impact tests, when the heat-affected zone is not part of the weld?

During the welding process, the heat-affected zone (HAZ) refers to those areas that have not been melted, but whose microstructure and properties have changed due to the high temperatures. Although the heat-affected zone is not part of the weld itself, it is a very important component in the welding process. Due to the rapid temperature changes in the heat-affected zone, this can cause the material to harden or become brittle, thereby affecting the mechanical properties of the entire welded structure. Impact testing is primarily conducted to evaluate the toughness and fracture resistance of welded joints under sudden impacts or loads. The properties of the heat-affected zone directly affect the overall quality and safety of the welded joint. If the heat-affected zone experiences a decline in performance due to the thermal effects during welding, it can become a weak point in the welded structure in practical use, prone to cracking or failure. Therefore, including impact tests on the heat-affected zone in welding procedure qualification is intended to comprehensively evaluate the performance of welded structures and ensure welding quality and safety. .

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Normal weld condition: Strength of heat-affected zone ≤ Strength of base material ≤ Strength of weld

Welding procedure qualification involves the evaluation of weld joints, including the heat-affected zone.

My personal opinion is as follows: 1) As mentioned on floor 5, the welded joint includes the heat-affected zone； 2) Whether impact resistance is required is related to the type of base material. For now, it pertains to welding ordinary low-alloy steel bases; although the heat-affected zone is located on the base material, the entire welding process involves temperature increases and decreases (the temperature is higher the closer it is to the weld line). This can be considered a very brief instantaneous heat treatment process (although the temperature involved is not that of conventional heat treatment). At the microscopic level, the grain size increases, and all these factors affect the mechanical properties of the heat-affected zone (especially impact resistance) as well as its corrosion resistance. The heat-affected zone represents the weak point in the properties of the joint, which is why impact tests are necessary ； As for some high-alloy steels (such as austenitic stainless steels), they already possess good toughness, and in terms of impact performance, they can meet the required standards even under the aforementioned conditions. Therefore, generally, there is no need to test the impact properties of the heat-affected zone, just as with the base material; however, special cases such as temperatures below -196 degrees or specific technical requirements may arise. The above is for reference~~~