[L1] 1 Overview of Heat Affected Zone [L2] 1) Definition [L4] - A zone formed within a few mm outside the weld, where the base metal undergoes phenomena similar to direct heat treatment due to welding heat. [L4] - The Heat Affected Zone is a part of the base metal that is not melted but whose metallographic structure and mechanical properties are changed by welding heat or cutting heat. [L5] * Especially when the maximum heating temperature passes the low transformation point (A1) of steel, significant changes occur in the steel structure. [L4] - It is a region affected by heat over a wide range from the melting point and is called HAZ (Heat Affected Zone). [L4] - The mechanical properties and structural changes of the heat affected zone depend on the chemical composition of the base metal, cooling rate, welding speed, and results of preheating/post-heating. [L2] 2) Composition of Weld Heat Affected Zone [L3] ① Base Metal [L4] - The original material part that is not affected by heat. [L3] ② Embrittlement Zone [L4] - Zone from 500℃~ [L4] - Area where significant structural changes occur, but cannot be distinguished by microscopic examination. [L3] ③ Granular Pearlite Zone [L4] - From 700℃~ / 4.8mm from weld metal (Reference dimension) [L4] - Homogeneous coarse-grained area due to refinement of austenite grains. [L4] - Spheroidization of pearlite occurs, and martensite is formed upon rapid cooling. [L3] ④ Fine Grain Zone (Refined Zone) [L4] - From 900℃~ / 3.3mm from weld metal [L4] - Refined by recrystallization, possessing high toughness and good physical properties. [L3] ⑤ Coarse Grain Zone [L4] - From 1100℃~ / 2.8mm from weld metal [L4] - Area where austenite grains grow significantly due to overheating, resulting in marked coarsening. [L4] - Some Widmanstätten structures are found, and hardness is maximum. [L3] ⑥ Bond Zone [L4] - Boundary between weld metal and heat affected zone, forming Widmanstätten structure. [L3] ⑦ Weld Metal Zone [L4] - The part that melted and solidified, having a dendritic crystal shape. [L1] 2 Characteristics of Heat Affected Zone [L2] 1) Mechanical Properties [L3] ① Increase in Hardness [L4] - The hardness of the structure adjacent to the bond zone is the highest, and the maximum hardness is affected by the cooling temperature. [L3] ② Decrease in Toughness and Ductility [L4] - Elongation or toughness of the coarse grain zone significantly decreases due to martensite formation. [L3] ③ Embrittlement of Structure [L4] - There is a tendency for the impact value to increase rapidly in the coarse grain zone. [L4] - Although the embrittlement zone is not affected by heat, embrittlement due to strain aging occurs. [L2] 2) Defects in Heat Affected Zone [L4] - The material becomes brittle due to the heat affected zone, which may cause brittle fracture. [L4] - The designed durability may not be maintained due to significant changes in material properties. [L2] 3) Management of Heat Affected Zone [L4] - Control of HAZ cooling rate: Using Continuous Cooling Transformation (CCT). [L4] - Preheating and Post-weld Heat Treatment (PWHT). [L4] - Estimation and management of maximum hardness value by carbon equivalent (Brittleness management).