Effect of Heat Input on Microstructure and Properties in Heat Affected Zone of ASTM A572 GR.65 Steel

The correlation among heat input (HI), microstructure and properties in heat affected zone of ASTM A572 GR.65 steel (GR.65 steel) was investigated using optical microscopy (OM), Vickers hardness tester and Charpy-V-Notch (CVN) testing. The result shows that due to the restriction by the Nb(CN) precipitates, the grain size in coarse grain heat affected zone (CGHAZ) at a heat input of 10 kJ/cm is 75 µm, which is half as that at the heat input of 40 kJ/cm. It is indicated that weld preheating is beneficial to the toughness in CGHAZ at low heat input of 10 kJ/cm, while it appears the opposite effect at high heat input of 40 kJ/cm. The heat input has a great influence on dissolution of precipitated particles and the austenite grain size, which contributes a lot to the microstructure and properties of GR.65 steel.