Effect of Tempering on Microstructure, Strength and Toughness Gradient in Quenched Low-Alloy Medium-Thickness Steel Plate

To elucidate how tempering temperature influences through-thickness microstructure and strength–toughness gradients in an online direct-quenched (DQ) low-alloy medium-thick plate, a 25-mm plate was direct-quenched from 900 °C to <150 °C and tempered at 530 °C × 1.5 h or 580 °C × 1.5 h. Tensile and room-temperature Charpy V-notch impact testing and microstructure characterization were performed at the upper surface, mid-thickness, and lower surface. In the as-DQ state, the upper surface contained ferrite (F, ~60%) and granular bainite (GB, ~30%) with minor lath bainite (LB, ~10%) and a small amount of martensite/austenite (M/A). The mid-thickness and lower surface remained dominated by F + GB (mid-thickness: GB~50%, F~30%, M/A~20%; lower surface: F~85%, GB~15%); the mid-thickness showed the lowest yield strength/ultimate tensile strength (YS/UTS) of 498/675 MPa. In the as-DQ state, the room-temperature Charpy V-notch absorbed energies at the upper surface, mid-thickness, and lower surface were 223.23, 229.88, and 261.22 J, respectively, indicating a pronounced through-thickness variation (ΔE(max–min) ≈ 38 J). After tempering at 530 °C, the upper surface and mid-thickness developed an F + tempered sorbite (TS) microstructure (upper surface: F~70%, TS~30%; mid-thickness: F~60%, TS~40%), whereas the lower surface was mainly ferrite with a small amount of spheroidized carbides/tempered cementite (SC). The mid-thickness YS/UTS increased to 619/805 MPa, and the impact energies at the upper surface and mid-thickness increased to 240.62 J and 235.56 J, respectively, resulting in a reduced through-thickness gradient. After 580 °C tempering, recovery and polygonal ferrite formation dominated; surface yield strength increased but mid-thickness yield improvement was limited.