Alternative PWHT to Improve High-Temperature Mechanical Properties of Advanced 9Cr Steel Welds

Abstract

Creep-resistant 9Cr steels are extremely important in thermal power generation industry due to their marked resistance to creep and corrosion. The weldability of these alloys is critical since they are used in welded construction equipment. The required mechanical properties are achieved after post-weld heat treatment. This study examined the effect of different post-weld heat treatments on microstructure and mechanical properties of creep strength-enhanced 9Cr steel welding deposits. It was obtained with an experimental flux-cored arc welding wire used under protective gas (Ar-20% CO2). The heat treatments used were: (1) tempering (760 °C × 2 h), (2) solubilizing (1050 °C × 1 h) + tempering (760 °C × 2 h) and (3) solubilizing (1150 °C × 1 h) + first tempering (660 °C × 3 h) + second tempering (660 °C × 3 h). All-weld metal chemical composition was analyzed, and hot tensile tests were carried out at different temperatures. Charpy-V impact tests and Vickers microhardness measurements were also performed. Microstructures were studied using x-ray diffraction and optical and scanning electron microscopy. In all cases, a martensitic matrix with intergranular and intra-granular precipitates was detected. In the as-welded condition, δ-ferrite was also found. Microhardness dropped, and the impact energy increased with post-weld heat treatments. The highest hot tensile strength result was achieved with samples submitted to austenization at 1150 °C and double tempering at 660 °C.