Influence of Boron and Nitrogen on the Heat Affected Zone of Modified 9Cr-1Mo Steel:Gleeble Simulation Study

Boron is reported to improve Type IV cracking resistance of 9Cr-3W-3Co-V-Nb steel, with this improvement being attributed to the presence of uniform microstructure in the heat affected zone (HAZ) and its thermal stability during high temperature service. Similar results have been observed recently in modified 9Cr-lMo steel; hence, the present work aims to understand the role of boron and nitrogen in the microstructural evolution of the HAZ in this steel. For this purpose, using a Gleeble thermal-mechanical simulator, MAZ microstructures were simulated in normalized (1100°C/lh) and tempered (760°C/3h) modified 9Cr-lMo steels with different boron and nitrogen contents. The Gleeble simulation was carried out by subjecting the specimens to heating cycle at different peak temperatures in the range 875 to 1200°C using a heating rate of 45°C.s^-1. The effect of heating rate on the transformation temperatures was also studied using dilatometry. The simulated HAZ specimens were characterized using optical and scanning electron microscope, x-ray diffraction (XRD) and hardness measurements.

Microstructural examination of the Gleeble-simulated specimens shows that typical lath martensitic structure is present in all specimens of the boron-containing steel. However, in the 910°C simulated specimens of the boron-free steel lath martensite is absent and fine prior austenite grains are present. It is also observed that hardness increases marginally with increase in peak simulation temperature in the boron-containing steel. To understand the microstructural evolution in the HAZ, the crystallite size, lattice strain and dislocation density were estimated by analysis of XRD data. While variations in crystallite size are marginal, the lattice strain and dislocation density vary with peak simulation temperature. Further detailed microstructural analyses have clearly shown the stability of microstructure in the HAZ depends on the boron and nitrogen contents in the steel. The present paper presents and discuss the results of this experimental investigation.