The Effect of Long-Term Ageing on Microstructural Properties and Laves Phase Precipitation of Welded P91 and P92 Steels

Creep resistant 9% Cr steels are frequently used in civil engineering projects of supercritical coal-fired power plants operating at temperatures of superheated steam around 610 degrees C. This work focus on analysis of microstructure degradation and degradation of mechanical properties of orbital narrow-gap welding of P91 and P92 steels. Nowadays welding technology of orbital narrow-gap using tungsten inert gas is not limited to welding of high-pressure steam pipelines. This method is highly productive and efficient and leads to an increase in productivity, quality, and repeatability of welding process. It replaces the original method of manual welding with coated electrode into a standard bevel, which is a time-consuming process that costs a high amount of weld metal. Welds are degraded at 650 degrees C/30 000 h without applied stress and the effect of degradation is evaluated across the weld based on the observed changes in microstructure and hardness. Attention is focused mainly on precipitation and coarsening of Laves phase in the main areas of heat affected zone (HAZ): coarse grain HAZ, fine grain HAZ, and intercritical HAZ. The results show that frequency and rate of Laves phase coarsening in the fine grain and coarse grain HAZ are higher than in the base material.