Abstract
The transformation behavior of nitrogen bearing 25Cr-13Ni-1Mo and 21Cr-11Ni-2.5Mo steel weld metals containing 1.5% to 5% delta-ferrite has been studied after heat-treatment in the temperature range 700°C to 1000°C. The extent of the delta-ferrite transformation was estimated magnetically using a ferrite scope. The microstructural changes occurring in the delta-ferrite were examined using a transmission electron microscope and a scanning electron microscope, and it was found that the delta-ferrite transformed to sigma-phase, chi-phase and austenite during aging in the temperature range 700°C to 1000°C. The rate of transformation was significantly high at temperatures 850°C to 900°C, and was, however, rather low at lower or higher temperatures. The transformation behavior of delta-ferrite in the temperature range examined could be described by the Johnson-Mehl's equation y=l-exp (-ktn). Based on these results, a time-temperature parameter has been developed for representing by means of a single variable, the combined effects of time and temperature on the transformation of delta-ferrite during aging. Available. data are analyzed, which result in indicating that it is possible to predict the transformation behavior of delta-ferrite in the weld metal after heat-treatment or long time service at elevated temperatures using the newly developed time-temperature parameter.
