Round Robin test on measurement of phase transformation temperatures in 9Cr1Mo steel

Abstract

Modern heat resistant 9–12% Cr steels require optimised heat treatments and welding strategies to receive the best mechanical and long-term creep properties during the fabrication of power plant components. Therefore, the phase transformation temperatures—especially the austenite and martensite transformation temperatures—have to be well known to define optimised heat treatment and interpass temperatures as well as heating and cooling rates. Since phase transformations are influenced by the chemical composition of the materials and other numerous factors, it is important to pay attention to the circumstances surrounding the determination of these temperatures. To determine specific phase transformation temperatures various methods and procedures like dilatometry, single-sensor differential thermal analysis (SS DTA) or differential scanning calorimetry (DSC) were established. The presented comparative study was initiated to investigate and quantify possible scattering within the measured values of participating institutions. Therefore, the commercially available martensitic heat resistant steel P91 was used to compare specific phase transformation temperatures determined by several participating institutes and laboratories. Two different simplified temperature cycles were defined to identify possible scattering and differences within the determined phase transformation temperatures. Furthermore, possible differing results regarding the evaluation of a given dilatometry data set by various institutes and laboratories were discussed. The presented round robin test shows that institutes and laboratories although using standard methods of analysis—which are said to be state of the art—are reporting variable values for the critical phase transformation temperatures in the steel of interest. It is also shown that the amount of scattering can vary widely depending strongly on the specific phase transformation temperature which has to be determined.