Root Cause of Degradation in the Creep Strength of Martensitic Steel

Creep curves of Grade 91 and 92 steels were analyzed by applying an exponential law to the temperature, stress, and time parameters to investigate the formation process of the Z-phase, which lowers the long-term rupture strength of high-Cr martensitic steel. The activation energy (Q ), activation volume (V ), and Larson–Miller constant (C ) were obtained as functions of creep strain. At the beginning of creep, sub-grain boundary strengthening occurs because of dislocations that are swept out of the sub-grains, and this is followed by strengthening owing to the rearrangement of M23C6 and the precipitation of the Laves phase. Heterogeneous recovery and subsequent heterogeneous deformation start at an early stage of transient creep near several of the weakest boundaries because of the coarsening of the precipitates; this results in the simultaneous decreases in Q , V , and C  even in transient creep. Further, this activity triggers an unexpected degradation in strength because of the accelerated formation of the Z-phase even in transient creep. The stabilization of M23C6 and the Laves phase is important to mitigate the degradation of the long-term rupture strength of high-strength martensitic steel. The stabilization of the Laves phase is especially important for Cr-Mo systems because Fe2Mo is easily coarsened at approximately 600 °C compared to Fe2W in Grade 92 steel.

Development of questionnaire instrument to assess students’ transformative competencies in science learning

According to the OECD Education 2030 project, students in every country need to be ready as agents of change in the future. Three transformative competencies (Creating New Value, Reconciling Tensions and Dilemmas, Taking Responsibility) which are expected to develop students’ awareness, responsibility and innovation are used as focus of this study. A descriptive research for developing a valid and reliable questionnaire to assess the transformative competencies was conducted to investigate these competencies of junior high school students in Indonesia. It consists of 39 statements with a 5 Likert scale representing each transformative competency. A total of 445 students from 10 schools representing five provinces participated which consisted of students from 7th grade (222), 8th grade (91), and 9th grade (132) with the proportion of male (174) and female (271) students. The questionnaire was distributed on-line in sequence. The analysis results in that the questionnaire has a high reliability index of 0.96 for Cronbach’s Alpha (IBM SPSS Statistics 25), 0.94 for Person Reliability and 0.97 for Item Reliability (Winsteps Rasch Model) and good validity index for all items in the questionnaire, because the calculated r value is greater than the r table value. Correlation is significant at the 0.01 level (2-tailed). In general, students’ responses are on a “fairly typical of me” scale, except for TC4 statement “I can offer new solutions when there are problems while studying science”, which is “not very typical of me”. These results provide opportunities for further study to increase student creativity in science learning.

Numerical Prediction of Creep Rupture Life of Ex-Service and As-Received Grade 91 Steel at 873 K

Infallible creep rupture life prediction of high temperature steel needs long hours of robust testing over a domain of stress and temperature. A substantial amount of effort has been made to develop alternative methods to reduce the time and cost of testing. This study presents a finite element analysis coupled with a ductility based damage model to predict creep rupture time under the influence of multiaxial stress state of ex-service and as-received Grade 91 steel at 873 K. Three notched bar samples with different acuity ratios of 2.28, 3.0 and 4.56 are modelled in commercial Finite Element (FE) software, ABAQUS v6.14 in order to induce different stress state levels at notch throat area and investigate its effect on rupture time. The strain-based ductility exhaustion damage approach is employed to quantify the damage state. The multiaxial ductility of the material that is required to determine the damage state is estimated using triaxiality-ductility Cock and Ashby relation. Further reduction of the ductility due to the different creep mechanisms over a short and long time is also accounted for in the prediction. To simulate the different material conditions: ex-service and as-received material, different creep coefficients (A) have been assigned in the numerical modelling. In the case of ex-service material, using mean best fit data of minimum creep strain rate gives a good life prediction, while for new material, the lower bound creep coefficient should be employed to yield a comparable result with experimental data. It is also notable that ex-service material deforms faster than as-received material at the same stress level. Moreover, higher acuity provokes damage to concentrate on the small area around the notch, which initiates higher rupture life expectancy. It also found out that, the stress triaxiality and the equivalent creep strain influence the location of damage initiation around the notch area.

Magnetic Methods for the Identification of Incorrect Microstructures in Grade 91 Power Station Steels

Grade 91 steels have been used in power generation for more than 20 years in high temperature, high pressure applications such as steam piping, headers and tubing because it provides superior creep and oxidation resistance at elevated temperatures. The mechanical properties of the material are dependent on the creation of a martensitic microstructure, however incorrect heat treatment during manufacture, installation or repair can result in a weak ferritic or semi-ferritic microstructure which can cause premature component failure. Currently, components with incorrect, weak microstructures are identified using hardness testing; a manual technique which is prone to error. This work details a series of tests carried out at the University of Manchester to assess the suitability of multi-parameter magnetic testing for the identification of incorrect microstructures. The tests stem from a workshop organized by the Electric Power Research Institute (EPRI) where three sets of samples (eight pipe sections, eight tube sections and eight unidentified tube sections) with different microstructures were circulated world-wide. The results of the work show that the magnetic measurement techniques employed in these tests have the potential to provide a basis for the development of a portable NDE system for the identification of incorrect microstructures in Grade 91 plant components. The developed system would enable fast scanning of components with very little surface preparation along with digital data storage, improving on current manual hardness testing.