STUDY ON HEAT-RESISTING STEEL (VIII)

Abstract

The precipitation phenomena of Timken 16-25-6 alloy, the heat resisting steel for gas turbine, have already been reported in this journal. The effect of solution-treatment was studied and reported in relation to aging in (I) and (VI) of this series. In this report, the solution phenomena of precipitated particles were researched. There were nine kinds of samples, each having different amounts of C, Mo, and N contents, including the standard 16-25-6 alloy. For the purpose of studying the influence of varied contents of added elements on solution-treatment, eight groups were made by various combination of the nine samples. (see Table 1)
Before solution-treatment, which is the purpose of this reports, all samples were treated to finish or almost finish the precipitation in two ways (A) and (B). In (A), samples were heated at a rather high temperature (ex. 950°C, 900°C, and 850°C etc.) for more than several hours respectively after usual solution-treatment (1150°C×1h), and then kept at 700°C more than 100 hours. This meant the coagulating of precipitated particles, besides finishing the usual precipitation. In (B), samples were aged at 800°C for from 100 hours to 150 hours after usual solution-treatment. (B) meant the samples got stable high hardness and general precipitation more or less by aging. Samples, treated in (A) way, were heated at three grades of temperature 1000°C, 1150°C, and 1230°C for 2 honrs respectively for solution-treatment. Samples, treated in (B) way, were heated at three, grades of temperature, 1000°C, 1100°C, and 1200°C for from 10 minutes to 7.5 hours. All samples, which were solutiontreated in each temperature and time, were tested by Vickers hardness tester and observed through microscope to study the disolving phenomena of precipitated particles.
In (A), the precipitated particles were not yet disolved by heating at 1000°C for 2 hours, but disolved at 1150°C and annealing twins were observed. When the samples were heated at 1230°C annealing twins were easily seen and grain coarsening was also observed.
In (B), the softening of hardness by heating was almost parallel with the disolvening phenomena of precipitated particles. Some samples showed that the heating temperature 1000°C was not enough for solution-treatment of precipitated particles. The process of disolving of precipitated particles was clearly observed during heating at. 1100°C. The temperature 1200°C was too high to watch the process, because solution-treatment finished and annealing twins appeared after 10 minutes heating. The effect of C, Mo, and N content in 16-25-6 alloy on solution-treatment was also discussed The samples of low carbon showed rather low. hardness at both aged condition and solution-treated condition. More Mo samples meant more difficulties than with the low Mo samples. N showed similar influence with Mo, except the samples denitrolized by the addition of Ti. In this report, the articles which were precipitated artificially were disolved for the purpose of test, so that the conclusion from this data can not always be applied to the, case of practical solution-treatment under factory conditions.