STUDY ON HEAT-RESISTING STEELS (VII)

Abstract

The influence of temperature and time during the solution-treatmen on age-hardening was studied. There were four kinds of samples with varying chemical compositions. Three of them were standard Timken 16-25-6 alloys, varying nitrogen content from 0.04%, 0.10% to 0.16%. Another one was denitrized by adding Ti. After hot-forgingto bars, they were solution-treated at 1100°C (2010°F), 1150°C (2100°F), and 1200°C (2190°F) for 40 minutes, 1 hour 3 hours, 6 hours and 10 hours respectively. The hardness was measured and the microstructure was observed immediately after solution-treatment. The hardnesswas lower when it was solution-treated at a higher temperature and for a longer time. Sometimes the residual influence of hot-forging on grain boundary was observed even after solution-treatment. Some solution-treatment, operated at a rather low temperature or for a short time could not remove completely the influence of hot forging on grain shape, but another solution-treatment at a high temperature or for a long time removed the forged shape ofthe grain boundary.
The influence of the hot forging before solution-treatment, observedin grain boundary after solution-treatment, was more easily removed in sample of 0.04% N than in sample of 0.16% N.
All samples, solution-treated under different conditions, were aged at 800°C, (1470°F) and hardness was measured from 1 hour to 200 hours. The samples which were solution-treated at a higher temperature for a long time had lower hardness than the samples heated at a lower temperature for a shorter time. But, when they were heated at 800°C (1470°F) and reached stable maximum hardness after 30-100 hours aging, their hardness became almost uniform. This means that a sample, completely solution-treated, hardened in a larger range and reached the maximum stable hardness of the other aged samples which started from a higher hardness immediately after solution-treatment at a lower temperature and a shorter time heating, overcoming the difference of hardness at beginning of aging process.
The microstructure of all samples was observed after aging at 800°C (1470°F) for 200 hours. The grain-size was larger and the coagulated size of precipitated particles were more or less larger and the distribution of precipitated particles was more typical of the general precipitation type of the sample of solution-treatment at a higher temperature and longertime than the sample at a lower temperature for a shorter time.