STUDY ON TIMKEN 16-25-6 TYPE HEAT-RESISTING ALLOY (I)

Influences of Solution-Treatment and of Nitrogen on High-Temperature Age-Hardening

Abstract

The influences of solution-treament (1000°-1250°C) and of nitrogen (0.04-0.16%) on the high-temperature age-hardening phenomena (600°-950°C) of the Timken 16-25-6 heat-resisting alloy were studied.
The results were as follows;
(1) The influence of nitrogen on the hardness of solution-treatment was little. In microstructure, almost homogeneous solid solution was obtained by the solution-treatment at 1200°C for 1 hour. As the nitrogen content was higher the, temperature at which the majority of precipetated small particles were dissolved became lower.
Grain growth began at 1100°C and generally the higher the nitrogen content was, the lower became the temperature of grain growth and the more serious the degree of grain growth. The solution-treatment seemed suitable at 1170°C 1 hour.
(2) The maximum age-hardening was obtained at 700°C. Generally the higher the nitrogen content was, the slower became the rate of hardening, however at the temperature above 850°C the influence of nitrogen on it became small. As for microstructure, at 600°C precipitation arose mainly at the grain-boundaries, but at 700°C they precipitated mainly at the cleavages and "striated structure" was obtained and at the temperature above 800°C, precipitated particles globulized and at 900°C conspicuous coarsening of precinitated particles began.
(3) In order to obtain the complete high temperature age-hardening, when the nitrogen content was higher, the higher solution-treatment temperature became necessary. For the age-hardening at 700°C for 70 hours, at the solution-treatment temperature under 1100°C specimen C was minimum, but at 1200°C the influence of mitrogen was remarkable and except specimen A, the higher the nitrogen content was, the smaller became the degree of agehardening.
(4) As for age-hardening at 700°C for 500 hours, in case of 1200°C for the solution treatment temperature the higher the nitrogen content was, the smaller became the degree of hardening, while at 1250°C for the solution treatment temperature, the higher the carbon plus nitrogen content was, the more became the degree of hardening for the age-hardening at 600°C for 1000 hours, the rate of hardening was very slow and at times over 500 hours the hardeness of all specimens approached constant value independent of nitrogen content. The microstructure after the age-hardening at 600°C for 1000 hours was "striated structure" and the microstructure at 700°C for 500 hours was similar to it at 750°C for 70 hours.