Manabu Tanaka1, Ohmi Miyagawa2, Tsune-aki Sakaki2 and Dai Fujishiro3
1Graduate School, Tokyo Metropolitan University, Tokyo
The precipitation behavior of 21-4 N austenitic valve steels, in which the grain boundary reaction is apt to occur owing to the high concentrations of carbon and nitrogen, has been investigated at various aging temperatures ranging from 600 to 1050°C. At higher temperatures above about 850°C, the grain boundary reaction preceded the general precipitation occurred in the interior of grains and its rate was fast. However, it slowed down with decreasing temperature and only the general precipitation occurred below about 700°C.
In order to study the driving force of the grain boundary reaction, the pre-aging was applied at 700°C to the solution treated steels prior to higher temperature aging. The grain boundary reaction took place at a much slower rate during aging at higher temperatures even in short time pre-aged steels because of the decreased supersaturation in the austenite matrix and the increased pinning force acting on the reaction front caused by finely dispersed particles due to pre-aging. It was also found that in the pre-aged steels the grain boundary nodules ceased to grow at higher supersaturation as a result of the increased pinning force for growth. For the same reason, the grain boundary reaction was considerably suppressed by cold working after the solution treatment. As regards to the effect of the cooling procedure from the solution temperature, a higher growth rate of the reaction was observed in air-cooled steels than the water-quenched ones.
A small addition of phosphorus provided the sites for homogeneous precipitation and consequently very fine precipitates were uniformly formed in the matrix in phosphorus-containing steels. In phosphorus-containing steels, however, the observed growth rate of the reaction was somewhat higher than that in steels of standard composition. It was suggested that in phosphorus-containing steels an additional driving force caused by coherent precipitation strain had a strong influence on the rate of the reaction.
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