20% Cr弁用鋼の研究

Si, Ni, CおよびCrの影響について

抄録

The silchrome steel has been widely used for exhaust valves in the motor industry. However, recent experience has indicated that the products of combustion of leaded petrols exerted a rather marked corrosive attack on it, and that when such petrol was used, a steel containing higher chromium should be used.
To investigate the influence of Si, Ni, C and Cr on the properties of 20% Cr valve steel, the authors measured the critical point, the sectional diagram, the quenched and tempered hardness, the TTT-diagram, the retained austenite, the dimensional changes, and the mechanical properties at elevated temperatures.
The results obtained were as follows:
(1) The critical point was raised with the silicon and the chromium content, and lowered with the Ni addition.
(2) The hardenability increased with the nickel addition, and decreased with the Si content. The TTT-diagram showed only the pearlitic transformation within any reasonable duration.
(3) The quantity of the retained austenite after air-hardening from 1050°C increased with the nickel and the carbon content. The silicon content up to 2.8% increased the amount of the retained austenite, then more silicon addition decreased this amount. This retained austenite decomposed at 550°C and resulted in the volume expansion.
(4) The full hardness was obtained by air-cooling from 1050°C, and the austenitizing temprature for obtaining the maximum hardness was lowered with the silicon content, and when the silicon content was as high as 3.3%, the steel was not hardenable.
The hardness after 44h. tempering at a temperature between 600 to 750°C increased with the silicon, nickel and carbon content.
(5) As the carbon content increased, the tensile strength increased. The impact value decreased at room and elevated temperature. The impact value was increased with the Si content up to 2.3%, then decreased with more silicon addition, and also the nickel addition more than 2.1% decreased the impact value.