In order to prolong the life and to strengthen mechanical properties of exhaust valves made of high-N-19Cr-8Ni valve steel which was widely used for automotive industry, the authors investigated the influence of C, Si, Ni, W, Mo, V, Nb, Cu and N on various properties of this valve steel. Some properties measured were hardness-change by solution treatment, aging and cold working, mechanical properties at room and elevated temperatures, rupture strength, magnetic permeability, oxidation resistance in the air at elevated temperature and corossion resistance to liquid PbO at 1000...
The experimental results obtained were as follows:
(1) The as-quenched and the as-aged hardnesses, the yield and the tensile strength at room temperature and the rupture strength were increased, but the impact strength were decreased with C contents.
(2) With the alloy containing 037-319% Si, as the Si was increased, the hardness after solution treatment, the tensile strength at room temperature and the oxidation resistance were increased. Especially the oxidation resistance in air at elevated temperature was excellent with the alloy containing 2-3% Si. The as-aged hardness, the impact strength and the corossion resistance to liquid PbO were decreased with the alloy containing more than 1% Si.
(3) The impact strength, the rupture strength and the tensile and yield strength at elevated temperature were increased by addition of Ni more than 8%.
(4) When the carbide-forming elements such as W, Mo, and Nb were added, the hardness, the mechanical properties and the rupture strength were increased, but the impact strength was decreased. When the sum of W+Mo contained were more than 1% or when Nb added was more than 05%, the strength at room temperature was decreased, but the rupture strength was enhanced by addition of these elements. The hardness and the strength were decreased by V addition.
(5) The hardness and the strength were decreased, but the rupture strength was definitely increased by Cu addition.
(6) When about 0'3% N was added, the hardness as solution-treated and the strength at room and elevated temperatures were extremely improved.