Hiromitsu Endoh, Mitsuo Mutsuura and Takemi Murayama
Technical Center, Nittan Valve Co. Ltd., Hadano 257-0031
Engines for passenger cars have intake valves and exhaust valves. Especially, exhaust valves are exposed to emission after combustion in cylinders: they are in the corrosive environment with sulfuric acid, hydrochloric acid, nitric acid or the adhesion of residue. Such an environment causes various kinds of corrosion. Recently, the result that we investigated the valve which had broken in motion, sulfuric acid corrosion phenomenon was found out on the neck taper. It occurred to an exhaust valve for a diesel engine made of SUH35 steel, of the austenitic heat resistant steel with solution and aging treatment. The corrosion was going ahead in the grain-boundary choosing-ly and on the grain-boundary, the corrosive substance such as sulfur and oxygen were detected. Also, the corrosion had the characteristic that we could observe metallic gloss on the surface and the valve didn't degrade by the corrosion and so on. It is expected that such a grain-boundary corrosion declines material strength remarkably. Therefore, we thought that we need to reproduce corrosion in order to clarify the mechanism of this corrosion phenomenon.
This study was to experimentally reproduce the grain-boundary corrosion which occurred to the valve, with various corrosion experiments. Moreover, we examined about the relation between the corrosion and the fatigue strength by performing fatigue examination in the test pieces which had reproduced corrosion. The results we have obtained are as follows.
(1) The grain-boundary corrosion which occurred to the valve could be reproduced experimentally in performing the corrosion examination with the condensed liquid of sulfuric acid.
(2) It has been proved that grain-boundary corrosion's occurrence caused the fatigue strength decline.
(3) It has been admitted that fatigue strength has downward tendency with increasing of the grain-boundary corrosion depth.
(Received April 26, 1999; In Final Form August 17, 1999)
engine valve, condensed liquid, sulfuric acid, corrosion, grain-boundary, austenitic heat resisting steel, experimental reproduction, fatigue strength, corrosion depth
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