Failure analysis of spring of hydraulic operated valve

The breakage of spring of a valve causes the non functionality of a hydraulically operated system which pours steel into tundish (controls the flow of steel before putting into a caster) in a steel manufacturing plant. The valve is operated with two springs of diameter 0.3 mm among which one was broken. The spring failed after 5 years of service which had expected life of 10 years. The fracture surface under SEM showed striations confirming fatigue failure. The chemistry of the spring was confirmed to be o be a plain high carbon steel consistent with the requirements of ASTM A228 “Steel wire,Music quality” which is a cold drawn steel wire for mechanical spring. The microstructural analysis showed crack associated with scale and crow feet. The presence of crow feet like surface defect acted as stress concentration for fatigue crack initiation. The valve spring activated100 times per hour causing fatigue loading of the spring. The etched condition shows drawn pearlite typical structure of cold drawn spring and hardness was also found to be satisfactory. The presence of crow feet surface defect came from the wire drawing stage due to insufficient lubrication and considered to be manufacturing defect. The inspection of a spring this kind for critical applications should be robust to avoid future failures