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Occasional failures were experienced in spool-type valves used in a hydraulic system. When a valve would fail, the close-fitting rotary valve would seize, causing loss of flow control of the hydraulic oil. The rotating spool in the valve was made of 8620 steel and was gas carburized. The cylinder in which the spool fitted was made of 1117 steel, also gas carburized. Investigation (visual inspection, low magnification images, 400x images, metallographic exam, and hardness testing) supported the conclusion that momentary sliding contact between the spool and the cylinder wall caused unstable retained austenite in the failed cylinder to transform...

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