Metallurgical Failure Analysis of a Fractured Steam Control Valve Stem

One fractured steam control valve stem was provided for metallurgical failure analysis. The specified material of manufacture for the steam control valve stem was Incoloy 901 alloy. The valve stem was nitrided, but the nitriding process was masked at the location of the fracture. The valve was reported from a throttling type service in a steam turbine subject to steam-induced turbulence while in a free state. The steam was reportedly held at a temperature of 1050°F under a pressure of 2400 psig. When the valve was in a closed position, the stem was subject to compression or buckling forces under a seating load of 13,000 lbs. The valve was in a guide bushing with a diameter of 2 in. It was requested that the fractured steam control valve stem could be evaluated to determine potential causes for failure. The scope of the project consisted of visual inspection, scanning electron microscopy with energy dispersive x-ray spectroscopy, tensile testing, hardness testing, chemical analysis, and metallographic analysis.