Brittle Fracture of a Clapper Weldment for a Disk Valve Due to Improper Filler Metal

Abstract

The clapper in a 250 mm diam disk valve (made from ASTM A36 steel, stress relieved and cadmium plated) fractured at the welded joint between the clapper and a 20 mm diam support rod (also made of same material). The valve contained a stream of gas consisting of 55% H2S, 39% CO2, 5% H2, and 1% hydrocarbons at 40 deg C and 55 kPa during operation. Voids on the fracture surface and evidence of incomplete weld penetration were revealed by examination. Brittle fracture was indicated by the overall appearance through some fatigue beach marks were observed. Very narrow bands of high hardness were revealed at the edges of the weld metal. It was revealed by chemical analysis of this band that a stainless steel filler metal had been used which produced mixed composition at the weld boundaries. The plating material was revealed to be nickel by chemical analysis. It was concluded that clapper failed by fatigue and brittle fracture because it was welded with an incorrect filler metal. A clapper assembly was welded with a low-carbon steel filler metal, then cadmium plated.