Analysis of fracture in the heat affected zone of a weld in cast iron

An investigation was conducted into the fracture of a large valve body. The fracture initiated at a weld repair on the exterior surface of the valve. Metallography revealed the valve body was made from gray cast iron. Energy dispersive x-ray spectroscopy of a sample of weldment showed the filler material was a nickel-iron alloy appropriate for use on cast iron. The heat affected zone (HAZ) was narrow and contained plate martensite and retained austenite, indicating rapid cooling and a lack of preheat or post-heat of the weld. Cracking was observed originating at porosity at the weld root in the martensitic region and propagating along the graphite flakes. Ultimately fracture of the valve was determined to have been progressive cracking initiating in the HAZ due to improper welding parameters for gray cast iron.