Broken Exhaust Valve

A broken exhaust valve from the cylinder of a motor car had a 30-mm disk diam and 8-mm stem diam. The site of the fracture was directly where the valve cone joined the cylindrical stem. Both the cone and the stem were heavily scaled in the vicinity of the fracture; in some parts the scale has flaked off. Furthermore, the rim of the disk was badly damaged by secondary mechanical action. The core of the valve had a very fine austenitic microstructure with precipitations of numerous granular and very fine, mostly rounded carbides and tine segregation bands. A hard alloy facing was welded on to the valve seat. Fracture was a consequence of fatigue corrosion cracking, itself strongly promoted by the presence of sulphur compounds. The origin of these corrosive sulphur compounds could not be explained.