Abstract
Microstructures and martensite transformation start (Ms) temperatures of the synthetic weld heataffected zone of two types of nodular graphite cast irons, FCD40 and FCD60, are investigated. The thermal cycles of the test pieces, 3 mm in diameter and 10 mm in length, are simulated to those of real arc welding; test pieces are rapidly heated to a temperature about 1100°C in 4 seconds, then cooled to room temperature. Implant cracking tests using smoothed 6 mm in diameter specimens FCD40 are also carried out with and without preheating.
When the peak temperature of the specimens were made higher, the Ms temperature became lower; from about 300°C to below 200°C. Though two types of cast irons had different matrices at room temperature, the Ms temperatures of those were nearly same. This means that, in spite of very short holding time, the minimum carbon content of matrices of both irons become nearly equal, when they are heated to high temperature near melting point. This concept was proved from the results of the implant tests of FCD40 showing same critical strength of welds, about 28OMPa, and martensitic fracture structure as previous work for FCD50 and FCD60.
