Characterization of High-silicon alloy ductile iron in very thick sections

Sammanfattning: Valmet AB has ha foundry located I Karlstad, Sweden. The foundry has specialized in large components in ductile and gray iron. This report is a part of an evaluation of a new alloy that Valmet has produced. The alloy is called S550 where 550 is the expected tensile strength in a cast on sample. S550 is a ferritic ductile alloy with 4,15-4,25% silicone. In this experiment the alloy is casted in very large sections to better match the intended final product. The purpose of the report is to promote a better understanding of how silicon works in large castings. Ultimate tensile strength, yield strength and elongation will be measured. The microstructure will be mapped and described. This as a step to ensure that the material is ready to use to produce castings in large dimensions. In the experiment, the microstructure is examined from 3 charges. Mechanical properties are collected from the 3 charges by widened specimens from 3 details of 2785 kg/piece and 4 drilled samples from 4 details about 600 kg/piece. The samples are processed to test bars according to standard SS-EN1563: 2012. Thereafter the rods are dragged in a tensile testing machine to collect data. The microstructure is mapped through light optic microscopy. The samples where machined according to SS-EN1563:2012. Tensile testing machine was used to record the mechanical values. Microstructure was mapped using a light-optic microscope. The material achieved a tensile strength of 544 MPa on average which is below the expected value. The yield strength was 436 MPa in average. The material shows low dispersion, mainly in fracture limit and yield limit. The elongation in average 12.8% varied in the different geometries but with lower variations within the same geometry. The alloy got better mechanical values on Detail B that had a shorter cooling time. In Detail A, micro porosity and slag were found which adversely affected their mechanical properties. The ground mass is considered as ferritic. The graphite nodes had a size of 82μm and a nodule density of 75 nodules / mm2. Most nodules had graphite form VI, small amount of graphite V and III have been found. No degenerated graphite forms as chunky or exploded graphite has been found.