THE INFLUENCE OF POROSITY AND CARBON CONTENT ON THE FRACTURE TOUGHNESS OF SOME SINTERED STEELS

Abstract

Fracture toughness (K_IC) and mechanical properties at room temperature (RT) and at 200 K were evaluated for a range of sintered steels. In a series of quenched and tempered samples prepared from a lowalloy, atomized powder (Höganäs ATST-A) with 0·45 wt.-%C and density varying between 6·7 and 7·8 g/cm³ (powder forged), the K_IC varied between 28 and 80 MN/m^3/2 at RT and between 34 and 58 MN/m^3/2 at 200 K. There was a correlation between the K_IC and the yield strength of the porous materials, due to the fact that fracture of the specimen is effected by plastic instability on a micro scale. Adding carbon (0–0·84%C) and copper (2%) to a sponge-iron powder and sintering to a final density of 6·8 g/cm³ gave a material with a fracture toughness of ～34 MN/m^3/2 when the C content was >0·6%. At lower C contents the toughness was considerably increased, but it was not possible to obtain a valid. K_IC determination. In this investigation a new type of fracture-toughness specimen was used, the RCT specimen, diameter 75 mm and thickness 29 mm; by using this type as opposed to the ordinary CT variant, specimens ～50% larger could be used.