The effect of chromium content on dimensional change, tensile strength, and hardness of sintered chromium steel was investigated.
The amount of carbon added in the form of graphite powder was 1.0, 1.2, and 1.4 weight percent. Between 1 and 4% chromium was added to each carbon content as high carbon Fe-Cu powder with 60% Cr. 0.85% of zine-stearate was added as a lubricant to the mixture and the powders were mixed for 1hr. in a V-cone mixer.
The mixed powders were compacted into tensile specimen under a pressure of 5t/㎠. These compacts were presintered for 30min. at 650℃, given a final sintering treatment at 1200℃ for 1hr. in hydrogen gas with a dew point of-15℃ to - 20℃. They were cooled to room temperature at a cooling rate of 50℃/min.
The specimens were buried in a mixture of alumina & carbon powder and sintered in graphite boats with graphite covers for the minimizing of decarburization by hydrogen gas and producing uniform structure through the cross section of the specimen.
The amounts of combined carbon were estimated from the sintered microstructure. For Fe-C samples without the addition of ferrochrome, the carbon content of the samples was about 0.4 0.5% lower than the amount added as graphite. The samples with ferrochrome added had somewhat higher carbon content because of the carbon introduced with the ferrochrome powder.
The specimens with chromium addition show growth after sintering. The amount of growth increases with chromium content.
For the specimens with varying chromium content compacted at 5t/㎠ and sintered the tensile strength increases with chromium content, reaches a maximum, and then decreases. This decrease is attributed mainly to the decreasing sintered density due to the growth during sintering for specimens with high chromium. It is estimated that for samples of constant sintered density, the tensile strength of samples containing chromium will be generally higher than those of chromium free samples.
Hardness also increases initially with chromium content and then remains constant. When the growth during sintering is taken into account, hardness is expected to show continuous increase with chromium content for the same sintered density.
The maximum value of tensile strength and hardness was found for a sample with 1% chromium to which 1.4% graphite had been added. It had a density of 6.9g/cc, a tensile strength of 60kg/㎟ and a hardness of $HR_B$ 90.
The microstructures of the sintered specimens are presented and an attempt is made to relate these to the mechanical properties and dimensional changes after sintering.
