Effect of small addition of chromium on structure and properties of low carbon structural steels

Abstract

An attempt has been made to sfudy the effects of chromium and nickel on the structure and properties of 0.13 o4 carbon steels. The austenite grain coarsening behaviour of these steels have been first studied. The carburiz-ation technique has been adopted to reveal the prior austenite grain boundaries. It is observed from the vmiation of prior austenite grain size with temperature in the steels indicates that the undissolved particles of chromium carbide, Cr2C, refine the austenite grain size at temperatures below 1000 0 C but the effect decreases with increasing temperature. Nickel does not produce any austenite grain refinement. In presence of nickel, CrzC particles are less effective than the Cr2C particles in absence of nickel in the refinement of austenite grain size. The standard tensile and Charpy V-notch specimens made from steel bars were heat-treated at temperatures corresponding to a common austenite grain size cooled at four different rates were tested to obtain data on tensile and impact properties. The ferrite grain size of the steels after tansfonnation suggests that the precipitating particles of Cr2C are excellent in ferrite grain size refinement and also nickel refines ferrite grain size. In presence of nickel Cr2C precipitates are less effective than CrzC precipitates in absence of nickel in the refinement of ferrite grain size. At all cooling rates, Cr2C precipitates increase the yield strength. As the cooling rate increases, the yield stergth increment due to Cr2C precipitates increases. In presence of nickel the contribution of CrzC to yield stength is more than that of CrzC in absence of nickel at slow cooling rate. Nickel in solution contributes to yield shength. Ni and Cr are both dehimental to toughness. Cr is more detrimental than Ni. In presence of Ni , Cr is less harmful to toughness than Cr in absence ofNi. Both chromium and nickel decrease the ductility of low carbon steel, but cbromium is more harmful than nickel.