The effects of copper on the metallurgical, mechanical, and fracture properties of 0.90 carbon rail steels.

摘要

The effects of variations in copper content on the metallurgical, mechanical, and fracture properties of pearlitic rail steels with a base composition (in wt pct) of 0.9 C, 1.0 Mn, 0.35 Si, and 0.01 Ti were evaluated. Six industrial heats with copper content ranging from (in wt pct) 0.07 to 0.85 were cast, re-heated, rolled and air-quenched with identical industrial processing parameters to produce full rails of the 136RE section. The materials were tested to determine the influence of copper content on austenitic grain size, pearlitic interlamellar spacing, microstructure, hardenability, hardness profile, tensile and yield strength, Charpy U-notch impact toughness, K1c fracture toughness, and fatigue crack growth rate according to standard ASTM testing methodologies.;The austenitic grain size, as determined by the McQuaid-Ehn method, suggested that copper does not influence the austenite grain growth characteristics in the temperature range of the test for the steels evaluated. Jominy end-quench hardenability testing showed that copper acts to delay pearlite transformation to a small degree. Pearlite interlamellar spacing measurements, determined via scanning electron microscopy, indicated that increased copper content refines the interlamellar spacing, which agrees with the Jominy hardenability data. The refinement in pearlite interlamellar spacing with increasing copper content increased hardness and strength according to a Hall-Petch type relationship. Both the impact toughness and fatigue crack growth rates were essentially independent of copper content. There was a slight decrease in fracture toughness with increasing copper content most likely due to the increase in yield strength with increasing copper content. Nonetheless, copper does not diminish the strength-toughness balance in pearlitic rail steels, and copper appears to primarily act as another hardenability element during the production of steel rails. The results of this study suggest that the current maximum allowable copper content of 0.4 wt pct specified by the American Railway Engineering and Maintenance-of-Way Association (AREMA) may be too restrictive for modern steel rail production technology.