AGING BEHAVIOUR OF COPPER BEARING HSLA-100 (GPT) STEEL

Abstract

Investigations have been carried out on the development of copper bearing HSLA-100 (GPT) steel with composition (weight percent): C-0.05, Cu-1.23, Ni-1.77, Mo-0.51, A1-0.025, Mn-1.00, V-.004, P-.009, S-0.01, Si-0.34, Cr-0.61, Ti-0.003, Al-0.025, Nb-0.037, Sb-0.003, As-0.005, which was received from Naval Research Laboratory, Washington (USA) under an Indo-US Technical Collaboration Programme. The steel was given treatment, which comprise of solution treatment/austenitization followed by oil quenching and tempering/aging at various temperatures. The effects of various processing parameters have been described with the help of mechanical property measurement, optical microscopy and scanning electron microscopy (SEM). The HSLA-100 (GPT) steel was first austenitized at temperatures 1000°C for 180 minutes and then oil quenched. The austenitized specimens at 1000°C were tempered/aged at 400°C for varying lengths of time, from as low as 1 minutes to as high as 1200 minutes. As a result of aging, martensite is transformed into acicular ferrite and precipitation of carbide takes place at grain boundaries as well as within the laths/grains. The precipitation of microalloying element preferably Nb(Nb,Ti)C carbide particles initially form at the lath/grain boundaries in the peak aged condition and later on in the laths/grains. The content of Nb in precipitates increases with increasing aging time. Copper is always present in the carbides precipitates, retained austenite and in the matrix, where no separate precipitation of copper in this steel could be observed. Peak hardness is observed after aging for a particular aging time of 360 minutes. The microstructure at peak aged condition shows ferrite structure along with coherent precipitates of carbides of microalloying elements containing copper. Highest concentration of carbides particles has been observed in the peak aged condition of this steel. At the peak hardness level good hardness is observed at the cost of toughness. It has been observed that prolonged aging (aging beyond peaks) causes the formation of polygonal/acicular ferrite and coarse carbide particles. These precipitates particles in the over aging condition are rendered incoherent with the matrix of ferrite causing restoration of toughness by promoting homogeneous deformation and retarding early initiation of quasi-cleavage fracture.