Equal-Channel angular pressing of high-carbon steel

Abstract

This master thesis deals with the research to the effects of severe plastic deformation on the development of the microstructure and the mechanical properties of three low-alloyed high-carbon steels. The steels investigated include a hypo-eutectoid, an eutectoid and a hyper-eutectoid steel containing 0.61, 0.81 and 1.22 weight-percent carbon. The microstructure of the eutectoid steel consists only of pearlite, whereas that of the hypo-eutectoid and the hyper-eutectoid steels also contains several volume-percent of ferrite and grain boundary cementite, respectively. Several experiments were conducted using equal-channel angular pressing at a processing temperature of 500 °C (773 K). The die configuration consisted of a channel angle of 120 ° and a corner angle of 0 °, yielding an effective deformation of 0.67 per pass. The microstructure was analysed and the mechanical properties were assessed after each pass. Samples were analysed both in the initial and in the deformed conditions by means of hardness testing, qualitative and quantitative image analysis using secondary electrons in a scanning electron microscope and the assessment of the crystallographic orientation using electron backscattered diffraction. The results revealed the formation of very complex microstructures due to the severe deformation. The cementite lamellae deform plastically and gradually break up into tiny segments of the size of coarse second-phase particles. Besides, small pieces of pearlite regroup to form smaller clusters in the hyper-eutectoid steel. A subgrain structure is formed in the ferrite phase and in the ferrite lamellae, exhibiting a subgrain boundary misorientation that progressively rises from low- to high-angle boundaries with increasing deformation. It appeared that the size of the subgrains is largely controlled by the interlamellar distance of the cementite lamellae and that the subgrain boundary misorientation is determined by the local amount of deformation. The me-chanical properties exhibit dramatic changes; from 351 MPa to 1,066 MPa for the yield strength and from 743 MPa to 1,141 MPa for the ultimate tensile strength, but contradictory values for the elongation at fracture.