INFLUENCE OF MICROSTRUCTURE ON TENSILE PROPERTIES OF SPHEROIDIZED ULTRAHIGH-CARBON (1.8-PCT-C) STEEL

Ultrahigh-carbon steel (UHCS) containing 1.8 pct carbon was processed to create microstructures consisting of fine-spheroidized carbide part icles (0.2- to 1.5-mum size range) within a fine-grained ferrite matri x (0.3- to 5-mum range) through a variety of thermomechanical processi ng and heat-treatment combinations. Tensile ductility, yield, and frac ture strengths, and strain-hardening behavior were evaluated at room t emperature. Yield strengths ranged from 640 to 1450 MPa, and uniform t ensile elongation ranged from 3 to 23 pct. Quantitative analyses revea led that a Hall-Petch type relationship exists between the yield stren gth and the ferrite grain size and carbide particle size within grain interiors. The fracture strength, on the other hand, was found to be u niquely dependent on the coarse carbide particle size typically found at grain boundaries. Data from other investigators on spheroidized car bon steels were shown to correlate well with the data for the UHCS (1. 8 pct C) material. It was shown that the tensile ductility will increa se when the difference between the fracture strength and the yield str ength is increased and when the strain-hardening rate is decreased. Th e basis for the trends observed is that the tensile ductility is limit ed by the fracture process that appears to be dictated by the nucleati on of cracks at large carbide particles. The results obtained indicate that UHCSs have significant potential for sheet applications where hi gh strength and good ductility are primary requirements.