A feasibility study on the development of hard and strong bainitic steels in medium and high carbon low alloy steels by short transformations

Abstract

High strength bainitic steels are the new kind of steels with good combination of strength and toughness. These steels are very much required as replacement for conventional quenched and tempered steels for several applications. The bainitic steels eliminate the two stage hardening and tempering processes required of quenched and tempered steels and also avoid the possible crack formation during hardening. Most of the bainitic steels are produced by adding substantial amounts of nickel or manganese to low and medium carbon steels and through very long duration of bainitic transformation, as high as 20 days. Long processing time makes these steels not viable for industrial production. The main objective of this research is to study the feasibility of developing a novel nano-structured type of bainitic steel with good combination of mechanical properties. This is to be achieved by optimizing the steel composition to get carbide-free bainitic microstructure by isothermal transformation conducted at around 225 oC to 325 oC for short durations in the range 2 to 3 hrs. The resultant steel is expected to have a hardness value above 600 HV, an yield strength around 1000 MPa, ultimate tensile strength around 1100 MPa, a minimum elongation of 12% and a Charpy energy of about 20 - 40 J at 25oC. Some very common applications of bainitic steels involve rail transportation and defence applications, mainly for making armor vehicle coverings. Two chemical compositions were considered with different concentrations of carbon (C), aluminium (Al) and cobalt (Co). For both the steels, it is required to find out the minimum time required to produce bainite with above said properties. The addition of Al and Co is believed to be necessary to produce the desired non-carbidic bainitic structure at very low temperatures within a reasonable transformation time. A small quantity of niobium (Nb) is added to form strong and tiny carbides and thus fine grained structure which in turn gives high strength and toughness. Accordingly, two steel compositions of C: 0.45 and 0.8%, Si: 1.5 - 1.75%, Mn 2 - 2.5%, Cr: 1.0 – 1.26%, Mo: 0.2 – 0.3%, Nb: 0.04 - 0.06%, Al + Co: up to 2.5%, were arrived at for a detailed experimental work for this study. The designed steels were melted in ambient atmosphere in a high frequency induction furnace and made as ingots. These ingots were initially hot forged into plates of 238x140x62 mm and then hot rolled into plates of 760x200x15 mm. The actual chemical composition of this alloy was checked by both wet chemical and Spectrometer analysis. The rolled plates were then subjected to homogenization annealing at 1250 oC for 24 hours in a high temperature muffle furnace in high purity nitrogen atmosphere to get uniform microstructure that is free from massive carbides. The martensite start temperature (Ms) and bainite start temperature (Bs) of this steel were calculated empirically and the same are 175 oC and 330oC respectively. The austempering temperatures were selected between 200 and 350oC and austempering time range was selected between 30 and 180 min, since a minimum of 30 min is required as incubation time to start bainitic transformation. Small samples of size 25x25 mm were cut from the homogenized and machined plates and were subjected to austempering treatment. This process involves austenitizing the samples at 900 oC for about 1 hour in a controlled atmosphere resistance type muffle furnace and then immediately transferring the samples to a molten salt bath maintained at the austempering temperature and holding for a given duration. The microstructures due to different heat treatments and transformations were characterized by light optical metallography at different magnifications after 2% Nital etching. Further they were analysed by scanning electron microscopy (SEM) attached with energy dispersive spectroscopy (EDS) for high resolution microscopy and for elemental analysis. Thin foil transmission electron microscopy (TEM) and X–ray diffraction (XRD) analysis were carried out for the determination of the phases formed in the microstructure and their quantification. Vickers hardness was tested at 30 kg load. Charpy impact testing was carried out to evaluate the room temperature toughness. Tension testing was carried out to know the strength and ductility of the steels. Mixture of bainitic ferrite, unresolved martensite and stabilized blocky austenite was observed in different proportions in all microstructures of high carbon bainitic steel and the tensile strength values were about 580 – 600 MPa and the room temperature Charpy toughness values were about 10 J for almost all conditions. The reduction in strength and toughness of the steel may be due to coarse grained microstructure, defects like inclusions and micro porosities, blocky nature and high volume percentage of the retained austenite and mixed microstructures containing bainite and martensite due to incomplete transformation within the short transformation durations. A relatively refined microstructure containing mixture of bainitic ferrite, unresolved martensite and stabilized blocky austenite is observed in different proportions in all microstructures of medium carbon bainitic steel. The yield strength is about 800 – 1000 MPa, tensile strength is about 1000 – 1250 MPa and the room temperature Charpy toughness values are about 18 – 23J for almost all conditions. Medium carbon bainitic steels are showing better properties comparatively than high carbon steels within three hours of austempering treatments.