Effect Of Alloying Elements Al And Si On Microstructures And Mechanical Properties Of The Ultrahigh Carbon Steels

Ultrahigh carbon steels (UHCSs)are the steels which contain carbon between those of conventional steels and cast irons, in which the continuous network carbides precipitate along primary austenite grain boundaries under routine processing, and then greatly increase brittleness of the steels. However, it revealed that mechanical properties of the steels can be greatly improved by obtaining microstructures fine equiaxed ferrite matrix and spheroidal carbides.UHCSs with chemical compositions of C=1.5～1.9%,Cr=1.3～1.7%,Al=1.0～4.0%or Si=2.0～3.0% were prepared by vacuum melting. The ingots are forged at 1050～1100℃into bars with diameter of 20~25mm, and the forged bars are used as starting materials. Heat treatment, microstructure analysis and mechanical-property measurements were conducted for the UHCSs.It revealed that (1) Alloying elements Al and Si can restrain continuous network carbides precipitation along primary austenite grain boundaries as well as refine pearlite microstrucutures and thin its lamellar space, but Al Al is prior to Si on both aspects. For example, the pearlite lamellar space of UHCS-2.83Si is 0.2μm, that of UHCS-2.61Al is 0.072μm. (2) Al has also great effect on pearlite morphology, such as there is much split lamellar carbides in pearlite colony as Al<2.0%. Actually, the coarse cementite pieces seen under lower magnification is made up of ferrite and much thinner carbides pieces with homogeneous lamellar space under higher magnification in pearlite as Al>2.0%. But Si has hardly effct on it. (3) However, Si is prior to Al in promoting graphitization of UHCSs. For instance, small amount of thin graphite lamellar is precipitation in UHCS-2.13Si and small amount of small-scale cluster flocculent graphite is precipitation in UHCS-4.03Al including carbon. But it is existence that much large-scale lamellar graphite in UHCS-2.83Si. (4) UHCSs containing Al all exhibit good forging property even when it comes to UHCS-4.03Al precipitating graphite. In contrary, UHCSs containing Si, in which lamellar graphite is precipitation, exhibit bad forging property, especially, it cann't be forged into bars for UHCS-2.83Si. (5)UHCSs with chemical compositions of C=1.5~1.7%, Cr=1.5~1.6%,Al=2.3~4.0 can obtain spheroidized carbides at the temperature range of 820~880℃.The plasticity of these UHCSs is obviously improveed and it exists yield point elongation in stress-strain curve under spheroidization. It revealed that its plasticity was greatly affected by second phase particles type. For example, UHCS-4.02Al, whose microstructures is large amount of lamellar carbides at 820℃for 2h, small amount of thin lamellar carbides at 850℃for 2h and complete spheroidization carbides at 880℃for 2h, has high ambient strength (σs≈700MPa,σb≈1000MPa) and low plasticity (δ<10%) at all spheroidizing conditions for intermetallics. However, UHCS-2.61Al containing major second phase Fe3C can obtain high ambient strength ofσs≈700MPa,σb≈1000MPa and good plasticity ofδ=18% after spheroidization at 820℃for 2h and at 850℃for 3h.