Papers by Author: C.K. Syn

Abstract: The exceptional high hardness of lath martensite in quenched Fe-C steels is explained by the Engel-Brewer valence electron theory for crystal structures. The theory predicts the transformation sequence FCC-HCP-BCC with FCC iron as Fe3v, HCP iron as Fe2v, BCC iron as Fe1v and carbon as C4v. Electronic compatibility requires transformation from FCC to HCP to form two separate components. Carbon-rich clusters of C4v with 8 Fe3v atoms are distributed uniformly in a carbon-free matrix of HCP Fe2v atoms. The carbon-iron clusters are viewed as particle-like, calculated as 0.63 nm in size, and is responsible for the high strength of martensite. The carbon-free region experiences shear deformation during FCC to HCP transformation leading to work hardened fine grains. Subsequent transformation to BCC iron maintains the same size carbon cluster with additional shearing deformation during HCP to BCC formation in the carbon-free region. Tempering studies of quenched martensite are shown to support the carbon-iron cluster model.

Abstract: The properties of ultrahigh carbon steels (UHCS) are strongly influenced by aluminum additions. Hardness studies of quenched UHCS-Al alloys reveal that the temperature for the start of transformation increases with increases in aluminum content. It is shown that this change is a function of the atomic percent of solute and of the valence state when comparisons are made with UHCSs containing silicon and tin as solutes. The thermal expansion of UHCSs with dilute aluminum additions shows no discontinuity in the vicinity of the ferrite-austenite transformation temperature. This is the result of a three phase region of ferrite, carbides and austenite. The slope of the expansion curve is higher in the austenite range than in the ferrite range as a result of the dissolution of carbon in austenite with temperature. Processing to achieve a fine grain size in UHCS-Al alloys was principally by hot and warm working (HWW) followed by isothermal warm working (IWW). The high temperature mechanical properties of a UHCS-10Al-1.5C material show nearly Newtonian-viscous behavior at 900 to 10000C. Tensile elongations of 1200% without failure were achieved in the 1.5%C material. The high oxidation corrosion resistance of the UHCS-10Al materials is described.

Abstract: The work of Frommeyer on electrical conductivity measurements in pearlitic steels is reviewed to provide insight into microstructures developed during wire drawing. Electrical conductivity measurements were made as a function of drawing strain (up to ε = 6.0) for wires with strength exceeding 3500MPa. The results show that electrical conductivity increases during wiredrawing to a maximum value, then decreases with further deformation finally reaching a steady state value that is equal to the original conductivity. The initial increase is the result of pearlite plate orientation in the direction of wire-drawing, which makes the path of conduction through the ferrite plates more accessible. At a critical strain the cementite plates begin to fragment and the electrical conductivity decreases to a steady state value that is the same as that observed prior to wire drawing. With increasing strain, the cementite particles are refined and the strength increases due to the reduction in inter-particle spacing. It is concluded that the electrical conductivity of the wires is solely dependent on the amount of iron carbides provided they are randomly distributed as plates or as particles. An estimate was made that indicates the carbide particle size is approximately 3 - 5 nm in the steady state range of electrical conductivity.

Abstract: The body-centered tetragonal (BCT) structure in quenched Fe-C steels is usually illustrated to show a linear change in the c and a axes with an increase in carbon content from 0 to 1.4%C. The work of Campbell and Fink, however, shows that this continuous linear relationship is not correct. Rather, it was shown that the body-centered-cubic (BCC) structure is the stable structure from 0 to 0.6 wt%C with the c/a ratio equal to unity. An abrupt change in the c/a ratio to 1.02 occurs at 0.6 wt%C. The BCT structure forms, and the c/a ratio increases with further increase in carbon content. An identical observation is noted in quenched Fe-N steels. This discontinuity is explained by a change in the transformation process. It is proposed that a two-step transformation process occurs in the low carbon region, with the FCC first transforming to HCP and then from HCP to BCC. In the high carbon region, the FCC structure transforms to the BCT structure. The results are explained with the Engel-Brewer theory of valence and crystal structure of the elements. An understanding of the strength of quenched iron-carbon steels plays a key role in the proposed explanation of the c/a anomaly based on interstitial solutes and precipitates.