C.K. Syn

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 particles (0.2- to 1.5-μm size range) within a fine-grained ferrite matrix (0.3- to 5-μm range) through a variety of thermomechanical processing and heat-treatment combinations. Tensile ductility, yield, and fracture strengths, and strain-hardening behavior were evaluated at room temperature. Yield strengths ranged from 640 to 1450 MPa, and uniform tensile elongation ranged from 3 to 23 pct. Quantitative analyses revealed that a Hall-Petch type relationship exists between the yield strength and the ferrite grain size and carbide particle size within grain interiors. The fracture strength, on the other hand, was found to be uniquely dependent on the coarse carbide particle size typically found at grain boundaries. Data from other investigators on spheroidized carbon 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 increase when the difference between the fracture strength and the yield strength is increased and when the strain-hardening rate is decreased. The basis for the trends observed is that the tensile ductility is limited by the fracture process that appears to be dictated by the nucleation of cracks at large carbide particles. The results obtained indicate that UHCSs have significant potential for sheet applications where high strength and good ductility are primary requirements.

Pearlite in ultrahigh carbon steels: Heat treatments and mechanical properties

Two ultrahigh carbon steel (UHCS) alloys containing 1.5 and 1.8 wt pct carbon, respectively, were studied. These materials were processed into fully spheroidized microstructures and were then given heat treatments to form pearlite. The mechanical properties of the heat-treated materials were evaluated by tension tests at room temperature. Use of the hypereutectoid austenite-cementite to pearlite transformation enabled achievement of pearlitic microstructures with various interlamellar spacings. The yield strengths of the pearlitic steels are found to correlate with a predictive relation based on interlamellar spacing and pearlite colony size. Decreasing the pearlite interlamellar spacing increases the yield strength and the ultimate strength and decreases the tensile ductility. It is shown that solid solution alloying strongly influences the strength of pearlitic steels.

Diamond turning of L-arginine phosphate, a new organic nonlinear crystal

We have demonstrated that single point diamond turning can be used to generate high optical quality finished surfaces on a new organic nonlinear crystal, L-arginine phosphate (LAP). The proper choice of cutting conditions can produce surfaces with <5-A rms local roughness. Local softening or melting near the cutting tool tip may play a key role in the machining process by ensuring that material is removed by ductile cutting rather than brittle fracture. At the same time, the low melting temperature of LAP makes lubrication and cooling especially important to prevent extensive melting and tool fouling. In spite of the presence of a weak cleavage plane in LAP, the surface quality is relatively insensitive to crystallographic orientation. Tool wear is apparently negligible, so that surface flatness is governed by the stability of the diamond turning machine. These results suggest that it may be possible to fabricate large aperture LAP harmonic converters for use in inertial confinement fusion lasers.

The case for ultrahigh-carbon steels as structural materials

Ultrahigh-carbon steels (UHCSs) are low-alloyed plain carbon steels containing 1–2.1% carbon. These steels have remarkable structural properties when processed to achieve fine ferrite grains with fine spheroidized carbides. They can be made superplastic at intermediate temperatures. Further, they can be made hard with compression toughness and strong with good tensile ductility at ambient temperatures. Contrary to conventional wisdom, UHCSs are ideal replacements for currently used high-carbon (0.5–1 % carbon) steels because they have comparable ductility but higher strength and hardness. In this article, examples of structural components formed from fine-grained spheroidized UHCSs are illustrated, and other potential structural applications are reviewed. These steels can be laminated with other metal-based materials to achieve superplasticity, high impact resistance, exceptionally high tensile ductility, and improved fatigue behavior.

Layer thickness effect on ductile tensile fracture

Laminated metal composites containing equal volume percentage of ultrahigh carbon steel (UHCS) and brass were prepared in three different layer thicknesses (750, 200, and 50 μm) by press- bonding and rolling at elevated temperature and were tensile tested at ambient temperature. A dramatic increase in tensile ductility (from 13 to 21 to 60 pct) and a decrease in delamination tendency at the UHCS-brass interfaces were observed as the layer thickness was decreased. The layer thickness effect on ductility is attributed to residual stress whose influence on delamination is decreased as the layer thickness is decreased. Suppression of delamination inhibits neck for- mation in the UHCS layers, allowing for extended uniform plasticity. For a given layer thick- ness, the tensile ductility decreases as the ratio of hardness of component layers is increased.

Processing, structure, and properties of a rolled, ultrahigh-carbon steel plate exhibiting a damask pattern

Abstract A plate of ultrahigh-carbon steel (UHCS) was processed by hot and warm rolling, according to the Wadsworth–Sherby mechanism, to produce damask surface markings. The surface markings produced by this industrial processing method are similar to those of historical Damascus steels, which are also of hypereutectoid composition. The microstructure of the UHCS with damask contains fine, spheroidized carbides and a discontinuous network of proeutectoid carbides along former-austenite grain boundaries, which give rise to a surface pattern visible with the unaided eye. Tensile tests at room temperature measured tensile strengths and ductilities, which depend on sample orientation relative to the rolling direction of the plate. Hot and warm rolling causes a directional microstructure, giving rise to both an elongated, directional damask pattern and a directional dependence for strength and ductility. A maximum tensile ductility of 10.2% was measured at 45° relative to the rolling direction. The plate material was subjected to heat treatments creating pearlitic and martensitic microstructures, which retain visible damask patterns.

Enhancing tensile ductility of a particulate-reinforced aluminum metal matrix composite by lamination with Mg-9%Li alloy

A laminated metal composite has been made by press bonding alternating layers of a particulate-reinforced aluminum metal matrix composite (MMC), 6090/SiC/25p, and a highly ductile Mg-9%Li alloy. The mechanical properties including tensile ductility were evaluated for the 50-50 volume fraction laminated composite. The tensile yield strength of the composite obeyed the rule of averages in agreement with other laminates based on aluminum and ultrahigh carbon steel. Lamination of the Al MMC material is shown to increase the tensile ductility from 8.0% to 11.5%. The ductility of the laminated composite was found to be limited by interlayer reaction products developed during processing which led to delamination. Thus necking of the MMC Al layers could occur leading to premature failure.

Microstructure in adiabatic shear bands in a pearlitic ultrahigh carbon steel

Abstract Adiabatic shear bands, obtained in compression deformation at a strain rate of 4000 s−1, in a pearlitic 1·3%C steel, were investigated. Shear bands initiated at 55% compression deformation with the width of the band equal to 14 μm. Nano-indentor hardness of the shear band was 11·5 GPa in contrast to the initial matrix hardness of 3·5 GPa. The high strength of the shear band is attributed to its creation from two sequential events. First, large strain deformation, at a high strain rate, accompanied by adiabatic heating, led to phase transformation to austenite. Second, retransformation upon rapid cooling occurred by a divorced eutectoid transformation (DET). The result is a predicted microstructure consisting of nano size carbide particles within a matrix of fine ferrite grains. It is proposed that the DET occurs in iron–carbon steels during high rate deformation in ball milling, ball drop tests and in commercial wire drawing.

Fracture behavior of spheroidized hypereutectoid steels

A fracture model for spheroidized hypereutectoid steels is developed based on the concept that the stress in the ferrite matrix is the driving force for crack initiation at grain boundaries within the coarse carbides. The ferrite matrix fracture stress, {sigma}{sub f,ferr} is calculated by averaging the ferrite stress using upper and lower bound concepts, and by utilizing the fracture strength of the carbide. The analyses and results indicate that the fracture behavior followed a classical fracture mechanics relation in that the fracture strength is a unique function of the reciprocal of the square root of the carbide particle size with {sigma}{sub f,ferr} equal to zero at infinite carbide (crack) size. It is concluded that the fracture strength of the iron-iron carbide composite is enhanced by: (i) increasing the strength of grain boundaries within carbides; (ii) decreasing the average carbide size; and (iii) increasing the carbide volume fraction.

The c/a Ratio in Quenched Fe-C and Fe-N Steels - A Heuristic Story

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.