Large strain work hardening and textures

The mechanism of formation of nanostructure and dissolution of cementite in a pearlitic steel during high pressure torsion

Characterization of the strength and microstructure of heavily cold worked CuNb composites

Microstructure and strengthening mechanisms in cold-drawn pearlitic steel wire

Atomic-scale mechanisms of deformation-induced cementite decomposition in pearlite

Pearlite with its lamellae oriented mainly parallel to the longitudinal direction was prepared by Bolling's method of transformation in a steep temperature gradient. The Fe-0.7 pct Mn-0.9 pct C pearlite was drawn into wire and also into strip in dies designed to minimize macroscopically nonuniform deformation. Cross sections of the drawn wires and strip were examined by conventional and high-voltage transmission electron microscopy and were analyzed by quantitative metallography for a) average interlamellar spacing, b) distribution of interlamellar spacings, and c) orientation relationship between the cementite lamellae and the slip systems in the ferrite. The strength of pearlite is proportional to the reciprocal square root of the average interlamellar spacing, and the proportionality constant analogous to the Hall-Petch constant (k) is related to the strength of the cementite lamellae. If the stress for the propagation of slip through the cementite is assumed constant, a Hall-Petch type of equation can be derived for the strengthening of the pearlite against slip in the ferrite by piled-up groups of dislocations. Evidence for the plastic deformability of cementite is presented; sufficiently thin cementite plates were fully plastic. The exponential strain hardening of drawn pearlitic wires and of rolled pearlite is explained in terms of locally inhomogenous deformation revealed by the lack of fragmentation of the lamellae.

Deformation of pearlite

A new process for casting and welding carbon steels is described in which carbon diffuses isothermally or adiabatically within an intimate mixture of solid low carbon steel and high carbon liquid iron to effect solidification and subsequent homogenization with respect to carbon. Advantages over conventional casting processes and products result from 1) 150 to 200°C lower casting temperature; 2) reduced solidification shrinkage, obviating the need for risers in most cases; and 3) more rapid solidification, especially for castings with large ratios of volume to area. In its most versatile form the process involves low pressure forced infiltration of a mold filled with preheated spherical low carbon steel particles by a higher-carbon liquid. The process can reliably produce castings with greater than 99 pct of theoretical density; solidification time typically range from a few seconds to several minutes; and tensile strengths as high as 185 ksi with 15 pct reduction of area to break have been attained. The ductility of such castings is approximately one order of magnitude more sensitive to total oxygen content than the ductility of wrought steels, probably because of cavitation nucleated by oxides during solidification of the pools of liquid trapped between the shot particles. An analysis of the kinetics of the infiltration and solidification is performed for steel casting by diffusion of carbon, manganese or heat in iron. The iron-carbon system is most tractable; steel casting by thermal diffusion has also been demonstrated but no attempt was made to test the iron-manganese system.

Steel casting by diffusion solidification

An aggregate of nonwettable refractory particles can act as an off-on valve to control the flow of liquid metals. The performance of such a particle valve is evaluated experimentally and theoretically. The breakthrough pressure, PBR, which is the pressure differential required to initiate flow through the particle valve has been theoretically derived: PBR = -10.928 [γLV/d] cos θ′A where γLV is the liquid-vapor surface tension, d is the uniform particle diameter, and θ′A is the apparent advancing contact angle between the liquid and solid. Experimentally determined breakthrough pressures for the Sn/Al2O3 and Hg/Al2O3 systems agree with and confirm the derived expression. Mechanisms which may lead to premature breakthrough. of the liquid through the valve are: i) failure due to direct impingement of liquid droplets on the particle valve, ii) failure by augmented hydrostatic pressure caused by an impinging liquid droplet, and iii) failure due to buoyancy forces acting on individual particles. Critical operating limits for each mechanism are given.

George Langford

Papers

Deformation of pearlite

Steel casting by diffusion solidification

Performance of porous refractory valve for liquid metals