W. S. Owen

Strain Hardening of Hadfield Manganese Steel

The plastic flow behavior of Hadfield manganese steel in uniaxial tension and compression is shown to be greatly influenced by transformation plasticity phenomena. Changes in the stress-strain (σ−ε) curves with temperature correlate with the observed extent of deformation twinning, consistent with a softening effect of twinning as a deformation mechanism and a hardening effect of the twinned microstructure. The combined effects give upward curvature to the σ−ε curve over extensive ranges of plastic strain. A higher strain hardening in compression compared with tension appears to be consistent with the observed texture development. The composition dependence of stacking fault energy computed using a thermodynamic model suggests that the Hadfield composition is optimum for a maximum rate of deformation twinning. Comparisons of the Hadfield steel with a Co-33Ni alloy exhibiting similar twinning kinetics, and an Fe-21Ni-lC alloy deforming by slip indicate no unusual strain hardening at low strains where deformation is controlled by slip, but an unusual amount of structural hardening associated with the twin formation in the Hadfield steel. A possible mechanism of anomalous twin hardening is discussed in terms of modified twinning behavior (pseudotwinning) in nonrandom solid solutions.

Nitrogen strengthening of a stable austenitic stainless steel

Abstract The low-temperature flow stress of mechanically stable austenitic stainless steels increases with increasing concentration of nitrogen in solution and with decreasing temperature. This phenomenon has been studied in a series of FeNiCrMo alloys with nitrogen contents between 0.04 and 0.36 wt% by measuring the flow stress and the thermal activation parameters for plastic flow as a function of stress, plastic strain and nitrogen concentration in stress relaxation and strain-rate change experiments. Care is taken, when analyzing the data, to distinguish between athermal and thermal effects. The significant increase of the athermal flow stress with increasing nitrogen concentration is attributed to short-range ordering of chromium and nitrogen atoms. The thermally activated component of the flow stress is also dependent on the nitrogen concentration and is thought to be due to localized, predominantly modulus interactions between lattice disturbances in the immediate vicinity of nitrogen atoms and slip dislocations. The thermally activated component is suitably described by Friedels model of solid solution strengthening.

Strain aging of austenitic Hadfield manganese steel

Abstract Strain aging of Hadfield steel is discussed in terms of the interstitial octahedron, local-order model, which defines order as the probability that a C atom in an octahedral cluster of metal atoms has n (an integer between 0 and 6) Mn nearest neighbors. Equilibrium order is assessed by a Monte Carlo procedure using pair exchange energies derived from an established thermodynamic database and a Boltzmann distribution function. The disorder produced by the passage of a slip dislocation, the resulting change in free energy and, consequently, the stress opposing dislocation motion are calculated both for a single isolated dislocation and for a sequence of dislocations moving on the same slip plane. The model is extended to analyze aging effects involving diffusion of carbon before or during deformation. It is assumed that, during aging, atoms on the metal sublattice are frozen on sites determined either by the high-temperature equilibrium anneal or by prior deformation. Only diffusion of carbon is allowed. The fully aged condition at selected aging temperatures is simulated using a Monte Carlo procedure to assess local order when the free energy of the system is minimum (para-equilibrium). It is shown that the increase in strength on aging is a direct result of the relatively small thermal energy at the aging temperature favoring an increase in the number of Mn–C atom pairs. The predictions of the model are supported by the results of static aging experiments and the model provides a complete phenomenological description of dynamic strain aging in Hadfield steel.

Mobility of martensitic interfaces

Using a dislocation model of interfacial structure, kinetic theories of dislocation motion are adapted to predict the mobility of martensitic interfaces. Defining generalized driving forces and activation parameters, analytical models are developed which describe the kinetics of motion controlled by various types of obstacle interactions. The behaviors of martensitic interfaces and slip dislocations in identical microstructures are compared. For a lattice-invariant shear by slip, the martensitic interface behaves similarly to a collection of glide dislocations. The interface/obstacle interaction is much weaker if the martensite is internally twinned, giving a higher relative mobility.

The development of some dual-phase steel structures from different starting microstructures

The development of dual-phase structures with different morphologies has been studied in detail by intercritical annealing specimens of a steel containing 0.11 pct C and 1.6 pct Mn with different microstructures before annealing. The kinetics of formation of the two-phase structure at the annealing temperature and the redistribution of substitutional solute elements were measured in specimens quenched from the intercritical annealing temperature. The structure before annealing was either banded ferrite-pearlite, homogenized ferrite-pearlite, lath martensite, spheroidal cementite dispersed in ferrite, or austenite. No measurable partitioning of silicon or molybdenum, present in the steel in small concentrations, was found. However, close to equilibrium partitioning of manganese occurred on annealing specimens with either a ferrite-pearlite or a lath martensite structure, but during the separation of ferrite from austenite in step-quenched specimens there was no partitioning. Surprisingly, measurements of manganese concentrations using an electron beam of 1 nm diameter at intervals of 5 nm or less revealed the presence of narrow spikes in the concentration profile at many ferriteaustenite interfaces in specimens with a ferrite-pearlite or martensite starting structure as well as in those step-quenched from austenite. In some instances, a minimum in the concentration profile was found in ferrite, adjacent to a maximum at an interface. Thus, adsorption of manganese at ferriteaustenite interfaces produces concentrations in excess of the concentrations indicated by the equilibrium diagram. The probable diffusion processes controlling the kinetics of transformation in the different microstructures are identified.

A model of the thermoelastic growth of martensite

Eshelbys continuum elastic model of the strain field of a coherent inclusion in an infinite matrix is adapted to describe the strain field associated with a circular plate of ellipsoidal cross-section transformed martensitically. Initially, the plate and the austenitic matrix are assumed to be elastically Isotropic, but with different moduli. The model is also used to calculate the strain field of a pair of parallel, ellipsoidal, martensite plates and of a single, isolated, square, flat plate. Finally, the treatment is extended to reveal the effects of elastic anisotropy. A criterion for thermoelastic growth of an isolated plate of martensite in terms of the permissible plastic yielding is proposed. Using the model of the strain fields, contours of the yield envelopes associated with an isolated plate of martensite formed in Fe3Pt are calculated as a function of the atomic order in the parent, austenitic phase. It is concluded that thermoelastic growth is to be expected when the long-range order, S, is greater or equal to 0.2, in agreement with experimental observations. The equilibrium thickness of an isolated thermoelastic plate at Ms and the associated chemical driving force are found to be much smaller than for plates formed irreversibly. Quantitatively, the predictions of the model applied to the ‘ordered’ alloy are in good agreement with experimental measurements. Clearly, Invar elastic softening is a major factor determining thermoelastic behavior in Fe3Pt. In addition, the model is used to explore the reduction in elastic accommodation strain which can be achieved by the interactions of two or more plates arranged edge-to-edge or in stacks of self-accommodating plates. The influence of the shape of the individual plates is also discussed. Calculations using the anisotropic model for an isolated plate in the form of an oblate spheroid show that the effects of the large elastic anisotropy, which develops on cooling partially ordered Fe3Pt, on the equilibrium thickness of the thermoelastic plate and the chemical driving force at Ms are surprisingly small.

The dependence of some tensile and fatigue properties of a dual-phase steel on its microstructure

Five different dual-phase microstructures have been produced in a steel containing 0.11 C, 1.60 Mn, 0.73 Si. By careful design of the heat treatment schedules, it was arranged that all the specimens had the same fine austenite grain size before intercritical annealing and that they all contained close to 30 vol pct martensite and a negligible fraction of retained austenite after annealing. The microstructures were characterized by measurement of the mean ferrite path and the density and distribution of the extrinsic transformation accommodation dislocations in the substructure of the ferrite. Specimens representative of each of the microstructures were tested in tension at room temperature. The strength, work-hardening, and fracture of three of them were examined in detail and correlated with the microstructural parameters. The microstructural features which most influence the tensile properties are identified. Two specimens, representative of the finest and coarsest microstructures, were the subject of a detailed exploration of the initiation of persistent slip and microcracks and of crack propagation in bending fatigue. The fatigue endurance limit was also measured. It is shown that the initiation of persistent slip is primarily influenced by the heterogeneity in the density of the dislocations in the ferrite. The microstructural features of greatest importance in controlling the formation and propagation of microcracks are the local dislocation density and the mean ferrite path.

Mobility of the β1-γ 1 ′ martensitic interface in Cu-Al-Ni: Part I. Experimental measurements

The mobility of theβ1-γ1′ martensitic interface in thermoelasticβ-Cu-Al-Ni alloys has been investigated using stress-assisted single interface transformation over the temperature range 180 to 410 K. The imposed interfacial velocities varied between 10-6 and 10-2 ms-1. The kinetic behavior is found to be consistent with thermally-activated interfacial motion, although an anomalous temperature dependence is observed below 210 K. This low temperature anomaly is attributed to the effect of the experimentally verified elastic softening. At higher temperatures, measured activation energies of 1.5 × 10-20 to 3.5 × 10-20 J and activation volumes of 103 to 104 atomic volumes are rationalized in terms of the rate-controlling interaction of the moving interface with fine-scale lattice displacements, the tweed structure, observed by transmission electron microscopy. Coarse particles ofγ- and 2H-phase, detected by the TEM, are found to give rise to noncontact particle/interface interactions which are too long-range to be surmounted by thermal activation. Consequently, they produce athermal friction stresses opposing interface motion.

Precipitation and Recrystallization in Some Vanadium and Vanadium-Niobium Microalloyed Steels

Static precipitation and recrystallization following hot compression of austenite and the interactions between the two processes have been studied in a set of aluminum-killed HSLA steels containing 0.1 pct carbon, [0.016 - 0.026] pct nitrogen and 0.1 or 0.2 pct vanadium. Two steels containing both vanadium (0.1 and 0.2 pct) and niobium (0.03 pct) were included for purposes of comparison. The compression and the static tests were all carried out isothermally at temperatures between 800 and 900 °C. The course of recrystallization was followed by measurements of the rate of softening and by optical metallography of specimens quenched from the test temperature after different times. Precipitation was studied by measurements of the rate of hardening, by transmission electron microscopy of thin foils, carbon and aluminum extraction replicas, and by X-ray dispersion and energy-loss spectroscopy from individual precipitates.The temperature of the nose of theC-curve for precipitation in vanadium steels is much lower than that in niobium steels, as is the temperature, TR, below which no recrystallization occurs in short times. Precipitation occurs both at austenite grain boundaries and in the grains (matrix precipitation). The former starts early and the precipitates grow rapidly to an approximately constant size; the matrix precipitates grow more slowly and are responsible for the observed hardening of the austenite. The relevance of various models proposed for the retardation and arrest of recrystallization of austenite are discussed.In the steels containing vanadium and niobium the precipitates contain both heavy elements: (V,Nb) (C,N). The Nb/V ratio in the matrix precipitates is different than in the parent austenite. The grain-boundary precipitates, however, contain the same Nb/V ratio as the parent austenite. The rate of hardening exhibits a reverseC-curve behavior, being more rapid than in the corresponding vanadium steels at higher temperatures and about the same at lower temperatures.

The Kinetics of Carbon Clustering in Martensite.

The rate of clustering of carbon atoms into regions of ordered Fe4C has been studied by Mössbauer spectroscopy. The activation energy associated with this process is 89,500 ± 12,000 J per mole (21.4 kcal per mole).