A. H. Advani

Combined effects of deformation (strain and strain state), grain size, and carbon content on carbide precipitation and corrosion sensitization in 304 stainless steel☆

Abstract In this investigation we have examined the combined effects of temperature, aging time, carbon content, grain size, strain, and strain state (uniaxial versus rolling deformation) on sensitization development in 304 stainless steel. In general, increasing the carbon content, strain, and strain state (uniaxial → rolling) decreases the sensitization time. Reducing the grain size also accelerates the rate of sensitization development. In addition, for sufficiently small grain sizes, even with superimposed straining, the grain boundaries become dominant in Cr diffusion kinetics.

Mechanisms of deformation-induced grain boundary chromium depletion (sensitization) development in type 316 stainless steels

Deformation accelerates the development of grain boundary chromium depletion (GBCD), or sensitization, in type 316 austenitic stainless steels (SS). Quantitative assessment of the degree of sensitization (DOS) using the electrochemical potentiokinetic reactivation (EPR) test indicates that the acceleration in GBCD is a function of the amount of strain in the material and temperature of isothermal sensitization treatment. A systematic increase in strain from 0 to 20 pct yields a continuous increase in EPRDOS values below 700°C, while at higher temperatures, a threshold strain of 6 to 10 pct is required to cause accelerated GBCD development. Straining SS above 20 pct also produces higher amounts of chromium depletion, though the (intergranular) sensitization susceptibility of the material could not be quantitatively evaluated due to the presence of grain matrix or transgranular corrosion. Classical C-curve precipitation-sensitization behavior was also noted for strained and unstrained materials, though strain moved the C-curves to the left. Microstructural evaluation of sensitization revealed a systematic increase in grain boundary and twin boundary corrosion on EPR attack surfaces with strain, which corroborated the deformation-induced acceleration of EPRDOS. A time-temperature-strain dependence of transgranular corrosion was also identified on EPR-etched samples strained above 20 pct. These were also reflected in transmission electron microscope (TEM) observations of higher grain boundary carbide precipitation on strainedvs unstrained specimens and site-specific carbide precipitation on deformation sites in the material. Kinetic and thermodynamic modeling of deformation effects on carbide precipitation and depletion development in type 316 SS indicated that strain induces a reduction in the activation barrier to diffusion (Q)a and thermodynamic barrier to nucleation (ΔG*) during the precipitation-depletion process. The lowering ofQa with strain caused chromium diffusivity and depletion development to be accelerated in strainedvs unstrained materials and appears to be due to increased dislocation pipe diffusion with strain. Reduction of ΔG* with strain was related to an increase in the free energy change of the grain boundary (ΔG)gb and accelerated carbide precipitate nucleation in deformed SS. The effect of strain on the kinetics and thermodynamics of the precipitation-depletion process decreases with increasing temperature.

Characterization and comparison of microstructures in the shaped-charge regime: copper and tantalum

Abstract Light microscopy scanning electron microscopy, and transmission electron microscopy techniques were employed, along with a novel technique for building up small, recovered jet fragments using electrodeposition of copper, to examine specific segments of fabricated shaped charge liner cones and corresponding, residual jet fragments. Oxygen-free electronic copper and tantalum shaped charged regimes (linerr cones and recovered jet fragments) were compared, and a reduction in the average grain size of recovered jet fragments as compared to the starting liner cones was a consistent observation. The average grain sizes for all cones was 35μm, and the maximum grain reduction occurred for an annealed, equiaxed tantalum cone, which resulted in a residual jet fragment grain size between 1 and 5μm. This is indicative of dynamic recrystallization during jet elongation and microstructure evolution.

Dynamic modelling of material and process parameter effects on self-propagating high-temperature synthesis of titanium carbide ceramics

A dynamic, finite-difference model evaluation of titanium carbide (TiC) ceramic processing by self-propagation high-temperature synthesis (SHS) has revealed that material and process parameters have a significant influence on SHS reaction propagation kinetics. Examination of the effects of Ti∶C stoichiometry variations and the presence of pre-reacted TiC diluents in the initial (Ti+C) reactant powder mix indicates that off-stoichiometric ratios and dilution additions tend to lower SHS reaction velocities. Increasing compact preheat temperatures, lowering powder particle sizes and raising compact packing densities, on the other hand, cause a significant increase in this reaction variable. Model results are supported by experimental studies on dilution, stoichiometry and preheat temperature effects on SHS velocity, and other literature data on packing density and particle size effects on this parameter. Simulations also suggest that effects of these initial conditions are due to their influence on adiabatic reaction temperatures and heat transfer patterns produced during the process. Critical selection of initial material and process conditions thus appears to be of vital importance during SHS processing of TiC ceramics.

Combustion synthesis and subsequent

Explosive densification following combustion synthesis of titanium and graphite powder mixtures has been used to fabricate bulk compacts (100-mm diameter × 20-mm thick) of TiC ceramics. A model rocket ignitor was used to initiate the combustion reaction in ≈65 pct dense green pressed reactants of titanium and carbon powder mixtures. Upon completion of reaction, the reacted mass was allowed to cool. After the desired time delay (td) and while the reacted mass was still above the ductile-brittle transition temperature, an explosive charge was detonated in contact with a steel driver plate to transmit the pressure into the reacted mass and consolidate it to solid density. Temperature-time cooling profiles for the reacted material were developed using calculations based on a heat flow model. The explosive loading conditions, namely, the densification pressure controlled by the ratio of explosive charge mass (C) to driver plate mass (M) ratio (C/M) and thetd between the combustion reaction completion and explosive detonation, were observed to critically affect the density and the microstructure of the final compacted reaction product.

Deformation-induced microstructure and martensite effects on transgranular carbide precipitation in type 304 stainless steels

Abstract Plastic deformation of 304 stainless steel (SS) induces transgranular (TG) carbide precipitation, which is critically dependent on deformation-induced microstructural changes occurring during thermal treatment of the SS. Uniaxial deformation of the 304 SS to 40% strain produces a high density of intersecting micro-shear bands composed of heterogeneous bundles of twin-faults and about 12–17% strain-induced α′-martensite at the intersections of the twin-faults. Thermal treatment of 670°C for 0.1–10 h, however, results in a rapid annihilation/transformation of the strain-induced martensite and the concurrent formation of zones containing mixed thermal martensite laths and fine-grained austenite, though the thermal martensite also decreases with increasing heat treatment time. Simultaneous with these thermomechanically-induced microstructural changes, TG chromium-rich carbides form at intersections of twin-faults and on fine-austenite or thermal martensite boundaries in the SS; however, no correlation between strain-induced α′-martensite and carbides was observed in this work. The mechanisms of deformation-induced microstructure and (strain-induced and thermal) martensite effects on TG carbide precipitation in 304 SS are discussed.

Deformation effects on intragranular carbide precipitation and transgranular chromium depletion in type 316 stainless steels

Abstract Grain matrix or transgranular (TG) chromium depletion in type 316 stainless steels (SS) has been observed to be strain and heat treatment dependent, yielding C-curve behavior on a time-temperature-strain-TG corrosion map. The strain effect dominates above 16 to 20 percent showing increased TG corrosion and decreased time to develop TG depletion with increasing strain level. Higher temperatures also reduce the time to develop grain matrix attack, while isothermal holding for longer times produces higher amounts of TG corrosion in SS. Transgranular depletion is caused by precipitation of carbides within grain-matrix regions. Specifically, defect sites created during straining are favored locations for intragranular precipitation in SS. Regions of high dislocation density, deformation twin-stacking fault planes and twin-fault intersections were identified to be preferred sites for grain matrix precipitation during transmission electron microscopy (TEM) of 16 to 35 percent deformed SS.

Residual microstructure of a shaped-charge jet fragment

A recovered copper shaped-charge jet fragment has been built up by copper electrodeposition for the first time to allow it to be systematically sectioned and polished for detailed observations by optical and electron microscopy. The residual jet fragment microstructure was observed to have a recrystallized grain structure and dislocation substructures similar to those in the undeformed copper shaped charge liner cone. However, the average grain size in the recrystallized jet fragment was 15 μm compared to 45 μm for the liner. More significantly, however, SEM examination near the tips (or ends) of the jet fragment exhibited voids and coalesced void tunnels elongated axially within the fragment geometry, which are believed to have resulted during jet elongation and breakup by diffusion and viscous growth at high strain and strain rate. The observation of additional porosity in the interior of the jet fragment is supported by the lack of any similar observations in the surrounding and supporting (built up) copper electrodeposit.

Influence of near-surface microstructures on the transient current response in Fe-Cr-ni alloys during scratch tests

Metals and alloys are protected from the corrosive environments in which they are used by the presence of a passive film on the surface. However, the passivity of a metal surface cannot be preserved if mechanical forces rupture this protective film from the surface. Thus, during corrosive wear, the process of wear essentially destroys the passive film, which then leaves a bare metal surface for the corrosion reaction to start taking place once again. In an industrial application of a metal, the process of passivation, depassivation, and repassivation kinetics must be completely understood if the life of a metal part is to be prolonged. The purpose of this article is to study the kinetics involved in the corrosive wear of Fe-Cr-Ni alloys by the scratch technique and to determine the relationship between the electrochemical response of these alloys and the near-surface microstructures.

Combined effects of strain and grain size on carbide precipitation and sensitization in 304 stainless steel

The dramatic effects of uniaxial strain on sensitization in austenitic stainless steels is now well documented. Many parameters influence the occurrence of sensitization and the degree of sensitization (DOS) is thus a function of microstructural characteristics (which of course includes the grain size) and chemical composition, including the carbon content (which is the most critical compositional element controlling sensitization). However, there have been no studies evaluating simultaneous effects of deformation (strain) and grain size on sensitization development. A study of two grain sizes and carbon content variations by Pascali, et al found that desensitization was delayed at the larger grain size (68 [mu]m) in comparison to the smaller grain size (17[mu]m), especially at aging temperatures near 600 C. In this study the authors present some preliminary results of the combined effect of grain size and uniaxial strain on sensitization (DOS) in 304 stainless steel.