Austenite Lattice Research Articles

Phase transformation induced by classical ion implantation in austenitic stainless steel

AbstractStructural modifications induced by Cr, Ar, N, Xe, Mo, Ag and Pb ion implantations in a 316LVM austenitic stainless steel within a 70‐nm thick surface layer were studied by grazing incidence X‐ray diffraction. In the implanted layer, austenite lattice expansion was first observed, then ferrite formation and in some cases amorphization. Atomic force microscopy (AFM) was used to study changes in surface topography under ion implantation: roughening, grain boundaries revelation, holes formation. Magnetic force microscopy (MFM) was used to distinguish on the surface ferrite domains when they were formed. Ferrite started to grow at grain boundaries and covered all the surface of the sample in some cases. A theoretical damage level was estimated using SRIM‐2003 computer program. The amount of ferrite and the amorphization rate were considered with respect to the damage level brought in the implanted layer by ion introduction. Existence of damage level thresholds for ferrite formation (200 displacements per atom (dpa)) and amorphization (400 dpa) were considered as evidence. These thresholds were independent of the nature of implanted element and the fluence of implantation. Copyright © 2008 John Wiley & Sons, Ltd.

In situ X-ray phase analysis and computer simulation of carbide dissolution of ball bearing steel at different austenitizing temperatures

This paper presents the results of in situ X-ray diffraction experiments and computer simulation of AISI 52100 steel during soaking at different austenitizing temperatures. A study of carbide dissolution kinetics and austenite lattice parameter variations has been executed. In situ X-ray diffraction investigations were carried out with a rotating anode and an area detector to allow a good time resolution. For the computer simulation, a Johnson–Mehl–Avrami equation was used. Lattice parameters of austenite obtained by X-ray diffraction show good agreement with values calculated from a model based on neutron diffraction experiments. Carbide contents measured by X-ray diffraction are close to those obtained by simulation. But discrepancies are present which are essentially due to local heterogeneity of the chemical composition.

A study of the carbon distribution in retained austenite

Cold-rolled and annealed transformation-induced plasticity (TRIP) steels were overaged to modify the carbon concentrations (Cγ) in retained austenite. Experimental Cγ values were directly obtained by electron energy loss spectroscopy and compared with data derived from X-ray diffraction measurements of the austenite lattice parameter (aγ). In this way, we evaluated the different expressions available in the literature relating Cγ to aγ.

Controlled dissolution of colossal quantities of nitrogen in stainless steel

The solubility of nitrogen in austenitic stainless steel was investigated thermogravimetrically by equilibrating thin foils of AISI 304 and AISI 316 in ammonia/hydrogen gas mixtures. Controlled dissolution of colossal amounts of nitrogen under metastable equilibrium conditions was realized, with nitrogen contents as high as corresponding to an occupancy of y N=0.61 of the interstitial sublattice, i.e., about 38 at. pct N. Associated with the dissolution of these unprecedented nitrogen contents in an austenitic matrix a reversible volume expansion of the austenite lattice occurred for y N > 0.17. A simplistic model based on a statistical distribution fo the nitride forming elements over the octahedrons constituting the solid state agrees favorably with the experimental data.

The Effect of Boron on Hot Ductility of Nb-microalloyed Steels

Two grades of the Nb-containing steel, one modified with B, were examined in order to study the effect of B on the hot ductility. Since the hot ductility loss of the Nb-containing steel during the continuous casting is manifested as surface cracking, the surface thermal schedule including in situ melting was taken into account. The results revealed that addition of B improves the hot ductility of austenite. Two mechanisms for such improvement are proposed that are not based on the presence of intragranular ferrite. These mechanisms concern with depletion of strengthening elements in the austenite lattice through faster kinetics of grain boundary precipitation and non-equilibrium segregation of precipitants.

Modelling of grazing incidence X-ray diffraction spectra from Mo-implanted stainless steel: Comparison with experimental data

Mo ion implantation at 150 keV in an austenitic stainless steel leads to a pseudo-Gaussian concentration depth profile as predicted by calculation using TRIM code and confirmed by RBS analyses. The structural modification of the implanted layer was investigated using grazing incidence X-ray diffraction (GIXD) for different incidence angles. The X-ray austenite peak depends strongly on fluence and incidence angle, and presents one or two peaks as a function of incidence angle. Using the concentration depth profile of implanted element, the information depth profile of diffracted intensity, and a linear relationship between Mo concentration and lattice parameter, we predicted the γ diffracted X-ray peaks. The experimental X-ray diffraction peaks are comparable to the predicted ones. Mo implantation induces a continuous expansion of austenite lattice, which follows the depth profile concentration in the implanted layer even if the signature of this expansion is the apparition of a new austenite peak.

Characterization of a carburized surface layer on an austenitic stainless steel

Characterization tests were made of a commercial carburization treatment that has shown promise for hardening the surfaces of the austenitic stainless steel target vessel of the Spallation Neutron Source against cavitation erosion and pitting caused by the action of pulsed pressure waves in the liquid mercury target. The findings support most of the provider’s promotional claims. The high surface hardness and the thickness of the hardened layer are validated, as are the hardness-depth profiles. The austenite lattice of the layer is enlarged and placed in a state of compressive stress and the surface is plastically distorted by the treatment. The corrosion resistance of the surface in selected acid media is greater than that for untreated austenite. The treated surface is not brittle and is quite resistant to cracking during straining. The maximum carbon content of the layer is measured at 3–4.5 wt%, versus 6–7 wt% cited by the provider. Contrary to the provider’s assertion that all of the carbon is contained in supersaturated solid solution in the austenite phase, some of the carbon is present in an iron carbide phase located non-uniformly at the very surface. Inclusion stringers and δ-ferrite phase in the carburized layer are more prone to corrosion and may provide preferential sites for cavitation pitting.

Analysis of lattice parameter changes following deformation of a 0.19C-1.63Si-1.59Mn transformation-induced plasticity sheet steel

Austenite and ferrite lattice parameters were monitored using X-ray diffraction subsequent to deformation in uniaxial and biaxial tension and plane straining of a 0.19C-1.63Si-1.59Mn transformation-induced plasticity (TRIP) sheet steel. Details from peak position results suggest the presence of stacking faults in the austenite phase, especially after deformation in uniaxial tension. The results also indicate residual stress or composition effects (through changes in the average carbon concentration due to selective transformation of lower carbon regions of austenite). Compressive residual stresses in the ferrite matrix were measured, and found to increase with increasing effective strain in specimens tested in biaxial tension and plane strain. Strain partitioning between softer ferrite and harder austenite (and possibly bainite or martensite) may be responsible for these residual compressive stresses in the ferrite, although volume expansion from the γ → α′ transformation and texture gradients through the sheet thickness are also possible contributors.

In-situ stability study of nitrocarburized 304 stainless steel during heating

Plasma immersion ion implantation (PIII) has been employed for nitrocarburizing 304 stainless steel. The sample was treated at relatively low gas pressure of nitrogen and acetylene in the range of 10−3 mbar. The microstructure variation, the austenite lattices spacing and the phase transformations were studied in-situ during heating up to 800 °C and after cooling, using synchrotron X-ray diffraction. Glow discharge optical spectroscopy (GDOS), optical microscopy and hardness profile measurements have been used before and after thermal treatment to analyse the nitrocarburized layer.

Kinetics of low temperature plasma carburizing of austenitic stainless steels

A low temperature plasma carburizing technique has recently been developed to engineer the surfaces of austenitic stainless steels for combined improvement in wear and corrosion resistance. The resultant carburized layer is characterized by the supersaturation of carbon in austenite lattices, the much-increased hardness and wear resistance, and most importantly its superior corrosion resistance. This paper presents recent experimental results on the kinetics of this novel process, in terms of the growth of the precipitation-free layer and its variation with processing temperature, time and substrate material. This work demonstrates that the low temperature carburizing process is a diffusion-controlled process, and only when the processing temperature is sufficiently low can a precipitation-free layer be produced. In addition, the chemical compositions of the substrate material also affect the formation and kinetics of the precipitation-free layer. By proper process control and material selection, a high-quality carburized layer 10–50 μm thick can be produced at temperatures between 400 and 500 °C for wear protection in highly corrosive environments.

Effect of Nitrogen Doping on Sliding Wear Behaviour of Stainless Steel Coatings

Nitrogen-doped austenitic stainless steel coatings have been deposited onto a mild steel substrate by the magnetron sputtering technique, with the purpose to enhance the tribologica1 and chemical properties of the steel. The resultant coatings with different degrees of nitrogen doping have been characterised by X-ray diffraction, scanning electron microscopy and nanoindentation. Dry sliding friction and wear tests have been conducted to investigate the tribological behaviour of the coating-substrate systems. The results show that nitrogen doping stabilises and expands the austenite lattices, which leads to a significant hardening effect in the coating without formation of harmful nitride compound phases. With an appropriate amount of nitrogen doping, the coating exhibits much enhanced sliding wear resistance. However, excessive nitrogen doping leads to the deterioration in tribological performance due to coating embrittlement and reduced coating-substrate integrity.

High-resolution electron microscopy study of ledge structures and transition lattices at the austenite–martensite interface in Fe-based alloys

In order to clarify how the lattices of austenite and martensite are connected at the interface on the atomic scale in the region within the width of the transformation twins or between slipped planes, a high-resolution electron microscopy study of the austenite–martensite interface was performed for Fe–23.0Ni–3.8Mn, Fe–30.5Ni–10Co–3Ti and Fe–8.8Cr–1.1C (mass %) alloys. Martensitic transformations occur at a low temperature in these alloys and exhibit various transformation characteristics, such as isothermal and athermal transformations, Kurdjumow–Sacks and Nishiyama orientational relationships, and a reversible mobile interface and a non-reversible interface in the reverse transformation. There were three important findings: (1) the (121)f interface is made up of two component planes, i.e. the terrace plane (111)f (//(011)b) and the ledge (0 0)f, and the average numbers of atom rows of [ 01]f (//[ 1]b) on these planes are 2.7–4.9 on (111)f and 1.3–2.6 on (0 0)f. These are different depending on the alloy, but the ratio of the atom rows of the former to the latter is about 2:1, in accordance with the observed habit plane of (121)f; (2) the neighbouring (0 0)f ledges are separated by about five to ten planes of (112)b; and (3) transition lattices from fcc to bcc (or bct) exist at the interface and the thickness of this transition lattice region is 0.2–0.8 nm. It is reasonably assumed that screw dislocations with a Burgers vector of 1/2[ 01]f (//1/2[ 1]b) reside at the ledge of the interface, which produces the lattice invariant deformation when they move with the interface. The complete scheme for the relaxation of the strain at the interface is considered to be as follows. First, transformation twins are introduced to relax the strain at the interface on the macroscopic scale, in accordance with the phenomenological theory, then screw dislocations are brought in at the ledges to release the strain between each of the twin-related martensite crystals and the austenite, and, finally, transition lattices are generated at the interface to relax the strain on the most microscopic scale, i.e. at the atomic level.

A Dilatometry Study of the Austenitization and Cooling Behavior of Ductile Iron Meant for the Production of Austempered Ductile Iron (ADI)

An understanding of the kinetics of transformation during austenitization, cooling, and austempering of ductile iron is critical to achieving the desired microstructures and ultimately mechanical properties in austempered ductile iron (ADI). To this end, dilatometry experiments have been carried out to study the austenitization and cooling behavior of an unalloyed ductile iron. When a typical austenitization temperature of 900°C is used, unlike in steels, there is an initial expansion of the specimen, which levels off as the soaking time is increased. This occurs despite the fact that the temperature remains constant. This phenomenon, hitherto unreported, highlights the subtle differences between the austenitization of ductile irons and steels. The initial expansion is attributed to the increase in austenite lattice parameter, arising from the diffusion of carbon from the graphite nodules. The levelling off signals the saturation of austenite with carbon and can therefore be used as an indicator of the appropriate austenitization time. Studies of the cooling behavior of unalloyed ductile iron have also shown that the dilatometer can be used as a tool for determining the minimum cooling rates, which guarantee the formation of ausferrite during austempering. When ductile iron is appropriately heat-treated based on results from dilatometry studies, the mechanical properties obtained are typically superior and consistent.

Stability under temperature of expanded austenite developed on stainless steel AISI 316L by ion nitriding

Ion nitriding of steels is a process widely used on steels because it can give the surface important improvements in their tribological and mechanical properties attributable to the iron nitridings formation. On the other hand, on stainless steels, this improvement can normally be obtained by sacrificing their resistance to the corrosion. Nevertheless, several years ago, a new type of phase known as expanded austenite on austenitic stainless steels has been developed, a phase that have shown a high micro-hardness conserving at the same time the resistance to corrosion. In this work, we study the ion nitriding conditions to obtain this type of phase on stainless steel AISI 316L. We have found that the expanded austenite was completely developed after only 30 min of ion nitriding. We obtain images of the surface treated, showing the austenitic nature of the phase, with slip bans that confirms the stress state originated in the expansion of the original austenitic lattice during the ion nitriding process. Further studies of thermal treatment at different temperatures have shown that this expanded austenite developed on AISI 316L is stable for 40 h at temperatures of 300 °C, conserving their structure, X-ray diffractogram and a micro-hardness between 1200 and 1400 HV 0.025.

Low temperature plasma nitrocarburising of AISI 316 austenitic stainless steel

Plasma nitrocarburising was carried out at a low temperature to improve the surface hardness of AISI 316 austenitic stainless steel without degradation of its corrosion resistance. It was found that nitrogen and carbon atoms can simultaneously be dissolved into the austenite lattice during the nitrocarburising process, forming a nitrogen and carbon supersaturated solid solution, which has a face centre tetragonal (fct) structure (i.e. the S phase). GDS analysis showed that the maximum nitrogen and carbon concentrations in the nitrocarburised S-phase layer occur at different depths from the surface. The nitrocarburised layers have not only high hardness like the nitrided layer, but also high thickness and gradually reduced hardness profile like the carburised layer.

Xe and Mo ion implantation on austenitic stainless steel: structural modification

Xe ions and Mo ions with 49-keV energy were implanted into an austenite stainless steel at doses between 1×10 15 and 35×10 15 ions cm −2 . Grazing incidence X-ray diffraction was used to analyze the surface structure of implanted sample. Xe implantation went through differentiation of two types of austenite crystallites and formation of constrained ferrite. For low implantation doses, Mo first inserted into austenite cells inducing austenite lattice expansion. Then, as the Mo dose increased, this expansion was combined with the differentiation of two types of austenite crystallites. Above 6×10 15 ions cm −2 a new bcc structure appeared: a ferrite with Mo was formed. High dose implantation leads to partial amorphisation.

Xe and Mo ion implantation on austenitic stainless steel: structural modification

Xe ions and Mo ions with 49-keV energy were implanted into an austenite stainless steel at doses between 1×10 15 and 35×10 15 ions cm −2. Grazing incidence X-ray diffraction was used to analyze the surface structure of implanted sample. Xe implantation went through differentiation of two types of austenite crystallites and formation of constrained ferrite. For low implantation doses, Mo first inserted into austenite cells inducing austenite lattice expansion. Then, as the Mo dose increased, this expansion was combined with the differentiation of two types of austenite crystallites. Above 6×10 15 ions cm −2 a new bcc structure appeared: a ferrite with Mo was formed. High dose implantation leads to partial amorphisation.

Neutron Diffraction Study of the Low-Temperature Ageing of Martensite in Dual-Phase Steel

Abstract Neutron diffraction measurements were used to obtain a better understanding of the ageing behaviour in dual-phase ferrite + martensite steels. Therefore, the influence of different ageing treatments on the tempering of low-alloyed 0.4 and 0.7% C FeMnCrMo-martensite and 0.1% C FeMnCrMo dual-phase steels was investigated. On tempering at 170 and 350°C, the sequential precipitation of η- and θ-carbides, as well as the loss of tetragonality was observed in the martensite. The decrease in austenite lattice parameter after tempering is due to stress relaxation in the material. These observations offer an acceptable explanation for the large yield stress increase after ageing at 170°C.

Lattice Changes Generated by Electrochemically Induced Surface Annealing of Austenitic Stainless Steel

Measurement of the X-ray diffraction patterns of 304L stainless steel after subjecting it to an electrochemically induced surface annealing process in concentrated nitrite solutions shows that the austenite lattice contracts during processing relative to the untreated alloy. The surface annealing procedure, which involves alternate anodic and cathodic potential pulses, thereby induces a small increase in the density of the metal surface. The observations are not due to processes involving ingress of reduced hydrogen. The results of comparative experiments in alternative electrolytes show that ingress of hydrogen into the steel expands the austenite lattice.

Transformation Textures in Unstable Austenitic Steel

During the martensitic transformation in elastic-plastic materials, the local transformation strain as well as the plastic flow inside austenite are strongly related with the crystallographic orientation of the austenitic lattice. Two mechanisms involved in these materials, i.e., plasticity by dislocation motion and martensitic phase formation are coupled through kinematical constraints so that the lattice spin of the austenitic grains is different from the one due to classical slip. In this work, the lattice spin ω˙eA of the austenitic grains is related with the slip rate on the slip systems of the two phases, γ˙A and γ˙M, the evolution of the martensite volume fraction f˙ and the overall rotation rate Ω˙ of the grains. This new relation is integrated in a micromechanical model developed for unstable austenite in order to predict the evolution of the austenite texture during TRansformation Induced Plasticity (TRIP). Results for the evolution of the lattice orientation during martensitic transformation are compared with experimental data obtained by X-ray diffraction on a 304 AISI steel.