Metastable Austenite Phase Research Articles

High-temperature mechanical properties improvement on modified 9Cr–1Mo martensitic steel through thermomechanical treatments

In the framework of the development of generation IV nuclear reactors and fusion nuclear reactors, materials with an improved high temperature (≅650 °C) mechanical strength are required for specific components. The 9–12%Cr martensitic steels are candidate for these applications. Thermomechanical treatments including normalisation at elevated temperature (1150 °C), followed by warm-rolling in metastable austenitic phase and tempering, have been applied on the commercial Grade 91 martensitic steel in order to refine its microstructure and to improve its precipitation state. The temperature of the warm-rolling was set at 600 °C, and those of the tempering heat-treatment at 650 °C and 700 °C thanks to MatCalc software calculations. Microstructural observations proved that the warm-rolling and the following tempering heat-treatment lead to a finer martensitic microstructure pinned with numerous small carbide and nitride particles. The hardness values of thermomechanically treated Grade 91 steel are higher than those of the as-received Grade 91. It is also shown that the yield stress and the ductility of the thermomechanically treated Grade 91 steel are significantly improved compared to the as-received material. Preliminary creep results showed that these thermomechanical treatments improve the creep lifetime by at least a factor 14.

Ageing study of Cu–Al–Be hypoeutectoid shape memory alloy

Thermal ageing at constant temperature (350 °C) and under systematically designed temperature-varying conditions were performed on the metastable austenitic phase of hypoeutectoid Cu–Al–Be shape memory alloy. Thermal precipitations and their effects on the alloy global microstructure were studied by DSC, XRD and SEM techniques. Precipitations of equilibrium phases (α + γ 2) within the temperature range of 330–370 °C, and their generalization by a discontinuous mechanism for an ageing time of 864 ks (240 h), were identified and analyzed. The results of this study are expected to benefit the applications of copper-based shape memory alloys under various thermal conditions.

Semi Phenomenological Modelling of the Behaviour of Trip Steels- Application to Sheet Metal Forming

A semi-phenomenological model is developed based on a mean-field description of the TRIP behaviour for the simulation of metal forming processes [1]. Within the metastable austenitic phase exhibiting martensitic transformation, a mean field approach is introduced at the phase level with the concept of Mean Instantaneous Transformation Strain (MITS) accompanying transformation. Within the framework of the thermodynamics of irreversible processes, a transformation criterion and martensitic volume fraction evolution are determined. A multiscale scheme is used to model the behaviour of TRIP steels. The model reproduces the increase in ductility and the multiaxial behaviour of a multiphased TRIP steel. In this study, this new model is implemented in the finite element code Abaqus Explicit for metal forming simulations. The simulations are compared to experimental data.

Microstructure formation in rapidly solidified AISI 304 stainless steel strip

It is recognised that the solidification sequence in austenitic stainless steels during strip casting plays an important role in the tendency to hot cracking. In order to investigate the competitive growth between the stable ferrite phase and the metastable austenite phase to optimise the microstructures obtained by strip casting, experiments using symmetrical water-cooled copper mould were designed to simulate strip casting of AISI 304 stainless steels. It was found that primary phase transition from stable ferrite (δ) to metastable austenite (γ) takes place adjacent to the surface of the strip due to high heat extraction rate. Phase selection theory, based on the criterion of the highest interface temperature, was used to explore the primary phase prediction. The experimental results are compared with the theoretical predictions.

Surface and mid-plane texture evolution in austenite phase of cold rolled austenitic stainless steels

Austenite stainless steels with stable and metastable austenite phase were studied for the evolution of texture during cold rolling. After solution annealing treatment AISI 304L and 316L austenitic stainless steels (ASS) were unidirectionally cold rolled to different degrees of thickness reductions up to 90% of cold reduction. Texture measurements were made at the surface as well as at the mid-plane sections by X-ray diffraction technique. Distinctly two different types of textures were noted at the surface and mid-plane section. The surface texture showed texture transition from copper type to Brass type whereas the mid-plane section showed typical Brass type texture from the very beginning of deformation.

Neutron diffraction analysis of martensite ageing in high-carbon FeCMnSi steel

Abstract A high-carbon metastable austenite phase was quenched in liquid nitrogen to obtain fresh athermal martensite. Different ageing treatments were performed in order to study the transformation of the martensite phase and the formation of carbides. Neutron diffraction experiments give detailed information on the transformations during the annealing treatment itself. The tetragonal athermal martensite transformed immediately, i.e. during the heating to the ageing temperature, to cubic martensite. Ageing at 400°C resulted in the decomposition of the austenite in ferrite and carbides and the formation of bainite. Those carbides could be determined as -carbides transforming in -carbides after extended annealing times. As was expected, ageing at 170°C resulted in the formation of a small amount of carbides while the austenite phase remained stable.

Influence of strain-induced phase transformation on the surface crystallographic texture in cold-rolled-and-aged austenitic stainless steel

Stainless steels (SSs) having a stable and metastable austenitic phase were studied to see the influence of strain-induced phase transformation in the metastable austenitic stainless steel on the evolution of texture during cold rolling and aging. AISI 304L and 316L SS plates were unidirectionally cold rolled up to a 90 pct reduction and aged at different aging temperatures. The strain-induced transformation of austenite to α′-martensite phase and the evolution of texture in both the phases were studied as a function of rolling reduction as well as aging temperature in the metastable 304L austenitic stainless steel. The X-ray diffraction (XRD) technique was employed to quantify the volume fractions and characterize the texture of austenite and martensite phases in the rolled and aged conditions. Results are compared with the texture evolution in the stable austenitic 316L SS.

Stress Evolution in Sputter-deposited Fe–Pd Shape-memory Thin Films

Fe–Pd films with Pd content varying between 26 and 30 at.% have been deposited by means of magnetron sputtering of elemental Fe and Pd targets. As-deposited films are highly supersaturated solid solutions of Pd in Fe that have a body-centered-cubic crystal structure and a very fine grain size. Substrate curvature measurements indicate that the films undergo an irreversible densification when heated above 100 °C. This densification is attributed to a structural change that is also observed in other supersaturated systems with a substantial atomic size difference between the constituents. It is possible to retain the high-temperature austenite phase at low temperature by annealing the films at 900 °C followed by rapid cooling. Depending on film composition, this metastable austenitic phase transforms to either a body-centered tetragonal (bct) or a face-centered tetragonal (fct) martensite around room temperature. Substrate curvature measurements show that formation of the fct martensite is reversible, while that of bct martensite is not. The fct transformation occurs at lower Pd content and higher temperature than reported for bulk materials. Both the fct and the fcc phase show a strong Invar effect at lower temperature and Pd content than observed in the bulk.

Characterisation of Microstructures, Deformation, and Fatigue Damage in Different Steels Using Magnetic Barkhausen Emission Technique

The characterization of microstructures, mechanical properties, deformation, damage initiation, and growth by Non-Destructive Evaluation (NDE) techniques is assuming a vital role in various industries because of the growing awareness of the benefits that can be derived by using NDE techniques for assessing the performance of various components. NDE is widely applied for assessment of material degradation, where investment in new plants is not cost-effective and safe operational life of existing plants needs to be extended. In recent years, various advanced NDE techniques have been successfully employed for characterization of defects and microstructural features such as grain size, texture, nucleation and growth of second phases, assessment of tensile, creep and fatigue properties, deformation, and damage. With the advent of fracture mechanics concepts, microstructure, defects as well as stresses must be quantitatively characterized to have reliable and fail-safe materials and components. Any alteration in the microstructure, which reduces the life or performance, should be predicted sufficiently in advance in order to ensure safe, reliable, and economic operation of the components. This is possible when one realizes that the interaction of the nondestructive probing medium with the material depends on the substructural/microstructural features such as point defects, dislocations, voids, micro and macro cracks, secondary phases, texture, residual stress, etc. In this paper, use of Magnetic Barkhausen Emission technique for characterization of microstructures, deformation, and fatigue damage in different steels would be discussed. When a ferromagnetic material is subjected to a varying magnetic field, the magnetic flux densidy during magnetization varies in discrete steps as the magnetic domain walls have to overcome various types of obstacles during their movement. The discrete changes in magnetization induce electric voltage pulses in pickup coil placed near the surface of ferromagnetic materials. This noise like voltage pulses were, firstly observed by Barkhausen in 1919 and this phenomenon is named as magnetic Barkhausen Noise (MBN) or magnetic Barkhausen emission (MBE). Based on the two stage magnetization process modeled by the authors, the size of the grain/lath and carbides in ferritic steels could be correlated with MBE parameters. The influence of morphology of cementite (lamellar and spheroidized) on the MBE behaviour has been understood. Various stages of tensile deformation, viz. (i) perfectly elastic, (ii) micro-plastic yielding, (iii) macroyielding, and (iv) progressive plastic deformation in ferritic steels could be identified using the MBE parameters. The hardening depth and its quality with respect to the microstructure in induction hardened specimens could also be established using the MBE technique. MBE is found to be highly sensitive in detection of stress induced martensite in stainless steels with metastable austenite phase. Different stages of fatigue damage in 9Cr-1Mo steel, viz. (i) cyclic hardening, (ii) softening, (iii) saturation, and (iv) crack initiation could be identified using the MBE parameters.

Experimental research on fatigue crack growth in powder composite nickel-bearing steels

Micromechanical aspects of fatigue crack growth propagation in powder heterogeneous materials such as porous composite nickel-bearing steels are discussed. It is found that the residual porosity of powder steels (usually 3-5%) is favourable to damage initiation under cyclic loading and damage growth due to fatigue crack propagation. High strength austenitic phases of powder composite steel generated by the sintering process because of microscale volumes rich in nickel are formed near nickel powder particles and these microscale austenitic volumes are favourable to fatigue damage growth. The powder nickel-bearing steel microstructure contains ferrite and austenite phases and pores in the iron-carbon-nickel base system. In addition, under cyclic loading, the meta-stable austenite phase is turned into a carbide-ferrite substructure, resulting in a change in the fatigue crack propagation process. Considering that the lifetime of functional materials in friction wearing parts or cutting tools is defined through the crack propagation time, the experimental research into fatigue crack resistance for powder nickel-bearing steels presented in this paper should have substantial practical importance.

Analysis of heat flow and microstructure evolution during spray deposition of Fe‐3C‐1.5Mn alloy

A theoretical model for the concomitant solidification of droplets and preform during spray deposition has been proposed, based on heat‐flow analysis. It has been unambiguously demonstrated that cooling rates approaching those in the rapid solidification (RS) regime can only be achieved when the droplets are still in free flight during the deposition process. The cooling rates in the droplets range from 104–106 Ks−1 depending upon their size for the experimental conditions employed in the present studies. In contrast, the model predicted cooling rates for the deposits in the region of 103–104 Ks−1. A hypoeutectic Fe‐3C‐1.5Mn‐0.3Si has been chosen as an experimental alloy for studies relating to microstructural characterization. The microstructure of powder developed fully during solidification of droplets in free flight revealed dendritic morphology of the metastable austenitic phase, whereas the spray‐deposited alloy exhibited characteristic homogeneous and refined substructure. The evolution of microstructure during spray deposition as also during atomization has been compared and discussed by invoking the proposed model.

The microstructures and composition profiles of iron implanted with combinations of Ni, Mo and B ions

TEM, AES and backscattered CEMS were used to investigate the microstructures and the composition depth profiles of the iron surface layers implanted with Ni +, Mo + and B + ions, respectively, or their combinations. The ion energy ranged between 50 and 150 keV and the ion dose ranged between 5×10 15 and 4×10 17 ions cm −2. The implantations were carried out at room temperature and the pressure in the target chamber was equal or lower than 1.3×10 −3 Pa. A relatively thick amorphous layer formed, particularly, after combined implantations of three ion species. Metastable HCP carbides Fe 2C and FCC austenitic phase were also observed. Subgrains were found within the original grains. In the case of ausstenite formation, microtwins were also seen. A carburized layer containing up to 20–30 at% of carbon near the surfaces was revealed by AES profiling. Under the conditions of multiple implantations, the profiles of different implanted elements were shown to affect each other. The influence of the radiation damage on the carbon in-diffusion and the formation of the amorphous layers is also discussed in the paper.

MARTENSITE FORMATION AND THE RELATED TOUGHNESS IN 304 STAINLESS STEEL DURING LOW TEMPERATURE FATIGUE

Abstract— Effect of transformed martensite during low temperature fatigue in metastable 304 stainless steel on low temperature toughness were investigated. Low temperature fatigue was mainly conducted at R= 0.1 with maximum stress of 1030 MPa at −196°C. The amount of martensite during fatigue cyclic deformation was related to the reduction of impact toughness and to the apparent fracture toughness. Beyond 30% of martensite formation, a strong decrease of low temperature impact toughness was found. This phenomenon is related to the phase stability of the metastable austenitic phase.

Fe-Cr-C hardfacing alloys for high-temperature applications

The microstructure and phase chemistry of a Fe-34Cr-4.5C wt% hardfacing alloy has been investigated using transmission electron microscopy and microanalytical techniques. The microstructure is found to consist of large primary M7C3. carbides in a eutectic mixture of austenite and more M7C3. The results indicate that the microstructure of the undiluted alloy becomes configurationally frozen at a temperature of about 1150° C during deposition by the manual metal arc welding technique. This allows the metastable austenite phase to contain a large chromium concentration (≈ 16 to 17 wt %), thus imparting good corrosion and oxidation resistance. Experimental data on the partitioning of chromium, manganese and silicon between the carbide phases are discussed in the context of the high-temperature stability of the alloy.

Metastable Austenite Phase in Rapidly Quenched Fe-Cr-C Alloys

By rapid quenching technique, metastable austenitic alloys with high strength and hardness have been found in Fe-Cr-C ternary system. This formation range is limited to about 1.0-2.2wt%C and 7-30wt%Cr. The austenite phase has ultra-fine grains of about 0.2μm in diameter. Their Vickers hardness, 0.2% proof stress and tensile fracture strength increase with the amounts of carbon and chromium, and the maximum values reach about 630DPN, 1500 and 1550MPa, respectively. These alloys are so ductile that no crack is observed even after closely contacted bending test. In addition, the changes in microstructure and mechanical properties of the tempered austenitic alloys have been investigated and it has been observed that a large secondary hardening occurs at about 870K due to phase transformation from austenite to equilibrium structure of ferrite and M7C3 pearlite. Thus the present alloys may be attractive as a fine gauge high-strength wire or plate.