Original Austenite Research Articles

Influence of original austenite grain size on tensile properties of a high-manganese transformation-induced plasticity (TRIP) steel

Original austenite grain size and stacking fault energy (SFE) of a high-Mn transformation-induced plasticity (TRIP) steel were varied by annealing at different temperatures. The microstructural evolution during cooling and tensile deformation were investigated to explain the work hardening behavior. The study suggested that the amount of ε-martensite was increased with annealing temperature. When SFE was greater than 13.3mJ/m2, twin may form in austenite on cooling; and ε-martensite cannot be thermally induced in austenite once its SFE was higher than 20.6mJ/m2. Work hardening behavior of the steel annealed at different temperatures could be divided into two stages using Hollomon analysis. The grain refinement strengthening and deformation induced γ → ε transformation rendered 800°C annealed sample to have the largest work hardening exponent in Stage-I, in this case ultimate tensile strength was also highest. In Stage-II, significant deformation-induced ε → α′ transformation in samples annealed at high temperatures resulted in greater improvement of work hardening exponent. The elongation of the steel was also improved with annealing temperature because local stress concentration was effectively released by ε → α′ transformation.

Cryorolling impacts on microstructure and mechanical properties of AISI 316 LN austenitic stainless steel

Microstructure evolution and mechanical properties of AISI 316 LN austenitic stainless steel (SS) after cryorolling with different strains were investigated by means of optical, scanning and transmission electron microscopy, X-ray diffractometer, microhardness tester, and tensile testing system. The deformation-induced martensite transition and the deformation microstructure occurred during cryorolling process were always composed of high-density dislocations, deformation twins, and deformation-induced martensites. Following the strain, the dislocation density in deformation microstructure approached saturation state and the volume fraction of deformation twins combined with deformation-induced martensites increased significantly. At the 70% strain, original austenite was transformed into martensite completely. Further increasing the strain to 90% would refine the martensitic lamellae to nanoscale. The deformation degree also led to remarkable increase of the strength and hardness of the cryorolled SS, and drastic reductions of the elongation. Due to the cryorolling, the tensile fracture morphology changed from typical ductile rupture to a mixture of quasi-cleavage and ductile fracture.

Overall Hardening of Solid-Rolled Wagon Wheels by Volume Quenching and Surface Plasma Processing

In relation to solid-rolled wagon wheels the advantages of differentiated quenching of all wheel elements are shown in comparison with quenching of only wheel rim rolling surface. It should be noted that larger possibilities to enhance performance characteristics of heavy duty details and products can be fulfilled using the technology of overall hardening including the differentiated quenching and surface plasma processing. It is established that along with the improvement of mechanical properties plasma processing after volume quenching allows for a considerable increase of crack resistance of surveyed grades in comparison with volume quenching without plasma processing. The martensite structure of the strengthened layer during such overall processing in comparison with the martensite structure of volume and strengthened base is characterized by high level of particle size that is specified by reduction of the original austenite grain due to very high heating and cooling speeds as well as small time of steel stay at high temperatures. In order to increase hardness and resistance against cracks generation plasma processing and preliminary volume quenching are recommended. A composite working layer with high wear and crack growth resistance and relatively soft and plastic core are developed in details and products volume quenched when surface plasma strengthening.

Microstructural and Crystallographic Insights in a Martensite/Austenite Dual Phase Ni–Mn–Sb Alloy

For Ni–Mn–Sb multifunctional alloy, its microstructure and crystallographic information are decisive factors for its multiple magnetic field induced properties, such as magnetic field induced shape memory effect, magneto‐caloric effect, exchange bias effect, and so on. While, studies on such field are rarely conducted. In the present work, a thorough study on microstructural features and crystallographic characteristics has been conducted in a martensite/austenite dual phase coexisting polycrystalline Ni50Mn37Sb13 alloy. Results show that the martensite is self‐organized in plates in the original austenite. The intra‐plate martensite presents a fine lamellar microstructure and each fine lamella corresponds to one martensite variant. Crystallographic orientation analysis indicates that each martensite plate exists four differently oriented martensite variants and they can form three types of twins, type I, type II, and compound twin. Trace analysis results show that the interface between adjacent variants are their twinning plane K1. Further investigation on martensitic transformation orientation relationship reveals that the Pitsch orientation relationship, specified as //and //, is the effective one from austenite to martensite. All the above results offer basic microstructural and crystallographic information on Ni–Mn–Sb alloys and can be useful for further investigation on property optimization of these alloys.

Effect of Ti on T15M composite coating fabricated by laser cladding technology

In this article, the T15M (belonging to a kind of high speed steel powder) composite coating with different Ti content was prepared by laser cladding technology (LCT). The microstructure and properties of the samples were investigated by scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), automatic micro hardness tester and multifunctional friction and wear testing machine. The results show that the microstructure of the T15M composite coating is mainly composed of martensite, austenite and carbides which mainly distribute on the original austenite grain boundaries. With the increase of titanium element, the shape of carbides become more spherical and the carbides on the grain boundaries change from continuous to discontinuous which attributes to the destruction of carbon concentration balance at the interface. In contrast to the T15M composite coating without adding titanium, the carbides distribution in the coating are more uniform relying on preferential homogeneous nucleation of titanium compounds in solidification. By hardness test, it can be discovered that the addition of titanium to T15M composite coating has no significant effect on the average hardness, but the fluctuation range of the microhardness value decreases. After the sliding wear test, both the coefficient of friction and the mass loss after wear are reduced with the addition of titanium.

The Effect of Twice Thermal Cycle on the Microstructure and Mechanical Properties of Coarse Grain Region in X80 Pipeline Steel

Based on the multi-pass welding characteristics of X80 pipeline steel, the influence of twice thermal cycle on the microstructure and mechanical properties of coarse grain region in X80 pipeline steel was investigated. The thermal cycles of weld coarse grain region with different peak temperature for the second thermal cycle were simulated with the Gleeble-3500 thermal/mechanical simulator. The Charpy impact absorbed energy for toughness was measured, and the corresponding optical micrographs and electron micrographs were systematically investigated to study the effect of the peak temperature on microstructure and impact toughness in the coarse grain region. The results of simulated experiment showed that the microstructure in heat affected zone of coarse grain region is granular bainitic and bainitic ferrite. When the peak temperature of the second thermal cycle is 800°C, the types of microstructure and the grain size of original austenite have no change. However, it forms network microstructure with chain structure in grain boundary and the reduction of toughness may be affected by the M–A constituents. With the peak temperature of 1000°C, the micro structure is composed of granular bainitic and a little bainitic ferrite. In this case, the grain size of austenite can be significantly fined, being helpful to increase the impact toughness.

Intergranular Corrosion Resistance and Microstructure of Laser-Metal Active Gas Hybrid Welded Type 301L-MT Stainless Steel Joint

The microstructure and intergranular corrosion resistance of 6 mm thick Type 301L-MT stainless steel joints welded by laser-MAG (metal active gas) hybrid welding were investigated. Double loop electrochemical potentiokinetic reactivation measurement (DL-EPR) and electrochemical impedance spectroscopy (EIS) were conducted to study the corrosion behavior of laser-MAG hybrid welded (LMHW) joints in a slightly oxidative medium (a solution of 0.5 mol/L H2SO4 + 0.01 mol/L KSCN). The results show that the weld metal mainly consists of austenite and δ-ferrite, while the base metal (BM) and heat affected zone are the rolled structure including the original austenite and lath martensite. The different parts of LMHW joints present diverse intergranular attack resistance as a result of the microstructural difference. BM shows the best corrosion resistance with average Ir/Ia ratio (Ir is the reactivation peak current density, and Ia is the activation peak current density) of not more than 1%. The results indicate that...

Comparison of microstructure and property of high chromium bearing steel with and without nitrogen addition

Microstructure and property of bearing steel with and without nitrogen addition were investigated by microstructural observation and hardness measurement after different heat treatment processing. Based on the microstructural observation of both 9Cr18 steel and X90N steel, it was found that nitrogen addition could effectively reduce the amount and size of coarse carbides and also refine the original austenite grain size. Due to addition of nitrogen, more austenite phase was found in X90N steel than in 9Cr18 steel. The retained austenite of X90N steel after quenching at 1050 °C could be reduced from about 60% to about 7% by cold treatment at −73 °C and subsequent tempering, and thus finally increased the hardness up to 60 HRC after low temperature tempering and to 63 HRC after high temperature tempering. Furthermore, both the wear and corrosion resistance of X90N steel were found much more superior than those of 9Cr18 steel, which was attributed to the addition of nitrogen. It was proposed at last that nitrogen alloying into the high chromium bearing steel was a promising way not only to refine the size of both carbides and austenite, but also to achieve high hardness, high wear property and improved corrosion resistance of the stainless bearing steel.

Crystallographic analysis of the martensitic transformation in medium-carbon steel with packet martensite

Based on X-ray diffraction studies of the martensite texture in a single martensite packet, exact orientation relationships between the orientations of martensite crystallites and the original austenite single crystal in medium-carbon steel 37KhN3A have been determined to be as follows: (011)α||(1; 0.990; 1.009)γ to an accuracy of \( \pm 0.15^\circ ,{\left[ {01\overline 1 } \right]_\alpha }||{\left[ {1;1.163; - 2.133} \right]_\gamma }\) to an accuracy of ±0.15°. It has been shown that the orientation relationships proved to be almost the same as in the Fe–31% Ni alloy with a twinned martensite with close lattice parameters. Therefore, the conclusion has been drawn that the mechanism of the lattice deformation upon the martensitic transformation is the same in both alloys. It is described as follows. The lattice deformation occurs by shear on the (111) plane in the \({\left[ {11\overline 2 } \right]_{_\gamma }}\) direction and is accompanied by an additional change in the dimensions in the mutually perpendicular directions \({\left[ {11\overline 2 } \right]_{_\gamma }},\left[ {111} \right],\;and{\left[ {1\overline 1 0} \right]_{_\gamma }}\). The invariantlattice deformation is implemented by slip in martensite on the planes of the (112)α type in the direction \({\left[ {\overline 1 \overline 1 1} \right]_\alpha }\). One of the 24 crystallographically equivalent variants of the transformation mechanism has been considered. Apart from this type of deformation, an additional deformation of martensite is possible that does not change its orientation. It has been shown that the orientation of the martensite crystallite calculated via the phenomenological theory of the martensitic transformations (PTMT) differs by approximately 1° from the experimentally determined orientation. This refers to both the lath and twinned martensite. In the twinned martensite, the invariant plane obtained in the PTMT calculations and the habit plane coincide. In lath martensite of 37KhN3A steel, the invariant plane of the martensite crystal obtained in PTMT calculations deviates by ~25° from the orientation of the surface of the martensite plate (habit plane), which is close to the (111)γ plane. An explanation of this phenomenon is given.

Identification and Characterization of Intercritical Heat-Affected Zone in As-Welded Grade 91 Weldment

A metallurgical method is proposed for locating the intercritical heat-affected zone in the as-welded Grade 91 steel. New austenitic grains, preferentially formed along the original prior austenite grain boundaries, are characterized to contain finer M23C6 carbides and higher strain levels than the original prior austenite grains. Kurdjumov–Sachs Group 1 variant pairs, with a low misorientation of 7 deg within a martensitic block, are identified as the dominant variants in the new PAGs.

Observation austenite memory and significant enhancement of tensile properties during cyclic reverse martensite transformation in a Fe-Ni-C TRIP steel

In this study, the influence of reverse martensite transformation (reverse transformation) on microstructure development and mechanical properties of Fe-24Ni-0.3C metastable austenitic TRIP steel was investigated. Microstructural characterization by electron backscatter diffraction (EBSD) system proved that large amount of low angle boundaries appeared after 1-cycle of reverse transformation (γ→α→γ). It is also found that the 1-cycle reversely transformed austenite and original austenite exhibited similar shape, size and orientations indicating that austenite memory appeared during reverse transformation. By increasing the number of reverse transformation cycle, fraction of low angle boundaries significantly increased. Uniaxial tensile test exhibited that yield and ultimate tensile strengths significantly improved even by 1-cycle reverse transformation comparing to the starting material. In addition, further continuation of reverse transformation up to 5- or 7-cycle causes gradual increase in yield and ultimate tensile strengths as well. The significant improvement in yield strength should be originated from increasing the dislocation density that are introduced during reverse transformation.

Crystal structure of modulated martensite and crystallographic correlations between martensite variants of Ni50Mn38Sn12alloy

A comprehensive study on the crystal structure, the microstructure and the crystallographic features of the martensite in an Ni50Mn38Sn12alloy has been conducted in the present work. The results show that the martensite possesses a 4O modulated structure. The martensite is organized into broad plates in the original austenite grain. The plates contain irregularly shaped colonies with two characteristic microstructural patterns: a classical lamellar pattern and a herringbone pattern. Crystallographic analyses by scanning electron microscopy/electron backscatter diffraction demonstrate that in each colony there are four orientation variants (A, B, C and D) and they form three types of twins (type I, type II and compound twin). The interfaces between corresponding variants are coincident with their twinning planeK1. The interface planes of the compound twin pairs A&D and B&C can have one or two different orientations, which leads to the two microstructural patterns. The corresponding variants in neighboring colonies within one broad plate (intra-plate colonies) possess close orientations, but the type I and the type II twin relationships are interchanged. The variants in neighboring colonies situated in adjacent plates (inter-plate colonies) are type I or type II twin related but with some angular deviations. The plate interface is defined by the {221} plane of the variant pair with largest thickness. The results of the present work provide comprehensive microstructural and crystallographic information on modulated martensite in NiMnSn alloys that is useful for the understanding of their specific functionalities and helpful for further investigation on property optimization of these materials.

Weldability of 1000 MPa Grade Ultra-low Carbon Bainitic Steel

Maximum hardness test in weld heat-affected zone (HAZ), oblique Y-groove cracking test and mechanical property test of welding joint of 1000 MPa grade ultra-low carbon bainitic steel were carried out, so as to research the weldability of the steel. The results show that the steel has lower cold cracking sensitivity, and preheating temperature of 100 °C can help completely eliminate cold cracks, generating good process weldability. The increase of preheating temperature can reduce the hardening degree of heat-affected zone. The strength of welding joint decreases and hardness reduces when heat inputs increase, and excellent mechanical properties can be obtained when low welding heat inputs are used. Fine lath bainites of different orientations combined with a few granular bainites that effectively split the original coarse austenite grains are the foundation of good properties.

A comparative study of the microstructure and properties of 800 MPa microalloyed C-Mn steel welded joints by laser and gas metal arc welding

The differences in microstructure and mechanical properties of laser beam welded (LBW) and gas metal arc welded (GMAW) joints of 800MPa grade Nb-Ti-Mo microalloyed C-Mn steel of 5mm thickness were studied. The study suggested that the microstructure in welded seam (WS) of GMAW was acicular ferrite and fine grained ferrite, whereas lath martensite (LM) was obtained in WS of LBW, where inclusions were finer and did not act as nucleation sites for acicular ferrite. The microstructure of coarse-grained HAZ (CGHAZ) obtained using the two welding methods was LM and granular bainite (GB), respectively. The original austenite grain size in CGHAZ of LBW was 1/3 of GMAW. The microstructure of fine-grained HAZ and mixed-grained HAZ using the two welding methods was ferrite and M-A constituents, while that of LBW was significantly fine. The hardness of LBW welded joints was higher than the base metal (BM), which was the initiation site for tensile fracture. The tensile fracture location of GMAW welded joints was in WS. The impact toughness of LBW welded joints was excellent and the impact absorption energy was similar to BM.

Microstructure Evolution and Precipitation Behavior of 0Cr16Ni5Mo Martensitic Stainless Steel during Tempering Process

The microstructure, morphology of precipitates and retained austenite and the volume fraction of retained austenite in 0Cr16Ni5Mo stainless steel during the tempering process were analyzed using optical microscope (OM), transmission electron microscope (TEM), X-ray diffraction (XRD) and scanning transmission electron microscope (STEM). The results show that the microstructure of the tempered steel is mainly composed of tempered martensite, retained austenite, and delta ferrite. In the case of samples tempered from 500 to 700 °C, the precipitates are mainly M23 C6, which precipitate along the lath martensite boundaries. The precipitate content increases with the tempering temperature. During the tempering process, the content of retained austenite initially increases and then decreases, the maximum content of retained austenite being 29 vol. % upon tempering at 600 °C. TEM analysis of the tested steel reveals two morphology types of retained austenite. One is thin film-like retained austenite that exists along the martensite lath boundary. The other is blocky austenite located on packet at the boundary and the original austenite grain boundary. To further understand the stability of reversed austenite, the Ni content in reversed austenite was measured using STEM. Results show a significant difference in nickel concentrations between reversed austenite and martensite.

Microstructure and Properties of Weld CGHAZ under Different Heat Input for X90 Pipeline Steel

Two kinds of industry trial X90 pipeline steels which have different chemical composition were chosen as test objects, and the grain coarsening, microstructural characteristics and the variation rules of low-temperature impact toughness in weld CGHAZ of this two steel under different welding heat input were studied by physical thermal simulation technology, SEM, optical microscope and Charpy impact tests. The results showed that the microstructure in weld CGHAZ of 1# steel was mainly bainite ferrite (BF) and most of the M/A constituents were blocky or short rod-like; the grains of 2# steel were coarse and there was much granular bainite (GB). Meanwhile M/A constituents became coarser and their morphology changed from block to long bar; alloy content of X90 pipeline steel under different weld heat input had great effect on the grain size of original austenite. When heat input was lower than 20KJ/cm, the impact toughness in CGHAZ of lower alloy content pipeline steel was good; as heat input increased, impact toughness in CGHAZ of 1# steel increased to the values between 260J and 300J when heat input was between 20KJ/cm and 25KJ/cm and the dispersion of impact energy was small. The impact toughness of 2# steel decreased gradually and the impact energy had the obvious dispersion.

Austenite grain refinement during load-biased thermal cycling of a Ni49.9Ti50.1 shape memory alloy

A near-equiatomic NiTi shape memory alloy was subjected to a variety of thermomechanical treatments including pure thermal cycling and load-biased thermal cycling to investigate microstructural evolution of the material under actuating conditions. In situ and post mortem scanning transmission electron microscopy (STEM) was used to study the effects of stress on the development of defect substructures during cycling through the martensitic transformation. High temperature observations of the austenite phase show rapid accumulation of dislocations and moderate deformation twinning upon thermomechanical cycling. Additionally, TEM-based orientation mapping suggests the emergence of fine crystallites from the original coarse austenite grain structure. A possible mechanism is proposed for the observed grain refinement based on the crystallographic theory of martensite transformation.

Austenite Grain Refinement by Reverse α′ → γ Transformation in Metastable Austenitic Manganese Steel

Microstructure of metastable austenitic manganese steel after reverse transformation treatment was investigated using optical microscopy, X-ray diffraction (XRD), electrical resistivity and hardness testing. Austenite grain refinement was successfully achieved by a two-step heat treatment. First, martensite was produced by cooling the solution-treated samples to −196 °C. Then, the deep cryogenic treated samples were heated to 850 °C upon slow or rapid heating. The mean size of original austenite grain was about 400 μm. But the mean size of equiaxed reversion austenite was refined to 50 μm. Microstructure evolution and electrical resistivity change showed that martensite plates underwent tempering action upon slow heating, and the residual austenite was decomposed, resulting in the formation of pearlite nodules at the austenite grains boundaries. The refinement mechanism upon slow heating is the diffusion-controlled nucleation and growth of austenite. However, the reverse transformation upon rapid heating was predominated by displacive manner. The residual austenite was not decomposed. The plate a-phase was carbon-supersaturated until the starting of reverse transformation. The reverse transformation was accompanied by surface effect, resulting in the formation of plate austenite with high density dislocations. The refinement mechanism upon rapid heating is the recrystallization of displacive reversed austenite.

Mossbauer and XRD characterization of the phase transformations in a Fe-Mn-Al-C-Mo-Si-Cu as cast alloy during tribology test

In present study Fe-29.0Mn-6Al–0.9C-1.8Mo-1.6Si-0.4Cu (%w) alloy was obtained after melted in an induction furnace, and then molded as an ingot. From the as cast ingot it were cut samples for the different characterization measurements. The microstructure of the as-cast sample is of dendritic type and its XRD pattern was refined with the lines of the austenite, with a big volumetric fraction, and the lines of the martensite, with small volumetric fraction. The Mossbauer spectrum of the sample was fitted with a broad singlet which corresponds to disordered austenite. After the tribology test, its XRD pattern was refined with the lines of two austenite phases, one similar to the previous one and other with bigger lattice parameter. The total volumetric fraction of the austenite is smaller than that obtained for sample without wear. It was added the lines of the martensite phase with bigger volumetric fraction than that of the previous sample. The Mossbauer spectrum of the weared sample was fitted with two paramagnetic sites which correspond to the two Fe austenite phases and a hyperfine magnetic field distribution which is associated to the disordered original martensite and the new one which appears in the surface as a consequence of the wear process. These results show that during wear process the original austenite phase is transformed in martensite and in a new austenite phase. The increases of the martensitic phase improves mechanical properties and wear behavior.

Phase Transformation Behavior and Microstructural Control of High-Cr Martensitic/Ferritic Heat-resistant Steels for Power and Nuclear Plants: A Review

The martensitic/ferritic steels have been used as boiler and turbine materials in power plants, and also been selected as potential materials for structural materials in nuclear reactors. In this paper, the kinetic analysis of the martensite formation and microstructural control of high-Cr martensitic/ferritic steels are reviewed. A modular approach, incorporating Fisher partitioning nucleation and anisotropic growth for impingement, was proposed to describe the martensite formation kinetics under different cooling rates. The kinetic analysis suggested a thermal-activated growth feature occurring during the martensitic transformation of martensitic steels. The microstructure can be tuned by composition optimization and various combinations of heat treatment parameters (temperature, time, severe and minor deformation). For the application in power plant, the potential of boundary-design, refinement of original austenite grain size and the final martensitic lath, pinning effect of stable carbides, in improving the performances of martensitic/ferritic steels at elevated temperatures should be investigated more thoroughly. Furthermore, efforts should be made to explore the effects of retained austenite on the improvement of high-temperature creep strength. For the application of nuclear plants, attempts should also be made to produce Fe powders with uniformly distributed oxide particles by chemical reactions.