Complete Martensitic Transformation Research Articles

Effect of Mo and Cr on the Microstructure and Properties of Low-Alloy Wear-Resistant Steels.

Low-alloy wear-resistant steel often requires the addition of trace alloy elements to enhance its performance while also considering the cost-effectiveness of production. In order to comparatively analyze the strengthening mechanisms of Mo and Cr elements and further explore economically feasible production processes, we designed two types of low-alloy wear-resistant steels, based on C-Mn series wear-resistant steels, with individually added Mo and Cr elements, comparing and investigating the roles of the alloying elements Mo and Cr in low-alloy wear-resistant steels. Utilizing JMatPro software to calculate Continuous Cooling Transformation (CCT) curves, conducting thermal simulation quenching experiments using a Gleeble-3800 thermal simulator, and employing equipment such as a metallographic microscope, transmission electron microscope, and tensile testing machine, this study comparatively investigated the influence of Mo and Cr on the microstructural transformation and mechanical properties of low-alloy wear-resistant steels under different cooling rates. The results indicate that the addition of the Mo element in low-alloy wear-resistant steel can effectively suppress the transformation of ferrite and pearlite, reduce the martensitic transformation temperature, and lower the critical cooling rate for complete martensitic transformation, thereby promoting martensitic transformation. Adding Cr elements can reduce the austenite transformation zone, decrease the rate of austenite formation, and promote the occurrence of low-temperature phase transformation. Additionally, Mo has a better effect on improving the toughness of low-temperature impact, and Cr has a more significant improvement in strength and hardness. The critical cooling rates of C-Mn-Mo steel and C-Mn-Cr steel for complete martensitic transition are 13 °C/s and 24 °C/s, respectively. With the increase in the cooling rate, the martensitic tissues of the two experimental steels gradually refined, and the characteristics of the slats gradually appeared. In comparison, the C-Mn-Mo steel displays a higher dislocation density, accompanied by dislocation entanglement phenomena, and contains a small amount of residual austenite, while granular ε-carbides are clearly precipitated in the C-Mn-Cr steel. The C-Mn-Mo steel achieves its best performance at a cooling rate of 25 °C/s, whereas the C-Mn-Cr steel only needs to increase the cooling rate to 35 °C/s to attain a similar comprehensive performance to the C-Mn-Mo steel.

Исследование усталостных свойств и трибологические испытания стали AISI 316 после кручения под высоким давлением

The effect of high-pressure torsion process in a new die on the fatigue properties of corrosion-resistant steel AISI 316 has been studied. The deformation was carried out at cryogenic and room temperatures. The number of deformation cycles was 8. The initial blank had a ring shape with a diameter of 76 mm, a width of 3.5 mm and a thickness of 3 mm. It is revealed that the fatigue strength of both samples increases due to structure refinement and twinning in austenite during high pressure torsion deformation, partial martensitic transformation and increase of the share of more angular boundaries during cyclic deformation. The main factor of higher fatigue limit of AISI 316 steel after cryogenic deformation in comparison with room temperature is more intensive grinding of microstructure and presence of increased share of large-angle boundaries and more complete martensitic transformation.

Pressure tuning reverse martensitic transformation in the Mn0.9Co0.1NiGe half-Heusler alloy

Here we investigate the structural properties of the Mn0.9Co0.1NiGe half-Heusler alloys under pressure up to 12 GPa by Synchrotron angle-dispersive x-ray diffraction (XRD). At room temperature and pressure, the compound exhibits only the hexagonal NiIn2-type structure. Lowering the temperature to 100 K at ambient pressure induces an almost complete martensitic phase transformation to the orthorhombic TiNiSi-type structure. With increasing pressure, the stable orthorhombic phase gradually undergoes a reverse martensitic transformation. The hexagonal phase reaches 85% of the sample when applying 12 GPa of pressure at T = 100 K. We further evaluated the bulk modulus of both hexagonal and orthorhombic phases and found similar values (123.1 ± 5.9 GPa for hexagonal and 102.8 ± 4.2 GPa for orthorhombic). Also, we show that the lattice contraction induced is anisotropic. Moreover, the high-pressure hexagonal phase shows a volumetric thermal contraction coefficient α v ∼ −8.9(1) × 10−5K−1 when temperature increases from 100 to 160 K, evidencing a significant negative thermal expansion (NTE) effect. Overall, our results demonstrate that the reverse martensitic transition presented on Mn0.9Co0.1NiGe induced either by pressure or temperature is related to the anisotropic contraction of the crystalline arrangement, which should also play a crucial role in driving the magnetic phase transitions in this system.

Additive manufacturing of Ni-Mn-Sn shape memory Heusler alloy – Microstructure and magnetic properties from powder to printed parts

Ni-Mn-based Heusler alloys such as Ni-Mn-Sn show both elastocaloric and magnetocaloric effects during the magneto-structural phase transition, making these materials interesting for solid-state cooling applications. Material processing by additive manufacturing can overcome difficulties related to the machinability of the alloys, caused by their intrinsic brittleness. Since the magnetic properties and transition temperature are highly sensitive to the chemical composition, it is essential to understand and monitor these properties over the entire processing chain. In the present work, the microstructural and magnetic properties from gas-atomized powder to post-processed Ni-Mn-Sn alloy are investigated. Directed energy deposition was used for processing, promoting the evolution of a polycrystalline microstructure being characterized by elongated grains along the building direction. A complete and sharp martensitic transformation can be achieved after applying a subsequent heat treatment at 1173 K for 24 h. The Mn-evaporation of 1.3 at.% and the formation of Mn-oxide during DED-processing lead to an increase of the transition temperature of 21 K and a decrease of magnetization, clearly pointing at the necessity of controlling the composition, oxygen partial pressure and magnetic properties over the entire processing chain.

An integral transformation model for the combined calculation of key martensitic transformation temperatures and martensite fraction

Understanding the process of martensitic transformation and accurately predicting the fraction of retained austenite and martensite after athermal martensitic transformation are of great significance for optimizing the mechanical properties of various advanced high strength steels. This paper proposes an integral transformation model for the description of both the martensitic transformation temperature and its volume fraction considering both chemical driving force and non-chemical free energy. The proposed model was validated in both incomplete and complete martensitic transformation systems with different austenite grain sizes. Compared with the conventional Koistinen and Marburger model, the integral transformation model can predict the Ms and Mf temperatures as well as the transformation fraction in an integrated manner and depict the relationship between martensite fraction and temperature as a more realistic S-shaped curve. This can be attributed to the fact that the integral transformation model is developed from a generic energy viewpoint and takes into account not only the chemical driving force but also the significant non-chemical free energy contribution.

Transient Nucleate Boiling Process Used for Obtaining Super Strong Carbon Steels and Irons

Based on self–regulated thermal process, in the paper four types of thermomechanical treatments are considered. The first is a high temperature thermomechanical treatment (HTTMT) followed by complete martensitic transformation. The second is a low temperature thermomechanical treatment (LTTMT) plus martensitic transformation. The third is the high and low temperature thermomechanical treatment (HTTMT and LTTMT) plus martensitic transformation. And the last includes HTTMT and LTTMT plus bainitic transformation to obtain super strong and ductile materials. It is shown in the paper that listed technologies are enough intensive to obtain very strong and ductile materials using plain high carbon steels. A detailed consideration of all processes in the paper will motivate engineers to perform mentioned technologies in forging shops to receive super strong and ductile materials without costly alloying that saves energy and alloying elements. The paper discusses the opportunity of preventing martensite transformation to receive fine and nano–bainitic microstructure during intensive quenching. A hypothesis is forwarded that explains possible technology used in 8th and 9th centuries in the Middle East to manufacture Damascus steel. The secret of Damascus steel could be the duration of transient nucleate boiling process needed for preventing martensite transformation during forging of steel.

Transient Phase-Driven Cyclic Deformation in Additively Manufactured 15-5 PH Steel.

The present work extends the examination of selective laser melting (SLM)-fabricated 15-5 PH steel with the 8%-transient-austenite-phase towards fully-reversed strain-controlled low-cycle fatigue (LCF) test. The cyclic-deformation response and microstructural evolution were investigated via in-situ neutron-diffraction measurements. The transient-austenite-phase rapidly transformed into the martensite phase in the initial cyclic-hardening stage, followed by an almost complete martensitic transformation in the cyclic-softening and steady stage. The compressive stress was much greater than the tensile stress at the same strain amplitude. The enhanced martensitic transformation associated with lower dislocation densities under compression predominantly governed such a striking tension-compression asymmetry in the SLM-built 15-5 PH.

Thermokinetic EMF during a reverse phase transition in titanium nickelide as a way of information recording

The external factors that influence on the thermokinetic EMF value in the Ti – 50 at.% Ni samples were determined. A method for setting thermokinetic EMF in certain sections of the TiNi wire was developed. The thermokinetic EMF value was measured directly using a digital millivoltmeter MNIPI V7-72. The sections of the Ti – 50 at.% Ni wire samples were subjected to tensile tests on a tensile machine IP 5158-5. On the basis of calorimetric studies, the kinetics of martensitic transformations was investigated. It was found that the direct phase transition affects the thermokinetic EMF value of the Ti – 50 at.% Ni during thermal cycling. Thermal cycling in the temperature range of the complete martensitic transformation causes the thermokinetic EMF value reduction by 0.16 mV by the 15th temperature cycle. The degradation of the thermokinetic EMF value by 0.04 mV took place during thermal cycling in the temperature range of the incomplete martensitic transformation by the 70th thermal cycle. The thermokinetic EMF value was restored to 0.22 mV with increasing temperature to 240 °С, as in the case of annealing at temperatures of 400÷800 °С. The thermokinetic EMF value is associated with a change in physical and mechanical properties of the alloy during thermal cycling. It is characterized by a change in stages of the phase transition and a shift of the characteristic temperatures. On the basis of the obtained experimental data, a method was proposed for a purposeful setting of extended TiNi wire sections with the thermokinetic EMF value from 0 to 0.6 mV, using different methods of influence on its value (thermal cycling, deformation, temperature change in heating zone). The proposed technical solution can be used as a method for information recording.

Влияние обратного термоупругого фазового превращения на напряжение начала структурного превращения в никелиде титана

The experimental study results of the dependence in the stress value of the onset of the structural deformation on the degree of the reverse phase transformation occurring after the complete direct martensitic transformation in titanium nickelide are described. It is established that these results are qualitatively and quantitatively correctly described in the framework of the model according to which the maximum value of the intensity of the intrinsic inelastic deformation of the martensitic part of the representative volume of the shape-memory alloy is used as a parameter of isotropic hardening.

Effects of annealing temperature on microstructure and mechanical behavior of conventionally and severely deformed Fe–24Ni steel by accumulative roll bonding

In this research, the effects of annealing temperature (500, 550, and 600 °C) on the microstructure and mechanical properties of a conventionally (1 cycle) and severely (6 cycles) deformed Fe–24Ni martensitic steel, processed by accumulative roll bonding (ARB), were investigated. Electron backscattered diffraction (EBSD) analysis revealed that annealing at 500 and 550 °C for 30 min was insufficient for completing the reverse martensite transformation, and consequently, the microstructure consisted of austenite and martensite. However, complete reverse martensitic transformation has been accomplished after annealing at 600 °C for 30 min for both deformed samples. Both yield strength (YS) and ultimate tensile strength (UTS) were increased after first and sixth cycle of rolling. Subsequent, annealing led to a decrease in YS and UTS with improvement in uniform elongation ( e u ). In addition, the work hardenability was also improved by annealing the deformed samples.

Exact solutions for isothermal cyclic torsional loading of a circular SMA bar exploiting the shape memory effect

Two simple and fully analytical models are presented for a SMA bar or wire of circular cross section subjected to fully-reversed cyclic torsional loading, by taking into consideration the reorientation of the martensitic variants occurring during unloading and reverse loading. The process is assumed to take place at constant temperature between the start temperatures of the martensitic and austenitic transformations. In the first part of this work, the same shear modulus is taken both for Martensite and Austenite. In the second part, different elastic shear moduli are considered for the two phases. The volume fractions of both positive and negative twisted Martensite are assumed to evolve linearly with the shear stress. The bar is initially in a state of Austenite. As the applied torque is increased the martensitic transformation starts from the outer surface and then it extends inwards. If the maximum applied torque is large enough, then the complete Martensitic transformation takes place in the outer region of the cross section. During unloading and subsequent reverse loading the martensitic reorientation process may occur starting from the boundary between the fully martensitic outer region and the intermediate transforming region. Particular attention is focused on modeling the unloading and reverse loading processes. At each stage, the radial distributions of shear stress and Martensite variant are calculated analytically. A closed form relation between the applied torque and the angle of twist is presented for the entire process in the case of equal shear moduli, and only for the loading and elastic unloading stages in the case of different shear moduli. The approach is then validated against analytical, numerical and experimental results available in the literature for the direct loading–unloading process. Application to the seismic response of dissipative systems based on SMA helical springs is also envisaged.

Analytical modeling of the shape memory effect in SMA beams with rectangular cross section under reversed pure bending

An analytical model is developed for a prismatic SMA beam with rectangular cross section subjected to alternating bending at temperature below the austenitic transformations. The loading path consists in a loading-unloading cycle under bending and then under reversed bending. Two opposite martensitic variants take place, whose volume fractions evolve linearly with the axial stress. Different Young’s moduli are taken for the austenitic and martensitic phases. As the bending moment is increased, the martensitic transformation starts from the top and bottom and then it extends inwards. If the maximum applied bending moment is large enough, then the complete Martensitic transformation takes place at the upper and lower parts of the cross section. During unloading and the following reversed bending, reorientation of the Martensite variant into the opposite one takes place starting from the boundary between the fully martensitic region and the intermediate transforming region. Special attention is devoted to calculate analytically the axial stress and Martensite variant distributions within the cross section at each stage of the process. A closed form moment-curvature relation is provided for loading and elastic unloading and in integral form for the rest of the process. The approach is then validated by comparison with analytical results available in the literature.

Numerical analysis of NiTi actuators with stress risers: The role of bias load and actuation temperature

NiTi wires and thin ribbons are used in shape memory actuators as active elements undergoing repeated thermal cycles. The NiTi actuators operate either under constant force (CF loading constraint) or they act against a variable force (VF loading constraint) from an elastic bias spring. Any stress riser represents a danger for potential actuator failure upon cycling. Although NiTi actuators always include stress risers either inherently in their design or as material flaws, their effect on actuation performance has, however, not been studied thoroughly yet. This issue is addressed in the present paper through numerical analysis of the effects of actuation load type (CF or VF loading constraint), maximum temperature, and stiffness of the bias springs (in VF loading constraint) on the local evolution of martensitic transformation (MT) and mechanical fields around the notch upon the cyclic thermomechanical loading of a thin NiTi shape memory notched ribbon. The analyses clearly show a strong amplification effect of the transformation strain on the stress concentration factor. The results reveal that the evolution of the forward MT upon cooling causes a sharp stress peak around the notch-tip the magnitude of which depends on the bias spring stiffness while upon heating, the stress at the notch-tip area relaxes due to the reverse MT of the surrounding bulk. Moreover, the simulations indicate that any overheating during the actuation is harmful to the notch-tip as the complete reverse MT at the notch-tip has stress rising effect while incomplete MT at the notch-tip has stress relaxing effect.

Effect of Composition, Mechanical Alloying Temperature and Cooling Rate on Martensitic Transformation and Its Reversion in Mechanically Alloyed Stainless Steels

Stainless steels with Fe/Cr/Ni ratios of 74/18/8, 71/17/12, and 55/20/25 were produced from elemental powders by high energy mechanical alloying at both room and cryogenic temperatures. The effect of mechanical alloying temperature on martensitic transformation, the reversion of deformation-induced martensite-to-austenite upon annealing, and the influence of cooling rate on the thermal stability of reversed austenite upon cooling to room temperature were investigated in detail by in-situ and ex-situ X-ray diffraction (XRD) experiments, transmission electron microscopy (TEM) and Thermo-Calc simulations. A relative comparison of stainless steels after room temperature mechanical alloying indicated that the low nickel-containing steel underwent an almost complete martensitic transformation. However, martensitic transformation by deformation through mechanical alloying at room temperature would not be possible with increasing nickel contents but was created partially at cryogenic temperature, the degree of which depended on the steel composition. The in-situ XRD studies exhibited that the deformation-induced martensite completely transformed to austenite at elevated temperatures. The complete reverse transformation temperature simulated by Thermo-Calc software was found to be lower than that of the experimentally determined ones. Additionally, the different cooling rates from the reversed austenite demonstrated that the slower cooling increased the thermal stability of reversed austenite at room temperature.

Martensitic transformation and mechanical behavior of a medium-entropy alloy

Diffusionless martensitic transformation (MT) exerts one of the most significant influences on the mechanical properties of alloys. However, the application of martensitic transformation to improve mechanical performance was seldom involved in the manufacture of high-entropy alloys (HEAs) and medium-entropy (MEAs) alloys. In this work, an innovative non-equiatomic MEA, Fe42Co42Cr16, was proposed with incorporation of martensitic transformation during water quenching and plastic deformation. Water quenching for the alloy in the high-temperature single-phase region produced a partial MT; i.e., transformation of γ–FCC austenite phase into an ε-HCP martensite phase, responsible for the coexistence of γ and ε phases in the dual-phase (DP) alloy. Another triple-phase (TP) alloy, including γ–FCC austenite, ε-HCP martensite and B2-BCC precipitates, was obtained by quenching the alloy in γ+B2 phase region. Owing to the low intrinsic stacking fault energy (γI), both DP (γI=10.9 mJ/m2) and TP (γI=12.2 mJ/m2) alloys involved the complete polymorphic MT process during plastic deformation; i.e., the transformation of the γ–FCC austenite phase into the α-BCT martensite phase with an intermediate ε-HCP martensite phase. Due to the transformation-induced plasticity effect and precipitation strengthening, the produced TP alloy exhibited a yield strength above 1 GPa with a total elongation of as high as 25%.

Effect of Cooling Rate on the Microstructure of a Pressure Vessel Steel

The present article thoroughly explores the effect of cooling rate on various microstructural features of a reactor pressure vessel steel (20MnMoNi55). For this, dilatometric experiments were performed at different cooling rates. Optical microscopy and electron backscatter diffraction were used to measure different microstructural parameters. Continuous cooling from 0.15 to 0.3 °C/s revealed ferrite and bainite; only bainite is seen from 1 to 2 °C/s, both bainite and martensite were found in the regime 5 to 15 °C/s, and complete martensitic transformation was observed at cooling rate 20 °C/s and above. With increase in cooling rate, the fraction of low angle boundary decreased and that of high angle boundary increased. The nature of grain size distribution was found to be cooling rate sensitive.

Increasing efficiency of plasma hardening by local cooling of surface by air with negative temperature

The martensitic transformation interval of some hypoeutectoid, all eutectoid and all hypereutectoid steels covers to a large extent the region of negative temperatures. Due to the fact that the plasma hardening operation is carried out in workshops where the minimum temperature is +20 °С, the surface temperature of the part after plasma heating cannot reach negative values. Because of this, the temperature range of the martensitic transformation is not fully used and in the hardened structure there is a certain amount of austenite, which has not undergone martensitic transformation. This circumstance reduces the hardness of the hardened layer and often low tempering is required to convert residual austenite to tempered martensite, which lengthens and makes the heat treatment more expensive. Complete or almost complete martensitic transformation is possible if the surface heated by the plasma beam is immediately cooled to a negative temperature.It is shown that local cooling of the hardened surface to a temperature of –40 °C can be carried out by air using the Ranque-Hilsch tube, which significantly expands the possibilities of full hardening for eutectoid and hypereutectoid steels. The studies consisted in heating the surface with a plasma stream to a temperature of 750 °C and 900 °C. The temperature was changed by the plasma torch current and by changing the velocity of the plasma flow spot moving along the sample surface. The experiments were carried out on steels 45 (0.45 % C), U8 (0.8 % C) and U10 (1 % C). The study of the structures was carried out on a MIM-7 microscope with a video camera and with the image displayed on the screen. The approximate quantitative composition of austenite, martensite, and associated structures was determined by the areas on the screen.During plasma hardening of steel 45 from a temperature of 900 °C using the Ranque-Hilsch tube, there is practically no residual austenite in the structure. When hardening U8 steel, residual austenite is detected in a small amount. When hardening U10 steel, the amount of residual austenite is approximately 15 %. Local surface cooling allows high-quality hardening of steels of most grades, regardless of the carbon content.

LASER SURFACE HARDENING OF AISI 1055 STEEL IN WATER SUBMERGED CONDITION

Underwater laser hardening might produce better surface mechanical properties than conventional laser hardening in air due to additional cooling effect by water. However, it has not been studied in detail. This study investigates the effect of water layer on laser surface hardening of AISI 1055 steel. It is found that laser surface hardening is feasible with water layer up to 3[Formula: see text]mm above the steel surface. A higher surface hardness is achieved during underwater processing. This is attributed to fast cooling by water which facilitates complete martensitic transformation. Nevertheless, the hardened area is smaller than that in conventional laser hardening in air due to attenuation of laser energy. Above 3[Formula: see text]mm, the laser beam is severely attenuated due to formation of vapor plume. Furthermore, it is found that surface oxidation cannot be prevented completely even during underwater treatment, and the water movement results in random distribution of metal slag on the surface.

Application of self-piercing nuts during hot forming of 22MNB5

The increasing use of hot-formed steels for structural components in lightweight construction requires solutions to create mounting points into the thin blank of high-strength steel. Compared to welding nuts, self-piercing nuts are often used due to advantages for the mechanical properties. The problems of setting these elements in hot-formed steels like 22MnB5 are high process forces and often limited undercuts, which are produced during the joining process. In this regard, the application of the self-piercing nut during the hot forming process of 22MnB5 is the focus of the investigation. The particular challenge is to find out the desired process parameter in a defined temperature window. Thus, the ductile austenitized 22MnB5 is exploited, while the local shape of the deformed blank in contact with the self-piercing nut is realized. A newly developed process enables insertion of the self-piercing nuts by different joining conditions. In order to evaluate the efficiency of the new process, various aspects are recorded. To achieve a successful hot forming process by a complete martensitic microstructure transformation, a minimum cooling rate of 27 K/s is provided. Furthermore, it has to be assured, that there is no thermal influence on the nut element, while the blank and the self-piercing nut are strongly heated. Otherwise, this can lead to a change in the strength class of the nut. For this purpose, hardness measurement is used to analyze the microstructure development. The mechanical behavior is described by torsion- and pull-out tests.

Complete martensitic transformation sequence and magnetic properties of non-stoichiometric Ni2Mn1.2Ga0.8 alloy by first-principles calculations

Abstract The complete martensitic transformation sequence and magnetic properties of the non-stoichiometric Ni2Mn1.2Ga0.8 alloy have been revealed by the first-principles calculations. The calculated martensitic transformation sequence of the Ni2Mn1.2Ga0.8 alloy during cooling is Paramagnetic Austenite (PA) → Ferromagnetic Austenite (FA) → 7M → NM, which is consistent with the experimental observations. When the cubic FA phase transforms to a 7M or NM martensitic phase, a significant reduction of the down-spin total density of states occurred at the Fermi level. This makes the martensitic phase more stable than the FA phase at low temperature.