Effect of high magnetic fields on the martensite transformation

The effect of high magnetic fields up to 132 kOe on the martensite transformation has been investigated in two alloy steels, 52100 bearing steel and a type 410 stainless steel. In both cases the martensite start temperature is raised by the application of a magnetic field, and the increase inM _s is linear with field. The rate of formation of martensite is not affected by the field. Numerical values for the entropy of the austenite-martensite reaction can be obtained from the experimental results, and are in reasonable agreement with previous results and with theoretical calculations. Richard Fields was formerly a student.     

On Various Aspects of Decomposition of Austenite in a High-Silicon Steel During Quenching and Partitioning

Using a Gleeble thermomechanical simulator, a high-silicon steel (Fe-0.2C-1.5Si-2.0Mn-0.6Cr) was laboratory hot-rolled, re-austenitized, quenched into the M _s–M _f range, retaining 15 to 40 pct austenite at the quench stop temperature (T _Q), and annealed for 10 to 1000 seconds at or above T _Q in order to better understand the mechanisms operating during partitioning. Dilatometer measurements, transmission electron microscopy, and calculations showed that besides carbon partitioning, isothermal martensite and bainite form at the partitioning temperature. While isothermal martensite formation starts almost immediately after quenching with the rate of volume expansion dropping all the time, the beginning of bainite formation is marked by a sudden increase in the rate of expansion. The extent of its formation depends on the partitioning temperature following TTT diagram predictions. At the highest partitioning temperatures martensite tempering competes with partitioning. Small fractions of bainite and high-carbon martensite formed on cooling from the partitioning temperature. The average carbon content of the austenite retained at room temperature as determined from XRD measurements was close to the carbon content estimated from the M _s temperature of the martensite formed during the final cooling.     

On the Selection of the Optimal Intercritical Annealing Temperature for Medium Mn TRIP Steel

A model is proposed to predict the room temperature austenite volume fraction as a function of the intercritical annealing temperature for medium Mn transformation-induced plasticity steel. The model takes into account the influence of the austenite composition on the martensite transformation kinetics and the influence of the intercritical annealing temperature dependence of the austenite grain size on the martensite start temperature. A maximum room temperature austenite volume fraction was obtained at a specific intercritical annealing temperature T _M. Ultrafine-grained ferrite and austenite were observed in samples intercritically annealed below the T _M temperature. The microstructure contained a large volume fraction of athermal martensite in samples annealed at an intercritical temperature higher than the T _M temperature.     

Microstructure of stainless steel after heat treatment: Data from atomic-force microscopy

The micro- and nanostructure of 40Kh13 stainless steel is studied by optical, scanning electron, and atomic-force microscopy. The images of the steel’s structure and phase composition in three different states (after annealing, quenching, and high-temperature tempering) are compared. The optical images of the ferrite–pearlite structure with considerable content of (Cr, Fe)₂₃C₆ globular carbides obtained after annealing are compared with the results of scanning electron and atomic-force microscopy. It is found that the qualitative conclusions regarding the microstructure of the steel obtained by atomic-force and scanning electron microscopy not only agree with the results of optical microscopy but also provide greater detail. Data from the scanning electron microscope indicate that large carbides are located at the boundaries of ferrite grains. Some quantity of carbides may be found within the small ferrite grains. The size of the inclusions may be determined. The structure formed after quenching consists of coarse acicular martensite. Images from the atomic-force microscope show the acicular structure with greater clarity; three-dimensional images may be constructed. The undissolved carbides are also globular. The size of the martensite plates may be determined. The structure of the steel after high-temperature tempering (tempering sorbite) is formed as a result of the decomposition of martensite to ferrite–carbide mixture, with the deposition of regular rounded carbides. As confirmed by spectral analysis, the individual and row carbides (Cr, Fe)₂₃C₆ that appear contain chromium, which rapidly forms carbides. This structure is stronger than martensite. Data from uniaxial tensile tests are presented for all the states; the hardness HB is determined.     

Influence of heat treatment on the structure and mechanical properties of chrome steel with unstable austenite

The structure and mechanical properties of 35Kh12G3MVFDR steel are investigated. After normalization or quenching, the steel contains up to 35 vol % austenite and may be assigned to the martensitic–austenitic class. On heat treatment—tempering, isothermal holding, or isothermal quenching—the austenite is converted to martensite within 2 h. The martensite in 35Kh12G3MVFDR steel is more thermally stable: the first signs of its conversion to sorbitic structure are observed after 25-h isothermal quenching at 640°C, and its complete decomposition requires 50 h. The breakdown of martensite is accompanied by decrease in the high-temperature strength and hardness. Aging of the quenched and tempered 35Kh12G3MVFDR steel at 670–720°C lowers the hardness from 61–65 HRA to 55–60 HRA after 1600–3200 h and the yield point at 20°C from 1350 MPa to 750–850 MPa. Likewise, the yield point at 720°C declines from 310 MPa to 160–230 MPa after 600 h and then stops. The state of the martensitic structure of 35Kh12G3MVFDR steel determines its creep resistance at 700°C. For example, the martensite remains in the steel structure after relatively brief isothermal quenching (up to 24 h at 640°C), and consequently the creep limit σ^700°C _0.1%/h is no lower than after simple quenching with subsequent high tempering: 86.2 ± 9.4 MPa and 89.3 ± 8.8 MPa, respectively. At the same time, in response to the decomposition of martensitic structure as a result of prolonged aging (1600 h at 670°C), σ^700°C _0.1%/h declines to 63.9 ± 7.1 MPa. In contrast to martensite, the austenite in 35Kh12G3MVFDR steel is thermally unstable and is converted to martensite after only 1–2 h of heating, depending on the temperature.     

Effect of microstructure on plane-strain fracture toughness of aisi 4340 steel

Commercially available AISI 4340 steel has been studied to determine the effect of transformation structures on plane-strain fracture toughness (K _IC). Martensitic and bainitic steels with wide variation in the prior austenitic grain size, and steels having two different mixed structures of martensite and bainite were investigated. Microstructures were examined by optical and transmission electron microscopy. Fracture morphologies were characterized by scanning electron microscopy. The significant conclusions are as follows: in a martensitic or lower bainitic steel in which well-defined packets were observed, the packet diameter is the primary microstructural factor controllingK _IC. The steel's property is improved with increased packet diameter. If the steel has an upper bainitic structure, the packet is composed of well-defined blocks, and the block size controls theK _IC property. When the steel has a mixed structure of martensite and bainite, the shape and distribution of the second phase bainite have a significant effect on theK _IC property. A lower bainite, which appears in acicular form and partitions prior austenite grains of the parent martensite, dramatically improves theK _IC in association with tempered martensite. If an upper bainite appearing as masses that fill prior austenite grains of the parent martensite is associated with tempered martensite, it significantly lowers the K_IC.     

The effect of quench rate on the properties and morphology of ferrous martensite

The effect of high quench rate on theM _s temperature, percent transformed, martensite morphology and austenite hardness has been studied for several Fe-Ni-C steels. For these steels the quench rate was varied only in the austenite region. TheM _s temperature was found to increase with increased quench rate for both high- and low carbon steels while the percent transformation increased or decreased depending upon the morphology of the steel. No variations in martensite hardness were found in the as-quenched condition, but a difference in tempering rate was found between fast and slow quenched specimens. Austenite hardness decreased slightly with increasing quench rate while the martensite morphology changed from lath to plate. Parallel aligned plate structures were observed which resemble a twinned lath morphology. It was demonstrated that the actual difference between this morphology and a true lath morphology is the self-accommodating nature of the lath structure. The morphology changes were compared to the measured changes in martensite properties in order to identify the mechanism of the morphology shift. It was concluded that for these alloys the morphology was controlled by the austenite shear mode.     

Influence of laser- electromagnetic field composite strengthening on hardness of 45 steel

The influence of rotating magnetic field on hardness of 45 steel was studied. The 45 steel which was under the process of laser transformation hardening was magnetized by self- made magnetic enforcement device. The effect of magnetic frequency, magnetic time on hardness of 45 steel was studied. The microstructure of 45 steel laser transformation hardening layer was analyzed by metallographic microscope, and its microscope mechanism of properties?? improvement was considered. Experiment results show that surface hardness values of 45 steel samples are all improved under proper magnetic processing parameters. A part of retained austenite is transformed into martensite because of decrease of martensite activation energy when rotating magnetic field is applied. Dislocation redistributes and becomes more uniform. Microstructure of the sample is more compact.     

In-Situ High-Energy X-Ray Diffuse-Scattering Study of the Phase Transition of Ni2MnGa Single Crystal under High Magnetic Field

The defects-related microstructural features connected to the premartensitic and martensitic transition of a Ni₂MnGa single crystal under a high magnetic field of 50 KOe applied along the [ 1[`1]0 ] \left[ {1\bar{1}0} \right] crystallographic direction of the Heusler phase were studied by the in-situ high-energy X-ray diffuse-scattering experiments on the high energy synchrotron beam line 11-ID-C of APS and thermomagnetization measurements. Our experiments show that a magnetic field of 50 KOe applied along the [ 1[`1]0 ] \left[ {1\bar{1}0} \right] direction of the parent Heusler phase can promote the premartensitic transition of Ni₂MnGa single crystal, but puts off martensite transition and the reverse transition. The premartensitic transition temperature (T _PM ) increases from 233 to 250 K (−40 to −23 °C). The martensite transition start temperature (M _s ) decreases from 175 to 172 K (−98 to −101 °C), while the reverse transition start temperature (A _s ) increases from 186 to 189 K (−87 to −84 °C). The high magnetic field leads to a rapid rearrangement of martensite variants below the martensite transition finish temperature (M _f ). The real transition process of Ni₂MnGa single crystal under the high magnetic field was in-situ traced.     

Maximum magnetic permeability of magnetically hard steel and the limiting hysteresis loop

The proposed formula relates the maximum magnetic permeability μ _m of magnetically hard steel to its coercive force and residual magnetization. The calculation results may be used in nondestructive quality assessment of heat treatment and in magnetic structural analysis of steel, instead of the measurement of μ _m , which is laborious and less precise.     

Deformation Induced Martensite Formation and its Effect on Transformation Induced Plasticity (TRIP)

The knowledge of the stress‐ and deformation‐induced martensite formation in metastable austenitic steels including the formation temperatures and amounts formed is of considerable importance for the understanding of the transformation induced plasticity. For this purpose a stress‐temperature‐transformation (STT) and a deformation‐temperature‐transformation (DTT) diagram have been developed for the steel X5CrNi 18 10 (1.4301, AISI 304). It is shown that the M_d‐temperature for γ→?, ?→α', γ→?→α’ and γ→α’ martensite formation is defined by two stress‐temperature curves which show a different temperature dependence. They specify the beginning and the end of the deformation‐induced martensite formation in the range of uniform elongation. The intersection point defines the corresponding M_d‐temperature. The stress difference which results from the stresses for the end and the beginning of the martensite formation shows positive values below the M_d‐temperature. It defines the amount of martensite being formed. When the M_d^γ→? temperature is reached and the formation of the first deformation‐induced amount of ?‐martensite appears, an anomalous temperature dependence of the maximum uniform elongation starts. The highest values of the maximum uniform elongation are registered for the tested steel in the immediate vicinity of the M_d^γ→α' or the M_d^γ→?→α' temperature ‐ similar as in other metastable austenitic CrNi steels. At this temperature the highest amount of deformation‐induced ?‐phase exists. The transformation plasticity in the test steel is considerably caused by the deformation‐induced ? and α’ martensite formation. Using the new evaluation method, the increase of plasticity ΔA (TRIP‐effect) and strength ΔR can be quantified.     

Stress- and strain-induced formation of martensite and its effects on strength and ductility of metastable austenitic stainless steels

The effects of deformation-induced formation of martensite have been studied in metastable austenitic stainless steels. The stability of the austenite, being the critical factor in the formation of martensite, was controlled principally by varying the amounts of carbon and manganese. The formation of martensite was also affected by different test and rolling temperatures, rolling time, and various reductions in thickness. The terms “stress-induced” and “strain-induced” formation of martensite are defined. Experimental results show that low austenite stability resulted in stress-induced formation of martensite, high work-hardening rates, high tensile strengths, low “yield strengths,” and low elongation values. When the austenite was stable, plastic deformation was initiated by slip, and the work-hardening rate was too low to prevent early necking. A specific amount of strain-induced martensite led to an “optimum” work-hardening rate, resulting in high strengthand high ductility. For best results processing should be carried out aboveM _d and testing betweenM _d andM _s. Mechanical working aboveM _d had a negligible effect on the yield strength betweenM _d andM _s when the austenite stability was low, but its effect increased as the austenite became, more stable. Serrations appeared in the stress-strain curve when martensite was strain induced.     

The Effects of Composition and Aging on the Martensite and Magnetic Transformations in Ni-Fe-Ga Ferromagnetic Shape Memory Alloys

The martensite and magnetic transformations in Ni-Fe-Ga ferromagnetic shape memory alloys are very sensitive to both alloy chemistry and thermal history. A series of Ni-Fe-Ga alloys near the prototype Heusler composition (X₂YZ) were used to investigate how the martensite and magnetic transitions change with alloy composition and isothermal aging above and below the B2/L2₁ ordering temperature. Calorimetry and magnetometry were employed to measure the martensite transformation temperatures and Curie temperatures. Compositional variations of only a few atomic percent result in martensite start temperatures and Curie temperatures that differ by about 230 and 35 K, respectively. Aging a Ni₅₃Fe₁₉Ga₂₈ alloy for 3600 seconds at various temperatures shifts the martensite start temperature and the Curie temperature by almost 70 K. Transmission electron microscopy investigations were conducted on the aged Ni₅₃Fe₁₉Ga₂₈ alloy. The considerable variations in the martensite and magnetic transformations in these alloys are discussed in terms of microstructural differences resulting from alloy chemistry and aging treatments. This article is based on a presentation made in the symposium “Phase Transformations in Magnetic Materials: Magnetic Shape Memory Alloys which occurred March 14, 2006, during the TMS Spring Meeting in San Antonio, TX, under the auspices of the ASMI/MPMD-Phase Transformations, EMPMD/SMD-Chemistry & Physics of Materials, and EMPMDNanomaterials Committees."     

A comparison of common and new methods to determine martensite start temperature using a dilatometer

This paper analyses the start of the martensitic transformation in 4140 steel from the point of view of six definitions, and discusses in detail the implications based on the better understanding of progression of the transformation. The application of two relatively new techniques (cooling curve analysis-CCA and dilation curve analysis-DCA) is among the methods studied. These new techniques allow for a more rigorous quantification of microstructural constituents at each step of the transformation. Experiments consisted of dilatometric analysis of 12 samples of 4140 steel with prior austenite grain sizes from 16 to 44?µm that were rapidly quenched in the dilatometer to form martensite. The results indicate that DCA and CCA are superior to traditional methods used to determine the martensite start temperature. The practical choice of 10% martensite fraction in CCA and DCA yielded M_s values statistically undistinguishable from ASTM A1033 or the tangent method. The practical choice of 1% martensite fraction in CCA and DCA yielded M_s values comparable to the offset method. The important implication of this finding is that M_s values determined with empirical methods should not be confused with the temperature of first appearance of martensite; instead, they correspond to martensite fractions of the order of 10%.     

Effect of Mn Addition on Microstructural Modification and Cracking Behavior of Ferritic Light-Weight Steels

In the present study, effects of Mn addition on cracking phenomenon occurring during cold rolling of ferritic light-weight steels were clarified in relation to microstructural modification involving κ-carbide, austenite, and martensite. Four steels were fabricated by varying Mn contents of 3 to 12 wt pct, and edge areas of steel sheets containing 6 to 9 wt pct Mn were cracked during the cold rolling. The steels were basically composed of ferrite and austenite in a band shape, but a considerable amount of κ-carbide or martensite existed in the steels containing 3 to 6 wt pct Mn. Microstructural observation of the deformed region of fractured tensile specimens revealed that cracks which were initiated at ferrite/martensite interfacial κ-carbides readily propagated along ferrite/martensite interfaces or into martensite areas in the steel containing 6 wt pct Mn, thereby leading to the center or edge cracking during the cold rolling. In the steel containing 9 wt pct Mn, edge cracks were found in the final stage of cold rolling because of the formation of martensite by the strain-induced austenite to martensite transformation, whereas they were hardly formed in the steel containing 12 wt pct Mn. To prevent or minimize the cracking, it was recommended that the formation of martensite during the cooling from the hot rolling temperature or during the cold rolling should be suppressed, which could be achieved by the enhancement of thermal or mechanical stability of austenite with decreasing austenite grain size or increasing contents of austenite stabilizers.     

节镍无磁不锈钢Cr18Ni6Mn3N的组织及性能

研究了节镍无磁不锈钢Cr18Ni6Mn3N的热轧及固溶后的力学性能和耐蚀性能,分析了其固溶和时效析出后的组织演变规律、冷变形过程中形变诱发马氏体相变及其磁性能.结果表明:该不锈钢的固溶组织为单相奥氏体,其力学性能和耐蚀性能均高于SUS304不锈钢;800℃保温4 h后,在晶界析出粒状氮化物,随着保温时间延长,逐渐沿晶界凸起片层状析出物并向晶内生长,保温20 h后,凸出的片层状析出物直径达20μm.冷轧压下率18.3%时尚未发现形变诱发马氏体组织,随着变形量增大,马氏体含量增多,磁导率上升,但与相同条件下的SUS304不锈钢相比,冷轧板固溶后相对磁导率可降至1.002,因此可用于低成本无磁不锈钢领域.      

The shape memory effect and pseudoelasticity in polycrystalline Cu-Zn alloys

The shape memory effect associated with the reverse transformation of deformed martensite, pseudoelastic behavior involved in stress-induced martensite formation and the reversion of strained martensite after an applied stress is relaxed aboveA _f have been studied. Grain size and specimen geometry effects have been related to the above phenomena. Although recoverable strains as high as 10.85 pct were observed in coarse-grained (“bamboo” type) specimens, the shape memory effect is restricted in fine-grained specimens because of permanent grain boundary deformation and intergranular fracture which occurs at relatively low strains. A fine grain size also acts to suppress pseudoelastic behavior because permanent, localized deformation is generated concurrent with the formation of stress-induced martensite which inhibits reversion of the latter upon release of stress. The apparent plastic deformation of martensite belowM _f can be restored by transforming back to the original parent phase by heating toA _f (shape memory) or alternatively, can be recovered belowM _f by applying a small stress of opposite sign. Martensite deformed belowM _f with the same stress maintained while heating persists aboveA _f, but reverts to the parent phase in a pseudoelastic manner when the stress is relieved. The athermal thermoelastic martensite, which forms in groups composed of four martensite plate variants, undergoes several morphology changes under deformation. One of the variants within a plate group cluster may grow with respect to the others, and eventually form a single crystalline martensitic region. At a later stage pink colored deformation bands form in the same area and join up with increasing stress, resulting in thermally irreversible kinks. The clusters of plate groups may expand like grain growth or contract as a whole during deformation, or act as immobile “subgrains” which lead to permanent deformation at their boundaries. Stress-induced martensite usually forms as one variant of parallel plates which join up with increasing stress to form single crystalline regions. Further stress leads to pink colored deformation bands, similar to those in the deformed athermal martensite. Other similarities and differences between the stress-induced and athermal martensite have been investigated and are discussed.     

Structure and mechanical properties of hot-deformed low-carbon martensitic steel

The structural changes in low-carbon martensitic 15Kh2G2NMFBA steel induced by its hot forging in the temperature range 1150–850°C have been studied. The calculated cracking resistance parameter I_c is in agreement with its experimental value. A relation is found between the lath sizes in the martensite structure and the change in the impact toughness characteristics. A combined regime of hot deformation and hot treatment of the low-carbon martensitic steel is proposed to form submicrometer-sized structural elements and high strength and impact toughness characteristics.     

Magnetic Field-Induced Precipitation Behaviors of Alloy Carbides M2C,M3C,and M6C in a Molybdenum-Containing Steel

The effect of a 12-T high magnetic field on alloy carbide precipitation in an Fe-C-Mo alloy during tempering at an intermediate temperature was investigated. Thin foils and carbon extraction replicas of the treated specimens were examined by transmission electron microscopy (TEM). The results show that the applied high field effectively promoted the precipitation of (Fe,Mo)₆C alloy carbide. The concentration of Fe atom in Fe_6?x Mo _x C carbide is increased whereas that of Mo atom decreased when the high magnetic field was applied. However, the high magnetic field almost had no detectable influence on the atom concentration in (Fe,Mo)₂C and (Fe,Mo)₃C carbides. First principle calculations have been performed to calculate the magnetic moment per iron atom of the carbides to explore the origin of the effect of the magnetic field. The influence of the high magnetic field on the precipitation behaviors of alloy carbides was closely related to the magnetic moment of (Fe,Mo)₂C, (Fe,Mo)₃C, and (Fe,Mo)₆C. The magnetic field promotes the formation of the carbides with high total magnetic moment. The effect of the high magnetic field on the substitutional solute atom (Fe and Mo) concentration change in the three alloy carbides was attributed to their magnetization differences per Fe atom.     

Numerical study of transverse lattice waves connected with martensitic transformation

The behavior of the transverse lattice wave connected with the nucleation process of martensite is studied, taking into account the anharmonicity, by use of one-dimensional model for the crystal. The key factor which determines the behavior of the transverse lattice waves according to the model is its amplitudeA with respect to the critical amplitudeA _c. As far asA < A _c, the presence of the transverse lattice wave does not lead to the initiation of the martensite. Depending on the variation ofA andA _c with temperature, the model suggests two alternative processes for the initiation of the martensitic transformation. 1) In case the variation ofA _c with temperature is larger than the variation ofA with temperature, the transverse lattice wave develops into the stable nuclei of the martensite, as soon asA _c becomes smaller thanA with decrease of temperature. 2) In case the variation ofA with temperature is larger than the variation ofA _c with temperature, the transverse lattice wave represents the heterophase fluctuation whenA « A _c. The stable martensite is formed with the reduction ofA as temperature is reduced. The presently available experimental data suggest the process 1) as the nucleation mechanism of the martensite. The prominent role of the interface energy between the martensite and the austenite in determining both the structure of the martensite and the dispersion relationship of the transverse lattice wave in the austenite is pointed out.