Laser surface treatment of tool steels

Laser surface treatment is a promising technique for improving the wear and corrosion resistance of materials. In the case of tool steels, laser surface treatment is preferably carried out in the liquid state to allow for complete dissolution of carbides. This paper concerns the application of laser melting to the surface treatment of AISI 420 and 440C martensitic stainless steels and sintered AISI T15 high-speed steel. Usually, laser-melted tool steels contain martensite, retained austenite and carbides. In steels containing large proportions of ferrite-forming alloying elements, -ferrite may also be observed. When applied to sintered steels, laser treatment leads to the elimination of residual porosity. The proportion of retained austenite in laser-melted steels is much higher than in conventionally treated steels. However, the hardness is high because austenite is strengthened by solid solution, dislocations and small grain size. The high volume fraction of retained austenite usually prohibits the application of tool steels in the laser-treated condition. Austenite may be eliminated by multiple tempering treatments at temperatures in the range 550–650°C. During tempering, carbides precipitate within austenite and martensite, and austenite transforms to martensite on cooling or isothermally to ferrite. Strong secondary hardening is often observed and the temperature of the secondary hardening peak of laser-surface-melted steels is higher than after conventional heat treatment.     

Magnetic glass in shape memory alloy: Ni(45)Co(5)Mn(38)Sn(12)

The first order martensitic transition in the ferromagnetic shape memory alloy Ni(45)Co(5)Mn(38)Sn(12) is also a magnetic transition and has a large field induced effect. While cooling in the presence of a field this first order magnetic martensite transition is kinetically arrested. Depending on the cooling field, a fraction of the arrested ferromagnetic austenite phase persists down to the lowest temperature as a magnetic glassy state, similar to the one observed in various intermetallic alloys and in half doped manganites. A detailed investigation of this first order ferromagnetic austenite (FM-A) to low magnetization martensite (LM-M) state transition as a function of temperature and field has been carried out by magnetization measurements. Extensive cooling and heating in unequal field (CHUF) measurements and a novel field cooled protocol for isothermal MH measurements (FC-MH) are utilized to investigate the glass like arrested states and show a reverse martensite transition. Finally, we determine a field-temperature (HT) phase diagram of Ni(45)Co(5)Mn(38)Sn(12) from various magnetization measurements which brings out the regions where thermodynamic and metastable states coexist in the HT space, clearly depicting this system as a 'magnetic glass'.     

Mössbauer investigation of austenite formation together with Cr depletion in aged turbine blade steels

Mössbauer spectroscopy and hardness measurements were used to study annealing effect on turbine blade steels. Hyperfine field distribution method was applied to follow the changes in the concentration of alloying elements being in the martensite after various heat treatments. Our results imply that upon annealing at a given temperature (400–640°C), formation of austenite takes place (similarly as found earlier [1] in some cases), simultaneously with a significant depletion (up to 4%) of Cr (and other alloying elements) in the martensite.     

Cu对Ni50Mn36In14相变和磁性的影响

文章研究了Cu替代部分Ni对铁磁性形状记忆合金Ni₅₀Mn₃₆In₁₄相变和磁性的影响规律. 研究表明,在Ni_50-xCu_xMn₃₆In₁₄中,随着Cu含量的增加,相变温度逐渐降低. Cu含量低于5%时,奥氏体的磁性强于马氏体的磁性, 母相和马氏体相的饱和磁化强度的差值ΔM随着Cu含量的增加而增大. 当Cu含量x=4.5时, ΔM迅速增加到80 emu/g, 并在该材料中观察到了磁场驱动的马氏体到奥氏体的转变,显示了该材料作为磁驱动磁电阻材料的潜在应用前景.当Cu含量高于5%时,奥氏体保持铁磁状态, 马氏体相由反铁磁状态变为铁磁状态,马氏体的磁性强于奥氏体的磁性, ΔM大大削弱,磁场驱动性质消失.     

Mössbauer studies of medium-carbon,high-chromium martensitic steels

⁵⁷Fe Mössbauer effect spectroscopy is employed to determine the relationship between the microstructure and the mechanical properties of martensitic steels with base composition Fe-10wt%Cr-0,26wt%C. The microstructure consists predominantly of two phases: martensite and austenite. The effect of low concentrations of both Mn and Ni on the structure and the mechanical properties of these steels is studied. The results indicate that Mn and Ni additions are equally effective in increasing the fraction of retained austenite. The austenite is an important phase since it is considered to be beneficial to the toughness of steel. However, we find that the impact toughness first decreases and then increases as a function of the fraction of austenite.     

Orientation relationship in various 9%Cr ferritic/martensitic steels–EBSD comparison between Nishiyama–Wassermann,Kurdjumov–Sachs and Greninger–Troiano

EBSD measurements were carried out on four different martensitic steels (T91, P92, EM10 and Eurofer) in various metallurgical conditions (nine different microstructural states). The usual orientation relationships (ORs) between the parent austenitic phase and the resulting martensite in martensitic steels are those of Nishiyama–Wassermann (NW) and Kurjumov–Sachs (KS). The present study first proposes a methodology based on the combined analysis of the misorientation distribution, the pole figures (PFs) and the angle/axis pairs. This methodology leads to the conclusion that neither NW nor KS relationships are able to account for all the features observed whatever the material under study. A third OR proposed by Greninger and Troiano (GT) proves to describe the relationship between austenite and ferrite in all four different martensitic steels much more accurately.     

Mössbauer study of turbine blade steels

Mössbauer investigations were carried out on low carbon steels containing 12–13.5% Cr and 3–5% Ni in order to get information about the reason of cracking and fracture which take place during the use of turbine blade wheels. The quantity of retained austenite determined from the Mössbauer spectra of steels was low (<1%) in the cracked and fractured basic materials. Comparing this value with those being considerable in quenched (≈11%) as well as in annealed state (≈5%) of the same sample, we can conclude that the transformation of the austenite taking place during the working of turbine blade wheel can be associated with the cracking and the fracture. We found an anomalous increase of the quantity of the austenite in steel samples (quenched from different temperature between 700 and 1000 °C and aged at 450–600 °C) aged again at 450–550 °C. On the basis of the evaluation of Mössbauer spectra of the steels, information can be obtained about the changes in the concentration of alloying elements being in martensite at the various heat treatments.     

Microstructure and properties of high chrome steel roller after laser surface melting

Laser surface melting of high chrome steels was achieved by a 5 kW continuous wave CO₂ laser. The microstructure of the laser surface-melted steels was investigated by scanning electron microscopy, transmission electron microscopy and X-ray diffractometry, and the hardness profiles were determined by a Vickers hardness tester. The corrosion behavior in 3.5% NaCl solution was studied by electrochemical corrosion equipment. The large carbides of high chrome steels are completely dissolved and ultrafine dendrites of austenite with submicroscopic M₂₃C₆ carbides precipitation are formed in the melted zone. The austenite in the melted zone has a high tempering stability. The corrosion resistance of the laser surface-melted steels is significantly improved due to the dissolution of carbides and the increase of the alloying elements in the solid solution as well as the large amount of austenite.     

An efficient model for laser surface hardening of hypo-eutectoid steels

This paper presents a model able to predict the austenization of hypo-eutectoid steels during very fast heat cycle such as laser hardening. Laser surface hardening is a process highly suitable for hypo-eutectoid carbon steels with carbon content below 0.6% or for low alloy steels where the critical cooling rate is reached by means of the thermal inertia of the bulk. As proposed by many authors, the severe heat cycle occurring in laser hardening leads to the pearlite to austenite microstructures transformation happening to a temperature much higher than the eutectoid temperature A_c1 and, afterwards, all the austenite predicted during the heating phase become martensite during quenching. Anyway, all these models usually generate a predicted hardness profile into the material depth with an on-off behavior or very complicated and time consumed software simulators. In this paper, a new austenization model for fast heating processes based on the austenite transformation time parameter I_p→a is proposed. By means of the I_p→a parameter it is possible to predict the typical hardness transition from the treated surface to the base material. At the same time, this new austenization model also reduces the calculation time. I_p→a was determined by experimental tests and it was postulated to be constant for low-medium carbon steels. Several experimental examples are proposed to validate the assumptions and to show the accuracy of the model.     

Anomalous Electrical Conductivity and Magnetization in Fe–Cr–Ni Austenite–Martensite Alloys

Technical Physics - We have measured the temperature dependence of the resistivity and magnetization of a special steel of the Fe–Cr–Ni austenite–martensite class in a wide...     

Crystallographic variant selection of martensite during fatigue deformation

Metastable austenitic stainless steels are prone to form deformation-induced martensite under the influence of externally applied stress. Crystallographic variant selection during martensitic transformation of metastable austenite has been investigated thoroughly with respect to the interaction between the applied uniaxial cyclic stress and the resulting accumulated plastic strain during cyclic plastic deformation. The orientation of all the Kurdjomov–Sachs (K-S) variants has been evaluated extensively and compared with the measured orientation of martensite with their corresponding interaction energies by applying the elegant transformation texture model recently developed by Kundu and Bhadeshia. Encouraging correlation between model prediction and experimental data generation for martensite pole figures at many deformed austenite grains has been observed. It has been found that both the applied uniaxial cyclic stress and the accumulated plastic strain are having strong influence on crystallographic variant selection during cyclic plastic deformation. Patel and Cohen’s classical theory can be utilized to predict the crystallographic variant selection, if it is correctly used along with the phenomenological theory of martensite crystallography.     

Grain refinement and strengthening of austenitic stainless steels during large strain cold rolling

The microstructure/texture evolution and strengthening of 316?L-type and 304?L-type austenitic stainless steels during cold rolling were studied. The cold rolling was accompanied by the deformation twinning and micro-shear banding followed by the strain-induced martensitic transformation, leading to nanocrystalline microstructures consisting of flattened austenite and martensite grains. The fraction of ultrafine grains can be expressed by a modified Johnson-Mehl-Avrami-Kolmogorov equation, while inverse exponential function holds as a first approximation between the mean grain size (austenite or martensite) and the total strain. The deformation austenite was characterised by the texture components of Brass, {011}<211>, Goss, {011}<100>, and S, {123}<634>, whereas the deformation martensite exhibited a strong {223}<110> texture component along with remarkable γ-fibre, <111>∥ND, with a maximum at {111}<211>. The grain refinement during cold rolling led to substantial strengthening, which could be expressed by a summation of the austenite and martensite strengthening contributions.     

Bulk and Surface Quenching of Structural Steel: Morphological Analysis of the Structure

Structures formed in medium-carbon low-alloy steels during bulk quenching from furnace heating and surface quenching initiated by a low-power high-current electron beam are investigated by the methods of diffraction electron microscopy. The influence of the carbon concentration, initial austenite grain size, and cooling rate on the morphology of martensitic crystals and self-tempered carbide particles, long-range stress fields inside a packet and plates, and their dislocation substructure is analyzed. The temperature intervals for the formation of high-temperature plate martensitic crystals and packet (lath) martensite are estimated. It is demonstrated that the structure formed at ultrahigh heating and cooling rates is determined mostly by the morphology of martensite in the initial steel samples.     

Effects of Cu on the martensitic transformation and magnetic properties of Mn_(50)Ni_(40)In_(10) alloy

The structures, the martensitic transformations, and the magnetic properties are studied systematically in Mn50Ni40-xCuxIn10, Mn50-xCuxNi40In10, and Mn50Ni40In10-xCux alloys. The partial substitution of Ni by Cu reduces the martensitic transformation temperature, but has little influence on the Curie temperature of austenite. Comparatively, the martensitic transformation temperature increases and the Curie temperature of austenite decreases with the partial replacement of Mn or In by Cu. The magnetization difference between the austenite phase and the martensite phase reaches 70 emu/g in Mn50Ni39Cu1In10; a field-induced martensite-to-austenite transition is observed in this alloy.     

Determination of the normal and anomalous hall effect coefficients in ferromagnetic Ni50Mn35In15 − x Si x Heusler alloys at the martensitic transformation

The magnetization, the electrical resistivity, the magnetoresistance, and the Hall resistivity of Ni₅₀Mn₃₅In_{15 ? x} Si _x (x = 1.0, 3.0, 4.0) Heusler alloys are studied at T = 80-320 K. The martensitic transformation in these alloys occurs at T = 220?C280 K from the high-temperature ferromagnetic austenite phase into the low-temperature martensite phase having a substantially lower magnetization. A method is proposed to determine the normal and anomalous Hall effect coefficients in the presence of magnetoresistance and a possible magnetization dependence of these coefficients. The resistivity of the alloys increases jumpwise during the martensitic transformation, reaches 150?C200 ??? cm, and is almost temperature-independent. The normal Hall effect coefficient is negative, is higher than that of nickel by an order of magnitude at T = 80 K, decreases monotonically with increasing temperature, approaches zero in austenite, and does not undergo sharp changes in the vicinity of the martensitic transformation. At x = 3, a normal Hall effect nonlinear in magnetization is detected in the immediate vicinity of the martensitic transformation. The temperature dependences of the anomalous Hall effect coefficient in both martensite and austenite and, especially, in the vicinity of the martensitic transformation cannot be described in terms of the skew scattering, the side jump, and the Karplus-Lutinger mechanisms from the anomalous Hall effect theory. The possible causes of this behavior of the magnetotransport properties in Heusler alloys are discussed.     

Heusler合金Mn50–xCrxNi42Sn8的相变、磁性与交换偏置效应

研究了Mn50–xCrxNi42Sn8 (x=0, 0.4, 0.6, 0.8)多晶样品的相变、磁性和交换偏置效应.结果表明,该系列合金在室温下都具有非调制的四方马氏体结构.马氏体逆相变温度随Cr含量增加而逐渐降低. 20 k Oe磁场下的M-T曲线表明,该系列合金的磁性比较弱.两相之间的磁性差最大为△M=7.61 emu/g.磁性的变化主要与Mn-Mn间距的变化以及Ni(A位)-Mn(D位)间杂化作用的强弱有关.在低温下,马氏体相的磁性随着Cr含量增加而增强.在500 Oe的外加磁场作用下,从室温冷却到5 K,在Mn50Ni42Sn8合金中观察到高达2624 Oe的交换偏置场.随着Cr含量的增加,交换偏置场逐渐减小.当Cr含量x=0.8时,随着冷却场的增加, 5 K时的交换偏置场先迅速增加然后逐渐减小.当冷却场为500 Oe时,交换偏置场最大.这主要归因于自旋玻璃态与反铁磁性区域的界面交换耦合作用的变化.     

Magnetic transitions and structure of a NiMnGa ferromagnetic shape memory alloy prepared by melt spinning technique

A ferromagnetic shape memory alloy with nomial composition Ni_52.5Mn_24.5Ga₂₃ (at%) was developed by a melt spinning technique. The as-spun ribbon showed dominant L2₁ austenitic (cubic) structure with a splitting of the primary peak in the X-ray diffractogram indicating the existence of a martensitic feature. The quenched-in martensitic plates were revealed in transmission electron microscopy. An increase of magnetization at low temperature indicated a martensite to austenite transformation and its reverse with a drop in magnetization during the cooling cycle. Higher magnetic fields propel martensite–austenite transformation spontaneously.     

Study of the surface structure of tempered carbon steel

A CEMS and XMS study of carbon steel has shown that the microstructure of the bulk material as well as the surface depends on the temperature at which the steel was tempered. Ion milling of the surface results in no change to the surface microstructure which corresponds closely to the bulk properties. However, after polishing the surface with 3μm diamond, the surface composition is changed to a large extent. Polishing decreases the amount of retained austenite and also the fraction of interstitial carbon within the austenite on the surface of the non-tempered steel. A related increase in the amount of surface martensite is observed. The effects of polishing extends to depths of over 1000 Å. Both X- and θ-carbides are present in each of the tempered steels and only the amount of θ-carbide is seen to increase as the samples are tempered to decrease hardness. Tempering to higher temperatures decreases the martensite content and a corresponding increase is seen in the θ-carbide only. Polishing also removes a large feaction of the carbide from the surface and this may account for the anomalously high wear resistance previously determined for the steel.     

Crystalline damage growth during martensitic phase transformations

A thermomechanical model is developed within a large deformation setting in order to simulate the interactions between martensitic phase transformations and crystalline damage growth at the austenitic grain level. Subgrain information is included in the model via the crystallographic theory of martensitic transformations. The damage and transformation characteristics are dependent of the specific martensitic transformation systems activated during a loading process, which makes the model strongly anisotropic. The state of transformation for the individual transformation systems is represented by the corresponding volume fractions. The state of damage in the austenite and in the martensitic transformation systems is reflected by the corresponding damaged volume fractions. The thermodynamical forces energetically conjugated to the rate of volume fraction and the rate of damaged volume fraction are the driving forces for transformation and crystalline damage, respectively. The expressions for these driving forces follow after constructing the specific form of the Helmholtz energy for a phase-changing, damaging material. The model is used to analyze several three-dimensional boundary value problems that are representative of microstructures appearing in multiphase carbon steels containing retained austenite. The analyses show that the incorporation of damage in the model effectively limits the elastic stresses developing in the martensitic product phase, where the maximum value of the stress strongly depends on the toughness of the martensite. Furthermore, in an aggregate of randomly oriented grains of retained austenite embedded in a ferritic matrix the generation of crystalline damage delays the phase transformation process, and may arrest it if the martensitic product phase is sufficiently brittle. The response characteristics computed with the phase-changing damage model are confirmed by experimental results.     

Micromechanical characterisation of TRIP-assisted multiphase steels by in situ neutron diffraction

The flow behaviour of the constitutive phases in multiphase steels, possibly exhibiting a mechanically-induced phase transformation (TRIP effect), is investigated using neutron diffraction conducted during uniaxial tensile loading. The BCC and FCC lattice strains of several specimens containing different amounts ferrite, bainite, martensite and metastable retained austenite are measured along elastic and plastic deformation. The validity of the measurements, as well as the strengthening resulting from the TRIP effect, are evaluated on the basis of overall mechanical equilibrium.