CHARACTERISTICS OF MARTENSITE IN A HIGH CARBON FERROALLOY

Some characteristics of plate martensite in a 1.03% C ferroalloy have been studied by usingthe transmission electron microscopy.The habit plane of the plate martensite in this ferroalloywas found to be close to{224}_f.The morphology,distribution,coalescence and curving ofmartensite as well as the substructure in both martensite and austenite have been observed.The mechanism of both nucleation and growth of the martensite have been discussed.     

Thermal-expansion anisotropy of orthorhombic martensite in the two-phase (α + β) titanium alloy

Anisotropy of the thermal expansion coefficient (TEC) has been revealed along the axes of the crystal lattice of the α″ titanium martensite in the two-phase (α + β) titanium alloy of grade VT16 (Ti–3Al–5V–4.5Mo, wt %). It has been established by the method of in situ X-ray diffraction analysis that the lattice parameter b of the orthorhombic martensite obtained by quenching from different temperatures decreases upon heating. The TECs along the axes of the crystal lattice of the martensite obtained by quenching from different temperatures have been calculated. It has been shown that the uniaxial extension of bars of the VT16 alloy quenched for the metastable β phase with relative deformations of 0.7, 1, 2, 3, 4, 5, 6, and 8% leads to the formation of the deformation-induced martensite with an axial texture along the b direction of the martensite lattice. In the course of dilatometric studies of the deformed bars, it has been established that there are two temperature intervals (from–100 to +70°C and from 150 to 300°C) with a low TEC. In the first interval, the value of the TEC varies from–2 × 10^–6 to +8 × 10^–6 K^–1 and is determined by the volume fraction of the oriented α″ martensite. This Invar effect is one-dimensional and is manifested along the b axis of the martensite.     

304不锈钢应变诱发α''''马氏体相变及对力学性能的影响

借助于C射线衍射，研究了C、Mn、Cr和Ni含量对304奥氏体不锈钢拉伸力学性能和应变诱发马氏体相变倾向的影响。结果表明：C、Mn、Cr和Ni在允许的成分范围内变化，应变诱发α’马氏体相变倾向差异很大，这导致屈服强度和抗拉强度复杂的变化，尽管应变诱发α’马氏体相变使加工硬化速率提高，相变可以诱发塑性，但相变速率较快，相变倾向较大的钢塑性反而下降，此外，由于室温变形还增大热诱发马氏体相变倾向，从而限制了C、Mn、Cr和Ni下限钢在高精度和低温环境下构件的应用。     

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 借助于X射线衍射，研究了C、Mn、Cr和Ni 含量对304奥氏体不锈钢拉伸力学性能和应变诱发 马氏体相变倾向的影响。结果表明：C、Mn、Cr和Ni在允许的成分范围内变化，应变诱发α′马氏体相变倾向差异很大，这导致屈服强度和抗拉强度复杂的变化，尽管应变诱发α′马氏体相变使加工硬化速率提高，相变可以诱发塑性，但相变速率较快，相变倾向较大的钢塑性反而下降，此外，由于室温变形还增大热诱发马氏体相变倾向，从而限制了C、Mn、Cr和Ni下限钢在高精度和低温环境下构件的应用。     

高速钢中贝氏体的二次硬化特征

研究了6-5-4-2高速钢中贝氏体和马氏体回火后的力学性能和组织结构。结果表明,贝氏体比马氏体具有更高的二次硬化效应以及抗回火稳定性和冲击韧性。透射电镜观察发现,马氏体在二次硬化峰温度范围回火时,有Fe_3C析出,而贝氏体铁素体和奥氏体中只析出弥散的MC和M_2C合金碳化物。这表明在贝氏体长大过程中存在合金元素的扩散。     

Microstructural stability and creep rupture strength of the martensitic steel P92 for advanced power plant

Japanese 9% Cr steel containing 0.5% Mo and 1.8% W (P92) has been investigated. Quantitative microstructural analyses using TEM of thin foils and extraction double replicas have been carried out after different austenitization and tempering treatments and after creep deformation at 600 and 650°C. Statistical quantitative analyses were undertaken to determine the dislocation density within the martensite laths, the width of the martensite laths/subgrains and the size and distribution of the carbide and carbonitride precipitates. Correlation of the results of the microstructural investigation with the creep rupture properties allowed the stability of the microstructure during the high temperature exposure to be assessed. The consequences of microstructural changes for the extrapolation of creep rupture data to design lifetimes are considered. Taking into account the strengthening mechanisms which remain effective in long term testing, the 100000 h stress rupture strength of P92 at 600°C is estimated at 115 MPa.     

Nanoscale austenite reversion through partitioning,segregation and kinetic freezing: Example of a ductile 2 GPa Fe–Cr–C steel

Austenite reversion during tempering of a Fe–13.6 Cr–0.44 C (wt.%) martensite results in an ultra-high-strength ferritic stainless steel with excellent ductility. The austenite reversion mechanism is coupled to the kinetic freezing of carbon during low-temperature partitioning at the interfaces between martensite and retained austenite and to carbon segregation at martensite–martensite grain boundaries. An advantage of austenite reversion is its scalability, i.e. changing tempering time and temperature tailors the desired strength–ductility profiles (e.g. tempering at 400 °C for 1 min produces a 2 GPa ultimate tensile strength (UTS) and 14% elongation while 30 min at 400 °C results in a UTS of ～1.75 GPa with an elongation of 23%). The austenite reversion process, carbide precipitation and carbon segregation have been characterized by X-ray diffraction, electron back-scatter diffraction, transmission electron microscopy and atom probe tomography in order to develop the structure–property relationships that control the material’s strength and ductility.     

SECONDARY HARDENING OF BAINITE IN HIGH SPEED STEEL

The mechanical properties and structure of bainite and martensite after tempering m 6-5-4-2high speed steel have been investigated.It was found that the secondary hardening effect andresistance to heat softening as well as impact toughness are higher in the bainite than in themartensite.TEM observations show that the(Fe,M)_3C precipitated in the martensite aftertempering at temperature near the peak of hardness-tempering temperature curve.However,no(Fe,M)_3C but the dispersive MC and M_2C precipitated in the ferrite and austenite ofbainite after tempering at the same temperature.This indicates that the diffusion of alloy ele-ments forming carbide might exist during the process of bainite growing.     

Effect of Martensite Morphology on Tensile Deformation of Dual-Phase Steel

Three morphologies of martensite in dual-phase microstructure of 0.2% C steel were obtained by different heat treatment cycles. These morphologies consisting of grain boundary growth, scattered laths, and bulk form of martensite have their distinct patterns of distribution in the matrix (ferrite). In tensile testing martensite particles with these distributions behaved differently. A reasonable work hardening was gained initially during plastic deformation of the specimens. The control on ductility was found to depend on the alignment of martensite particles along the tensile axes. The increased surface area contact of martensite particles with ferrite, in grain boundary growth and scattered lath morphologies, facilitated stress transfer from ductile to hard phase. The ductility in the later part of deformation was dependent on the density of microvoids in the necked region. The microvoids are formed mostly by de-cohesion of martensite particles at the interface. The fracture of martensite particles is less prominent in the process of microvoid formation which predicts high strength of martensite.     

Role of the structure and texture in the realization of the recovery strain resource of the nanostructured Ti-50.26 at %Ni alloy

In this work, we have studied the nanostructure, the crystallographic texture, and the crystal lattice of the martensite of the Ti-50.26 at % Ni alloy subjected to a thermomechanical treatment, which includes cold rolling, warm (at 150°C) rolling, intermediate and post-deformation annealings (at 400°C) in different combinations. To calculate the resource of the recovery strain in the approximation of a polycrystal, we suggested and employed a method based on the sufficiently complete allowance for the orientation distribution function of the initial B2 austenite and on the assumption on the realization of the most favorable orientational variant of martensite in each grain. The calculated values of the resource of the recovery strain have been compared with the experimental data and have been analyzed along with the results of the determination of the recovery stresses and parameters of the loading-unloading diagram. Estimations have been made of the role of the structural and textural factors in the realization of the recovery strain of the nanostructured Ti-50.26 at % Ni alloy. To achieve the maximally high recovery strain, one should focus on obtaining a nanocrystalline structure in combination with a sharp texture, which ensures the maximum transformation deformation in the direction of tension.