Structural features of martensite α-phases in alloys of uranium with transition metals

The structures and formation kinetics are considered for martensite α phases in diffusionless transformation of the γ and β phases of alloys between uranium and transition metals. The general features of the {ie118-1} martensite phase and of the martensite in a steel have been determined by metallography, x-ray structure analysis, and dilatometry, and the same has been done for the martensite {ie118-2} and {ie118-3} phases and martensite in indium-thallium and gold-cadmium alloys. Kinetic features are given for the two-stage diffusionless {ie118-4} transformation in uranium-molybdenum and uranium-niobium alloys, in which the second stage {ie118-5} has the characteristics of a thermoelastic martensite transformation that is realized not only as the temperature varies but also in response to external stress. Bochvar All-Russia New Materials Research Institute, Russian Federation State Scientific Center. Translated from Atomnaya énergiya, Vol. 86, No. 2, pp. 113–120, February, 1999.     

Investigation on the precipitation behavior of M3C phase in T91 ferritic steels

As traditional 9-12% Cr heat-resistant ferritic steels, T91 steels have been considered as candidate reduced-activation materials for nuclear engineering applying due to its excellent creep resistance and high resistance to void formation during neutron irradiations at elevated temperature. Needle-like M₃C precipitates are produced during the routine normalizing process before tempering. Differential scanning calorimetry and infrared radiation thermometer have been employed to study the precipitation behavior of the secondary M₃C particles upon subsequent cooling process after austenization. Various austenization conditions (holding time, temperature and the subsequent cooling rate) were carried out to clarify effect of normalizing condition on the formation of the M₃C phase. In spite of various austenization conditions applied, it is found that the precipitation of M₃C phase is depends only on the cooling rate applied. Furthermore, the precipitation of M₃C phase occurs before the onset of the martensite transformation, which is contrary to the previous statement that it takes place during the auto-tempering stage after martensitic transformation. The above observation points out that the precipitation of M₃C would produce an effect on the subsequent martensitic transformation behavior, leading to the formation of wide martensite laths with a low dislocation density.     

Variation of martensite phase transformation mechanism in minor-stressed T91 ferritic steel

The effects of compressive stress applied at different temperatures on martensite transformation process of the T91 steel were studied by high-resolution differential dilatometer. The stress applied above 850 °C exhibits no influence on the martensite formation. The stress applied below 850 °C not only facilitates the formation of martensite, but also enhances the onset temperature of the martensite transformation. There are two different transformation mechanisms occurring: when the compressive stress is applied at high temperature, the mechanism of strain-induced martensite transformation takes place, as a result, the microstructure tends to be refined with irregular grain boundary. When the compressive stress is applied at low temperature, the stress-induced martensite transformation occurs, and its morphology is similar to that of thermal-activated martensite. In addition, it is summarized that 200 MPa is the critical stress and 440 °C is critical temperature for the onset of the stress-induced martensite transformation for the investigated T91 ferritic steel.     

Thermoelastic analyses of cladding on beam transient of lead–bismuth cooled accelerator-driven system

The thermoelastic analyses of cladding for lead–bismuth cooled accelerator-driven system (ADS) are conducted for the beam transients. The beam transients are considered to be caused by the abnormal behavior of the accelerator and are peculiar to ADS. The program of the thermoelastic analyses is developed for the evaluation of the stresses of the cladding. This program is intended to analyze a fuel pin of a cylindrical model, and solves the thermoelastic problem by the use of the finite-element-method. The beam transients are analyzed by employing the ADS dynamic calculation code and the program of the thermoelastic analyses for the ADS designed by Japan Atomic Energy Agency. As a result, the transformation of the beam shape does not cause the cladding failure. However, the ductile failure is caused by the beam incident position change in several seconds. These results are also compared with those of the creep analyses conducted in the previous study, and both the creep and the ductile failure are revealed to be caused by the beam incident position change. Consequently, the beam incident position change is concluded to have a high risk of cladding failure.     

Sensitivity of the magnetization curves of different austenitic stainless tube and pipe steels to mechanical fatigue

In meta-stable austenitic stainless steels, fatigue is accompanied by a partial strain-induced transformation of paramagnetic austenite to ferromagnetic martensite [G.B. Olsen, M. Cohen, Kinetics of strain induced martensite nucleation, Metall. Trans. 6 (1975) 791-795]. The associated changes of magnetic properties as the eddy current impedance, magnetic permeability or the remanence field may serve as an indication for the degree of fatigue and therefore the remaining lifetime of a component, even though the exact causal relationship between martensite formation and fatigue is not fully understood. However, measuring these properties by magnetic methods may be limited by the low affinity for strain-induced martensite formation. Thus other methods have to be found which are able to detect very small changes of ferromagnetic contents. With this aim the influence of cyclic strain loading on the magnetization curves of the austenitic stainless tube and pipe steels TP 321, 347, 304L and 316L is analysed in the present paper. The measured characteristic magnetic properties, which are the saturation magnetization, residual magnetization, coercive field and the field dependent permeability (AC-magnetization), are sensitive to fatigue and the corresponding material changes (martensitic transformation). In particular, the AC-magnetization was found to be very sensitive to small changes of the amount of strain induced martensite and therefore also to the degree of fatigue. Hence we conclude that applying magnetic minor loops are promising for the non-destructive evaluation of fatigue in austenitic stainless steel, even if a very small amount of strain induced martensite is formed.     

The continuous cooling transformation behaviour of zirconium-niobium-oxygen alloys

The continuous cooling transformation behaviour of zirconium alloys containing up to 5 at% niobium and 3 at% (5300 ppm) oxygen has been examined using thermal analysis and metallographic techniques. Two types of reactions were found; nucleation and growth reactions which showed the familiar C-curve kinetics, and the athermal martensite reaction. The nucleation and growth reactions were identified as the formation of α-zirconium at the β-grain boundaries and the transformation βzr → αZr+βNb throughout the grains. Increasing niobium content lowered the temperature and increased the time required to obtain the thermal arrests. Increasing oxygen raised the temperature of both reactions and caused grain boundary nucleation to occur sooner and nucleation within the grains to occur later. The martensite start temperature was not affected by oxygen but decreased linearly with niobium content.     

Development of Oxide Dispersion Strengthened Steels for FBR Core Application, (II)

Previously manufactured oxide dispersion strengthened (ODS) ferritic steel cladding tubes had inferior internal creep rupture strength in the circumferential hoop direction. This unexpected feature of ODS cladding tubes was substantially ascribed to the needle-like grain structure aligned with the forming direction. In this study, the grain morphology was controlled by using the martensite transformation in ODS martensitic steels to produce an equi-axial grain structure. A major improvement in the strength anisotropy was successfully achieved. The most effective yttria addition was about 1 mass% in improving the strength of the ODS martensitic steels. A simple addition of titanium was particularly effective in increasing the strength level of the ODS martensitic steels to that of ODS ferritic steels.     

A508—3钢回火时显微组织的变化

对 A508-3钢中粒状贝氏体回火转变的研究表明,粒状贝氏体组织具有较高的回火稳定性。回火时板条状贝氏体铁素体基体产生回复及再结晶,并有 Mo_2C 析出;岛状组织中马氏体和奥氏体分解而析出渗碳体。还讨论了显微组织变化对机械性能的影响。     

Mossbauer study of martensitic transformation andcollective magnetic excitations in Fe_9Ni．1 fine particles

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Depth distribution of martensite in xenon-implanted stainless steels

The amount of stress-induced martensite and its distribution in depth in xenon-implanted austenitic stainless-steel poly- and single crystals have been measured by Rutherford backscattering and channeling analysis, depth-selective conversion-electron Mössbauer spectroscopy, cross-sectional transmission electron microscopy and X-ray diffraction analysis. In low-nickel 17/7, 304 and 316 commercial stainless steels and in 17:13 single crystals the martensitic transformation starts at the surface and develops towards greater depth with increasing xenon fluence. The implanted layer is nearly completely transformed, and the interface between martensite and austenite is rather sharp and well defined. In high-nickel 310 commercial stainless steel and 19:15 and 19:20 single crystals, on the other hand, only insignificant amounts of martensite are observed.