Microstructural study in graphitised hypereutectoid cast and commercial steels

In the present paper, past research work with new and/or improved processes for willing graphitisation in steels is reviewed. Experiments were carried out to study the carbide dissociation in two different hypereutectoid steels (cast and commercial steels) during graphitisation process by annealing primary martensitic structures at 670°C. Graphite phase evolution during graphitisation treatment was investigated by light optical and scanning electron microscopy and energy dispersive X-ray analyses. It has been reported with promising results that a uniform distribution of alloying elements is found around the graphite particles, which resulted in cast steel. Furthermore, graphite particles in the cast steel were observed to be larger and more spherical than that in commercial steel, which seems to be due to lower Mn/S ratio in cast steel composition.     

Influence of sulphur and vanadium additions on toughness of bainitic steels

Abstract

The Charpy impact toughness of bainitic steels, in the longitudinal direction, was increased by appropriate additions of sulphur and vanadium. The Charpy impact value in the longitudinal direction was highest when the sulphur content was 0.1 wt-%. A suppression of austenite grain growth by manganese sulphides was recognised when the austenitising temperature was higher than 1373 K. Furthermore, it was found that bainite packets were refined by an increase in sulphur content, because manganese sulphides existing in the austenite grains promoted bainite transformation. The main effect of the vanadium addition was probably to enhance the above effect by precipitation of vanadium carbonitrides on the manganese sulphides. Accordingly, an increase in toughness with additions of sulphur and vanadium may be attributed to bainite packet refinement resulting from the suppression of austenite grain growth and the promotion of intragranular bainite transformation by manganese sulphides.     

Effects of aluminum additions on the microstructure and mechanical properties of alumina-forming austenitic stainless steels

The mechanical properties, including tensile and impact properties at different testing temperatures of alumina-forming austenitic steels (25 % nickel, 20 % chromium) with different aluminum contents (0, 2.5 %, 5 % and 8 %) were investigated. Scanning and transmission electron microscopy together with tensile and impact properties tests were conducted. The results showed that the tensile strength of steels at 298 K increased obviously along with aluminum contents increasing, while plasticity decreased at the same, which attributed to the higher volume fraction and number density of spherical NiAl precipitation together with main ferrite in matrix. In addition, spherical NiAl particles dispersed easily in ferrite. In particular, the ultimate tensile strength of the sample with 8 % aluminum could reach 1398 MPa, with the elongation of 14 % at 298 K. However, NiAl precipitations would lose strengthening effects at high temperatures, but the plasticity could be improved. In addition, the sample with 5 % aluminum showed better comprehensive properties by comparison to other samples, and the ultimate tensile strength was 1018 MPa and 491 MPa at 298 K and 973 K with the elongation of 26 % and 43 %, respectively, enabling it to be promising material for industrial application in advanced nuclear systems.     

Wear behaviour and microstructure evolution of pearlitic steels under block-on-ring wear process

Three kinds of pearlitic steels containing different interlamellar spacing and mechanical properties were obtained by different heat treatments. The wear behaviours of the steels and evolution of microstructures under different wear conditions were analysed by block-on-ring wear tests. The results showed that shortening air-cooling time and accelerating cooling rate after rolling dramatically refined the pearlite interlamellar spacing and then enhanced the hardness and strength of the pearlitic steels. The pearlitic steel with a high strength of 1537?MPa was obtained and exhibited the best wear resistance than others. The deformed and fractured cementite in the subsurface layer partially spheroidised induced by deformation and frictional heating. A special layer containing spherical carbide formed and reduced the wear rate of the steel.     

Relationship between fatigue limit and Vickers hardness in steels

In this work, a method to predict the fatigue limit by using Vickers hardness measurements is proposed. Tests carried out in small regions of different annealed, quenched and quenched-tempered alloy steels allowed an improvement of the empirical Murakami-Endo's equation.In this method, the plastic deformation caused by the indentation is assumed to be the defect from which the process of initiation and propagation of cracks originate, analogously to small cracks.Fatigue limits for four kinds of steels in different metallurgical states (annealed, quenched and quenched-tempered) were estimated in two different ways, and the obtained values were compared to the experimental ones. A good correlation between Vickers hardness and the fatigue limits estimated by direct plastic deformation zone measurements using optical microscopy was envisaged.     

Effect of microstructure on non-linear behavior of ultrasound during low cycle fatigue of pearlitic steels

Influence of microstructural changes on the second harmonics of sinusoidal ultrasonic wave during low cycle fatigue (LCF) deformation in pearlitic steel was studied. Fatigue tests were interrupted and at every interruption, non-linear ultrasonic (NLU) parameter (β) was determined. Microstructures of cyclically deformed specimens at various cycles were examined by transmission electron microscopy (TEM). The variation of β with fatigue cycles was correlated with the microstructural changes and the results were explained through the variation in dislocation morphology and carbon content of the steel.     

Influence of small vanadium and cobalt additions on microstructure and properties of ductile iron

Abstract

Ductile irons containing additions of up to 0·44% Vand 0·48%Co have been prepared and tested in the as cast state and after aferritising heat treatment. In the heat treated condition the microstructure of the alloyed irons consists of graphite nodules in aferrite matrix that contains afine dispersion (20–80 nm in dia.) of V₄C₃ particles. These carbide particles produce dispersion strengthening and refine the grain size of the ferrite, resulting in an iron of intermediate strength and high ductility.

MST/3079     

Influence of vanadium concentration on the microstructure and magnetic properties of V-doped ZnO thin films

Vanadium doped ZnO thin films (Zn_1 − xV_xO, where x = 0.05 or x = 0.13) were grown on c-cut sapphire substrates using pulsed laser deposition technique. Their structure and magnetic properties were examined in relation to the doping concentration. All deposited films were highly oriented along the c-axis and exhibited ferromagnetic behavior with a Curie temperature up to 300 K. The crystal structure was found to be better for layers with lower vanadium concentration. The films had a porous fine-grained microstructure and a column-like character as the V concentration was reduced. A weak dependence of magnetization on temperature was observed. The saturation magnetization was found to be strongly dependent on the crystal structure, grain size and V-ion concentration.     

Effect of vanadium micro-alloying on the microstructure evolution and mechanical properties of 718H pre-hardened mold steel

The effects of different contents of vanadium(V)(0.1,0.2,and 0.3 wt%) on the microstructure evolution and mechanical properties of 718H steel were investigated.The precipitate was characterized by means of atom probe tomography(APT) and bright-field transmission electron microscopy(TEM).The increase in V content has great benefits for strength,but has an adverse effect on impact toughness.The strength increase can be attributed to the influence of V addition on dislocation density,misorientation gradient,and fine scale MC precipitates.Precipitation strengthening mainly contributes to the V-added steel by analyzing various strengthening mechanisms.However,fine scale MC precipitates can pin dislocation leading to a decrease in its mobility.A large number of immovable dislocations will increase the dislocation accumulation,internal stress and brittle cracking,resulting in a gradual decrease in impact toughness with the V addition.In addition,compared with V-free steel,the dissolved V content in austenite decreases the grain boundary energy and inhibits the diffusion of the C atoms,ultimately reducing the transformation range of pearlite(P).     

Measurement of pearlite interlamellar spacing in hypereutectoid steels

In hypereutectoid steels, determination of interlamellar spacing is essential to microstructural characterization, since both static and dynamic properties have been related to the pearlitic structure. Generally, quantitative analysis of the size of a pearlite colony and its interlamellar spacing is performed using optical and electron microscopy techniques. In this work, laser scanning confocal and field emission scanning electron microscopes were used to examine interlamellar spacing in isothermally transformed hypereutectoid steels to benefit from increased resolving power and statistical significance capabilities as compared to light optical and transmission electron microscopes, respectively.     

Effect of heat treatment on microstructure,hardness and rollability of V55Ti30Ni15 alloy membranes

The effect of heat treatment on the microstructure, hardness and rollability of V₅₅Ti₃₀Ni₁₅ alloy membranes has been investigated in this study. The microstructure resulting from different heat treatment conditions has a great influence on hardness. Fine NiTi particles precipitate from the supersaturated V-matrix solid solution at temperatures above 600 °C, increase in quantity until 800 °C, then dissolve back into the V-matrix with a further increase in temperature up to 950 °C. The resultant hardness decreases with temperature until 800 °C, and then increases from 800 to 950 °C. In the present study, a comparison has been made between the rollability of the as-cast and the heat treated state selected for deformation at different rolling temperatures. The percent reduction in thickness of the heat-treated alloy (800 °C/18 h) has been found to be up to 30% higher than that of the as-cast alloy, even at room temperature (cold rolling).     

Effects of precipitates on strength and toughness of vanadium structural steels

Abstract

The effects of precipitates formed by the addition of vanadium, nitrogen, and aluminium on the strength and toughness in 0·15%C steels were studied by cooling suitable steels at varying rates from temperatures giving a constant austenite grain size. Yield strength increments resulting from precipitation were derived from the experimental results. Similar increments were produced by vanadium carbide (VC) and vanadium carbonitride (V(C,N)) and in each case a reduction in cooling rate resulted in reduced strengthening because of increased particle coarsening. In the presence of aluminium rather less strengthening was obtained from V(C,N) precipitates. No strengthening resulted from the presence of aluminium nitride (AIN) particles. When the carbon content of the steel is greatly in excess of the stoichiometric quantity, the carbonitride formed is relatively low in nitrogen and the yield strength increments caused by low nitrogen V(C,N) and VC are similar. Impact transition temperature increments resulting from precipitation were also derived from the experimental data. These showed that V(C,N) precipitates are less detrimental to transition temperatures than are VC precipitates giving comparable yield strength increments. Shelf energies were also reduced to a greater extent by VC than by V(C,N). The presence of AIN particles had little effect on shelf energy, but had a detrimental effect on transition temperature. The high nitrogen V(C,N) precipitates are less detrimental to both impact transition temperatures and upper shelf energies of the steels than are low nitrogen V(C,N) or VC precipitates. Removal of soluble nitrogen in the form of nitrides has been confirmed as a means of improving the toughness of steels. Aluminium has been found to be more effective than vanadium in the removal of soluble nitrogen.

MST/935     

Effect of deformation temperature on microstructure and mechanical behaviour of warm working vanadium microalloyed steels

Plane strain compression tests of two V microalloyed steels and one plain C–Mn steel have been done to analyse the influence of the deformation temperature, in the warm working range, on the final microstructure and subsequent mechanical behaviour. In the case of V microalloyed steels, the reheating temperature has an effect on the amount of vanadium in solution prior to deformation. This factor influences the austenite evolution during warm deformation and the transformation during cooling. As a consequence, in the microalloyed steels complex multiphase microstructures are obtained that lead to a wide range of strength–toughness combinations. In contrast, in the case of the plain C–Mn steel minor effects are observed in the deformation range from 800 to 870 °C.     

Optimized infrared switching properties in thermochromic vanadium dioxide thin films: role of deposition process and microstructure

This work deals with high efficient optical switching properties at 68 °C of thermochromic vanadium dioxide (VO₂) thin films deposited on amorphous silica substrates. VO₂ thin films were deposited by radio frequency reactive sputtering process. Conditions of deposition were optimized making use of parameters such as film thickness, gas ratio and substrate temperature. Process was optimized adjusting the distance between target and substrate, and dimensions of target and substrates, to obtain a good uniformity and reproducibility of the layers. X-Ray diffraction patterns and scanning electron microscopy convincingly illustrated that VO₂ thin films could grow on amorphous silica substrates with a specific preferential crystal orientation: the [001]_M crystallographic direction of oxygen octahedral chains is parallel to the substrate plane and corresponds with vanadium-vanadium links (insulating state) or with a maximum of electron delocalization (metal state). Optical switching properties in the mid-infrared range are discussed: transmittance, reflectance and emissivity values are strongly modified at the thermochromic transition temperature (Tc=68 °C). A maximum of optical transmittance contrast is observed for a thickness of 120-nm, then interpreted in terms of absorption law. Using a specific software, the n and k optical indices are determined and used to simulate the variation of transmittance vs. film thickness.     

Homogenisation of austenite during non-equilibrium heating in hypereutectoid steels

The homogenisation (Stage III) of austenite during non-equilibrium heating of hypereutectoid steels has been characterised for the first time through dilatometric strain measurements and electron microscopic techniques. It has been found that the homogenisation temperature (A_ch) is significantly higher than the cementite dissolution temperature (A_cm). Though there is a complete conversion of low-temperature constituents to austenite above the A_cm, a significant segregation of carbon in the austenite is observed below the A_ch. Performing heat treatment with an incomplete austenitisation between the A_cm and the A_ch temperature can affect the subsequent on-cooling phase transformations. It is proposed that the A_ch temperature should be used as the full austenitisation temperature in hypereutectoid steels.     

Effect of carbon concentration on tensile behaviour of pearlitic steels

Abstract

A quantitative relationship between flow stress and microstructure is studied for pearlitic steels incorporating 0.39 - 0.77 wt-% carbon. The distribution of true lamellar spacing (S ₀) is determined. It is found that S ₀ depends on carbon concentration and pearlite transformation temperature accompanying a considerable distribution. The 0.2% proof stress is described as a function of the averaged S ₀ but the influence of the accuracy in S ₀ measurement precludes satisfactory prediction of the 0.2% proof stress. High work hardening corresponds to the generation of phase stress caused by misfit plastic strain between ferrite and cementite. The stress partitioning behaviour between ferrite and cementite is verified by in situ neutron diffraction during tensile deformation.     

Ga对HDDR磁粉热变形磁体磁性能和微观结构的影响

利用HDDR工艺制备出Nd32FebalBGax(x=0.0、0.2%、0.4%、0.6%、0.8%(质量分数))磁粉,并且对HDDR磁粉进行热压/热变形处理制备出全密度各向异性磁体。研究了热变形温度和Ga含量对Nd-Fe-B热变形磁体磁性能的影响,观测了不同Ga含量热变形磁体的微观结构,探讨了微量元素Ga的添加对用HDDR磁粉制备的热变形磁体微观结构和磁性能的影响机制。研究发现,Ga的添加能够明显减小热变形磁体的主相晶粒尺寸,改善磁体的微观织构,并可以同时提高热变形磁体的剩磁和矫顽力。当Ga含量为0.6%(质量分数)时,热变形磁体的磁能积达到最大值228.3kJ/m3。     

Modelling of dynamic recrystallisation kinetics in austenitic stainless and hypereutectoid steels

Abstract

Two important parameters for dynamic recrystallisation can be derived from changes in the strain hardening rate: the critical strain for initiation of dynamic recrystallisation and the point of maximum softening. In the present work, these values are determined from stress–strain data obtained by compression testing over the range of 900–1100°C. The resulting strains are used to derive a kinetic model of dynamic recrystallisation for two materials: 304 austenitic stainless steel and a hypereutectoid plain carbon steel. The values of the mechanical parameters used to define the proposed model are confirmed with the aid of metallographic analysis of the recrystallised microstructures.     

Effects of TiC formation on the microstructure and hardness of wear-resistant steel

In this study, as-cast low alloy wear-resistant steels with Ti contents of 0.6 and 3.0?wt-% were prepared at different solidification cooling rates, and the effects of TiC formation on their microstructures, hardness values, and wear resistances were investigated. The obtained phase diagrams and experimental data revealed that the TiC phase was formed via a divorced eutectic reaction in a residual liquid phase during solidification. The larger Ti content promoted the formation of TiC particles, while the solidification cooling rate represented the main influencing factor. Moreover, the produced TiC phase increased the wear resistance of the studied steel and decreased the matrix hardness, suggesting a new method for enhancing its mechanical properties through the formation of a hard ceramic phase.