FE simulation for the forging process of TC6 alloy disc utilising a microstructural model

The interaction between the deformation behavior and microstructural evolution is the main characteristic in the forging process of a titanium alloy and the interaction can be researched using FE simulation. In this paper, a constitutive equation, considering the effects of grain size on the deformation behavior, is established for high temperature deformation of titanium alloys. Also, FE simulation of deformation behavior combined with heat transfer and grain size in the forging process of TC6 alloy disc is carried out. By FE simulation, the deformation distribution and grain size were illustrated for the forging of a TC6 alloy disc at 920 °C deformation temperature and 2.0 mm/s of hammer velocity. The calculated deformation load and grain size are in a good agreement with the experimental results.     

On the variation of grain size and fractal dimension in an austenitic stainless steel

Samples of an AISI type 316L stainless steel were subjected to different treatments to promote changes in their microstructure. The specimens were heated in a box furnace set at four different temperatures for 30 min and cooled in air to room temperature by placing them in water after the cycle was completed. The samples were prepared following standard metallographic procedures, the microstructure was revealed with an electrolytic etchant, and the average grain size in each sample was determined by the mean line intercept technique. Images from the microstructures were digitized and fed into a personal computer for their fractal analysis by box counting. Two different approaches were used to obtain the fractal dimension of the structure, yielding to similar values in both cases. It was found that the fractal dimension of the microstructure increased with the reduction in grain size.     

EFFECT OF GRAIN SIZE ON COLLECTIVE DAMAGE OF SHORT CRACKS AND FATIGUE LIFE ESTIMATION FOR A STAINLESS STEEL

In the short crack regime of the fatigue process, grain boundaries in steels are barriers against crack growth. In this paper, we use: (1) a method involving crack density; and (2) a method of dimensional analysis, to evaluate the effects of grain size and grain-boundary resistance on short crack behaviour and fatigue life. The results show that the fatigue life increases with a decrease in grain size and an enlargement in the obstacle effect of a grain boundary. An experimental investigation is consequently performed and four groups of stainless steel specimens are used with different grain sizes. The experimental measurements show the dependence of fatigue properties on grain size, which are in good agreement with the theoretical results.     

A unified constitutive model for quasi-static flow responses of pure Ta and Ta-W alloys

This study aims to develop a unified constitutive model describing the flow behaviors of pure Ta and Ta-W alloys over a wide range of strain rate and temperature. The model was based on the previously suggested model by Nemat-Nasser and Isaacs [3] and was further extended to capture the hardening effects caused by the changes in solute contents and concentrations, and grain size; while interstitial solutes (C, N, and O) increased the thermal stress, substitutional solute (W) increased both the thermal and athermal stresses. The results showed that the developed model can provide a reasonable prediction on the flow behaviors of pure Ta and Ta-W alloys (Ta-2.47W, Ta-5.2W, and Ta-9.8W) with different grain sizes of 0.3-45 μm over a strain rate range of 1-10⁻⁶ s⁻¹ and a temperature range of 77-900 K without any change in the material constants used.     

A study on the mechanism of serrated grain boundary formation in an austenitic stainless steel

Measurement of the activation energy for the formation of serrated grain boundaries (GB) has been carried out to understand its underlying formation mechanism in an AISI 316 stainless steel. The apparent incubation time necessary to initiate grain boundary serration was obtained at different aging temperatures, and the apparent activation energy for serration was carefully calculated from the Arrhenius relationship between incubation time and aging temperature. The activation energy for GB serrations in this alloy was measured to be approximately 148 ± 20 kJ mole⁻¹, which is consistent with the activation energy for lattice diffusion of carbon in γ-iron (142 kJ mole⁻¹). This result indicates that GB serration could be controlled essentially by the lattice diffusion of carbon to grain boundaries. Based on the through-thickness observation of serrated GBs, a straight boundary began to serrate from the surface at an early stage of the aging treatment, and then the serrated parts propagated throughout the entire grain boundary.     

分形理论在金相学中的应用

简要介绍了分形理论的基本概念,论述了分形理论在珠光体球化、钢中夹杂物、钢的晶粒度和碳化物形态等金相学上的若干应用。     

Constitutive modeling of nitrogen-alloyed austenitic stainless steel at low and high strain rates and temperatures

In this paper, microstructures-based constitutive relations are introduced to simulate the thermo-mechanical response of two nitrogen-alloyed austenitic stainless steels; Nitronic-50 and Uranus-B66, under static and dynamic loadings. The simulation of the flow stress is developed based on a combined approach of two different principal mechanisms; the cutting of dislocation forests and the overcoming of Peierls–Nabarro barriers. The experimental observations for Nitronic-50 and Uranus-B66 conducted by Guo and Nemat-Nasser (2006) and Fréchard et al. (2008), respectively, over a wide range of temperatures and strain rates are also utilized in understanding the underlying deformation mechanisms. Results for the two stainless steels reveal that both the initial yielding and strain hardening are strongly dependent on the coupling effect of temperatures and strain rates. The methodology of obtaining the material parameters and their physical interpretation are presented thoroughly. The present model predicts results that compare very well with the experimental data for both stainless steels at initial temperature range of 77–1000 K and strain rates between 0.001 and 8000 s⁻¹. The effect of the physical quantities at the microstructures on the overall flow stress is also investigated. The evolution of dislocation density along with the initial dislocation density contribution plays a crucial role in determining the thermal stresses. It was observed that the thermal yield stress component is more affected by the presence of initial dislocations and decreases with the increase of the originated (initial) dislocation density.     

ZrO_2对不锈钢焊条工艺性能的影响

在国产不锈钢焊条配方基础上添加新成分ＺｒＯ＿２，通过测定焊条表面温度、熔滴尺寸以及用高速摄影法观察熔滴过渡形态，探讨了ＺｒＯ＿２对焊条工艺性能的影响。试验证明，焊条药皮中加入约４％的Ｚｒｎ＿２将使熔滴尺寸细化，表面温度下降；焊条综合工艺性能明显改善。     

Development and validation of a 3D computational tool to describe concrete behaviour at mesoscale. Application to the alkali-silica reaction

A finite element analysis of the large deformation of three-dimensional polycrystals is presented using pixel-based finite elements as well as finite elements conforming with grain boundaries. The macroscopic response is obtained through volume-averaging laws. A constitutive framework for elasto-viscoplastic response of single crystals is utilized along with a fully-implicit Lagrangian finite element algorithm for modeling microstructure evolution. The effect of grain size is included by considering a physically motivated measure of lattice incompatibility which provides an updated shearing resistance within grains. A domain decomposition approach is adopted for parallel computation to allow efficient large scale simulations. Conforming grids are adopted to simulate flexible and complex shapes of grains. The computed mechanical properties of polycrystals are shown to be consistent with experimental results for different grain sizes.     

Effect of grain size on the hydrogen-assisted cracking in duplex stainless steels

The embrittlement behavior of 2205 duplex stainless steels with two different grain sizes in 26 wt% NaCl (pH 2) under cathodic potential were investigated by slow strain rate testing. The electrochemical permeation technique was used to characterize the permeation rate and effective diffusivity of hydrogen. The results indicated that both the effective diffusivity and the susceptibility of hydrogen embrittlement were lower for the finer grain size specimen. Ultimate tensile strength (UTS) and uniform elongation (UEL) decrease linearly with decreasing logarithm of strain rate. The dependence of UTS and UEL on the logarithm of strain rate was higher for the finer grain specimen. The microstructural examination revealed that internal cracks resulted from hydrogen embrittlement of the ferrite phase under cathodic charging conditions were arrested by austenite in duplex stainless steels.     

Prediction of hot deformation behaviour of Fe-25Mn-3Si-3Al TWIP steel

Hot deformation behaviour of Fe-25Mn-3Si-3Al twinning-induced plasticity (TWIP) steel was investigated by hot compression testing on Gleeble 3500 thermo-mechanical simulator in the temperature range from 800 to 1100 °C and at strain rate range from 0.01 to 5 s⁻¹, and the microstructural evolution was studied by metallographic observations. The results show that the true stress-true strain curves exhibit a single peak stress at certain strain, after which the flow stresses decrease monotonically until the end of deformation, showing a dynamic flow softening. The peak stress level decreases with increasing deformation temperature and decreasing strain rate, which can be predicted by the Zener-Hollomon (Z) parameter in the hyperbolic sine equation with the hot deformation activation energy Q of 405.95 kJ/mol. The peak and critical strains can also be predicted by Z parameter in power-law equations, and the ratio of critical strain to peak strain is about 0.7. The dynamic recrystallization (DRX) is the most important softening mechanism for the experimental steel during hot compression. Furthermore, DRX procedure is strongly affected by Z parameter, and the decreasing of Z value leads to more extensive DRX.     

07Cr12NiMoVNb耐热钢疲劳性能研究

对07Cr12NiMoVNb钢进行了常温和550℃旋转弯曲疲劳试验,结果表明,此钢不同晶粒度时疲劳断裂行为相近,室温疲劳强度与静强度关系符合奥金格经验公式,高温疲劳强度与静强度有关,但直线关系的系数不同于常温的,需重新总结。晶粒度大小影响疲劳强度,但室温表现的不明显,550℃表现为细晶钢疲劳强度高,这正好与蠕变行为相反,粗晶钢表现出较高的蠕变强度。     

氧化法检验钢的奥氏体晶粒度的研究

采用氧化法和晶粒边界腐蚀法对几种不同的碳钢和合金钢的奥氏体晶粒的显示进行了对比试验。试验结果表明,该两种方法对碳钢的显示效果无甚差别,对合金钢而言,氧化法较晶粒边界腐蚀法所显示的奥氏体晶粒要细。对氧化法中存在的问题提出了一些看法。     

Modelling of flow stress of dual phase steel under warm tensile deformation

Abstract

In order to evaluate the flow stress of dual phase high strength steel in warm forming processes, the flow stress behaviours of dual phase steel were investigated at different temperatures and strain rates. A mathematical model is proposed to predict the stress–strain curves of dual phase steel during warm tension by employing the hyperbolic sine function of Zener–Hollomon parameter, which can describe the relationship among temperature, strain rate and flow stress under warm tensile deformation correctly. Material constants in the equation are expressed in polynomial form of strain. Parameters in the polynomials were obtained by least square method. The predicted stress–strain curves are in good agreement with experimental results, which confirms that the proposed model is accurate.     

Stress induced martensite transformation texture in AISI 304 austenitic stainless steel

Abstract

The phenomenological theory of martensitic transformation was applied to a tension induced martensitic transformation in an AISI 304 austenitic stainless steel in order to estimate the transformation texture. Input data were obtained from the published literature. Calculated pole figures were constructed assuming a variant selection process based on Patel and Cohen’s theory, which emphasises that a mechanical component of free energy is the driving force for martensitic transformation at temperatures above martensite start M_s. The results showed a remarkably good match between the calculated and published measured data.     

Effect of Nb on long-term creep life of JIS SUS304HTB and JIS SUS347HTB steels – heat-to-heat variation and life assessment of stainless steels

Heat-to-heat variation in creep life has been investigated for the 9 heats of JIS SUS 304HTB (18Cr–8Ni steel) and also for the 9 heats of JIS SUS 347HTB (18Cr–12Ni–Nb steel) in the NIMS Creep Data Sheets, mainly taking the effect of Nb into account. The heat-to-heat variation in creep life of 304HTB is mainly caused by the variation in precipitation hardening due to fine NbC carbides at short times, while it is mainly caused by the variation in available nitrogen concentration, defined as the concentration of nitrogen free from AlN and TiN, at long times. The heat-to-heat variation in creep life of 347HTB is mainly explained by the variation of boron concentration, 3–27 ppm, but not by the variation of solution temperature, Nb/C atomic ratio and phosphorus concentration. Boron reduces the coarsening rate of fine M₂₃C₆ carbides along grain boundaries, which enhances the grain boundary precipitation hardening.