遗传算法在无钴高强韧钢设计中的应用

提出一种利用残缺实验数据建立无钴高强韧钢力学性能与钢的合金成分及热处理条件关系模型的方法,依据该方法建立了人工神经网络模型,并应用跗算法,对无钴高强韧钢进行了优化设计,实践表明所提出的方法是有效的,为研制新材料开辟了一条新途径。     

Fracture toughness of high speed steels

Abstract

Fracture toughness testpieces made from tool steels M2, T1, A2, and S1 were given various heat treatments in an attempt to establish the requirements for maximum toughness without undue loss of hardness. The three point bend, crack opening displacement method was used to obtain values of plane strain fracture toughness K_Ic. The required fatigue precrack was initiated by sparking the root of the machined notch with an electric marking pen. Steels M2 and T1 gave K_Ic values in the range 20–28 MN m^?3/2 and heat treatment had little effect. Steels S1 and A2 showed, respectively, marked and slight improvements in toughness when the hardness levels were reduced to about 520 HV. Steel S1 gave the highest toughness recorded (48 MN m^?3/2). All toughness values for steel A2 were lower than expected. This was considered to result from the rapid cooling imposed by the nitrogen gas quench on the very small specimens employed in the work.

MST/1159     

无钴高强韧钢力学性能研究

本文利用人工神经网络方法研究了化学成分和热处理条件对G50钢力学性能的影响,通过电镜观察分析了合金元素间交互作用形成的组织和第二相的作用,结果C、Cr、Mo提高钢强度,Ni、Mo、Si降低钢强度,C,Mn降低韧性,Cr,Mo,Ni提高韧性.     

Microstructural features affecting fracture toughness of high strength steels

The fracture toughness of quenched and tempered steels, such as AISI 4340, AISI 4130 and 300M, can be increased by 50–100% by minor changes in heat treating procedures. Certain microstructural features, particularly blocky ferrite, upper bahnte and twinned martensite plates, are deleterious to fracture toughness. Similarly, the presence of undissolved carbides and sulfide inclusions, which act as crack nuclei, can lower fracture toughness by 25–50%. Other microstructural constituents, such as lower bainte, autotempered martensite, and retained austenite can enhance fracture toughness. By controlling the amounts and distributions of the microstructural constituents, the fracture toughness values of AISI 4340, AISI 4130 and 300M can be raised to the fracture toughness level of 18Ni maraging steel at equivalent values of yield strength.     

A statistical micromechanical model of multiple cracking for ultra high toughness cementitious composites

A new statistical micromechanical model of multiple cracking is proposed in which a general expression of the fiber bridging stress laws in the crack plane is established. In this model, the random distribution properties of fibers are considered. And the Weibull function is adopted to represent the flaw size distribution. The relationships of stress versus strain and crack width versus strain are proposed. The formulas of the crack width, crack space, strain capacity and fracture energy density at the end of multiple cracking processes are also deduced. The validity of the proposed model was demonstrated by experimental results.     

On the embrittlement of fracture toughness specimens of two high strength steels

Hydrogen embrittlement of two commercial materials, a 1$\text{1}\text{2}$ Ni-Cr-Mo steel and a 5 Cr-V-Mo steel both quenched and tempered to a nominal ilrength of 1.60 × 10³MN/m² has been studied. Frecture toughness testing was carried out on each steel in three conditions, (a) heat-treated and tested in air, (b) thermally charged with hydrogen and tested in air, and (c) uncharged and tested a an atmosphere of dry hydrogen. The effects of hydrogen pressure and loading rate on embrittlement were examined. The experimental results show that thermal charging had a greater embrittling effect on the 1$\text{1}\text{2}$ Ni-Cr-Mo steel than on the 5 Cr-V-Mo steel although the latter had a higher hydrogen concentration. In a hydrogen environment the 1$\text{1}\text{2}$ Ni-Cr-Mo steel was much more sensitive to embrittlement than the 5 Cr-V-Mo steel.     

Fracture toughness of laminated steels

Lamination occurs spontaneously in the transverse direction in many commercially available steel plates, if the transverse stresses are sufficiently high. Previous investigations have indicated that lamination is often accompanied by an improvement in the fracture toughness of the plate material. In the vicinity of the crack tip, the stress concentration is so large that the bond between adjacent layers will break before crack propagation sets in. If these layers are sufficiently thin, a state of plane stress is approached near the crack tip. In the present study, the influence of layer thickness and bond strength on the fracture toughness is investigated. It is shown that lamination does improve the toughness, if certain conditions in these variables are fulfilled. This offers a possibility to build up structures with high yield stress and high fracture toughness at the same time, since the permissible defect size to prevent unstable crack growth need not be uncomfortably small.     

A new method for determining the fracture toughness of main pipeline steels

One of the fundamental aims of fracture mechanics is to define fracture toughness K_IC of a material. Hence, the ASTM E399 standard was developed. However according to the standard, large‐sized specimens are required to determine the fracture toughness of low alloy carbon steels. ASTM E1921 standard was developed on the fracture toughness of ferritic steels. In this study, a new method was proposed to determine the fracture toughness of ferritic steels. The purpose of the present paper is to compare the results of the method with the experimental results. Two steels that are used in gas and oil main pipelines were investigated in this study.     

A new high temperature life correlation model for austenitic and ferritic stainless steels

Low-cycle fatigue tests on 429EM ferritic stainless steel and 316L austenitic stainless steel were carried out in a wide range of temperatures from room temperature to 750 °C. The Tomkins fatigue life model was applied to correlate the fatigue life with crack propagation rate and this model matched well with the fatigue life of 429EM stainless steel but not for the 316L stainless steel. A new life prediction model was developed to consider the temperature effect on fatigue life. The predictions show good agreement with experimental results for both materials. The predicted lives were within a±2X scatter band at all test temperatures.     

A model for austenitisation of hypoeutectoid steels

In the field of phase transformations in steels, much attention has been paid to the transformation of austenite into diverse product phases but, until recently not much work has been done on the formation of austenite during heating. There are few published models dealing with the transformation of eutectoid or hypoeutectoid steels with a starting microstructure which is a mixture of ferrite and pearlite.The aim of the present work was to use phase transformation theory to develop a model for austenite formation which takes into account the chemical composition and microstructure of the steel studied, and thermal history experienced. Classic nucleation theory and diffusion-controlled growth equations are used to determine the progressive transformation of the different phases into austenite.A phase transformation model with sound physical basis as the one presented in this work can be used to determine the effects of various parameters in the reaction involved, like microstructure (grain size, pearlite spacing), composition, heating rate and others. Another direct application of this model is the generation of CHT (continuous heating transformation) diagrams for specific steels, which are a useful reference in research, as well as in many industrial processes.     

A model for predicting the influence of warm pre-stressing and strain ageing on the cleavage fracture toughness of ferritic steels

A micromechanistic model of warm pre-stressing is extended to predict the combined effects of warm pre-stressing and strain ageing on the cleavage fracture toughness of ferritic steels. The crack tip stress distribution after a cycle of pre-straining and strain ageing is estimated by superposition of the appropriate monotonic loading stress distributions. The Ritchie, Knott and Rice model of cleavage fracture and its associated fracture criterion are employed in conjunction with the crack tip stress distribution to predict the critical stress intensity factor after warm pre-stressing and strain ageing. Illustrative calculations are presented, based upon the published material's properties of a high nitrogen mild steel. Available experimental data for pressure vessel steels bear out the form of the predictions. At low temperatures, and after heavy pre-loads, the benefits of warm pre-stressing dominate strain ageing induced embrittlement and the toughness is apparently enhanced. At higher temperatures, or after small pre-loads, however, strain ageing dominates and the apparent toughness is reduced. Various assumptions and approximations inherent in the model are discussed. These generally tend to render the predictions conservative. Finally it is noted that the model should be equally applicable to the prediction of the combined effect of warm pre-stressing and neutron irradiation on the cleavage fracture toughness of ferritic steels.     

Comparison of mode I and mode II elastic-plastic fracture toughness for two low alloyed high strength steels

In the present work, mode I and mode II tests were carried out on two low alloyed high strength steels. An asymmetrical four point bend specimen and J _II-integral vs. crack growth resistance curve technique were used for determining the mode II elastic-plastic fracture toughness, J _IIc · J _II-integral expression of the specimen was calibrated by finite element method. The results indicate that the present procedure for determining the J _IIc values is easy to use. Moreover, the mode I fracture toughness J _Ic is very sensitive to the rolling direction of the test steels, but the mode II fracture toughness J _IIc is completely insensitive to the rolling direction of the steels, and the J _IIc /J _Ic ratio is not a constant for the two steels, including the same steel with different orientations. Finally, the difference of the fracture toughness between the mode I and mode II is discussed with consideration of the different fracture mechanisms.     

A mechanical model to predict elastic-plastic fracture toughness in high strength materials

A mechanical model of crack initiation and propagation, which is based on the actual mechanism of ductile fracture in high strength materials, is proposed. Assuming that a crack initiates when the equivalent stress at a distance ρ from the crack tip reaches a critical value $\text{}\text{?}$gsf, an equation for predicting fracture toughness J_IC is obtained. From comparison between the predicted values and the experimental results, it is found that the distance ρ corresponds to the spacing of micro-inclusions. The temperature dependence of fracture toughness J_IC estimated according to the derived equation is given in an Arrhenius form of equation and is nearly consistent with the experimental results.     

HAZ fracture toughness of titanium containing steels

Abstract

Steels containing various combinations of microalloying elements (Nb, V, and Ti) were welded at heat inputs from 3 to 6 kJ mm^?1. It was shown by detailed crack tip opening displacement fracture toughness testing of coarse grained heat affected zone (HAZ) regions in single pass weld deposits that the poorest toughness properties were exhibited by steel containing a combination of Nb, V, and Ti. Steel microalloyed with only titanium had the best HAZ fracture toughness at all heat input levels. Detailed microstructural analysis, grain size measurement, hardness, and precipitation in HAZ regions were evaluated to explain the fracture toughness properties observed.

MST/887     

A model for the toughness of epoxy-rubber particulate composites

Epoxy resins are toughened significantly by a dispersion of rubber precipitates. Microscopic examinations of propagating cracks in epoxy-rubber composites reveal that the brittle epoxy matrix cracks, leaving ligaments of rubber attached to the two crack surfaces. The rubber particles are stretched as the crack opens and fail by tearing at large, critical extensions. This fracture mechanism is the basis of a new analytical model for toughening. An increase in toughness (G _IC) of the composite is identified with the amount of elastic energy stored in the rubber during stretching which is dissipated irreversibly (e.g. as heat) when the particles fail. The model predicts the failure strain of the particles in terms of their size. It also relates the toughness increase to the volume fraction and tearing energy of the rubber particles. Direct measurements of the tearing strains of rubber particles, and toughness data obtained from epoxy-rubber composites, are in good agreement with the model. The particle-stretching model provides a quantitative explanation, in contribution to existing qualitative theories, for the toughening of epoxy-rubber composites.     

Systems design of high performance stainless steels I. Conceptual and computational design

Application of a systems approach to the computational materials design led to the development of a high performance stainless steel. The systems approach highlighted the integration of processing/structure/property/ performance relations with mechanistic models to achieve desired quantitative property objectives. The mechanistic models applied to the martensitic transformation behavior included the Olson–Cohen model for heterogeneous nucleation and the Ghosh–Olson solid-solution strengthening model for interfacial mobility. Strengthening theory employed modeling of the coherent M₂C precipitation in a BCC matrix, which is initially in a paraequilibrium with cementite condition. The calibration of the M₂C coherency used available small-angle neutron scattering (SANS) data to determine a composition-dependent strain energy and a composition-independent interfacial energy. Multicomponent pH-potential diagrams provided an effective tool for evaluating oxide stability. Constrained equilibrium calculations correlated oxide stability to Cr enrichment in the metastable spinel film, allowing more efficient use of alloy Cr content. The composition constraints acquired from multicomponent solidification simulations improved castability. Then integration of the models, using multicomponent thermodynamic and diffusion software programs, enabled the design of a carburizable, secondary-hardening martensitic stainless steel for advanced bearing applications. This revised version was published online in June 2006 with corrections to the Cover Date.     

Assessment of the fracture toughness of cast steels

Estimates of the fracture toughness in terms of the critical stress intensity factorsK _C andK _IC are made for a 1Cr steel, a 1/2Cr-1/2Mo-1/4V steel, a 1 1/2Mn-Ni-Cr-Mo steel and a 1 1/2 Ni-Cr-Mo steel all in cast form. The methods used are linear elastic fracture mechanics,J-integral and crack opening displacement methods. The last two methods are applied in combination with an electrical potential method to detect the initiation of fracture.