Application of strain gradient plasticity theory to model Charpy impact energy of functionally graded steels using modified stress-strain curve data

Functionally graded ferritic and austenitic steels were produced through electroslag refining by setting the austenitic and carbon steels with appropriate thickness as electrode. Charpy impact energy of the specimen has been studied and modeled regarding the mechanism-based strain gradient plasticity theory. The hardness of each layer was related to the density of the dislocations of that layer and then by using a linear relation, the predicted hardness was related to its corresponding yield stress. Afterwards; by assuming Holloman relation for the corresponding stress-strain curves, tensile strengths and tensile strains of the constituent layer were determined via numerical method. By using load-displacement curves acquired from instrumented Charpy impact tests on primary specimens, the obtained stress-strain curves from uniaxial tensile tests were modified. Charpy impact energy each layer was related to the corresponding area under modified stress-strain curve of that layer and finally by applying the rule of mixtures, Charpy impact energy of functionally graded steels was determined. The obtained results of the proposed model are in good agreement with the experimental ones.     

Application of strain gradient plasticity theory to model Charpy impact energy of functionally graded steels

Functionally graded ferritic and austenitic steels were produced through electroslag refining by setting the austenitic and carbon steels with appropriate thickness as electrode. Charpy impact energy of the specimen has been studied and modeled regarding the mechanism-based strain gradient plasticity theory. The yield stress of each layer was obtained by the density of the statistically stored dislocations of that layer and by assuming Holloman relation for the corresponding stress–strain curves, tensile strength of the constituent layer were determined via numerical method. Charpy impact energy of each layer was related to the corresponding area under stress–strain curve of that layer and finally by applying the rule of mixtures, Charpy impact energy of functionally graded steels was determined. The obtained results of the proposed model are in good agreement with the experimental ones.     

Simulation Charpy impact energy of functionally graded steels by modified stress-strain curve through mechanism-based strain gradient plasticity theory

In the present work, Charpy impact energy of functionally graded steels produced by electroslag remelting composed of graded ferritic or austenitic layers in both crack divider and crack arrester configurations has been modeled by finite element method. The yield stress of each layer was related to the density of the statistically stored dislocations of that layer and assuming by Holloman relation for the corresponding stress-strain curves, tensile strengths of the constituent layers were determined via numerical method. By using load-displacement curves acquired from instrumented Charpy impact tests on primary specimens, the obtained stress-strain curves from uniaxial tensile tests were modified. The data used for each layer in finite element modeling were predicted modified stress-strain curves obtained from strain gradient plasticity theory. A relatively good agreement between experimental results and those obtained from simulation was observed.     

管线钢冲击吸收功测量的尺寸效应

对20钢、16Mn钢和X70钢不同宽度试样在-80～40℃范围内进行夏比冲击试验,研究了非标准尺寸试样冲击功与标准试样冲击功之间的关系,比较了大小试样韧脆转变温度的不同;把试验结果与ASTM标准、BS标准进行对比,不同尺寸试样的韧脆转变温度漂移程度相差很大。并对上平台冲击功的换算公式做了修正。     

Simulation of impact energy in functionally graded steels by mechanism-based strain gradient plasticity theory

Charpy impact energy of functionally graded steels composed of graded ferritic or austenitic layers which were produced by electroslag remelting in both crack divider and crack arrester configurations has been modeled by finite element method. The yield stress of each layer was related to the density of the dislocations of that layer and by assuming Holloman relation for the corresponding stress-strain curve, tensile strength and tensile strain of that layer were determined. Cubic elements were joined together to build the standard Charpy impact specimen. The data used for each cubic element in finite element modeling was the predicted stress-strain curve obtained from strain gradient plasticity theory. After applying the impact loading, a relatively good agreement between experimental results and those obtained from simulation was observed.     

摆锤冲击试验结果的统计分析

鉴于摆锤冲击试验结果分散性大，用多试样求平均值的方法有时仍不能正确地反映材料真实的抗冲击性能。为了得到更加可靠的结果，借助数理统计方法，对某低合金钢TL和LT两种取向试样的冲击功Ak值进行了分析。结果表明，TL和LT两种取向试样的Ak值基本均呈正态分布，而它们对应可靠度为0．9时的安全冲击功分别为62．7J和72．6J。     

Modeling ductile to brittle transition temperature of functionally graded steels by fuzzy logic

In this article, a model based on fuzzy logic (FL) for predicting ductile to brittle transition temperature of functionally graded steels in both crack divider and crack arrester configurations has been presented. Functionally graded steels containing graded ferritic and austenitic regions together with bainite and martensite intermediate layers were produced by electroslag remelting. For purpose of building the model, training and testing using experimental results from 140 specimens produced from two basic composites were conducted. The used data as inputs in FL models are arranged in a format of six input parameters that cover the FGS type, the crack tip configuration, the thickness of graded ferritic region, the thickness of graded austenitic region, the distance of the notch from bainite or martensite intermediate layer, and temperature. According to these input parameters, in the FL, the ductile to brittle transition temperature of each FGS specimen was predicted. It has been found that FL model will be valid within the ranges of variables. The training and testing results in the FL model have shown a strong potential for predicting the ductile to brittle transition temperature of each FGS specimen.     

借助准标样测算夏比冲击试验测量不确定度的方法

为解决在测定材料冲击吸收功出现用户验收结果与出厂检验结果不一致的问题，提出了用准标样测算冲击试验测量不确定度的方法。这种方法可作为双方在进行冲击值比对分析时都能接受的评定方法。     

Influence of sulphur additions on impact properties of bainitic or martensitic steels

Abstract

Effects of sulphur addition on the Charpy impact properties of various continuously cooled bainitic steels with different prior austenite grain size, hardness, and content of retained austenite were investigated and compared with martensitic steels. The impact properties of 1473 K austenitised bainitic steels were improved with increasing sulphur content up to 0·1 wt-%, while the impact properties of martensitic steels were deteriorated with increasing sulphur content. The crack initiation energy of bainitic steels increased with the increase of sulphur content because the structure units surrounded by the high angle boundaries were refined with the increase of manganese sulphide inclusions which caused the expansion of ductile fracture area. On the other hand, the impact energy, particularly the crack propagation energy, of martensitic steels decreased with increasing sulphur content because the nucleation sites of voids increased with the increase of manganese sulphide inclusions in the ductile fracture region.     

Developing higher strength microalloyed medium carbon steel capable of being fracture split after laser transformation notching

Abstract

High proof strength (>600 MPa) microalloyed medium carbon steels are being developed to replace high carbon steels in the production of internal combustion engine components which are manufactured by processes involving fracture splitting. The target steels are required to achieve a balance between the ability to fracture when initiated by a novel laser transformation notch (LTN) and the properties required by the application. In this study, the effect of phosphorus (P) and sulphur (S) and the depths of LTN on the fracture process of these forged steels were investigated by using an instrumented Charpy impact tester. The impact specimens were notched by a fibre laser with no removal of material. The depth of the notch showed a significant influence on the force–displacement curves, the Charpy impact energy and the associated fracture surfaces. The steel with combined high S and P contents showed the lowest Charpy impact energy of 2·9 J when the depth of LTN was >0·4 mm compared to 5·5 J with the steel containing only a higher S content and 11·2 J for the reference base steel with normal S and P contents. It was observed that MnS inclusions in the originally forged materials were redistributed to the resolidified grain boundaries in the melted region of the LTN. The distribution of P could not be identified in the LTN but presumably it also segregated to the interdendritic regions and the columnar grain boundaries during freezing. The steel containing enhanced contents of S and P was shown to be potentially suitable for fracture splitting and higher load applications.     

Effect of Layers Position on Fracture Toughness of Functionally Graded Steels in Crack Divider Configuration

In the present study, fracture toughness of functionally graded steels in crack divider configuration has been modeled. By utilizing plain carbon and austenitic stainless steels slices with various thicknesses and arrange- ments as electroslag remelting electrodes, functionally graded steels were produced. The fracture toughness of the functionally graded steels in crack divider configuration has been found to depend on the composites’ type together with the volume fraction and the position of the containing phases. According to the area under stress-strain curve of each layer in the functionally graded steels, a mathematical model has been presented for predicting fracture toughness of composites by using the rule of mixtures. The fracture toughness of each layer has been modified according to the position of that layer where for the edge layers, net plane stress condition was supposed and for the central layers, net plane strain condition was presumed. There is a good agreement between experimental results and those acquired from the analytical model.     

仪器化冲击法研究几种金属材料的冲击性能

利用仪器化冲击试验机进行了D36、14MnVTiRe和10CrNiCu船体结构钢的系列冲击试验，结果表明：这几种船体结构钢具有较低的韧脆转变温度。用裂纹扩展功取作为韧脆转变温度的判据更加准确。韧脆转变前，断裂能量主要消耗于裂纹的扩展；韧脆转变后，裂纹形成功Wi明显大于裂纹扩展功Wp，断裂能量主要消耗于裂纹的形成。     

微量Ce元素对0.23C-0.36Si-0.56Mn钢显微组织和力学性能的影响

为研究Ce元素对钢筋力学性能的作用机制,用中频感应熔炼炉制备含不同微量Ce的0.23C-0.36Si-0.56Mn钢样.通过OM、SEM及EDS等分析测试手段,研究微量Ce对钢材微观组织和力学性能的影响.研究结果表明:当钢液中Ce含量在0~0.010%时,钢液洁净度提高;铁素体晶粒尺寸由19.65μm减小到9.65μm;夹杂物面积由0.058%减小到0.028%,10μm左右带尖角的Al2O3和长条状Mn S的夹杂物变为1μm左右球形86.72%Al2O3-13.28%Ce2O3和Ce固溶Mn S复合夹杂物;试样力学性能随微Ce量的增加而明显改善,当Ce含量为0.01%时,钢材的抗拉强度、屈服强度和冲击功比不加Ce时分别提高了16.95%、20.81%和91.94%,断后延伸率(21.85%)超过HRB400国标要求(17%).     

Untersuchung des Strahlverschleißmechanismus von Metallen

Problems of Mechanism of Erosion in Abrasive Stream. The results of investigation obtained by different methods (metallographical and electron microscopy, X-ray analysis, microhardness testing, profile measurement) are presented in the paper. Changes in surface layers prior to chip formation and its separation have been investigated. The specimens, mainly of soft and hardened steels were tested for wear in the stream of abrasive particles with a hardness greater than that of the specimens. The data of investigations show that the so-called polydeformation wear mechanism is of prime importance with soft steels subjected to erosion (mean cycle number n = 4 … 10). In the case oif hardened steels separation of the wear particles is experienced at greater and medium impact angles along the ?white layers”?. The cycle number of particle separation with hardened steels constitutes 1 … 2 only. In the case of quite small impact angles microcutting predominates for all metals.     

国产低屈服点钢材循环加载试验研究

为研究不同荷载作用下国产低屈服点钢的材料力学行为,对LY100、LY160及LY225钢材共46个试件进行了单调拉伸试验及12种不同加载制度的循环加载试验。对国产低屈服点钢的单调性能、滞回性能、破坏形式、延性特征等进行了分析,并与其他结构钢材的力学性能进行了对比。结果表明,低屈服点钢在循环荷载作用下有明显循环强化现象,塑性变形能力强,且与普通钢材相比延性及耗能能力突出。该试验结果为后续研究低屈服点钢本构模型提供基础。     

Hot Forging of Nitrogen Alloyed Duplex Stainless Steels

Duplex stainless steels are gaining global importance because of the need for a high strength corrosion resistant material. Three compositions of this group were selected with three different nitrogen contents ciz, 0.15 wt pct （alloy 1）, 0.23 wt pct （alloy 2） and 0.32 wt pct （alloy 3）. The steels were melted in a high frequency induction furnace and hot forged to various reductions from 16% to 62%. In this work, the effect of hot forging on the ferrite cont.ent, hardness, yield strength, impact strength and grain orientation （texture） were studied. Fracture analysis on all the forged specimens using SEM reveals that a size reduction of 48% results in maximum ductility and impact strength as well as minimal ferrite content and grain size. Thus the mechanical properties are found to have a direct correlation to ferrite content and grain size. The highest impact strength was observed in specimens with the smallest grain size, which was observed in specimens forged to 48% reduction in size.     

Comparison of mechanical properties of martensite/ferrite and bainite/ferrite dual phase 4340 steels

Different microstructures were produced by heat treatment of 4340 steel. These microstructures are bainite, martensite, ferrite–martensite and ferrite–bainite. Mechanical tests were carried out at room temperature. The results showed that steel with bainite–ferrite microstructure has better ductility and charpy impact energy than steels with martensite–ferrite and full bainite microstructures. But yield and tensile strengths of this steel are less than the yield and tensile strengths of the other two steels. Hardness measurements showed that their hardness is the same. Fracture surface observations of tensile specimens showed increase in toughness of bainite–ferrite in comparison to martensite–ferrite and full bainite microstructures.     

Impact behavior of different stainless steel weldments at low temperatures

A comparative study was made of the fracture behavior of austenitic and duplex stainless steel weldments at cryogenic temperatures by impact testing. The investigated materials were two austenitic (304L and 316L) and one duplex (2505) stainless steel weldments. Shielded metal arc welding (SMAW) and tungsten inert gas welding (TIG) were employed as joining techniques. Instrumented impact testing was performed between room and liquid nitrogen (?196 °C) test temperatures. The results showed a slight decrease in the impact energy of the 304L and 316L base metals with decreasing test temperature. However, their corresponding SMAW and TIG weld metals displayed much greater drop in their impact energy values. A remarkable decrease (higher than 95%) was observed for the duplex stainless steel base and weld metals impact energy with apparent ductile to brittle transition behavior. Examination of fracture surface of tested specimens revealed complete ductile fracture morphology for the austenitic base and weld metals characterized by wide and narrow deep and shallow dimples. On the contrary, the duplex stainless steel base and weld metals fracture surface displayed complete brittle fracture morphology with extended large and small stepped cleavage facets. The ductile and brittle fracture behavior of both austenitic and duplex stainless steels was supplemented by the instrumented load–time traces. The distinct variation in the behavior of the two stainless steel categories was discussed in light of the main parameters that control the deformation mechanisms of stainless steels at low temperatures; stacking fault energy, strain induced martensite transformation and delta ferrite phase deformation.     

Materials characterization and mechanical properties of graded tool steels processed by a new co‐spray forming technique

In order to meet the requirements of micro cold forming tools, a new co‐spray forming process has been applied to produce graded materials from two different tool steels in this study. The two steel melts were atomized and co‐sprayed simultaneously onto a flat substrate, resulting in a flat graded deposit when the two sprays were overlapped. To eliminate porosity and break up carbide network, the graded deposits were further hot rolled. The resultant graded tool steels were investigated with respect to porosity, element distribution, microstructure, hardness, strength, and toughness. The degree of overlapping of the two sprays determined the concentration gradient of the chemical elements in the deposits. The overlapping of the spray cones also contributed to low porosity in the gradient zone of the deposits. The porosity in the graded deposits could be essentially eliminated by means of hot rolling. The carbides and grain structures of the hot rolled tool steels were fine and homogeneous. By means of combining different tool steels in a single deposit, different microstructures and properties were combined.     

Weld overlay cladding of high strength low alloy steel with austenitic stainless steel – Structure and properties

The present work aims at studying structure–property correlations in a weld overlay clad high strength low alloy steel with austenitic stainless steel of American Institute for Steel and Iron (AISI) 347 grade. Optical microscopy studies revealed that the interface between the two steels was nearly flat. The base plate had ferrite plus bainite microstructure adjacent to the interface and tempered bainite/martensite structure away from the interface. Grain coarsening and decarburization were observed near the interface. The stainless steel exhibited austenite dendritic structure. Tensile strength, notch-tensile strength and charpy impact energy of the base plate were found to be higher than those for the interface. The microhardness was observed to be maximum on the clad layer near interface. The shear bond strength of the weld overlay-interface was higher than the shear strength of the base plate. Fractography was carried out using scanning electron microscope on tensile, notch-tensile and shear bond test specimens of the interface as well as shear test specimens of the base plate. It revealed the presence of predominantly dimpled rupture. Charpy impact specimens of the interface failed in mixed mode while impact specimens of the base plate failed in ductile mode. Electron probe microanalysis across the bond interface indicated linear change in concentrations of Cr, Ni, Mn, Cu, Mo, Nb and Si between the levels appropriate to the clad layer and base metal.