纵向变厚度EH40钢板的组织和性能

采用拉伸、冲击、硬度、OM和TEM等方法研究纵向变厚度EH40钢板的组织和性能。结果表明,由于EH40钢板的薄端和厚端的制备工艺过程不同,其纵向的组织和性能呈现多样化;随着钢板厚度的增加屈服强度由534 MPa降低至489 MPa,抗拉强度由599 MPa降低至569 MPa。在-60℃进行冲击实验时,钢板薄端的冲击吸收能量大于200 J,而厚端的冲击吸收能量出现波动。钢板厚端的晶粒尺寸比薄端的粗大,贝氏体的含量低。在30 mm和40 mm位置全厚度都有贝氏体组织,厚度为8 mm时50 mm位置的贝氏体组织全部消失。薄端和厚端的析出相均为(Nb,Ti)C,但是薄端析出相的数量多、尺寸小,厚端析出相的数量少、尺寸大。     

A572 Gr.50厚板对接焊缝断裂性能研究

A572 Gr.50厚板常用于锅炉钢结构大板梁的加工制作,其对接焊缝易存在焊接裂纹缺陷。为评定含缺陷的锅炉钢结构大板梁的安全性,通过焊接接头模拟制作实际大板梁下翼缘的A572 Gr.50厚板对接焊缝,分别对其母材、焊缝金属及热影响区材料进行了系列单轴静力拉伸试验、冲击韧性试验和直三点弯曲断裂韧度试验,并结合有限元分析对A572 Gr.50厚板对接焊缝存在裂纹缺陷时的断裂性能进行了研究。研究结果表明:厚板对接焊缝母材、焊缝和热影响区材料基本力学性能均能满足规范要求,均具有良好的塑性变形能力;随温度的降低,母材、热影响区及焊缝处的夏比冲击功减少,但均具有良好的冲击韧性;比较而言,母材的抗低温冷脆性能最好,焊缝最差;母材、焊缝和热影响区3个区域中焊缝的断裂韧度最差;厚板对接焊缝接头的焊缝区是大板梁焊接缺陷安全性评估的重点控制区域;基于断裂力学,可以运用有限元软件方便的对带裂缝的工作状态下工作的钢结构构件的断裂性能进行分析,保证其安全性。     

桥梁高性能钢HPS485W断裂韧性试验研究

为了研究桥梁高性能钢的断裂韧性,对中国舞阳钢厂生产的高性能钢HPS 485W进行了一系列的断裂韧性试验研究。通过夏比V形缺口冲击试验得到HPS 485W在不同温度下的冲击韧性,并应用Boltzmann函数求解韧脆转变温度曲线,试验结果表明:与传统桥梁用钢相比,HPS 485W冲击韧性更高且韧-脆转变温度更低。由HPS 485W延性断裂韧度(J_IC)试验,测得板厚18mm和28mm的HPS 485W试样的J积分值,试验结果表明HPS 485W具有优良的断裂韧性。对8mm和14mm的HPS 485W进行裂纹尖端张开位移(CTOD)试验,试验结果表明HPS 485W有良好的塑性和韧性。基于HPS 485W拉伸试验获得的应力-应变曲线结果,运用失效评定曲线方法对CTOD试验值进行评定,HPS 485W的母材断裂韧性(CTOD)落在评定曲线的合格范围内。该文的研究为高性能钢桥的安全评定和防断裂设计提供了试验依据。     

CTOD‐R curves of the metal‐clad interface of API X52 pipes cladded with an Inconel 625 alloy by welding overlay

Crack‐tip opening displacement resistance curves (CTOD‐R) of the substrate/cladding interface of an API 5 L X52 steel pipe internally coated with Inconel 625 applied by TIG (GTAW) welding were experimentally evaluated. A small pipe section with 168 mm of outer diameter and 22.5 mm of thickness was internally coated with a 15 mm thick layer of Inconel 625 corrosion resistant alloy. Tension test specimens were obtained from both substrate and cladding, as well as compact tension test specimens (C(T)) for the evaluation of CTOD‐R curves. The fracture testing specimens were notched at the interface in RL orientation. In addition to fracture and tensile testing, microstructural characterization was conducted at the interface using optical microscopy (OM) and scanning electron microscopy (SEM). Qualitative chemical composition scanning and microhardness determination were also performed. The results indicated high fracture toughness for the substrate/cladding interface and the absence of low toughness regions at the interface of the tested samples.     

Effects of welding residual stresses and phosphorus segregation on cleavage delamination fracture in thick S355 J2 G3+N steel plate

Due to residual tensile stresses acting through the material’s thickness a cleavage fracture occurred in the central part of a thick steel plate during the cooling of a welded T-joint. Steel in the normalized rolled state, without non-metallic inclusions and with a very good contraction in the “Z” axis, is brittle because of its banded, ferrite–pearlite microstructure, which is dependent on phosphorus segregation. A normalizing heat treatment greatly improved the toughness of the central part of the thick plate and prevented its cracking during the cooling of the welded T-joint.     

Investigation on the mechanical properties and fracture toughness of graphite

In the present study, mechanical properties and fracture toughness of graphite as a brittle material were investigated. At first, some specimens were examined in two perpendicular directions to derive Young's modulus and ultimate tensile strength. Then, graphite fracture toughness tests were conducted using some three‐point bending specimens with a sharp machined V‐notch by two different methods. The first method is based on the applied force at the moment of fracture, and the second one uses energy released during the test. Moreover, a technique was adopted to reduce differences between the two methods. It was observed that considering the effect of dehydration of the specimens, the fracture toughness was reduced by about 8%. Finally, crack growth simulation of the experiment was performed and indicated that finite element analysis predicts about 25% lower crack length values when critical energy release rate is utilized as a crack growth criterion instead of fracture toughness. In other words, the required input displacement for crack growth would be overestimated by using the critical energy release rate criterion.     

Characterization of single crystal silicon and electroplated nickel films by uniaxial tensile test with in situ X-ray diffraction measurement

In this study, mechanical properties of micron‐thick single crystalline silicon (Si) and electroplated nickel (Ni) films at intermediate temperatures are investigated by means of X‐ray diffraction (XRD) tensile testing. The developed tensile test technique enables us to directly measure lateral (out‐of‐plane) elastic strain of microscale crystalline specimen using XRD during tensile loading, and determines Young's modulus, Poisson's ratio and tensile strength of the Si and Ni specimens. The specimens, measuring 10 μm thick, 300 μm wide and 3 mm long, are prepared through a conventional micro‐machining process, and the ultraviolet lithographie galvanoformung abformung (UV‐LIGA) process including a molding and an electroplating. The Si specimens, showing brittle fracture at room temperature (R.T.), have average Young's modulus and Poisson's ratio of 169 GPa and 0.35, respectively, in very good agreement with analytical values. The Ni specimens, showing ductile fracture, have those of 190 GPa and 0.24, lower than bulk coarse grained Ni. Young's moduli of both the Si and Ni specimens decrease with increasing temperature, but Poisson's ratios are independent of temperature. The influence of specimen size on elastic‐plastic properties of the specimens is discussed.     

The effect of fatigue pre‐cracking forces on fracture toughness

Testing procedures for the determination of the fracture toughness of a material by monotonic loading of fatigue pre‐cracked specimens are well established in standards such as BS 7448, BS EN ISO 15653, ISO 12135, ASTM E1820 and ASTM E1921. However, a review of these standards indicates a wide range of permitted fatigue pre‐cracking forces, whilst the underlying assumption in each standard is that the pre‐cracking conditions do not affect the fracture toughness determined. In order to establish the influence of different fatigue pre‐cracking forces on the fracture toughness, tests were carried out on specimens from an API 5L X70 pipeline steel. Single‐edge notch bend specimens of Bx2B geometry were notched through thickness and tested at temperatures of +20 °C, ?80 °C and ?140 °C to show the fracture behaviour in different regions of the fracture toughness ductile‐to‐brittle transition curve. Fatigue pre‐cracking was conducted on a high‐frequency resonance fatigue test machine over a range of pre‐cracking forces permissible within the various standards and beyond. The results showed that an excessively high pre‐cracking force can result in a significant overestimation of the value of fracture toughness for material exhibiting brittle behaviour, whilst very low fatigue pre‐cracking forces appeared to result in an increase in scatter of fracture toughness. A review of standards indicated that there was a possibility to misinterpret the intention of the ISO 12135 standard and potentially use excessively high pre‐cracking forces. Suggested clarifications to this standard have therefore been proposed to avoid the risk of overestimating fracture toughness.     

Thermal-shock-induced crack arrest of two low-alloy steels

Tests were performed on a C-Mn-Nb steel (E 36) and a C-Mn-Ni-Mo steel (A 50B) to determine the fracture toughness either at crack initiation, K_1c or at crack arrest, K_1a, under a very severe thermal shock. The experimental set-up was designed in such a way that it could provide enough flexibility to investigate various factors, including the specimen size effect in brittle fracture and the variations of K_1c or K_1a with temperature.

The thermal shock experiments were carried out either on small discs (thickness 19 mm) or on larger cylinders (height 220 mm) with an inner diameter and an outer diameter of 46 or 50 mm and 150 mm respectively, containing at their external periphery either a longitudinal sharp notch (0.04 mm) for the cylinders or a fatigue crack for the discs. These specimens are cooled to liquid nitrogen temperature until a homogeneous temperature distribution is reached. Then they are heated up by an induction coil set in the centre of the inner hole. The induction coils were designed to maintain purely radial heating of the specimens in order to induce axisymmetric thermal stresses. Typically, the experimental set-up is able to develop radial temperature gradients as large as 250°C in 20 s in the large cylinders and 500°C in 5 or 10 s in the thinner discs. Under the influence of these thermal gradients, which produce tensile hoop stress at the external periphery of the specimens, a crack is initiated from the notch or the initial fatigue precrack, which propagates very rapidly (-100 j1s) over a distance of a few centimetres and then stops.

The temperature distribution measured continuously during the experiments is used as the input for the numerical calculations. Finite element method calculations were performed to determine the variations of the hoop stress and those of the stress intensity factor across the wall thickness. Results obtained on both materi.als are given. In A50B steel it is shown that the apparent fracture toughness K_1c determmed on these large test pieces is smaller than the toughness measured on smaller convsntional specimens. This size effect is explained in terms of a local approach of brLttle cleavage fracture based on Weibull statistics.     

Long-term service effect on the toughness of pipeline steel 17GS

A transit oil pipeline, 720 mm in diameter, 334 km in length, and made of steel 17GS, after 30 years of service was twice non-destructively tested with intelligent Pipeline Inspection Gauges (PIGs). Two segments of this pipeline were cut in order to investigate the steel structure, composition, and mechanical properties and to compare these properties with those of `archive' material. Metallographic and microprobe analyses revealed no significant difference between the `archive' and `aged' materials. Specimens for testing tensile mechanical properties, for fatigue-crack growth rate measurements, and for a Charpy test were used. All those specimens were fabricated from both the base and weld metal in longitudinal and circumferential directions. At room temperature, the ultimate strength of the aged steel was found to be 5% higher than that of the `archive' material, whereas the yield point was 18% higher for the aged material. Charpy energy changed more significantly; decreasing by approximately 40% at room temperature. In addition to conventional mechanical properties, new characteristics such as `microcleavage stress' R _MC (minimum brittle fracture stress) and the toughness index K _T were determined. It has been found that a long period of operation does not affect the values of the steel microcleavage stress R _MC and does not influence the hardening exponent significantly. The use of the local approach to fracture has shown that an increase in yield stress is the main cause of the decrease in its toughness after long-term service. It is shown that a minor (15%) increase in the yield stress results in a rise (by 40°) in the Charpy transition temperature and may cause a twofold reduction in the fracture toughness K _IC.     

Anwendung des instrumentierten Kerbschlagbiegeversuchs zur Ermittlung von Referenztemperaturen nach dem Master‐Curve‐Konzept

Application of the Instrumented Impact Test for the Determination of Reference Temperatures Using the Master Curve Concept The instrumented impact test is suitable for the determination of fracture mechanical parameters. In this paper the determination of the dynamic fracture toughness values in the lower ductile‐to‐brittle transition region is presented. The fracture toughness is determined at the onset of cleavage fracture and evaluated by the Master Curve (MC) concept. The MC concept allows to quantify the variation of fracture toughness with the temperature within the lower ductile‐to‐brittle transition region. Limit curves of fracture toughness for defined failure probabilities and a reference temperature can be determined using this method. This paper presents the application of the master curve concept to the reference temperature determination through the thickness of reactor pressure vessel (RPV) steel plate. The reference temperatures determined dynamic fracture toughness values (T₀^dy) are compared with quasi‐static reference temperatures (T₀^st) and Charpy‐V transition temperatures (TT). T₀^dy, T₀^st and TT increase from the surface to the middle of the RPV steel plate. Compared with T₀^st, the T₀^dy values are higher approximately 70 to 90 K.     

Effects of plate thickness on fatigue fracture behaviors of an aluminum alloy (Al‐Cu‐Mg)

High cycle fatigue properties of 2124 aluminum alloy plates with different thickness were investigated by determining fatigue S?N curves, fatigue crack growth rates and fracture toughness of 2124‐T851 aluminum alloy plates with the thickness of 30 mm, 40 mm and 55 mm, respectively. Fatigue fracture behaviors of alloy plates were also analyzed and discussed using scanning electron microscope morphology observation, energy spectrum analysis, X‐ray diffraction phase analysis and transmission electron microscopy observation in this paper. The results indicate that plate thickness affects the comprehensive fatigue properties of 2124 aluminum alloy plates. Thinner plate achieves better comprehensive fatigue properties. Due to the different amount of deformation during hot rolling, the variation of microstructure of alloy plates with different thickness mainly concentrates on the difference of grain sizes, substructure and volume fraction of grain boundaries. The thinner the plate, the smaller the grain sizes and therefore the thinner plate produces a higher volume fraction of grain boundaries and substructure, and a greater resistance to fatigue crack growth, thus thinner plate exhibits better fatigue properties.     

The microstructure and mechanical properties of prolonged and lower temperature aged Fe–Ni–Mn–Mo–Ti–Cr maraging steel

In this study, the microstructure and mechanical properties of Fe–Ni–Mn–Mo–Ti–Cr maraging steel at low temperature and prolonged aging condition were investigated. Optical and scanning electron microscopy examinations, tensile and hardness tests were conducted to study the microstructure, aging behavior and mechanical properties of the cold‐rolled steel. The results showed that aging of cold rolled Fe–Ni–Mn–Mo–Ti–Cr maraging steel resulted in the formation of Mo rich and Ti rich Lave phase precipitates. Existence of many dislocation cores due to cold rolling and subsequently, low temperature aging caused to formation of uniform distribution of very fine precipitates. The presence of these precipitates increased the yield and ultimate tensile strengths but couldn't improve the uniform tensile ductility. This alloy showed ultra‐high fracture stress of about 1950 MPa with a negligible tensile elongation (about 2 %) at the peak aged condition. The fractographic studies indicated this alloy shows semi‐brittle fracture in the subsequent aging treatment.     

Fracture toughness analysis of laser-beam-welded superalloys Inconel 718 and 625

In this study, two 3.2‐mm thick Ni‐base superalloys, Inconel 718 and 625, have been laser‐beam‐welded by a 6‐kW CO₂ laser and their room temperature fracture toughness properties have been investigated. Fracture toughness behaviour of the base metal (BM), fusion zone (FZ) and heat affected zone (HAZ) regions was determined in terms of crack tip opening displacement (CTOD) using compact tension‐type (C(T)) specimens. Laser‐beam‐weld regions showed no significant strength overmatching in both alloys. Ductile crack growth analysis (R‐curve) also showed that both materials exhibited similar behaviour. Compared to the BM there is a slight decrease in fracture toughness of the fusion and the HAZ.     

Use of the small punch test to determine the ductile‐to‐brittle transition temperature of structural steels

The small punch test (SPT) consists in punching very small square specimens, measuring 10 × 10 mm² and 0.5‐mm thickness, until fracture using a 2.5‐mm‐diameter hemispherical punch. Different specimens of a structural steel were tested from room temperature to cryogenic temperatures in order to determine its ductile‐to‐brittle transition temperature (DBTT). The DBTT obtained in SPT (DBTT_STP) is much lower than the DBTT obtained by means of Charpy specimens (DBTT_CVN). The variation of the mechanical parameters calculated from the SPTs with temperature was also calculated and the operative fracture micromechanisms defined using a scanning electron microscope.     

Influence of retrogression and reaging on fracture toughness of 7010 aluminium alloy

Abstract

Compact tension fracture toughness specimens of high strength aluminium alloy 7010 have been tested after the application of retrogression and reaging (RRA) heat treatments. Short transverse orientation specimens, 25 mm in thickness were cut from a large rectilinear open die forging. Two retrogression temperatures were investigated, 200 and 240 ° C, and the RRA treatments were applied to material in the solution treated and cold compressed (W52) condition. Varying the retrogression time and temperature strongly influences the fracture toughness and does so to a degree that is greater than would be expected from the change in tensile properties alone. The fracture toughness, tensile properties and work hardening characteristics of alloy 7010 are reported and their dependence on retrogression time and temperature is described.     

钢结构厚板的冲击韧性研究

随着钢材在我国大跨度桥梁工程中广泛应用,钢材板厚也日益增加。对于厚板钢材,厚度增大使钢板的应力应变状态发生变化,中心偏析现象严重,更容易降低钢板厚度方向的韧性,且在低温环境下,这种现象表现尤为突出。而现行相关规范缺乏对厚钢板冲击试验规定。根据厚板低温冲击韧性试验数据,对钢板厚度变化对冲击韧性影响进行了探讨,并且利用Boltzmann函数对试验结果进行拟合,为钢结构厚板工程中冲击韧性取值提供参考作用。     

Experimental and numerical investigation of thickness effect on ductile fracture toughness of steel alloy sheets

Effect of thickness on ductile fracture toughness of plates made of steel alloy GOST 08Ch22N6T is investigated experimentally. Multiple specimen tests for determining fracture toughness have been conducted using compact tension (CT) specimens with thicknesses of 1.25, 1.64 and 4.06 mm according to standard test method ASTM E813. The results show the significant effect of thickness on fracture toughness. It is observed that in low thickness, J_c increases with the thickness increase until it reaches a maximum; however, further increase in the thickness causes the J_c-value to decrease. Two-dimensional finite element analysis is also performed to reproduce the experimental results. The comparison shows a very good agreement.     

Thermal cycles and their effects during friction stir welding of AA7075 thicker plates with and without in-process cooling

Friction stir welding of AA 7075 plates in three different thicknesses such as 10, 16 and 25 mm at natural convection condition was carried out successfully without defects. Water cooled friction stir welds were also produced on 16 mm thick plates. The thermal cycles at different locations of the plate, during the friction stir welding process, were predicted using a three-dimensional thermal model. Mechanical properties of the welds were evaluated using tensile and hardness tests. Weld microstructures were also examined with optical and transmission electron microscopes. The weld hardness values and tensile properties were found to decrease with increase in plate thickness. The use of water cooling was found to improve the weld properties to some extent, although not to the level of base metal. The reasons for this behavior are discussed, correlating thermal cycles, mechanical properties, fracture locations and precipitate morphology.     

Direct metal deposition (DMD) of H13 tool steel on copper alloy substrate: Evaluation of mechanical properties

H13 tool steel powder was clad on copper alloy substrate both directly and using 41C stainless steel (high Ni steel) powder as a buffer layer by direct metal deposition (DMD). Cu-steel bimetallic die casting and injection molding tools are of high interest for reduction of cycle time by efficient heat extraction due to high thermal conductivity of copper. The mechanical properties of these bimetallic structures were investigated in terms of bond strength, impact energy and fracture toughness. The bond interfaces of these claddings showed porous and crack free transition regions. The bond strength was higher in the directly clad H13 tool steel compared to the H13 tool steel clad with 41C stainless steel as buffer layer. The fracture morphology in tensile test specimens showed ductile dimple fracture. Presence of necking just below the interface depicted the softening of substrate in heat affected zone (HAZ) during cladding. The Charpy impact energy is little higher in the 41C stainless steel buffered specimens compared to the directly clad H13 tool steel specimens but the fracture toughness results showed reduction of fracture toughness in the 41C stainless steel buffered specimens due to the low strength in the tensile test. However the fracture toughness value was in the ductile region for both deposits.