Effect of microstructure on the low temperature toughness of high strength pipeline steels

Microstructure observations and drop-weight tear test were performed to study the microstructures and mechanical properties of two kinds of industrial X70 and two kinds of industrial X80 grade pipeline steels. The effective grain size and the fraction of high angle grain boundaries in the pipeline steels were investigated by electron backscatter diffraction analysis. It is found that the low temperature toughness of the pipeline steels depends not only on the effective grain size, but also on other microstructural factors such as martensite-austenite （MA） constituents and precipitates. The morphology and size of MA constituents significantly affect the mechanical properties of the pipeline steels. Nubby MA constituents with large size have significant negative effects on the toughness, while smaller granular MA constituents have less harmful effects. Similarly, larger Ti-rich nitrides with sharp corners have a strongly negative effect on the toughness, while fine, spherical Nb-rich carbides have a less deleterious effect. The low temperature toughness of the steels is independent of the fraction of high angle grain boundaries.

Mechanical properties and fracture toughness of rail steels and thermite welds at low temperature

Brittle fracture occurs frequently in rails and thermite welded joints, which intimidates the security and reliability of railway ser- vice. Railways in cold regions, such as Qinghai-Tibet Railway, make the problem of brittle fi＇acture in rails even worse. A series of tests such as uniaxial tensile tests, Charpy impact tests, and three-point bending tests were carried out at low temperature to investigate the mechanical properties and fracture toughness of U71Mn and U75V rail steels and their thermite welds. Fracture micromechanisms were analyzed by scanning electron microscopy （SEM） on the fracture surfaces of the tested specimens. The ductility indices （percentage elongation aider frac- ture and percentage reduction of area） and the toughness indices （Charpy impact energy Ak and plane-strain fracture toughness Kic） of the two kinds of rail steels and the corresponding thermite welds all decrease as the temperature decreases. The thermite welds are more critical to fracture than the rail steel base metals, as indicated by a higher yield-to-ultimate ratio and a much lower Charpy impact energy. U71Mn rail steel is relatively higher in toughness than U75V, as demonstrated by larger Ak and Klc values. Therefore, U71Mn rail steel and the corresponding thermite weld are recommended in railway construction and maintenance in cold regions.

Low temperature impact toughness and fracture mechanism of cast QT400-18L ductile iron with diferent Ni additions

Different contents of Ni(0.3wt.%to 1.2wt.%)were added to the QT400-18L ductile iron to investigate the effect of Ni addition on the impact toughness of cast ductile irons at low temperatures.The impact toughnesses of the samples at room and low temperatures were tested.The microstructures and fractographs were observed.Results show that with the increase of Ni addition there is a general trend of refinement of the ferrite matrix while the nodule density shows no obvious change.When the Ni content is 0.7wt.%,the matrix structure is the refined ferrite with a very small fraction(about 2%)of pearlite near the eutectic cell boundaries.When the Ni content is further increased,the fraction of pearlite increases significantly and reaches more than 5%when 1.2wt.%Ni is added.The impact toughness at room temperature increases as the content of Ni increases from 0.3 wt.%to 0.7 wt.%,but decreases as the Ni content further increases to 1.2wt.%due to the increase of pearlite fraction.The maximum value of the impact work is 18.5 J at room temperature with 0.7wt.%Ni addition.The average value of the impact work is still more than 13 J even at-30℃.In addition,the fracture mechanism changes from ductile manner to brittleness as the testing temperature decreases from 20℃to-60℃.

Relationship between Low Temperature Toughness and Microstructure in Low Activation Fe-Cr-Mn(W,V)Steel for Fusion Reactors

Fe-Cr-Mn (W, V) austenite steel was researched in order to supply a theory base for the first wall materials of fusion reactors.Experiments included vacuum melting, forging, annealing, solution treatment, Charpy impact tests and microstructure observation. Theresults show that the imped value decreases with the test temperature decreasing. In this system, there is ductile/brittle transition. Themechanism of this decrease of the impact value is considered to be due to γ - ε transformation in sub-stable austenite steel and stoppingoverlapping sacking fault by grain boundaries in stable austenite steel.

Low temperature impact toughness of laser hybrid welded joint of high strength low alloy steel

High strength low alloy steel with 16 mm thickness was welded by using high power laser hybrid welding. Microstrueture was characterized by using optical microscopy, scanning electron microscopy （ SEM ） , transmission electron microscopy （TEM） and selected area electron diffraction （SAED）. Low temperature impact toughness was estimated by using Charpy V-notch impact samples selected from the upper part and the lower part at the same heterogeneous joint. Results show that the low temperature impact absorbed energies of weld metal are （202,180,165 J） of upper samples and （178,145,160 J） of lower samples, respectively. All of them increase compared to base metal. The embrittlement of HAZ does not occur. Weld metal primarily consists of refined carbide free bainite and a little granular bainite since laser hybrid welding owns the character of low heat input. Retained austenite constituent film ＂locates among the lath structure of bainitie ferrite. Refined bainitic ferrite lath and retained austenite constituent film provide better low temperature impact toughness compared to base metal.

Fracture toughness and fracture behavior of SA508-Ⅲ steel at different temperatures

The fracture toughness of SA508-Ⅲ steel was studied in the temperature range from room temperature to 320℃ using the J-integral method. The fracture behavior of the steel was also investigated. It was found that the conditional fracture toughness （JQ） of the steel first decreased and then increased with increasing test temperature. The maximum and minimum values of do were 517.4 kJ/m^2 at 25℃ and 304.5 kJ/m^2 at 180℃, respectively. Dynamic strain aging （DSA） was also observed to occur when the temperature exceeded 260℃ with a certain strain rate. Both the dislocation density and the number of small dislocation cells effectively increased because of the occurrence of DSA; as a consequence, crack propagation was more strongly inhibited in the steel. Simultaneously, an increasing number of fine carbides precipitated under high stress at temperatures greater than 260℃. Thus, the deformation resistance of the steel was improved and the Jo was enhanced.

Impact Toughness Scattering of Bainitic Steel in the Ductile-brittle Transition Temperature Region

The impact toughness scattering in the ductile-brittle transition temperature （DBTT） region was experimentally examined on mixed and homogeneous grains of low alloy high strength bainitic steel under dynamic loading conditions. The results revealed that the mixed grain microstructure had larger impact toughness scattering than the homogeneous one, and the impact toughness scattering was mainly caused by the scattering in the cleavage fractttre stress σf. The value of σf. is related to the size of the microcrack formed in the bainitic packet. When a bainitic packet-sized microcrack propagates from one bainitic packet into the adjacent packet, cleavage fracture occurs. The cleavage fracture is controlled by the few coarse packets in the microstructures, and the σf scattering is influenced by the varied distances/relative locations between these coarse packets, and homogenizing the distribution of fine bainitic packet sizes is an effective way to reduce the impact toughness scattering in the DBTT region.

Effects of low temperature on properties of structural steels

The experiments were carried out to measure the mechanical properties ofthree grades of structural steels (Q235A, 16Mn and Q390E steel) at low temperature. It was shownthat the strength of the steels increases while the plasticity and toughness decrease as temperaturedrops. In the transitional area the toughness drops rapidly with temperature. Among the threestructural steels, Q390E steel has the best toughness and the lowest sensitivity.

Effects of Annealing Process on Crystallization and Low Temperature Resistance Properties of PP-R Composites

The effects of the annealing process on the mechanical properties and crystallization behaviors of polypropylene random copolymer（PP-R） composites were investigated using differential scanning calorimetry（DSC）, wide-angle X-ray diffraction（WAXD）, and dynamic mechanical analysis（DMA）, and scanning electron microscopy（SEM）. The experimental results indicated that the annealing process significantly influenced the comprehensive properties of PP-R composites. At temperatures below 23 ℃, the impact strength of the PP-R composites annealed at 120 ℃ for 6 h was relatively high at 74.73 k J/m^2, which was 16.8% higher than that of the samples annealed at 80 ℃ for 6 h. At low temperatures（-30-0 ℃）, the impact strength ranged from approximately 13.31 k J/m^2 to 54.4 k J/m^2. In addition, the annealing process conducted at 120 ℃ for 6 h improved the crystalline structure and low-temperature toughness of the PP-R composites and induced α-form to β-form crystal transformation. The work provides a possible method to reinforce and toughen the semicrystalline polymer at low temperatures（-30-0 ℃） by annealing.

CHARACTERISTIC TRANSITION TEMPERATURE OF BRITTLENESS OF LOW CARBON STEEL

On the basis of analysis of low carbon steel fracture macro-features and micro-processes at low temperature,the definition was given of the characteristic transition temperature of brittleness,T_(pm),and its physical meaning was expounded.Discussion was carried out of phys- ical characteristic of variation at T_(pm) in respect of the fracture toughness and property of crack arrest.In addition,research was made on the application of T_(pm),which can give infor- mation about the transition of the fracture toughness,the property of crack arrest and critical crack size,to estimation the brittleness of low carbon steel at low temperatures.

Application Research on Toughness Index CTOD in Design of Railway Steel Bridge

Railway steel bridge belongs to large-scale weld structures suffered with cyclic dynamic stress generated by the train. In recent years, the section of bridge member becomes bigger, plate becomes thicker, connection form becomes more complicated and steel bridge is applied to wider districts even in the lower temperature environment. Thus, fatigue and fracture problems become more serious. On the basis of CTOD （crack tip open displacement） test data of 372 specimens tested in different temperatures, this paper discusses research work about fracture proof design that involves how to determine the criterion of CVN （Charpy V-notch） impact toughness by establishing the relationship between CTOD and CVN, how to prevent from brittle fracture by stress control in railway steel bridge design based on COD （crack open displacement） design curve through the test data and how to do the fatigue design for railway steel bridge at -50 ℃ of design temperature in an easy way. The method of fatigue design at -50 ℃ environment has been used for railway steel bridge structure of Qinghai-Tibet Railway in China.

Effect of Yttrium on Toughness of Weld Metal Deposited by TiO_2－CaO Type Electrode

Effect of yttrium on low temperature toughness of weld metal deposited by TiO2-CaO type electrode has been investigated by transfering yttrium from welding pool to weld with fluxcored wire.The results show that the decrease in surface tension of liquid iron in welding pool and the variation in density of inclusion in weld are main causes for toughness improvement effect on weld by yttrium.

Fracture toughness and safety assessment of X65 offshore pipeline welded joints

In this investigation, the mechanical properties and low-temperature fracture toughness of API 5L X65 offshore pipeline welded joints were studied. Structure Integrity Assessment Procedure (SINTAP)—Failure Assessment Diagram (FAD) method was applied to the pipe structure with surface flaw at the weld toe. According to the ISO standard BS7448, the CTOD fracture toughness of the welded joints was determined at the temperature of 0℃. For the heat-affected zone (HAZ) specimens, post-test metallographic analysis was performed to verify that the tip of the crack was located in the coarse crystal zone in order to confirm the validity of the above results. The failure lines of analysis level 1 and 3 of weld metal were derived from the results of the mechanical property test. The assessment was performed, considering the maximum lay stress, residual stress conservatively assumed to be uniform tensile stress, and minimum CTOD value. The results of the assessment showed that pipeline structure with a surface flaw (the height and length are respectively 2.2mm and 5mm) at the weld toe is safe. This study lays the foundation of application of SINTAP to pipeline structure assessment.

Analytical Investigation of Prior Austenite Grain Size Dependence of Low Temperature Toughness in Steel Weld Metal

Prior austenite grain size dependence of the low temperature impact toughness has been addressed in the bainitic weld metals by in situ observations.Usually,decreasing the grain size is the only approach by which both the strength and the toughness of a steel are increased.However,low carbon bainitic steel with small grain size shows a weakening of the low temperature impact toughness in this study.By direct tracking of the morphological evolution during phase transformation,it is found that large austenite grain size dominates the nucleation of intragranular acicular ferrite,whereas small austenite grain size leads to grain boundary nucleation of bainite.This kinetics information will contribute to meet the increasing low temperature toughness requirement of weld metals for the storage tanks and offshore structures.

Energy Separation and Explicit Dynamic Analysis of Low Temperature Impact Toughness of Transmission Tower Material Q420B

To determine the physical significance of the impact toughness parameters and accurately characterize the low temperature impact toughness of transmission tower material Q420 B,the finite element model of Charpy impact test is established on the basis of experiment.The simulation and test results are verified,and the specimen fracture is analyzed by scanning electron microscope.The formation and growth mechanism of the crack are dynamically analyzed.On this basis,energy separation method is used to investigate the effect of low temperature on impact toughness.The results show that the simulation and test results are in good agreement,and the ductile-brittle transition temperature of Q420 B is about-50 ℃.The breaking process of the specimen is divided into the crack formation and propagation.When temperature drops from 20 to-60 ℃,the crack propagation energy decreases from 51.0 to 11.9 J,the crack formation energy reduces from 39.9 to 15.8 J,and the fracture time of the material drops from 1.8 to 0.6 ms.

Microstructure and Mechanical Properties of Nb-B bearing Low Carbon Steel Plate: Ultrafast Cooling versus Accelerated Cooling

The microstructure and mechanical properties of low carbon bainite high strength steel plate were studied via different cooling paths at the pilot scale. There was a significant increase in mechanical properties, and notably, the yield strength, tensile strength, and toughness at-40 ℃ for the tested steel processed by ultra-fast cooling were 126 MPa, 98 MPa and 69 J, respectively, in relation to steel processed by accelerated cooling. The ultra-fast cooling rate not only refined the microstructure, precipitates, and martensiteaustenite（M/A） islands, but also contributed to the refinement of microstructure in thick plates. The large size M/A constituents formed at lower cooling rate experienced stress concentration and were potential sites for crack initiation, which led to deterioration of low-temperature impact toughness. In contrast, the acicular ferrite and lath bainite with high fraction of high-angle grain boundaries were formed in steel processed by ultra-fast cooling, which retarded cleavage crack propagation.

焊接方法对奥氏体不锈钢焊接接头低温冲击性能的影响

为了研究奥氏体不锈钢焊接接头的低温冲击性能对低温压力容器的安全性影响,本文通过-196℃的低温冲击试验,对比研究了钨极气体保护焊(GTAW)、熔化极气体保护电弧焊(GMAW)、埋弧焊(SAW)3种焊接方法对S30408和S31608两种奥氏体不锈钢焊接接头低温冲击性能的影响。对于S30408焊接接头,热影响区的低温冲击性能小于母材与焊缝区,成为薄弱位置;3种焊接方法中,GTAW获得的焊接接头在热影响区的冲击性能处于最优水平。对于S31608焊接接头的低温冲击性能,焊缝、热影响区与母材的差异较小;3种焊接方法中,SAW获得的焊接接头在冲击吸收能量上与母材结果相近,能够获得焊接接头较好的综合冲击性能。通过对比不同焊接工艺下焊接接头断口形貌特征表明,焊接接头不同区域冲击断口形貌特征与低温冲击性能的结果相吻合。本文研究工作对于奥氏体不锈钢低温压力容器焊接工艺的选择具有参考意义。

玄武岩微筋对磷酸镁修补砂浆弯曲性能的增强增韧效应研究

为了探究玄武岩微筋(Basalt minibar)对磷酸镁修补砂浆(MPCRM)的增强增韧改性效果,通过四点弯曲试验分析纤维类型、纤维掺量、基体水胶比及养护条件对MPCRM弯拉强度和弯曲韧性的影响规律。试验结果表明:玄武岩微筋对MPCRM的弯曲增强增韧性能优越,且其增韧效果优于玄武岩单丝纤维;当玄武岩微筋掺量不低于1.0%(体积分数)时,MPCRM弯拉强度和弯曲韧性均随着玄武岩微筋掺量增加而提高;随着水胶比提高,玄武岩微筋增强MPCRM的弯曲韧性降低,弯拉强度先增加后减小,水胶比为0.18和体积掺量为1.5%的玄武岩微筋增强MPCRM的弯拉强度最大;掺入玄武岩微筋能够部分弥补低温养护MPCRM的受冻损伤。

船用LNG储罐高锰奥氏体低温钢埋弧焊焊丝焊剂CHW-SMn/CHF610的研制

通过实验研制了一种-196℃超低温船用LNG储罐高锰奥氏体低温钢埋弧焊焊丝焊剂CHW-SMn(φ4.0)/CHF610,焊剂为氟碱型CaF_(2)-CaO-MgO-Al_(2)O_(3)渣系,碱度BⅡW在2.5~3.5。该焊丝焊剂组合具有优良的焊接工艺性能,焊接电弧稳定,焊缝脱渣容易,焊缝成形美观。该焊丝焊剂组合焊缝金属力学性能稳定,屈服强度516 MPa、抗拉强度735 MPa、延伸率49.5%,-196℃冲击功KV2均值为125 J,且杂质含量低,具有良好的抗裂纹性能,完全满足超低温高锰钢LNG储罐焊缝金属的要求。

900 MPa级管线钢热处理工艺优化及韧性波动分析

通过三因素三水平正交热处理试验,探索新型900 MPa级低合金高强韧管线用钢的最优热处理工艺,借助OM、SEM、TEM等手段,对管线钢韧性波动原因进行了全面分析。结果表明,各因素(淬火温度、回火温度、回火时间)对钢的力学性能(冲击功、强度性能和延伸率)的影响具有不同优先级,确认了最优工艺参数为淬火温度890℃、回火温度590℃、保温时间60 min。随着回火温度的升高和保温时间的延长,析出物数量增多且趋于球化,同时组织软化,位错密度降低,这些因素是管线钢韧性提高的主要原因。相比之下,细小弥散分布的夹杂物对韧性的提升贡献较小。尽管回火温度升高和保温时间延长也会导致析出相尺寸增大和组织粗化,使管线钢韧性降低,但这种降低效果远小于前述增韧因素带来的正面影响。