Effect of Cu addition on microstructure and impact toughness in the simulated coarse-grained heat-affected zone of high-strength low-alloy steels

The effects of Cu content on microstructure and impact toughness in the simulated coarse-grained heat-affected zone (CGHAZ) of high-strength low-alloy steels were investigated. It has been observed that the microstructure in the simulated CGHAZ of Cu-free steel is dominated by a small proportion of acicular ferrite and predominantly bainite with martensite–austenite constituent. Whereas, in the 0.45 and 1.01% Cu-containing steels, the acicular ferrite increased significantly due to the effective nucleation on intragranular inclusions with outer layer of MnS and CuS. The formation of acicular ferrite is attributed to superior high heat-affected zone impact toughness in the 0.45% Cu-containing steel. Furthermore, the increasing martensite–austenite constituent and ε-Cu precipitates in the simulated CGHAZ of 1.01% Cu-containing steel caused degradation in impact toughness.     

Effect of Post-weld Heat Treatment on Microstructure and Fracture Toughness of 30CrMnSiNi2A Steel Welded Joints

The electron beam local post-weld heat treatment (EBLPWHT) is a rather new method that provides the advantages of high precision, flexibility and efficiency, energy saving and higher productivity. This paper studies the effect of two post-weld heat treatment processes on the microstructure, mechanical properties and fracture toughness of an electron beam welded joints in 30CrMnSiNi2A steel. EBLPWHT, in a vacuum chamber, immediately after welding and a traditional furnace whole post-weld heat treatment (FWPWHT) were compared. The experimental results show that, after EBLPWHT treatment, the main microstructure of weld was changed from coarse acicular martensite into lath rnartensite, HAZ was changed from lath martensite, bainite into lower bainite, and base metal was changed from ferrite and pearlite into upper bainite and residual austenite. The microstructures of different zones of joints in FWPWHT condition were tempered sorbite. The properties of welded joints can be improved by the EBLPWHT in some extent, and especially largely for the fracture toughness of welded joints. However the value of fracture toughness of base metal is comparatively low, so appropriate heat treatment parameters should be explored in the future.     

Effects of Carbon on the CG HAZ Toughness and Transformation of X80 Pipeline Steel

X80 pipeline steel produced by TMCP has high strength and high toughness with ultrafine grain microstructure. The microstructure coarsens and the toughness worsens at the coarse grained (CG) HAZ apparently after weld simulation. The experimental results indicated that the bainitic ferrite and the second phases formed at cooling are differently as the variation of carbon in base metal. In low carbon steels, the bainitic ferrite laths are long and narrow, the second phases are complex including residual austenite, martensite, the M-A constituent and the Fe3C carbide. The formation of Fe3C carbide is the main reason of the poor toughness in CG HAZ. The ultralow carbon in base metal, however, can improve the CG HAZ toughness through restraining the formation of carbides, decreasing the M-A constituent, increasing the residual austenite content, which are beneficial to the CG HAZ toughness.     

Effects of Mn and Ti on microstructure and inclusions in weld metal of high strength low alloy steel

Abstract

The influences of alloying elements on chemical composition of non-metallic inclusions, impact toughness and microstructure in weld metals of high strength low alloy steels have been studied. Results indicated that microstructure had changed from a mixture of acicular ferrite, proeutectoid ferrite, ferrite side plates and microphases to a mixture of acicular ferrite, bainite and microphases due to the addition of Mn and Ti. The impact toughness of weld metal was improved correspondingly. The volume fraction and composition of inclusions both influenced the proportion of acicular ferrite. Mn and Si based oxide globular inclusions located at the boundary of acicular ferrite plates in the weld metal produced using C–Mn–Si–Cu wire. When Mn and Ti were added to welding wires, the inclusions within acicular ferrite plates permitted fewer primary acicular ferrite plates to grow into relatively larger dimensions. Secondary acicular ferrites nucleating on pre-existing ferrite plates refined microstructure effectively.     

氮含量对钒微合金钢粗晶热影响区组织和韧性的影响

采用焊接热模拟的方法,研究了氮含量对实验钢焊接粗晶热影响区(CGHAZ)显微组织和韧性的影响规律。结果表明:随着氮含量的增加,CGHAZ的组织从晶界铁素体、贝氏体和侧板条铁素体转变成针状铁素体、多边形铁素体和少量的贝氏体,且铁素体晶粒细化;CGHAZ韧脆转变温度(FATT₅₀)先降低后升高,屈服强度升高。氮含量从0.004 4%增加到0.009 4%时,有效晶粒尺寸减小,导致CGHAZ的FATT₅₀降低;氮含量从0.009 4%增加到0.019 0%时,CGHAZ中固溶氮、屈服强度增量对FATT₅₀的综合作用大于晶粒的细化作用,导致FATT₅₀升高。      

Relationship between microstructure and hydrogen induced cracking behavior in a low alloy pipeline steel

Hydrogen induced cracking(HIC) behaviors of a high strength pipeline steel with three different microstructures, granular bainite lath bainite(GB + LB), granular bainite acicular ferrite(GB + AF), and quasi-polygonal ferrite(QF), were studied by using corrosion experiment based on standard NACE TM0284. The HIC experiment was conducted in hydrogen sulfide(H_2S)-saturated solution. The experimental results show that the steel with GB + AF and QF microstructure present excellent corrosion resistance to HIC, whereas the phases of bainite lath and martensite/austenite in LB + GB microstructure are responsible for poor corrosion resistance. Compared with ferrite phase, the bainite microstructure exhibits higher strength and crack susceptibility of HIC. The AF + GB microstructure is believed to have the best combination of mechanical properties and resistance to HIC among the designed steels.     

Structure Character of M-A Constituent in CGHAZ of New Ultra-Low Carbon Bainitic Steel under Laser Welding Conditions

800 MPa grade new ultra-low carbon bainitic （NULCB） steel is the recently developed new generation steel. The microstructure in the coarse-grained heat affected zone （CGHAZ） of NULCB steel under laser welding conditions was investigated by thermal simulation. The influence of the cooling time from 800℃ to 500℃.t8/5 （0.3-30 s）, on the microstructure of the CGHAZ was discussed. The experimental results indicate that the microstructnre of the CGHAZ is only the granular bainite which consists of bainitic ferrite （BF） lath and M-A constituent while t8/5 is 0.3-30 s. The M-A constituent consists of twinned martensite and residual austenite, and the change of the volume fraction of the residual austenite in the M-A constituent is very small when t8/5 is between 0.3 and 30 s. The morphology of the M-A constituent obviously changes with the variation of t8/5.As t8/5 increases, tile average width, gross and shape parameter of the M-A constituent increase, while the line density of the M-A constituent decreases.     

Study on carbide-bearing and carbide-free bainitic steels and their wear resistance

Carbide-free and carbide-bearing bainitic steels have been obtained. The relationship between the bainitic microstructure and wear resistance has been studied. Results show that carbide-free upper and lower bainitic microstructures obtained in the steel with Si?+?Al mainly consist of bainitic ferrite and retained austenite. Carbide-bearing upper and lower bainitic microstructures obtained in the steel without Si?+?Al consist of bainitic ferrite, carbide and trace amounts of retained austenite. The carbide-free bainite exhibits higher strength and toughness than carbide-bearing bainite, especially the toughness. Under lower wear loading, carbide-bearing lower bainite (LB) exhibits higher wear resistance. Under higher wear loading, carbide-free LB exhibits higher wear resistance, which results from the improved surface hardness due to strain-induced martensitic transformation from the retained austenite.     

Electron backscattering diffraction study of acicular ferrite,bainite, and martensite steel microstructures

Abstract

This study deals with acicular ferrite, bainite, and martensite microstructures observed in three low alloy steels. Electron backscattering diffraction (EBSD) was used to assess crystallographic features of these microstructures. In each area studied by EBSD mapping, ‘crystallographic packets’ defined as clusters of points sharing the same crystallographic orientation were compared with ‘morphological packets’ observed in the corresponding light micrograph. Microtexture studies suggested that acicular ferrite and upper bainite grow with Nishiyama– Wassermann relationships with the parent austenite phase, whereas lower bainite and martensite consist of highly intricate packets having Kurdjumov–Sachs relationships with the parent phase. In all cases three highly misoriented texture components were found within each former austenite grain. Electron backscattering diffraction also gave information about the cleavage and intergranular reverse temper embrittlement fracture mechanisms of these steels. In conclusion, it is shown that EBSD is a powerful tool for studying phase transformation and fracture mechanisms in steels on a microscopic scale.     

回火温度对两相区退火海工钢组织和性能的影响

对690 MPa级海工钢进行“淬火+两相区退火+回火”三步热处理,研究了回火温度对其组织和性能的影响、分析了力学性能变化与组织演变和残余奥氏体体积分数之间的关系。结果表明：回火后实验钢的显微组织为回火贝氏体/马氏体、临界铁素体和残余奥氏体的混合组织。随着回火温度的提高贝氏体/马氏体和临界铁素体逐渐分解成小尺寸晶粒,而残余奥氏体的体积分数逐渐增加;屈服强度由787 MPa降低到716 MPa,塑性和低温韧性明显增强,断后伸长率由20.30%增至29.24%,-40℃下的冲击功由77 J提升至150 J。残余奥氏体体积分数的增加引起裂纹扩展功增大,是低温韧性提高的主要原因。贝氏体/马氏体的分解和残余奥氏体的生成,引起组织细化、晶粒内低KAM值位错的比例逐渐提高和小角度晶界峰值的频率增大,使材料的塑性和韧性显著提高。     

Tensile fracture behaviour of microstructurally engineered Cu bearing high strength low alloy steel

Abstract

An attempt has been made to highlight the influence of precipitation and microstructural constituents on tensile fracture behaviour in Cu bearing HSLA 100 steel. Variations in the microconstituents have been incorporated in the steel by engineering the microstructures through thermal treatments consisting of solutionising, water quenching and aging at various temperatures. The microstructure in quenched condition consists of mainly lath martensite, bainite and acicular ferrite besides little amount of retained austenite, carbides and carbonitrides. Aging up to 500°C facilitated fine coherent ?-Cu precipitation that lost its coherency at >550°C. Simultaneously, recovery and recrystallisation of martensite and acicular ferrite occurred at higher temperatures. The formation of new martensite islands occurred on aging at >650°C. Carbides, carbonitrides and retained austenite remained essentially unchanged. Tensile tests were conducted at a slow strain rate to study the tensile fracture behaviour of the steel. Microstructural and fractographic evidences indicating that coherent Cu precipitate causes the brittleness in the material in initial stages of aging whereas loss of coherency of Cu precipitate in later stages results in the reappearance of ductility in the material.     

氮对钒微合金钢粗晶热影响区(CGHAZ)的组织和性能的影响

用热模拟方法研究了氮含量对钒微合金钢粗晶热影响区(CGHAZ)的组织和性能的影响。结果表明,氮含量为0.0031%或0.021%时,CGHAZ的韧性较差。氮含量0.0031%时CGHAZ中有少量的Ti(C,N),晶界铁素体(GBF)较少,晶内有大量尺寸较大的侧板条铁素体(FSP),解理裂纹沿FSP的直线扩展使其韧性较差。氮含量0.021%时在CGHAZ中生成了较为粗大的(Ti, V)(C, N)和GBF,解理裂纹沿GBF扩展使其韧性较差。氮含量为0.012%时低温韧性较好,在CGHAZ中生成了大量细小的(Ti, V)(C, N)粒子,且GBF尺寸相对较小,晶内有大量的针状铁素体(AF)。这些因素都有利于阻止裂纹扩展,使其低温韧性显著提高。     

Fracture toughness of nanostructured railway wheels

This paper describes nanostructured railway wheels made of Si–Mn–Mo–V low-carbon steel through an advanced metallurgy process and fabrication technology. The microstructure of the wheels, particularly in the rim portion, is composed of carbide-free bainite that consists of bainitic ferrite laths and retained austenite films along the lath boundaries. The thickness of the laths and films is in nanometer scale. For comparison, traditional pearlite–ferrite wheel steels are also investigated. Test results show that carbide-free bainite steel is superior to pearlite–ferrite steel not only in yield strength but also in fracture toughness. Theoretical explanation of these phenomena is also elucidated.     

Microstructural evolution and tensile behaviour of medium carbon microalloyed steel processed through two thermomechanical routes

Abstract

A multiphase microstructure was obtained in a medium carbon microalloyed steel using two step cooling (TSC) from a lower than usual finish forging/rolling temperature (800–850°C). A low temperature anneal was then used to optimise the tensile properties. A multiphase microstructure (ferrite–bainite–martensite) resulted from forging as well as rolling. These were characterised using optical and scanning and transmission electron microscopy. X-ray diffraction, transmission electron microscopy and hardness measurements were used for phase identification. Tensile properties and work hardening curves were obtained for both the forged and the rolled multiphase variants. A Jaoul–Crussard (J–C) analysis was carried out on the tensile data to understand the basic mode of deformation behaviour. Rolling followed by the TSC process produced a uniform microstructure with a very fine grain boundary allotriomorphic ferrite, in contrast to the forged variety, which contained in addition coarse idiomorphic ferrite. The volume fraction of ferrite and its contiguity ratio in the rolled microstructure were greater than in the forged grade. The rolled microstructure exhibited a better combination of strength and toughness than that of the forged material. The rolled steel work hardened more than the forged variety owing to its fine, uniform (bainite–martensite and ferrite) microstructure. Retained austenite present in these steels underwent a strain induced transformation to martensite during tensile deformation. The J–C analysis of the work hardening rates revealed typical three stage behaviour in both varieties during tensile deformation.     

Microstructure and mechanical properties of C–Si–Mn(–Nb) TRIP steels after simulated thermomechanical processing

Abstract

Continuous and discontinuous cooling tests were performed using a quench deformation dilatometer to develop a comprehensive understanding of the structural and kinetic aspects of the bainite transformation in low carbon TRIP (transformation induced plasticity) steels as a function of thermomechanical processing and composition. Deformation in the unrecrystallised austenite region refined the ferrite grain size and increased the ferrite and bainite transformation temperatures for cooling rates from 10 to 90 K s^-1. The influence of niobium on the transformation kinetics was also investigated. Niobium increases the ferrite start transformation temperature, refines the ferrite microstructure, and stimulates the formation of acicular ferrite. The effect of the bainite isothermal transformation temperature on the final microstructure of steels with and without a small addition of niobium was studied. Niobium promotes the formation of stable retained austenite, which influences the mechanical properties of TRIP steels. The optimum mechanical properties were obtained after isothermal holding at 400°C in the niobium steel containing the maximum volume fraction of retained austenite with acicular ferrite as the predominant second phase.     

Cu对9Ni钢强度和低温韧性的影响

研究了Cu含量(质量分数)对9Ni钢强度和低温韧性的影响,并结合显微组织观察和精细结构分析了含铜9Ni钢的强韧化机理.结果表明,经过淬火+两相区淬火+回火(QLT)处理,Cu含量由0提高到1.5%,9Ni钢的室温屈服强度和抗拉强度分别提高约150和105 MPa;随着Cu含量的提高-196℃低温冲击功呈现先增加后降低的趋势,Cu含量为1.0%时达到最高值157 J,而所有含铜9Ni钢的冲击功均保持在较高的水平。随着Cu含量的增加,钢中二次回火马氏体增加而铁素体减少;颗粒或短杆棒状Cu析出物在基体上析出,组织强化与析出强化共同使钢的强度提高。同时,Cu的加入提高了二次回火马氏体板条边界上的逆转奥氏体含量,并富集于逆转奥氏体中提高其稳定性,从而提高了钢的低温韧性。     

正火型V-N-Ti和Nb-V-Ti海工钢焊接粗晶热影响组织和韧性研究

用焊接热模拟方法研究了V-N-Ti和Nb-V-Ti微合金化正火型海工钢模拟粗晶热影响区(CGHAZ)组织和韧性的变化规律。结果表明,组织的不同使V-N-Ti设计正火型海工钢的模拟CGHAZ韧性比Nb-V-Ti钢的好。对于V-N-Ti钢,较高的N含量提高了富Ti(Ti, V)(C, N)粒子析出温度和铁素体形核能力,使模拟CGHAZ原始奥氏体晶粒和(取向差角为15°)晶粒细化,并生成能阻止或使解理裂纹的偏转细小多边形铁素体,因此具有良好的低温韧性。而Nb-V-Ti钢模拟CGHAZ原奥氏体晶界上的链状M-A、粗大的原始奥氏体晶粒和有效晶粒尺寸,是模拟CGHAZ韧性差的原因。     

X90钢级管线钢预精焊接头的性能研究

为综合研究X90管线钢的焊接性,选用国内某钢厂轧制的X90管线钢卷板,利用预精焊工艺制备试验钢管4根,采用金相分析、扫描电镜(SEM)断口分析、夏比V型缺口冲击试验、拉伸、弯曲、硬度等试验,研究了焊接接头各个区域的组织和性能.试验结果表明:内外焊缝区组织均为针状铁素体,热影响区(HAZ)粗晶区晶粒粗化严重,主要组织为粒状贝氏体和贝氏体铁素体,在原奥氏体晶界和贝氏体板条内部存在块状或条状的(马氏体-奥氏体)M-A组元;HAZ冲击功离散性较大,出现了单值较低(45 J)的试样,SEM断口分析呈现典型的解理断裂特征;焊接接头抗拉强度805~815 MPa,断裂位置均在HAZ;焊接接头反弯试样易在HAZ出现裂纹和脆断现象;HAZ硬度在220~250 HV之间,较母材下降30 HV左右.HAZ是X90预精焊钢管焊接接头的薄弱环节,为提高X90管线钢的焊接稳定性,应重点研究精焊内外热循环双热影响亚区的组织转变和脆化机理.     

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.     

Multiphase microstructure formation and its effect on fracture behavior of medium carbon high silicon high strength steel

This work investigated the evolution of multiphase microstructure and impact fracture behavior of medium carbon high silicon high strength steel subjected to the austempering treatment at 240,360,and 400 ℃.The results show that martensite,bainite,and retained austenite (RA) are the main microstructural phases.The austempering treatments at 360 and 400 ℃ caused the formation of carbon-poor ferrite in the matrix,and the transformation of ultrafine bainite into coarse lath bainite and granular bainite,respectively.Thick filmy RA was distributed between bainite laths.The polygonal martensiteaustenite islands and blocky RA formed along the grain boundaries.The average carbon concentration in the matrix decreased with the temperature increase,while the impact toughness initially increased and then dropped with temperature.The quasi-cleavage brittle fracture dominated the impact fracture mechanism of the sample austempered at 240 ℃ by forming tearing surfaces and tearing steps.The microcracks disappeared in the RA on the prior austenite grain boundaries.On the other side,the fracture surface of the sample austempered at 360 ℃ exhibited ductile fracture with deep dimples and brittle fracture with cleavage river patterns.The polygonal martensite-austenite islands or blocky RA constrained the microcracks.After austempered at 400 ℃,the brittle fracture was dominant,showing river patterns,and the microcracks propagated through the granular bainite without any resistance.