Microscopic analysis of Fe–Cr alloy produced by single roll strip casting

Abstract

In the present investigation, the microstructures and mechanical properties of Fe–Cr alloy prepared by single roll strip casting were studied. Optical microstructure showed subgrain boundaries inside large grains. Cracks were observed along the grain boundary. Scanning electron microscopy, X-ray diffraction study confirms the formation of chromium carbide at the grain boundary in the case of as cast alloy. Electron backscatter diffraction showed preferred orientation of grains in the as cast alloy. Carbides and undesired phases were not observed in heat treated alloy. Pores present in as cast samples expand after heat treatment process. Mechanical properties, like tensile strength, yield strength, elongation and hardness, of Fe–Cr single roll strip casting alloys were improved after heat treatment.     

Effect of boron concentration on the solidification microstructure and properties of Fe‐Cr‐B alloy

In this article, the effect of boron concentration (B = 0, 0.4, 0.8, 1.4 and 2.0 respectively) on the solidification microstructure and properties of Fe‐Cr‐B alloy containing 0.35% C, 10–12% Cr, 0.5–0.8% Si, and 0.7–1.0% Mn was studied by means of the optical microscope (OM), the scanning electron microscope (SEM), X‐ray diffraction (XRD), Rockwell hardness and Vickers hardness tester. The results indicate that the microstructure of boron‐free sample consists of the martensite, and its hardness reaches 59.2 HRC. When a small amount of boron element was added, the eutectic phase of network structure generated along the grain boundary. The amount of eutectic phase increases when the boron concentration increases. Moreover, the eutectic phase is the mixture of boride and boron carbide. The boride is Fe₂B and the boron carbide is (Cr, Fe)₇(C, B)₃. Compared with boron‐free sample, the Rockwell hardness of the samples with different boron concentrations are all higher, above 62 HRC, and the hardness grow up with the increase of boron concentration. When the boron concentration reaches to 1.4%, the Rockwell hardness of the alloy is 65.7 HRC, which is the highest in this study. When the boron concentration rises to 2.0%, the hardness has no obvious change.     

热丝TIG堆焊Inconel 625层组织成形研究

采用热丝钨极惰性气体保护焊,用ENiCrMo-3型焊丝在Q235B钢管内表面进行堆焊试验。利用金相显微镜、扫描电镜、X射线衍射仪对堆焊层微观组织进行观察和元素成分分析,利用显微硬度计测定试样硬度分布。结果表明,堆焊层组织主要为γ-Ni单相奥氏体,在晶界处有碳化物析出;堆焊层组织自熔合线至顶部的基本形态依次为平面晶、胞晶、树枝晶、等轴晶及顶部横向柱状晶;熔合线附近有少量魏氏体组织形成;试样硬度自基材至堆焊层方向呈递增趋势;上层组织在晶界位置贫Cr较下层严重,从金相看其晶粒受到的侵蚀作用更明显。     

Suppression of chromium depletion and sensitization in austenitic stainless steel by surface mechanical attrition treatment

The effects of surface nanocrystallization via surface mechanical attrition treatment (SMAT) on degree of sensitization (DOS) of an austenitic stainless steel were investigated by means of double loop electrochemical potentiokinetic reactivation (DLEPR) test. The treated sample with grain size about 10 nm showed very low degree of sensitization value which can be considered as the non-sensitized material. This is mainly due to the formation of twin boundaries in the microstructure of the SMATed sample which weren't susceptible to carbide precipitation because of their regular and coherent atomic structure and extreme low grain boundary energy as compared with those of other grain boundaries.     

Effect of heat treatment on microstructure and property of Fe‐Cr‐B alloy

In this article, the effect of heat treatment in different quenching temperature on microstructure and hardness of Fe‐Cr‐B alloy was studied, by contrast with boron‐free Fe‐Cr alloy. The results indicated that microstructure of boron‐free Fe‐Cr alloy consisted of the martensite and a few (Cr, Fe)₇C₃ type carbide. The microstructures had no obvious change with the increase of quenching temperature, but its hardness increased from 51.5 HRC to 60.8 HRC. When boron element was added into the Fe‐Cr alloy, the netlike eutectic structure began to break and spheroidizing after quenching, in which the borocarbide turned into spherical groups and network Fe₂B phase was broken. Moreover, the portion of martensite increased, and the amount of secondary carbide decreased, and the size of secondary carbide began to largen after quenching. When the quenching temperature reached 1100°C, secondary carbide particles dissolved in the matrix wholly. The hardness of Fe‐Cr‐B alloy increased with the increase of quenching temperature below 1050°C. The hardness of sample containing 2.0% B and quenching at 1050°C reached 66.7 HRC. The hardness of Fe‐Cr‐B alloy had no obvious change when quenching temperature continued to increase. After tempered at 200°C, the microstructure of Fe‐Cr‐B alloy had no significant change and its hardness had slight decrease. The hardness of sample containing 2.0% B tempered at 200°C reached 63.9 HRC.     

P92钢625℃持久实验过程中试件特征部位相参量的变化EI北大核心CSCD

采用扫描电镜背散射电子成像(SEM-BSE)、X射线能谱(EDS)、电子背散射衍射(EBSD)、透射电镜(TEM)衍衬像和复相分离技术(MPST)分别研究P92钢在625℃持久实验条件(110~180 MPa)下试件特征部位(断口附近和夹持端)相参量与力学性能的变化。结果表明:在相同持久时间下,与可视为无应力作用的试件夹持端相比,应力集中的断口附近M23C6相和Laves相析出量较大,M23C6相粗化且马氏体板条变宽,基体中Cr,W元素的浓度和小角度晶界的比例较小,马氏体向铁素体转化程度较大,硬度下降。由此可见,无应力时效过程中试样所发生的显微组织演化及其所对应的性能不能表征相同时间条件下有应力部位的真实情况。     

Effects of vanadium additions on microstructure and hardness of hypereutectoid pearlitic steels

An investigation has been carried out on the effects of vanadium additions on the microstructure and hardness values of hyper-eutectoid pearlitic steels with carbon contents up to 1.15 wt.%, for a range of isothermal transformation conditions. As in the case of eutectoid steels with vanadium additions, the predominant carbide species at the grain boundaries is cementite, rather than vanadium carbide, even at the higher vanadium levels (0.2 wt.%). However, the extent of the pro-eutectoid grain boundary cementite network is progressively reduced as the vanadium content is increased, in spite of the hypereutectoid composition. According to SEM and TEM investigations, the morphology of the grain boundary carbide is less continuous in the higher vanadium alloys, and consequently less deleterious to mechanical properties. The microstructural investigation indicates that the cementite particles form independently at many different places on the grain boundary, consistent with vanadium addition increasing the driving force for nucleation. Vanadium additions also effectively decrease the austenite grain sizes. An increment in hardness values has been achieved because of vanadium additions, which has potential to improve other mechanical properties in hypereutectoid pearlitic steels, because of these modifications in microstructure.     

Microstructure and Properties of Cu-Cr-Zr Alloy after Rapidly Solidified Aging and Solid Solution Aging

The structure and properties of Cu-Cr-Zr alloy were studied after rapidly solidified aging and solid solution aging. At the early stage of aging (500℃for 15 min), the hardness and the conductivity of the alloy rapidly solidified are 143 HV and 72% IACS, respectively. Under the same aging condition, the hardness and electrical conductivity of the alloy solid solution treated can reach 86 HV and 47% IACS, respectively. The microstructure was analyzed, and the grain size after rapid solidification is much smaller than that after solid solution treatment. By rapidly solidified aging the fine precipitates distribute inside the grains and along the grain boundary, while by solid solution aging there are large Cr particles along the grain boundary.     

航空齿轮钢16Cr3NiWMoVNbE的真空低压渗碳

对航空齿轮钢16Cr3NiWMoVNbE进行真空低压渗碳热处理,研究了真空渗碳、淬火、冰冷处理以及回火工艺对材料的组织和性能的影响。结果表明：实验钢经渗碳淬火处理后,从表面到心部的组织可分为碳化物区、碳化物与针状马氏体混合区、针状马氏体区和心部板条马氏体区。在碳化物区的晶界有大量的块状Cr碳化物析出,在析出位置Ni元素较少。在针状马氏体和板条马氏体基体中细小的析出物为Nb、V、Mo微合金元素的碳化物。从渗碳钢表面到心部,随着碳浓度的降低硬度曲线呈现先升高后降低的趋势,渗层深度为0.95 mm。冰冷处理使残余奥氏体进一步转化为马氏体,使实验钢的硬度大幅度提高。     

微束等离子多层熔覆NiCrBSi的试验研究

为了在降低熔覆层裂纹敏感性的同时获得低稀释率熔覆层,用微束等离子弧热源在低碳钢圆管表面熔覆12层NiCrBSi,成形后利用扫描电镜(SEM)、能谱仪(EDS)和X射线衍射仪(XRD)对多层熔覆的熔覆层的微观组织、元素分布及相组成进行了分析.结果表明,采用微束等离子弧熔覆得到的多层熔覆主要由γ-Ni,Ni3B,Cr21.34Fe1.66C6和Cr2B3相组成,其中的强化相弥散分布;无气孔和裂纹,层与层间结合致密,分层未见明显;焊缝组织均匀,晶粒细小,稀释率低,热影响区很小.     

Effect of chromium carbides on electrical resistivity of nickel based alloys by low temperature aging

Abstract

To clarify the physical and mechanical properties of nickel based alloys (Alloy 600 and 690), the effects of chromium carbides on the electrical resistivity and hardness were examined after low temperature aging following cold working. In Alloy 600, chromium carbide precipitates were dispersed at grain boundaries and within the grains and the electrical resistivity and hardness increased markedly after short time aging. In Alloy 690, chromium carbides precipitated mainly at grain boundaries and the increases in electrical resistivity and hardness were relatively small, even after long aging times. The most probable mechanism causing the large increases in electrical resistivity in Alloy 600 is the segregation of solute atoms, such as carbon and nitrogen atoms, to dislocation loops around chromium carbides introduced by cold working.

MST/939     

On the phenomenon of grain-boundary hardening in iron

Microhardness measurements on a fully recrystallized Fe-0.02 at. % W alloy show the existence of a region extending some 40 μm on either side of the grain boundaries, where the hardness is up to 35% higher than that of the grain. The magnitude of the hardening is partly controlled by the effective resolution of the hardness test because of the steeply increasing hardness near the boundaries. In addition, for small grain sizes, overlapping hardening profiles from the boundaries may affect the grain-interior hardness. The position of the boundary, with respect to the metallographic surface, significantly affects the width of the hardening; this width is increased on that side of the boundary where its inclination with respect to the surface of the specimen is increased. The results show that the excess, hardening at the boundary is a real hardening effect comparable to other hardening effects and that we should expect a marked inhomogeneity of bulk mechanical properties even for a relatively pure alloy.     

Structure and hardness of laser-processed Fe-0.2%C-5%Cr and Fe-0.2%C-10%Cr alloys

The effects of high quench rates achieved through a laser surface-alloying process on the martensitic transformations of Fe-0.2%C-Cr steels (up to 10wt% chromium) were investigated. The microstructural variables: martensitic morphology and its substructure; amount of retained austenite; and carbide precipitation were characterized by optical metallography and thin foil transmission electron microscopy (TEM). The microstructures exhibited were fully lath martensitic type, the substructure of which consisted of dislocations. The morphology and substructure of martensite were influenced neither by the chromium content of the alloy nor by the laser parameters (or melt depth) employed. Thin films of retained austenite were observed at packet and lath boundaries of martensite and at prior austenite grain boundaries. The amount of retained austenite was found to decrease with decrease in melt depth. TEM studies also revealed the presence of more or less continuous cementite films both at the lath boundaries and within the laths. Microhardness measurements had shown that the hardness increased with increase in chromium content of the alloy but appeared to be independent of melt depth.     

Inconel 600合金中不同类型晶界处铬的浓度

应用TEM、EDS和EBSD等技术研究了Inconel 600合金在715℃时效过程中不同类型晶界处碳化物的结构、形貌和晶界附近Cr浓度的分布。结果表明,不同类型晶界处碳化物的结构和形貌有较大的不同：在Σ3_c晶界析出的碳化物很少,在Σ3_i晶界析出不规则形状的M₂₃C₆碳化物,在Σ9晶界析出较大的M₂₃C₆碳化物颗粒,在Σ27晶界随机析出粗大的M₇C₃碳化物颗粒。富Cr碳化物在晶界的析出使晶界附近贫铬,在相同的时效条件下晶界的Σ值越高其附近贫铬越严重。随着时效时间的延长各晶界附近贫铬区的宽度不同程度地增大,时效15 h贫铬区的深度最大,时效50 h后深度不同程度地减小。     

Effect of long-term service exposure on microstructure and mechanical properties of Alloy 617

The present work was carried out to investigate the effect of long-term service exposure on microstructure and mechanical properties of a gas turbine hot gas path component, made of Alloy 617. The results showed significant service-induced microstructural changes, such as excessive grain boundary Cr-rich M₂₃C₆ carbides formation and some oxidation features in the exposed material in compare with the solution-annealed material. Also it was found that the yield strength and hardness of the alloy have increased while the ductility of the alloy has decreased. In the similar test conditions, the stress-rupture life of the exposed alloy decreased considerably compared to the solution-annealed sample, which could be attributed to the microstructural degradation, especially formation of continuous M₂₃C₆ carbides on grain boundaries.     

Improvement of toughness of high chromium white cast iron: duplex ferritic-austenitic matrix

It is attempted to enhance the impact toughness of industrially used high chromium white cast iron (WCI) without sacrificing wear resistance. The microstructure is engineered by cyclic annealing to obtain features such as duplex grain matrix, where austenite envelops ferrite grain, refined M₇C₃ carbide. The newly cast and heat-treated alloy shows remarkable impact toughness i.e. 13J with improved wear resistance. The fracture micro-mechanism is studied through extensive scanning electron microscopy and it is ascertained that enhanced impact toughness results from crack arrest at duplex grain boundaries. A few other toughness enhancing features are also discussed. The results are compared with standard ASTM grade Class-III high chromium WCI and are found to be encouraging.     

成形角度对选区激光熔化4Cr5MoSiV1钢组织和性能的影响EI北大核心CSCD

选区激光熔化(SLM)成形4Cr5MoSiV1钢具有良好的强/硬度以及耐磨性是提高其使用寿命的重要保证,为优化SLM成形4Cr5MoSiV1钢的组织和性能,研究不同成形角度下4Cr5MoSiV1钢试样的显微组织、显微硬度、拉伸性能和耐磨性。结果表明:随成形角度的增加,试样熔道间的热量累积程度降低,晶粒尺寸减小,细晶强化作用增强,故试样的显微硬度升高。随成形角度的增加,拉伸试样的滑移搭接面数量增多,滑移程度增加,且熔道边界处分正应力值降低,故试样的抗拉强度和断裂伸长率均升高。磨损试样的磨损机制以黏着磨损和氧化磨损为主,且随成形角度增加,试样的耐磨性升高。同一成形角度下,试样底层表面经多次热量累积后,其细晶强化和固溶强化作用减弱,显微硬度和耐磨性均降低。SLM成形4Cr5MoSiV1钢试样的显微硬度、耐磨性和拉伸性能呈正相关,45°成形角度下试样的力学性能最高,抗拉强度最高为1576.5 MPa,断裂伸长率最高为17%,顶层表面的显微硬度最高为608.4HV,顶层表面的磨损率最低为4.95×10^(-9)kg·N^(-1)·m^(-1)。     

A Comparative Study on Crystal Structure and Magnetic Properties of Fe-Mn-Si and Fe-Mn-Si-Cr Alloys

In this study, the crystal structure and magnetic properties of Fe-30Mn-6Si (wt.%) and Fe-30Mn-6Si-5Cr (wt.%) alloys were compared by using X-Ray Diffraction (XRD) and Physical Properties Measurement System (PPMS) measurements. Detailed analysis of diffractograms at room temperature demonstrates that the Cr-free sample contains austenite and martensite phases, but for Cr-added sample the martensite phase disappears. According to micro hardness measurements, the presence of chromium decreased the hardness of the alloy. The magnetic saturation values at room temperature were measured as 11.32 emu/g for Fe-30Mn-6Si (wt.%) alloy and 18.34 emu/g for Fe-30Mn-6Si-5Cr (wt.%) alloy. The addition of Cr increased the magnetic saturation value of FeMnSi alloy while for both systems the hysteresis loop was quite narrow. As a result, both alloys exhibited soft magnetic characteristic.     

烧结Cr15高铬铸铁组织与性能的研究

为研发耐磨性能优良、成本相对低廉的高铬铸铁,本文分别以亚共晶、过共晶的水雾化Cr15高铬铸铁粉末为原料,采用超固相线液相烧结工艺制备了烧结高铬铸铁(SHCCI),并对其显微组织、力学性能和冲击磨粒磨损工况下的耐磨性能进行对比研究。结果表明,烧结高铬铸铁主要由M7C3碳化物、马氏体和奥氏体组成;在亚共晶烧结高铬铸铁中,通过电解腐蚀萃取的M7C3碳化物三维形貌呈珊瑚状,沿晶界均匀分布,材料抗冲击耐磨性能优良;在过共晶烧结高铬铸铁中,优先形成的初生碳化物可能成为共晶碳化物的生长基底,形成核-壳结构的M7C3碳化物,沿晶界相互连接呈网状,严重割裂基体。亚共晶、过共晶烧结高铬铸铁的力学性能分别为:硬度HRC63.9、HRC64.3,冲击韧性7.92、3.04 J/cm^2,抗弯强度2112.65、1624.87 MPa。     

Fe—Mn—Cr合金晶间腐蚀电子理论研究

为了从电子层面揭示Fe—Mn—Cr合金晶间腐蚀的物理本质,采用递归法计算了合金的原子埋置能、格位能、亲和能等电子结构参数,探索合金晶间腐蚀机理。研究表明：Cr在晶内稳定性很低,Cr在晶界和表面稳定性较高,基体中的Cr首先扩散到晶界,并通过晶界扩散至合金的表面。Cr元素减小费米能级差,抑制合金的晶间腐蚀。O-Cr间的亲和能为负数,表明氧与Cr之间有相互作用,生成Cr的氧化物。当氧化膜达到一定厚度可起到保护合金的作用。碳与Cr的亲和能也为负数,且其数值比氧与Cr间的亲和能更负。合金中碳优先与Cr形成化合物,在晶界析出,造成晶界贫Cr,使合金晶间腐蚀加重。