Effect of tempering temperature on Z-phase formation and creep strength in 9Cr–1Mo–V–Nb–N steel

The effect of the tempering temperature on Z-phase formation and creep strength in 9Cr–1Mo–V–Nb–N steel was examined with particular attention to the precipitation sequence of MX, M₂X, and Z-phase during creep exposure. Tempering at a lower temperature provided a high dislocation density and a fine lath structure. Tempering at 953, 1003, and 1038 K provided [M₂₃C₆, M₂X, NbX], [M₂₃C₆, M₂X, NbX, VX], and [M₂₃C₆, NbX, VX] phases, respectively. The creep strength of steel tempered at 953 K was the highest among the steels studied, even in the long term. No large decrease in creep strength was observed in steel tempered at 953 K. The Z-phase was observed after long-term creep in steel tempered at 1003 or 1038 K. In steel tempered at 953 K, a VX rather than a Z-phase formed during creep, and this was accompanied by consumption of M₂X. Retardation of the Z-phase formation can retard the creep strength degradation in steel tempered at 953 K.     

Electron microscopy and hardness study of a semi-solid processed 27 wt%Cr cast iron

A semi-solid processed 27 wt%Cr cast iron was studied by electron microscopy and its microstructure was related to the hardness. In the as-cast condition, the primary proeutectic austenite was round in shape while the eutectic M₇C₃ carbide was found as radiating clusters mixed with directional clusters. Growth in the [0 0 1]M₇C₃ with planar faces of {0 2 0}M₇C₃ and was usually observed with an encapsulated core of austenite. Destabilisation heat treatment followed by air cooling led to a precipitation of secondary M₂₃C₆ carbide and a transformation of the primary austenite to martensite in the semi-solid processed iron. Precipitation behaviour is comparable to that observed in the destabilisation of conventional cast iron. However, the nucleation of secondary M₂₃C₆ carbide on the eutectic M₇C₃ carbide was observed for the first time. Tempering after destabilisation led to further precipitation of carbide within the tempered martensite in the eutectic structure. The maximum hardness was obtained after destabilisation and tempering heat treatment due to the precipitation of secondary carbides within the martensite matrix and a possible reduction in the retained austenite.     

Microstructural transformations of heat affected zones in duplex steel welded joints

The influence of the welding thermal conditions exemplified by heat input and heat treatment after welding on the structure of the heat affected zone (HAZ) UNS S31803 has been analysed. The post weld treatment was used to create the precisely defined thermal conditions for the decomposition of primary phases in the HAZ, by a multi-layer welding thermal cycle stimulation. Detailed analyses of the microstructure and chemical composition of the phases in the different post welded conditions were performed by scanning electron microscopy (SEM) combined with energy dispersive spectrometry (EDS) and transmission electron microscopy (TEM). Three types of secondary precipitates have been observed: secondary austenite (γ₂), carbides: M₂₃C₆ and M₇C₃. The dependence of the secondary austenite volume fraction and morphology in the HAZ on thermal cycle have been interpreted. The eutectoid decomposition of the primary phases in the analysed thermal conditions was confirmed.     

Microstructure characteristics of Ni-based WC composite coatings by laser induction hybrid rapid cladding

To avoid low cladding rate and cracks of cladding layer, laser induction hybrid rapid cladding (LIHRC) has been put forward in the paper. The microstructure characteristics of Ni-based WC composite coatings at the different laser scanning speed were investigated. For low laser scanning speed, the growth of γ-nickel was characterized by coarse columnar dendrites and eutectics, blocky W₂C + Fe₃W₃C carbides, and bar-like (W, Cr, Ni)₂₃C₆ carbides were formed. With increasing laser scanning speed, the growth of γ-nickel presented the fine dendrites and eutectics, the only blocky mixed carbides were precipitated and identified as W₂C + FeW₃C + W₆C_2.54 carbides. With further increasing laser scanning speed, the growth of γ-nickel was characterized by cellular crystals and eutectics, the only blocky carbides were identified as W₂C + W₆C_2.54. Moreover, experimental results showed that the efficiency of LIHRC was increased much four times higher than that of laser cladding without preheating, ceramic–metal composite coatings detected were free of cracks and had a good metallurgical bonding with substrate.     

The effect of niobium on the microstructure of ferritic stainless steel

The effect of different amounts of Nb and of homogenization on the ferritic stainless steels containing 17–18 wt.% Cr was investigated with scanning electron microscopy (SEM), optical microscopy, energy-dispersive spectrometry (EDS), X-ray diffraction (XRD) and differential thermal analysis (DTA). It was observed that M₂₃C₆, NbC and sigma phase formed in these steels. In addition, the formation of Nb₂C was observed in the sample containing 3.0 wt.% Nb. While the amount of Nb increased from 0.5 to 3.0 wt.% Nb, the microhardness of the matrix and the amount of M₂₃C₆ decreased and the toughness of the samples increased. After homogenization, the increase in the toughness of the samples containing 1.5–3.0 wt.%Nb was considerable and impressive.     

Recrystallization in a directionally solidified cobalt-base superalloy

The recrystallization behavior in a range of annealing temperature from 1020 to 1280 °C in a directionally solidified cobalt-base superalloy was studied. Local recrystallization first appeared at 1040 °C. The recrystallized volume increased rapidly as increasing the annealing temperature. Pinning effect of all carbides (M₂₃C₆, M₇C₃ and MC) was observed and large amount of twin formed at low annealing temperature. The size of the recrystallized grains increased significantly at high annealing temperatures accompanied with the sharp decrease of twin. The effect of annealing temperature and the role of carbide and twin on the development of the recrystallization were discussed.     

Microstructure of Fe-Cr-C Hardfacing Alloys with Additions of Nb, Ti and, B

The abrasive wear of machine parts and tools used in the mining, earth moving, and transporting of mineral materials can be lowered by filler wire welding of hardfacing alloys. In this paper, the microstructures of Fe-Cr-C and Fe-Cr-C-Nb/Ti hardfacing alloys and deposits and those of newly developed Fe-Cr-C-B and Fe-Ti-Cr-C-B ones are described. They show up to 85 vol.% of primarily solidified coarse hard phases; i.e., Carbides of MC-, M₇C₃-, M₃C-type and Borides of MB₂-, M₃B₂-, M₂B-, M₃B-, M₂₃B₆-type, which are embedded in a hard eutectic. This itself consists of eutectic hard phases and a martensitic or austenitic metal matrix. The newly developed Fe-Cr-C-B alloys reach hardness values of up to 1200 HV and are harder than all purchased ones. The primary solidification of the MB₂-type phase of titanium requires such high amounts of titanium and boron that these alloys are not practical for manufacture as commercial filler wires.     

The relationships between wear behavior and thermal conductivity of CuSn/M7C3–M23C6 composites at ambient and elevated temperatures

The effect of FeCr (M₇C₃–M₂₃C₆) particles on the wear resistance of a CuSn alloy was investigated under 125 N load, and 300–475 K temperature interval. Sliding tests were performed to investigate the wear behavior of FeCr_p-reinforced CuSn metal–matrix composites (MMCs) against DIN 5401 in a block-on-ring apparatus. The CuSn/FeCr_p MMCs, which were prepared by addition of 5, 10, 15 and 20 vol.% of FeCr_p, were produced by powder metallurgy and the size of the particles was taken as 16 μm. The powders were uniaxially cold compacted by increasing pressure up to 250 Mpa. The dry sliding wear tests were carried out in an incremental manner, i.e. 300 m per increment and 3500 m total sliding length. The wear-test results were used for investigation of the relationship between weight loss, microstructure, surface hardness, friction coefficient, particle content and thermal conductivity. Finally, it was observed that FeCr_p reinforcement is beneficial in increasing the wear resistance of CuSn MMCs. FeCr particles in MMCs also tend to reduce the extent of plastic deformation in the subsurface region of the matrix, thereby delaying the nucleation and propagation of subsurface microcracks     

Microstructural characterization of high-carbon ferrochromium

Light optical and scanning electron microscopy techniques were used for high-carbon ferrochromium microstructural analysis. Different microstructures were observed for industrially and laboratory-produced ferroalloys. Primary carbides of M₇C₃ with chromium ferrite were found in the industrially produced, slowly solidified, and cooled ferroalloy, while primary M₇C₃ carbides accompanied a eutectic mixture of M₇C₃ carbides and chromium ferrite in the laboratory-melted and in the water-solidified and water-cooled materials. Different microstructural arrangements are directly related to the friability properties of this material, which characterizes its resistance to abrasion on handling and impact. In ferrochromium upgraded by carbon content reduction, the eutectic M₇C₃ hexagonal carbides are partly replaced by M₂₃C₆ dendritic carbides. The presence of dendritic carbides in the ferrochromium eutectic microstructure can be interpreted as a proof of a lower carbon content, raising the commercial value of the ferroalloy. The hexagonal M₇C₃ carbides exhibited a central hollow along the longitudinal axis, and on metallographic samples they looked like screw nuts. A model of the solidification mechanism for such crystals is proposed.     

热作模具钢DM的高温稳定性和热疲劳性能

研究了热作模具钢DM的高温稳定性和热疲劳性能。结果表明,DM钢在620℃热稳保温过程中马氏体板条内的薄片状M₃C型碳化物逐渐向条块状M₇C₃型碳化物转变,在板条的边界生成M₇C₃、M₂₃C₆型碳化物。DM钢的短循环周次热疲劳性能受控于位错重排和湮灭,长循环周次热疲劳性能受控于碳化物的粗化程度。DM钢中M₃C、M₇C₃、M₆C型碳化物的生成自由能分别为27765.5 J/mol、3841.5 J/mol、-7138.1 J/mol,表明在热稳保温与热疲劳试验过程中碳化物的演变机理一致,发生了M3C→M7C3→M6C类型演变。     

TEM study on precipitation and transformation of secondary carbides in 16Cr–1Mo–1Cu white iron subjected to subcritical treatment

High chromium white irons solidify with a substantially austenitic matrix supersaturated with chromium and carbon. The subcritical heat treatment can destabilize the austenite by precipitating chromium-rich secondary carbides and other special carbides. In the as-cast condition the eutectic carbides are (Fe,Cr)₇C₃ and (Fe_4.3Cr_2.5Mo_0.1)C₃. The initial secondary carbide precipitated is (Fe,Cr)₂₃C₆ after heat-treating at 853 K for 10 h. There are MoC, Fe₂MoC and -carbide precipitating, and (Fe,Cr)₂₃C₆ transforms to M₃C after 16 h at 853 K. The -carbide and (Fe,Cr)₂₃C₆ accomplish transformation to M₃C and the matrix changes from martensitic to pearlitic after 22 h at 853 K. Thereby, in the subcritical heat treatment process, the initial secondary carbide precipitated is (Fe,Cr)₂₃C₆, followed by -carbide, MoC and Fe₂MoC. In addition, there are two in situ transformations from (Fe,Cr)₂₃C₆ and -carbide to M₃C carbides.     

Plastic deformation of Fe–Al polycrystals strengthened with Zr-containing Laves phases: I. Microstructure of undeformed materials

The microstructures of several Fe-rich Fe–Al–Zr alloys have been studied as a basis of investigating the mechanical behaviour, which is subject of Part II. The alloys with only low Zr contents show microstructures with a relatively soft matrix and a hard skeleton along the grain boundaries, the latter being residual eutectics containing the matrix phase and the Zr(Fe,Al)₂ Laves phase. Scanning electron microscopy, orientation imaging microscopy as well as transmission electron microscopy and diffraction are used to study the grain sizes, the orientation relationships between the grains and the phases and the crystallography of the Laves phase. With higher Zr contents above about 10 at.%, the matrix is formed by the Zr(Fe,Al)₂ Laves phase.     

Sintering behaviour and microstructure development of T42 powder metallurgy high speed steel under different processing conditions

High speed steel powders (T42 grade) have been uniaxially cold-pressed and subsequently densified through different sintering routes including: supersolidus liquid phase sintering (SLPS) under vacuum and different nitrogen pressures (0.2, 0.9, and 8 bar) and through solid state sintering (SSS) by hot isostatic pressing (HIP). HIP temperatures as low as 850 °C led to near full densification of the material (>98% theoretical density) with average size of M₆C and MC carbides lower than 1 μm and grain size ≈3 μm. Pressureless sintering under different nitrogen pressures (up to 0.39 wt.%N absorption) led to a significant reduction of the optimum sintering temperature (OST) and a pronounced increase in the sintering window (SW) as compared to vacuum sintering. Pressureless sintering under 8 bar N₂ led to a further reduction in OST together with the precipitation of massive eutectic structures. Therefore, the SW was judged to be negligible. The response of the as-sintered materials to the heat treatment is basically determined by the amount of C available in the matrix prior to quenching and the grain size. The highest hardness achievable for the sintering conditions evaluated ranges 700–1100 HV2 after austenitizing at 1100 °C, oil quenching and multitempering at 500–550 °C.     

Characterizations of WC–10Co nanocomposite powders and subsequently sinterhip sintered cemented carbide

Ultrafine WC–Co cemented carbides, combining high hardness and high toughness, are expected to find broad applications. In this study, WC–10Co–0.4VC–0.4Cr₃C₂ (wt.%) nanocomposite powders, whose average grain size was about 30 nm, were fabricated by spray pyrolysis-continuous reduction and carbonization technology. The as-prepared nanocomposite powders were characterized and analyzed by chemical methods, scanning electron microscopy (SEM), transmission electron microscopy (TEM), BET analysis and atomic force microscopy (AFM). Furthermore, “sinterhip” was used in the sintering process, by which ultrafine WC–10Co cemented carbides with an average grain size of 240 nm were prepared. The material exhibited high Rockwell A hardness of HRA 92.8, Vickers hardness HV₁ 1918, and transverse rapture strength (TRS) of 3780 MPa. The homogeneously dispersed grain growth inhibitors such as VC, Cr₃C₂ in nanocomposite powder and the special nonmetal–metal nanocomposite structure of WC–10Co nanocomposite powder played very important roles in obtaining ultrafine WC–10Co cemented carbide with the desired properties and microstructure. There was an abundance of triple junctions in the ultrafine WC–10Co cemented carbide; these triple junctions endowed the sintered specimen with high mechanical properties.     

TiCp/Cr15高铬铸铁复合材料的制备与性能

以TiC_p粉末和水雾化Cr15高铬铸铁粉末为原料,采用粉末冶金液相烧结技术制备TiC_p增强高铬铸铁复合材料。研究了TiC_p含量对高铬铸铁的物相组成、显微组织和力学性能的影响。研究结果表明,全致密的TiC_p增强高铬铸铁基体复合材料的构成相为TiC、M₇C₃型碳化物、马氏体和少量奥氏体;随着TiC_p添加量增大,金属基体逐步呈孤岛状,并在其中析出越来越多的M₇C₃型碳化物,同时TiC_p逐步呈连续网状分布;同时,其硬度稳步提升,而抗弯强度和冲击韧性降低。当TiC_p添加量为20wt%时烧结态复合材料具有最佳综合力学性能。此时硬度为HRC 66.8 ,冲击韧性为6.86 J/cm2,抗弯强度为1 343.10 MPa。当TiC_p添加量为25wt%时硬度达到最大值HRC 67.20 。      

The effect of post weld heat treatment on the properties of 6061 friction stir welded joints

Post weld heat treatment (PWHT) of Friction Stir Welds (FSW) was carried out at solutioninsing temperatures of 520, 540, and 560°C followed by ageing at 175°C or 200°C. It was found that the weld (stir) region exhibited very coarse grains after the PWHT. The hardness was found to be uniform across the weldment after the PWHT. The samples failed after PWHT during root bend test. Heat treatments to reduce the grain size did not reduce the brittleness of the welds. The brittleness was attributed to the presence of precipitate free zones adjacent to the grain boundaries and the equiaxed structure of the weldment microstructure and the failure was due to a ductile intergranular fracture mechanism.     

Microstructural changes inside the lamellar structures of alloy ZA27

The microstructural changes inside the lamellar structure of the cast and aged alloy ZA27 were studied using TEM, XRD and SEM techniques. Using TEM, the network of transitional phase η_m^′ was determined to be of an fcc crystal structure inside the lamellae η phase during ageing at 150 °C. The mechanism of the decomposition of the η phase lamellae can be summarized as follows: η → η_m^′ + η′ →  + η. The adjacent co-existence of the ε phase and the T′ phase inside the phase lamellae confirmed that a four phase transformation,  + ε → T′ + η, had occurred during the prolonged ageing.     

WC-氧化石墨烯/Ni复合熔覆层的制备及形成机制

采用真空熔覆技术制备了WC-氧化石墨烯(GO)/Ni复合熔覆层,运用扫描电镜、能谱仪、X射线衍射仪观察并分析在不同温度下熔覆层内显微形貌的变化与物相组成。结果表明:在ZG45表面制备了组织致密、与基体形成良好冶金熔合的WC-GO/Ni复合熔覆层;熔覆层的微观结构组成从表面至基体依次是约1.5 mm厚的复合层、360 μm左右的过渡层、50 μm左右的扩散熔合层和100 μm左右的扩散影响层,其主要组成相有Cr₇C₃、FeNi₃、WC、Cr₂₃C₆、Ni₃Si、C、Fe₇W₆、γ-Ni固溶体等,FeNi₃、Fe₇W₆分散在冶金熔合带,扩散影响区主要组织为珠光体;复合区的物相尺寸小于界面区的物相尺寸,熔覆层形成过程中复合区的金属颗粒变化先于界面区,凝固时熔化不完全的颗粒表面长出团簇物(Cr₇C₃/Cr₂₃C₆),随着保温长大逐渐变成针状物镶嵌在Ni基固溶体中。      

Influence of Mn doping on microstructure and DC-accelerated aging behaviors of ZnO–V2O5-based varistors

The microstructure, electrical properties, dielectric characteristics, and DC-accelerated aging behavior of the ZnO–V₂O₅–MnO₂ system sintered were investigated for MnO₂ content of 0.0–2.0 mol% by sintering at 900 °C. For all samples, the microstructure of the ZnO–V₂O₅–MnO₂ system consisted of mainly ZnO grain and secondary phase Zn₃(VO₄)₂. The incorporation of MnO₂ to the ZnO–V₂O₅ system was found to restrict the abnormal grain growth of ZnO. The nonlinear properties and stability against DC-accelerated aging stress improved with the increase of MnO₂ content. The ZnO–V₂O₅–MnO₂ system added with MnO₂ content of 2.0 mol% exhibited not only a high nonlinearity, in which the nonlinear coefficient is 27.2 and the leakage current density is 0.17 mA/cm², but also a good stability, in which %ΔE_1 mA = −0.6%, %Δ = −26.1%, and %Δtan δ = +22% for DC-accelerated aging stress of 0.85E_1 mA/85 °C/24 h.     

Elaboration, microstructure and reactivity of Cr3C2 powders of different morphology

Cr₃C₂ powders have been prepared by heat-treatment of metastable chromium oxides of controlled morphology in H₂---CH₄ atmosphere. Starting with these highly reactive oxides allows formation of Cr₃C₂ at 700 °C. The reaction is pseudomorphic and different grain shapes (needles, rods, spheres and polyhedra) have been obtained. The size distribution is narrow and the grain size is generally of the order of a few tens of micrometers, but the “spheres” are in fact made up of aggregates of small platelets about 1.5 μm wide and 0.7 μm thick. The oxidation in air of the carbides was studied by thermal analyses (TGA, DTG and DSC) and was found to proceed in four steps in the 250–700 °C range. The differences observed between the carbides are related to their morphology and texture.