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。     

Microstructural study and wear behavior of ductile iron surface alloyed by Inconel 617

In this research, microstructure and wear behavior of Ni-based alloy is discussed in detail. Using tungsten inert gas welding process, coating of nearly 1–2 mm thickness was deposited on ductile iron. Optical and scanning electron microscopy, as well as X-ray diffraction analysis and electron probe microanalysis were used to characterize the microstructure of the surface alloyed layer. Micro-hardness and wear resistance of the alloyed layer was also studied. Results showed that the microstructure of the alloyed layer consisted of M₂₃C₆ carbides embedded in Ni-rich solid solution dendrites. The partial melted zone (PMZ) had eutectic ledeburit plus martensite microstructure, while the heat affected zone (HAZ) had only a martensite structure. It was also noticed that hardness and wear resistance of the alloyed layer was considerably higher than that of the substrate. Improvement of wear resistance is attributed to the solution strengthening effect of alloying elements and also the presence of hard carbides such as M₂₃C₆. Based on worn surface analysis, the dominant wear mechanisms of alloyed layer were found to be oxidation and delamination.     

Nonequilibrium microstructures and their evolution in a Fe–Cr–W–Ni–C laser clad coating

The laser-solidified microstructural and compositional characterization and phase evolution during tempering at 963 K were investigated using an analytical transmission electron microscope with energy dispersive X-ray analysis. The cladded alloy, a powder mixture of Fe, Cr, W, Ni, and C with a weight ratio of 10:5:1:1:1, was processed with a 3 kW continuous wave CO₂ laser. The processing parameters were 16 mm/s beam scanning speed, 3 mm beam diameter, 2 kW laser power, and 0.3 g/s feed rate. The coating was metallurgically bonded to the substrate, with a maximum thickness of 730 μm, a microhardness of about 860 Hv and a volumetric dilution ratio of about 6%. Microanalyses revealed that the cladded coating possessed the hypoeutectic microstructure comprising the primary dendritic γ-austenite and interdendritic eutectic consisted of γ-austenite and M₇C₃ carbide. The γ-austenite was a non-equilibrium phase with extended solid solution of alloying elements and a great deal of defect structures, i.e. a high density of dislocations, twins, and stacking faults existed in γ phase. During high temperature aging, in situ carbide transformation occurred of M₇C₃ to M₂₃C₆ and M₆C. The precipitation of M₂₃C₆, MC and M₂C carbides from austenite was also observed.     

Effect of carbon content on microstructure and corrosion behavior of hypereutectic Fe–Cr–C claddings

The aim of this study was to examine the influence of carbon content on the microstructures and corrosion characteristics. The results showed that the hypereutectic microstructure comprised primary (Cr,Fe)₇C₃ carbides and the eutectic colonies [γ-Fe + (Cr,Fe)₇C₃]. The amounts of primary (Cr,Fe)₇C₃ carbides increased from 33.81 to 86.14% when carbon content increased from 3.73 to 4.85 wt%. The corrosion resistance of the hypereutectic alloy with 4.85 wt% C was about 20 times higher than that with 3.73 wt% C. The galvanic corrosion occurred in all claddings due to difference of corrosion potential between primary carbide and austenite. The dense distribution of primary carbides could retard the austenitic matrix from selective corrosion. The austenite dissolved the Fe²⁺ ions and formed a Cr₂O₃ film under 3.5% NaCl aqueous solution.     

Improved surface properties of D2 steel by laser surface alloying

The wear and the high-temperature oxidation resistance of the D2 steel (Fe-1.5 C-12 Cr-0.95 Mo-0.9 V-0.3 Mn) were increased by laser surface alloying after coating the surface with SiC or Cr₃C₂ powder. The surface alloys exhibit two microstructures: hypoeutectic and hypereutectic, respectively, all containing iron solid solutions and iron-chromium carbides, (Fe,Cr)₇C₃. The oxidation resistance of these alloys was measured in isothermal and cyclic conditions, and was shown to increase with silicon or chromium additions, particularly due to the formation of a chromia scale with excellent behaviour during thermal shoks. The surface alloy obtained with Cr₃C₂ also has shown a better resistance to wear due to its hypereutectic microstructure.     

Microstructure and properties of Ti–Nb–V–Mo-alloyed high chromium cast iron

The correlations of microstructure, hardness and fracture toughness of high chromium cast iron with the addition of alloys (titanium, vanadium, niobium and molybdenum) were investigated. The results indicated that the as-cast microstructure changed from hypereutectic, eutectic to hypoeutectic with the increase of alloy contents. Mo dissolved in austenite and increased the hardness by solid solution strengthening. TiC and NbC mainly existed in austenite and impeded the austenite dendrite development. V existed in multicomponent systems in forms of V alloy compounds (VCrFe₈ and VCr₂C₂). With the increase of alloy additions, carbides size changed gradually from refinement to coarseness, hardness and impact toughness were increased and then decreased. Compared with the fracture toughness (6 J/cm²) and hardness (50·8HRC) without any alloy addition, the toughness and hardness at 0·60 V–0·60Ti–0·60Nb–0·35Mo (wt%) additions were improved and achieved to 11 J/cm² and 58·9HRC, respectively. The synergistic roles of Ti, Nb, V and Mo influenced the solidification behaviour of alloy. The refinement of microstructure and improvement of carbides morphologies, size and distribution improved the impact toughness.     

Fe–15 wt-%Cr–X wt-%C hardfacing surface layer: wear resistance and its enhanced mechanism with C additive

Abstract

An electrode with different C additives was developed. The microstructure of the hardfacing surface layer was observed by optical microscopy. The phase structure was determined by X-ray diffraction. The hardness and wear resistance of the hardfacing surface layer were measured respectively. The worn-out surface and three-dimensional morphology were observed by field emission scanning electron microscope equipped with energy dispersive X-ray spectrometry. The relation curve between mass fraction of M₇C₃ carbide and temperature was calculated according to thermodynamics software Thermo-Calc. The results show that, with the increase in C additive, the hardfacing surface layer changes from a hypoeutectic structure to a hypereutectic one. Meanwhile, the primary phase changes from austenite to carbide. The hardness and wear resistance of the hardfacing surface layer increase gradually with the increase in C additive, and when the C additive is 25 wt-%, they are the largest. With the increase in C content, the precipitation temperature of M₇C₃ decreases from 1280 to 1260°C, while the maximum amount of M₇C₃ increases instead, which is from 0·154 to 0·313. The reason of the improvement for wear resistance of the hardfacing surface layer is that the carbide initiates and the amount of M₇C₃ increases.     

Microstructure evolution of HP40-Nb alloys during aging under air at 1000 °C

Two as-cast HP 40 alloys provided by different manufacturers were aged at 1000 °C under laboratory air. They had the same as-cast microstructure consisting of austenite dendrites delineated by a network of eutectic Nb-rich MC and Cr-rich M₇C₃ carbides. After aging for several months, they showed similar microstructures in the bulk materials, though M₇C₃ carbides have been replaced by M₂₃C₆ carbides. As expected, a sub-surface zone depleted in chromium has appeared where a tetragonal CrNbC could be identified in both materials. However, the composition of the transition zones between the surface and the bulk materials differed, mainly because one of the materials underwent significant nitrogen pick-up with associated precipitation of M₆(C,N) and M₂(C,N) phases. On the contrary, the other alloy did show only one intermediate zone with a mix of CrNbC, M₂₃C₆ and MC carbides. A full account of the microstructures observed in the aged materials is given.     

Effects of Mo on microstructure of as-cast 28 wt.% Cr–2.6 wt.% C–(0–10) wt.% Mo irons

Microstructures of as-cast 28 wt.% Cr–2.6 wt.% C irons containing (0–10) wt.% Mo with the Cr/C ratio of about 10 were studied and related to hardness. The experimental irons were cast into dry sand molds. Microstructural investigation was performed by light microscopy, X-ray diffractometry, scanning electron microscopy, transmission electron microscopy and energy-dispersive X-ray spectrometry. It was found that the iron with about 10 wt.% Mo was eutectic/peritectic, whereas the others with less Mo content were hypoeutectic. The matrix in all irons was austenite, partly transformed to martensite during cooling. Mo addition promoted the formation of M₂₃C₆ and M₆C. At 1 wt.% Mo, multiple eutectic carbides including M₇C₃, M₂₃C₆ and M₆C were observed. M₂₃C₆ existed as a transition zone between eutectic M₇C₃ and M₆C, indicating a carbide transition as M₇C₃(M_2.3C) → M₂₃C₆(M_3.8C) → M₆C. At 6 wt.% Mo, multiple eutectic carbides including M₇C₃ and M₂₃C₆ were observed together with fine cellular/lamellar M₆C aggregates. In the iron with 10 wt.% Mo, only eutectic/peritectic M₂₃C₆ and M₆C were found without M₇C₃. Mo distribution to all carbides has been determined to be increased from ca. 0.4 to 0.7 in mass fraction as the Mo content in the irons was increased. On the other hand, Cr distribution to all carbides is quite constant as ca. 0.6 in mass fraction. Mo addition increased Vickers macro-hardness of the irons from 495 up to 674 HV₃₀. High Mo content as solid-solution in the matrix and the formation of M₆C or M₂₃C₆ aggregates were the main reasons for hardness increase, indicating potentially improved wear performance of the irons with Mo addition.     

Influence of secondary carbides precipitation and transformation on the secondary hardening of laser melted high chromium steel

The influence of secondary carbides precipitation and transformation on the secondary hardening of laser melted high chromium steels was analyzed by means of scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The microstructure of laser melted high chromium steel is composed of austenite with supersaturated carbon and alloy elements and granular interdendritic carbides of type M₂₃C₆. Secondary hardening of the laser melted layer begins at 450 °C after tempering, and the hardness reaches a peak of 672HV at 560 °C and then decreases gradually. After tempering at 560 °C, a large amount of lamellar martensite was formed in the laser melted layer with a small quantity of thin lamellar M₃C cementite due to the martensitic decomposition. The stripy carbides precipitating at the grain boundaries were determined to be complex hexagonal M₇C₃ carbides and face centered cubic M₂₃C₆ carbides. In addition, the granular M₂₃C₆ carbides and fine rod-like shaped M₇C₃ carbides coexisted within the dendrites. As a result, the combined effects of martensitic transformation, ultrafine carbide precipitations, and dislocation strengthening result in the secondary hardening of the laser melted layer when the samples were tempered at 560 °C.     

Microstructural features of an iron-based laser coating

A high toughness wear resistant coating is produced by laser clad Fe-Cr-W-Ni-C alloys. The microstructural and compositional features of the laser-solidified microstructures and phase evolutions occurring during high temperature tempering at 963 K were investigated by using analytical electron microscopy with energy dispersive X-ray analysis. The clad coating possesses the hypereutectic microstructure consisted of M₇C₃+(+ M₇C₃). During high temperature aging, the precipitation of M₂₃C₆ and M₂C in austenite and in situ transformation of dendritic M₇C₃ to M₂₃C₆ and eutectic M₇C₃ to M₆C occurred. The laser clad coating reveals an evident secondary hardening and superior impact wear resistance.     

Microstructural and mechanical properties evolutions of plasma transferred arc deposited Norem02 hardfacing alloy at high temperature

Plasma-transferred-arc welded Norem02, an iron-based hard-facing alloy, was characterised. Its microstructure and chemical composition were investigated using optical microscopy, scanning electron microscopy (with electron probe microanalysis), electron backscattering diffraction, and X-ray diffraction. The microstructure of the as-deposit alloy consists of a dendritic austenite structure with ferrite islets at dendrites centres, with an interdendritic eutectic region containing austenite, M₇C₃ and M₂₃C₆ carbides and zones containing Mo-rich precipitates. Tensile behaviour of Norem02 was characterised and completed by dilatometry tests in welding process temperature range. No significant phase transformation was detectable during mechanical testing. Different heat treatment cycles of ageing at high temperatures (until 1100 °C) were carried out for different durations. The microstructure of Norem02 heated at 1100 °C was not significantly affected by a short time (15 s) treatment whereas changes were observed for longer durations (2 h), although hardness remains almost unchanged.This work tends to demonstrate that for this alloy metallurgical evolution during the welding process has very little influence on mechanical properties.     

Effect of M7C3→M23C6 transformation on fracture behaviour of cast ferritic stainless steels

Abstract

The fracture behaviour of three 29 wt-%Cr ferritic steels, two containing zirconium and titanium respectively, has been investigated in the as cast condition and after annealing at 660°C for different times up to 2210 h. The fracture energy and the mode of fracture depend on both the morphology and the nature of the eutectic, which consists of carbides and ferrite. In the as cast condition, fracture is predominantly transgranular cleavage and it can be associated with the discontinuous morphology of the M₇C₃ carbides present in the eutectic as coarse particles surrounded by the eutectic ferrite. After prolonged heating, the ambient fracture energy decreases and the interdendritic mode of fracture is enhanced. This change in fracture mechanism is associated with transformation of the M₇C₃ to M₂₃ C₆ carbides. The M₂₃ C₆ carbides, unlike the coarse M₇C₃ carbides, form a continuous network within the eutectic mixture and constitute an easy path for crack propagation. The zirconium and titanium additions result in a more massive morphology of the carbides in the eutectic mixture and accelerate the M₇C₃ to M₂₃C₆ transformation during the heat treatments, enhancing the interdendritic mode of fracture both in the as cast and in the annealed condition.

MST/1734     

Effect of boron on the microstructure of near-eutectic Al–Si alloys

The effect of boron on the microstructure of a near-eutectic Al–Si alloy (ZL109) was investigated by scanning electron microscopy (SEM) and electron beam microprobe analysis (EPMA). It was found that α-Al dendrites and eutectic clusters were significantly refined by the addition of boron. Another interesting discovery is that the near-eutectic alloy exhibited hypereutectic structure characteristics when the level of boron added exceeds 0.3%, i.e., primary Si is precipitated in the eutectic microstructure. A new type of nucleation substrate for the primary Si is found, Al_xCa_mB_nSi. This appears to be the main reason for the precipitation of primary Si.     

Refining effect of nitrogen on M7C3 carbides in hypereutectic Fe–25Cr–4C–0.5Ti–0.5Nb hardface coatings

Hypereutectic Fe–Cr–C–Ti–Nb coatings with N additives were developed by surface-hardening welding (hardfacing). The experimental results showed that the primary M₇C₃ carbides were refined by the N additives in the coatings. Based on the micro-morphologies of M₇C₃ and (Ti,Nb)(C,N), the (Ti,Nb)(C,N) was present inside the primary M₇C₃ carbides, and they were tightly combined. The mismatch between the (010) crystal plane of M₇C₃ and the (110) crystal plane of (Ti,Nb)(C,N) was 6.15%, which indicated that (Ti,Nb)(C,N) was moderately effective as a heterogeneous nucleus of M₇C₃ carbides. Therefore, the preferentially precipitated (Ti,Nb)(C,N) in the Fe–Cr–C–Ti–Nb coating was the heterogeneous nucleus of the primary M₇C₃ carbides and thereby refined the primary M₇C₃ carbides .     

Laser in-situ synthesis of mixed carbide coating on steel

A 2.5 KW Nd:YAG laser was employed to modify the surface of a AISI 1010 steel deposited with a precursor powder mixture of Fe, Ti, Cr and C. In-situ formation of TiC and chromium carbides [M₇C₃ (M = Fe, Cr) and Cr₇C₃] was observed as function of laser processing power at constant scan speed. Although TiC was present in all the samples, the chromium carbides were absent in samples processed at certain laser powers. Corresponding to this behavior, variation in mechanical properties of the coating was observed. The hardness and wear properties of the samples without chromium carbides was inferior in comparison to samples with both TiC and chromium carbides.     

Relationship between microstructure,hardness and corrosion resistance in 20 wt.%Cr, 27 wt.%Cr and 36 wt.%Cr high chromium cast irons

The microstructures, hardness and corrosion behavior of high chromium cast irons with 20, 27 and 36 wt.%Cr have been compared. The matrix in as-cast 20 wt.%Cr, 27 wt.%Cr and 36 wt.%Cr high chromium cast irons is pearlite, austenite and ferrite, respectively. The eutectic carbide in all cases is M₇C₃ with stoichiometry as (Cr_3.37, Fe_3.63)C₃, (Cr_4.75, Fe_2.25)C₃ and (Cr_5.55, Fe_1.45)C₃, respectively. After destabilization at 1000 °C for 4 h followed by forced air cooling, the microstructure of heat-treatable 20 wt.%Cr and 27 wt.%Cr high chromium cast irons consisted of precipitated secondary carbides within a martensite matrix, with the eutectic carbides remaining unchanged. The type of the secondary carbide is M₇C₃ in 20 wt.%Cr iron, whereas both M₂₃C₆ and M₇C₃ secondary carbides are present in the 27 wt.%Cr high chromium cast iron. The size and volume fraction of the secondary carbides in 20 wt.%Cr high chromium cast iron were higher than for 27 wt.%Cr high chromium cast iron. The hardness of heat-treated 20 wt.%Cr high chromium cast iron was higher than that of heat-treated 27 wt.%Cr high chromium cast iron. Anodic polarisation tests showed that a passive film can form faster in the 27 wt.%Cr high chromium cast iron than in the 20 wt.%Cr high chromium cast iron, and the ferritic matrix in 36 wt.%Cr high chromium cast iron was the most corrosion resistant in that it exhibited a wider passive range and lower current density than the pearlitic or austenitic/martensitic matrices in 20 wt.%Cr and 27 wt.%Cr high chromium cast irons. For both the 20 wt.%Cr and the 27 wt.%Cr high chromium cast irons, destabilization heat treatment gave a slight improvement in corrosion resistance.     

Effect of K/Na on microstructure of high-speed steel used for rolls

In the present work, the effect of K/Na on microstructure of high-speed steel (HSS) used for rolls was investigated utilizing Hi-scope video microscope (HSVM) and electron probe microanalyser (EPMA). As-cast microstructure of the alloy is mainly composed of pearlite matrix, M₇C₃, M₂C and MC eutectic carbides. The carbides are connected or placed next to each other to form a network along grain boundaries. After K/Na modification, the morphology, size and distribution of carbides change greatly. The carbide network tends to break, and all carbides are refined and distributed homogeneously in the matrix. The mechanism of K/Na modification on microstructure of the alloy is also discussed.     

Thermal oxidation-resistant surface alloys processed by laser alloying on 35 NCD 16 ferritic steel

The hardness and the high-temperature oxidation resistance of the low-alloyed ferritic steel 35 NCD 16 (Fe-0.38 C-1.8 Cr-4 Ni, wt%) were increased by laser surface alloying of Cr₃C₂ or Cr₃C₂ and SiC. The obtained surface alloys always exhibit primary dendrites ( Fe solid solution with Cr and Si + martensite) and an interdendritic eutectic containing Fe + martensite + M₇C₃ carbides (M=Fe, Cr). In the first case, the formation of iron chromite, FeCr₂0₄, in contact with the coating accounts for the good oxidation resistance. In the second case, the beneficial influence of silicon lies in the formation in the presence of oxygen of a thin chromia scale only, with local silicon enrichment, showing excellent barrier properties. Kinetic and structural observations are discussed in the light of thermodynamics and diffusion processes.     

Fe-Cr-C hardfacing alloys for high-temperature applications

The microstructure and phase chemistry of a Fe-34Cr-4.5C wt% hardfacing alloy has been investigated using transmission electron microscopy and microanalytical techniques. The microstructure is found to consist of large primary M₇C₃. carbides in a eutectic mixture of austenite and more M₇C₃. The results indicate that the microstructure of the undiluted alloy becomes configurationally frozen at a temperature of about 1150° C during deposition by the manual metal arc welding technique. This allows the metastable austenite phase to contain a large chromium concentration ( 16 to 17 wt %), thus imparting good corrosion and oxidation resistance. Experimental data on the partitioning of chromium, manganese and silicon between the carbide phases are discussed in the context of the high-temperature stability of the alloy.