铸态中铬硅铸铁价电子结构与M7C3体积分数

为提高中铬铸铁共晶碳化物中M7C3比例,研究了中铬硅铸铁(290Cr8Si2).建立了铸态中铬硅铸铁基体的价电子结构,运用固体与分子经验电子理论(EET)分析了中铬硅铸铁中Si的作用.分析和实验结果表明,中铬硅铸铁基体含C、Cr、Si的γ-Fe晶胞中,C原子与Si原子的结合力强于C原子与Cr原子的结合力,较高的含Si量降低了铸铁基体的含Cr量,提高了共晶碳化物的含Cr量,进而提高了共晶碳化物中M7C3的比例.耐磨损中铬硅铸铁(290Cr8Si2)共晶碳化物(M7C3 M3C)中M7C3占94.2%(体积分数),明显高于中铬铸铁(290Cr8Si1)共晶碳化物中的M7C3的71.7%(体积分数).     

钨铬合金白口铸铁中碳化物三维形貌及结晶特征

通过观察分析钨铬铸铁中碳化物的三维形貌,研究了M_3C、M_7C_3、M_6C三种碳化物的结晶特征。初生M_3C以平板状生长;亚共品、过共晶铸铁中共晶M_3C分别以初生奥氏体、初生M_3C为衬底与奥氏体构成离异共晶和莱氏体。初生M_7C_3呈六角形棒状体以螺旋方式轴向生长的同时向心生长;共晶M_7C_3与奥氏体协同生长。初生M_6C为八面体尖端相连的枝晶形态呈“锚状”生长。     

16Cr2Mo1Cu高铬铸铁在亚临界处理中硬化行为研究

研究了亚临界处理对16Cr2Mo1Cu高铬铸铁的组织转变和性能的影响,并利用X射线衍射分析、磁性法和硬度测定法分析了硬化机制.研究表明:16Cr2Mo1Cu高铬铸铁的铸态组织由残余奥氏体、马氏体和M7C3型共晶碳化物组成,其相对含量分别为77.0%,7.2%和15.8%;在亚临界处理过程中,基体组织中的残余奥氏体析出二次碳化物并在冷却过程中转变为马氏体,使该合金在560～600℃的亚临界处理过程中出现二次硬化;在适当的处理温度和保温时间下,16Cr2Mo1Cu高铬铸铁可得到最高的硬度.     

等离子表面高铬高碳耐磨层的研究

采用等离子表面合金化技术,在20钢表面渗铬,并进行双辉等离子渗碳,形成高铬高碳合金层.利用GDS、XRD、OM、SEM研究了合金层成分、相组成及组织形貌,并通过摩擦试验对合金层耐磨性进行了分析.研究结果表明:表面高碳高铬层含铬量和含碳量以及碳化物的质量分数(40%以上)高于一般冶金高铬铸铁;渗层主要包含M23C6和M7C3型碳化物,这些碳化物均匀弥散分布,尺寸通常在1μm左右,并无共晶碳化物组织;合金层表面显微硬度达到1000～1600 HV,耐磨性比GCr15轴承钢提高8.6倍.     

热处理回火温度对Cr26高铬铸铁组织与性能的影响

目前,有关淬火后回火温度对Cr26高铬铸铁组织及性能的研究报道不多。为此,采用XRD、OM、SEM、TEM和电子拉力试验机和洛式硬度计,研究了回火温度对Cr26高铬铸铁调质处理前后的显微组织和力学性能的影响。结果表明:调质处理前Cr26高铬铸铁中碳化物类型有M_7C_3、M_(23)C_6和M_3C_2;调质处理后Cr26高铬铸铁的显微组织得到明显改善,基体上弥散分布着细小的碳化物;抗拉强度和硬度值随回火温度的增加而降低,延伸率有所提高;回火温度为560℃左右时,抗拉强度、延伸率和硬度值分别为1 294 MPa、8.02%和38.6 HRC,有良好的力学性能。     

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 。      

添加Mo对高铬铸铁组织及亚临界处理硬化作用的影响研究

为提升高铬铸铁的综合性能,以无Mo高铬铸铁和含1.0％(质量分数)Mo高铬铸铁为研究对象,对比研究了添加Mo元素对铸态高铬铸铁组织结构以及550℃亚临界处理对其组织及硬度的影响.研究表明:2种铸态高铬铸铁的主要组成相均为奥氏体、马氏体及(Cr,Fe)7C3,但含Mo高铬铸铁中奥氏体含量明显增多.铸态高铬铸铁均以亚共晶方...     

液相烧结工艺制备高钒钢

采用气雾化粉末+压制+超固相液相烧结(SLPS)工艺制备钒含量(质量分数)约为10%的高钒钢,研究了烧结工艺对致密化行为、显微组织演变、相构成与分布和力学性能的影响规律。结果表明,烧结温度的影响最全面,保温时间主要影响碳化物的析出量。烧结高钒钢的基体为针状M和少量残余γ,在基体中有VC、复合型碳化钼和碳化铬等碳化物,VC大多呈球形,分布在晶界和晶粒内部。随着烧结温度的提高和保温时间的延长,晶粒和碳化物逐渐粗化,各类碳化物的析出越来越充分,而复合型碳化物的析出对高钒钢的强度和冲击韧性有不利影响。烧结高钒钢具有优秀的综合力学性能:硬度HRC 65-68,冲击韧性高于6 J/cm2,抗弯强度高于1800 MPa。     

等离子渗铬及复合强化处理提高耐磨性能的研究EI

利用铬与-αFe无限互溶,含铬的共晶碳化物与二次碳化物形成机理不同,首先在Q235钢和45钢表面渗入合金元素铬,形成高铬合金扩散层,表面含铬量达到40%(质量分数,下同)以上,并在一定范围内使表面合金层铬含量呈梯度分布。利用铬是碳化物形成元素,进行等离子渗碳,因为碳化物形成温度低,在高铬合金层中固体形核长大,所形成的含铬碳化物弥散、细小、均匀。虽表面含碳量达2.8%以上,但是没有共晶碳化物。经淬火及回火处理,表面硬度在HV1800以上。磨损实验表明:与对磨淬火GCr15钢相比,耐磨性能提高7倍以上。     

镍基合金粉末光束堆焊层的微观组织及强化机理

采用X射线衍射，SEM，EDAX及显微硬度和洛氏硬度等分析手段研究了含碳量为1．0％的NiCrBSi系自熔合金粉末光束堆焊层的微观组织及强化机理，结果表明，采用光束镍基合金粉末堆焊可在铁碳合金表面获得与基体冶金结合良好，无裂纹，轻度稀释的强化层，堆焊热输入对堆焊层稀释率及合金元素烧损的影响程度决定了堆焊层微观组织及物相组成，小热输入堆焊时，堆焊金属经度稀释（η=3.5%），其显微组织由少量初生的γ－Ni和大量的γ－Ni Bi3B Ni3Si三相共晶组成的亚共晶基底，以及在基底上分布着大量的Cr23C6,(Cr,Fe)7C3高硬度相组成，采用大热输入堆焊，堆焊金属稀释率达12％，堆焊层由大量的γ－（Fe,Ni0枝晶和少量γ－(Fe,Ni) M7C3共晶组成，在堆焊层中未发现一次碳化物的析出，在光束粉末堆焊层中大量高硬度M23C6,M7C3共晶组成，在堆焊层中未发现一次碳化物的析出，在光束粉末堆焊层中大量高硬度M23C6，M7C3型碳化物和Ni3B,Ni3Si共晶相的析出以及合金元素在γ相中的过饱和固溶是其是以强化的主要原因，与TIG堆焊相比，采用相近热输入所获得的光束粉末堆焊层的耐磨性能提高了3倍以上。     

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.     

Microstructural characteristics of gas tungsten arc synthesised Fe-Cr-Si-C coating

Abstract

The gas tungsten arc (GTA) method was used to synthesise Fe-Cr-Si-C alloy coatings, and processing effects on the coating were investigated experimentally. Coatings were developed on an AISI type 1040 steel substrate. Four different regions were obtained in the surface coating; and in these regions either a hypoeutectic or a hypereutectic microstructure was found. The hypoeutectic microstructure consisted of primary dendrites of austenite (γ) phase and eutectic M₇C₃ (M=Cr,Fe) carbides. On the other hand, the hypereutectic microstructure consisted of M₇C₃ primary carbides and eutectic. A hypoeutectic or hypereutectic microstructure was determined by the combination of particularly carbon concentration, solidification rate, and extent of substrate melting. The higher hardness of the hypereutectic microstructure is attributed especially to the formation of M₇C₃ primary carbides. The lower hardness of the hypoeutectic microstructure is related to three effective parameters: first, the presence of γ phase in the primary dendrites; second, excessive dilution from the base material; and third, relatively low concentrations of chromium and carbon.     

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.     

The influence of vanadium on fracture toughness and abrasion resistance in high chromium white cast irons

The influence of vanadium on wear resistance under low-stress conditions and on the dynamic fracture toughness of high chromium white cast iron was examined in both the ascast condition and after heat treatment at 500 °C. A vanadium content varying from 0.12 to 4.73% was added to a basic Fe-C-Cr alloy containing 2.9 or 19% Cr. By increasing the content of vanadium in the alloy, the structure became finer, i.e. the spacing between austenite dendrite arms and the size of massive M₇C₃ carbides was reduced. The distance between carbide particles was also reduced, while the volume fraction of eutectic M₇C₃ and V₆C₅ carbides increased. The morphology of eutectic colonies also changed. In addition, the amount of very fine M₂₃C₆ carbide particles precipitated in austenite and the degree of martensitic transformation depended on the content of vanadium in the alloy. Because this strong carbide-forming element changed the microstructure characteristics of high chromium white iron, it was expected to influence wear resistance and fracture toughness. By adding 1.19% vanadium, toughness was expected to improve by approximately 20% and wear resistance by 10%. The higher fracture toughness was attributed to strain-induced strengthening during fracture, and thereby an additional increment of energy, since very fine secondary carbide particles were present in a mainly austenitic matrix. An Fe-C-Cr-V alloy containing 3.28% V showed the highest abrasion resistance, 27% higher than a basic Fe-C-Cr alloy. A higher carbide phase volume fraction, a finer and more uniform structure, a smaller distance between M₇C₃ carbide particles and a change in the morphology of eutectic colonies were primarily responsible for improving wear resistance.     

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.     

Effect of aluminum on the primary carbides of a hypereutectic high chromium cast iron

The effect of aluminum on the primary M₇C₃ carbides of a hypereutectic high chromium cast iron containing 4.0 wt% carbon and 20.0 wt% chromium was studied by means of optical microscopy, scanning electron microscope (SEM), energy dispersive X‐ray spectrometry (EDX), water quenching, and differential thermal analysis (DTA). Compared with specimen without aluminum addition, the primary carbides were all refined when different amount of aluminum was added into the melts, but the primary carbides in specimen with 0.3 wt% aluminum were the finest. With the addition of aluminum, aluminum element enriched at the boundary of primary carbides during solidification and was beneficial for the refinement of primary carbides. However, the increase of primary carbide growth time with the increase aluminum content had adverse effect on the refinement of the primary carbides. The comprehensive influence of those two factors leaded to the result that the primary carbides in specimen 1 with 0.3 wt% aluminum were the finest.     

Development of microstructure during heat treatment of high carbon Cr–Mo steel

Abstract

Stainless steels containing enhanced chromium and carbon contents are particularly attractive for applications requiring improved wear and corrosion resistance. The as cast microstructure of such steels is composed mainly of ferritic matrix along with a network of interdendritic primary carbides. It has been shown that heat treatment of these steels results in microstructures that contain more than one type of carbide. A selective dissolution technique has been employed to isolate carbides from the matrix. Scanning electron microscope and X-ray diffraction studies of the as cast steels have shown that the primary carbides are essentially of M₇C₃ type, whereas in heat treated specimens both M₇C₃ (primary) and M₂₃C₆ (secondary) type carbides have been observed. The relative amounts of these carbides are found to be dependent on the heat treatment temperature. In addition, nucleation of austenite occurs above 950°C and at ～1250°C the matrix transforms entirely to austenite, which is retained completely on quenching to room temperature.     

Matrix microstructure effect on the abrasion wear resistance of high-chromium white cast iron

The two-body abrasion resistance of high-chromium white cast iron was investigated as a function of cast iron microstructure. Different microstructures were obtained by means of heat treatment. The chromium and carbon content were chosen in order to have different matrix microstructures (austenitic, martensitic and ferritic) with the same amount of eutectic carbide (M₇C₃). The results show that the cast iron with an austenitic matrix has the best wear resistance. The good wear resistance of this material is due to strong work hardening of the austenitic matrix resulting in a hardness which exceeds that of other structures. The effect of abrasive paper deterioration on abrasion has also been investigated.     

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.     

On the structure and properties of wear- and corrosion-resistant nickel-chromium- tungsten-carbon-(silicon) alloys

In some applications, for chemical and physical reasons hard nickel-based alloys have to be used instead of cobalt-based alloys but boron must be avoided. The nickel-chromium-tungsten-carbon system with and without silicon was therefore studied in several concentration ranges at 1050°C with respect to structure, phase, hardness and corrosion and wear resistance. Alloys containing 2% carbon, 10% tungsten and more than 10% chromium are composed of a nickel solid solution and an M₇C₃ carbide in both cast and homogenized (1050°C, 180 h) conditions. On increasing the tungsten content up to 20% the M₂C carbide becomes dominant, and this is associated with a remarkable increase in the hardness of the alloys. Additions of 2% silicon do not change the M₇C₃ and M₂C carbides present. In some cases a carbon-stabilized silicide M₅Si(C) was observed. Silicon additions decrease the liquidus temperature range relatively little, but they affect particle shape and size and the grain size distribution. The relation of various chromium, tungsten and silicon contents to corrosion and wear resistance was studied. The corrosion resistance depends on the chromium content of the nickel solid solution but also on carbide formation (tungsten and carbon content). The silicon content of the nickel solid solution is important too.Because their liquidus temperature is close to 1300°C the alloys cannot be used as self-fluxing and fusing powders for flame spraying but they can be sprayed by plasma torches and they can, of course, be welded.