Effects of carbides on fatigue characteristics of austempered ductile iron

Crack initiation and growth behavior of an austempered ductile iron (ADI) austenitized at 800 °C and austempered at 260 °C have been assessed under three-point bend fatigue conditions. Initiation sites have been identified as carbides remaining from the as-cast ductile iron due to insufficient austenization. The number of carbides cracking on loading to stresses greater than 275 MPa is critical in determining the failure mechanism. In general, high carbide area fractions promote coalescence-dominated fatigue crack failure, while low area fractions promote propagation-dominated fatigue crack failure. Individual carbides have been characterized using finite body tessellation (FBT) and adaptive numerical modeling (Support vector Parsimonious Analysis Of Variance (SUPANOVA)) techniques in an attempt to quantify the factors promoting carbide fracture. This indicated that large or long and thin carbides on the whole appear to be susceptible to fracture, and carbides that are locally clustered and aligned perpendicular to the tensile axis are particularly susceptible to fracture.     

Correlation of microstructure and failure initiating sites in an AISI M2 high-speed steel using Vickers indentation

In this study, the relation of crack initiation and propagation to microstructure was shown in an AISI M2 high-speed steel using Vickers hardness indentation. Observations were carried out on the cracks generated from the corners of the square pyramid indentation in both the as-quenched, the quenched and tempered specimens. It is shown that cracks are nucleated from the corners of the indentation by either cleavage of the carbide or by decohesion between the carbide and the matrix, depending on the size of the carbide. A nucleated crack propagates by tearing of the martensitic matrix between the cracked carbides ahead of the indentation corner. It is shown that the crack resistance of the M2 steel is determined by the toughness of the matrix rather than the size and distribution of the carbides in the matrix.     

Fracture toughness and fatigue behavior of matrix II and M-2 high speed steels

The effects of heat treatment and of the presence of primary carbides on the fracture toughness,K _Ic and the fatigue crack growth rates,da/dN, have been studied in M-2 and Matrix II high speed steels. The Matrix II steel, which is the matrix of M-42 high speed steel, contained many fewer primary carbides than M-2, but both steels were heat treated to produce similar hardness values at the secondary hardening peaks. The variation of yield stress with tempering temperature in both steels was similar, but the fracture toughness was slightly higher for M-2 than for Matrix II at the secondary hardening peaks. The presence of primary carbides did not have an important influence on the values ofK _Ic of these hard steels. Fatigue crack growth rates as a function of alternating stress intensity, ΔK, showed typical sigmoidal behavior and followed the power law in the middle-growth rate region. The crack growth rates in the near threshold region were sensitive to the yield strength and the grain sizes of the steels, but insensitive to the sizes and distribution of undissolved carbides. The crack growth rates in the power law regime were shifted to lower values for the steels with higher fracture toughness. SEM observations of the fracture and fatigue crack surfaces suggest that fracture initiates by cleavage in the vicinity of a carbide, but propagates by more ductile modes through the matrix and around the carbides. The sizes and distribution of primary carbides may thus be important in the initiation of fracture, but the fracture toughness and the fatigue crack propagation rates appear to depend on the strength and ductility of the martensite-austenite matrix.     

Statistical aspects of cleavage fracture in steel

The fracture of mild steel in the cleavage range has been evaluated using a weakest link statistical model, assuming the preexistence of a distribution of cracked carbides. The model provides a rationale for the critical fracture distance, viz., the distance from the crack tip at which the probability of cleavage cracking exhibits a maximum. The critical distance depends on the size distribution and volume fraction of carbides. The model also predicts trends in K,_ic with material properties: flow strength, cracked carbide size and volume fraction, and grain size. The resultant temperature dependence of K,_ic is shown to derive exclusively from the temperature dependence of the flow stress, as in prior models. The effects of microstructure on K,_ic depend primarily on the size distribution of cracked carbides, with additional influences of the grain size and of the volume fraction of carbides.     

Microstructure and toughness in high speed tool steels: the influence of hot reduction and austenitization temperature

Measurements of transverse rupture strength are reported for AISI M2, AISI M 35, AISI T 15 and ASP 30 produced by conventional ingot metallurgy, sintering of cold pressed powders, and hot rolling of HIP billets. Also reported are the size distributions of undissolved carbides and the effects of hot reduction and austenitization temperature on these and on the transverse rupture strength of test bars from the centre and the periphery of the bar stock, and of test bars oriented longitudinally and transversally to the rolling direction. Fracture initiating defects were identified and their size measured by SEM fractography. The results are discussed in terms of a fracture mechanical model for transverse rupture strength. It is inferred that in ingot-derived materials, fracture is initiated by micro-cracks formed by subcritical crack growth within the carbide stringers. In powder metallurgy materials, large carbides or carbide clusters initiate fracture. The model explains the dependence of transverse rupture strength on carbide distribution, degree of hot working, austenitization temperature, and specimen orientation. It is emphasized that transverse rupture strength is suitable for characterizing the defect population of a tool steel, but not the fracture resistance of small stressed volumes such as the cutting edge of a tool bit.     

基于Weibull分布的冲压模具用硬质合金耐冲击疲劳性能研究

通过小能量多次冲击方法研究了五种常用冲压模具用硬质合金的冲击疲劳行为,利用Weibull统计分布分析材料的冲击疲劳性能,采用扫描电镜观察断口形貌。试验结果表明:从硬质合金多次冲击疲劳试验的断口可观察到特征明显不同的三个区域——裂纹稳定生长的微粗糙区,裂纹激烈扩展的光亮区,最后断裂的平滑区。晶粒度1.4μm、钴含量20%的硬质合金,其耐冲击疲劳性能最好。     

H13钢中一次碳化物的特征及控制进展

首先结合H13钢的成分特点肯定了H13钢优异的材料性能,随后总结了一次碳化物与H13钢使用寿命之间的关系。进一步系统地论述和研究了H13钢中一次碳化物的特征,包括二维和三维形貌、热稳定性、析出机理等。最后对比了4种H13钢中一次碳化物的控制手段,包括成分优化、冷速控制、Mg处理和稀土处理。相关论述和研究工作能够对钢中一次碳化物的合理优化起到一定的启发作用。      

热处理对钢结硬质合金TLMW50中复式碳化物形态与分布的影响

利用SEM对烧结态和热处理态钢结硬质合金TWLM50微观组织中复式碳化物形态与分布进行研究。结果表明：烧结态钢结硬质合金TLMW50微观组织中主要复式碳化物Fe3W3C、Fe2W2C相形状基本呈现长条状,宽约10—20μm之间,颗粒长度可达10～60μm之间,且复式碳化物的晶粒上还有细小的三角状二次碳化物析出。经过1050℃淬火,250℃回火复式碳化物颗粒的棱角有所钝化,条形颗粒明显减少。二次碳化物析出数量明显减少,大部分二次碳化物溶于或与初生复式碳化物发生反应,形成复式碳化物,且随机分布在初生复式碳化物上。热处理导致钢结硬质合金TWLM50硬度提高到65HRC,耐磨性显著提高。     

The influence of Mo2C morphology and distribution on the fatigue crack initiation and propagation behavior of Fe-C-Mo dual-phase steels

Dual-phase microstructures consisting of ferrite with carbides (Mo₂C) surrounding equiaxed martensite packets have been developed in two alloys, Fe-O. 2C-4Mo and Fe-O. 2C-2Mo. These alloys were chosen because of the presence of two distinct carbide morphologies: (1) a needle-shaped interphase carbide structure, and (2) a fibrous carbide structure. Isothermal transformations were used to control the carbide morphology and distribution in the ferritic regions of the dual-phase microstructures. In the present research the effects of changes in carbide structure on low cycle fatigue (LCF) and fatigue crack growth (FCG) behavior were studied. Crack initiation was observed at prior austenite grain boundaries in the fibrous microstructure, and along intrusion/extrusion defects in the interphase needle microstructures for LCF tests. TEM studies revealed a carbide free region at prior austenite grain boundaries where initiation occurs for the fibrous case. The cyclic stress/strain response of the ferritic portions of the microstructure is determined by the ability of the carbides to homogenize the strain found there. This affects the stress/strain distribution in the composite ferrite-martensite microstructure by changing the hardness ratio of the two phases and subsequently alters the fatigue crack growth behavior and the macroscopic cyclic stress/strain response. Strain localization was also found to affect the roughness induced closure found for fatigue crack growth tests for low R tests (R = 0.1).     

Influence of Rare Earth Elements on Microstructure and Mechanical Properties of Cast High-Speed Steel Rolls

The influence of rare earth （RE） elements on the solidification process and eutectic transformation and mechanical properties of the high-V type cast, high-speed steel roll was studied. Test materials with different RE additions were prepared on a horizontal centrifugal casting machine. The solidification process, eutectic structure transformation, carbide morphology, and the elements present, were all investigated by means of differential scanning calorimetry （DSC） and scanning electron microscopy energy dispersive spectrometry （SEM-EDS）. The energy produced by crack initiation and crack extension was analyzed using a digital impact test machine. It was found that rare earth elements increased the tensile strength of the steel by inducing crystallization of earlier eutectic γ-Fe during the solidification process, which in turn increased the solidification temperature and thinned the dendritic grains. Rare earth elements with large atomic radius changed the lattice parameters of the MC carbide by forming rare earth carbides. This had the effect of dispersing longpole M C carbides to provide carbide grains, thereby, reducing the formation of the gross carbide and making more V available, to increase the secondary hardening process and improve the hardness level. The presence of rare earth elements in the steel raised the impact toughness by changing the mechanism of MC carbide formation, thereby increasing the crack initiation energy.     

Experimental Testing and Computer Simulations of Ductile Fracture in Tool Steels

Ductility was determined in experimental four‐point bending tests of smooth specimens of tool steel. The tool steels had different contents of carbides and carbide sizes and with a hardness of approximately 60HRc. Two of the materials tested were produced powder metallurgically, one was spray formed and one was conventionally uphill ingot cast. Carbide size distribution analysis was performed on planar polished sections of each material. Correlation between carbide microstructure and ductility performance was obtained. The fracture mechanisms were investigated with fractography. A 3D FE‐model was used to simulate the four‐point bending tests and thereby analyse the matrix flow curve. Also the strain at failure was analysed for each material when simulations were performed based on experimental data. SEM‐images of the materials carbide microstructure were used to create 2D FE‐models. The models simulated crack initiation and propagation by removing elements in the steel matrix as the plastic strain reached a critical level. With three variants, simulations of crack initiation and propagation at carbides were investigated. That was carbides with no cohesion to matrix, carbides fixed to the matrix and carbides with internal cracks. Comparison of strains at failure for the 2D and the 3D FE‐models showed good correlation.     

稀土对余热淬火低合金耐磨蠕铁组织和性能的影响

研究了稀土对余热淬火低合金耐磨蠕铁中石墨形态及碳化物尺寸的影响。试验表明，稀土能使低合金铸铁组织中的板条状碳化物细化，其原因主要和细小均匀分布在基体中的蠕墨对碳化物长大起阻碍作用有关。生产实践表明，这种稀土变质低合金铸铁在余热淬火处理条件下能获得较好的韧性及耐磨性。     

Spheroidization and Ostwald Growth of Carbide in Isothermal Process of Hot-Deformed High Carbon Chromium Cast Steel

Ｈｉｇｈ ｃａｒｂｏｎｃｈｒｏｍｉｕｍｃａｓｔｓｔｅｅｌｈａｓ ｇｏｏｄｓｔｒｅｎｇｔｈ ,ｈａｒｄｎｅｓｓａｎｄｗｅａｒ ｒｅｓｉｓｔａｎｃｅ ,ａｎｄｉｔｈａｓｂｅｅｎｗｉｄｅｌｙｕｓｅｄｉｎｔｈｅｆｉｅｌｄｏｆｈｉｇｈ ｓｐｅｅｄａｂｒａｓｉｏｎａｎｄｅｘ ｔｒｕｓｉｏｎ .Ｔｈｅｃａｒｂｉｄｅｓｉｎｃａｓｔｓｔｅｅｌａｒｅｄｉｓｔｒｉｂｕｔｅｄｉｎｔｈｅｆｏｒｍｏｆｃｏｎｔｉｎｕｏｕｓｎｅｔｗｏｒｋｓｔｒｕｃｔｕｒｅ ,ｗｈｉｃｈｌｅａｄｔｏｔｈｅｂｒｉｔｔｌｅｎｅｓｓｏｆｓｔｅｅｌ,ｌｉｍｉｔｉｎｇ…     

Investigation of Carbide Precipitation Process and Chromium Depletion during Thermal Treatment of Alloy 690

For the purpose of studying the effect of heat treatment on carbide morphology and chromium concentration distribution, which are critical to the resistance of alloy 690 to stress corrosion cracking (SCC), a series of thermal treatments was performed. A model taking into account the intercorrelated dynamic process between the carbide precipitation and chemical diffusion of the chromium atom from matrix to grain boundary (GB) was constructed on the basis of classical nucleation theory, Kolmogorov–Johnson–Mehl–Avrami law, and diffusion theory. The validity of this model was evaluated by comparing the simulated results of the carbide average size and chromium concentration near the GB with the corresponding measured results. A discontinuous factor was introduced based on the relation linking the interdistance between the carbides and the carbide average size; thus, the carbide morphology and chromium concentration could be predicted by this model. According to the results of the experiments and simulations, a carbide discontinuous factor smaller than 2.2 together with the chromium concentration at the GB higher than a critical value (21 wt pct) were essential for the corrosion resistance ability of the alloy, and then some proper heat-treatment conditions were obtained through predicting the value of the two variables. In addition, the effects of the grain size and composition variation on the carbide discontinuous factor and chromium concentration profile were simulated. The results indicated that an intermediate grain size of approximately 31.8 to ~63.5 μm was beneficial for effectively improving the resistance of the alloy to SCC. Simultaneously, the carbon content should be adjusted near 0.02 pct, and the chromium content should be the highest possible in its chemical composition scale.     

高温渗碳轴承钢旋弯疲劳裂纹萌生与扩展行为

 为了提高航空轴承的服役寿命,借助QBWP-10000X型旋转弯曲疲劳试验机,研究了高温渗碳轴承钢的旋转弯曲疲劳性能和裂纹萌生扩展行为。结果表明,钢的中值疲劳强度达到913.3 MPa。有效渗层中大量M₂₃C₆和少量M₆C碳化物显著提高了试验钢的表面硬度,渗层不同碳浓度导致马氏体先后发生相变而形成408 MPa表面压应力,进而提高了钢的疲劳性能。疲劳裂纹主要萌生在表面缺陷和次表面碳化物,分别占比71.4%和 28.6%。萌生裂纹缺陷特征尺寸及承载应力对应力强度因子和循环次数影响显著,深犁沟形状由于涉及应力集中而直接影响疲劳循环次数,承受相同加载应力碳化物特征尺寸越大,循环次数越低。裂纹萌生后沿渗碳层碳化物边界快速扩展同时向芯部缓慢扩展,最后在试样疲劳源对侧近边缘区域发生准解理和韧性混合断裂。     

镁对GH3625合金一次碳化物析出的影响

 GH3625合金中碳的质量分数约为0.05%,由于含有较高的铌、钼和铬元素,合金中会形成 MC型、M₆C型和M₂₃C₆型碳化物,在冶炼凝固过程中由于选分结晶的原因,易产生碳化物偏聚问题。因为碳化物回溶温度偏高,在其可锻温度区间内很难消除,所以会导致合金棒材中存在碳化物条带状聚集的问题,对其服役性能影响较大。利用Thermal-calc热力学软件计算分析了GH3625合金平衡析出相及一次碳化物的析出规律,利用金相显微镜、扫描电镜等研究了镁对GH3625合金一次碳化物形貌、尺寸及分布的影响。结果表明,GH3625合金的基体为单一的奥氏体相,MC型碳化物作为一种高温析出相,直接从液相析出,其富含铌元素,其次还有少量的钛、钼等元素。而随着凝固温度的降低,铌质量分数逐渐升高;未加镁时,GH3625合金在二次枝晶间析出了大量长条状的一次碳化物,其平均直径和面积较大;加入质量分数为0.014%的镁后,镁通过改变碳化物相与基体相的比界面能关系,不仅有效地改善了合金枝晶间和晶界碳化物的分布及形貌,还减小了一次碳化物的尺寸;当镁质量分数增加到0.037%时,一次碳化物分布更加均匀弥散,此时镁细化、球化碳化物的效果最好;同时,试验合金在水冷和空冷的条件下,由于冷却速率比炉冷时更大,一次碳化物析出尺寸也相对更细小。     

Fatigue crack initiation and propagation in a quenched and tempered niobium bearing HSLA steel

The fatigue crack initiation and propagation behavior of a niobium bearing HSLA steel heat treated to give two tempered martensitic microstructures presumably with and without fine niobium carbides has been studied by light microscopy, electron microscopy, and strain gage measurements of plastic zone deformation. The high cycle, stress controlled fatigue life of the steel in both heat treated conditions was quite similar with the steel presumably containing the fine niobium carbides having slightly better resistance at low stress amplitudes. This slightly better high cycle resistance is associated with better resistance to fatigue crack initiation for this heat treatment. The fatigue crack propagation behavior of the steel was the opposite. The steel presumably containing the fine niobium carbides exhibited a much faster fatigue crack growth rate than that without them. The difference in growth rates is explained in terms of the plastic work expended during the propagation of the fatigue crack.     

以紫钨为原料制备超细WC-Co硬质合金

超细硬质合金具有高硬度、高耐磨性等优异性能,保证超细硬质合金的晶粒度小而且均匀的一个关键因素就是以粒度细小、分布均匀的超细WC粉末为原料。超细WC粉末的制备过程中,常用的氧化物原料为蓝钨,以紫钨为原料的制备工艺报道较少。采用相同的工艺,分别以蓝钨和紫钨为原料制备出超细WC粉末,并采用相同工艺制备出超细硬质合金,对两种产品性能进行对比,发现以紫钨为原料制备出的超细硬质合金晶粒度小,强度和硬度高,具有较好综合性能。     

Correlation between Microstructure and Mechanical Properties of High C‐Cr Cast Steel

The effect of carbide morphology and matrix structure on abrasion resistance of cast alloyed steel with 2.57% C, 16.2% Cr and 0.78% Mo was studied in the as‐cast and heat treated conditions. Samples were austenitized at three different temperatures of 980, 1050 and 1250 °C for 15 minutes and followed by tempering at 540 °C for 3 hours. The austenitizing temperature of 980 °C revealed fully martensitic structure with little amount of retained austenite, while at 1050 °C the matrix was austenitic with massive amount of coarse secondary carbides. The austenitic matrix with very fine secondary carbides was developed at 1250 °C. The maximum abrasion resistance was obtained at 1050 °C due to the highest structure hardness and existence of both eutectic and secondary carbides in larger size than the formed groove by the abrasive particles during the wear test. On the other hand, the as‐cast pearlitic structure showed high wear rate by an applied load of up to 0.2 bar, followed by very rapid increase in wear rate with higher applied loads. It could be considered that the austenitizing temperature of 1050 °C showed better combination of abrasion resistance and toughness in comparison with other heat treatment cycles.