等温淬火温度对超细贝氏体钢组织及耐磨性的影响

设计了一种0.7C的低合金超细贝氏体钢,并通过膨胀仪、二体磨损实验、光学显微镜、扫描电镜、X射线衍射、激光扫描共聚焦显微镜及能谱仪,研究了不同等温淬火温度对超细贝氏体钢的贝氏体相变动力学、微观组织以及干滑动摩擦耐磨性的影响,揭示超细贝氏体钢在二体磨损条件下的耐磨性能和磨损机理.研究结果表明,不同等温温度下的超细贝氏体钢都由片层状贝氏体铁素体和薄膜状以及块状的残留奥氏体组成;随着等温温度的升高,超细贝氏体的相变速率提高,相变孕育期及相变完成时间缩短,但贝氏体铁素体板条厚度增加,残留奥氏体含量增加,硬度值有所降低;超细贝氏体钢磨损面形貌以平直的犁沟为主,主要的磨损机理为显微切削;不同等温温度下所获得的超细贝氏体的耐磨性能都优于回火马氏体,且随着等温温度的降低,耐磨性能提高.其中在250℃等温所获得的超细贝氏体钢具有最优的耐磨性能,其相对耐磨性为回火马氏体的1.28倍.这主要与超细贝氏体钢中贝氏体铁素体板条的细化及磨损过程中残留奥氏体的形变诱导马氏体相变（TRIP）效应有关.      

金属材料在酸性介质中的腐蚀电化学行为研究

由于金属材料的腐蚀电化学行为可以准确反映出金属材料的耐腐蚀性能特征,为此提出金属材料在酸性介质中的腐蚀电化学行为研究。首先通过金属材料在酸性介质中的腐蚀电位-时间曲线分析得知,纯金属材料与合金材料在酸性介质中的腐蚀电化学过程主要表现为:腐蚀程度随着酸性介质浓度的增加而增加;然后通过金属材料在酸性介质中电化学阻抗谱分析得知,随着酸性介质浓度的提高,金属材料腐蚀电流密度增加,并且腐蚀速度加快,钝化加强,以此完成了对金属材料在酸性介质中的腐蚀电化学行为研究。     

奥氏体等温淬火温度对两种不同磨擦体系条件下球铁耐磨性的影响

本文通过中温和高温等温淬火温度（L）评价了等温淬火球铁（ADI）作为一种耐磨材料用于生产机器零件的研究结果。进行了几次现场磨损试验和低应力实验室磨损试验（ASTMG-65）,同时用光学显微镜和x-射线衍射观察其显微组织特点,得出的结论为,在实验室条件下,随Ta的升高,ADI表现出极好的耐磨性。然而,在干砂／橡胶轮磨损装置（ASTMG65）,在低应力磨损条件下表现出相反的一面。在ADI显微组织中,亚稳相和韧性奥氏铁素体（转变完和没有转变完的奥氏体＋铁素体）是影响性能的主要因素。另外,检测到表面有高的变形能力,用x-射线衍射测定了奥氏体转变为马氏体的过程。这两个因素的作用,当奥氏体等温淬火温度提高时,使奥氏体铁素体显微组织硬度没有降低,大大提高了抗磨性,同时提高了冲击韧性。     

淬火温度对不同铬含量的低碳马氏体不锈钢组织和性能的影响

摘要：为研究淬火温度对不同铬含量的马氏体不锈钢组织和性能的影响,采用高温共聚焦显微镜(CLSM)、光镜(OM)、扫描电镜(SEM)、万能拉伸机、显微硬度计等方法对材料组织和性能进行了测试及表征。随着淬火温度的升高,不锈钢淬火后的晶粒尺寸都变大,计算确定了13%Cr和14%Cr不锈钢的晶界迁移能分别为113.62和125.92J/mol。13%Cr不锈钢经过淬火后显微组织为板条马氏体,回火后的组织为回火马氏体。但是,14%Cr不锈钢在1200℃淬火后生成了板条马氏体和少量的高温铁素体,并且在回火后高温铁素体并未消失,会对后续性能产生影响。淬火温度对不锈钢的强度影响不大。不锈钢中的铬质量分数从13%增加至14%,马氏体不锈钢强度增加,但伸长率有所降低。马氏体不锈钢的硬度随淬火温度的升高而下降,这主要与晶粒尺寸有关。     

淬火分配工艺对低碳低合金钢组织与性能的研究

研究了0.15C-Mn-Si-Cr低碳低合金钢在Ms点以下不同温度预淬火-碳分配工艺(QP工艺)及贝氏体转变对钢组织与性能影响。结果表明,实验钢经QP处理后获得贝氏体/马氏体复相组织,与淬火回火钢相比能获得更多的残余奥氏体量,随着淬火碳分配温度的升高,钢中残余奥氏体量增加,等温温度超过310℃后,钢中析出碳化物,残余奥氏体量减少。在250℃预淬火温度等温碳分配淬火,钢的冲击韧性显著高于传统的淬火回火钢。     

添加微量铈对热镀锌层耐腐蚀性的影响

在热镀锌镀液中加入微量铈,对其所形成的镀层在酸性和中性介质中的耐腐蚀性进行了测定和分析.结果表明,在镀液中加入约0.05%的金属铈,能显著提高锌镀层在酸性介质中的耐腐蚀性能;在镀液中加入约0.1%的金属铈,锌镀层在中性介质中的耐腐蚀性能最好.     

稀土硅铁合金对硼铸铁性能的影响

本文研究了稀土硅铁合金不同加入量对硼铸铁机械性能和耐蚀性的影响。结果表明，当加入量为０．４％时，σｂ＝２８４ＭＰａ，ＨＢ＝２２０，耐（酸）腐蚀性能最好。而且随温度升高，稀土硼铸铁耐（酸）腐蚀速度增大，随介质中ｐＨ值减小，耐（酸）腐蚀速度也增大。     

淬火温度对NM400马氏体耐磨钢残余应力的影响

采用热处理实验、X射线衍射(XRD)、光学显微镜(OM)、扫描电镜观察(SEM)等方法,研究了不同淬火温度对NM400马氏体耐磨钢的组织、硬度与表面残余应力的影响规律。结果表明,随着淬火温度的升高,原始奥氏体逐渐均匀化并粗化长大,实验钢表面轧制方向的残余应力逐渐增大。淬火温度940℃以下,随着温度升高,马氏体板条束逐渐增多,板条块逐渐减少,实验钢硬度随淬火温度的升高而增加;淬火温度1 150℃时,马氏体板条块增多,马氏体板条粗化,表面硬度明显下降。     

淬火温度对M4粉末高速钢组织和性能的影响

为了研究淬火温度对M4粉末高速钢组织和性能的影响, 利用光学显微镜观察高速钢试样的金相组织, 对淬火组织的晶粒度进行评级, 并对回火组织中碳化物的组成和分布进行统计; 采用洛氏硬度计和材料万能试验机测试试样的硬度和抗弯强度。结果表明: 随淬火温度的升高, M4粉末高速钢淬火后硬度先上升后下降, 在1200 ℃时出现最大值HRC62.9;淬火态试样的晶粒度随淬火温度的升高而降低。经三次回火后M4粉末高速钢硬度值较淬火态均有提高, 且随淬火温度的升高, 先增高后下降, 在淬火温度为1190 ℃时达到最大值HRC66.4。随淬火温度的升高, 回火态试样的抗弯强度逐渐下降, 碳化物聚集长大倾向明显, 尺寸均匀性下降。M4粉末高速钢的最优淬火温度区间为1180~1190 ℃。     

钢铁炉窑低温烟气中金属材料的耐腐蚀特性研究

采用自主设计的耐腐蚀特性试验台研究了钢铁炉窑低温烟气对五种常见金属材料的耐腐蚀特性,分析了不同烟气对金属材料腐蚀特性的影响。结果表明:金属材料在焦炉烟气中的耐腐蚀性强于热轧炉烟气;随着温度的升高,金属管材的锈层厚度和管壁膨胀率均逐步减少,20g的耐腐蚀性弱于ND钢;不锈钢的耐腐蚀性能由大到小依次为316L316304,其腐蚀速率随温度升高而下降,不锈钢在焦炉烟气中的腐蚀速率为32.5~146.2μm/a;金属管材在两种烟气中的腐蚀主要为硫腐蚀、碱金属腐蚀和煤焦油的腐蚀,但金属管材在两种烟气中的腐蚀表现形式有所不同。     

Mechanical behaviour of an austempered ductile iron

Austempering of a ferrite-pearlitic grade of ductile iron was carried out to assess the potential use of the material for crank shaft application reported. A commercial material was austempered at 340°C to realize the properties. The austempered ductile iron gave good strength although the ductility values were lower. The material developed had complete ausferritric structure free of pearlite. The various phase constitution and phase transformation associated with the treatment and during mechanical deformation was examined. Using XRD analysis the volume fraction of the austenite in the matrix was estimated. The various aspects of processing a commercial cast iron during ausetmpering, the phase transformation, microstructural evolution have been examined along with the property of the material. The mechanical behaviour of the material and the scope for further improvement is discussed.     

Neutron diffraction study of austempered ductile iron

Crystallographic properties of an austempered ductile iron (ADI) were studied by using neutron diffraction. A quantitative phase analysis based on Rietveld refinements revealed three component phases, α-Fe (ferrite), γ-Fe (austenite), and graphite precipitate, with weight fractions of 66.0, 31.5, and 2.5 pct, respectively. The ferrite phases of the samples were found to be tetragonal,14/mmm, with ac/a ratio of about 0.993, which is very close to the body-centered cubic (bcc) structure. The austenite phase had C atoms occupying the octahedral site of the face-centered cubic (fcc) unit cell with about 8 pct occupancy ratio. A strong microstrain broadening was observed for the two Fe phases of the samples. The particle sizes of the acicular ferrite phase were studied by using small angle neutron scattering. The analysis suggested a mean rod diameter of 700 A. The scattering invariant predicts a ferrite volume fraction consistent with the powder diffraction analysis. A textbook case of nodular graphite segregation, with average diameters ranging from 10 to 20 μm, was observed by optical micrography.     

An analysis of retained austenite in austempered ductile iron

Data from the literature have been analyzed to understand aspects of the retained austenite in austempered ductile irons, especially its relationship with the transformation mechanism of bainite. The final and initial carbon concentrations in austenite, and C _γ ⁰ , respectively, are important in determining the maximum extent of reaction, and hence, the amount of austenite and and bainitic ferrite and C _γ ⁰ data have been expressed in terms of chemical compositions and reaction temperature, with reasonable agreement between experimental and predicted results. It is demonstrated that, in connection with the lever rule, the calculated and C _γ ⁰ values can be employed to predict the microstructural constituents of austempered ductile irons.     

Dependence of fracture toughness of austempered ductile iron on austempering temperature

Ductile cast iron samples were austenitized at 927 °C and subsequently austempered for 30 minutes, 1 hour, and 2 hours at 260 °C, 288 °C, 316 °C, 343 °C, 371 °C, and 399 °C. These were subjected to a plane strain fracture toughness test. Fracture toughness was found to initially increase with austempering temperature, reach a maximum, and then decrease with further rise in temperature. The results of the fracture toughness study and fractographic examination were correlated with microstructural features such as bainite morphology, the volume fraction of retained austenite, and its carbon content. It was found that fracture toughness was maximized when the microstructure consisted of lower bainite with about 30 vol pct retained austenite containing more than 1.8 wt pct carbon. A theoretical model was developed, which could explain the observed variation in fracture toughness with austempering temperature in terms of microstructural features such as the width of the ferrite blades and retained austenite content. A plot of K _IC ² against σ _y (X _γ, C _γ)^1/2 resulted in a straight line, as predicted by the model.     

Microstructure and mechanical properties of copper alloyed austempered grey cast iron

Austempered grey cast iron (AGI) has emerged as a major engineering material in recent years because of its attractive mechanical properties. The main aim of this investigation is to assess the mechanical properties of copper alloyed AGI. Alloyed grey cast iron specimens are subjected to austempering heat treatment at six different temperatures for four different time periods. The resulting microstructures have been evaluated and characterised by means of light microscope and scanning electron microscope and X-Ray diffraction analysis. The microstructural features of AGI such as austenite content and its carbon content have been also found to influence the hardness, tensile properties and elongation. Both duration of the austempering time and the austempering temperature affect the mechanical properties of AGI. The hardness, tensile strength and ductility initially increase, and thereafter it decreases on longer periods of austempering. On the other hand hardness, tensile strength decreases as increasing austempering temperature, while ductility increases. The best combination of hardness 380BHN and strength 332?MPa; observed at 927°C of austenitising and 260°C of austempering temperature for 60?min.     

Influence of microstructure on fracture toughness of austempered ductile iron

An investigation was carried out to examine the influence of microstructure on the plane strain fracture toughness of austempered ductile iron. Austempered ductile iron (ADI) alloyed with nickel, copper, and molybdenum was austenitized and subsequently austempered over a range of temperatures to produce different microstructures. The microstructures were characterized through optical microscopy and X-ray diffraction. Plane strain fracture toughness of all these materials was determined and was correlated with the microstructure. The results of the present investigation indicate that the lower bainitic microstructure results in higher fracture toughness than upper bainitic microstructure. Both volume fraction of retained austenite and its carbon content influence the fracture toughness. The retained austenite content of 25 vol pct was found to provide the optimum fracture toughness. It was further concluded that the carbon content of the retained austenite should be as high as possible to improve fracture toughness.     

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.     

The effect of testing temperature on fracture toughness of austempered ductile iron

The effect of testing temperature (− 150 °C, 25 °C, and + 150 °C) on the fracture toughness of austempered ductile iron (ADI) was studied. Specimens were first austenitized at 900 °C for 1.5 hours and then salt-bath quenched to 360 °C or 300 °C, for 1, 2, or 3 hours of isothermal holding before cooling to room temperature. The resulting matrices of the iron were of upper-ausferrite and lower-ausferrite. It was found that raising the testing temperature to 150 °C from ambient improved the fracture toughness by 18, 30, and 7 pct for the as-cast/lower-ausferrite ADI/upper-ausferrite ADI, respectively. Lowering the testing temperature to −150 °C produced a decrease of −15, −35, and −48 pct. Optical microscopy, X-ray diffraction analysis, and scanning electron microscopy (SEM) fractography were applied to correlate the toughness variation with testing temperatures.     

Influence of stepped austempering process on the fracture toughness of austempered ductile iron

This research studied the effect of a two-step austempering process on the fracture toughness of ductile iron and compared it to that of the conventional upper- and lower-ausferrite austempered ductile irons (ADIs). The results showed that such a two-step austempering heat-treatment process yielded a fracture-toughness value equivalent to that of the upper-ausferrite ADI, while the hardness was maintained at the level of lower-ausferrite ADI. This provided a unique combination of high toughness with good hardness (strength) properties for the ADI with a two-step austempering. Optical microscopy, scanning electron microscopy (SEM), and X-ray diffraction analysis were performed to correlate the properties attained to the microstructural features.     

Embrittlement of austempered nodular irons: Grain boundary phosphorus enrichment resulting from precipitate decomposition

The microstructures, mechanical properties, and fracture behavior were characterized for a series of Mg treated nodular cast iron specimens austenitized at 1170, 1255, and 1340 K and subsequently austempered at 640 K. The ductility and toughness of the alloy decreased as austenitization temperatures were increased, which is contrary to the behavior anticipated from the observed micro-structural evolution. Fractographic and surface chemical analyses demonstrated that the mechanical property degradation was associated with embrittlement of the austenite grain boundaries by phosphorus. The primary mechanism of grain boundary phosphorus enrichment does not appear to be equilibrium segregation, and an alternative mechanism based on the decomposition of P rich precipitates is proposed and discussed. Formerly with Michigan Technological University, Houghton, MI