Ni在球墨铸铁中的行为及作用

采用电子探针、图像处理和差热分析仪等测定观察了Ｎｉ在球铁中的偏析分布特性，研究分析了Ｎｉ对铸态态球铁组成比例，Ｆｅ－Ｃ－Ｓｉ平衡相图Ｍｓ点和贝氏体转变动力学的影响及其机制，结果表明：Ｎｉ在球铁中呈连续负偏析，其加入可以改变铸态球铁中各组成相的比例，使得Ｍｓ点降低，奥氏体稳定性增加，等温转变过程中，贝氏体转变速度减慢。     

等温淬火对奥贝球铁上贝氏体转变动力学的影响

本文研究了等温淬火过程中奥氏体化温度和等温淬火温度对奥贝球铁上贝氏体第一阶段转变速度的影响及其机制；观察与测定了不同等温转变时奥贝球铁的组织与性能。结果表明，升高奥氏体化温度和等温淬火温度，使上贝氏体第一阶段转变速度减慢，在该转变结束时，奥贝球铁组织与性能最稳定。     

Ni在奥贝球铁上贝氏体第一阶段转变中的作用及其机制研究

采用光学显微镜、扫描电镜、透射电镜、电子探针、X射线衍射仪、图象处理仪、磁性秤、差热分析仪、磨粒磨损机、拉伸和冲击试验机等,研究分析了Ni、Mn在奥贝球铁中的行为、分布和对奥贝球铁热处理工艺参数的影响。 深入探讨了 Ni及 Ni与 Mn复合作用、奥贝球铁处理工艺参数对奥贝球铁贝氏体转变动力学、热稳定性、组织形貌、性能的影响与机制。 系统提出并论述了金相法、硬度法、磁性法、残余奥氏体量法四种确定贝氏体第一阶段转变表面结束时间和实际结束时间的有效方法和原理。 发现并解释了奥贝球铁中存在的网球状石墨及其形成机制和作用。 提出了Ni、Mn对奥贝球铁组织均匀性、强韧化和贝氏体转变动力学的复合作用机制。指出了保证奥贝球铁生产工艺稳定性和最佳综合性能的关键因素。     

Fe—Ni合金预贝氏体转变的研究

用扫描俄歇显微探地，Ｘ射线衍射扫描秀射电镜对Ｆｅ－Ｎｉ合金预贝氏体转变进行了研究。结果表明，在中温等温时，奥氏体内出现了成分偏聚，形成了贫Ｎｉ区和富Ｎｉ区，贝氏体在贫Ｎｉ区形成。     

马氏体基体含碳量对马氏体球铁磨损性能的影响

针对马氏体球铁,研究了马氏体基体不同含碳量(不同奥氏体化温度)对两体磨损和冲击磨料磨损耐磨性的影响。结果表明:在软磨料作用下的高应力两体磨损中,马氏体球铁以疲劳机制破坏,硬磨料作用下以切削机制破坏;在有较大冲击载荷作用的动载磨料磨损中,马氏体球铁存在有抗冲击疲劳剥落的最佳马氏体含碳量;石墨球的存在对耐磨性有害。     

60Si2Mn钢中的上贝氏体

本文用光学显微镜、电子显微镜和 X 射线衍射仪,对60Si2Mn 钢经870℃×20分钟奥氏体化后在320～500℃等温不同时间所获得的组识形态和结构进行了研究。认为这种钢的上贝氏体在等温形成的前期阶段为无碳化物贝氏体或称 BI 型上贝氏体,后期阶段为典型粒状贝氏体。粒状贝氏体的形成是 BI 型上贝氏体中贝氏体铁素体长大与富碳奥氏体为减少界面能而进行球化过程的结果。BI 型上贝氏体中奥氏体量达22%,其含碳量为1.52%,经-78℃处理和300℃以下回火显示了高度的稳定性。此外,初步探讨了 BI 型上贝氏体的机械性能。     

贝氏体相变区等温停留对低合金TRIP钢相组成的影响

用膨胀法及XRD法研究了经780℃临界间加热后20Mn2SiMoV钢在贝氏体转变区域等温温度和等温时间对相组成、残余奥氏体含碳量(CA残)及其机械稳定性的影响。结果表明，随着等温温度的提高，残余奥氏体量(体积分数φA残，下同)增加、含碳量减小；随着等温时间的增加．残余奥氏体先增加后减少；在没有析出碳化物的前提下，残余奥氏体含碳量随着等温时间的增加而增加，在380℃等温50min时残余奥氏体达到最高(22％)；在340℃等温9．2min时奥氏体的机械稳定性最好。     

铸态奥氏体—贝氏体球墨铸铁的组织与性能

探讨了在砂型连续冷却条件下获得的铸态奥氏体－贝氏体球铁的组织特点，断口特征和力学性能。结果表明，这种球铁具有优良的综合力学性能，是一种很好的新型抗磨材料。     

65Si2MnWA钢下贝氏体碳原子调幅分解及有序化的TEM研究

利用Ｓｉ元素阻碍碳化物析出的作用，对６５ＳｉＭｎＷＡ钢进行适当热处理，获得无碳化物析出的下贝氏体组织，通过透射电镜分析了贝氏体基体衍射花样特征，进而考察其碳含量，结果表明，该钢下贝氏体转变初期存在碳原子调幅分解及有序化过程，其碳原子行为与马氏体回火早期阶段碳原子行为相似，说明该钢下贝氏体基体含碳量是过饱和的。     

贝氏体钢精细结构的TEM观察

使用ＴＥＭ研究了钢中贝氏体的精细结构及碳化物的形貌与分布，加入微量元素使贝氏体铁素体组织明显细化。贝氏体铁素体由亚单元或亚块组成，碳化物形貌不一，分布在条间，片内和亚块边界。贝氏体铁素体内有极为丰富的精细组织。文中讨论了贝氏体的相变机制。     

Investigation of bainitic isothermal transformation products in unalloyed ductile cast iron

The properties of the products of isothermal transformation of undercooled austenite into bainite in unalloyed ductile cast iron were investigated using X-ray diffraction. The following parameters were investigated: the fraction of austenite in the cast iron matrix, the crystal lattice parameter, and the width of the diffraction lines of the α and γ phases. The structures were studied using a TEM. It was observed that the temperature T_A and time τ_A of the isothermal transformation significantly influence the nature of the α and γ phases. The transformations are determined by the diffusion of carbon, and the maximum carbon content is approximately twice the equilibrium carbon content at the austenitising temperature. The lattice parameter of the α phase in the range of T_A studied decreases with increasing cooling time but increases in the upper bainite range. The increase in this lattice parameter results from the typical process of bainitic transformation during the retained austenite eutectoidal reaction (stage III). The crystal structure of the γ phase in the upper bainitic region is more perfect than in the lower range. Within the investigated temperature range of T_A, bainitic ferrite continually improves its crystal structure.

MST/3104     

Metallography of bainitic transformation in austempered ductile iron

Abstract

An unalloyed nodular cast iron has been used to investigate the development of microstructure on heat treating in the bainite temperature region. Specimens were austenitised at 900°C for 1·5 h, then austempered for 1, 2, or 3 h at 250,300, and 350°C, respectively, and examined by light, transmission electron, and scanning electron microscopy. Experimental results indicate a microstructure consisting of a stable, highly enriched, retained austenite with one of two lower bainitic ferrite morphologies. One of these morphologies is carbide free acicular ferrite for specimens austempered at 350°C for 1 h and the other is bainitic ferrite in which carbide is distributed within the ferrite produced by different heat treatment conditions. Austempering at 350°C for 2 h and at 300°C for 1 and 2 h resulted in the formation of transition carbides in bainitic ferrite platelets. The η carbide was formed at 350°C for 2 h by precipitation from a bainitic ferrite supersaturated with carbon. By contrast, ? carbide was associated with austempering at 300°C for 1 and 2 h and precipitates either on the austenite twin/bainitic ferrite boundaries or within the bainitic ferrite. The fracture mode of tensile and impact specimens in the austempered condition was fully ductile compared with as cast specimens, which had mixed fracture characteristics.

MST/1646     

Austempered ductile iron heat treatment for machine hammer peening of milled tool surfaces

Austempered ductile iron (ADI) has become a competitive material to conventional steels. In addition to its favorable price the main reasons are its mechanical properties can be adjusted over a wide range via heat treatment. Austempered ductile iron consists of ferrite, graphite and metastable austenite. Tailoring its microstructure (phase fractions, stability) with regard to the application is an important challenge. A cast iron used for forming dies is EN‐JS2070. In earlier studies it could be shown that EN‐JS2070 can be transformed into austempered ductile iron [1]. Machine hammer peening, causes martensitic transformation of the metastable austenite and leads to hard and smooth surfaces. Focus of this study is to optimize the microstructure with regard to machine hammer peening process. Before and after machine hammer peening the sample surfaces were characterized using optical and laser microscopy, X‐ray diffraction and hardness measurement. It could be shown that a combination of high amount of metastable austenite with a high carbon content leads to the best results in surface roughness and hardness.     

Effect of Ni on Sintering Characteristics of Short Cast Iron Fibre

Nickel,as the carrier through which Fe atomsdiffuse,obviously enhances the densification ofsintered compact containing short cast iron fibre.When Ni content is above 4.5 wt-% there are someaustenites in microstructure at room temperature,discontinuous cementite networks at the interfacesbetween austenites and pearlites,free graphite pre-cipitated in austenite and pores andWidmannst(?)tten carbide precipitated in austenite.When Ni content is about 12 wt-%,it leads to de-crease in densification ratio and change in fracturemorphology of sintered compact from ductile tobrittle.     

合金元素Ni对铸铁短纤维烧结特性的影响

Ni 作为铁元素扩散的载体,显著加快了铸铁短纤维烧结体的致密化。当镍含量高于4.5%时,显微组织中发现了奥氏体,在奥氏体与珠光体边界析出断网状渗碳体,在奥氏体晶内析出魏氏碳化物及游离石墨,引起致密化速率下降,断裂方式由延性断裂转化为脆性断裂。     

Microstructure and toughness of CuNiMo austempered ductile iron

The effect of austempering on the microstructure and toughness of nodular cast iron (designated as CuNiMoSG) alloyed with molybdenum, copper, nickel, and manganese has been studied. Light microscopy (LM), scanning electron microscopy (SEM), and X-ray diffraction technique were performed for microstructural characterization, whereas impact energy test was applied for toughness measurement. Specimens were austenitised at 860 °C, then austempered for various times at 320 and 400 °C, followed by ice-water quenching. Austempering at 320 °C produces a microstructure consisting of a mixture of acicular bainitic ferrite and the stable carbon-enriched austenite. In this microstructure ε-carbides are also identified after austempering up to 5 h. Fracture mode is changed from ductile to brittle with the prolonged time of austempering at 320 °C. The highest impact energy (115 kJ) corresponds not only to ductile fracture, but also to the maximum value of the volume fraction of retained austenite. Only martensitic structure was observed during austempering at 400 °C, inducing brittle fracture and significantly low-impact energy (10–12 kJ).     

Nanoausferritic matrix of ductile iron

Abstract

Austempered ductile iron is known for its excellent mechanical properties resulting from special phase composition and austempering heat treatment. Typical microstructure consists of ferrite plates of micrometre size submerged in untransformed austenite matrix. It has been recently shown that by use of appropriate chemical composition of cast iron and well targeted heat treatment parameters, it is possible to reduce ferrite plates width to submicron or even nanometric size. This creates the potential to achieve even higher mechanical properties of austempered ductile iron. The paper describes the influence of applied heat treatment parameters on microstructure of selected austempered ductile iron grades. Conditions necessary to reduce size of phases to a nanometric scale by heat treatment in austempered ductile iron are discussed.     

Multiphase matrix structure of unalloyed austempered ductile iron

A multiple phase structure characterised by a mixture of lenticular prior martensite (PM), fine needle bainitic ferrite and film retained austenite (RA) of an unalloyed ductile iron is developed. The designed austempering consists of initial rapid quenching to 210, 200 and 180°C respectively and finally austempering at 220°C for 240 min. The optimum mechanical properties, with a tensile strength of 1330?MPa, an elongation of 3.1% and 422HB, can be achieved by controlling the volume fraction of PM to 12.3% and the RA content to 18.1%. This is mainly attributed to PM that can accelerate the subsequent bainitic transformation and promote refinement of multiphase colonies.     

Abrasive wear property of bainitic nodular cast iron in laser processing

The effects of the different austenitizing and isothermal transformation temperature and time on the wear resistance in bainitic ductile iron has been investigated and compared with surface hardening by laser processing, to find the best wear resistance for the bainitic ductile iron. It was found that the wear resistance of the lower bainitic ductile iron was better than upper bainitic ductile iron, and that the factor affecting hardness and wear properties the most was the isothermal transformation temperature. After surface hardening by laser processing, the hardening reached RC 55 or more, no evidence of any difference between the lower and the upper bainitic ductile iron being found.