Effect of scanning speed during PTA remelting treatment on the microstructure and wear resistance of nodular cast iron

The surface of nodular cast iron （NCI） with a ferrite substrate was rapidly remelted and solidified by plasma transferred arc （PTA） to induce a chilled structure with high hardness and favorable wear resistance. The effect of scanning speed on the microstructure, micro-hardness distribution, and wear properties of PTA-remelted specimens was systematically investigated. Microstructural characterization in-dicated that the PTA remelting treatment could dissolve most graphite nodules and that the crystallized primary austenite dendrites were transformed into cementite, martensite, an interdendritic network of ledeburite eutectic, and certain residual austenite during rapid solidifica-tion. The dimensions of the remelted zone and its dendrites increase with decreased scanning speed. The microhardness of the remelted zone varied in the range of 650 HV0.2 to 820 HV0.2, which is approximately 2.3-3.1 times higher than the hardness of the substrate. The wear re-sistance of NCI was also significantly improved after the PTA remelting treatment.

Effects of Light and Heavy Rare Earths on Anti-Degradation of Nodular Cast Iron

The effects of rare earths with different contents on anti-degradation of Y base heavy rare earths-magnesium and cerium base light rare earths-magnesium nodular cast iron were studied. Curves of the relationship between the spheroidization grade of liguid iron treated by different nodulizers and holding time were obtained. The results show that the more the remains of the rare earth, the longer the anti-degradation time, the liquid iron was treated either by Y-Mg-Si or by Ce-Mg-Si nodulizer. When the rare earth remains are above certain quantity, the spheroidization grade starts to decline. When liquid iron treated by either Y-Mg-Si or Ce-Mg-Si nodulizer with proper remains of rare earths about 0.04% 0.06%（mass fraction）, the anti-degradation is the best, and the remains of Mg being about 0.04% -0.07% （mass fraction）. Rare earths-Mg nodulizer with moderate RE remains is better than that of Mg-Si nodulizer in anti-degradation property. However, Y-Mg-Si nodulizer with moderate remins of Re is better than Ce-Mg-Si nodulizer. Sometimes Ce-Mg- Si nodulizer can be used for the production of heavy section nodular cast iron.

Centrifugal Casting of High Speed Steel/Nodular Cast Iron Compound Roll Collar

The centrifugal casting of compound HSS/nodular cast iron roll collar was studied,and the factors affecting transition zone quality were analyzed.The pouring temperature and interval in pouring are the main factors affecting transition zone quality.By controlling process parameter and flux adding during casting,high quality roll collar was obtained.The cause,why in the casting of HSS part,segregation appears easily,was analyzed and the countermeasure eliminating segregation was put forward,the measure eliminating heat treatment crackling was also put forward.

Effect of nodularity on mechanical properties and fracture of ferritic spheroidal graphite iron

Ferritic spheroidal graphite irons with nodularity from 72%to 96%were prepared.The relationship between the nodularity and the mechanical properties of the ferritic spheroidal graphite iron was investigated.The effect of nodularity on the mechanical properties and tensile fracture of the cast iron were studied.Results showed that the tensile strength Rm,yield strength Rp0.2,elongation to failure A5,and impact energy KV2 of the cast iron had a good linear relationship with its nodularity.Nodularity and annealing treatment would obviously affect the fracture characteristics of ferritic spheroidal graphite iron.The annealed ferritic spheroidal graphite iron with 93%nodularity showed a completely ductile rupture.With the decrease of nodularity from 93%to 72%,the cleavage fracture area ratio increased gradually from 0%to 8.3%.Compared with as-cast ferritic spheroidal graphite iron,annealing treatment reduced the cleavage fracture area of the ferritic spheroidal graphite iron.

Analysis of reasons causing riser feeding failure in nodular iron castings production

In addition to mold rigidity and metallurgical quality of iron melting, the main reasons causing riser feeding failure in nodular iron castings production are: (a) open and cold metal flowing-over risers were adopted; (b) riser location was not proper; (c) riser was too small or/and not enough high; (d) ingates did not freeze up instantly as soon as pouring finished; (e) there’re isolated hot spots in the casting which are not connected with feeding channel of the riser; (f) the feeding channel of castings with small size and thin sections is too narrow for feeding liquid to enter casting; and so on.

SIZE DISTRIBUTION AND BEHAVIOUR OF VC PARTICLES IN FERRITIC NODULAR CAST IRON

Size distribution and precipitation of VC particles in ferritic nodular cast iron have been examined by techniques of ultrasonic screening,small angle X-ray scattering and chemical dissolution.The VC particles are sized over a wide range.The superfine ones, dispersed in the ferritic matrix,are the majority.A saturated dispersion with ultrafine VC particles may be approached as the V content increasing in the iron.Thi5 seems to play an important role in the precipitation strengthening for ferritic nodular cast iron.

Strength distribution at interface of rotary-friction-welded aluminum to nodular cast iron

The morphology,size and composition of intermetallic compound at the interface of Al 1050 and nodular cast iron were studied by electron microprobe analysis(EMPA)and scan electron microscopy(SEM),respectively.The bond strength of the interface was measured by the tensile tests and the morphology of the fracture surface was observed by SEM.The observation of the interface reveals that there are two distinct morphologies:no intermetallic compound exists in the central area at the interface;while numbers of intermetallic compounds(FexAly)are formed in the peripheral area due to the overfull heat input.The tensile tests indicate that the distribution of strength in radial direction at the interface is inhomogeneous,and the central area of the interface performs greater bond strength than the peripheral area,which proves directly that the FexAly intermetallic compounds have a negative effect on the integration of interface.The morphology on the fracture surface shows that the facture in the central area at the interface has characteristic of the ductile micro-void facture.So it is important to restrain the form of the intermetallic compound to increase the bond strength of the Al 1050 and nodular cast iron by optimizing welding parameters and the geometry of components.

PIXE ANALYSIS OF GRAPHITE PHASE IN NODULAR CAST IRON

The graphite phase has been extracted from the nodular cast iron to measure the concentration of trace element in it. The impurity phase was removed from the extracted matter by dissolving it with HCl and HCl+H2O2 to get the pure graphite phase. PIXE measurements were performed with thick targets of the pure graphite phase. Differences in the trace element concentrations between the graphite phase and the matrix have been observed. The effect of Ti and As in graphite phase on the nodularization rate and the mechanical properties have been studied.

LASER ALLOYING ON CHROMIUM-COATED SURFACE OF NODULAR CAST IRON

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Study on numerical simulation of nodular graphite iron microstructure formation

In this paper, the mathematical and physical model was developed based on thermodynamics and solidification theory before the eutectoid transformation of nodular graphite iron occurred. The Local Element Substitute and Magnification Method was brought forward and 3-dimensional numerical simulation program based on the model and the new assistant algorithm was developed and used to calculate the samples. Results of calculation have good agreement with experimental data. To display the microstructure formation during solidification of nodular graphite iron, a 2-dimensional numerical simulation program combined with the result of the 3-dimensional numerical simulation of experimental samples was compiled.

Research on nodular iron joint welded with nickel-iron electrode by the moire fringe method

Nickel-iron type cast iron electrodes are widely used for welding QT600-3 nodular iron.Nevertheless,the research work has demonstrated that the strength of these heterogeneous welded joints is low.In this paper,the uni- axial tension of the joints is studied by the Moire fringe method,and the dis- placement,strain and stress in the total fields are obtaind.Based on the uneven distribution of strain and stress,the reason of crack initiation and propagation is discussed.Through examining the appearance of the fracture by scanning elec- tron microscope,a lot of spherical substances distributed on the fracture surface are found,which may be another reason leading to cracking of the welded joint under the lower tensile stress.These new findings will help to improve the properties of nickel-iron type cast iron electrodes and the strength of the welded joint.

Feasibility and practice of nodular iron casting feeder-less production

The volumetric changes of castings and dimension changes of mould cavity occurring during liquid cooling and solidification of nodular iron castings were described. The feasibility and prerequisites to realize feeder-less production of nodular iron castings was analyzed and proved with practical examples. It was pointed out that the feeder-less foundry method is by no means a feeding-less method, and it was emphasized that adopting high carbon equivalent, high rigidity mould, simultaneous and synchronous solidification, and intensifying cooling capacity of the mould to increase feeding effect of the gating system are important to successfully realize feeder-less production of nodular iron castings.

Strength Characteristics of Inoculated and Nodularised Thin Wall Ductile Iron Castings

Carbide precipitates in Thin Wall Ductile Iron (TWDI) used for automotive applications needs to be eliminated or reduced for improved strength, ductility, crack propagation resistance and good machinability. Ductile iron thin section profiles (≤3 mm) present danger of massive carbide precipitations in the as-cast sample. Precipitated carbide phase is brittle and negatively affects the mechanical properties of the iron matrix. The suppression of carbide formation is associated with the nucleating properties of the nodularizer and innoculant alloys. This treatment is vital in ensuring that carbide precipitation, flake graphite structure and non-nodular graphite phases are reduced or completely eliminated in the TWDI castings. Therefore, the temperature and technique of treatment would influence the yield of the process, and ultimately the mechanical properties. In this study, the effect of nodularization and inoculation treatment temperature on the microstructure and mechanical properties of TWDI castings is examined. The results indicate that good nodularity and nodule count with better percent elongations are achieved using low treatment temperatures in descending order of 1490°C, 1470°C and 1450°C, but have negative effect at lower treatment temperature of 1430°C. However, TWDI castings have superior properties in terms of nodule counts and nodularity at 1450°C. Treatment temperature does not produce significant influence on ultimate tensile strength (UTS) and hardness of TWDI castings. TWDI castings show poor nodularity, nodule count and ductility at higher inoculation treatment temperatures of 1550°C, 1530°C and 1510°C.

工艺参数对球墨铸铁和低碳钢激光焊接的影响

为了提高球墨铸铁和低碳钢焊接接头力学性能,解决容易开裂的问题,采用激光焊接的方法对球墨铸铁和低碳钢进行焊接,研究了工艺参数对其焊接接头的组织和性能影响。结果表明,随着激光功率逐渐增加或焊接速率的逐渐变缓,试验件抗拉强度呈现出先增大后减小的变化趋势;当焊接功率为4250 W和焊接速率为2.4 m/min时,试样件焊接接头的强度处于极值400 MPa;在球墨铸铁侧热影响区内形成3种壳体结构(双壳结构、单壳结构和无核结构);焊缝区组织主要由树枝晶组织、少量的马氏体和莱氏体组织构成;焊缝区存在微裂纹,通过添加镍基材料可获得无裂纹焊缝,并提升抗拉强度约40 MPa,达到母材的95%。该研究对后续球墨铸铁与低碳钢高功率激光焊接工艺优化是有帮助的。

随流孕育对高镍奥氏体球墨铸铁的影响

介绍了高镍奥氏体球墨铸铁排气歧管铸件的结构及首次交付的样件在试验中的断裂问题,通过生产试验,验证了排气歧管断裂的原因是在浇注过程中不进行随流孕育,导致铸件金相中产生碎块状石墨,使铸件力学性能下降,加速了铸件开裂失效的风险。最后指出:(1)高镍奥氏体球墨铸铁件在生产过程中,除了保证球化处理严格按照工艺要求外,在浇注过程中还必须进行随流孕育;(2)在保证球化处理过程符合工艺要求的前提下,随流孕育会进一步增加铁液在凝固过程中石墨结晶的核心,增加共晶团数,避免碎块石墨的产生。

球墨铸铁表面激光熔覆层的组织及耐腐蚀性能研究

目的解决用于乏燃料贮运的球墨铸铁筒体容器因内壁腐蚀导致服役寿命短的问题,提高球墨铸铁表面的硬度及耐腐蚀性能。方法利用送粉式激光熔覆技术在球墨铸铁表面制备4种不同组分的316L不锈钢、Ni625合金、410L不锈钢及NJ30合金熔覆层。采用金相显微镜、X射线衍射仪、扫描电子显微镜、能谱仪表征熔覆层的微观组织、物相组成、元素分布及腐蚀形貌。借助显微硬度计、电化学工作站测试熔覆层的显微硬度及耐腐蚀性能。结果4种合金熔覆层均成形质量良好,无裂纹及明显的气孔缺陷,并与基体形成了良好的冶金结合。熔覆层底部至顶部出现相似的组织形态,但晶粒尺寸存在较大的差异。316L、Ni625、410L及NJ30显微硬度显著高于基体,分别为基体的1.74、1.92、2.35及2.4倍,熔覆层内部固溶体及硬质相的生成是提高硬度的显著因素。在3.5%的NaCl溶液中,4种熔覆层的耐腐蚀性由好到差为316L、Ni625、410L、NJ30。腐蚀形貌显示,316L及Ni625熔覆层在电化学测试环境下具有良好的抗腐蚀性能。结论316L及Ni625合金熔覆层具有良好的综合性能,适用于提高大型球墨铸铁容器的表面改性。

高镍奥氏体球墨铸铁涡壳的铸造工艺优化

介绍了高镍奥氏体球墨铸铁涡壳铸件的结构及铸造工艺,针对试制过程中出现的表面缩孔、缩松、显微缩松以及组织中的碎块状石墨、表面恶化层、碳化物等问题进行了工艺优化,最后指出:(1)高镍奥氏体球墨铸铁液态收缩量大、石墨化膨胀弱,冒口设计遵循顺序凝固原则是防止涡壳产生缩孔、缩松缺陷的有效措施;(2)涡壳厚处和舌尖易形成碎块状石墨,强化孕育可防止缺陷的产生;(3)表面恶化层超标,可通过较高的残余镁量和砂芯刷涂料进行防止;(4)由于冷却速度较慢,舌尖部位碳化物析出数量较多,良好的孕育可减少碳化物析出量,热处理也可改变碳化物体积分数及其形态。

大型球墨铸铁蜗轮铸件的生产工艺

介绍了大型球墨铸铁蜗轮铸件的铸造工艺设计及生产过程,利用华铸CAE软件对球墨铸铁蜗轮铸件的两种工艺方案充型及凝固过程进行了数值模拟分析,经过对比,选择中心轴孔端面引入的工艺方案二进行进一步优化。为了满足力学性能要求,选用纯净的原材料、合理的化学成分,采用包内孕育与随流孕育相结合的处理方式。通过对生产的10件铸件进行表面质量及力学性能检测,结果显示:铸件加工后,中心轴孔、键槽、外圆齿圈位置未见缩孔、缩松、夹渣等铸造缺陷,铸件的金相组织和力学性能均符合技术要求。

固溶强化铁素体QT500-14在转向器壳体中的应用

介绍了Si固溶强化铁素体球墨铸铁QT500-14的化学成分设计、合理的熔炼工艺以及铁液处理工艺,对单铸Y型试块和转向器壳体铸件本体取样分析发现,利用Si固溶强化的铁素体基体可以保证QT500-14汽车转向器壳体的基体组织和球化级别符合要求,从而满足力学性能及硬度等指标的要求。不同w(Si)量对材料组织和性能影响的研究数据显示:随着w(Si)量的增加,石墨有细化趋势,球径逐渐减小,固溶强化效果愈加明显,抗拉强度逐渐增强、韧性呈小幅下降趋势;Si在共晶转变时,有利于石墨的析出,降低石墨的尺寸,提高石墨的圆整度,减少对基体的割裂作用。

两种随流孕育剂在厚大球铁件上的应用对比

采用250 mm厚的方形铸块和D型试附铸试块模拟风电铸件厚大部位,对比分析了两种特效随流孕育剂处理后的试块金相组织和力学性能。试验结果显示:硫氧孕育剂孕育处理的试块抗拉强度和屈服强度要好于硅铝孕育剂孕育处理的试块,且硫氧孕育剂孕育处理的试块石墨数量要高于硅铝孕育剂孕育处理的试块;采用硅铝孕育剂孕育处理的D型试块低温冲击性能要好于硫氧孕育剂孕育处理的D型试块;两种孕育剂孕育处理的250 mm试块的冲击性能差异不大;两种孕育剂孕育处理的250 mm铸块几何心部的石墨数量都要高于上下表面的石墨数量。