Erosion behavior of high silicon bainitic structures: II: High silicon steels

The erosion behavior of two austempered high silicon (2.5 wt.% Si) steels, one containing 1 wt.% Mn and the other 1 wt.% Ni, has been compared with the mechanical property variations after austempering for various times in the upper bainite temperature range (420 °C). For both steels the erosion resistance is a maximum for austempering times near the end of stage II. The erosion resistance correlates directly with the tensile toughness and with the percentage elongation divided by the percentage reduction in area and inversely with the hardness in all three stages of austempering. It is shown that the strain-hardening coefficient peaks at the minimum erosion rate and that the high strain-hardening character of the retained austenite in these austempered steels is beneficial to erosion resistance. Comparison with the austempered cast irons in Part I of this study shows that the graphite nodules of the cast irons have no significant effect upon erosion rate.     

Effect of structural features on erosion resistance of cast irons

Erosion resistance of four types of cast iron of different microstructures and graphite morphologies (viz., grey cast iron, compacted graphite iron, spheroidal graphite iron and austempered ductile iron) was evaluated in three different erosive media. Results indicate that austempered ductile iron has the highest erosion resistance in all three media, followed by spheroidal graphite iron, compacted graphite iron and grey cast iron, in that order. Graphite morphology has a significant effect on the erosion resistance of these irons in quartz-water and iron oxide-oil slurry. However, the matrix microstructure determines the erosion resistance of these irons in quartz-oil slurry. The parameter H/E (which is the ratio of the Brinell hardness number to Young's modulus of the material) has been found to be a good indicator of erosive wear in quartz-oil slurry.     

球化孕育处理工艺对下贝氏体铸铁组织特性及力学性能的影响

采用真空中频感应炉,通过改变球化孕育处理工艺获得具有不同形态石墨的铸造试样,将铸件加热到860 ℃保温2 h,使用20 ℃的硝酸钠、亚硝酸钠饱和溶液将其连续冷却至室温,并在250 ℃低温回火2 h后空冷,获得具有不同形态石墨的下贝氏体基体组织。研究了球化孕育处理工艺对石墨球化效果的影响,以及石墨形态对热处理显微组织和力学性能的影响。结果表明：球化处理温度及时间对铸铁中的镁吸收率有显著影响,超过1 500 ℃时,镁烧损加剧,当球化处理温度为1 450 ℃,镁吸收率达到64.2%;孕育剂添加工艺也是影响铸铁中石墨的形态和分布的重要因素,当一次孕育和二次孕育添加量分别为0.8%和1.2%时,石墨的形态以球状为主,尺寸细小且分布均匀,石墨的球化率为93%,球化级别达到2级,尺寸级别为9级;另外,均匀分布的球状石墨有利于提高热处理后下贝氏体铸铁的综合力学性能。     

A metallurgical evaluation of tool wear and chip formation when machining pearlitic grey cast irons with dissimilar graphite morphologies

The need for superior in-service strength has meant that an increasing number of engineering components are now being made from pearlitic cast irons containing spheroidal graphite, rather than the more traditional cast irons containing flake graphite. Such changes of workpiece material have resulted in a rapid decline in tool life in many machining operations, particularly turning and facing.An investigation into the factors involved during chip formation which result in the observed patterns of tool wear is described in the work presented here. A series of turning tests were made on pearlitic grey cast irons containing flake (GA iron) and spheroidal (SG iron) graphite morphologies with cemented carbide (coated and uncoated) and ceramic tool materials. Built-up edge persisted to higher cutting speeds when cutting SG iron than GA iron, its periodic detachment causing attrition or fracture of the cutting edge. Smooth wear processes, probably caused by dissolution-diffusion and small strain discrete plastic deformation, were predominant on the rake and flank faces of the coated and ceramic tools when cutting both cast irons at high speed. Smooth wear was less rapid when cutting GA iron than SG iron because tool temperatures were reduced and “protective” nonmetallic layers, deposited from the chip-workpiece, interrupted dissolution-diffusion. When cutting SG iron, rapid wear of the uncoated cemented carbides was caused by attrition, while the relatively slower smooth wear, when cutting GA iron, was caused by dissolution-diffusion.     

Microscopic approach to micromechanism of damage in spheroidal cast iron

In this work, the observations of the fracture surface after standard tensile tests of several kinds of spheroidal cast irons, ferritic and austempered ductile irons, have been carried out by means of scanning electron microscopy. The local crack path in the area of graphite (G)/matrix (M) interface has been analyzed as affected by a matrix phase composition and the austempering treatment parameters. The obtained results allowed identifying some determination factors for debonding mode at the G/M interface and their role in a final damage mechanism. Some microstructural details in the microregions composed of graphite and matrix showed that the G–M debonding mode in the separation area of the G/M interface seems to be controlled by macroscopic properties of the alloy and by the morphology of G/M interface. On the other hand, the internal destruction of graphite nodule has been mainly determined by a structure and anisotropy of graphite crystal lattice.     

Sliding wear of graphite crystallized chromium white cast iron

The effects of sliding velocity, heat-treatment and graphite shape on sliding wear of graphite crystallized chromium white cast iron were studied. Two types of graphite crystallized chromium white cast irons having flaky or spheroidal and another type of 2.6C–15Cr white cast iron were prepared for this study. The effect of sliding velocity on wear resistance was studied by the Okoshi type and pin-on-disk type wear tests on materials which have experienced “as cast” and “heat-treated” conditions. The Okoshi type wear test results are divided into two relationships depending on sliding velocity or distance. Two regimes, initial wear and steady-state wear, existed for wear loss and sliding distance. A characteristic form of wear curve with a peak and a minimum was obtained when correlating wear loss and sliding velocity. The wear resistance of graphite crystallized chromium white cast irons were superior to that of 2.6C–15Cr white cast iron. In the results of pin-on-disk tests, there was no clear difference in the reported wear loss and friction coefficient among the alloys. However, an opposite tendency has appeared in the wear loss and friction coefficient: the wear loss value reached a peak in the wear curve at 0.52 m/s, while the friction coefficient reached a minimum at 0.52 m/s.     

Comparative research on wear characteristics of spheroidal graphite cast iron and carbon steel

Wear characteristics of a spheroidal graphite cast iron and a carbon steel were studied under atmospheric conditions at 25–400 °C. The spheroidal graphite cast iron presented obviously different wear behaviors from the carbon steel, which may be attributed to the presence of graphite. With an increase of ambient temperature, tribo-oxides of carbon steel substantially increased and its substrate softened, thus severe wear, oxidative mild wear, oxidative wear and extrusive wear took turns to prevail. However, compared with carbon steel in the same case, tribo-oxides were markedly reduced in the spheroidal graphite cast iron, thus oxidative mild wear and oxidative wear did not appear due to the lack of oxides. It is suggested that less tribo-oxides in the spheroidal graphite cast iron may be attributed to the reduction of graphite to tribo-oxides during sliding.     

Erosion behavior of high silicon bainitic structures: I: Austempered ductile cast iron

The erosion behavior of unalloyed austempered ductile cast iron has been studied as a function of austempering time for irons austempered in the upper bainitic temperature range (370 °C). A minimum erosion rate and iron matrix hardness occurs at the end of the stage II austempering time, where the microstructure consists of bainitic ferrite and retained austenite. Experiments show that erosion induces transformation of retained austenite at early times of stage II austempering and that maximum erosion resistance correlates with maximum work hardening of the substrate during the erosion process. Scanning electron microscopy studies of the eroded surfaces and the substrate revealed that the material removal in erosion occurs because of the cutting, ploughing and flake formation mechanisms which depend upon the microstructures and their mechanical properties.     

Wear mechanism and chemical composition optimization of complex-alloyed cast iron with spheroidal vanadium carbide under conditions of abrasive erosion

Using full factorial design in experiment 3², the contents of vanadium and chromium in heattreated V–Cr–Mn–Ni cast irons with spheroidal vanadium carbide have been optimized with regard to the conditions of quartz sand erosion. It has been found that, in the case of bulk quenching from 760°C (or in combination with a subsequent plasma surface hardening), the maximum wear resistance of cast iron is achieved at 5.0% V and 2.0–4.5% Cr and, in the case of bulk quenching from 840°C followed by cryogenic treatment (–196°C), at 5.0% V and 7.0–9.0% Cr. It has been shown that the wear mechanism of the investigated alloys consists of the repeated deformation (indentation) of the matrix accompanied with spalling of spheroidal carbides and with chipping of eutectic carbides. Spheroidal vanadium carbides provide an effective protection of dendrites regions against erosion due to their uniform distribution in the bulk of the alloys.     

球墨铸铁件微观组织的数值模拟

根据凝固动力学理论,建立和完善了球墨凝固过程中各阶段组织形成的数学模型。用试验试件进行了验证,并对实际生产球墨铸铁件的球墨数量和大小、珠光体和铁素体数量、硬度等进行了模拟,与定量金相分析结果进行了比较。结果表明模拟结果与分析结果吻合较好,同时,共析模型尚有待进一步改进以更准确地预测珠光体和铁素体数量。研究表明：球墨的形核与生长主要与过冷有关,共析阶段产生的珠光体与铁素体体积分数则由过冷度与球墨个数两者所决定。     

Abrasive wear resistance of spheroidal vanadium carbide cast irons

The effect of the chemical composition and heat treatment on the microstructure and abrasive wear resistance of V-Mn, V-Ni-Cr, and V-Mo spheroidal vanadium carbide cast irons (18–23 vol %) has been studied. The wear resistance has been determined under conditions of wear by abrasives with various hardnesses, i.e., corundum and quartz and compared to that of high-chromium cast iron with 13% Cr. It has been found that the advisability of using high-vanadium cast irons is governed by the hardness of the abrasive. When a hard abrasive, i.e., corundum was used, V-Mo cast iron with the maximum concentration of spheroidal VC carbides, which were uniformly distributed in the martensitic matrix, had the highest wear resistance. When a soft abrasive, i.e., quartz, was applied, high-chromium cast iron with a hardness of 68 HRC, which contained the largest amount of M₇C₃ carbides, was more wear-resistant. In the course of isothermal exposure at 300–1000°C, V-Ni-Cr and V-Mo cast irons with an austenitic structure had high resistance to phase and structural transformations. However, the properties and microstructure of V-Mo cast irons with a martensitic matrix depended strongly on the temperature of exposure during heat treatment.     

铸铁凝固期间电阻率的变化

作者们通过测量变压器式传感器中一次线圈内电流随时间的变化,研究了亚共晶白口铁、灰铸铁和稀土镁铸铁凝固期间电阻率的变化。发现,随着凝固行径和凝固过程中析出相的结晶构造不同,各种铸铁的电阻率变化特点也有很大差别。奥氏体和球状石墨的析出会造成试样电阻的降低或传播器一次线圈电流的升高,而莱氏体和片状石墨则引起相反的变化,在铸铁内所有的相和组成物中,石墨是决定电阻率变化的主要因素。作者们深信,本文所报导的实验结果有助于铸铁凝固机理的研究和生产过程的控制。     

Wear of spheroidal graphite cast irons for tractor drive train components

This study was prompted by a desire to improve the wear resistance of power transmission components in rear axle drives on commercial farm tractors. Reciprocating wear tests were conducted under lubricated and non-lubricated conditions on three spheroidal cast irons which varied in strength and hardness (designated GGG450, GGG600, and GGG700). Hemispherically tipped steel pins (designated 42CrMoS4/41CrS4) were used as the sliders. Except for the selection of the test duration, test procedures were similar to those described in ASTM Standard Test Method G133 for linearly reciprocating sliding. Among the three cast irons tested, the harder and stronger the alloy, the lower was its wear rate. Wear factors were approximately four orders of magnitude lower for experiments lubricated with fresh, fully formulated oil. There was a linear relationship between the Brinell hardness of the alloys and the negative logarithm of the wear factors that were expressed in mm³/N-m. Wear of lubricated test pins was not measurable due to the presence of deposits; however under non-lubricated sliding, the ratio of the wear of the flat specimen to that of the pin decreased as the hardness of the flat specimens approached that of the pin specimen.     

A new approach to calculate the nodule density of ductile cast iron graphite using a Level Set

This paper proposes a new approach with digital image processing techniques to determine the number of graphite nodules on samples of nodular cast iron by following the NBR 6913 standard. Counting errors and excessive inspection time are common problems if the procedure is performed without the assistance of computational systems. To overcome these drawbacks, we propose here an algorithm for graphite nodule segmentation based on the Level Set technique. The proposed approach and two other computational methods, Watershed and Region Growing, are compared with the results given by experts using optical microscopy (OM). The results of the proposed method were closer to those of the experts using OM than the other two computational methods. The proposed method presented greater accuracy and faster execution time than the traditional method by visual inspection.     

Effect of bulk heat treatment and plasma surface hardening on the microstructure and erosion wear resistance of complex-alloyed cast irons with spheroidal vanadium carbides

The results of an investigation of the effect of bulk quenching from temperature in the range of 760–1050°C, cryogenic treatment (–196°C) and surface plasma hardening on the abrasive-erosion wear of frugally alloyed V–Cr–Mn–Ni cast irons with spheroidal vanadium carbides have been presented in this article. It has been found that cast irons containing 5.0–7.5% V, 4.5–9.0% Cr, and 5.5–5.7% (total) of Mn and Ni after heat treatment have a 2–3-fold advantage in wear resistance compared to the prototype high-vanadium cast iron (11.9% V, 12.9% Mn). The maximum wear resistance of cast irons studied is achieved by quenching at 760°C followed by plasma surface hardening, as well as quenching at 840°C, followed by cryogenic treatment. These treatments result in the formation of an optimum microstructure that consists of spheroidal vanadium carbides, eutectic carbides M₇C₃, and a martensite-austenite matrix reinforced by secondary carbides. The increase in quenching temperature leads to an increase in the amount of residual austenite and decrease in the erosive wear resistance of cast irons.     

Microstructure and erosion resistance enhancement of nodular cast iron by laser melting

The surface of nodular graphite cast iron samples was melted by 50% overlapping passes from a 3 kW CW CO₂ laser. The objective was to modify the microstructure and improve the hardness and erosion resistance of the surface. The results showed that laser melting led to complete dissolution of the graphite nodules which on solidifying created an inter-dendritic network of ledeburite eutectic with a very fine structure, good homogeneity, and high hardness. Sand particle erosion experiments were carried out at impingement angles of 30°, 60°, and 90° using angular particles of size between 300 and 600 μm. The velocity of the sand particles was 50 m/s, which was controlled by the gas pressure and measured by the double-disc method. The erosion resistance of the laser treated nodular cast iron was 110 times greater than the untreated material. The erosion mechanism of the untreated nodular cast iron under normal and oblique angles was by severe plastic deformation and ploughing; whilst the mechanism for the treated specimens was by fatigue cracking. The improvements of erosion resistance after laser treatment were considered due to the very fine structure, high micro hardness (650 Hv0.1) with the resistance to plastic flow and to the dissolution of the graphite nodules.     

Erosion of AISI 4140 steel

The erosion behavior of AISI 4140 steel under various heat treatment conditions was investigated. A variety of microstructures, such as the primary and tempered martensites, varying proportions of martensite and bainite, cementite spheroids embedded in a ferrite matrix and ferrite and pearlite were obtained. The erosion tests were performed in a sand-blast-type test rig. Except in the region where temper embrittlement occurred, the erosion decreased with increasing tempering temperatures. Erosion decreased with the increasing percentage of bainite in the austempered condition and also with increasing tempering time during spheroidization. From the point of view of the mechanical properties, erosion decreased with increasing ductility and decreasing hardness or ultimate strength. The abraded surfaces were also studied using scanning electron microscopy.     

Effect of phosphide and matrix microstructures on the dry sliding wear of grey cast iron

The effect of a continuous phosphide network in matrices of pearlite, ferrite, martensite, and tempered martensite has been investigated on the dry wear of a grey iron, sliding at a speed of 1.5 m s⁻¹ with stresses of 0.5 and 2.0 MPa against cast iron. A running-in period was observed with a 0.2% P iron, whereas no running-in was observed with the 1.0% P irons. The presence of a continuous phosphide network reduced the wear rate of the pearlite iron by a factor of 0.25. In the weaker matrices (pearlite, ferrite, and tempered martensite) the phosphide network stiffened the matrix, fractured, and formed a particulate composite of phosphide in the deformed surface which resisted deformation. The wear rates and wear mechanisms of the irons are presented and discussed.     

优化组织球铁的力学性能及其微观断裂行为

根据球铁断裂过程中石墨及石墨-基体界面的微观力学行为,对球铁组织进行优化设计.以强相(马氏体)或强韧相(奥氏体-贝氏体组织)环包围石墨,基体组织为马氏体或奥氏体-贝氏体组织,加上适量的铁素体,并通过快速加热短时保温后淬火或等温淬火获得.实验结果表明,优化组织球铁具有很高的强韧性.采用扫描电镜动态拉伸观察优化组织球铁断裂过程,发现微裂纹在石墨-基体界面萌生并沿界面扩展,马氏体环或奥-贝环阻碍界面裂纹的萌生与扩展,基体中的硬相和软相分别提高球铁的强度和韧性,这些都有利于提高优化组织球铁的强韧性.     

铸铁气蚀破坏机制

从铸铁基体组织，石墨形态，合金元素等方面探讨铸铁气蚀性能的影响因素，分析空化的力学冲击作用与气蚀时的材料受力情况，研究铸铁气蚀裂纹核的形成与扩展乃至导致铸铁气蚀破坏的过程，剖析球墨铸铁的气蚀特点。