Correlation of microstructure and thermal fatigue property of three work rolls

This is a study of the thermal fatigue property in three centrifugally cast work rolls, i.e., a nickel-grain cast-iron roll (Ni-grain roll), a high-chromium cast-iron roll (Hi-Cr roll), and a high-speed steel roll (HSS roll). The thermal fatigue mechanism was investigated with a focus on the roll microstructure and the increase in tensile stress which led the specimen to fracture when it reached the tensile strength. The thermal fatigue test results indicated that the thermal fatigue property was best in the HSS roll, followed by the Hi-Cr roll and the Ni-grain roll, respectively, and that the thermal fatigue life of each roll decreased with the increase of the mean temperature or of the temperature range of the thermal fatigue cycle. The results were then interpreted based on the amount of primary carbides and the cyclic softening phenomenon associated with the exposed time to elevated temperatures. The coarse primary carbides on the specimen surface acted as fatigue crack initiation sites, as they cleaved at a low stress level to form cracks. The HSS roll, having the highest tensile strength and the smallest amount of primary carbides, thus showed better thermal fatigue property than the other rolls. For the improvement of the thermal fatigue property of the rolls, this study suggests a homogeneous distribution of primary carbides by reducing the carbide segregation formed along the solidification cell boundary and by optimizing of the roll-casting process.     

Solidification and solid-state transformation mechanisms in Si alloyed high-chromium white cast irons

Chromium white cast irons are widely used in environments where severe abrasion resistance is a dominant requirement. To improve the wear resistance of these commercially important irons, the United States Bureau of Mines and CSIRO Australia are studying their solidification and solid-state transformation kinetics. A ternary Fe-Cr-C iron with 17.8 wt pct (pct) Cr and 3.0 pct C was compared with commercially available irons of similar Cr and C contents with Si contents between 1.6 and 2.2 pct. The irons were solidified and cooled at rates of 0.03 and 0.17 K · s^-1 to 873 K. Differential thermal analysis (DTA) showed that Si depresses the eutectic reaction temperature and suggests that is has no effect upon the volume of eutectic carbides formed during solidification. Microprobe analysis revealed that austenite dendrites within the Si alloyed irons cooled at 0.03 and 0.17 K·s^-1 had C and Cr contents that were lower than those of dendrites within the ternary alloy cooled at the same cooling rate and a Si alloyed iron that was water quenched from the eutectic temperature. These lower values were shown by image analysis to be the result of both solid-state growth (coarsening) of the eutectic carbides and some secondary carbide formation. Hardness measurements in the as-cast condition and after soaking in liquid nitrogen suggest an increase in the martensite start temperature as the Si content was increased. It is concluded that Si’s effect on increasing the size and volume fraction of eutectic carbides and increasing the matrix hardness should lead to improved wear resistance over regular high-chromium white cast irons.     

Microstructure, Impact Fatigue Resistance and Impact Wear Resistance of Wear Resistant Low Cr-Si Cast Iron

Wear parts are greatly consumed in metallurgi-cal industry,fire power stations and cement works.The consumption of grinding balls and lining platesmay be the greatestunder such impactabrasive wearconditions.In China,annual consumption of ballsand plates is close to 1× 10 6 t.Therefore,develop-ing a new material for impact wear conditions is veryimportant.　　 The martensitic high chromium cast iron(WCr= 15 % ) is widely studied and used as wear resis-tance material[1— 8] ,especially for ma…     

石墨形态对含铸铁干摩擦磨损行为的影响

本文利用自制的干摩擦磨损试验机,系统地研究了不同石墨形态含磷铸铁在不同接触压力及不同摩擦速度条件下的干摩擦磨损行为。试验结果证明,合磷蠕墨铸铁具有最佳的耐磨性及最高的摩擦系数。随着接触压力的提高,含磷铸铁的磨损量呈直线增加,而随着摩擦速度的提高,含磷铸铁的磨损量首先增加,而后则大幅度下降。随着接触压力和摩擦速度的提高,含磷铸铁的摩擦系数首先大幅度降低,而后趋于一相对稳定值。随着石墨长宽比的增加,摩擦系数趋于相对稳定值的接触压力和摩擦速度呈直线增加。     

Modification of 27Cr cast iron with alloying yttrium for enhanced resistance to sliding wear in corrosive media

Yttrium has been found to be beneficial to the wear resistance of (Cr, Al)-containing alloys in corrosive environments. In this study, the performance of chill-cast 27Cr white iron alloyed with yttrium during sliding wear in different media, including tap water, dilute NaOH, and HNO₃ solutions, was investigated. Compared to Y-free 27Cr cast iron, the Y-containing cast iron showed improved wear resistance. The study demonstrated that chill-cast 27Cr white iron containing 1 wt pct yttrium performed the best and that too much yttrium, however, deteriorated the material. In order to understand the beneficial role that yttrium played, mechanical properties of Y-containing and Y-free cast irons and their passive films were investigated. The electron work function, corrosion, and anodic polarization behavior of the materials were also studied. It was clarified that the beneficial role of yttrium was attributable to improved passivation capability with enhanced passive film. For comparison, sandcast 27Cr white iron was also investigated.     

The Effect of Graphite Fraction and Morphology on the Plastic Deformation Behavior of Cast Irons

The plastic deformation behavior of cast irons, covering the majority of graphite morphologies, has not been comprehensively studied previously. In this investigation, the effect of graphite morphology and graphite fraction on the plastic deformation behavior of pearlitic cast irons has been evaluated. The investigation is based on tensile tests performed on various different cast iron grades, where the graphite morphology and volume fraction have been varied. Pearlitic steel with alloying levels corresponding to the cast irons were also studied to evaluate how the cast iron matrix behaves in tension without the effects of the graphite phase. It is concluded that as the roundness of the graphite phase increases, the strain hardening exponent decreases. This demonstrates that the amount of plastic deformation is higher in the matrix of lamellar cast iron grades compared to compacted and nodular cast iron grades. Furthermore, this study shows that the strength coefficient in flake graphite cast irons increases as the graphite fraction decreases due to the weakening effect of the graphite phase. This study presents relationships between the strain hardening exponent and the strength coefficient and the roundness and fraction of the graphite phase. Using these correlations to model the plastic part of the stress-strain curves of pearlitic cast irons, we were able to calculate curves in good agreement with experimentally determined curves, especially for gray cast irons and ductile iron.     

High Temperature Oxidation of a Work Roll Grade High-Chromium White Cast Iron

High-chromium white cast iron is a common material for the manufacture of the work rolls used in the finishing stands in hot strip mills. The high temperature oxidation behaviour of as-cast and heat-treated high-chromium white cast iron was studied by gravimetric means at 650 °C in atmospheres that included dry air and a mixture of dry air and water vapour. For both environmental and material conditions, the oxidation rate of the alloys was represented by parabolic trends and the oxidation phenomenon was more obvious when the samples were oxidised in water vapour. Water vapour also influenced the chemical composition of the oxides formed in the alloy and under this condition, the oxide scale formed on the alloy contained the oxides FeCr₂O₄, (Fe, Cr)₃O₄ and (Fe, Cr)₂O₃. In contrast, the alloys exposed to dry air conditions only experienced the formation of FeCr₂O₄ and (Fe, Cr)₂O₃. It was found that the presence of water vapour and the chromium content of the alloys increased the oxidation rate by a mechanism that involved chromium depletion of the oxide as a result of hydroxide formation. The differences observed in the oxidation behaviour of the alloy are discussed in terms of the composition of the oxidant atmosphere and the metallurgical condition of the specimens.     

Effect of Microstructure on Impact Fatigue Resistance and Impact Wear Resistance of Medium Cr-Si Cast Iron

 A great amount of iron grinding balls in tube mills have been consumed. Under this impact abrasive wear working condition, the failure of wear resistant alloying white irons grinding balls is mainly caused by fatigue spalling. The impact wear resistance of martensitic high chromium cast iron (Cr of 15%) is not high sometimes, but its cost is not low. Thus, medium Cr Si wear resistant cast iron is recommended. The influence of the iron on impact fatigue resistance and impact wear resistance is pronounced. Ball on ball impact fatigue test and high stress impact wear test of the grinding balls have been carried out. The results show that the impact fatigue resistance (IFR) and impact wear resistance (IWR) of medium Cr Si cast iron are superior to those of martensitic high chromium cast iron (Cr of 15%). The main reasons are that (1) the stress in medium Cr Si cast iron is released in the as cast state; (2) the matrix is fine pearlite with better toughness and plasticity; (3) the pearlite is more stable compared with a retained austenite under repeated impact load and less phase transformation can take place; (4) high silicon content improves the morphology of eutectic carbide; (5) there is no secondary carbide which results in less crack sources. All these factors are beneficial to improvement of impact fatigue spalling resistance. The eutectic carbide M7C3 is the main constituent to resist wear.     

The Effect of H13 Oxide Particles Supplied at the Interface Copper Alloy/Cast Iron Before Sliding on Tribological Behaviour and Friction Mechanisms

In high temperature hot forming processes (forging), the tool surfaces are the privileged places for mechanical, thermal and physico-chemical solicitations. More precisely, friction and wear play an important part in tool surface damage. The tool steel grades exhibit damages such as oxidation. Moreover oxide scales formed had a significant effect in failure forging tools on wear mechanisms and have to be considered in the wear model development. The damage caused by oxidation is very different according to the nature and the physical properties of formed oxide layers in the contact surfaces. The objective of the present work is to give an outline of the effects of the nature and the morphology of supplied X40CrMoV5-1 steel (AISI H13) oxides particles before slow sliding onto rubbing brass–steel surfaces on friction transition and wear mechanisms. It is the first to show the effects of thin flats plates having different micrometric size supplied before friction. In order to improve and to have a better insight into the wear phenomena taking place during the first instants of sliding, this work conducted has also demonstrated the role of this third body introduced before friction on the development and establishment of tribological circuit on the rubbing surface. In this paper, the wear investigations are carried out using a pad-on-disc tribometer. The pad-on-disc sliding wear experiments were performed at a load of 500 N and a sliding velocity of 0.065 mm/s in normal atmosphere. The pad is made of brass and the disc is made of cast iron lamellar. Static oxidation test used to evaluate samples oxidation characteristics consisted of one cycle of 70 h at 600 °C. In brief examination of rubbed pad surfaces after friction was conducted using scanning electron microscopy (SEM) to identify the wear mechanisms under oxidation surface. In addition, the samples structure and properties were examined by optical microscopy and SEM, profilometrical measurements and X-ray diffraction. Tribological results, correlated with microscopic observations, are conducted to establish a phenomenological model of wear mechanisms describing the evolution of the third body in contact. The effect of planning flats plates of third body on friction evolution was also discussed.     

热轧铸造高速钢工作辊表面氧化膜研究

研究了热轧铸造高速钢工作辊的辊面氧化膜形态、主要失效特征,分析了氧化膜失效对辊面粗糙度和磨损状态的影响.通过实物轧辊试样的连续变温氧化试验、恒温持续氧化试验和湿热氧化试验,研究了高速钢材质的氧化生长过程.采用SEM、XRD分析了氧化膜生长机制、分布形态、结合状态等的变化规律,进而提出了改进工作辊冷却效果的措施,为有效控制高速钢工作辊氧化膜状态和带钢断面质量奠定了基础.     

Abrasive Wear Behavior of High Chromium Cast Iron and Hadfield Steel-- A Comparison

 Abstract： Wear properties of two different crushers used for grinding raw materials of cement industry are compared using pin-on-disk wear test. The wear test was carried out with different loads on a pin. Abrasive wear behavior of two alloys was evaluated by comparing mass loss, wear resistance, microhardness and friction coefficient. The microstructure of the specimens was detected using optical microscope. The results showed that abrasive wear of high chromium cast iron is lower than that of Hadfield steel. Due to the presence of M7C3 carbides on the high chromium cast iron matrix, impact crushers exhibited higher friction coefficient.     

Microstructures and superplastic behavior of eutectic Fe-C and Ni-Cr white cast irons produced by rapid solidification

Superplastic behavior of two commercial grade white cast irons, eutectic Fe-C and Ni-Cr white cast irons, was investigated at intermediate temperatures (650 to 750 °C). For this purpose, rapidly solidified powders of the cast irons were fully consolidated by compaction and rolling at about 650 °C. The volume fractions of cementite in the eutectic cast iron and in the Ni-Cr cast iron were 64 pct and 51 pct, respectively, and both cast irons consisted of fine equiaxed grains of cementite (1 to 2 μm) and ferrite (0.5 to 2 μm). The cast iron compacts exhibited high strain-rate sensitivity (strain-rate-sensitivity exponent of 0.35 to 0.46) and high tensile ductility (total elongation of 150 pct to 210 pct) at strain rates of 10^-4 to 10^-3 s^-1 and at 650 °C to 750 °C. Microstructure evaluations were made by TEM, SEM, and optical microscopy methods. The equiaxed grains in the as-compacted samples remained unchanged even after large tensile deformation. It is concluded that grain boundary sliding (e.g., along cementite grain boundaries in the case of the eutectic cast iron) is the principal mode of plastic deformation in both cast irons during superplastic testing conditions. Formerly with the Department of Materials Science and Engineering, Stanford University Formerly Visiting Scholar, Department of Materials Science and Engineering, Stanford University     

New Method for Evaluating Thermal Wear of Rolls in Rolling Process

A new method was developed by a thermal wear machine to evaluate the thermal wear of roils in steel rolling process. The steel strip and rolls were simulated by upper and lower heating disks. The upper heating disk could he kept at a temperature of over 900 ℃ by induction heating. The pressure between the disks as high as 323.2 MPa could be achieved and the slipping rate could be 12. 7 %. The thermal wear of high speed steel （HSS） roll material, the wear rate of the HSS roll, and the SEM morphology of a worn HSS roll surface were investigated. This method was useful and could be employed to simulate friction and wear between strip and roll during the strip rolling process.     

高铬白口铸铁低速重载条件下的干滑动摩擦磨损特性

研究了不同类型碳化物和不同基体组织的高铬白口铸铁在低速（滑动速度为0.4187～1.0467m/s），重载（接触应力为1～21MPa）条件下与淬火40Cr钢（硬度HRC51～53）配副的干滑动摩擦磨损特性，结果表明，在（Fe,Cr)7C3、（Fe,Cr）3C和（Fe,Cr）33C63种碳化物中，（Fe,Cr）7C3有利于提高高铬白口铸铁的耐磨性，（Fe,Cr）3C有利于降低摩擦系数，共析组织，奥氏体和马氏体3种基体相比，共析组织基体使合金具有较高的摩擦系数，而奥氏体基体合金的耐磨性最好，存在一个临界摩擦应力，当摩擦应力大于此值时，磨损率急剧上升。     

Superplasticity in Rapidly Solidified White Cast Irons

Superplastic properties of three different composition white cast irons were investigated in the temperature range of 630 to 725 °C. Fine structures consisting of 1 to 2 μm ferrite grains were developed in these materials by consolidation of rapidly solidified powders at intermediate temperatures below the A₁ critical temperature. Tensile elongations of 1410 pct were found for a 3.0 pct C + 1.5 pct Cr white cast iron, 940 pct for a 3.0 pct C white cast iron, and 480 pct for a 2.4 pct C white cast iron when tested at 700 °C and at a strain rate of 1 pct per minute. The superplastic white cast irons exhibited a high strain rate sensitivity exponent,m, of 0.5 and activation energies for plastic flow were found to be nearly equal to the activation energy for grain boundary self-diffusion in iron. These observations are in agreement with the creep behavior of superplastic materials controlled by grain boundary diffusion. OSCAR A. RUANO, formerly with the Department of Materials Science and Engineering, Stanford University. LAWRENCE E. EISELSTEIN, formerly with the Department of Materials Science and Engineering, Stanford University.     

Evaluation of deterioration mechanisms and roll life of different roll materials

Evaluation of wear loss of rolls in operation is very difficult since the rolling conditions are hard to keep under control. To solve this problem a high temperature test rig was designed. A conventional roll material was compared to a hot work tool steel. The main wear mechanisms were different. The conventional roll grade was mainly subjected to thermal fatigue as could be expected. The hot work tool steel was mainly subjected to abrasive wear, and no evidence of thermal fatigue was found. The rate of material loss was 6 times higher in the hot work tool steel, but when the deterioration caused by thermal fatigue of the cast iron is taken into consideration, the deterioration rate was of the same order in both materials. The friction between the hot steel and the roll materials were of the same order. The frictional value should be a measure of the wear rate. Since the wear mechanisms are not equal, the coefficient of friction found in the test rig is only qualitatively related to roll wear.     

Effects of heat treatment and counterface on the wear behaviour of Cr21 cast iron

The unlubricated sliding wear test of high chromium white cast irons (HCCIs) was conducted using a pin-on-disc configuration under different heat treatments and different hardnesses of the counterface. With the increase of counterface hardness (20?HRC–47?HRC–54?HRC), the mass loss of the sample first increases then decreases. When the counterface hardness is 20?HRC, adhesion wear mainly takes place between the high chromium cast iron and the surface of 1045 steel. When the hardness is 47 or 54?HRC, first HCCI’ matrix wear takes place, then carbide bump flakes under alternating stress. The mass loss of the counterface decreases with the increase of hardness for the same sample. The mass loss of quenching, once tempering and twice tempering sample decreases gradually for the same counterface hardness, but fluctuation of the samples’ surface increased. The disc material is always softer than the pin material and results in a severe wear regime operation.     

Experimental Study on Surface Deterioration of Hot RollingMill Rolls under Near Service Conditions

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Roll Wear Evaluation of HSS,HiCr and IC Work Rolls in Hot Strip Mill

While it is acknowledged that roll wear is one of the most challenges to hot strip mills (HSM), very few studies which detail an exact prediction model have been published. The aim of this work is to evaluate and compare two prediction models with measured roll wear. The first prediction model, model 1, developed for a plate mill, was modified to use in strip rolling process. The second prediction model, model 2, is a simplified on‐line model. Data of two hot strip mills were used to investigate the influence of different rolling schedules. The rolls and strip properties were described and the rolling conditions were detailed. The influence of hot rolling factors, such as strip strength, roll grades, rolling temperature, rolling force, reduction and contact length, were also studied. When rolling with different work roll materials and strip grades, the modified prediction model has better prediction accuracy than the simplified model. The accuracy of both models becomes better at higher roll wear > 150 μm. HSS work rolls were confirmed to exhibit improvement of roll wear in comparison with HiCr rolls, the wear resistance was 3 to 4 times better. The influence of strip grade on roll wear was shown to be significant, with higher accuracy of the regression statistics for rolling with similar strip grades and lower regressed accuracy for rolling with mixed strip grades. The roll wear was evaluated at the centre of the barrel.     

Damping steel alloyed with chromium and vanadium for noise-combatting technology

This article evaluates the damping properties of a group of low-alloy steels used under conditions that expose machine parts to friction and wear. The properties of these steels are compared with the grades of structural cast iron that are most commonly used to obtain superior damping. The comparison showed that some of the new alloys are not only the equal of these grades of iron but even exceed them.