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Key Engineering Materials Vols.304-305(February 2005) pp.39-47on line at 2005Trans Teach publications, SwitzerlandHigh-temperature tensile and wear behaviour of microalloyed medium carbon steelNenad Ivezic and Tom Potokivezicn|potokteCollaborative Technologies Research CenterComputer Science and Mathematics DivisionOak Ridge National LaboratoryPhone: (423) 574-5200Fax (423) 241-6211AbstractPurpose to provide new observations about dynamic strain ageing in medium carbon microalloyed steels which are used for automotive applications.Design/methodology/approach The present work aims to provide theoretical and practical information to industries or researchers who maybe interested in the effects of dynamic strain ageing on mechanical properties of microalloyed steel. The sources are sorted into sections: introduction, experimental procedure results and discussion, conclusion.Findings Microalloyed medium carbon steel was susceptible to dynamic strain ageing where serrated flow is observed at temperatures between 200and 350. In this temperature regime ultimate tensile strength and proof stress exhibit maximum value, however, elongation to fracture showed a decrease until 250C, after which it increased.Obove 350C, a sharp decrease tensile strength and proof stress were observed.Abrasive wear resistance of the microalloyed medium carbon steel was also increased at temperatures between 200 and 350C due to dynamic strain ageing.Research limitations/implicationsA search of the literature indicated that although there is considerable volume of information related to dynamic strain ageing in mild steel or in low-carbon steel no extensive investigation has been made of dynamic strain ageing in microallyed steel due to the ease with nitrogen is combined AIN,VN,NBN,etc.which perhaps increase its implication.Practical implicationsA very useful source of information for industries using or planning to produce microalloyed steels.Originality/value This paper fulfils an identified resource need and offers practical help to the industries.Keywords Wear, Ageing (materials), Strain measurement, Tensile, Strength, SteelPaper type Research paperIntroductionThe development of microalloyed medium carbon steels has been one of the singificant advances in the 1970s 1.The main benefit of microalloyed steels lies in the prospect of important energy and cost savings in the manufacturing of forged components for automotive applications.In such steels, the strength levels and otherproperties achieved after cooling from hot working temperatures are reported to be comparable with those obtained from conventional quenched and tempered steels.Microalloying or the use of small additions of elements, for example, V, Nb, and Ti, in low-carbon steels has been successfully empoyed for large diameter pipelines, bridges and other construction applications 2. This has been extended to medium carbon steels for a variety of automotive engine and engineering applications. The microallying elements produce precipitation of carbonitrides in austenite, and the proeutectoid and pearlitic ferrite phases of the final microstructure to obtain grain refinement and precipitation strengthening. Vanadium microalloying is commonly employed, owing to its higher solid solubility in austenite as compared with niobium or titanium that can produce a major strengthening component3。If sufficient percentage of microallying elements such as V, Nb, Ti are not present, not all of the carbon and nitrides 4. Therefore, microalloying steel will show strain ageing due to interaction between free carbon and/or nitrogen with dislocatiions.Of course there is interstitial free steels that rely on the absence of uncombined carbon and nitrogen for formability.However,a search of the literature has indicated that no extensive investigation has been carried out into dynamic strain ageing in microallyed steel.the aim of the present study is,therefore ,to determine the effects of the dynamic strain ageing on the high-temperature tensile properties of the microallyed medium carbon steel.The influence of high temperatures on wear performence of steel has also been investigated in oder to compare the findings obtained from hegh-temperature tensile tests.Experimental materials and procedureThe composition of the steel used in this investigation was:Fe-0.28,C-0.30,Si-1.4,M n-0.02,P-0.01,S-0.08,V-0.03.The steel were received in the form of 36mm diameter billets5.After roll forging at 1,180C,the steel was firmly cooled to room temperature at a cooling rate of 27C/min.Tensile specimens with gauge length of 26.6mmand diameters of 4.1 mm were manufactured in the temperature range of 25-450C at a strain rate of 1.2X103/s using an Instron universal testing machine, model 1115.Each specimen was held for approximately 15 min at the testing temperature before testing began.The temperature was controlled to within+2C.After each test, data for load versus displacement were converted into engineering stress versus strain curves which were analysed to determine the proof stress at 0.2 per cent plastic deformation ,ultimate tensile strength and total elongation6.Wear performance tests of microallyed medium carbon steel was carried out at temperatures beween 25 and 450C using the metal-abrasive type wear test machine as shown in Figure 1.Wear test specimens with length of 30 mm and tip diameter of 3 mm were heated to the test temperatures in an electrical resistance heater and all the specimens were held approximately 30 min befor the test .Wear performance test specimens were rubbed on 250 mesh SiC paper under a pressure of 6MPa with a sliding speed of 0.24 m/s.During the test care was taken to hold samples in contact with fresh abrasive grains.Total sliding distance on abrasive paper was determined as 11m.The results of wear test were quantified as the weight loss of the specimens measured with 0.1mg sensitivity.The examination of steel microstructure andworn surfaces of the specimens were done using optical and scanning electron microscopes,respectively.The optical examination of specimens was carried out using a Nicon microscope capble of magnifications between 5X and 400X.Scanning electron microscopy was also used to examine tensile frature and worn surfaces of the specimens representing the warious testing conditions.Results and discussionFigure 2 shows microstructure of the microallyed medium carbon steel in optical microscope.It is seen that steel consists of equiaxed grains in mean linear intercept grain sizes of 8m.The measurement phase volume fraction also indicated that steel had 53 per cent ferrite for themicroallyed medium carbon steel,including proof stress at 0.2 per cent plastic deformation,UTS and percentage elongation to fracture.Itis noted that the the proof stress and UTS of the steel samples increased between 200 and 350C.However,percentage elongation decreased slowly until 250C,after which it increased steadily. The effects of temperature on tensile behaviour of microalloyed medium carbon steel is shown in Figure 3.With the increasing of temperature of deformation,strain hardening rate first increased and then serrated flow occured at 200C.As the temperature increases to 300C the frequency of serrations on the floe curves decreased,although the strain hardening rate increases slightly.Above 300C,serration began to disappear from the curves.It is generally accepted that these effects are due to interaction between mobile dislocations and active interstitial solutes,such as carbon and nitrogen.As show in figure 3,the strain hardening rate and thus the flow stress for a given strain and the UTS were the properties most affected by dynamic strain ageing.Dynamic strain ageing is acccompanied by a large increase in the strain hardening index n in the relationship =kn,where and are true stress and true strain,respectively.It has been show that there is a much greater increase in dislocation density for a given strain in the blue brittleness range than at room temperature and this effect is clearly responsible for much of the enhanced strain hardening rate. Presumably dislocation become immobilised by solute pinning and fresh dislocations have continually to be formed to maintain the applied strain rate. It is generally accepted that carbon and nitrogen are the main elements responsible for dynamic strain ageing7.The main differences between the strain ageing effects of carbon and nitrogen arise from their widely differing solubilities .The solubilities of carbon in ferrite is fairly low at 0-200C compared to nitrogen.Therefore,carbon strain ageing at low temperatures is normally negligible in slowly cooled steel.However,on ageing above 200C there is an evidence that fine carbide particles can redissolve to produce extensive strain ageing.As first shown by Glen(1957)and confirmed by Baird and Jamieson that the presence of substitutional solutes with an affinity for carbon or nitrogen extend,the dynamic strain ageing up to higher temperature8.The present results indicate that there is a strong interaction between dislocations and interstitial solutes (carbon) or solute pairs (M-C and V-C) which reduces the mobility of interstitial and shifts dynamic strain ageing to higher temperatures. Figure 4 shows the effect of test temperatures on the proof stress at 0.2 percent plastic deformation, ultimate tensile strength and percentage elongation to gracture.It is evident that steel exhibit an increase in proof stress and ultimate tendile strength between 200 and 350C consistent with dynamic strain ageing.Several investigators indicated that strength decreased from room temperature to about 100C,and then a slower decrease was observd in corresponding to about 275-300C.Thereafter,the changes in flow stress are small or negligible. The abrasive wear test results at different temperatures of the microalloyed medium carbon steels are shown in Figure 5 where weight loss versus temperature.In general, there is a continuously increase in weight loss versus temperature up to 200C.However ,steel samples showed minimum weight loss and maximum abrasion resistance between 200 and 350C over which serrated yielding ocurred due to dynamic strain ageing.In this temperature regime the steel samples exhibited 11 per cent higher abrasion resistance compared to room temperature.This indicates that dynamic strain ageing caused an improvement on abrasion resistance.The evidence presented confirms the existence of dynamic strain ageing.The increased UTS and abrasion resistance between 200 and 350C suggest that there is an interaction between dislocation and solute atoms or solute pairs which make dislocation movement more difficult and increase strain hardening.Conclusions Tensile tests and abrasive wear tests were carried out between 25 and 450 C to examine the effects of the dynamic strain ageing on mechanical properties of microalloyed medium carbon steels. The main conclusions from this study are as follows:1 Dynamic strain ageing occurs in tested steel during tensile testing in the temperature range 200-350C at a strain rate of 1200/s.This phenomena has a considerable effect on the elevated temperature mechanical properties.2 The proof stress at 0.2 percent plastic deformation and ultimate tensile strength of microalloyed medium carbon steel increase with temperature and reaches a maximum at around 200-350C before decreasing with further increase in temperature.In this temperature regime steel samples showed serrated yielding and lower ductility.These features could be attributed to dynamic strain ageing.3 The weight loss and maximum abrasion resistance were observed in between 200 and 350C over which serrated yielding occurred.The inference can be draw,therefore,that dynamic strain ageing caused an improvement on abrasion resistance.4 There is a strong interaction between dislocations and interstitial solutes or solute pairs (Mn-C and V-C) which reduces the mobility of interstitial and cause dynamic strain ageing to occur at temperatures between 200 and 350C. REFERENCE LIST1 H.C. Leung，“Neural Networks in Supply Chain Management,” 1995 Engineering Management Conference pp. 347-352, 1995.2 J.M. Swaminathan, S. Smith, and N. Sadeh，“A Multi-Agent Framework for Supply Chain Dynamics,” Proceedings of NSF Research Planning Workshop on AI & Manufacturing. Albuquerque, NM, 1996. /afs/project/ozone/www/supply-chain/supply-chain.html3 N. R. Jennings, K. Sycara, M.Wooldridge，“A Roadmap of Agent Research and Development,” Autonomous Agents and Multi-Agent Systems, vol. 1, no. 1, pp. 7-38, 1998.4 Process Specification Language (PSL): /psl5 M. Barbuceanu and M. S. Fox, “COOL: A Language for Describing Coordination in Multi Agent Systems.” In Proceedings of ICMAS95, San Francisco, CA, The AAAI press/The MIT Press, pp 17 - 24. 6 M. N. Huhns and M. P. Singh, “Multiagent Systems in Information-Rich Environments,” Cooperative Information Agents II, 1998.7 N. Ivezic and J. H. Garrett Jr., 1998. Machine learning for simulation-based support of early collaborative design. Artificial Intelligence for Engineering. Design, Analysis, and Manufacturing, 12, pp. 123-139.8 A. Goodall. “IBMs MemoryAgent,” Intelligence in Industry, pp.5-9, January 1999. http:/2-含微量合金元素中碳钢的高温抗拉强度和耐磨特性探究摘 要 为了提供被用来作为自动化应用的中碳合金钢的动态应变时效的新的探测方法。设计/方法/处理 目前的工作目标是为了动态应变可能时效的微量合金钢的机械性能感兴趣的工人或研究者提供理论和实践信息。信息源被分成几个部分：前言，实践过程，结果和讨论，结论。发 现 微量合金钢处于200-350时对动态应变时效是很敏感的。在这个温度范围内微量合金钢呈现出最大的极限抗拉强度和弹性极限应力，当温度达到250C时拉伸所导致的断裂开始减小，此后又逐渐增加。当温度达到350C以上时张力和耐力又急剧减小。由于动态应变时效，在200-350C之间，微量合金中碳钢的耐磨性也增强了。研究的局限性/本质 虽然有大量关于低碳合金钢动态应变时效的信息，但由于中碳钢极易与氮元素结合成AIN，VN，NBN等而导致对中碳钢的动态应变测试不易进行，这就可能增加了此次研究的意义。 实践意义 为使用或计划生产微量合金钢者提供一些有用的信息。创新/价值 对识别资源的需要和对工人的实践提供帮助。关键词 磨损；时效（原料）；应变测试；抗拉强度；张力；钢文章类型 研究性文章前言 微量合金中碳钢是二十世纪七十年代钢铁产业中的一大重要发展之一。微量合金钢的最主要优点在于为重要能源和自动化应用在制造业方面节省成本的美好前景。这种钢制品从高温到完成冷却所达到的硬度和其它特征足以与传统的淬火和回火钢相比较。 微量合金的应用于已有不小的基础，例如；钒、铌和钛，含有这些元素的低碳钢已成功应用于大直径管道，桥和其他建筑物。并且开始逐渐扩展到中碳钢的自动化工程和自动化应用。微量合金元素通过产生碳化合物沉淀和先共析体以及最终的纤维组织的珠光铁素体来达到细化晶粒和聚集沉淀的目的。与铌和钛合金相比，由于钒在奥氏体中具有高固溶性，因而钒合金通常被更广泛的使用。 如果有足够的微量合金元素，如钒，铌，钛还未出现。并不是所有的微量合金元素能够合成碳化物和氮化物。因此，由于没有与单体碳和氮的相互作用，微量合金钢将表现出应变时效。当然，有的合金钢在也能在无碳和氮的环境下形成。然而调查显示，至今尚没有对微量合金钢动态应变时效有过广泛的研究。此次的研究目的是为了了解含微量合金的中碳钢所具有的高温张力特性及其动态应变时效。为了与高温张力测试所获得数据相比较，高温下钢的耐磨性也被列为此次研究的对象之一。实验材料与程序 在这次的调查研究中钢的成分如下：铁（0.28%）碳（0.3%）硅（1.4%）锰（0.02%）硫（0.01%）S(0.08%) 钒（0.03%）；钢坯直径36毫米。经1180C加热后，钢坯在室温下以27C/分速率下冷却，张力样品以长26.6毫米和直径为4.1毫米纵向制造。实验在拉伸强度试验机上进行。起始温度在25-450C范围内，应变率为0.0012/秒。每种样品在实验开始之前被放置大约十五分钟来达到实验温度,温度被控制在-2C+2C之内.每次实验之后所得数据被转换成工程压力应变函数曲线用以分析0.2%塑性变形时钢的塑性变形，最大的张力和总功伸长量。 微量合金中碳钢的耐磨性实验温度范围是25-450C，采用磨损型实验机，如图1所示。耐磨实验所用材料长30毫米，尖端直径为3毫米，至于电阻上加热至实验温度。每种样品被放置大约30分钟，在