Key Engineering Materials Vols.304-305(February 2005) pp.39-47on line at http://www.scientific.net ◎2005Trans Teach publications, SwitzerlandHigh-temperature tensile and wear behaviour of microalloyed medium carbon steelNenad Ivezic and Tom Potok{ivezicn|potokte}@ornl.govCollaborative 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 250°C, after which it increased.Obove 350°C, 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 350°C 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 component[3]。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 billets[5].After roll forging at 1,180°C,the steel was firmly cooled to room temperature at a cooling rate of 27°C/min.Tensile specimens with gauge length of 26.6mmand diameters of 4.1 mm were manufactured in the temperature range of 25-450°C at a strain rate of 1.2X10ˉ3/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+2°C.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 elongation[6].Wear performance tests of microallyed medium carbon steel was carried out at temperatures beween 25 and 450°C 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 8μm.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 350°C.However,percentage elongation decreased slowly until 250°C,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 200°C.As the temperature increases to 300°C the frequency of serrations on the floe curves decreased,although the strain hardening rate increases slightly.Above 300°C,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 σ=kεn,where σ and ε 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 ageing[7].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 0-200°C compared to nitrogen.Therefore,carbon strain ageing at low temperatures is normally negligible in slowly cooled steel.However,on ageing above 200°C 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 temperature[8].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 350°C consistent with dynamic strain ageing.Several investigators indicated that strength decreased from room temperature to about 100°C,and then a slower decrease was observd in corresponding to about 275-300°C.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 200°C.However ,steel samples showed minimum weight loss and maximum abrasion resistance between 200 and 350°C 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 350°C 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 200-350°C 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 200-350°C 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 350°C 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 350°C. 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Goodall. “IBM’s MemoryAgent,” Intelligence in Industry, pp.5-9, January 1999. http://2-ins.com含微量合金元素中碳鋼的高溫抗拉強(qiáng)度和耐磨特性探究摘 要 為了提供被用來作為自動化應(yīng)用的中碳合金鋼的動態(tài)應(yīng)變時效的新的探測方法。設(shè)計/方法/處理 目前的工作目標(biāo)是為了動態(tài)應(yīng)變可能時效的微量合金鋼的機(jī)械性能感興趣的工人或研究者提供理論和實(shí)踐信息。信息源被分成幾個部分：前言，實(shí)踐過程，結(jié)果和討論，結(jié)論。發(fā) 現(xiàn) 微量合金鋼處于 200-350℃時對動態(tài)應(yīng)變時效是很敏感的。在這個溫度范圍內(nèi)微量合金鋼呈現(xiàn)出最大的極限抗拉強(qiáng)度和彈性極限應(yīng)力，當(dāng)溫度達(dá)到250°C 時拉伸所導(dǎo)致的斷裂開始減小，此后又逐漸增加。當(dāng)溫度達(dá)到 350°C 以上時張力和耐力又急劇減小。由于動態(tài)應(yīng)變時效，在 200-350°C 之間，微量合金中碳鋼的耐磨性也增強(qiáng)了。研究的局限性/本質(zhì) 雖然有大量關(guān)于低碳合金鋼動態(tài)應(yīng)變時效的信息，但由于中碳鋼極易與氮元素結(jié)合成 AIN，VN，NBN 等而導(dǎo)致對中碳鋼的動態(tài)應(yīng)變測試不易進(jìn)行，這就可能增加了此次研究的意義。 實(shí)踐意義 為使用或計劃生產(chǎn)微量合金鋼者提供一些有用的信息。創(chuàng)新/價值 對識別資源的需要和對工人的實(shí)踐提供幫助。關(guān)鍵詞 磨損；時效（原料） ；應(yīng)變測試；抗拉強(qiáng)度；張力；鋼文章類型 研究性文章前言微量合金中碳鋼是二十世紀(jì)七十年代鋼鐵產(chǎn)業(yè)中的一大重要發(fā)展之一。微量合金鋼的最主要優(yōu)點(diǎn)在于為重要能源和自動化應(yīng)用在制造業(yè)方面節(jié)省成本的美好前景。這種鋼制品從高溫到完成冷卻所達(dá)到的硬度和其它特征足以與傳統(tǒng)的淬火和回火鋼相比較。微量合金的應(yīng)用于已有不小的基礎(chǔ)，例如；釩、鈮和鈦，含有這些元素的低碳鋼已成功應(yīng)用于大直徑管道，橋和其他建筑物。并且開始逐漸擴(kuò)展到中碳鋼的自動化工程和自動化應(yīng)用。微量合金元素通過產(chǎn)生碳化合物沉淀和先共析體以及最終的纖維組織的珠光鐵素體來達(dá)到細(xì)化晶粒和聚集沉淀的目的。與鈮和鈦合金相比，由于釩在奧氏體中具有高固溶性，因而釩合金通常被更廣泛的使用。如果有足夠的微量合金元素，如釩，鈮，鈦還未出現(xiàn)。并不是所有的微量合金元素能夠合成碳化物和氮化物。因此，由于沒有與單體碳和氮的相互作用，微量合金鋼將表現(xiàn)出應(yīng)變時效。當(dāng)然，有的合金鋼在也能在無碳和氮的環(huán)境下形成。然而調(diào)查顯示，至今尚沒有對微量合金鋼動態(tài)應(yīng)變時效有過廣泛的研究。此次的研究目的是為了了解含微量合金的中碳鋼所具有的高溫張力特性及其動態(tài)應(yīng)變時效。為了與高溫張力測試所獲得數(shù)據(jù)相比較，高溫下鋼的耐磨性也被列為此次研究的對象之一。實(shí)驗(yàn)材料與程序在這次的調(diào)查研究中鋼的成分如下：鐵（0.28%）碳（0.3%）硅（1.4%）錳（0.02% ）硫（0.01% ）S(0.08%) 釩（0.03% ） ；鋼坯直徑 36 毫米。經(jīng) 1180°C加熱后，鋼坯在室溫下以 27°C/分速率下冷卻，張力樣品以長 26.6 毫米和直徑為 4.1 毫米縱向制造。實(shí)驗(yàn)在拉伸強(qiáng)度試驗(yàn)機(jī)上進(jìn)行。起始溫度在 25-450°C 范圍內(nèi)，應(yīng)變率為 0.0012/秒。每種樣品在實(shí)驗(yàn)開始之前被放置大約十五分鐘來達(dá)到實(shí)驗(yàn)溫度,溫度被控制在-2°C~+2°C 之內(nèi).每次實(shí)驗(yàn)之后所得數(shù)據(jù)被轉(zhuǎn)換成工程壓力應(yīng)變函數(shù)曲線用以分析 0.2%塑性變形時鋼的塑性變形，最大的張力和總功伸長量。微量合金中碳鋼的耐磨性實(shí)驗(yàn)溫度范圍是 25-450°C，采用磨損型實(shí)驗(yàn)機(jī)，如圖 1 所示。耐磨實(shí)驗(yàn)所用材料長 30 毫米，尖端直徑為 3 毫米，至于電阻上加熱至實(shí)驗(yàn)溫度。每種樣品被放置大約 30 分鐘，在 6Mp 壓力下耐模性試驗(yàn)樣品大約被磨 250 個網(wǎng)格。在試驗(yàn)期間垂直移動樣品的目的是為了保持樣品與被磨損顆粒的接觸。在磨損處總共滑動距離為 11 米，耐磨性實(shí)驗(yàn)的結(jié)果是樣品損耗重量 0.1mg。用光學(xué)顯微鏡和電子顯微鏡來分別檢查鋼的纖維組織和樣品的表面損壞情況。使用放大倍率為 5 倍~400 倍 Nicon 偏光鏡來檢查樣本，電子顯微鏡也用來檢查各種實(shí)驗(yàn)條件下樣本素產(chǎn)生的裂紋和表面損壞情況。結(jié)果和結(jié)論圖 2 顯示了在光學(xué)顯微鏡中觀察到得微量合金中碳鋼的纖維組織。在其中發(fā)現(xiàn)這種鋼含有有 53%的鐵素體和 47%的珠光體，表 1 中給出了微量合金中碳鋼的張力測試結(jié)果，其中包括每 0.2%塑性變形時試樣的彈性極限應(yīng)力，極限抗拉強(qiáng)度和裂紋伸長量。該表表明在 200-350°C 之間試樣的彈性極限應(yīng)力和極限抗拉強(qiáng)度增加，而到 250°C 時其伸長率將逐漸減小，之后又穩(wěn)定增加。圖 3 顯示了溫度對微量合金中碳鋼張力特性的影響，隨著溫度的增加，變形，張力緩慢增加，在 200°C 時開始出現(xiàn)鋸齒狀裂紋，當(dāng)溫度增加到 300°C 時，周期性的鋸齒狀變形曲線減少，盡管張力的增加相當(dāng)緩慢。當(dāng)溫度到達(dá) 300°C以上，鋸齒形狀開始從曲線上小消失，由此表明出現(xiàn)這種結(jié)果是由于位錯和節(jié)點(diǎn)溶質(zhì)相互作用造成的，如碳和氮。圖 3 表明由于動態(tài)張力時效影響了張力和極限抗拉強(qiáng)度，隨著機(jī)械硬化增加，動態(tài)應(yīng)變時效也增加，二者的關(guān)系是：σ=kεn，式中，σ 和 ε 分別為實(shí)際壓強(qiáng)和實(shí)際應(yīng)變。實(shí)驗(yàn)表明低溫下位錯密度的增加比室溫下更加顯著，這些影響都是由于機(jī)械硬化的增加而引起的，可能是由于舊位錯停止，而新的位錯繼續(xù)形成卻維持了外加的應(yīng)變率。通常碳和氮是動態(tài)應(yīng)變時效的主要元素，碳和氮對應(yīng)變時效的影響的主要差別是兩者極大的溶解度差別。與氮相比，在 0-200°C 之間的碳在鐵素體中的溶解度相當(dāng)?shù)?。因此，在溫度很低的冷鐵中，碳的應(yīng)變時效通常可以忽略不計。然而當(dāng)溫度達(dá)到 200°C 以上時，實(shí)驗(yàn)數(shù)據(jù)表明碳化物顆粒的再溶解將導(dǎo)致應(yīng)變時效的擴(kuò)大。這一觀點(diǎn)于 1957 年首先被格林提出，并在 1966 年被伯德和詹姆斯所證實(shí)：高溫下，由碳或氮形成的置換固溶體的相互吸引將導(dǎo)致動態(tài)應(yīng)變時效顯著增強(qiáng)。目前的實(shí)驗(yàn)結(jié)果表明位錯和間隙固溶體將減小高溫下的動態(tài)應(yīng)變效應(yīng)。圖 4 顯示了 0.2%塑性變形時實(shí)驗(yàn)溫度對極限抗拉強(qiáng)度和伸長率的影響。圖中顯示，當(dāng)溫度在 200-350°C 之間，鋼的彈性極限應(yīng)力與極限強(qiáng)度的增長與動態(tài)應(yīng)變時效相一致。幾個試樣顯示：當(dāng)溫度從室溫增加至 100°C 并緩慢冷卻時應(yīng)力的變化與溫度在 275~300℃時的情況是一致的，而這之后應(yīng)力的減小幾乎可以忽略不計。圖 5 顯示了在不同的溫度下的微量合金中碳鋼磨損試驗(yàn)所繪制的重量損失-溫度曲線。當(dāng)溫度到達(dá) 200°C 時，重量損失-溫度曲線有一段連續(xù)的增長。當(dāng)溫度處于 200-350°C 之間時重量損耗最小，耐磨性最好，并且由于動態(tài)應(yīng)變時效而在曲線上出現(xiàn)鋸齒狀彎曲。在這種溫度下，與室溫相比試樣的耐磨性提高 11%。這表明動態(tài)應(yīng)變時效將導(dǎo)致試樣耐磨性能的提高。至此已證實(shí)動態(tài)應(yīng)變時效的存在。在 200~350°C 時的極限抗拉強(qiáng)度和耐磨性表明：位錯和溶質(zhì)的相互作用將使加工硬化效果增強(qiáng)。結(jié)論至此我們完成了溫度介于 25-450°C 之間的磨損實(shí)驗(yàn)，實(shí)驗(yàn)探討了微量合金中碳鋼的動態(tài)應(yīng)變時效對其機(jī)械特性的影響。通過該實(shí)驗(yàn)我們得出了以下結(jié)論：1.溫度為 200-350°C 之間的張力實(shí)驗(yàn)中動態(tài)應(yīng)變時效以 1.2X10-3/S 的應(yīng)變率發(fā)生，這種現(xiàn)對提高材料的機(jī)械特性有很大的影響。2.當(dāng)微量合金中碳鋼塑性變形為 0.2%時試樣的彈性極限應(yīng)力及其極限抗拉強(qiáng)度隨溫度的提高而增強(qiáng)并在 200-350℃之間達(dá)到最大值。在該溫度下試樣表現(xiàn)出鋸齒狀彎曲和低的延展性。這些特征都?xì)w因于動態(tài)應(yīng)變時效。3.在 200°C 和 350°C 時，試樣的耐磨性最強(qiáng)，同時鋸齒彎曲出現(xiàn)，因此可以推斷動態(tài)應(yīng)變時效將使材料的耐磨性能提高。4.位錯與間隙固溶體間強(qiáng)烈的相互作用會減少節(jié)點(diǎn)的移動并在 250-350°C之間引起了動態(tài)應(yīng)變時效。