铸态42CrMo环坯在热轧及后续淬火回火过程中的组织和力学性能外文文献翻译、中英文翻译
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铸态42CrMo环坯在热轧及后续淬火回火过程中的组织和力学性能秦方成，李永堂，惠惠琪，魏小建（2016年5月26日提交; 2016年12月10日修订版; 2017年1月26日在线发布）铸造42CrMo环坯的热轧及其后续淬火和回火是在铸轧复合成形技术的基础上进行的。 通过光学显微镜和扫描电子显微镜研究进料速度和回火温度对显微组织的影响。 检查了轧制环的机械性能。 结果表明，当空载辊的进给速度增加时，晶粒退化程度略微变小，并且在轧制环的整个厚度上平均晶粒尺寸约为44m。 在中心层和最小扩展区附近的微观结构是不均匀的，其特征在于少量不规则和粗粮。 热轧环的强度和硬度高，塑性和韧性相对较低。 在低进给率下制造的环的裂纹中的凹坑的深度和直径大于用高进给速率制造的环的深度和直径。 在803 K淬火和回火后，在轧制环中不能观察到碳化物颗粒，但在863 K时回火和分散颗粒都会析出。因此，机械性能显着提高并满足技术要求淬火和回火后。 拉伸试样和冲击试样的断裂在较高的回火温度下具有规则和凹陷的特征，这表明韧窝断裂并且获得了强度，塑性和韧性的优良组合。关键词:铸态42CrMo环坯;进给速率;断口;热环轧制;力学性能;显微组织1.介绍42CrMo（美国等级：AISI 4140）是代表性的中碳和低合金钢之一1-3）。 由于其在强度，韧性和耐磨性方面的良好平衡，42CrMo轴承圈广泛应用于许多行业，特别是高速铁路，航空航天，汽车和风力发电应用中的关键部件。42CrMo轴承套圈通常采用热环轧（HRR）技术制造。 在HRR中，从动辊的旋转运动和惰辊的进给运动直接作用在环形毛坯上，以减小环形毛坯的壁厚，扩大直径并形成横截面。 在过去的几十年中，基于锻造状态材料的HRR工艺（参考文献4-8）和锻态42CrMo合金钢的变形行为9, 10）已被广泛研究。 Lin等人 （参考1, 11）通过等温压缩试验补偿应变，提出了锻造42CrMo合金的本构模型，并建立了其加工图以识别动态再结晶和流动不稳定区域。 重结晶为了揭示晶粒退化对变形参数的响应，建立了模型12, 13).现有生产42CrMo轴承套圈的工艺主要包括浇注锭，齿槽，锯切，镦粗，冲孔，HRR和热处理（参考文献14）。在HRR之前，原材料开始处于伪造状态。 然而，这种工艺仍存在多道加热，浪费材料和能量等缺点，并且由于齿槽，镦粗和冲压所需的大型成型机而导致成本高。 为了解决这些问题，提出了一种新型的铸造 - 轧制复合成形（CRCF）工艺，包括铸造环坯，HRR和热处理，如图1所示。 1 （参考14-16）。 与目前的工艺相比，CRCF已成为高端技术，具有许多优点，包括缩短加热时间，节省材料和能源，提高生产率并降低成本。 因此，在HRR中应完成铸态42CrMo环坯的几何尺寸成形和微观组织变更。 研究了温度，应变速率和时间间隔对铸态42CrMo合金流变应力的影响，16）使用中断的压缩测试。 Li等人 （参考14）通过考虑连续压缩试验和有限元方法分析了砂型铸件42CrMo的变形行为，并分析了微观组织演变与关键轧制参数之间的关系，包括空载轧辊进给速率，轧制速率和转速从动辊。工业测试表明，42CrMo合金环具有精确的几何尺寸和直接由铸造环坯热轧而成的良好的微观结构，并且机械性能满足标准要求（Ref14, 17）。 然而，目前，对铸态42CrMo合金的HRR工艺和随后的加热进行了研究图1用于生产42CrMo轴承套圈的铸造 - 轧制复合成形（CRCF）工艺表1铸态42CrMo环坯的化学组成（重量）C硅锰莫铬PS你铜铁0.440.280.7120.0070.120.08平衡图2铸态42CrMo钢的环坯（a）及其初始组织此外，广泛报道，裂解断裂行为明显受到粗碳化物和其他脆性颗粒的损害（参考文献18）。 通过适当的淬火和回火使碳化物分散的碳化物颗粒析出; 因此，机械性能，特别是低温冲击韧性也得到改善19-21）。 因此，为了消除不利的轧制残余应力，均匀化贯穿厚度的组织，并在HRR制造的环的强度和塑性 - 韧性之间获得适当的组合，有必要深入研究随后的淬火和回火过程（参考文献）18, 22）。 在这项研究中，进行了铸态42CrMo环坯的HRR及其后续淬火和回火（QT）。 研究了惰辊进料速率和回火温度对热轧环组织和力学性能的影响。 通过扫描电镜观察拉伸和冲击试样的断口形貌，断口形态为评估。2实验步骤本研究中使用的铸态42CrMo环坯是通过砂型铸造工艺制造的，浇注温度为1803-1813K，浇注速率为16.8-18.5公斤/秒。 环形毛坯的尺寸为/ 240 mm 9/120 mm9 45 mm。 铸态42CrMo环坯的化学成分（重量）和初始显微组织为：如表格所示1 和图。2， 分别。 如图2所示，可以清楚地观察到粗大的等轴晶粒和不均匀的微观结构。2（b），这表明平均晶粒尺寸约为87 lm。对铸态42CrMo环坯进行HRR测试使用D51-350辗环机。 环轧过程的示意图如图3所示。3（参考23）。 根据减少壁厚的原则（包括三个阶段：15mm-10mm-5mm），在该过程中采用了惰辊的不同进料速率。 HRR测试后，热轧环被空气冷却至环境温度。 随后，QT工艺的设计基于完QT的详细参数2。 根据ASTM标准（参考文献）24-26），沿热轧环的轧制方向分别制备拉伸试样和夏比冲击试样。使用基于ASTM标准E10-01的布氏（Brinell）机器进行硬度测量（参考文献：27）。 机械加工总长度为70毫米，标距长度为40毫米，有效直径为8毫米的拉伸试样。 在SHIMADZU AGS-X100机器上以5mm / min的恒定速度在室温下进行拉伸试验。 夏比冲击试样采用标准的V型缺口几何尺寸为10 9 10 9 55 mm3。 夏比冲击试验在JB-300B机器上以5.42m / s的冲击速度进行。 报告了每种测试条件下至少三个样本的平均值。用 VHX-600E 光学显微镜（ OM ）和 JSM-6510 扫描电镜（SEM）分别检测HRR制备的环和QT环的微观结构和碳化物形态。 标本是图3环轧过程示意图表2 HRR的参数和后续QT参数样品值准备用于使用标准金相技术的微观结构观察，例如在苦味酸溶液中研磨，抛光和蚀刻。 使用ASTM E209标准中描述的方法测量平均粒度（参考文献11）。 通过SEM目测评价拉伸试样和却贝冲击试样的断裂外观。3 结果与讨论3.1滚环的微观结构在HRR过程中，铸态环坯连续被拉入成形辊之间的间隙中，在受驱动辊和惰辊的摩擦和压力作用下经历复杂的应力和应变状态。 在图1中，4可以在轧制环的轴向观察到侧向扩展（参考文献1）5），这对纳米环的微观结构和机械性能有显着的影响。 数字5 显示了A1轧制环不同区域的微观结构。 平均晶粒尺寸通过线性截距法测量。 所谓的谷物，在外层和内层附近发现直径为32微米且具有均匀微观结构，并且在三角晶界区域中观察到小的再结晶晶粒。 然而，微观结构是inho-在中心层附近是不均匀的，其特征在于少量不规则和粗糙的颗粒，如图所示从动辊R1/空转辊R2/导向辊R3/ mm从动辊n的转速1/（r min-1）初始轧制温度T / K惰辊进给速度v /（mm s-1）滚动比率k铸造环毛坯尺寸/（mm 9 mm 9 mm）轧制环尺寸/（mm 9 mm 9 mm）淬火温度T1/ K 淬火保温时间t1/ h 回火温度T2/ K回火保温时间t2/ h340/52.5/52.5621373A13-2-1.5A24-3-22.6/240 9 /120 9 45A1/332 9 /272 9 51.1A2/331 9 /271 9 50.5112355863 5803 52.5图5图5 因此，中心层附近的约40m的平均晶粒尺寸大于外层和内层附近的平均晶粒尺寸。 尽管在最小扩展区域观察到了谷粒（图1）。4a），平均粒度为大而不规则的晶粒具有畸变晶界发现（图。5d）。 最少也会出现少量直径为50-54 lm的粗颗粒传播区域。 这主要是因为两者的抑制作用自由端表面的最大外部扩散和内部扩散很弱，导致低塑性应变和不均匀的微观结构，如图2所示。5(d).数字6 也显示了该微观结构的演变A2轧制环与A1轧制环相似，但前者的晶粒退化程度略小于后者，特别是在中心层。 随着进料速率的增加，贯通厚度的微观结构变成了（a）外层，（b）中心层，（c）内层，（d）最小扩展区域日益不均匀，平均晶粒尺寸约为44微米。 微观组织的演变表明中心层与少量的粗晶粒混合，不规则的晶粒在最小扩散区附近被发现。 粮食在粮食分配中的均匀性如图2所示，A2的最小扩展区域与A1的最小扩展区域相比略有改善。5（d）和6（d）。 这主要是因为每次通过应变随着惰辊进给速率的增加而增加，从而导致在铸态环坯整个厚度上发生足够的塑性变形。 在1373K的轧制温度下，再结晶的驱动力增加，软化效果逐渐增加。 因此，动态再结晶的速度加快，晶粒尺寸变小。 另外，最小扩展区域附近的少量不规则纹理与最大外部扩展和内部扩展对自由端面的抑制效应密切相关。 此外，加工硬化和变形的储能在较高的进料速率下增加，这导致铸态微结构的不充分恢复（Ref10, 13, 16）。 在这些地区，储存能量的增加不仅提供再结晶成核点，而且抑制异常晶粒生长。从以上分析可以得出结论，在HRR的初始阶段，表层存在比其他区域更大的应变。 随着从动辊和惰辊之间的间隙逐渐变窄，应变从表层扩展到中心层。 产生直径的连续扩大和厚度的减小。 在HRR工艺中，铸态42CrMo环坯的几何尺寸变化会导致复杂的组织演变，如回复，静态再结晶（SRX）和动态再结晶（DRX）。 如果应变达到临界值，DRX将发生; 如果没有，SRX会发生（参考文献）15）。 在严重的变形带和三角形晶界区域，由于DRX，观察到了新晶粒。 由于外部变形区域材料在旋转阶段仍然处于高温下，因此也产生偏动态再结晶。 因此，人们普遍认识到，HRR过程具有累积和多道次变形的特点。3.2 QT后滚环的微观结构与铸造环坯相比，铸态42CrMo环坯的主要微观组织特征是松散的和低密度的特征。 铸造环坯在受驱动辊和惰辊的摩擦和压力作用下经历复杂的应力和应变状态，导致轧制环中残余应力较大（参考文献21）。 因此，适当的QT工艺对消除不利的应力和改善整个轧制环厚度的微观结构稳定性至关重要（参考文献28, 29）。 数字7 显示了在863 K淬火和回火后，A1轧制环不同区域碳化物形貌的SEM显微照片。分散的碳化物颗粒沉淀在马氏体基体中，在不同区域显示不同的尺寸，形貌和分布。 在表层中，碳化物的特征是均匀的直径为0.1-0.15微米的微粒。 然而，在马氏体板条和边界内沉淀的碎片化碳化物形态被发现在中心层和最小扩展区域附近。 该如图4所示，在863K回火后，碳化物颗粒在A1轧制环中呈连续分布。（a）外层，（b）中心层，（c）内层，（d）最小扩展区域图7（a）外层，（b）中心层，（c）内层，（d）最小扩展区域此外，由于在基体底部存在少量的小碳化物，所以显微组织不均匀。 在冲击塑性变形过程中，部分连接的碳化物会导致很大的应力集中并进一步发展成微裂纹。 夏比冲击韧性也可以大大降低。（a）外层，（b）中心层，（c）内层，（d）最小扩展区域（a）外层，（b）中心层，（c）内层，（d）最小扩展区域数字8 提出了在863 K回火后A2轧制环不同区域碳化物形貌的SEM显微照片。约为直径0.08微米沿马氏体板条边界分布并延伸到板条内部。在整个厚度上呈现分散且均匀的颗粒.表3 HRR和QT后铸态42CrMo环坯的力学性能样品回火温度/ KYS，MPaUTS，MPa电话，RA，Ak，J硬度，HB标准值JB/T5000.6-2007510740-8801227200-250JB/T6396-2006550800135035241-302Q / LYCC（B）0014550800135027240-280滚环A1963117413.131.912.7391A295911535.73.310395QT之后A18636447762154106283803825966175472308A2863633772225488282803856996144743331特别是在最小蔓延区域（图2）。8d）。 因此，可以在轧制环中获得各向同性的特性。 这主要归因于在整个厚度上单位面积的塑性变形增加以及在较高进给速率（4-3-2mm / s）处出现硬化再结晶晶粒。 此外，随后的回火过程中，合金元素的固溶度随着碳化物在基质中的溶解而增加。 随着进给速度的增加，碳化物颗粒的优先析出位置通过在863K下回火从马氏体板条边界转变为板条内部，因此导致不明显的马氏体取向和韧性劣化（参考文献30, 31).数字9 和10 显示了分别在803K回火后A1和A2轧制环的不同区域中碳化物形态的SEM显微照片。 在A1和A2回火环的不同区域都不能观察到碳化物颗粒，这些区域保留在淬火板条马氏体的取向上。 根据回火条件，马氏体会发生不同的变化图11热轧环的工程应力 - 应变曲线和后续的QT工艺图12卷制环的拉伸断口形态：（a）A1，（b）A2（a）A1，（b）A2因为它是一个非常不稳定的结构（参考文献32）。 所有这些变化都会影响HRR制造的42CrMo环的强度和韧性33-35）。 马氏体和残余奥氏体的转变和溶解的活化能在803K的低回火温度下降低，这解释了为什么碳化物颗粒的数量显着减少。 因此，基体中的软化不充分，这意味着随着回火温度降低韧性也降低。3.3 机械性能和断裂形态微观结构的变化影响强度，塑性和韧性等因素。 为了评估直接从铸坯上热轧的42CrMo环的性能，拉伸试样的极限拉伸强度（UTS），屈服强度（YS），总伸长率（TEL）和面积减少量（RA） ，HRR和QT后的夏比冲击试样的夏比吸收能（Ak）总结于表3。 （中国标准）36, 37），试验结果表明，热轧环的UTS，YS和硬度要高得多，但塑性和韧性相对较低。 如表所示3，进料速率3-2-1.5 mm / s后铸态42CrMo环坯的YS和UTS分别为963 MPa和1174 MPa，TEL，RA和Ak分别为13.1，31.9和12.7 J。 当进给速率增加到4-3-2毫米/秒时，YS和UTS没有显示任何明显的变化，其值为959MPa和1153MPa。 但是，相应的TEL，RA和Ak分别下降到5.7，3.3和10J。 此外，本研究中UTS和YS的值优于HRR工艺制造的轴承环的值（参考文献17).在表中3，UTS，YS和硬度降低，并且通过QT过程显着改善了TEL，RA和Ak。 在803 K回火后，UTS，YS和硬度值仍然较高，但塑性和韧性值低于863 K回火的回火环。韧性降低的程度明显，因为碳化物颗粒确实在803 K回火后不会出现在A1和A2轧制环中。然而，所有的测试力学性能几乎满足轴承套圈的标准技术要求。 数字11 显示了热轧环的代表性工程应力 - 应变曲线和后续的QT工艺。其特征与表中总结的强度和延性值几乎相同3。 介绍了在 863 K淬火和回火后A1和A2轧制环变形后的明显屈服点。尽管QT后A1和A2轧制环的UTS值略低于Q / LYCC（B）0014中的标准值，但获得了强度，塑性和韧性的优良组合。数字12 介绍了A1和A2轧制环的拉伸断口形貌。 A1辗压环的断裂表现出准劈裂和凹陷的混合。 裂缝的特征是河流凹凸不平，小直径的等轴凹陷。 尽管A2轧环断裂也表现出少量的凹坑，但在A2轧环中以河型，剪切唇和撕裂山脊为主的裂隙型裂隙起着主导作用。 另外，断裂表面光滑且清洁，因此意味着典型的脆性断裂。 似乎A1卷轧环断裂深度和直径大于A2卷轧环的深度和直径，如图1所示。12。 这与表中所示的TEL和RA的结果一致3。 它也可以从图1中的冲击断口形态可以看出。13，A2轧制环的断裂由不规则剪切唇缘决定，穿过整个区域（见图1中的箭头）13B）。 微裂纹最初可能在这些区域产生，并在冲击变形过程中合并成裂纹15）。 最后，裂纹在剪切唇周围扩散到表层，从而导致横向颗粒破裂，并且韧性可以显着降低。数字14 和15 呈现QT后轧制环的拉伸和冲击断口形态。在较高的回火温度下，拉伸和冲击试样的断裂具有规则和凹陷的特征。 此外，在803 K回火的试样中观察到准解理和不规则断裂，特别是在A2轧制环上出现的一些步骤中，如图2所示。15（d）。 可以断定，断裂模式主要由拉伸试样的混合脆性断裂和韧性断裂以及冲击试样的脆性断裂决定。 先前已经报道了混合断裂模式（参考文献1）38, 39）。 与A2轧制环的冲击断口相比，A1轧制环的凹坑深度和直径也略大，这意味着A1的韧性比A2好。 因此，典型的韧性断裂在A1轧制环的拉伸和冲击断裂表面中起主要作用。 此外，断裂形态的变化与表中概括的机械性能一致。 4.结论研究了铸态42CrMo环坯热轧后淬火回火后的显微组织和力学性能。 扫描电镜显示拉伸试样和冲击试样的断口形貌和机制。 可以得出以下结论：1.当空转辊的进给速度增加时，晶粒补偿的程度稍微变小，并且在轧制环的整个厚度上平均晶粒尺寸约为44m。 中心层附近的微观结构不均匀且最小扩散区域，其特点是有少量不规则粗粒。2.HRR制造的环的UTS，YS和硬度高，塑性和韧性相对较低。 A1滚环的断裂表现为准解理和凹陷的混合，A2环轧制出现了河型，剪切唇和撕裂脊的准解理断裂。A1圆环的断口深度和直径大于A2圆环的深度和直径。3.在803 K淬火和回火后，碳化物颗粒不会出现在A1和A2轧制环的不同区域。 分散和分散的碳化物在马氏体基体中析出，并且其在整个轧制环厚度上的均匀分布通过在863K下回火获得。4.在803 K回火后，UTS，YS和硬度值仍然较高，但塑性和韧性值低于863 K回火的回火环。所有测试的机械性能，领带几乎满足轴承套圈的标准化技术要求。 拉伸试样和冲击试样的断裂在较高的回火温度下具有规则和凹陷的特征，这表明韧窝断裂并且获得了强度，塑性和韧性的优良组合。致谢本工作得到国家自然科学基金重点项目（批准号：51135007 ），国家自然科学基金（批准号： 51575371 和 51405325 ） ， 山 西 省 重 点 科 研 项 目 （ 批 准 号 ：03012015004） ）和山西省研究生教育创新工程（批准号：2016BY136）。参考文献1.YC Lin，MS Chen和J. 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Vyrostkova，Fe-Cr-Mo烧结钢的力学性能与微观结构关系的预测，Mater。 Des。，2012,35，p 619-625Microstructure and Mechanical Properties of As-cast42CrMo Ring Blank During Hot Rolling and SubsequentQuenching and TemperingFangcheng Qin, Yongtang Li, Huiping Qi, and Xiaojian Wei(Submitted May 26, 2016; in revised form December 10, 2016; published online January 26, 2017)The hot rolling of as-cast 42CrMo ring blank and its subsequent quenching and tempering were conductedbased on the casting-rolling compound forming technique. The effects of feed rate and tempering tem-perature on the microstructure were studied by optical microscopy and scanning electron microscopy. Themechanical properties of the rolled rings were examined. The results show that when the feed rate of theidle roll increases, the degree of grain refinement becomes slightly smaller and the average grain size isapproximately 44 lm through the whole thickness of the rolled ring. The microstructure is inhomogeneousnear the center-layer and minimum spread region, which is characterized by a small amount of irregularand coarse grain. The strength and hardness of the hot-rolled rings are high, and the plasticity andtoughness are relatively low. The depth and diameter of the dimples in the fracture of the ring fabricatedwith a low feed rate are larger than those of the ring fabricated with a high feed rate. The carbide particlescannot be observed in the rolled rings after the rings are quenched and tempered at 803 K, but the fine anddispersed particles are precipitated by tempering at 863 K. As a result, the mechanical properties aresignificantly improved and satisfy the technical demands after quenching and tempering. The fractures ofboth tensile and impact specimens are characterized by regular and fine dimples at a higher temperingtemperature, which indicates that a dimple fracture and an excellent combination of strength, plasticity andtoughness are obtained.Keywordsas-cast 42CrMo ring blank, feed rate, fracture, hot ringrolling, mechanical properties, microstructure1. Introduction42CrMo (American grade: AISI 4140) is one of therepresentative medium carbon and low alloy steels (Ref 1-3).Due to its good balance of strength, toughness and wearresistance, 42CrMo bearing rings are widely used in manyindustries, especially as critical components in high-speed rail,aerospace, automobile and wind power applications. The42CrMo bearing rings are commonly fabricated by using thehot ring rolling (HRR) technique. In HRR, the rotationalmotion of the driven roll and the feed motion of the idle roll actdirectly on the ring blank to reduce the wall thickness of thering blank, expand the diameter and shape the cross sec-tion. Over the last several decades, the HRR process based onas-forged state materials (Ref 4-8) and the deformationbehavior of as-forged state 42CrMo alloy steel (Ref 9, 10)have been extensively investigated. Lin et al. (Ref 1, 11)proposed the constitutive model of as-forged 42CrMo alloy bycompensating for strain using isothermal compression tests, andits processing maps were constructed to identify dynamicrecrystallization and flow instability domains. The recrystal-lization models were also established to reveal the response ofgrain refinement to deformation parameters (Ref 12, 13).The existing process for producing 42CrMo bearing ring ismainly composed of pouring ingot, cogging, sawing, upsetting,punching, HRR and heat treatment (Ref 14). Before HRR, thestarting material began in an as-forged state. However, thisprocess still has some disadvantages, such as multi-passheating, wasting material and energy, and a high cost due tothe huge forming machines required in cogging, upsetting andpunching. To solve the problems, a novel casting-rollingcompound forming (CRCF) process is proposed that includescasting a ring blank, HRR and heat treatment, as shown inFig. 1 (Ref 14-16). Compared with the current process, theCRCF has become a high-end technique with many advantages,including shorter heating times, considerable savings in bothmaterial and energy, productivity improvement and lower costs.Thus, the geometrical dimensions forming and microstructuremodification of as-cast 42CrMo ring blank should be completedin HRR. The effects of temperature, strain rate and interval timeon the flow stress of as-cast 42CrMo alloy were studied by Qi(Ref 16) using interrupted compression tests. Li et al. (Ref 14)analyzed the deformation behavior of sand casting 42CrMo byconsidering continuous compression tests and the finite elementmethod, and they then clarified the relationship between themicrostructure evolution and key rolling parameters, includingthe feed rate of the idle roll, rolling ratio and rotation speed ofthe driven roll. The industry tests indicate that the 42CrMoalloy rings with precise geometrical dimensions and soundmicrostructures hot-rolled directly from as-cast ring blank areobtained and that the mechanical properties satisfy the standardrequirements (Ref 14, 17). However, currently, research on theHRR process of as-cast 42CrMo alloy and subsequent heatFangcheng Qin, Yongtang Li, Huiping Qi, and Xiaojian Wei,School of Materials Science and Engineering, Taiyuan University ofScience and Technology, Taiyuan 030024, China. Contact e-mails: and liyongtang.JMEPEG (2017) 26:13001310?ASM InternationalDOI: 10.1007/s11665-016-2497-21059-9495/$19.001300Volume 26(3) March 2017Journal of Materials Engineering and Performancetreatment is rarely seen. Further, it is widely reported that thecleavage fracture behavior is significantly impaired by thecoarse carbides and other brittle particles (Ref 18). The fine anddispersed distribution carbide particles are precipitated byappropriate quenching and tempering; thus, the mechanicalproperties, particularly the low-temperature impact toughness,are also improved (Ref 19-21). Therefore, to eliminate theunfavorable rolled residual stress, homogenize the through-thickness microstructure and obtain a proper combinationbetween strength and plasticity-toughness of the HRR-fabri-cated ring, it is essential to conduct an in-depth study ofsubsequent the quenching and tempering process (Ref 18, 22).In this study, the HRR of as-cast 42CrMo ring blank and itssubsequent quenching and tempering (Q&T) were conducted.The influences of the feed rate of the idle roll and temperingtemperature on the microstructure and mechanical properties ofhot-rolled rings were studied. The fracture morphologies ofboth the tensile and impact specimens were visually observedby scanning electron microscopy, and the fracture modes wereevaluated.2. Experimental ProceduresThe as-cast 42CrMo ring blank used in this study wasfabricated by a sand mold casting process with a pouringtemperature of 1803-1813 K and a pouring rate of 16.8-18.5 kg/s. The dimension of the ring blank is /240 mm9/120 mm945 mm. The chemical composition (wt.%) andinitial microstructure of the as-cast 42CrMo ring blank areshown in Table 1 and Fig. 2, respectively. The coarse equiaxedgrains and inhomogeneous microstructure can be clearlyobserved, as shown in Fig. 2(b), which indicates the averagegrain size is approximately 87 lm.The HRR tests of as-cast 42CrMo ring blank were carriedout on a D51-350 ring rolling machine. The schematic diagramof the ring rolling process is shown in Fig. 3 (Ref 23).According to the principle of the reduction in wall thickness(includes three stages: 15 mm-10 mm-5 mm), the differentfeed rates of the idle roll were adopted in the process. The hot-rolled rings were air-cooled to ambient temperature after HRRtests. Subsequently, the Q&T processes were designed based ona complete austenitizing temperature and an improvement inthe plasticity and toughness (Ref 17, 20). The detailedparameters of HRR and subsequent Q&T are listed in Table 2.According to the ASTM standards (Ref 24-26), the tensilespecimens and Charpy impact specimens were prepared,respectively, along the rolling direction of the hot-rolled rings.Hardness measurements were taken using a Brinell machinebased on ASTM standard E10-01 (Ref 27). The tensilespecimens, which were 70 mm in total length, 40 mm in gagelength and 8 mm in effective diameter, were machined. Tensiletests were carried out on a SHIMADZU AGS-X100 machine ata constant speed of 5 mm/min at room temperature. TheCharpy impact specimens were in a standard V-notchedFig. 1Casting-rolling compound forming (CRCF) process for producing 42CrMo bearing ringTable 1Chemical composition of as-cast 42CrMo ring blank (wt.%)CSiMnMoCrPSNiCuFe0.440.280.7120.0070.120.08BalanceFig. 2Ring blank of as-cast 42CrMo steel (a) and its initial microstructure (b)Journal of Materials Engineering and PerformanceVolume 26(3) March 20171301geometry with a dimension of 10910955 mm3. Charpyimpact tests were conducted on a JB-300B machine at animpact speed of 5.42 m/s. The average of at least threespecimens for each testing condition was reported.The microstructures and carbide morphologies of both theHRR-fabricated rings and Q&T rings were examined by VHX-600E optical microscopy (OM) and JSM-6510 scanningelectron microscopy (SEM), respectively. The specimens wereprepared for microstructure observations using standard met-allographic techniques, such as being ground, polished andetched in picric acid solution. The average grain sizes weremeasured using the method described in the ASTM E209standards (Ref 11). The fracture appearances of tensilespecimens and Charpy impact specimens were visually eval-uated by SEM.3. Results and Discussion3.1 Microstructure of the Rolled RingDuring the HRR process, the as-cast ring blank is contin-uously drawn into a gap between the forming rolls, and itundergoes the complicated stress and strain states under theactions of friction and pressure from the driven roll and idleroll. In Fig. 4, the lateral spread can be observed in the axialdirection of the rolled ring (Ref 5), which has significant effectson the microstructures and mechanical properties of the finalring. Figure 5 shows the microstructures in different regions ofthe A1 rolled ring. The average grain size is measured by thelinear intercept method. The refined grains, which are approx-imately32 lmindiameterandhaveahomogeneousmicrostructure, are found near the outer-layer and inner-layer,and the small recrystallization grains are observed in triangulargrain boundary regions. However, the microstructure is inho-mogeneous near the center-layer, which is characterized by asmall amount of irregular and coarse grain, as presented inFig. 5(b). Thus, the average grain size of approximately 40 lmnear the center-layer is larger than that near the outer-layer andinner-layer. Although the refined grains are observed in theminimum spread region (Fig. 4a), the average grain size islarge, and irregular grains with distortion grain boundary arefound (Fig. 5d). A small quantity of coarse grains withdiameters of 50-54 lm is also exhibited in the minimumspread region. This is mainly because the restrain effect of bothmaximum outside spread and inside spread on the free endsurface is weak, resulting in the low plastic strain andinhomogeneous microstructure, as shown in Fig. 5(d).Figure 6 also shows that the microstructure evolution of theA2 rolled ring is similar to that of the A1 rolled ring but that thedegree of grain refinement in the former is slightly smaller thanthe latter, particularly in the center-layer. As the feed rateincreases,thethrough-thicknessmicrostructuresbecomeFig. 3Schematic diagram of ring rolling processTable 2Parameters of HRR and subsequent Q&TParametersSampleValuesRadius of driven roll R1/idleroll R2/guide roll R3/mm340/52.5/52.5Rotation speed of driven rolln1/(r min?1)62Initial rolling temperature T/K1373Feedrateofidlerollv/(mm s?1)A13-2-1.5A24-3-2Rolling ratio k2.6Dimensionofas-castringblank/(mm9mm9mm)/2409/120945Dimensionofrolledring/(mm9mm9mm)A1/3329/272951.1A2/3319/271950.5Quenching temperature T1/K11235Quenching holding time t1/h5Tempering temperature T2/K86358035Tempering holding time t2/h2.5Fig. 4Schematic diagram of lateral spread (a) and the rolled ring (b)1302Volume 26(3) March 2017Journal of Materials Engineering and Performanceincreasingly inhomogeneous, and the average grain size isapproximately 44 lm. The microstructure evolution indicatesthat the center-layer is mixed up with a small amount of coarsegrain and that the irregular grain is found near the minimumspread region. The homogeneity of grain distribution in theminimum spread region of the A2 is slightly improvedcompared to that of the A1, as shown in Fig. 5(d) and 6(d).This is mainly because the each-pass strain increases with anincrease in the feed rate of the idle roll, thus leading to thesufficient plastic deformation across the whole thickness of theas-cast ring blank. At the rolling temperature of 1373 K, thedriving force of recrystallization increases, and the softeningeffect gradually increases. Therefore, the rate of dynamicrecrystallization accelerates, and the grain size becomessmaller. In addition, a small amount of irregular grain nearthe minimum spread region is closely related to the restraineffect of both the maximum outside spread and the insidespread on the free end face. Further, the work hardening anddeformed storage energy increase at a higher feed rate, whichresults in the insufficient recovery of the as-cast microstructure(Ref 10, 13, 16). In those regions, the increased storage energynot only offers recrystallization nucleation points but alsosuppresses abnormal grain growth.From the above analysis, it can be concluded that in theinitial stages of HRR, more strain is found on the surface-layerthan in other regions. With a progressively narrowing gapbetween the driven roll and the idle roll, the strain is expandedfrom the surface-layer to the center-layer. A continuousexpansion in diameter and a reduction in thickness areproduced. The change in the geometrical dimensions of as-cast 42CrMo ring blank in the HRR process can result incomplicated microstructure evolution, such as recovery, staticrecrystallization (SRX) and dynamic recrystallization (DRX). Ifthe strain reaches the critical value, DRX will occur; if it doesnot, SRX will occur (Ref 15). In the severe deformation bandsand triangular grain boundary regions, the fine grains areobserved as a consequence of the DRX. The meta-dynamicrecrystallization is also generated because the outside defor-mation area materials are still under high temperature duringrotation stages. Thus, it is generally recognized that the HRRprocess is characterized by accumulative and multi-passdeformation.3.2 Microstructure of the Rolled Ring After Q&TThe loose and low density characteristics are the mainmicrostructure characteristic in as-cast 42CrMo ring blankcompared to the as-forged ring blank. The as-cast ring blankundergoes the complicated stress and strain states under theactions of friction and pressure from the driven roll and the idleroll, which results in the large residual stress in the rolled ring(Ref 21). Thus, the appropriate Q&T process is essential toeliminate unfavorable stress and improve microstructure sta-bility through the whole thickness of the rolled ring (Ref 28,29). Figure 7 shows SEM micrographs of carbide morphologyin different regions of the A1 rolled ring after being quenchedand tempered at 863 K. The dispersed carbide particles areprecipitated in the martensite matrix, which display differentsizes, morphologies and distributions in different regions. In thesurface-layer, the carbides are characterized by uniform andrefined particles with a size of 0.1-0.15 lm in diameter.However, fragmentized carbide morphologies,which areprecipitated within martensite laths and at boundaries, arefound near the center-layer and minimum spread region. Thecarbide particles display a continuous distribution in the A1rolled ring after being tempered at 863 K, as shown in Fig. 7(c)Fig. 5Through-thickness microstructures of the A1 rolled ring: (a) outer-layer, (b) center-layer, (c) inner-layer, (d) minimum spread regionJournal of Materials Engineering and PerformanceVolume 26(3) March 20171303and (d). Additionally, the microstructure is inhomogeneousbecause a small amount of small carbide is exhibited on thebottom of the matrix. During impact plastic deformation, thepartially connected carbides can result in a large stressconcentration and further develop into micro-cracks. TheCharpy impact toughness can also be greatly reduced.Fig. 6Through-thickness microstructures of the A2 rolled ring: (a) outer-layer, (b) center-layer, (c) inner-layer, (d) minimum spread regionFig. 7SEM micrographs of carbide morphology of the A1 rolled ring after being tempered at 863 K: (a) outer-layer, (b) center-layer, (c) inner-layer, (d) minimum spread region1304Volume 26(3) March 2017Journal of Materials Engineering and PerformanceFigure 8 presents SEM micrographs of carbide morphologyin different regions of the A2 rolled ring after being tempered at863 K. The refined carbides with the size of approximately0.08 lm in diameter are distributed along the martensite lathboundary and extended inside the lath. Dispersed and homo-geneous particles are exhibited across the whole thickness,Fig. 8SEM micrographs of carbide morphology of the A2 rolled ring after being tempered at 863 K: (a) outer-layer, (b) center-layer, (c) inner-layer, (d) minimum spread regionFig. 9SEM micrographs of carbide morphology of the A1 rolled ring after being tempered at 803 K: (a) outer-layer, (b) center-layer, (c) inner-layer, (d) minimum spread regionJournal of Materials Engineering and PerformanceVolume 26(3) March 20171305particularly in the minimum spread region (Fig. 8d). Thus, theisotropic properties can be obtained in the rolled ring. This isprimarily attributed to the plastic deformation per unit areathrough the whole thickness increasing and the refinedrecrystallization grains occurring at a higher feed rate (4-3-2 mm/s). Additionally, the solid solubilities of alloying ele-ments increase with the dissolution of carbides in the matrixduring subsequent tempering. With an increase in feed rate, thepreferential precipitation location for carbide particles trans-forms from the martensite lath boundary to inside the lath bytempering at 863 K, thus leading to inconspicuous martensiteorientation and deterioration in toughness (Ref 30, 31).Figure 9 and 10 show SEM micrographs of carbidemorphology in different regions of the A1 and A2 rolled ringsafter being tempered at 803 K, respectively. The carbideparticles cannot be observed in different regions of both theA1 and A2 tempered rings, which remain in the orientation ofthe quenched lath martensite. The martensite undergoesdifferent changes depending on the tempering conditionsFig. 10SEM micrographs of carbide morphology of the A2 rolled ring after being tempered at 803 K: (a) outer-layer, (b) center-layer, (c) in-ner-layer, (d) minimum spread regionTable 3Mechanical properties of the as-cast 42CrMo ring blank after HRR and Q&TSampleTempering temperature/KYS, MPaUTS, MPaTEL, %RA, %Ak, JHardness, HBStandard valuesJB/T5000.6-2007510740-8801227200-250JB/T6396-2006550800135035241-302Q/LYCC(B)0014550800135027240-280Rolled ringA1963117413.131.912.7391A295911535.73.310395After Q&TA18636447762154106283803825966175472308A2863633772225488282803856996144743331Fig. 11Engineering stress-strain curves of hot-rolled rings and sub-sequent Q&T process1306Volume 26(3) March 2017Journal of Materials Engineering and PerformanceFig. 12Tensile fracture morphologies of the rolled ring: (a) A1, (b) A2Fig. 13Impact fracture morphologies of the rolled ring: (a) A1, (b) A2Fig. 14Tensile fracture morphologies of the rolled ring after Q&T: (a) A1 tempered at 863 K, (b) A2 tempered at 863 K, (c) A1 tempered at803 K, (d) A2 tempered at 803 KJournal of Materials Engineering and PerformanceVolume 26(3) March 20171307because it is a highly unstable structure (Ref 32). All of thesechanges can affect the strength and toughness of the HRR-fabricated 42CrMo ring (Ref 33-35). The activation energy forthe transformation and dissolution of martensite and retainedaustenite decreases at a low tempering temperature of 803 Kand explains why the quantity of carbide particles is signifi-cantly reduced. Therefore, the softening in matrix is insuffi-cient, which implies that the toughness is also decreased as thetempering temperature decreases.3.3 Mechanical Properties and Fracture MorphologyChanges in microstructure affect the strength, plasticity andtoughness, among other factors. To evaluate the properties of a42CrMo ring that is hot-rolled directly from an as-cast blank,the ultimate tensile strength (UTS), yield strength (YS), totalelongation (TEL) and reduction of area (RA) of tensilespecimens, and Charpy absorbed energy (Ak) of Charpy impactspecimens after HRR and Q&T are summarized in Table 3.Compared to the Chinese standards (Ref 36, 37), the testingresults indicate that the UTS, YS and hardness of the hot-rolledrings are much higher but the plasticity and toughness arerelatively low. As shown in Table 3, the YS and UTS of as-cast42CrMo ring blank after HRR at the feed rate of 3-2-1.5 mm/sare 963 MPa and 1174 MPa, and the TEL, RA and Akare13.1%, 31.9% and 12.7 J, respectively. As the feed rateincreases to 4-3-2 mm/s, the YS and UTS do not show anyobvious changes, with values of 959 MPa and 1153 MPa.However, the corresponding TEL, RA and Akdecrease to 5.7%,3.3% and 10 J, respectively. In addition, the values of UTS andYS in this study are superior to those of bearing ringsmanufactured by the HRR process in (Ref 17).In Table 3, the UTS, YS and hardness are decreased, and theTEL, RA and Akare significantly improved by the Q&Tprocess. After being tempered at 803 K, the UTS, YS andhardness values are still relatively high, but the plasticity andtoughness values are lower than those of the rolled ringtempered at 863 K. The reduced degree of toughness is obviousbecause the carbide particle does not appear in both A1 and A2rolled rings after being tempered at 803 K. However, all of thetesting mechanical properties nearly satisfy the standardizedtechnical demands of bearing rings. Figure 11 shows therepresentative engineering stress-strain curves of the hot-rolledrings and subsequent Q&T process. The characteristics arealmost the same as the strength and ductility values summa-rized in Table 3. The obvious yield points upon deformation forboth the A1 and A2 rolled rings after being quenched andtempered at 863 K are presented. Although the UTS values ofboth the A1 and A2 rolled rings after Q&T are slightly lowerthan the standard value in Q/LYCC(B)0014, an excellentcombination of strength, plasticity and toughness is obtained.Figure 12 presents the tensile fracture morphologies of theA1 and A2 rolled rings. The mix of quasi-cleavage and dimpleis exhibited on the fracture of the A1 rolled ring. The fracture ischaracterized by a fluctuant river pattern and equiaxed dimpleswith small diameters. Although a small amount of dimpling isalso exhibited on the fracture of the A2 rolled ring, the quasi-cleavage fracture with river pattern, shear lip and tear ridgeplays a dominant role in the A2 rolled ring. In addition, thefracture surface is smooth and clean, thus implying a typicalbrittle fracture. It seems that the depth and diameter of dimplesin the fracture of the A1 rolled ring are larger than those of theA2 rolled ring, as shown in Fig. 12. This is in accordance withthe results of TEL and RA illustrated in Table 3. It also can beFig. 15Impact fracture morphologies of the rolled ring after Q&T: (a) A1 tempered at 863 K, (b) A2 tempered at 863 K, (c) A1 tempered at803 K, (d) A2 tempered at 803 K1308Volume 26(3) March 2017Journal of Materials Engineering and Performanceseen from impact fracture morphologies in Fig. 13, the fractureof the A2 rolled ring is determined by an irregular shear lip,which penetrates the whole region (see the arrow in Fig. 13b).The micro-cracks may generate initially in these regions andmerge into cracks during the impact deformation process (Ref15). Finally, the cracks spread around shear lip into the surface-layer, thus leading to trans-granular fracture, and the toughnesscan be dramatically reduced.Figure 14 and 15 present the tensile and impact fracturemorphologies of the rolled rings after Q&T, respectively. At ahigher tempering temperature, the fractures of both tensile andimpact specimens are characterized by regular and fine dimples.Further, the quasi-cleavage and irregular fracture are observedin the specimens tempered at 803 K, particularly in some of thesteps exhibited on the A2 rolled ring, as shown in Fig. 15(d). Itcan be concluded that the fracture mode is mainly governed bymixed brittle and ductile fracture for tensile specimens and bybrittle fracture for impact specimens. The mixed fracture modehas previously been reported (Ref 38, 39). Compared to theimpact fracture of the A2 rolled ring, the depth and diameter ofdimples of the A1 rolled ring are also slightly larger, whichimply that the toughness of A1 is superior to that of A2.Therefore, a typical ductile fracture plays a dominant role inboth tensile and impact fracture surfaces for the A1 rolled ring.Additionally, the variation in fracture morphologies is inaccordance with the mechanical properties summarized inTable 3.4. ConclusionsThe microstructures and mechanical properties of as-cast42CrMo ring blank after hot rolling and subsequent quenchingand tempering were studied. The fracture morphologies andmechanisms of tensile and impact specimens were revealed bySEM. The following conclusions can be drawn:1.When the feed rate of the idle roll increases, the degreeof grain refinement becomes slightly smaller, and theaverage grain size is approximately 44 lm through thewhole thickness of the rolled ring. The microstructure isinhomogeneousnearthecenter-layerandminimumspread region, which is characterized by a small amountof irregular and coarse grain.2.The UTS, YS and hardness of the HRR-fabricated ringsare high, and the plasticity and toughness are relativelylow. The mix of quasi-cleavage and dimpling is exhibitedon the fracture of the A1 rolled ring, and the quasi-cleav-age fracture with the river pattern, shear lip and tearridge is observed in the A2 rolled ring. The depth anddiameter of dimples in the fracture of the A1 rolled ringare larger than those of the A2 rolled ring.3.After being quenched and tempered at 803 K, the carbideparticle does not appear in different regions of both theA1 and A2 rolled rings. The refined and dispersed car-bides are precipitated in the martensite matrix, and itshomogeneous distribution through the whole thickness ofthe rolled ring is obtained by tempering at 863 K.4.After being tempered at 803 K, the UTS, YS and hard-ness values are still relatively high, but the plasticity andtoughness values are lower than those of the rolled ringtempered at 863 K. All of the tested mechanical proper-ties nearly satisfy the standardized technical demands ofbearing rings. The fractures of both tensile and impactspecimens are characterized by regular and fine dimplesat a higher tempering temperature, which indicates that adimple fracture and an excellent combination of strength,plasticity and toughness are obtained.AcknowledgmentsThis work was supported by the Key Program of NationalNatural Science Foundation of China (Grant no. 51135007), theNational Natural Science Foundation of China (Grant nos.51575371 and 51405325), the Key Research Project of ShanxiProvince (Grant no. 03012015004) and the Graduate EducationInnovation Project of Shanxi Province (Grant no. 2016BY136).References1. Y.C. Lin, M.S. Chen, and J. Zhang, Modeling of Flow Stress of42CrMo Steel Under Hot Compression, Mater. Sci. Eng. A, 2009, 499,p 88922. D. Chaouch, S. Guessasma, and A. Sadok, Finite Element SimulationCoupled to Optimisation Stochastic Process to Asses the Effect of HeatTreatment on the Mechanical Properties of 42CrMo4 Steel, Mater.Des., 2012, 34, p 6796843. S.I. Kim, Y. Lee, and S.M. Byon, Study on Constitutive Relation ofAISI, 4140 Steel Subject to Large Strain at Elevated Temperatures, J.Mater. Process. Technol., 2003, 140, p 84894. T.D. Kil, J.M. Lee, and Y.H. Moon, Quantitative FormabilityEstimation of Ring Rolling Process by Using Deformation ProcessingMap, J. Mater. Process. Technol., 2015, 220, p 2242305. H. Yang, M. Wang, L.G. Guo, and Z.C. Sun, 3D Coupled Thermo-Mechanical FE Modeling of Blank Size Effects on the Uniformity ofStrain and Temperature Distributions During Hot Rolling of TitaniumAlloy Large Rings, Comput. Mater. Sci., 2008, 44, p 6116216. C. Wang, H.J.M. Geijselaers, E. Omerspahic, V. Recina, and A.H. vanden Boogaard, Influence of Ring Growth Rate on Damage Develop-ment in Hot Ring Rolling, J. Mater. Process. Technol., 2016, 227, p2682807. S. Zhu, H. Yang, L.G. Guo, and R.J. Gu, Investigation of DeformationDegree and Initial Forming Temperature Dependences of Microstruc-ture in Hot Ring Rolling of TA15 Titanium Alloy by Multi-scaleSimulations, Comput. Mater. Sci., 2012, 65, p 2212298. X.H. Han, L. Hua, G.H. Zhou, B.H. Lu, and X.K. Wang, FESimulation and Experimental Research on Cylindrical Ring Rolling, J.Mater. Process. Technol., 2014, 214, p 124512589. Y.C. Lin, M.S. Chen, and J. Zhong, Constitutive Modeling for ElevatedTemperature Flow Behavior of 42CrMo Steel, Comput. Mater. Sci.,2008, 42, p 47047710. Y.C. Lin, M.S. Chen, and J. Zhong, Microstructural Evolution in42CrMo Steel during Compression at Elevated Temperatures, Mater.Lett., 2008, 62, p 2132213511. Y.C. Lin and G. Liu, Effects of Strain on the Workability of A HighStrength Low Alloy Steel in Hot Compression, Mater. Sci. Eng. A,2009, 523(12), p 13914412. Y.C. Lin, M.S. Chen, and J. Zhong, Effects of Deformation Temper-atures on Stress/Strain Distribution and Microstructural Evolution ofDeformed 42CrMo Steel, Mater. Des., 2009, 30, p 90891313. Y.C. Lin, M.S. Chen, and J. Zhong, Study of Metadynamic Recrys-tallization Behaviors in A Low Alloy Steel, J. Mater. Process. Technol.,2009, 209, p 2477248214. Y.T. Li, L. Ju, H.P. Qi, F. Zhang, G.Z. Cheng, and M.L. Wang,Technology and Experiments of 42CrMo Bearing Ring Forming Basedon Casting Ring Blank, Chin. J. Mech. Eng
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