双工位缸套激光淬火机及控制系统设计5张CAD图
双工位缸套激光淬火机及控制系统设计5张CAD图,双工,位缸套,激光,淬火,控制系统,设计,CAD
1、原始数据(资料):设计一台600 mm的回转工作台,数控系统采用MCS-51系列单片机(或微机)控制,伺服驱动采用步进电动机开环驱动,数控系统分辩率:0.005。原始数据:工作台尺寸: 600 mm最大工作负重: 200kg回转定位精度: 0.01X、Y轴重复定位精度: 0.005代码制: ISO输入方式: 增量值、绝对值通用自动升降速性能: 有2、毕业设计(论文)要求:(1)、任务要求根据毕业设计任务书中回转工作台技术要求,拟定回转工作台机械传动方案,完成机械关键零部件设计与计算、绘制装配图、和部分零件图;完成以MCS-51单片机控制的数控系统设计,绘制控制系统原理图。同时根据机械部分设计计算,电机及关键零部件选型计算,控制系统芯片选择等完成毕业设计设计说明书。主要任务如下:毕业设计(论文)开题报告;文献综述和外文翻译;回转工作台装配图,部分关键零件图,数控系统电气原理图;毕业设计说明书1、设计图样要求:设计原理正确,运用相关标准、查阅相关手册,正确处理好图、数字、符号、标准等的关系,图样完整准确。总体设计完整、图纸表达清晰、标注采用国家最新标准;完成整机装配图纸设计,保证结构方案最优化;完成部件图设计及传动系统设计;完成零件图设计,完成控制系统电气原理图设计。2、毕业设计说明书:设计依据可靠,参数选用合理,结构设计强度及刚度校核、计算准确,内容完整,中英文摘要与科技论文必须做到准确无误。对主要传动方案进行比较和选择、并可行性论证。对主要的零部件进行动力的计算,强度、刚度的校核;对电气原理图设计的说明。毕业设计说明书参考文献15篇以上,原则上所涉及的参考文献论文资料为近5年出版发表。设计成果要求:提交纸质资料(打印和部分手工绘制图纸)和电子文档资料。图纸使用AutoCAD软件绘制,文件为*.dwg格式。设计说明书资料为*.doc格式。1、毕业设计(论文)开题报告。2、毕业设计说明书1份,字数2-2.5万字。按山西能源学院本科毕业设计(论文)撰写规范执行。3、图纸:(1)数控回转工作台总装配图(A0)1张;(2)数控回转工作台关键零件图(图纸大小根据零件尺寸和复杂程度定)2-3张;(3)控制系统电气原理图(A1号)1张;4、文献综述与外文翻译:按山西能源学院本科毕业设计(论文)撰写规范执行。(1)文献综述:字数不少于3000字;(2)外文翻译:外文翻译必须与毕业设计课题相关,字数不少于5000字,并标明文章出处。1 蔡美琴编MCS-51系列单片机及应用M. 北京:高等教育出版社,20122 黄鹤汀主编机械制造装陪备M. 北京:机械工业出版社,20173 杜国臣主编机床数控技术M. 北京:机械工业出版社,20164 刘伟主编数控技术M. 北京:机械工业出版社,2019 5 刘建华主编,工程材料与机械制造. 北京:机械工业出版社,2019;6 成大先主编机械设计手册(第4卷)第四版M.北京:化学工业出版社20027 车洪麟、张素辉非标准机械设计(第一版)M.北京:机械工业出版社,2011;8 叶玉驹、焦永和、张彤机械制图手册(第5版)M. 北京:机械工业出版社,2019;9 屈波互换性与技术测量M. 北京:机械工业出版社,2014;10 闻邦春主编;机械设计手册(第6版)(1)(2) (3) (4)(5)(6)(7). 北京:机械工业出版社,2019;11 濮良贵、纪名刚机械设计(第8版)M.北京:高等教育出版社,2015;12 于永泗、齐民机械工程材料(第7版)M.大连:大连理工大学出版社,2007;13 刘宏新主编;机电一体化技术. 北京:机械工业出版社,2019;14 尹志强.机电一体化课程设计指导书M. 北京:机械工业出版社,2015;15 吴宗泽,高志.机械设计师手册(上册)(下册) 北京:机械工业出版社,2015;毕业设计(论文)题目:双工位缸套激光淬火机及控制系统设计 毕业设计(论文)要求及原始数据(资料):1、原始数据(资料):激光淬火,硬化层厚度为 500um,具有马氏体一奥氏体组织,硬度为 HV680-760,经过试验表明:激光硬化缸套的磨损比普通淬火生产的缸套磨损降低 37.5%。激光淬火有效硬化层的同一性很高,摩擦作用使奥氏体形变强化,沿着滑移面析出细小弥散的碳化物。这种淬火铸铁组织提高了摩擦表面的粘着磨损抗力.运行试验得出的结论是:激光淬火是强化缸套的有效方法,它与镀铬环结合使用时,提高了内燃机车走行公里。据报道,采用激光淬火可使内燃机车柴油机气缸套的耐磨性和抗擦伤能力提高 50%-100%。 要求: 本设计完成双工位缸套激光淬火机计及以 MCS-51 组成控制系统设计。 缸套最大尺寸为:200600mm 2、毕业设计(论文)要求:(1)、任务要求根据毕业设计任务书双工位缸套激光淬火机技术要求要求,拟定双工位淬火机械传动方案,完成机械关键零部件设计与计算、绘制装配图、和部分零件图; 完成以 MCS-51 组成控制系统设计,绘制控制系统原理图。 同时根据机械部分设计计算,电机及关键零部件选型计算,控制系统芯片选择等完成毕业设计设计说明书。主要任务如下:毕业设计(论文)开题报告;文献综述和外文翻译;激光淬火机装配图,部分关键零件图,激光淬火机控制电气原理图;毕业设计说明书。(2)、时间进度要求序 号 时间 周次 设计工作及要求 1 2021.3.22-2021.3.28 第 1 周 选择毕业设计题目,领取任务书 2 2021.3.29-2021.4.4 第 2 周 查阅文献资料,完成文献综述和外文翻译 3 2021.4.5-2021.4.11 第 3 周 撰写开题报告及答辩 4 2021.4.12-2021.5.9 第 4-7 周 方案设计、计算及图纸绘制 5 2021.5.10-2021.5.16 第 8 周 毕业设计中期检查 6 2021.5.17-2021.5.30 第 9-10 周 图纸进一步绘制、论文编写 7 2021.5.31-2021.6.6 第 11 周 论文查重、修改 8 2021.6.7-2021.6.13 第 12 周 打印装订、指导老师与评阅老师评阅、答辩 毕业设计(论文)主要内容:1、设计图样要求:设计原理正确,运用相关标准、查阅相关手册,正确处理好图、数字、符号、标准等的关系,图样完整准确。总体设计完整、图纸表达清晰、标注采用国家最新标准;完成整机装配图纸设计,保证结构方案确定最优化;完成部件图设计及传动系统设计;完成零件图设计,完成控制系统电气原理图设计。2、毕业设计说明书:设计依据可靠,参数选用合理,结构设计强度及刚度校核、计算准确,内容完整,中英文摘要与科技论文必须做到准确无误。对主要传动方案进行比较和选择、并可行性论证。对主要的零部件进行动力的计算,强度、刚度的校核;对电气原理图设计的工作原理说明。毕业设计说明书参考文献 15 篇以上,原则上所涉及的参考文献论文资料为近5 年出版发表。学生应交出的设计文件(论文):设计成果要求:提交纸质资料(打印和部分手工绘制图纸)和电子文档资料。图纸使用 AutoCAD 软件绘制,文件为*.dwg 格式。设计说明书资料为*.doc 格式。1、毕业设计(论文)开题报告。2、毕业设计说明书 1 份,字数 2-2.5 万字。按山西能源学院本科毕业设计(论文)撰写规范执行。3、图纸:(1) 双工位缸套激光淬火机总装配图(A0)1 张;(2) 双工位缸套激光淬火机关键部件装配图(A1)1 张、零件图(图纸大小根据零件尺寸和复杂程度定)2-3 张; (3) 控制系统电气原理图(A1 号)1 张; 4、文献综述与外文翻译:按山西能源学院本科毕业设计(论文)撰写规范执行。(1) 文献综述:字数不少于 3000 字;(2) 外文翻译:外文翻译必须与毕业设计课题相关,字数不少于 5000 字,并标明文章出处。 主要参考文献(资料):1 蔡美琴编MCS-51 系列单片机及应用M. 北京:高等教育出版社,20122 黄鹤汀主编机械制造装陪备M. 北京:机械工业出版社,20173 杜国臣主编机床数控技术M. 北京:机械工业出版社,20164 刘伟主编数控技术M. 北京:机械工业出版社,20195 闻邦春主编;机械设计手册(第 6 版)(1)(2) (3) (4)(5)(6)(7). 北京:机械工业出版社,2019;6 机械设计实用手册编委会机械设计实用手册(上)(下). 北京:机械工业出版社,20157 车洪麟、张素辉非标准机械设计(第 1 版)M.北京:机械工业出版社,2011;8 彭如恕现代工程制图(第 1 版)M.长沙:国防工业出版社,2006;9 屈波互换性与技术测量M. 北京:机械工业出版社,2014;10 孙桓、陈作模,机械原理(第 7 版)M.北京:高等教育出版社,2005;11 濮良贵、纪名刚机械设计(第 8 版)M.北京:高等教育出版社,2005;12 于永泗、齐民机械工程材料(第 8 版)M.大连:大连理工大学出版社,2007;13 水根机械制造工艺学(第 2 版)M.北京:清华大学出版社,2004;14 刘宏新主编.机电一体化技术. 北京:机械工业出版社,2019;15 黄鹤汀主编.金属切削机床设计. 北京:机械工业出版社,2015; 指 导 教 师 签 字 年月日教研室主任审查签字 年月日系 主 任 批 准 签 字 年月日 1研究意义: 早在二十世纪七十年代美国通用汽车公司就成功地将激光淬火技术应用于汽车转向器壳体及发动机缸套的热处理生产线,由于激光器成本高、且淬火工艺控制复杂等特点而未能广泛采用,但是,出于激光淬火技术具有传统热处理方式所不具备的优良特点所以它仍是一门面向二十一世纪有潜在应用前景的高新技术。激光淬火实时控制系统的研制对于提高激光淬火质量、推广激光淬火技术有很重要的现实意义,如果能够实现激光淬火工艺过程的自动控制,就可以用廉价、易于加工传统热处理方式对材料进行普通加工,而对于在工件的关键部分可用激光淬火来处理,以提高工件的加工质量,实现工件从成产工艺到加工成本的优化,延长工件的使用寿命。2基本内容: 1通过查阅和掌握大量有关文献和资料,熟悉和了解激光淬火机的发展技术以及国内外对此的研究现状。2. 通过查阅文献及相关资料,结合本课题任务书要求,初步确定双工位缸套激光淬火机的总体结构。3.根据激光淬火检测对象和控制参数,研究检测方法与原理,选择合理的检测控制方案。4.根据所确定的检测控制方案确定激光淬火机的总体结构,包括MCS-51控制系统。为了实现激光淬火过程的在线检测和控制,保证加工过程中硬化带均匀分布,对激光淬火控制系统进行硬件设计和系统集成,研究其各硬件设备的选型方法和集成方案并完成电机及关键零部件选型计算、控制系统芯片选择等。5.根据课题要求及激光淬火检测对象设计实验方案,利用实验结果分析检测系统的准确性,验证本文所设计的控制系统的可行性。6.完成机械关键零部件设计与计算、绘制装配图、和部分零件图;完成以MCS-51组成控制系统设计,绘制控制系统原理图。7.对本次课题设计进行总结,分析本次设计取得的主要成果,思考本次设计存在的一些问题,提出一些相应的进一步研究方向及意见等。3重点和难点及解决方法重点:1.激光淬火机各项基本原理的研究及选择2.各主要功能部分的方案确定3.相关功能部件的设计计算4.相关图纸绘制解决方法:1.多研读相关文献2.对激光淬火机各项基本原理进行分析研究3.熟悉相关制图软件的使用4提纲: 摘要第一章 前言1.1研究背景1.2国内外研究现状 1.2.1研究现状 1.2.2发展趋势1.3激光淬火技术概述第二章 激光淬火机设计思路2.1 激光淬火机应用原理2.1.1 激光淬火机的介绍2.1.2 激光淬火机的应用2.2 机械设计及相关计算第三章 MCS-51控制系统结构设计3.1系统硬件电路设计3.2 控制电路设计第4章 软件设计4.1 I/O端口的分配4.2 流程图设计4.3 程序设计第5章 调试仿真总结参考文献Microstructure refinement and properties of 1.0C-1.5Cr steel in a duplex treatment combining double quenching and laser surface quenchingABSTRACTThe 1.0C-1.5Cr steel was subjected to conventional quenching and laser surface quenching treatment. A process combing double quenching and laser surface quenching was proposed for enhancing surface hardness and obtaining finer microstructure. The cementite dissolution and grain growth behavior in the austenitizing process of single quenching, double quenching, and laser surface quenching were studied. The results indicated that compared with single quenching, mean diameter of undissolved cementite particles (UCP) was much finer in double quenching, and the final prior austenite grain size (PAGS) could be decreased by nearly 40% to about 4.5 m. Both grain and cementite particles near the surface will coarsen after laser surface treatment. Compared with single quenching, the PAGS within hardened layer can be decreased by at least 11% through double quenching, and the mean diameter of UCP at the bottom of hardened layer can be decreased by about 20%. Compared with conventional quenching, surface hardness was enhanced by about 20% through laser surface quenching, contributing to the wear resistance. However, the hard and brittle surface layer tends to be crack source during the impact process, leading to the deterioration of final impact toughness. Under the identical laser parameters, the impact absorbed energy is similar in both single and double quenching, which is about 25% of that before laser surface quenching. The impact absorbed energy can be increased from 22J to 28J by preheating at 160 during the laser surface quenching.Keywords:Double quenching;Laser surface quenching;Cementite;Grain size;Impact toughness ;Wear1. Introduction As a typical hypereutectoid steel, 1.0C-1.5Cr (SAE 52100) steel is extensively used in bearings, guide rails, and molds. In order to satisfy the complex working conditions, it is usual for this steel to be subjected to spheroidization annealing, quenching, and low temperature tempering treatment in sequence, which will finally produce a microstructure composed of martensite matrix, undissolved cementite particles, retained austenite, and nanoscale carbides 1,2. Its final mechanical properties can be controlled by adjusting these microstructure parameters. It is commonly accepted that the microstructure refinement is an effective method for improving the comprehensive mechanical properties of 1.0C-1.5Cr steel, especially its fatigue property. At present, there have been several research works surrounding the microstructure refinement of 1.0C-1.5Cr steel. For example, Beswick 3 reported that the prior austenite grain size (PAGS) could be decreased by imposing pre-cold deformation before quenching. Because lots of low angle dislocation cells form during cold deformation, which will increase the nucleation rate of austenite in the subsequent heating process, and thus lead to the refining of final prior austenite grain (PAG). Santos et al. 4 proposed that induction heating and repeated quenching were also feasible for decreasing the PAGS, and their work demonstrated that fatigue property could be enhanced obviously by refining PAG. Mizobe et al. 5 reported that repeated quenching from 850 could decrease the PAGS from 15 m to about 7 m. Li et al. 6,7 reported that thermo-mechanical control process could be used to refine the hot rolled and spheroidized microstructure, which would result in the slight decrease of size of both PAG and undissolved cementite particles (UCP) in the subsequent quenched microstructure. Recently, Salloom et al. 8 proposed that concurrent refinement of PAG and UCP could be achieved by a double quenching process, which consisted of austenitizing at 1050 followed by oil quenching and low-temperature tempering, and reheating to second austenitizing temperature of 850 followed by quenching and tempering. However, it may cause serious decarburization during austenitizing at 1050 , and microcracks may also form during the subsequent oil quenching 9. Lee et al. 10 reported that finer PAG could be obtained by lowering the second austenitizing tem-perature in the double quenching, whereas lower austenitizing tem-perature failed to enhance the final hardness. Therefore, they proposed that nitrocarburization could be conducted during the first austenitizing stage. However, it is time consuming and costly. In the above methods, repeated quenching, especially double quenching, is extremely prom-ising in refining the microstructure of 1.0C-1.5Cr steel. In order to further optimize microstructure and enhance mechanical properties, it is necessary to carry out the research works surrounding the cementite dissolution and grain growth in the double quenching process.It should be noted that both single quenching and double quenching belong to conventional hardening treatment, and usually result in the through-hardening of 1.0C-1.5Cr steel. However, in many working conditions, it is the surface that will bear complex stress and wear. Therefore, the optimization of surface properties of 1.0C-1.5Cr steel has received particular research attentions. There have been several methods for enhancing surface properties, such as surface thermo-chemical treatments (carburizing, carbonitriding, boronizing), shot peening, physical/chemical vapor deposition, laser surface treatment (quenching, cladding, shock peening) 1113. Due to the low cost, high efficiency, and excellent controllability, laser surface quenching is one of the promising methods for enhancing the surface properties of 1.01.5Cr steel. Basu et al. 14 developed a duplex treatment process for 1.01.5Cr steel, which consisted of austempering and laser surface quenching. It was found that laser surface quenching could enhance the surface hardness without affecting its bainitic core. Lin et al. 15 pro-posed that water cooling could be imposed during the laser surface remelting of 1.0C-1.5Cr steel, which would result in higher surface hardness and the formation of an ultrafine microstructure. Due to the beneficial effects on surface hardness, laser surface quenching could be employed to replace the carbonitriding process used in the double quenching treatment. However, until now, few works combined double quenching and laser surface quenching. In addition, the impact tough-ness of this steel should be also paid attention, especially when it is used in the bearings for high-speed rail or maglev train 16,17. However, the impact properties after laser surface quenching was seldom investigated.In this work, the 1.0C-1.5Cr steel was subjected to conventional single quenching, double quenching, and laser surface quenching. The dissolution and size variation of cementite particles, and grain growth behavior in the austenitizing process of conventional quenching and laser surface quenching were studied. The hardness, wear resistance, and impact toughness of 1.0C-1.5Cr steel were analyzed. In addition, the effect of preheating was also studied.2. Experimental procedure2.1. Materialswhich had a microstructure composed of spherical cementite particles and ferrite matrix. The corresponding chemical composition (wt.%) was: 1.0C, 0.28Si, 0.34Mn, 1.57Cr, 0.011P, 0.003S. The mean diameter of spheroidized cementite particles was about 0.34 m.2.2. Conventional quenching and laser surface quenchingIn order to study the cementite dissolution and grain growth behavior in single and double quenching, specimens with dimensions of 3 10 11 mm were prepared for conducting interrupted quenching, as shown in Fig. 1. For single quenching, some specimens were put into a tube furnace with the temperature of 800 and 850 for austeni-tizing, respectively, and held for 5 min, 20 min, 30 min, 60 min, and then they were oil quenched to room temperature. For double quench-ing, some specimens were first put into the furnace with the temperature of 850 (First austenitizing temperature), and held for 30 min, and oil quenched to room temperature. Subsequently, they were put into the furnace with the temperature of 800 (Second austenitizing temper-ature) again. They were oil quenched to room temperature after holding for 5 min, 20 min, 30 min, 60 min, respectively. It should be noted that the first austenitizing temperature and time in double quenching were fixed in this work, and thus the austenitizing temperature and time mentioned in the following text denote the second austenitizing tem-perature and time.Specimens with dimensions of 11 24 100 mm were also pre-pared. Similarly, they were put into the furnace with the temperature of 850 , and held for 30 min, and then oil quenched to room temperature. Subsequently, they were divided into two groups, and one group was directly tempered at 170 for 120 min, and the other group was put into the furnace with the temperature of 800 again. After holding at 800 for 20 min, they were also oil quenched to room temperature, and tempered at 170 for 120 min. These tempered specimens were further machined to 10 24 100 mm, which would remove the sur-face decarburized layers. Subsequently, they were subjected to laser surface quenching. Fig. 2 shows the schematic illustration of laser sur-face quenching. A fiber-coupled diode laser (Laserline) with spot size of 3 8 mm was used in this work. The scanning speed of laser is set to 20 mm/s, and the power was set to 1500 W and 2000 W, respectively. In addition, during the laser surface quenching, some specimens were preheated at 160 . Table 1 further shows the laser surface quenching parameters.2.3. Microstructure characterizationAll the specimens were cut, ground and polished. Subsequently, these specimens were etched with 4% nital. The microstructure was examined by optical microscope (Olympus BX53 M and Zeiss Axio Scope A1), scanning electron microscope (Zeiss PIGMA HV-01-043), electron probe (JEOL JXA8530F), and transmission electron microscope (FEI Tecnai G2 F20). TEM foils were prepared by electro-polishing using a solution composed of 10% perchloric acid and 90% ethanol. In addition, in order to observe the prior austenite grain boundaries, specimens were also etched at 65 for about 70 s with a solution composed of saturated picric acid and a small amount of Teepol wetting etchant. In this work, linear intercept method was used to measure the prior austenite grain size. In order to quantify the UCP, the cementite particles were colored using photoshop in advance, and then they were measured using Image-pro Plus. The volume fraction of UCP was assumed to be equal to its corresponding area fraction 18. Mean diameter of each UCP was calculated from its area by assuming a spherical shape 19. The number of cementite particles measured in each condition ranged from 600 to 2500, which depended on the austenitizing parameters.2.4. Hardness, wear resistance, and impact toughnessHardness was measured using a microhardness tester (HMV-2TADW) with a load of 0.3 kgf. Wear resistance was evaluated using a pin-on-desk wear test rig. The pin was a Si3N4 ceramic ball with 5 mm in diameter, as shown in Fig. 3a. The wear test was conducted at a rotational speed of 500 rpm, and rotational diameter of 5 mm. A normal load of 1.5 kg was imposed on the specimen during the wear test. The impact toughness was measured using a pendulum impact testing machine (SANS ZBC2452-B). The unnotched impact specimens with dimensions of 10 10 55 mm were used in this work 16. It should be noted that, for the specimens subjected to laser surface quenching, the hardened layer was in the opposite side with pendulum during the impact test, as shown in Fig. 3b.3. Results3.1. Microstructure in single and double quenching3.1.1. Cementite dissolution and sizeFig. 4 shows the microstructure of specimens in the single and double quenching treatment. It can be observed that there exist lots of spherical UCP in the martensite matrix. The volume fraction and size distribution of UCP are of great importance for the final mechanical properties. Usually, the increase of austenitizing temperature and extension of austenitizing time will result in the further dissolution of cementite. It is believed that the dissolution behavior of cementite will influence the size variation of UCP. In this work, the volume fraction and mean diameter of UCP under different austenitizing conditions were measured, as shown in Fig. 5.It can be seen from Fig. 5a, with the extension of austenitizing time from 5 min to 60 min, the volume fraction of UCP of the specimens austenitized at 800 C and 850 in single quenching decreased from 10.4% to 7.3%, and 8.0%2.4%, respectively. However, it decreased from 6.8% to 3.4% for the specimens subjected to second austenitizing at 800 in double quenching. Under the identical austenitizing tem-perature, the volume fraction of UCP is smaller in double quenching compared with single quenching. In addition, considering the volume fraction of cementite in the initial microstructure, it can be found that the volume fraction of UCP decreases continuously in the whole auste-nitizing process of single quenching, whereas it increases first, and then decreases in the later stage of second austenitizing process of double quenching. In other words, some new cementite forms at the early austenitizing stage, which can be observed in Fig. 4c. The size of these newly formed cementite particles is much smaller, which is about 0.1 m.Fig. 5b shows the variation of mean diameter of UCP in the auste-nitizing process. With the increase of austenitizing time, mean diameter of UCP decreases first, and then increases. In early austenitizing process of single quenching, the mean diameter of UCP of the specimen The difference in volume fraction and mean diameter of UCP in single and double quenching should be attributed to the different initial microstructure. For single quenching, the initial microstructure is composed of spherical cementite and ferrite matrix (Fig. 7a). For double quenching, the initial microstructure is mainly composed of UCP and martensite matrix, which is obtained through the previous first quenching. Compared with ferrite matrix, the martensite matrix is metastable, and it contains lots of dislocation and twin boundaries. In addition, carbon is supersaturated in martensite matrix, which induces the serious distortion of lattice. As a result, even at a quite low tem-perature (160 ), nano-carbides can form at the dislocation or twin boundaries within several minutes (Fig. 7b), and all these nano-carbides will transform into -carbides (i.e. cementite) 20,21. In this work, the measured values of volume fraction of cementite in the initial micro-structures of single and double quenching are 19.4% and 4.1%, respectively, which further indicates the initial microstructure of double quenching is supersaturated with carbon.3.1.2. Grain size in single and double quenchingFig. 8 shows the prior austenite grain boundaries of the specimens in single and double quenching. It is recognized that the grain growth is mainly driven by the decrease of interfacial energy. The growth process is a result of grain boundary migration, which is closely related to the diffusion of atoms 22,23. At higher austenitizing temperature and longer time, the PAGS tends to be larger due to the fact that effective diffusion distance of atoms is larger. The PAGS was further measured in this work. In single quenching, with the increase of austenitizing time from 5 min to 60 min, the PAGS of the specimens austenitized at 800 and 850 increases from 7.6 m to 8.1 m, and from 7.8 m to 8.6 m, respectively. There is no obvious difference in the PAGS under these two austenitizing temperatures. In double quenching, the PAGS of the specimens austenitized at 800 increases from 4.1 m to 4.7 m. Compared with single quenching, the PAGS is decreased by almost 40% in double quenching.3.2. Microstructure after laser surface quenching3.2.1. Morphology of the hardened layerAfter single quenching and double quenching, specimens were sub-jected to tempering, and laser surface quenching. Fig. 9 shows the microstructure after laser surface quenching. The hardened layer has a flat bottom except its two sides, which is because the heat dissipation is faster at two sides. Under identical laser parameters, the size of hard-ened layer is almost same, regardless of single and double quenching. When the laser power is 1.5 kW, the width of hardened layer is about 10am.It increases to about 10.6 mm at 2 kW, and preheating further in-creases its width to 11.1 mm. The microstructure of hardened layer is further examined, as shown in Fig. 10. There are few UCP at the surface, which is mainly due to the significant dissolution caused by higher surface temperature. In addition, the subsequent rapid cooling rate prevents the formation of cementite, which further leads to few UCP at the surface. The dissolution of cementite will leave the matrix super-saturated in carbon. Lots of UCP can be observed at the depth of 250 m and 625 m. In other words, a gradient microstructure was obtained after laser surface quenching.3.2.2. Dissolution and size of cementite within hardened layerThe volume fraction and mean diameter of UCP at different depths of hardened layer were measured, as shown in Fig. 11. It is quite obvious that with the increase of depth, the volume fraction of UCP increases, and mean diameter of UCP decreases. The increase of laser power, or preheating will accelerate the cementite dissolution, which is accom-panied with the coarsening of cementite. In addition, although the initial specimens were obtained through single and double quenching, respectively, there is no significant difference in the volume fraction of UCP at the same depth position of these two kinds specimens under identical laser parameters. However, compared with the specimens ob-tained through single quenching, the mean diameter of UCP at the bottom of hardened layer is obviously smaller in the specimens obtained through double quenching, which is decreased by about 20%. In other words, when the mean diameter of UCP is smaller in the initial micro-structure, the mean diameter of UCP is also smaller after laser surface quenching. Before laser surface quenching, the volume fraction of UCP in the specimens of single and double quenching was about 4.1% and 4.2%, respectively, and the corresponding mean diameter of UCP was about 0.24 m and 0.17 m, respectively. After laser surface quenching, at the deeper position, the volume fraction of UCP in the specimens of single and double quenching was at least 5.0% (Fig. 11a), which is slightly larger than that before laser surface quenching. In other words, some new cementite will form at the deeper position during the laser surface quenching. However, the mean diameter of UCP at the deeper position is quite close to that before laser surface quenching. On the whole, finer UCP can be obtained through the process combining double quenching and laser surface quenching.The cementite dissolution behavior is closely related to the temper-ature distribution during the laser surface quenching. However, it is very difficult to measure the temperature distribution along the depth. In this work, the temperature filed was calculated using ABAQUS based on the Fouriers heat conduction equation, as shown in Eq. (1).where , c, and is density, specific heat, and thermal conductivity, respectively. T is temperature, and t is time. Q is the latent heat of phase transformation. The values of , c, and is obtained through the JMatPro (version 7.0). The light intensity of laser heat source is thought to be distributed uniformly in this work. In addition, a 3-D model having size of 10 24 100 m
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