资源目录
压缩包内文档预览：(预览前20页/共52页)
编号：86402579
类型：共享资源
大小：11.13MB
格式：RAR
上传时间：2020-06-16
上传人：加Q294****549海量...
认证信息
个人认证
乐**（实名认证）
湖南
IP属地：湖南
39
积分
- 关 键 词：
-
2939
监控球外形罩模具设计
监控
外形
模具设计
- 资源描述：
-
2939 监控球外形罩模具设计,2939,监控球外形罩模具设计,监控,外形,模具设计
- 内容简介：
-
西安工业大学毕业设计(论文)题目申报、审核表(理工)（20102011学年）指导教师白瑀职 称讲师单位西安工业大学毕业设计(论文)题目监控球外形罩模具设计题目类型*综合应用题目来源科研 说明： 适用专业机械设计制造及其自动化完成地点西安工业大学是否重复否1选题背景（题目的意义与价值分析）。注塑模具主要用于热塑性塑料制品的成型，近年来也越来越多地用于热固性塑料制品的成型，注塑成型在塑料制品成型中占有很大比重，世界上塑料成型模具产量半数以上是注塑模具。在制造业日趋国际化的状况下，缩短产品开发周期和减少开发新产品投资风险，成为企业赖以生存的关键。2题目的基本要求及主要研究内容简介。（1）以最新版塑料模具设计手册为依据。 （2）模具型腔，型芯几何尺寸计算。确定浇注系统、冷却系统、脱模机构各个部分的具体尺寸、形状、位置等参数。 （3）用三维建模软件对零件进行建模，并进行尺寸转换（成型零件），完成型芯、型腔的设计。 （4）浇注系统、冷却系统、脱模机构各个部分的具体设计。 3对学生的知识、技能要求；完成本题目已具备条件，尚需要哪些条件？ 对学生知识背景的要求：1:具备机械制图、机械设计及机械原理基础知识2:Solidworks建模4教研室（系）审查意见：教研室（系）主任(签字)： 年 月 日5院(系)毕业设计(论文)工作领导小组审核意见： 院(系)主管领导(签字)： 年 月 日*注：1题目类型指工程设计科学实验软件开发理论研究综合，题目来源指科研生产实际自拟其它。若题目来源于教师的科研项目，请在“说明”处填写科研项目名称；若来源于生产/社会实际，请写明题目来源单位；若为其他，写明具体来源。2此表由各院（系）自行归档。西安工业大学毕业设计（论文）任务书系别 机电信息系 专业 机械设计制造及其自动化 班级B070203姓名李文昊学号B07020311 1.毕业设计（论文）题目： 监控球外形罩模具设计 2.题目背景和意义：注塑模具主要用于热塑性塑料制品的成型，近年来也越来越多地用于热固性塑料制品的成型，注塑成型在塑料制品成型中占有很大比重，世界上塑料成型模具产量半数以上是注塑模具。在制造业日趋国际化的状况下，缩短产品开发周期和减少开发新产品投资风险，成为企业赖以生存的关键。 3.设计(论文)的主要内容（理工科含技术指标）：(1) 模具型腔，型芯几何尺寸计算。确定浇注系统、冷却系统、脱模机构各个部分的具体尺寸、形状、位置等参数 （2）用三维建模软件对零件进行建模，并进行尺寸转换（成型零件），完成型芯、型腔的设计 (3) 浇注系统、冷却系统、脱模机构各个部分的具体设计； 4.设计的基本要求及进度安排（含起始时间、设计地点）： 第1周第3周：查阅资料，完成基础知识的积累和开题报告。 第4周第7周：模具型腔，型芯几何尺寸计算。 第7周第10周： 三维建模软件对零件进行建模。 第11周第14周：浇注系统、冷却系统、脱模机构各个部分的具体设计。 第15周第16周：完成毕业论文。 5.毕业设计（论文）的工作量要求 不少于15000字 实验（时数）*或实习（天数）： 无 图纸（幅面和张数）*： A0图纸1张 模具装配图 其他要求： 参考文献不少于 15篇（其中文期刊文献不少于5篇，外文文献不少于3篇，其中一篇外文文献原文作为翻译对象完成不少于1000词的英文翻译） 指导教师签名： 年 月 日 学生签名： 年 月 日 系（教研室）主任审批： 年 月 日说明：1本表一式二份，一份由学生装订入附件册，一份教师自留。2 带*项可根据学科特点选填。西安工业大学北方信息工程学院毕业设计(论文)开题报告题目： 监控球外形罩模具设计系 别 机电信息系 专 业 机械设计制造及其自动化 班 级 B070203 姓 名 李文昊 学 号 B07020311 导 师 白瑀 2010年 11月 20日51. 毕业设计（论文）综述（题目背景、研究意义及国内外相关研究情况）注塑模具主要用于热塑性塑料制品的成型，近年来也越来越多地用于热固性塑料制品的成型，注塑成型在塑料制品成型中占有很大比重，世界上塑料成型模具产量半数以上是注塑模具。在制造业日趋国际化的状况下，缩短产品开发周期和减少开发新产品投资风险，成为企业赖以生存的关键1。 当前的技术仅限于在三维建模的基础上进行设计，如果科技发展到一定水平，以后也许不需要三维建模，或许直接在电脑上生成产品，激光扫描出来后直接把工艺要求和设计方案显示出来2。这样可以更加省时省力。近年来，我国塑料模具工业发展迅速，体现在产品技术含量不断提高，制造周期不断缩短3。但与国外塑料模具的先进水平相比，依然存在不小的差距4。虽然，我国的塑料消费量已居世界首位，但人均消费量仅为工业发达国家的1/7，为工业中等发达国家的1/4。模具网CEO、国际模具及塑料供应商协会负责人罗百辉分析认为，这一方面反映出中外模具存在一定差异，另一方面也展现了我国塑料工业和塑料模具产业巨大的发展空间5。 在企业员工的整体素质上，德国、日本模具企业绝大多数员工是大学毕业或经过专门训练的，至少有10年的工作经验，模具企业技术人员比例很高，多数企业在25%以上，有些在50%以上6，不少企业的职工往往可以在技术与生产岗位上互换。国内模具企业的员工缺乏高新加工技术的培训和高端数控机床操作技能的培训，在某种程度上这也影响了高端设备的利用率。模具企业技术人员比例偏低，多数企业15%20%之间，且综合开发能力较低7。此外，德、日企业对新产品开发很重视，模具厂经常会与材料厂商、产品厂商共同开发；注塑机厂会与材料厂商共同研发新机型8；名牌塑料供应商会与名牌汽车公司联合研制以塑代钢的新产品。这种强强合作，使企业具有很强的研发能力。我国企业在创新、研发能力上差距更大9。国内的模具设计基本由一个人完成，而国外则是多人分工合作完成，这样制造出来的产品规格比较统一，零件利用率高，仪器的精密程度也比国内好，制造出来的塑料成品在外观和表面光洁度上都领先于我国10。因此，我们大学生更应该努力学习国外先进的制造工艺和技术，早日把国内的生产现状提高到更好的层次。2. 本课题研究的主要内容和拟采用的研究方案、研究方法或措施本课题主要研究监控球外形罩的注塑模设计。内容11：（1）以最新版塑料模具设计手册为依据。（2）模具型腔，型芯尺寸计算。确定浇注系统、冷却系统、脱模机构各个部分的具体尺寸、形状、位置等参数12。（3）用solidworks三维建模软件对零件进行建模，并进行尺寸转换（成型零件），完成型芯、型腔的设计。（4）浇注系统、冷却系统、脱模机构各个部分的具体设计13。基本方案如下表所示14：表1 模具方案比较方案浇注系统侧抽芯机构顶出机构冷却系统方案一潜伏式浇口斜滑块机构顶板顶出内冷却方案二侧浇口弯销机构顶板顶出内冷却方案三侧浇口斜导柱侧滑块顶块顶出外冷却方案确定：相对于潜伏式浇口，侧浇口的设计简洁明了；侧抽芯机构应该选择定位精度高的，故选择斜导柱侧滑块；而顶出机构中顶块顶出比较省料；由于冷却道设计在型腔上比较合适，故选择外冷却15。因此选择方案三最佳。表2技术参数：（单位: mm）半球直径200凸台外径220凸台内径210卡扣长20卡扣高8卡扣厚4卡槽宽60卡槽深73壁厚3卡扣外凸2凸面宽1 图1 零件图及尺寸3. 本课题研究的重点及难点，前期已开展工作重点：保证表面质量，光洁度以及开模、脱模安全难点：卡扣的侧抽芯以及如何保证脱模时不损坏表面质量4. 完成本课题的工作方案及进度计划（按周次填写）第1周第3周：查阅资料，完成基础知识的积累和开题报告。 第4周第7周：模具型腔，型芯几何尺寸计算。 第7周第10周： 三维建模软件对零件进行建模。 第11周第14周：浇注系统、冷却系统、脱模机构各个部分的具体设计。 第15周第16周：完成毕业论文。5 指导教师意见（对课题的深度、广度及工作量的意见） 指导教师： 年 月 日 6 所在系审查意见： 系主管领导： 年 月 日参考文献：1 张旭海，蒋建清，江静华，翟晓东，赵维垲。机械制造与自动化。2003年第05期：31-362 贾玉赞。小型塑料、橡胶模具设计浅议。太原科技，2002年第02期：10-123 阮雪榆,李志刚,武兵书,等 中国模具工业和技术的发展J 中国机械信息网 。2006（2）： 40-434 张荫朗。90年代塑料注射模发展趋向J 模具工业,1995（20）：10-115 徐维。冷作模具钢65Nb 的渗碳复合强韧化J 大连铁道学院学报,19971021 第B3版6 江静华,蒋建清,马爱斌,涂益友,翟晓东。室温盐液渗硫工艺研究与显微分析J 金属热处理,2002(12)：16-187 陈涛。塑料模具的动态仿真。现代制造工程。2009（3）15-178 郭幼丹。基于协同设计的塑料模具设计。龙岩学院学报2005(6):13-209 张鲁阳.以工程思维为主线组织教学内容.再谈模具专业材料学课程改革J.机械工业高教研究，2001（3）：48-51.10 蒋美丽.合理选用塑料模具的材料与热处理.提高模具使用寿命J.机床与液压，2004（1）：142-143.11 陈志刚.塑料模具设计M .北京：机械工业出版社 2002（4）：34-3412 武兵书.我国的模具材料及其应用技术J.机械工人（冷加工）2002（4）：16-19.13 Modelling of yield length in the mould of commercial plastics using artificial neural networks Materials & Design, Volume 28, Issue 1, 2007, Pages 278-28614 Development of Non-Quenched Prehardened Steel for Large Section Plastic Mould Original Research Article Journal of Iron and Steel Research, International, Volume 16, Issue 2, March 2009, Pages 61-6715 Relationships between tensile and fracture mechanics properties and fatigue properties of large plastic mould steel blocks Original Research Article: A, Volumes 468-470, 15 November 2007, Pages 193-200Materials Science and Engineering A 468470 (2007) 193200Relationships between tensile and fracture mechanics propertiesand fatigue properties of large plastic mould steel blocksDonato Firraoa, Paolo Matteisa, Giorgio Scavinoa, Graziano Ubertallia, Maria Giuseppina Iencob,Maria Rosa Pinascob, Enrica Stagnob, Riccardo Gerosac, Barbara Rivoltac,Antonio Silvestric, Giuseppe Silvac, Andrea GhidinidaDip. Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, IT-10129, Torino, ItalybDip. Chimica e Chimica Industriale, Universit a di Genova, IT-16146, Genova, ItalycDip. Meccanica, Politecnico di Milano, IT-23900, Lecco, ItalydLucchini Sidermeccanica S.p.A., IT-24065, Lovere (BG), ItalyReceived 31 March 2006; received in revised form 5 July 2006; accepted 7 July 2006AbstractMoulds for plastic automotive components such as bumpers and dashboards are usually machined from large pre-hardened steel blocks. Due totheir dimensions, the heat treatment produces mixed microstructures, continuously varying with the distance from the quenched surface, at whichfracture toughness and fatigue properties are not well known and generally lower than those corresponding to a fully quenched and temperedcondition. The response of the mould to defects (for example, microcracks due to improper weld bed deposition) and stresses during servicedepends on steel properties, that in turn depend upon the heat treatment and the microstructure. A pointwise determination of the tensile, CharpyV-notched, fracture toughness and rotating bending fatigue properties was carried out in a large block. High cycle fatigue was investigated bythe stair-case method. The samples were obtained from different depths of the blooms. The relationship between mechanical properties, fracturesurfaces morphology and microstructure was also investigated. 2007 Elsevier B.V. All rights reserved.Keywords: Plastic mould steel; Metallography; Mechanical properties; Fracture toughness; High cycle fatigue; Fractography1. IntroductionLarge forged steel blooms (typically 1m1m section andmore than 1m in length) are used to fabricate plastic moulds(Fig.1a),inturnemployedtoformautomotivecomponents,suchas bumpers and dashboards, made of thermoplastics, usuallypolypropylene or reinforced (ABS).Thestresspatternsappliedtothemouldsinservicearisefromthe polymers injection pressure and from the local thermal gra-dients, and could be enhanced by notch effects and by defectsof various origin (particularly weld bed depositions effectedwithout the proper heat treatment that could be recommendedfor smaller components). Stresses may be significantly raisedby abnormal operations, e.g., incomplete extraction of alreadyformed objects.Corresponding author. Tel.: +39 0115644663; fax: +39 0115464699.E-mail address: donato.firraopolito.it (D. Firrao).Each mould is expected to produce a few millions of piecesin its life, corresponding to the production run of one car model;thus, fatigue effects should also be considered. Wear induced bythereinforcedresinsflowmaybesevereandmaybeanadditionalcause for crack nucleation and propagation, with the flowingresin infiltrating cracks and acting as a wedge.For economic and logistic reasons, the traditional productioncycleofmechanicalcomponents(roughmachining,heattreatingandfinishing)hasbeenabandonedandcommonlysubstitutedbymachining pre-hardened blooms.The most commonly used steel grade is 1.2738 (or40CrMnNiMo8-6-4, ISO 4957 standard 1), a heat-treatable,0.4% C, high-hardenability, low-alloy steel (Table 1).The section of the bloom is usually comparable to the sec-tionoftheoriginalingot(becauselargeringotsarenotfeasible);thus, a sensible equivalent reduction ratio may be obtained onlyby repeated forging steps, each consisting of alternated elonga-tion and compression cycles. The total deformation is much lessthan that obtained in rolling and not comparable in the effects,0921-5093/$ see front matter 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.msea.2006.07.166194D. Firrao et al. / Materials Science and Engineering A 468470 (2007) 193200Fig. 1. View of half of an automotive bumper mold (a). Sampling plan to obtain38mm thick fracture toughness samples and smaller blanks cut at increasingdepths inside bloom C (b).because each step achieves only a limited reduction ratio (1.5 isa possible value).Depending on the size, dehydrogenization can take a fewdays, whereas the durations of the austenitizing and temperingstages can be as long as 12 days. Usually, blooms are austeni-tized in the 840880C temperature range, then quenched in oiland tempered in the 550600C temperature range (more thanone tempering stage may be applied) to obtain a final 330300HB hardness. Heating stages are usually executed in air.Since forging yields a rough shape with deep decarburationsoccurring during heating, external material removal (usuallyperformed in a commercial warehouse) may be up to 20mm(plusscale).Furthermore,bloomsmaybesawntorequestedsize(often asymmetrically); blooms for bumper moulds are usuallysawn to yield a U shape.The dimension of the blooms exceeds the very high hard-enability critical dimension of steel 1.2738 2,3, thus in largeoil-quenched blooms different microstructures occur at increas-ing depths, all of them being affected by the subsequenttempering.The final mould shape is obtained in the mould-machiningshop by chip-removal and/or electrical-discharge machining,followed by grinding with or without polishing in selected areasand by local surface treatments; upon request, shape correctionsare performed also by welding additions.Due to the sawing and machining operations, any of themicrostructures occurring at different positions in the originalbloom can be found at the mould face, where notch effects andweldingthermaldefectsareoftenpresent.Previousstudieshaveassessed the deleterious influence of the slack quench mixedmicrostructures upon the toughness of quenched and temperedlow alloy steels 4,5; nevertheless, whereas the fracture tough-ness of other tool steels was already studied 68, the sameproperty for the above steel has been analyzed only recentlyby the authors, by considering its relationship with the differentmicrostructuresoccurringinsidethelargeblooms2,9,10.Inthepresent work, these latter results (concerning not only fracturetoughness, but also hardness, tensile properties and resilience)are summarized and completed by an investigation of the steelfatigue behaviour and an in-depth analysis of the fracture sur-facestoassessthefracturemechanismsoccurringinthedifferenttest-specimens.2. ExperimentalThe examined bloom had the following original dimensions(after forging): 2970(L)1285(T)1190(S)mm. The L direc-tiondefinesthelongingotcastingandforgingaxis;propertiesintheSandTtransversedirectionsarethoughttobenotdifferentlyinfluenced by the casting and forging procedures.Series of 40mm thick fracture toughness specimens and ofsmallerblankspecimenswereobtainedatincreasingdepthfroma slab cut as outlined in Fig. 1b.The depth of a point is defined as its distance (in direction T)from the nearest forged and heat treated surface, or from a ref-erence one. For fracture toughness specimens, depth is reportedas that of the fracture initiation zone.Specimens were tested either in the as-received or in a re-heat-treated condition.The smaller blanks were used for metallography, Charpy-V, tension, and hardness testing; some of them were re-heat-Table 1Compositional limits for the plastic mould steel grade 1.2738 and heat chemical analysis of the examined bloomCCrMnNiMoSiSPGrade 1.273840CrMnNiMo8-6-40.350.40.40.030.03Examined bloom0.422.010.370.0020.006D. Firrao et al. / Materials Science and Engineering A 468470 (2007) 193200195treated. The broken halves of fractured toughness specimens,tested in the as-received condition, were used as such to yieldCharpy-V specimens and unnotched rotating bending fatiguespecimens with a minimum diameter of d=6mm; some of thelatter ones were tested in the re-heat-treated condition. Somefracture toughness specimens, fabricated from the original slab,were re-heat-treated before rupture; their broken halves wereused to yield tension specimens to assess the tensile propertiesof the re-heat-treated condition.The re-heat-treatments were performed to homogenize themicrostructure, improve the mechanical properties (due to a fullquench) and evaluate them as a function of the specimens origi-nal position inside the bloom. The samples were austenitized at860C for 45min, quenched, and tempered in two stages at 590and 550C, for 3h each. The already machined KIcsamples,together with some of the smaller blanks, were heated in vac-uumandquenchedinN2,whereastheKIctest-piecesfragments,usedtomachineunnotchedfatiguespecimens,whereheatedandquenched in air. The austenitizing and tempering temperatureswere similar to those adopted in industry.3. Results3.1. MicrostructuresThe previous austenitic grains, visualized by the BechetBeaujard etch 11, are homogeneous in size, isotropic, andabout 10?m wide, both at surface and at core.The actual microstructures were examined at increasingdepths along the 1285mm S dimension on ST parallel planesfrom one external surface to the other; a 2% Nital etch wasused.Lower cooling speeds at increasing depth during quenchingyielded different metallographic constituents going from sur-face to core: martensite (with some residual austenite, Fig. 2a),lower and upper bainite (Fig. 2bd), ultra-fine and fine pearlite(Fig.2candd).Bothmartensiteandbainitewereaffectedbythetemper. Pearlite does not occur up to 285mm depth; some rare,ultra-fine pearlite zones appear at 370mm and finally a signif-icant fraction of ultra-fine and fine pearlite is found at 450mmdepth. Close to the core, fine pearlite is the main constituent(Fig. 2c and d), but some bainite was still detected. The coremicrostructure was resolved by scanning electron microscopy(not reported).Themicrostructureofthere-heat-treatedspecimens,observed both on small blanks and on SENB fragments (in thelatter case either at one end or close to the fracture surface), istempered martensite in all the specimens.3.2. Tensile, Charpy-V, and KIctestsHardness,tensileandCharpy-Vtestswerecarriedoutinorderto completely characterize the mechanical properties.Hardness and tensile properties monotonically decrease atincreasing depths, consistently with the microstructure (Fig. 3).The work hardening coefficient variation across the originalbloom(Fig.3)isconsistentwiththemicrostructureversusdepthFig. 2. Metallographic constituents at increasing depths in bloom C; Nital etch.55mmdepth:temperedmartensitewithsomeresidualaustenitetransformeddur-ing tempering (a); 105mm depth: lower bainite modified by tempering, residualaustenite transformed during tempering (b); 450mm depth: fine and ultra-finepearlite, upper bainite modified by tempering (c); 650mm depth (core): finepearlite, upper bainite modified by tempering (showing coarser carbides, d).trend; in fact, it is known that softer structures have higherwork-hardening values.The hardness and tensile properties of the re-heat-treatedspecimens are much larger than the ones of the as-received con-dition; they slightly decrease with depth (Fig. 3). The fracturetoughness(LTorientation)intheas-receivedconditionincreasesfrom 34MPam1/2close to surface to 47MPam1/2close to196D. Firrao et al. / Materials Science and Engineering A 468470 (2007) 193200Fig. 3. Hardness, ultimate tensile strength (UTS), yield stress (YS) and fracturetoughness (KIc) vs. depth. As-received or re-heat-treated (RHT) samples. Hard-ness tests upon TL planes, uniaxial tensile tests with orientation L, LT fracturetoughness tests. Series of hardness tests along segments at 145mm or 440mmdistance from the blooms ST midplane.core, while re-heat-treated specimens exhibit much larger val-ues (mean value: 84MPam1/2), with a fairly large dispersion(Fig. 3). Sample dimensions were considered enough to avoidplastic hinges or excessive blunting of the crack front (thicknessequal to 40mm, height equal to 74mm).Charpy-V impact absorbed energies at room temperature areabout 10J for both core and surface specimens, whereas re-heat-treated specimens show improved values (Fig. 4). Fractureappearance transition temperatures were determined as beingTable 2Fatigue strength amplitudes at N=4.2106cycles (MPa) in rotating bendingfatigue testsSurvivalprobability (%)As-receivedRe-heat-treatedCoreSurfaceCoreSurface1051858163870650493195591760824700590469537577694about 270C for the material close to surface and about 150Cfortheoneclosetocore.FurtherCharpy-Vtestswereperformedat 1752C (Fig. 4), between the aforementioned transitiontemperatures, and close to the polymer injection temperature.The175CKVvaluesincreasewithdepth,consistentlywiththedecrease of the transition temperature from the surface to thecore.3.3. Fatigue testsRotating bending fatigue tests were carried out according tothe staircase method 12 on unnotched specimens both in theoriginalandinthere-heat-treatedcondition,selectingthemfromtwo significant positions: near surface (about 140mm from thesurface) and next to the core (about 560mm from the surface).It was chosen to determine the fatigue strength at 4.2 millioncycles,proportionatetotheautomotiveindustryproductionruns.The results are listed in Table 2, whereas an example of thestaircase method is shown in Table 3.The fatigue strength of the samples near surface are bet-ter than the ones determined by the samples next to the core,both in the original and re-heat-treated conditions. The re-heat-treatment has a very positive influence; in fact, fatigue lifeimproves of about 25% both for the samples near surface andnext to the core. These results, together with fracture tough-ness and tensile tests ones, demonstrate that heat treatmentsperformed on the massive blocks are not able to fully exploitthe quality of the steel.3.4. FractographyThe fracture surfaces of the Charpy-V specimens tested at175C were observed in order to identify the brittle fractureTable 3Staircasemethodforthesamplemachinedatabout560mmfromthesurfacein the original condition (“X” broken, “O” unbroken specimen)D. Firrao et al. / Materials Science and Engineering A 468470 (2007) 193200197Fig. 4. Charpy-V transition curves at surface and core and RT impact energy of re-heat-treated specimens.modes occurring in their central (macroscopically brittle) por-tion. The brittle fracture mode of the as-received specimens isalmost completely intergranular (microscopically brittle) at thelowest investigated depths (Fig. 5a); at increasing depths, theintergranular fracture is gradually substituted by cleavage orquasi-cleavage (Fig. 5b); finally, at core, intergranular (micro-scopically brittle) surfaces are absent and quasi-cleavage is thefracturemode;nevertheless,asignificantfractionofductilefrac-ture occurs also in the central macroscopically brittle region ofeach Charpy specimen (Fig. 5c). The brittle fracture mode ofthere-heat-treatedsamplestestedatRTisintergranularfracture,similartothatobservedintheas-receivedspecimensclosertothesurface (notwithstanding the large test temperature difference).These differences cannot be correlated directly to the impactabsorbedenergy,becausethelatteriscontrolledbythesurround-ing portion of macroscopically ductile fracture (whose surfacefraction increases with depth).All the KIcsamples (both as-received and re-heat treated)show macroscopically brittle surfaces, with negligible lateralshear lips, thus confirming the validity of the KIcmeasures.In all the as-received KIcsamples, the fracture initiationzone does not show significant signs of blunting. At 60mmdepth, the fracture follows the previous austenite boundariesselected by convenient orientation (probably weakened by ele-mental segregations due to the long heat treatment 13), andelsewhere it goes through the grains by cleavage (Fig. 6a andb), whereas only small transition zones between adjacent brit-tle domains show ductile dimples. The amount of intergranularbrittlefracturesteeplydecreaseswithdepth,beingpartiallysub-stituted by cleavage fracture, which is prevalent at 230 and395mm depths; upon increasing the depth from 230mm to core(645mm), areas of ductile fracture with shallow, 1?m dia. dim-ples, appear (Fig. 7a and b). These latter dimples are clearlydifferent in morphology respect to the smaller, but proportion-ally deeper, ones found in the aforementioned transition zones;since they occur in the same depth interval where bainite does,it can be inferred that they develop around bainite carbides. Inmany points the overall morphology of these ductile areas isonce more related to the previous austenite boundaries. Thus,the plain strain fracture of the as-received samples is at leastpartially controlled by the prior austenite grains, both in theFig. 5. Fracture surfaces of as-received Charpy-V specimens tested at 175C.Macroscopically brittle central portions of specimens obtained at increasingdepths. Gradual transition from intergranular (microscopically brittle) fracture(a and b) to cleavage (b); ductile and cleavage areas (c).198D. Firrao et al. / Materials Science and Engineering A 468470 (2007) 193200Fig. 6. Fracture surface of as-received 60mm depth KIcsample. Adjacent inter-granular (a) and cleavage (b) fracture areas.cases of microscopically brittle and of microscopically duc-tile surfaces. Moreover, from the 560mm and 140mm depthKIcspecimens (depth from opposite surface), two cross-sectionsamples were extracted in the centre of the fracture surfaces,polishedandetchedwiththeBechetBeaujardsolution;thefrac-ture path seems to follow, at times, the prior austenitic grainboundaries.The overall fracture morphology of the re-heat-treated KIcspecimens is mixed, with presence of both intergranular frac-Fig. 7. Fracture surface of as-received 395mm depth KIcsample. Cleavage andductile fracture.Fig. 8. Fracture surfaces of a re-heat-treated KIcspecimen (309mm depth,103MPam1/2). Large ductile region at the crack-tip.ture and quasi-cleavage. Nevertheless at the crack tip a ductileregion is present (Fig. 8), whose extension is directly correlatedtothefracturetoughness(15and50?m,respectively,for63and103MPam1/2).Thisductileregionmaybeinterpretedbyaseriesofridgesandvalleys,duetofracturealongsubsequentsegmentsof slip lines originating from the blunted notch. Some sphericaldefects were observed in ductile zones of fracture surfaces andwere identified prevalently as CaS by EDS analysis.The fatigue cracks of the rotating bending specimens werenucleatedateachspecimenssurfaceandinthemajorityofcasesare overall orthogonal to the specimen axis. The fatigue fracturesurfaces of the as-received specimens does not show striations,whereas those of the re-heat-treated ones show only some ill-defined striations. The macroscopically brittle overload fractureareasoftheas-receivedfatiguesamplesaremainlyintergranularat 140mm depth (Fig. 9a) and mainly by cleavage (but withsignificantductileareas)at560mmdepth(Fig.9b),whereastheoverloadfractureareasofthere-heat-treatedsamplesaremainlyintergranular, with some small ductile region, in both examinedpositions (Fig. 9c). Furthermore, all the fatigue samples show aductile shear lip (usually oblique to the specimen axis), due tothe rupture of the last ligament.The comparison of the macroscopically brittle overload frac-ture surfaces obtained in the Charpy-V, fracture toughness androtatingbendingfatiguetests,performeduponas-receivedsteel,evidences a common pattern, in respect to the depth inside theblooms, that can be summarized as follows.D. Firrao et al. / Materials Science and Engineering A 468470 (2007) 193200199Fig.9. Overloadfracturesurfacesoftherotatingbendingfatiguesamples.Inter-granuralfractureofasub-surfaceas-receivedspecimen(a).Cleavageandductilefracture surfaces in a close-to-core as-received specimen (b). Partially ductileintergranular fracture of a re-heat-treated (originally close-to-core) specimen(c).The prevalent brittle fracture mode of steel specimensobtainedatthelowestdepths(whichconsistmainlyoftemperedmartensite) is the intergranular fracture, that was observed inall the three different mechanical tests. The occurrence, in thesame metallurgical condition, in the KIctest-specimens, of asignificant fraction of transgranular cleavage, may be related tothe larger stress concentration occurring in these tests (duringthe unstable brittle propagation), which constrains the fracturesurface closer to the theoretical fracture plane, implying thetransgranular fracture of some grains.The re-heat-treated specimens generally show the same brit-tle fracture mode (intergranular rupture at times mixed withtransgranular cleavage) of the as-received material surface orsub-surface specimens, probably due to their similar overallmicrostructural condition (tempered martensite). At intermedi-ate depths, where the microstructure contains a rising and thenprevalent fraction of bainite, cleavage appears to be the mostcommon brittle fracture mode, but the macroscopically brittleareas contain a significant fraction of ductile (dimpled) rupture,thatincreasestowardthecore(wherethemicrostructureconsistsmainly of pearlite).3.5. Comparison of the as-received and re-heat-treatedfracture toughness resultsThe large fracture toughness increase obtained from the re-heat-treatment, in respect to the steel obtained from the surfaceof the blooms (that was confirmed also by the Charpy-V results)can be explained with the energy absorption occurring in theductile region observed at the crack tip in the re-heat-treatedsamples only. Nevertheless, it cannot be related to microstruc-turaldifferencesobservablebyopticalmetallography,thatshowsindistincttemperedmartensiteinbothcases.Nevertheless,adif-ference between the two microstructures may be hypothesizedin the carbide composition, morphology and distribution, on asub-micron scale; such a difference may be due to the differentcoolingcurves,throughamoreorlessintense(orabsent)carbideprecipitation from the austenite above Ms, or through a more orlessintenseauto-temperingofthemartensiteduringcooling.Asit regards the first hypothesis, it should be noted that the cool-ingspeeddifference,inthehightemperaturerange,betweenthebloomssurfaceandthere-heat-treatedspecimens,arenotlarge,because the latter ones were cooled in gas with a low drasticity.As it regards the second hypothesis, on the contrary, in may benoticed that the cooling of the blooms surface, at temperaturesbelowMs,maybeslowedbytheheatfluxcomingfromthecore,thus allowing for auto-tempering phenomena.4. ConclusionsMixed microstructures occur throughout the examinedbloom.Temperedmartensiteoccursonlyclosetothesurfaceandsteeply decreases with depth; upper and lower bainites (modi-fiedbytempering)aretheoverallprevalentconstituents;pearlitegradually appears at higher depths and becomes the main con-stituent at core.The hardness and the tensile properties significantly varyinside the bloom; yet, with the exception of a small coreregion, they are in overall compliance with the usually spec-ified values. It should be noticed that the minimum reportedvalues of the tensile properties does not correspond spatiallyto the minimum measured hardness values; by taking intoaccount three-dimensional effects, it should be concluded thatthe blooms minimum tensile properties are lower than the min-imum measured ones.The fracture toughness values of the as-received steel areexceptionally low (about 40MPam1/2) for a quenched and tem-pered steel, considering the achieved ultimate tensile strength.Theplain-strainfractureprevalentlyoccursbydecohesion,con-sistently with the fact that, at room temperature, this steelis in the brittle temperature range of its Charpy-V brittle-to-ductile transition curve. The moderate fracture toughness200D. Firrao et al. / Materials Science and Engineering A 468470 (2007) 193200increase at core can be attributed to energy consumption inthe ductile regions that appear, at increasing depth, in a lim-ited fraction of the fracture surfaces. They are absent closeto the surface; their occurrence increase going towards thecentre.Bycomparison,alltheindividuallyre-heat-treated(quenchedand tempered) samples resulted in values of fracture toughnessin excess of 60MPam1/2, and some over 100MPam1/2, evenat somewhat higher strength levels. Thus, the observed lowtoughness of the as-received specimens must be attributed tothe microstructures caused by the heat treatment, and in generalto the large dimensions of the blooms and of the moulds.Whereasthispaperconcernsonebloomonly,itisnecessarytonote that previously published Charpy-V results 10, obtainedon samples representative of another bloom of the same steelgrade, but obtained from a different manufacturer, has shownevidences of a remarkably higher toughness.TherotatingbendingfatiguestrengthamplitudesatN=4.2106cycles, about 560MPa for the steel close to thesurface and 495MPa for the steel close to the core, obtainedwith unnotched specimens, apparently scale with the steel ten-sile strength, rather than with its fracture toughness. The fatiguestrength for samples individually re-heat-treated increase ofabout 25%, keeping the differences due to the original location.The reasons of the latter fact are still object of discussion.The different macroscopically brittle (i.e., plastically con-strained) fracture modes of the examined
- 温馨提示:
1: 本站所有资源如无特殊说明，都需要本地电脑安装OFFICE2007和PDF阅读器。图纸软件为CAD,CAXA,PROE,UG,SolidWorks等.压缩文件请下载最新的WinRAR软件解压。
2: 本站的文档不包含任何第三方提供的附件图纸等，如果需要附件，请联系上传者。文件的所有权益归上传用户所有。
3.本站RAR压缩包中若带图纸，网页内容里面会有图纸预览，若没有图纸预览就没有图纸。
4. 未经权益所有人同意不得将文件中的内容挪作商业或盈利用途。
5. 人人文库网仅提供信息存储空间，仅对用户上传内容的表现方式做保护处理，对用户上传分享的文档内容本身不做任何修改或编辑，并不能对任何下载内容负责。
6. 下载文件中如有侵权或不适当内容，请与我们联系，我们立即纠正。
7. 本站不保证下载资源的准确性、安全性和完整性, 同时也不承担用户因使用这些下载资源对自己和他人造成任何形式的伤害或损失。
人人文库网所有资源均是用户自行上传分享，仅供网友学习交流，未经上传用户书面授权，请勿作他用。
川公网安备: 51019002004831号