Numerical Calculation and Experimental Research on Residual Stresses in Precipitation-hardening Layer of NAK80 Steel for Shot Peening

Shot peening can improve fatigue strength of materials by creating compressive residual stress field in their surface layers,and offers a protection against crack initiation and propagation,corrosion,etc.And fatigue fracture and stress corrosion cracking of NAK80 steel parts are improved effectively.Currently there lacks in-depth research in which the beneficial effect of the residual stress may be offset by the surface damage associated with shot peening,especially in terms of the research on the effective control of shot peening intensity.In order to obtain the surface residual stress field of NAK80 steel after shot peening,the samples are shot peened by pneumatic shot peening machine with different rules.The residual stress in the precipitation-hardening layer of NAK80 steel is measured before and after a shot peening treatment by X-ray diffraction method.In order to obtain true residual stress field,integral compensation method is used to correct results.By setting up analytical model of the residual stress in the process of shot peening,the surface residual stress is calculated after shot peening,and mentioning the reason of errors occurred between calculated and experimental residual stresses,which is mainly caused by the measurement error of the shoot arc height.At the same time,micro hardness,microstructure and roughness in the precipitation-hardening layer of NAK80 steel before and after shot peening were measured and surveyed in order to obtain the relation between shot peening strength and surface quality in the precipitation-hardening layer.The results show that the surface quality of NAK80 steel is significantly improved by shot peening process.The over peening effect is produced when the shot peening intensity is too high,it is disadvantageous to improve sample＇s surface integrity,and leading to reduce the fatigue life.When arc high value of optimal shot peening is 0.40 mm,the surface quality is the best,and the depth of residual stress in the precipitation-hardening layer reaches to about 450 μm.Numerical calculation is very useful to define the process parameters when a specific residual stress profile is intended,either to quantify the benefits on a specific property like fatigue life or to help on modeling a forming process like shot peen forming.In particular,the proposed parameter optimization in the progress of shot peening and effective control of the surface texture provide new rules for the quantitative evaluations of shot peening surface modification of NAK80 steel.

Effect of Air Pressure on Hardened Layer of U75V 60 kg/m Heavy Rail after Heat Treatment

The samples cut from U75V 60 kg/m heavy rail are heated to 900 ℃ in resistance furnace and then put into air spraying channel to be cooled for 80 s, and change air pressure from 0.16 MPa to 0.33 MPa, and observe the effect of air pressure on hardened layer. The thickness and hardness of hardened layer increases with the increase of air pressure, and the thickness is more than 24 mm at the center and top fillets of rail head, and more than 15 mm at the blow fillets of rail head when air pressure is more than 0.26 MPa. During the tempering after heat treatment, tempering temperature of rail head is more than 570 ℃ when air pressure is separately 0.16 MPa, 0.20 MPa and 0.23 MPa, which is higher than finishing temperature of pearlite transformation at the cooling rate of 3 ℃/s according to CCT curve of U75V steel. When air pressure is separately 0.26, 0.30 and 0.33 MPa, the tempering temperature is 529 ℃ lower than finishing temperature of pearlite transformation at the cooling rate of 3 ℃/s. Under this condition, pearlite transformation is finished totally, so in order to reduce air consumption and control the cost, proper air pressure for U75V 60 kg/m heavy rail heat treating should be 0.26 MPa; the cooling rate increases with the increase of air pressure, and the average cooling rate on the surface of rail head is more than 3.21 ℃/s when air pressure is more than 0.26 MPa, and the largest cooling rate occurs at the top fillets of rail head.

Impact toughness of a gradient hardened layer of Cr5Mo1V steel treated by laser shock peening

Laser shock peening（LSP） is a widely used surface treatment technique that can effectively improve the fatigue life and impact toughness of metal parts.Cr5Mo1 V steel exhibits a gradient hardened layer after a LSP process.A new method is proposed to estimate the impact toughness that considers the changing mechanical properties in the gradient hardened layer.Assuming a linearly gradient distribution of impact toughness,the parameters controlling the impact toughness of the gradient hardened layer were given.The influence of laser power densities and the number of laser shots on the impact toughness were investigated.The impact toughness of the laser peened layer improves compared with an untreated specimen,and the impact toughness increases with the laser power densities and decreases with the number of laser shots.Through the fracture morphology analysis by a scanning electron microscope,we established that the Cr5Mo1 V steel was fractured by the cleavage fracture mechanism combined with a few dimples.The increase in the impact toughness of the material after LSP is observed because of the decreased dimension and increased fraction of the cleavage fracture in the gradient hardened layer.

Sulfate Ions Immobilization of Calcined Layered Double Hydroxides in Hardened Cement Paste and Concrete

The sulfate ions immobilization behavior of calcined layered double hydroxides(CLDHs) in the hardened cement paste was investigated. The experimental results show that the sulfate ions in cement paste are immobilized by CLDHs to reconstruct the layered structure and aggregate around CLDHs. The immobilization amount of sulfate ions by CLDHs reaches 4.74×10^-3 mol/g, while the increasing amount indicates non-linear relation with the addition of CLDHs. The incorporation of CLDHs decreases the amount of ettringite formed to limit the expansion of cement paste, which decays the sulfate reaction to enhance the sulfate resistance of concrete.

Hardening Effect on Machined Surface for Precise Hard Cutting Process with Consideration of Tool Wear

During hard cutting process there is severe thermodynamic coupling effect between cutting tool and workpiece, which causes quenching effect on finished surfaces under certain conditions. However, material phase transformation mechanism of heat treatment in cutting process is different from the one in traditional process, which leads to changes of the formation mechanism of damaged layer on machined workpiece surface. This paper researches on the generation mechanism of damaged layer on machined surface in the process of PCBN tool hard cutting hardened steel Cr12MoV. Rules of temperature change on machined surface and subsurface are got by means of finite element simulation. In phase transformation temperature experiments rapid transformation instrument is employed, and the effect of quenching under cutting conditions on generation of damaged layer is revealed. Based on that, the phase transformation points of temperature under cutting conditions are determined. By experiment, the effects of cutting speed and tool wear on white layer thickness in damaged layer are revealed. The temperature distribution law of third deformation zone is got by establishing the numerical prediction model, and thickness of white layer in damaged layer is predicted, taking the tool wear effect into consideration. The experimental results show that the model prediction is accurate, and the establishment of prediction model provides a reference for wise selection of parameters in precise hard cutting process. For the machining process with high demanding on surface integrity, the generation of damaged layer on machined surface can be controlled precisely by using the prediction model.

超声滚压对DC53钢表面性能的影响

为了改善DC53冷作模具钢的表面性能,使用扫描电子显微镜、激光光谱共聚焦显微镜、显微硬度计、球-盘摩擦磨损试验机等设备分析了超声表面滚压加工工艺(USRP)的静压力、滚压道次、振幅、进给量对DC53冷作模具钢表面粗糙度、显微硬度、摩擦因数和磨损的影响规律。结果表明:与未处理DC53模具钢相比,USRP处理后DC53冷作模具钢表面产生了塑性变形,表面粗糙度由5.570μm最低可降为1.067μm,且在DC53冷作模具钢表面产生了晶粒细化现象,引入了梯度硬化层,表面硬度由基体的720 HV最大可提升为818.6 HV,硬化层深度最大可达300μm。摩擦因数降低,磨痕宽度减小,耐磨性能提高。通过极差分析法获得了对粗糙度、显微硬度、摩擦因数影响最大的工艺参数依次为进给量、滚压道次、静压力。通过灰色关联度分析法获得了工艺参数对表面性能的影响次序为静压力>进给量>滚压道次>振幅,最佳工艺参数组合为静压力400 N,滚压道次4次,振幅12μm,进给量0.05 mm。

20CrMnTiH钢渗碳齿轮弯曲疲劳性能研究

利用扫描电镜、金相显微镜、疲劳试验机和硬度计等测试手段研究20CrMnTiH钢渗碳齿轮弯曲疲劳性能。研究表明:20CrMnTiH钢渗碳齿轮主要沿齿根部位断裂。随着载荷的增加,弯曲疲劳寿命降低,当载荷在50~100kN时,齿轮疲劳寿命增加数倍,建议生产应用在此范围载荷内能发挥该材料最佳性能。

Modified Rammsonde tests in layered compacted snow

Investigation of the physical and mechanical properties of snow has long been a topic of interest to researchers as the construction of compacted-snow runways in Antarctica developed. In an attempt to assess the strength of layered compacted seasonal snow, penetration tests using modified Rammsonde were conducted in Harbin, China in early 2018. Compared with previous models, the modified Rammsonde is lighter overall, with improved resolution;thus, it is more suitable for seasonal snow;the mechanical structure was adjusted, and the reading of depth data is more convenient. A total of 74 penetration tests were carried out and the results were analyzed both qualitatively and quantitatively. The results of these analyses demonstrated the applicability of the device, and revealed that several factors affect the cone penetrometer's estimate of the strength of the layered compacted seasonal snow. Such factors include the confining pressure, penetration energy, and the snow material properties, particularly the compaction of the snow undergoing penetration. A linear relationship between the penetration pressure and snow density was also established. The effect of age hardening on the penetration pressure was studied and the microstructure of the snow particles was observed through a microscope. These analyses showed that the cone penetrometer and data processing methodology applied in this study enable a rapid estimate of strength in seasonal snow, and may be applied in Antarctica after further modification. This would provide a scientific basis for the design of China's Antarctic ice sheet airport.

人工缺陷下感应淬火重载铁路车轴损伤容限评价

目的研究不同感应淬火工艺对重载铁路车轴钢试样梯度结构的影响,阐明不同人工缺陷下感应淬火试样的疲劳断裂行为,并评价损伤容限,为重载铁路车轴感应淬火工艺设计和损伤容限评价提供理论基础和评价方法。方法在AAR-CM车轴钢试样表面采用不同感应淬火工艺参数制备不同深度淬硬层,采用X射线测量淬硬层残余应力分布,通过电火花和空气炮在正火态和感应淬火试样表面制备不同尺寸的电火花或冲击损伤,采用疲劳试验机研究损伤试样的弯曲疲劳性能,借助光学显微镜和扫描电镜分析试样的损伤机理和疲劳断裂行为,并对其疲劳性能进行评价。结果在3种工艺下感应淬火试样有效淬硬层深度分别为800、1200、1400μm。在电火花缺口尺寸较小时,感应淬火试样的疲劳强度比正火态试样高出50%。在冲击损伤尺寸较小时,感应淬火试样的疲劳强度比正火态试样高出40%。较小的尺寸冲击损伤对正火态和感应淬火处理试样的疲劳强度无影响。结论感应淬火可提高重载铁路车轴钢试样表面强度,进而提高其耐冲击性能,残余压应力减弱了电火花缺口试样和低速冲击损伤对试样疲劳强度的影响,高速冲击破坏了试样表面完整性,并削弱了残余压应力的作用,试样的疲劳强度急剧降低。文中设计的淬硬层深度对感应淬火试样损伤容限的影响较小。

45钢表面硬化层深度的高鲁棒性微磁定量预测方法

考虑多功能微磁检测系统对微磁参量的重复测试性能,研究利用系统对45钢表面硬化层深度进行高鲁棒性定量预测的方法。首先,利用测试数据的变异系数β统计方法,定量评价了系统对41项微磁参量的重复测试能力,结合指标β和微磁参量对硬化层深度的敏感性指标S,对微磁参量进行了筛选;其次,融合多项微磁参量建立了硬化层深度的前馈神经网络定量预测模型,提出了改善模型鲁棒性的建模策略及鲁棒性评价方法;最后,讨论了输入节点逐项剔除和有条件保留规则对模型鲁棒性的影响规律。与传统建模方法相比,利用规则剔除微磁参量项数为8时,模型的MAE均值和MAE值小于5%的模型数量P分别下降约68.8%和增加约150%,表明提出的建模策略可以有效改善仪器在45钢表面硬化层深度定量预测过程中的鲁棒性。

考虑淬硬铁磁构件过渡特性的磁滞回线建模方法

有效硬化层深度和过渡层范围是影响表面淬硬机械零部件性能的关键因素,也是产品质量控制过程中的重要测试内容。利用不同组织的磁特性差异,可发展出淬硬层的磁检测方法。相比传统金相观察法,具有无损、快速的技术优势,有望直接面向零部件进行淬硬层测试分析。本文提出利用玻尔兹曼函数描述沿深度方向材料磁滞特性参数的渐变规律。通过对多层材料进行离散分层处理,考虑层间磁场耦合,建立多层结构材料的T(x)磁滞回线模型。利用粒子群算法和提出的多层材料磁滞回线模型对逐层切割的含硬化层试样进行磁滞回线的反演计算,验证模型的准确性。提出的模型有望较为精确地描述多层材料的磁滞回线并可对硬化层深度和过渡层深度进行反演表征。

宽厚板轧机用Cr3钢支承辊淬硬层深度的理论预测与实验验证

差温热处理是决定支承辊工作层组织、应力分布和使用性能的关键工序。为精准控制宽厚板轧机用Cr3钢支承辊辊身淬硬层深度以进一步提高其寿命周期内的服役性能,利用ABAQUS有限元模拟软件计算了支承辊在差温热处理过程中的温度场分布,分析了距离辊身不同深度处的温度与显微组织演变,预测了差温处理过程中辊身淬硬层的深度。结果表明:当加热温度达到840℃及以上时,Cr3钢可获得耐磨性较佳的非珠光体组织;Cr3钢支承辊实验件的淬硬层深度达110 mm,表面硬度达到55 HSD以上,且距离表面110 mm深处的硬度降低不大于4 HSD;淬硬层深度的理论预测结果与实测结果吻合较好,可以实现支承辊淬硬层深度的精确控制,指导Cr3钢支承辊差温热处理工艺制定。

基于ANSYS的45钢表面激光淬火数值模拟

采用ANSYS软件模拟计算45钢表面激光淬火温度场的变化规律,预测淬火硬化层的深度和宽度,分析工艺参数对45钢表面激光淬火温度场的影响,并探讨工艺参数与45钢表面淬火硬化层深度和宽度之间的相互关系。结果表明:45钢表面的淬火硬化层的深度和宽度与激光输出功率成正比,与激光扫描速度和光斑直径成反比;45钢表面激光淬火的最佳工艺参数为激光输出功率900 W、激光扫描速度8 mm/s、光斑直径φ5 mm,此时,淬火硬化层的深度为0.508 mm、宽度为2.865 mm。

表面形貌对铝/镁热轧复合结合强度的影响

选用5052铝合金与AZ31B镁合金作为复合材料进行热轧复合,研究表面形貌对铝/镁热轧复合结合强度的影响。在热轧复合前使用不同直径钢丝刷对板材表面进行打磨处理,通过单向拉伸实验、扫描电镜以及激光共聚焦显微镜对打磨处理后板材的表面状态和形貌进行测试和观察。然后使用热轧复合法在25%、 35%和45%压下率下轧制制备出铝/镁复合板并通过万能试验机、扫描电镜、 X射线能谱仪对复合板的结合强度、微观形貌和元素分布进行测试和观察。最后通过轧卡和剥离实验对45%压下率不同直径钢丝刷打磨处理条件下铝/镁热轧复合过程中板材表面形貌的变化进行观察。结果表明,不同直径钢丝刷打磨处理仅改变了板材的表面形貌,未使板材表面形成硬质层。在25%和35%压下率下,铝/镁复合板的结合强度随着钢丝刷直径的增加而增加,但当压下率达到45%后,随着钢丝刷直径的改变,铝/镁复合板的结合强度稳定在71.2 MPa左右。通过改变板材表面形貌可以使铝合金和镁合金在低压下率下实现良好结合,但当压下率达到45%之后,板材表面形貌的变化不会再对铝/镁复合板的结合强度产生明显影响。

随车吊机销轴断裂原因

某随车吊机机械臂与液压杆的连接处销轴在运行过程中突然发生断裂事故。采用宏观观察、化学成分分析、金相检验、硬度测试、扫描电镜及能谱分析等方法分析了销轴断裂的原因。结果表明:在运行过程中,销轴外表面应力集中处萌生裂纹,裂纹沿淬硬层周向向两边快速扩展,最终汇合导致表面一周开裂;在交变载荷的作用下,周向裂纹沿作用力方向扩展,形成双源疲劳开裂;当销轴剩余有效强度不足时,销轴发生一次性快速断裂。

含梁场硬化层的填石路基路面结构优化

本文以广东某高速公路含梁场硬化层的填石路基为研究对象,提出梁场硬化层利用技术,并对硬化层上路面加铺结构进行优化设计和力学行为分析。结果表明:梁场硬化层顶面实测弯沉值比不含梁场硬化层填石路基顶面实测弯沉值小,均值为1.71×10^(-2)mm,其顶面当量回弹模量远大于同标高的填石路基顶面当量回弹模量,可直接利用硬化层并在硬化层上加铺路面结构;基于室内动三轴试验,确定了填石路基的材料回弹模量为308 MPa,PFWD实测不含硬化层的填石路基顶面当量回弹模量为116 MPa,梁场硬化层顶面当量回弹模量为1173 MPa,含梁场硬化层填石路基顶面当量回弹模量为314 MPa;梁场硬化层路面加铺结构优化方案满足规范要求;因保留了梁场硬化层,优化后的路面结构力学性能较优,优化后的CRC+AC复式路面加铺结构在力学性能上最优。

镍基单晶高温合金磨削表面白层的形成规律

表面完整性对零件的服役性能和寿命影响至关重要,但是目前仍缺乏关于单晶高温合金加工过程中微观组织演化机制的研究.本文对单晶高温合金磨削过程中的磨削力与温度进行采集,同时对亚表面微观组织以及显微硬度进行表征,研究了表面白层特性.结果表明,在试验参数范围内磨削力和温度随着磨削深度的增加呈现显著上升的趋势,白层和塑性变形层厚度随着磨削深度和进给速度的增加而增大.白层与塑性变形层之间发生元素扩散现象,导致白层富含Al和Ta元素.磨削之后表面显微硬度有所提高,其硬化程度随着磨削深度和进给速度的增加而提高.

Cr5M钢支撑辊回火脆性及整体浸入式调质处理研究

对Cr5M钢进行热处理实验,分析Cr5M钢回火脆性与硬度的关系及其组织结构对回火脆性的影响;据此提出满足辊面硬度要求的Cr5M钢支撑辊整体浸入式调质处理工艺,探究Cr5M钢支撑辊的整体浸入式调质处理效果。结果表明:Cr5M钢具有一定程度的回火脆性,回火脆性温度范围为300~500℃,400℃回火时Cr5M钢冲击吸收能量达到最低值(7.6 J);250℃附近回火,Cr5M淬火钢中的残余奥氏体(Ar)开始发生分解,分解结束温度约400℃,软韧相Ar的分解是导致回火脆性发生的主要因素;960℃淬火+615℃回火的整体浸入式调质处理可替代支撑辊的差温热处理或表面淬、回火热处理,经整体浸入式调质处理的Cr5M支撑辊的综合力学性能优良,表面硬度均匀,最大与最小肖氏硬度差值仅0.7 HSD,平均肖氏硬度为40.1 HSD,支撑辊工作层(淬硬层)深大于15 cm,工作层组织为回火索氏体+少量的铁素体,工作层硬度仅2.1 HSD。

高速精密轴承表面硬化及尺寸稳定化工艺研究

利用金相显微镜(OM)、显微硬度计和X射线衍射仪(XRD)等仪器对GCr15轴承钢在不同热处理工艺下的组织特征、硬化层梯度和残余奥氏体含量进行分析研究。结果表明,通过碳氮共渗淬火+(-70℃×2 h)冷处理+220℃×4 h回火热处理可实现轴承零件表面硬度>63 HRC、基体残余奥氏体含量<3%、表面残余奥氏体含量约6%的技术要求,保证零件尺寸精度的同时可避免因残余奥氏体含量过低而导致的轴承接触疲劳寿命下降。通过在原材料中添加微量Mo、V等强碳化物合金元素,并进行马氏体淬火和碳氮共渗两次淬火,基体的平均晶粒度由原来的8级细化至10级以上。

基于现场数据的全柔性沥青路面结构格栅增韧层施工工艺研究

为了明确全柔性沥青路面结构格栅增韧层最佳施工工艺,结合实体工程,通过直接剪切强度和疲劳试验,现场刚度和弯沉检测,对比了不同施工工艺下格栅增韧层层间剪切强度、剪切疲劳,以及现场刚度、弯沉状况,结果表明:格栅增韧层施工工艺对其结构承载能力具有重要影响,最佳施工工艺为先铺增韧格栅,再洒布粘层,固定钉距离60 cm最为合理。