【作者】 王琦；

【导师】 梁京；

【作者基本信息】 东北大学 ， 材料工程（专业学位）， 2019， 硕士

【摘要】 核电是清洁能源的首选之一,是未来国家电力系统的主要调整方向之一。采用核电应急柴油机发电机组确保核电站在意外断电的情况下能够迅速启动,降低堆芯余热,执行反应堆的安全停堆,可以避免核泄漏带来的安全隐患。由于应急柴油机发电机组结构复杂,其中凸轮轴作为柴油机的核心部件,在运行过程中承受着较大的扭矩和周期性的冲击载荷,并且凸轮轴在柴油机内部与多个零件衔接,因此对其强度、表面耐磨性表面以及精度和粗糙度有较高的要求。凸轮轴制造耗费的时间较长;需要高精度的模具;要经过车床进行高精度机加工等多道工序,采用传统的制造方法制造应急柴油发电机凸轮轴存在一定难度。激光直接沉积成形技术可以实现复杂零件的一次近终成形,其具有制造速度快,成本低,材料利用率高的特点。因此本文开展了激光直接沉积成形方法制备核电站应急柴油机凸轮轴材料的研究,以期获得激光工艺、热等静压及后热处理参数对12CrNi2合金钢材料显微组织及性能影响规律,为激光直接沉积核电应急柴油机凸轮轴获得基础数据支持。采用激光直接沉积成形成功制备出12CrNi2合金钢块体材料样品,并对其进行热等静压及常规后热处理,对显微组织进行调控,获得了较好的综合力学性能。通过OM、SEM、EPMA、TEM、XRD等检测方法对其显微组织进行表征;通过显微硬度计、万能材料试验机、冲击试验机等分析其性能。获得了激光直接沉积成形工艺参数以及热等静压及热处理工艺对其显微组织和性能的影响规律。实验结果表明:激光直接沉积12CrNi2样品成形性良好,无裂纹等其他缺陷,在其他激光工艺参数不变情况下,激光功率从700W上升至900W时,微孔数量略有减少,沉积态的样品的致密度有所上升,平均致密度稳定在98.2-98.3%之间(相对于锻造态的12CrNi2合金钢)。沉积态样品距上表层1-2 mm区域为马氏体组织,中部组织变化不大,为铁素体和粒状贝氏体组织。采用优化工艺条件(激光功率P=900W)制备的沉积态样品屈服强度665.2 MPa,抗拉强度703.7 MPa,延伸率15.6%。上表层马氏体组织平均显微硬度达到329 HV,中部稳定铁素体和粒状贝氏体组织平均硬度为223 HV。对激光直接沉积成形沉积态样品进行了两批次的热等静压处理,热等静压处理工艺分别为880℃保温4h和820℃保温2h,压力均选用120 MPa,采用氩气作为压力传递的介质,冷却速度为15℃/min冷至室温。热等静压处理后显微组织均为铁素体和珠光体组织,热等静压工艺为880℃保温4h时,珠光体含量为30.8%,珠光体团尺寸在13 μm左右,铁素体晶粒尺寸在16μm左右;热等静压工艺为820℃保温2h时,珠光体含量减少至19.2%,珠光体组织和铁素体的晶粒尺寸有所减小分别在12 μm和14μm左右,通过降低热等静压的温度和保温时间可以对热等静压组织进行调控。经820℃保温2h热等静压处理样品的机械性能较880℃保温4h的为佳,其平均显微硬度为155 HV,屈服强度381.2 MPa,抗拉强度488.8 MPa,延伸率26.7%。根据凸轮轴的使用需求,对热等静压态12CrNi2合金钢进行热处理,调整综合力学性能。沉积态样品在热等静压处理(820℃/2h,120MPa,15℃/min)后,经过850℃/30min油淬+300℃/2h热处理,获得的平均显微硬度为329 HV,屈服强度960.8MPa,抗拉强度1092.7 MPa,延伸率16.3%,冲击韧性达到140.26 J/cm2,综合力学性能满足凸轮轴的使用需求。初步获得不同热处理样品的硬度值规律,可对力学性能进行初步判断。更多还原

【Abstract】 Nuclear power is one of the first choices of clean energy and one of the main adjustment directions of national power system in the future.Nuclear emergency diesel engine generator set ensures that the nuclear power plant can start quickly under unexpected power outage,reduces the waste heat of the reactor core,and executes the safe shutdown of the reactor,which can avoid the potential safety hazards caused by nuclear leakage.Because of the complex structure of emergency diesel engine generator set,camshaft is the core component of diesel engine.During the operation,the camshaft bears large torque and periodic impact load,and links up with many parts in the diesel engine.Therefore,its strength,surface wear resistance,accuracy and roughness have higher requirements.It takes a long time to manufacture camshafts,requires high-precision moulds,and passes through many processes such as lathe,high-precision machining and so on.It is difficult to manufacture camshaft of emergency diesel generator by traditional manufacturing method.The laser direct deposition forming technology can realize the near-final forming of complex parts,which has the characteristics of high manufacturing speed,low cost and high material utilization.Therefore,this paper carried out the research of laser direct deposition forming method to prepare camshaft material for nuclear power plant emergency diesel engine.In order to obtain the influence of laser process,hot isostatic pressing and post-heat treatment parameters on the microstructure and properties of 12CrNi2 alloy steel.Obtain basic data support for laser direct deposition nuclear power emergency diesel camshaft.The 12CrNi2 alloy steel block material samples were successfully prepared by laser direct deposition,and then hot isostatic pressing and conventional post-heat treatment were carried out to control the microstructures and obtain better comprehensive mechanical properties.The microstructures were characterized by OM,SEM,EPMA,TEM and XRI),and the properties were analyzed by microhardness tester,universal material tester and impact tester.The effects of laser direct deposition process parameters and hot isostatic pressing and heat treatment on the microstructures and properties were obtained.The experimental results show the laser directly deposited 12CrNi2 sample has good formability and no other defects such as cracks.Under the condition of other laser process parameters,when the laser power is increased from 700W to 900W,the number of micropores is slightly reduced,the density of the deposited samples is increased,and the average density is stable between 98.2%and 98.3%.Relative to the forged 12CrNi2 alloy steel).The samples deposited by laser are martensite in the area of 1-2 mm from the upper surface,and the structure in the middle is ferrite and granular bainite.The yield strength of as-deposited samples prepared by optimizing process conditions(laser power P=900W)is 665.2 MPa,the tensile strength was 703.7 MPa,and the elongation was 15.6%.The average microhardness of the upper surface martensite is 329 HV,and the average hardness of the central stable ferrite and granular bainite is 223 HV.Two batches of hot isostatic pressing(HIP)treatment were carried out on the deposited samples formed by laser direct deposition.The HIP treatment processes were 880℃ holding for 4 hours and 820℃ holding for 2 hours,respectively.The pressure was 120 MPa.Argon was used as the medium for pressure transfer,and the cooling rate was 15℃/min.After hot isostatic pressing,the microstructures are ferrite and pearlite structure.When the hot isostatic pressing process is 880℃ for 4 h,the pearlite content is 30.8%,the size of pearlite structure is about 13 μm,and the size of ferrite is about 16 μm.When the hot isostatic pressing process is 820℃ for 2 h,the pearlite content is reduced to 19.2%,and the size of pearlite structure and ferrite is reduced by 12 μm and 14 μm,respectively.The structure of hot isostatic pressing can be controlled by reducing the temperature and holding time of hot isostatic pressing.The mechanical properties of the samples treated by hot isostatic pressing at 820℃ for 2 hours are better than those treated at 880℃ for 4 hours.The average microhardness is 155 HV,the yield strength is 381.2 MPa,the tensile strength is 488.8 MPa and the elongation is 26.7%.According to the requirement of camshaft,12CrNi2 alloy steel under hot isostatic pressing was heat treated to adjust its comprehensive mechanical properties.After hot isostatic pressing(820℃/2h),after heatment(850℃/30min+300℃/2h),The average microhardness obtained by heat treatment is 329 HV,yield strength 960.8 MPa,tensile strength 1092.7 MPa,elongation 16.3%,impact toughness 140.26 J/cm2.The hardness values of different heat-treated samples were obtained preliminarily,and the mechanical properties could be judged preliminarily.更多还原