李福泉,孟祥旭,章碧成,冯时
(先进焊接与连接国家重点实验室(哈尔滨工业大学),哈尔滨 150001)
摘要:
采用激光增材制造技术制备12CrNi2合金钢核电应急柴油机凸轮轴可以明显降低产品研发周期，优化产品结构.本研究采用光学显微镜(OM)、扫描电子显微镜(SEM)、X射线衍射仪(XRD)和电子万能试验机对激光增材制造12CrNi2合金钢热处理前后的组织和力学性能进行了研究.微观组织观察发现，试样存在带状特征和柱状晶形貌，圆弧带间为致密的冶金结合，顶部由于无后一道的重熔热作用，柱状晶形貌十分显著.沉积态组织主要由铁素体组成，试样不同部位的铁素体形貌存在一定的差异.经退火处理后，铁素体边界出现大量白色粒状析出物，延伸率显著提高，达到22.9%.经淬火及不同温度回火处理后，带状特征和柱状晶形貌消除，得到马氏体及其回火组织，抗拉强度达到1 000 MPa.与沉积态抗拉强度831 MPa相比，热处理态试样的抗拉强度显著提高.经固溶时效处理后，晶粒明显粗化.
关键词: 激光增材制造 12CrNi2合金钢 热处理 组织 拉伸性能
DOI：10.11951/j.issn.1005-0299.20180370
分类号:TG161
文献标识码:A
基金项目:国家重点研发计划项目(2016YFB1100200).
Effects of heat treatment on microstructure of 12CrNi2 steel fabricated by laser additive manufacturing
LI Fuquan, MENG Xiangxu, ZHANG Bicheng, FENG Shi
(State Key Laboratory of Advanced Welding and Joining(Harbin Institute of Technolog), Harbin 150001, China)
Abstract:
Laser additive manufauturing can significantly reduce the product development cycle and optimize the product structure of 12CrNi2 alloy steel nuclear power emergency diesel engine camshaft. In this study, the microstructure and mechanical properties of 12CrNi2 alloy steel before and after heat treatment were investigated through optical microscopy(OM), scanning electron microscopy(SEM), X-ray diffractometry(XRD), and electronic universal testing machine. Microscopic observation showed that the sample had banded features and columnar morphology. The circular arc bands were connected by dense metallurgical bands and the top of the sample had significant columnar morphology due to the lack of remelting heat treatment. The microstructure of the deposited state was mainly consisted of ferrite, whose morphologies were different at different parts of the sample. After annealing, a large number of bright granular precipitates appeared on the ferrite boundary, and the elongation was greatly improved(22.9%). After quenching and tempering at different temperatures, the banded features and the columnar morphology were eliminated, and the martensite and its tempering structures were obtained with the tensile strength reaching 1 000 MPa. Compared with the tensile strength of the deposited state of 831 MPa, the tensile strength was greatly improved after heat treatment. After solid solution aging treatment, the grains were significantly coarsened.
Key words: laser additive manufacturing 12CrNi2 alloy steel heat treatment microstructure tensile property
李福泉, 李福泉, 孟祥旭, 章碧成, 冯时. 不同热处理制度下激光增材制造12CrNi2钢组织研究[J]. 材料科学与工艺, 2019, 27(5): 14-20. DOI: 10.11951/j.issn.1005-0299.20180370.
LI Fuquan, LI Fuquan, MENG Xiangxu, ZHANG Bicheng, FENG Shi. Effects of heat treatment on microstructure of 12CrNi2 steel fabricated by laser additive manufacturing[J]. Materials Science and Technology, 2019, 27(5): 14-20. DOI: 10.11951/j.issn.1005-0299.20180370.
基金项目 国家重点研发计划项目(2016YFB1100200) 通信作者 李福泉，E-mail: lifuquan@hit.edu.cn 作者简介 李福泉(1972—)，男，博士，副教授 文章历史 收稿日期: 2018-11-21 网络出版日期: 2019-04-19
Contents Abstract Full text Figures/Tables PDF
不同热处理制度下激光增材制造12CrNi2钢组织研究
李福泉, 李福泉
, 孟祥旭, 章碧成, 冯时 先进焊接与连接国家重点实验室(哈尔滨工业大学)，哈尔滨 150001
收稿日期: 2018-11-21; 网络出版日期: 2019-04-19
基金项目: 国家重点研发计划项目(2016YFB1100200)
作者简介: 李福泉(1972—)，男，博士，副教授.
通信作者: 李福泉，E-mail: lifuquan@hit.edu.cn.
摘要: 采用激光增材制造技术制备12CrNi2合金钢核电应急柴油机凸轮轴可以明显降低产品研发周期，优化产品结构.本研究采用光学显微镜(OM)、扫描电子显微镜(SEM)、X射线衍射仪(XRD)和电子万能试验机对激光增材制造12CrNi2合金钢热处理前后的组织和力学性能进行了研究.微观组织观察发现，试样存在带状特征和柱状晶形貌，圆弧带间为致密的冶金结合，顶部由于无后一道的重熔热作用，柱状晶形貌十分显著.沉积态组织主要由铁素体组成，试样不同部位的铁素体形貌存在一定的差异.经退火处理后，铁素体边界出现大量白色粒状析出物，延伸率显著提高，达到22.9%.经淬火及不同温度回火处理后，带状特征和柱状晶形貌消除，得到马氏体及其回火组织，抗拉强度达到1 000 MPa.与沉积态抗拉强度831 MPa相比，热处理态试样的抗拉强度显著提高.经固溶时效处理后，晶粒明显粗化.
关键词: 激光增材制造 12CrNi2合金钢 热处理 组织 拉伸性能
Effects of heat treatment on microstructure of 12CrNi2 steel fabricated by laser additive manufacturing
LI Fuquan, LI Fuquan
, MENG Xiangxu, ZHANG Bicheng, FENG Shi State Key Laboratory of Advanced Welding and Joining(Harbin Institute of Technolog), Harbin 150001, China
Abstract: Laser additive manufauturing can significantly reduce the product development cycle and optimize the product structure of 12CrNi2 alloy steel nuclear power emergency diesel engine camshaft. In this study, the microstructure and mechanical properties of 12CrNi2 alloy steel before and after heat treatment were investigated through optical microscopy(OM), scanning electron microscopy(SEM), X-ray diffractometry(XRD), and electronic universal testing machine. Microscopic observation showed that the sample had banded features and columnar morphology. The circular arc bands were connected by dense metallurgical bands and the top of the sample had significant columnar morphology due to the lack of remelting heat treatment. The microstructure of the deposited state was mainly consisted of ferrite, whose morphologies were different at different parts of the sample. After annealing, a large number of bright granular precipitates appeared on the ferrite boundary, and the elongation was greatly improved(22.9%). After quenching and tempering at different temperatures, the banded features and the columnar morphology were eliminated, and the martensite and its tempering structures were obtained with the tensile strength reaching 1 000 MPa. Compared with the tensile strength of the deposited state of 831 MPa, the tensile strength was greatly improved after heat treatment. After solid solution aging treatment, the grains were significantly coarsened.
Keywords: laser additive manufacturing 12CrNi2 alloy steel heat treatment microstructure tensile property
激光增材制造技术是一种兼顾精确成形和高性能需求的一体化制造技术[1]，通常以合金粉末或丝材为原料，通过高功率的激光原位冶金熔化/快速凝固逐层堆积制造[2].钛合金的激光增材制造研究起步较早，成熟度也相对较高.而高强合金钢作为国民经济领域普遍采用的工业材料，因其合金化程度高，凝固和固态相变过程复杂，其增材制造工艺成熟度还远未达到钛合金的制造水平.激光增材制造合金钢可能存在由合金碳化物、残余奥氏体和马氏体等多相组织导致的组织应力，同时可能存在由于制造过程高温度梯度导致的较大热应力，这两类应力的存在极易导致构件出现变形和开裂等现象，因而需要通过热处理来改善组织，优化性能.
有关学者在对激光增材制造17-4PH不锈钢[3-6]、316L不锈钢[7-8]、HY100高强钢[9]和1Cr12Ni2WMoVNb马氏体不锈钢[10]等的研究中均发现了层带结构，沉积层层间为致密的冶金结合.激光增材制造是一个不断熔化沉积的过程，后道激光沉积会对前道激光沉积层存在热处理作用，从而导致激光增材制造钢微观结构的不均匀性[10].此外，研究还表明激光增材制造钢的力学性能存在各向异性，而通过热处理可以消除层带结构，均匀组织，减小各向异性，改善性能[10-12].目前针对12CrNi2合金钢的研究较少，中科院的董志宏[13]等人研究发现低O含量的12CrNi2合金钢粉末成型性好，可以抑制孔洞及氧化物夹杂的形成.东北大学的陈岁元[14]等人研究发现优化扫描速度可以使12CrNi2Re合金钢得到细小的粒状贝氏体组织，具有良好的综合力学性能.
目前，作为核电装备承载动力传输的核心金属构件，12CrNi2合金钢核电应急柴油机凸轮轴采用锻造制备.相较而言，采用激光增材制造技术制备12CrNi2合金钢核电应急柴油机凸轮轴可以明显降低产品研发周期，优化产品结构.本文基于12CrNi2合金钢的激光增材制造技术，探究了激光增材制造12CrNi2合金钢组织性能及热处理对其组织和力学性能的影响，以期为激光增材制造12CrNi2合金钢构件的应用提供实验支持.
1 实验实验采用惰性气体(氮气)雾化制备的12CrNi2钢粉，形貌如图 1所示，为大小不一的圆形颗粒，大粉末颗粒上粘有小粉末颗粒，粉末粒径在58~180 μm，实测流动性为19.0~20.5 s/50 g.合金钢粉末化学成分见表 1.
图 Figure1(Fig.Figure1)
图 1 12CrNi2合金钢粉末SEM照片Fig.1 SEM image of the 12CrNi2 alloy steel powder表1(Table 1)
表 1 12CrNi2合金钢粉末化学成分(质量分数/%)Table 1 Chemical compositions of 12CrNi2 alloy steel powder (wt.%) Cr Ni Mn Si C O
0.830 1.830 0.460 0.240 0.120 0.035
表 1 12CrNi2合金钢粉末化学成分(质量分数/%)Table 1 Chemical compositions of 12CrNi2 alloy steel powder
实验前用砂纸打磨基材以去除表面的氧化膜，而后用丙酮清洗表面油污.激光增材制造实验在laserline半导体激光增材制造系统上进行，采用激光直接沉积的方式进行，整个制备过程在氩气氛围下进行.试样制备过程的工艺参数为：激光功率900 W，扫描速度600 mm/min，送粉速率6.7 g/min，光斑直径2 mm，搭接率50%.在此参数下得到的试样块如图 2所示.
图 Figure2(Fig.Figure2)
图 2 激光增材制造12CrNi2合金钢Fig.2 Laser additive manufactured 12CrNi2 alloy steel激光增材制造加工后，使用电火花切割将12CrNi2钢试样与基板分离，然后将试样切割成15 mm×15 mm×4 mm的小试样块和拉伸试样进行热处理，在水平方向取拉伸试样进行拉伸试验，拉伸试样尺寸如图 3所示，热处理工艺见表 2.
图 Figure3(Fig.Figure3)
图 3 拉伸试样示意图Fig.3 Schematic of tensile sample表2(Table 2)
表 2 激光增材制造12CrNi2合金钢试样热处理工艺Table 2 Heat treatment process of laser additive manufactured 12CrNi2 alloy steel Sample state Processing parameter As-deposited -
Annealing 600 ℃×0.5 h/FC
Quenched 600 ℃×0.5 h /FC+900 ℃×0.5 h /AC+860 ℃×0.5 h/WC
Low temperature tempering 600 ℃×0.5 h /FC+900 ℃×0.5 h /AC+860 ℃×0.5 h/WC+200 ℃×0.5 h/AC
High temperature tempering 600 ℃×0.5 h /FC+900 ℃×0.5 h /AC+860 ℃×0.5 h/WC+500 ℃×0.5 h/AC
Solution 600 ℃×0.5 h/FC+1100 ℃×1 h/WC+600 ℃×0.5 h/FC
表 2 激光增材制造12CrNi2合金钢试样热处理工艺Table 2 Heat treatment process of laser additive manufactured 12CrNi2 alloy steel
金相试样经预磨抛光后采用4%HNO3酒精溶液对金相试样进行10~20 s的腐蚀，而后采用VHX-1000E超景深光学显微镜、GX71倒置式光学显微镜和Merlin Compact扫描电子显微镜进行组织分析.使用达芬奇D8 ADVANCE型射线衍射仪对沉积态试样进行测试.采用HXD-1000TM型维氏硬度计进行显微硬度的测试，载荷200 g，加载时间10 s.利用Instron-5569电子万能材料试验机进行拉伸性能测试，加载速度0.5 mm/min，而后用扫描电子显微镜观察断口形貌.
2 结果与分析2.1 沉积态组织激光增材制造12CrNi2合金钢XRD谱如图 4所示，可以看到，沉积态钢主要由铁素体组成，无奥氏体和其他碳化物.由于XRD结果仅能测出组成占比5%以上的物相，因而具体的物相分析还需要进一步的实验分析确定.
图 Figure4(Fig.Figure4)
图 4 激光增材制造12CrNi2合金钢XRD谱图Fig.4 XRD pattren of as-deposited additive manufactured 12CrNi2 alloy steel沉积态钢中下部宏观组织形貌(纵截面)如图 5(a)所示，沿垂直于扫描方向存在明显的带状特征，层间为致密的冶金结合.各激光沉积层均呈现由底部指向中心的柱状晶形貌，柱状晶内部由大量的细晶铁素体和块状铁素体组成，如图 5(b)所示.由于后道激光沉积层的热处理作用，在上下两个沉积层间发现了铁素体均匀化区域，无柱状晶形貌，由大量的细晶铁素体组成，见图 5(c).
图 Figure5(Fig.Figure5)
图 5 激光增材制造12CrNi2合金钢沉积态试样宏观组织Fig.5 Macrostructure of as-deposited laser additive manufactured 12CrNi2 alloy steel: (a) marostructure at the bottom and center of the sample; (b) columnar morphology at the bottom and center of the sample; (c) fine ferrite between layers of the deposit; (d) fine ferrite region at the top of the sample; (e) fine ferrite at the top of the sample; and (f) columnar morphology at the top of the sample由图 5(d)可以发现，试样中上部存在无明显柱状晶区域.高倍下观察，发现该区域均由细晶铁素体组成，铁素体块大小十分均匀，如图 5(e)所示.而在试样顶部区域，如图 5(f)所示，同样发现了典型的柱状晶形貌和带状特征，沉积层间的回火区十分明显.上述组织形态分布的形成是由于在激光沉积过程中，试样的中下部与基板相连，散热较快，温度梯度大，形成了明显的带状特征与柱状晶形貌.而在试样中上部，散热较慢，受热均匀，温度梯度小，加上下一道激光沉积层对其的热处理作用，没有明显的带状特征及柱状晶形貌.在试样的顶部，与保护气体氩气直接接触，对流散热快，且没有下一道激光沉积层的热处理作用，形成了典型的柱状晶形貌.
2.2 热处理态组织由于马氏体和铁素体的X射线衍射峰十分接近，通过X射线衍射实验很难对其加以区分，本文主要通过热处理态的组织形貌对组织进行分辨及讨论.沉积态试样经退火处理后，仍由铁素体组成，且铁素体形貌变化不大，试样依然存在带状特征和柱状晶形貌.在扫描电镜下观察，在铁素体边界发现了细小颗粒状的白色析出物，如图 6(a)所示，相关文献研究表明其可能为(Cr、Fe)23C6[15].
图 Figure6(Fig.Figure6)
图 6 激光增材制造12CrNi2合金钢热处理试样宏微观组织Fig.6 Macrostructure and microstructure of heat treated laser additive manufactured 12CrNi2 alloy steel: (a) microstructure of annealing state sample; (b) macrostructure of quenched state sample; (c) microstructure of quenched state sample; (d) microstructure of low temperature tempering state sample; (e) microstructure of high temperature tempering state sample; and (f) OM of solution state sample淬火态、低温回火和高温回火态试样带状特征消失，柱状晶转变为等轴晶，宏观形貌如图 6(b)所示.淬火态试样由细小致密的板条状马氏体和部分未溶的铁素体组成(图 6(c))，高温回火态试样马氏体发生了回复和多边化，板条束逐渐分解，在原马氏体边界及内部析出了大量碳化物(图 6(e))，而低温回火态试样马氏体板条束变化不明显，仅在晶界和晶粒内部析出了少量碳化物(图 6(d)).
固溶态试样的带状特征及柱状晶形貌得到消除，柱状晶转变为等轴晶，但是与淬火态、低温回火和高温回火态试样相比，晶粒发生了粗化，如图 6(f)所示.
2.3 热处理对拉伸性能的影响激光增材制造12CrNi2合金钢热处理前后的拉伸性能如图 7所示.图 7结果表明，与沉积态相比，退火后试样的拉伸强度有所下降，但延伸率提高了49.87%.在退火条件下，铁素体边界析出了大量碳化物，铁素体含碳量降低，如图 6(a)所示，而晶界析出强化对强度的补偿作用远低于固溶强化作用的削弱，同时退火后残余应力大幅度降低，两者的综合作用导致强度的降低和延伸率的显著提高.淬火、低温回火和高温回火热处理消除了带状特征，得到马氏体及其回火组织，试样的强度显著提高，抗拉强度最高达到了1 221 MPa，与沉积态相比提高了46.94%.淬火得到的板条马氏体由于C原子的固溶强化、相转变产生的位错亚结构强化以及马氏体自回火引起的时效强化导致试样的高强度.将淬火、低温回火和高温回火态试样的拉伸性能进行对比，发现回火温度越高，强度越低.这是由于回火温度越高，在原马氏体边界析出的碳化物越多，如图 6(d)、(e)所示，得到的回火类马氏体含碳量越低，C原子的固溶强化效应越弱，同时得益于残余应力的释放，试样表现出强度降低，塑性提高.虽然碳化物的析出，有一定的弥散强化效果，但其影响小于固溶强化作用的减弱.而固溶态试样可能由于高温下保温时间长，导致晶粒粗化和材料严重氧化脱碳，强度反而有所下降.
图 Figure7(Fig.Figure7)
图 7 激光增材制造12CrNi2合金钢热处理前后拉伸性能Fig.7 Tensile properties of as-deposited and heat treated laser additive manufactured 12CrNi2 alloy steel图 8为沉积态拉伸试样断口的宏观和微观形貌，可以发现，断口均由中心的纤维状区和四周的剪切唇区组成，断口截面与拉伸轴线呈约45°.纤维状区无金属光泽，凹凸不平，剪切唇表面光亮.观察断口的微观形貌，发现其由两种韧窝组成，一种是纤维状区受正应力拉断的等轴韧窝，另一种是剪切唇区受切应力剪断的剪切韧窝.这说明试样在断裂过程中受拉应力先在中心形成韧窝，韧窝聚集成裂纹，之后裂纹汇集更多的韧窝逐步形成纤维状区，裂纹扩展至近表面时，三向拉应力转变为平面应力状态，这时裂纹沿最大切应力方向扩展，最大切应力与外加载荷呈45°，于是形成沿45°方向的斜断口，即剪切唇区.
图 Figure8(Fig.Figure8)
图 8 激光增材制造12CrNi2合金钢沉积态试样断口形貌Fig.8 Fracture surface morphology of as-deposited laser additive manufactured 12CrNi2 alloy steel sample: (a) macroscopic morphology; (b) center equiaxed dimple; and (c) surrounding shear dimple图 9为热处理态试样断口的微观形貌，可以看到，热处理试样的断口形貌与沉积态一致，均由大量的韧窝组成.在高倍下进行观察，发现退火态试样的等轴韧窝较沉积态深，这与拉伸性能中退火态延伸率高于沉积态一致；而其余热处理态试样等轴韧窝较浅，其延伸率均低于沉积态和退火态.
图 Figure9(Fig.Figure9)
图 9 激光增材制造12CrNi2合金钢热处理前后试样等轴韧窝形貌.Fig.9 Equiaxed dimple as-deposited and heat treated laser additive manufactured 12CrNi2 alloy steel sample: (a) as-deposited state; (b) annealing state; (c) quenched state; (d) low temperature tempering state; (e) high temperature tempering state; and (f) solution state3 结论1) 激光增材制造12CrNi2合金钢存在带状特征和柱状晶形貌，沉积态组织由大量铁素体组成，由于散热条件的差异和激光沉积层的热处理作用，试样不同位置铁素体形貌存在差异.试样中下部及顶部柱状晶形貌和带状特征比较明显，中上部则无明显的带状特征和柱状晶形貌.
2) 在退火态试样铁素体边界发现白色粒状析出物，淬火、低温回火和高温回火态试样带状特征和柱状晶形貌消失，得到相应的马氏体及其回火组织，500 ℃回火时在原马氏体边界析出了大量碳化物.而固溶态试样由于在高温下保温时间长，晶粒发生粗化.
3) 退火态试样强度较沉积态有所下降，但延伸率提高了49.87%.淬火、低温回火和高温回火态试样强度逐渐降低，但比沉积态试样强度增加，淬火态试样强度比沉积态提高了46.94%.热处理前后试样均属于韧性断裂，断口形貌由中心的等轴韧窝和四周的剪切韧窝组成，不同热处理状态试样韧窝深浅不一.
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