热处理对激光选区熔化18Ni300马氏体时效钢微观组织和力学性能的影响
In recent years, significant progress has been made in preparing conformal cooling dies for die casting using additive manufacturing technology. Among these advancements, 18Ni300 maraging steel has been widely applied because of its excellent forming characteristics. Currently, most research on selective laser melting (SLM) manufacturing of 18Ni300 maraging steel has primarily focused on the changes in microstructure after a heat treatment and the influence of precipitate phases on the strength, with limited emphasis on the impact of toughness. However, toughness plays a crucial role in determining the service life and safety of the molds.
Although previous studies have explored reverse-austenite, systematic research on the toughness of 18Ni300 is currently lacking. Therefore, this study aims to systematically investigate the impact of the aging and solution temperatures on the microstructure and mechanical properties of 18Ni300 maraging steel. Additionally, it will specifically analyze the influence of reverse-austenite on the strength and plasticity of 18Ni300 maraging steel prepared using SLM technology. This study clarifies the relationship between the process, structure, and performance of 18Ni300 maraging steel, and proposes an optimal heat-treatment system. These findings offer valuable guidance for the practical application of this steel in various industries.
In this study, 18Ni300 powder was used as the raw material. Experimental samples were obtained through selective laser melting (SLM) using an appropriate method. Following the formation, the samples were subjected to various heat treatments. The bulk samples were ground and polished with sandpaper, followed by etching with a 4% nitric-acid solution in alcohol. The microstructure was examined using optical microscopy (OM) and scanning electron microscopy (SEM). The mechanically polished samples were additionally polished with SiO2 and the crystal structure of the material was analyzed using electron backscatter diffraction (EBSD). X-ray diffraction (XRD) was utilized to analyze the phase composition and determine its content. Finally, tensile tests were conducted at room temperature using a universal testing machine and the corresponding fracture surfaces were observed.
The morphologies of the tested samples are shown in Figure 3. The printed sample displays distinct fish-scale-like fusion pools and lath martensite structures, whereas the honeycomb-like microstructure is not discernible in the SEM image. Following the aging treatment, the boundaries of the fusion pools in the samples become indistinct, and the boundaries of the honeycomb-like microstructure in the SEM image begin to dissolve. In the solution and aging-treated samples, the boundaries of the fusion pools vanish completely, and the martensite is transformed into a more refined structure. Additionally, the honeycomb-like microstructure observed in the SEM image also completely disappears.
The XRD analysis of the samples reveals that the phase composition of the as-printed sample comprises martensite and residual austenite, whereas the aged sample consists of martensite, residual austenite, and reverse-austenite. Almost the entire microstructure of the solution- and aging-treated sample is composed of martensite. Figure 5 shows that the highest amount of reverse-austenite is observed in the aged sample. Furthermore, Table 3 indicates that the sample aged at 490 °C exhibits the highest content of reverse-austenite.The mechanical properties of the sample are closely correlated with the reverse-austenite content, as depicted in Figure 8. Notably, the sample aged at 490 °C exhibits greater toughness with only a marginal reduction in strength. However, the relationship between austenite and the strength toughness of 18Ni300 is not a simple linear correlation because of factors such as precipitates and the martensite morphology. Overall, it is evident that reverse-austenite significantly enhances the toughness and marginally decreases the strength. With an increase in the reverse-austenite content from 0.1% to 6.9%, the elongation after fracture improves by 72.5%, whereas the tensile strength decreases by 2.3%.
The printed samples of 18Ni300 maraging steel manufactured by SLM display a distinct molten pool and a microstructure comprised of coarse martensite and a small proportion of residual austenite. Following the aging treatment, a ductile phase called reverse-austenite is generated. After the post-solution and aging treatments, the microstructure exhibits uniform and dense plate-like martensite with no notable presence of the austenite phase. A direct aging treatment at 490 °C is considered the optimal heat-treatment process for achieving an ideal balance between strength and toughness. At this temperature, the microstructure exhibits the highest reverse-austenite content (volume fraction: 7.7%). The ultimate tensile strength is 2012.8 MPa, and the elongation after fracture reaches a peak value of 6.9%. Therefore, a direct aging treatment at 490 °C is regarded as the most optimal heat-treatment process.
The fine reverse-austenite within the maraging steel manufactured via SLM serves as a toughening phase, enhancing the toughness without significantly compromising the strength. With an increase in the reverse-austenite volume fraction from 0.1% to 6.9%, the elongation after fracture experiences a 72.5% improvement, albeit at the expense of a 2.3% decrease in the ultimate tensile strength. Thus, the reverse-austenite is advantageous for achieving exceptional overall mechanical properties in maraging steel manufactured via SLM. The fine reverse-austenite plays a pivotal role in enhancing themaraging steel. However, in the maraging steel manufactured via SLM using 18Ni300, precipitation strengthening constitutes the primary mechanism with a limited effective range of precipitation temperatures. Further research is necessary to increase the reverse-austenite content, while maintaining adequate precipitation strengthening.
1 引言
增材制造(AM)技术不需要传统的刀具和夹具以及多道加工工序,可以快速精密地制造出任意复杂形状的零件,实现零件的“自由制造”,同时缩短了加工周期,而且产品结构越复杂,其制造优势越显著[1]。激光选区熔化(SLM)是增材制造技术中非常有前景的一种成形工艺[2],由于其成形件致密性好并且具有冶金结合组织及尺寸精度较高的特点,在国内外备受关注。该技术由德国Froounholfer研究院于1995年首次提出,现已初步形成了一定的产业规模[3-4]。在航天领域,SLM技术主要用于航天发动机等各种复杂金属零部件的成形;在汽车制造领域,SLM技术被应用于热交换器等各种复杂金属构件的制造;在模具制造领域,SLM技术主要用于成形带有内流道的复杂随形冷却的模具[4-5];在医疗领域,SLM技术用于人体骨骼植入物、口腔义齿等的成形以及骨骼修复。目前,SLM 制造行业正处于快速发展阶段,可用于SLM制造的金属材料及工艺研发都取得了一定成果。
马氏体时效钢以其高强度、高韧性以及优良的工艺性能被广泛应用于航空航天、模具等领域,其中18Ni300马氏体时效钢是一种高合金低碳超高强度钢,具有高强韧性、良好的焊接性能和冷热加工性能等特点[6]。采用SLM制造18Ni300马氏体时效钢不仅可以一步成形,不会被复杂形状所限制,还省去了后续加工环节,而且成形态样品的力学性能接近锻件[7-10]。研究人员在金属材料的增材制造方面做了许多基础性工作,如:李时春等[11]总结了增材制造成形件的微观组织结构特征,发现晶粒形态主要有胞状晶、柱状晶、树枝晶和等轴晶等;Mao等[12]研究了SLM工艺参数对18Ni300马氏体时效钢致密度、显微组织演变、纳米析出行为和力学性能的影响,并建立了工艺参数和致密度之间的关系模型,该模型可以有效预测出最优加工参数组合;董福元等[13]研究了热处理对SLM 18Ni300马氏体时效钢力学性能的影响,结果发现时效或固溶+时效处理试样出现了强度增大、韧性下降的现象,但他们没有阐释微观组织演变和力学性能之间的关系;Mei等[8]研究了激光-粉末床熔合(L-PBF)18Ni300马氏体时效钢的微观组织随时效温度的变化,结果显示,490 ℃时效时,密集析出的Ni3Ti颗粒主要通过Orowan机制产生峰值强度。但是,目前人们对增材制造金属材料热处理的研究还不完善,大多只是在传统热处理方法中选取某一个热处理制度对样件进行处理,对比研究其与打印态样品的组织和性能,而且时效温度基本上在480~500 ℃范围内选取,探究欠时效、峰时效、过时效的影响,对最佳热处理制度及样件韧性改善还缺乏系统性探究。此外,马氏体不锈钢在回火过程中会发生一种逆转变。马氏体不锈钢在Ms点(马氏体转变的起始温度)到Ac1点(奥氏体转变的起始温度)之间进行回火或时效处理时,马氏体组织会直接切变成奥氏体,这种奥氏体在室温甚至更低的温度下都能够稳定存在。为将该奥氏体与马氏体形成时受空间限制被挤压、分割而留下的残余奥氏体区分开来,依据其形成特点,将其称为“逆转奥氏体”[13-14]。刘振宝等[14]研究了马氏体时效不锈钢中逆转奥氏体的析出与长大行为,结果表明:超高强度马氏体不锈钢时效后强度显著上升;薄膜状逆转奥氏体沿马氏体板条界非连续析出;一定数量和尺寸的逆转奥氏体对改善钢的韧性起着重要作用,是超高强度马氏体不锈钢强度很高又能保持良好韧性的重要原因。
综上可知:SLM成形18Ni300马氏体钢的时效温度在480~500 ℃之间;时效过程中会析出强化相(主要是Ni3Ti和Ni3Mo),强化相能够提高钢的强度和硬度,同时,马氏体边界或原奥氏体边界会形成细小的逆转奥氏体,逆转奥氏体的形成可在一定程度上改善钢的塑韧性。因此,笔者系统研究了时效温度(480~500 ℃)、固溶温度(820~880 ℃)对18Ni300马氏体时效钢微观组织及力学性能的影响规律,重点分析了逆转奥氏体对SLM 18Ni300马氏体时效钢强度和塑性的影响,阐明了工艺、组织、性能之间的关系,给出了最佳的热处理制度。本研究对18Ni300马氏体时效钢的实际应用具有一定的指导意义。
2 试验材料及方法
2.1 试验材料
试验所用18Ni300粉末由江苏威拉里新材料科技有限公司提供,其成分见
表 1. 18Ni300马氏体时效钢的化学成分
Table 1. Chemical composition of 18Ni300 maraging steel
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图 1. 18Ni300粉末的形貌和粒径分布图。(a)形貌;(b)粒径分布图
Fig. 1. Morphology and particle size distribution of 18Ni300 powder. (a) Morphology; (b) particle size distribution
2.2 组织结构观察与性能测试
热处理采用上海瑞晶机械设备有限公司生产的RGQ1400-50高温气氛马弗炉,直接时效(DA)处理工艺分别为480 ℃×6 h、490 ℃×6 h、500 ℃×6 h;固溶+时效(SA)处理工艺分别为820 ℃×1.5 h+490 ℃×6 h、850 ℃×1.5 h+490 ℃×6 h、880 ℃×1.5 h+490 ℃×6 h。
表 2. SLM成形18Ni300马氏体时效钢的热处理工艺
Table 2. Heat treatment process of SLM formed 18Ni300 maraging steel
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3 结果与分析
3.1 热处理对微观组织的影响
打印态及不同热处理态18Ni300马氏体时效钢的显微组织如
图 3. 不同状态18Ni300钢的微观组织。(a)~(c)OM图;(d)~(f)SEM图
Fig. 3. Microstructures of 18Ni300 steel in different states. (a)‒(c) OM images; (d)‒(f) SEM images
由
3.2 热处理对逆转奥氏体的影响
为了测量打印态和热处理态18Ni300钢组织中马氏体和奥氏体的体积分数,进行了XRD试验,得到的衍射图谱如
图 4. 不同状态18Ni300钢的 XRD衍射图谱。(a)DA2态;(b)不同状态
Fig. 4. XRD patterns of 18Ni300 steel in different states. (a) DA2 state; (b) different states
表 3. 不同热处理后18Ni300马氏体时效钢中α和γ相的体积分数及硬度值统计表
Table 3. Volume fraction of α and γ phases and hardness of 18Ni300 maraging steel after different heat treatments
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图 5. 不同状态18Ni300马氏体时效钢的EBSD图。(a)~(c)IPF;(d)~(f)相分布图
Fig. 5. EBSD diagrams of 18Ni300 maraging in different states. (a)‒(c) Inverse pole figures (IPFs); (d)‒(f) phase distributions
根据文献[19-20],马氏体时效钢在时效处理过程中会产生一定数量的逆转奥氏体,一般情况下,逆转奥氏体的尺寸较小,其可在不明显降低强度的情况下作为韧化相改善马氏体时效钢的韧性。
3.3 热处理对18Ni300力学性能的影响
3.3.1 对洛氏硬度的影响
图 6. 不同状态的18Ni300马氏体时效钢的洛氏硬度
Fig. 6. Rockwell hardness of 18Ni300 maraging in different states
3.3.2 对拉伸性能的影响
SLM制备的18Ni300马氏体时效钢在不同热处理后的拉伸性能如
表 4. 不同热处理后18Ni300马氏体时效钢的拉伸性能
Table 4. Tensile properties of 18Ni300 maraging steel after different heat treatments
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图 7. 不同状态18Ni300马氏体时效钢的拉伸性能。(a)工程应力-工程应变曲线;(b)拉伸性能统计柱状图
Fig. 7. Tensile properties of 18Ni300 maraging steel in different states. (a) Engineering stress-engineering strain curves; (b) statistical histogram of tensile properties
490 ℃直接时效处理后的试样具有最高的逆转奥氏体含量,其抗拉强度为2012.8 MPa,高于所有直接时效处理试样的平均抗拉强度值2008 MPa,且其断后伸长率为6.9%,高于所有直接时效处理试样的断后伸长率。480 ℃直接时效处理后,试样的抗拉强度为2024.0 MPa,也高于所有直接时效处理试样的平均抗拉强度值2008 MPa,但其断后伸长率只有5.6%,其抗拉强度比490 ℃直接时效处理试样高了0.56%,但断后伸长率却降低了18.8%。因此,为了获得更大增幅的断后伸长率,即更佳的综合力学性能,选择490 ℃直接时效处理工艺为SLM制备18Ni300马氏体时效钢的最优热处理工艺。
不同状态18Ni300试样的拉伸断口如
图 9. 不同状态18Ni300马氏体时效钢的拉伸断口形貌。(a)PS;(b)DA2;(c)SA2
Fig. 9. Tensile fracture morphology of 18Ni300 maraging steel in different states. (a) PS; (b) DA2; (c) SA2
4 结论
SLM 18Ni300马氏体时效钢打印态试样熔池清晰,组织为粗大的马氏体和少量残余奥氏体;时效处理后生成了强韧化相——逆转奥氏体;固溶+时效处理后,组织为均匀致密的板条状马氏体,无明显的奥氏体相。
490 ℃直接时效处理后可以获得最优的强韧性匹配,此时组织中逆转奥氏体含量最高(体积分数达到了7.7%),抗拉强度为2012.8 MPa,断后伸长率最高(6.9%)。490 ℃直接时效处理为最优热处理工艺。
SLM 18Ni300马氏体时效钢中细小的逆转奥氏体为强韧化相,可以在不明显降低强度的同时提升韧性。随着逆转奥氏体体积分数从0.1%增加到6.9%,断后伸长率提升了72.5%,抗拉强度降低了2.3%。因此,逆转奥氏体的存在有利于SLM 18Ni300马氏体时效钢获得优良的综合力学性能。
细小的逆转奥氏体对于提升马氏体时效钢极为重要,但SLM 18Ni300马氏体时效钢的强化机制主要为析出强化,且有效的析出温度区间不大,如何在保证析出足够强化相的同时增加逆转奥氏体的含量仍需进一步研究。
[1] Lewandowski J J, Seifi M. Metal additive manufacturing: a review of mechanical properties[J]. Annual Review of Materials Research, 2016, 46: 151-186.
[2] 郑志军, 毛凌燕, 董智豪. 增材制造316L不锈钢组织各向异性对耐蚀性能的影响[J]. 中国激光, 2023, 50(4): 0402012.
[3] Tian X Y, Wu L L, Gu D D, et al. Roadmap for additive manufacturing: toward intellectualization and industrialization[J]. Chinese Journal of Mechanical Engineering: Additive Manufacturing Frontiers, 2022, 1(1): 100014.
[4] 姜海燕, 林卫凯, 吴世彪, 等. 激光选区熔化技术的应用现状及发展趋势[J]. 机械工程与自动化, 2019(5): 223-226.
Jiang H Y, Lin W K, Wu S B, et al. Application status and development trend of laser selective melting technology[J]. Mechanical Engineering & Automation, 2019(5): 223-226.
[5] 杨永强, 吴伟辉, 来克娴, 等. 金属零件选区激光熔化直接快速成形工艺及最新进展[J]. 航空制造技术, 2006, 49(2): 73-76, 97.
Yang Y Q, Wu W H, Lai K X, et al. Newest progress of direct rapid prototyping of metal part by selective laser melting[J]. Aeronautical Manufacturing Technology, 2006, 49(2): 73-76, 97.
[6] 周隐玉, 王飞, 薛春. 3D打印18Ni300模具钢的显微组织及力学性能[J]. 理化检验(物理分册), 2016, 52(4): 243-246.
Zhou Y Y, Wang F, Xue C. Microstructure and mechanical properties of 3D printing 18Ni300 die steel[J]. Physical Testing and Chemical Analysis (Physical Testing), 2016, 52(4): 243-246.
[7] Guo W F, Guo C, Zhu Q. Heat treatment behavior of the 18Ni300 maraging steel additively manufactured by selective laser melting[J]. Materials Science Forum, 2018, 941: 2160-2166.
[8] Mei X Y, Yan Y, Fu H D, et al. Effect of aging temperature on microstructure evolution and strengthening behavior of L-PBF 18Ni300 maraging steel[J]. Additive Manufacturing, 2022, 58: 103071.
[9] 李虎, 赵伟江, 李瑞迪, 等. 增材制造马氏体时效钢的研究进展[J]. 中国激光, 2022, 49(14): 1402102.
[10] 管航, 王小新, 董志家, 等. 18Ni300模具钢粉末3D打印工艺研究[J]. 模具技术, 2020(2): 1-6.
Guan H, Wang X X, Dong Z J, et al. Research on 3D printing process of 18Ni300 die steel powder[J]. Die and Mould Technology, 2020(2): 1-6.
[11] 李时春, 莫彬, 肖罡, 等. 金属材料的激光增材制造微观组织结构特征及其影响因素[J]. 激光与光电子学进展, 2021, 58(1): 0100007.
[12] Mao Z F, Lu X D, Yang H R, et al. Processing optimization, microstructure, mechanical properties and nanoprecipitation behavior of 18Ni300 maraging steel in selective laser melting[J]. Materials Science and Engineering: A, 2022, 830: 142334.
[13] 董福元, 侯俊峰. 热处理对SLM 18Ni300马氏体时效钢力学性能的影响[J]. 真空科学与技术学报, 2021, 41(6): 562-565.
Dong F Y, Hou J F. Effect of heat treatment on mechanical properties of SLM 18Ni300 maraging steel[J]. Chinese Journal of Vacuum Science and Technology, 2021, 41(6): 562-565.
[14] 刘振宝, 杨志勇, 梁剑雄, 等. 超高强度马氏体时效不锈钢中逆转变奥氏体的析出与长大行为[J]. 金属热处理, 2010, 35(2): 11-15.
Liu Z B, Yang Z Y, Liang J X, et al. Growth behavior and precipitation of reverted austenite in ultra-high strength marageing stainless steel[J]. Heat Treatment of Metals, 2010, 35(2): 11-15.
[15] 金赟. 选区激光熔化18Ni300成形及热处理前后组织与性能的研究[D]. 兰州: 兰州理工大学, 2019: 6.
JinY. Research on microstructures and properties of selective laser melting 18Ni300 and heat treatment[D]. Lanzhou: Lanzhou University of Technology, 2019: 6.
[16] Bai Y C, Zhao C L, Wang D, et al. Evolution mechanism of surface morphology and internal hole defect of 18Ni300 maraging steel fabricated by selective laser melting[J]. Journal of Materials Processing Technology, 2022, 299: 117328.
[17] Chen B, Huang Y, Gu T, et al. Investigation on the process and microstructure evolution during direct laser metal deposition of 18Ni300[J]. Rapid Prototyping Journal, 2018, 24(6): 964-972.
[18] 许大杨, 陈婉琦, 万继方, 等. 时效温度对SLM 18Ni300马氏体时效钢显微组织和力学性能的影响[J]. 金属热处理, 2023, 48(2): 144-150.
Xu D Y, Chen W Q, Wan J F, et al. Effect of aging temperature on microstructure and mechanical properties of SLM 18Ni300 maraging steel[J]. Heat Treatment of Metals, 2023, 48(2): 144-150.
[19] 朱静, 赵瑛伟, 潘天喜, 等. 18Ni(250级)马氏体时效钢中的逆转变奥氏体的研究[J]. 钢铁, 1981, 16(8): 41-45.
Zhu J, Zhao Y W, Pan T X, et al. Investigation on reverse austenite in 18Ni(250 grade) maraging steel[J]. Iron and Steel, 1981, 16(8): 41-45.
[20] Casati R, Lemke J, Tuissi A, et al. Aging behaviour and mechanical performance of 18-Ni 300 steel processed by selective laser melting[J]. Metals, 2016, 6(9): 218.
[21] Kempen K, Yasa E, Thijs L, et al. Microstructure and mechanical properties of selective laser melted 18Ni-300 steel[J]. Physics Procedia, 2011, 12: 255-263.
Article Outline
向超, 张涛, 吴文伟, 邹志航, 孙勇飞, 刘金鹏, 徐小蕾, 韩恩厚. 热处理对激光选区熔化18Ni300马氏体时效钢微观组织和力学性能的影响[J]. 中国激光, 2024, 51(16): 1602302. Chao Xiang, Tao Zhang, Wenwei Wu, Zhihang Zou, Yongfei Sun, Jinpeng Liu, Xiaolei Xu, Enhou Han. Effect of Heat Treatment on Microstructure and Mechanical Properties of Selective Laser Melted 18Ni300 Maraging Steel[J]. Chinese Journal of Lasers, 2024, 51(16): 1602302.