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通过JMatPro软件中的Phase Transformation模块对Fe-0.0029C-0.0150Si-0.0150Mn-0.0096Cr-0.0120Ni- 0.0100Al-0.0050Mo(质量分数)钢的冷却转变(continuous cooling transformation, CCT)和等温转变(time temperature transformation, TTT)曲线进行模拟,其中奥氏体化温度设置为Ae3+50℃,晶粒尺寸设置为9μm,最终模拟结果如图 3a和图 3b所示。根据CCT曲线,珠光体冷却转变曲线与1℃/s冷却曲线未相交,因此可避免奥氏体化后的试样转移过程中生成珠光体组织,且激光粉末沉积过程中冷却速度较大(可达1×104℃/s),故沉积过程可避免珠光体转变等扩散型相变发生;根据TTT曲线,试样钢的马氏体开始转变温度(martensite start,MS)TMS=281.9℃,较低的TMS一方面可以降低激光粉末沉积过程中的残余拉应力积累,避免应力过大导致的开裂和变形,另一方面也对应了较低的贝氏体等温温度,低的贝氏体等温温度有利于试样获得更好的组织和性能。TTT曲线也显示贝氏体转变可在4.8h内完成,这为制备过程中等温工艺参量的设定提供了参考依据。
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图 4为热轧前后不同工艺试样的金相照片。由图 4a所示的激光粉末沉积态试样的金相照片可以看出,材料沉积态组织均匀细小,无夹杂无裂纹,无晶间偏析及第二相存在,但可观察到气孔,气孔等缺陷的存在对材料力学性能存在负面影响;通过热微轧工艺可消除气孔,根据图 4b~图 4d所示微轧后不同等温工艺试样的金相照片,可以观察到试样组织中无气孔、裂纹以及析出相等缺陷。试样中无明显偏析及碳化物生成是因为,Si元素可有效抑制碳化物的析出,Si为非碳化物形成元素,在碳化物中溶解度极低,在贝氏体等温过程中,Si在渗碳体形核过程会扩散至铁素体与渗碳体的边界,最终在边界处富集,从而抑制了碳化物的析出,故在沉积态和后处理之后,试样中均无碳化物析出。
图 5为不同等温工艺试样微观组织SEM照片。可以看出,所有试样微观组织均由贝氏体和残余奥氏体(retained austenite, RA)组成,组织均匀细小,无碳化物析出。根据图 5a,可以观察到280℃-3h试样组织由大量细长贝氏体板条以及部分块状残余奥氏体组成,由于等温时间较短,块状残余奥氏体较多;随着等温时间延长,贝氏体转变更加完全,根据图 5b,280℃-5h试样微观组织中贝氏体板条增多,块状残余奥氏体减少且尺寸降低,更多薄膜状残余奥氏体分布在贝氏体板条之间,根据图 5d所示的280℃-5h试样的局部放大图可以看出,贝氏体板条尺寸已达到纳米尺度;根据图 5c,虽然330℃-5h试样微观组织与280℃-5h试样基本一致,但贝氏体板条尺寸逐渐增大,块状残余奥氏体数量增多且尺寸也增大,这是由于低等温温度下,C元素扩散较慢,贝氏体长大速度较低,贝氏体形核率相对更高,贝氏体板条数量增加,部分块状残余奥氏体会被分割成更细的薄膜状残余奥氏体,故较低的等温温度有利于获得更加细小的贝氏体板条以及更多的薄膜状残余奥氏体。
图 6为不同等温工艺试样的XRD分析结果。可以看出,所有试样衍射峰均由α峰和γ峰组成,即试样的微观组织均由贝氏体和残余奥氏体组成,无碳化物析出,此结果与SEM实验的观测结果基本一致。根据(1)式和(2)式[16],计算得不同工艺试样中残余奥氏体的体积分数以及残余奥氏体中的碳含量,计算结果列于表 1中。
Table 1. Volume fraction of RA and C content in RA of the specimens obtained at different process
specimens volume fraction of RA/% C content in RA/% 280℃-3h 15.84 1.24 280℃-5h 12.69 1.19 330℃-5h 18.21 1.04 $ {\mathit{V}_\mathit{i}} = \frac{1}{{1 + \mathit{G}({\mathit{I}_\mathit{\alpha }}/{\mathit{I}_\mathit{\gamma }})}} $
(1) $ {\mathit{C}_\mathit{\gamma }} = \frac{{{\mathit{\alpha }_\mathit{\gamma }} - 3.555}}{{0.044}} $
(2) 式中,Vi为各个γ峰的残余奥氏体体积分数,Vi的平均值为试样的残余奥氏体体积分数; Iα和Iγ分别为α峰和γ峰的积分强度; G值为比例常数,取值各有不同(Iα(200)/Iγ(200)时取为2.5,Iα(200)/Iγ(220)时取为1.38,Iα(211)/ Iγ(200)时取为1.19,Iα(211)/ Iγ(220)时取为0.06)[1];Cγ为各个γ峰残余奥氏体中的碳含量,Cγ的平均值即为试样残余奥氏体中的碳含量; aγ为各个γ峰残余奥氏体的晶格常数。
根据表 1可知,280℃-3h试样残余奥氏体体积分数高于280℃-5h试样,由于280℃-3h试样处于不完全转变状态,而280℃-5h试样由于等温时间充足,贝氏体相变更加完全,故贝氏体钢残余奥氏体体积分数随着等温时间的延长而降低,但等温温度较低时,碳元素扩散能力不足,故残余奥氏体中碳元素含量变化并不明显;在贝氏体相变完全状态下,随着等温温度升高,碳原子扩散能力增强,残余奥氏体体积分数增加,根据杠杆原理,残余奥氏体中碳含量降低,且高等温温度试样中含有部分低碳含量的块状残余奥氏体,故330℃-5h试样相对于280℃-5h试样残余奥氏体体积分数增加而残余奥氏体中碳含量降低。
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图 7中为不同工艺拉伸试样的工程应力应变曲线。具体拉伸性能数据列于表 2中。在拉伸过程中,较细的贝氏体组织可以有效增加钢的强韧性,薄膜状残余奥氏体则有利于增加钢的韧性,但块状残余奥氏体过多对钢的性能会产生不利影响。由工程应力应变曲线可以看出,280℃-5h试样抗拉强度以及延伸率均最高,而280℃-3h试样拉伸性能相对较弱,这表明等温时间较短时,贝氏体转变不充分导致贝氏体含量较低,故在相同等温温度条件下,低等温时间试样抗拉强度较低;低等温时间试样中含有部分块状残余奥氏体,易在拉伸过程的屈服阶段发生马氏体相变,产生应力应变强化效应,但由于此类块状残余奥氏体含量较少且尺寸较大,强化效果不明显且较易引起应力集中,故低等温时间试样延伸率也较低;随着等温温度升高,贝氏体板条逐渐变粗导致强度开始下降,虽然随着等温温度升高组织中残余奥氏体体积分数有显著提升,但是由于含碳量变化不大,组织中出现更多贫碳的块状残余奥氏体,故330℃-5h试样延伸率也低于280℃-5h试样。
Table 2. Tensile properties of the specimens obtained at different process
specimens tensile strength/MPa yield strength/MPa elongation/% 280℃-3h 1209 1054 11.6 280℃-5h 1248 1037 14.5 330℃-5h 1181 1034 12.5 图 8为不同工艺试样的平均显微硬度变化图。可以看出,试样的平均显微硬度随着等温时间升高而增加,而随着等温温度上升略微降低,280℃-3h试样平均显微硬度最低,仅为442.7HV,这表明随着等温时间的延长,贝氏体含量上升,在贝氏体钢中代表较软相的残余奥氏体体积分数降低,故试样的平均显微硬度增加,280℃-5h试样平均显微硬度最高,达到494.5HV;330℃-5h试样平均显微硬度达到468.4HV,相对于280℃-5h试样略微降低,这表明等温温度升高引起贝氏体组织粗化,且残余奥氏体体积分数大幅度增加,虽然部分为块状残余奥氏体,会由于不稳定性在变形过程发生马氏体相变,但由于此类块状残余奥氏体含量相对较少,对硬度的强化并不明显,故330℃-5h试样平均显微硬度略微降低。
激光粉末沉积过程中,能量密度、激光扫描速率以及送粉速率对沉积过程的高温应力应变循环有显著影响,该循环过程会改变贝氏体钢最初的组织形态,对热处理之后的组织性能均有较大影响。本文中对以上工艺参量并未做深入优化,仅对后热处理相关参量进行相关讨论,最佳工艺参量的确认基于相关碳钢材料的经验以及实验结果。
激光粉末沉积中碳高强贝氏体钢组织与性能研究
Microstructure and properties of medium-carbon high strength bainite steel fabricated by laser powder deposition
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摘要: 为了实现高强贝氏体钢零部件增材修复的目的,采用激光粉末沉积+等温热处理的方法制备了一种中碳高强贝氏体钢,其合金成分(质量分数)为Fe-0.0029C-0.0150Si-0.0150Mn-0.0096Cr-0.0120Ni-0.0100Al-0.0050Mo。使用扫描电子显微镜、X射线衍射仪对贝氏体钢的微观组织进行表征,采用拉伸试验机以及显微维氏硬度仪对材料的力学性能进行表征,试样沉积后在280℃盐浴等温处理5h具备最佳综合性能,平均显微硬度达到494.5HV,抗拉强度达到1248MPa、屈服强度达到1037MPa,延伸率达到14.5%。结果表明,不同工艺参量试样微观组织均由贝氏体板条以及残余奥氏体组成,且组织均匀,无碳化物、偏析以及气孔夹杂等缺陷,但等温时间过低以及等温温度过高都会导致组织性能劣化。该研究为高强零部件的增材修复提供了参考。Abstract: In order to realize the additive preparation and surface repair of high strength bainitic steel parts, amedium-carbon high strength bainite steel was prepared by laser powder deposition and salt bath isothermal treatment with different process parameters. The alloy composition (mass fraction) was Fe-0.0029C-0.0150Si-0.0150Mn-0.0096Cr-0.0120Ni-0.0100Al-0.0050Mo.The microstructure of the steel wascharacterized by scanning electron microscope and X-ray diffraction, and the mechanical properties of steel were tested through tensile testing machine and Vicker's microhardness tester. The effect of isothermal temperature and isothermal time on microstructure and properties of the steel during salt bath isothermal process was analyzed. The best integrated mechanical properties were observed after deposition and salt bath isothermal at 280℃ for 5h, the average microhardness, tensile strength, yield strength, andelongationwas 494.5HV, 1248MPa, 1037MPa, and 14.5%, respectively. The results showe that the microstructure of the samples with different process parameters is composed of lath-like bainite and retained austenite, and the microstructure is uniform without defects such as carbide, segregation, pore and inclusion, but too low isothermal time or too high isothermal temperature would lead to the deterioration of the microstructure. This study provides a reference for the additive and repair of high strength parts.
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Table 1. Volume fraction of RA and C content in RA of the specimens obtained at different process
specimens volume fraction of RA/% C content in RA/% 280℃-3h 15.84 1.24 280℃-5h 12.69 1.19 330℃-5h 18.21 1.04 Table 2. Tensile properties of the specimens obtained at different process
specimens tensile strength/MPa yield strength/MPa elongation/% 280℃-3h 1209 1054 11.6 280℃-5h 1248 1037 14.5 330℃-5h 1181 1034 12.5 -
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