Elevated temperature tensile deformation behavior of powder metallurgy Fe−2Cu−0.5C steels
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摘要:
使用Gleeble-3500型热模拟机对粉末冶金Fe−2Cu−0.5C钢在变形温度850~1000 ℃、应变速率0.1~10.0 s−1下进行高温拉伸测试,定量分析变形温度、应变速率对粉末冶金钢高温拉伸变形行为的影响。通过应力应变曲线计算了该粉末冶金钢高温拉伸过程的断裂功,建立了断裂功与变形温度、应变速率的数学模型。基于Hensel-Spittel模型和BP神经网络模型建立了该粉末冶金钢的本构模型,用于表征其高温拉伸流变行为,并将两种模型的预测结果进行比较。结果表明:所建立的断裂功模型能够描述该粉末冶金钢在不同变形温度和应变速率下的抗断裂能力。Hensel-Spittel本构模型的预测值与实验值的平均绝对相对误差为3.16%,决定系数为0.9743,而BP神经网络模型的预测值与实验值的平均绝对相对误差为0.17%,决定系数为0.9999,说明BP神经网络模型的预测能力更强,能更好地表征粉末冶金Fe−2Cu−0.5C钢的高温拉伸流变行为。
Abstract:The elevated temperature tensile tests of the powder metallurgy (P/M) Fe−2Cu−0.5C steels at the deformation temperatures of 850~1000 ℃ and strain rates of 0.1~10.0 s−1 were carried out by Gleeble-3500 thermal simulator. The effects of deformation temperature and strain rate on the elevated temperature tensile deformation behaviors of the P/M steels were quantitatively analyzed. The fracture work of the P/M steels during the elevated temperature tensile was calculated using the stress-strain curves. The mathematical model characterizing the relationship of fracture work, deformation temperature, and strain rate was established. The Hensel-Spittel model and BP neural network model were used to establish the constitutive equation of the P/M steels to characterize the flow behaviors during the elevated temperature tensile. The predicted results of the two models were compared. The results show that the fracture work model can describe the capacity of the P/M steels to resist fracture at different deformation temperatures and strain rates. The average absolute relative error between the predicted values of Hensel-Spittel constitutive model and the experimental values is 3.16%, and the coefficient of determination is 0.9743. While the average absolute relative error and the coefficient of determination between the predicted values of BP neural network model and the experimental values are 0.17% and 0.9999, respectively, indicating that the BP neural network model has the stronger predictive capacity and can characterize the elevated temperature tensile deformation behaviors of the P/M Fe−2Cu−0.5C steels better.
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双相不锈钢将奥氏体不锈钢所具有的优良韧性和焊接性与铁素体不锈钢所具有的较高强度和耐氯化物应力腐蚀性能结合在一起,被广泛应用于海洋工程装备[1]。选区激光熔化(selective laser melting,SLM)是一种增材制造领域的近净成形技术,较快的冷却速度可以避免σ相等有害相的析出[2],可得到形状复杂、高密度、组织均匀、无需后续机加工的不锈钢零件。近年来,研究人员对选区激光熔化工艺制备双相不锈钢展开了一系列的研究。Davidson和Singamneni[3−5]研究了选区激光熔化制备UNS S32750超级双相不锈钢,分析了激光能量密度、铁素体含量、磁性能之间的关系,结果显示,打印后的孔隙会显著影响晶粒取向和晶粒长大,晶粒各向异性导致选区激光熔化试样比锻件试样具有更高的比饱和磁化强度。Hengsbach等[6]研究了选区激光熔化UNS S31803超级双相不锈钢的显微组织和力学性能,发现大量位错的出现使得热处理后的再结晶组织占主导地位,由于高位错密度和氮化物的存在阻碍了位错运动,使得试样获得了更高的抗拉强度和较低的断后伸长率。Saeidi等[7]研究了选区激光熔化制备SAF 2507超级双相不锈钢,得到了高密度和优异的力学性能,相对密度达99.5%,抗拉强度、屈服强度、硬度分别为1321 MPa、1214 MPa、HV 450。Shang等[8−9]用选区激光熔化工艺制备了UNS S32707双相不锈钢,打印态组织铁素体面积比98.2%,这是因为冷却速度较快,奥氏体没有足够的时间从铁素体相中形核析出,后续需要进行固溶处理调整两相比例。Sotomayor等[10−11]以316L与430混合粉末作为原材料,使用粉末注射成形工艺制备了双相不锈钢,研究了粉末注射成形双相不锈钢烧结过程中的显微组织演变,研究发现烧结后γ相含量远低于预期。实验采用316L和430两种不锈钢粉末以1:1比例混合,在低真空下不同温度(950~1300 ℃)烧结1 h,γ相衍射峰的强度随烧结温度的升高而降低,直到1100 ℃时完全消失。γ相含量的降低可能与烧结过程中Ni元素从γ相向α相扩散有关。本文以316L与430混合粉末作为研究对象,使用选区激光熔化成形技术制备双相不锈钢试样,研究了固溶处理温度对试样显微组织、力学性能、耐腐蚀性能的影响。
1. 实验材料及方法
选用气雾化法制备的316L和430不锈钢球形粉末为原料,按质量比50:50混合得到316L/430双相不锈钢混合粉末,命名为50A。混合粉末的粒度分布如图1所示,其中D50为36 μm,化学成分如表1所示。
表 1 双相不锈钢粉末的化学成分(质量分数)Table 1. Chemical composition of the 316L/430 duplex stainless steel powders% Cr Ni Mn Mo C Si Fe 17.00 6.00 1.50 1.25 0.06 0.50 余量 使用华曙高科FS121M金属3D打印机将50A粉末打印成形,打印参数为激光功率190 W、扫描速度800 mm·s−1、铺粉厚度0.01 mm、扫描间距0.05 mm,能量密度为89.96 J·mm−3。选区激光熔化扫描策略如图2所示,扫描角度为45°,工作室气氛为氮气气氛,相对压力为2~5 kPa。利用电子比重计测量打印件的相对密度为99.7%。采用ONH-2000氧氮氢分析仪测定打印试样中的含氮量(质量分数)为0.05%。依据所测含氮量和表1中双相不锈钢粉末化学成分,使用JMatPro软件模拟试样组织随温度变化的规律,模拟得到相图如图3所示,取950~1350 ℃作为温度区间,每隔50 ℃对3D打印后的试样进行固溶处理,保温时间为1 h。打印试样命名为DY,固溶处理试样命名为950GR、1000GR、1050GR、1100GR、1150GR、1200GR、1250GR、1300GR、1350GR。
图4是拉伸试样尺寸,厚度为2 mm。使用180#、600#、1200#砂纸打磨方片试样,然后用2.5、1.0 μm的金刚石抛光喷雾抛光试样表面,用去离子水冲洗干净后,再用质量分数20%的NaOH溶液电解腐蚀方片,电压为10 V,时间为5 s,最后用乙醇冲洗。使用Axio Vert.A1研究级倒置数字显微镜观察金相组织,将铁素体标记为α,奥氏体标记为γ。通过金相截线法计算铁素体晶粒的尺寸。采用CMT4204型微机控制电子万能试验机进行拉伸试验,测量试样的抗拉强度、屈服强度、断后伸长率。利用HV-30型维氏硬度计测量试样的硬度。选择三电极系统进行电化学实验,评价试样的耐腐蚀性能,其中参比电极为饱和甘汞电极(saturated calomel electrode,SCE),辅助电极为铂片电极。使用CHI660C型电化学工作站进行动电位极化曲线测试,电解质溶液为质量分数3.5%的NaCl水溶液。
2. 结果与讨论
2.1 显微组织
使用Axio imaging金相分析软件计算DY试样和固溶处理试样的铁素体与奥氏体两相面积比,结果如表2所示。根据表2中的数据选择DY、1150GR、1250GR、1350GR,作为进一步研究对象,其中1250GR的两相面积比接近1:1。图5是不同加工工艺的金相组织。从图5(a)中可看出条状组织呈直角相互交错排布,这是激光采用跳转变向扫描方式形成的显微组织,铁素体晶粒取向各不相同,产生大量位错。从图5(b)~图5(d)中可以发现,随着固溶处理温度的升高,铁素体含量逐渐增加,当温度升高时,N稳定奥氏体的作用减弱,进而使得两相转变速度加快,铁素体发生再结晶,使得铁素体含量急剧上升[12‒13],这与图3中相图模拟结果一致。当温度为1250 ℃时,铁素体与奥氏体的面积比最接近1:1,为45.7:54.3。经过截线法计算,1150GR、1250GR、1350GR试样中铁素体晶粒尺寸分别为8、18、38 μm,随着固溶温度的升高,试样中再结晶形成的铁素体晶粒尺寸逐渐长大。
表 2 打印态和固溶态试样铁素体与奥氏体两相面积比Table 2. Area ratio of ferrite and austenite for the printing state and the solution treatment specimens试样 α:γ(面积比) DY 99.0:1.0 950GR 13.5:86.5 1000GR 14.6:85.4 1050GR 13.7:86.3 1100GR 12.0:88.0 1150GR 23.9:76.1 1200GR 31.2:68.8 1250GR 45.7:54.3 1300GR 60.4:39.6 1350GR 80.7:19.3 2.2 力学性能
试样应力–应变曲线如图6所示,由图可知,DY试样的屈服强度较高,为843 MPa;1150GR的抗拉强度较高,为994 MPa;1250GR的断后伸长率较高,为25%,比其他试样的塑性好。从图5(a)中可明显观察到DY试样晶粒取向不同的铁素体晶粒,这种组织交错的结构堆垛层错能较高,在拉伸过程中,大量位错阻碍变形,导致试样的屈服强度较高。从图5(b)中可以看出,试样中铁素体晶粒尺寸为8 μm,与其他固溶处理试样相比,尺寸较小,细晶强化效果显著,因此1150GR的抗拉强度较高。从图5(c)中可以看出铁素体晶粒长大,且数量减少,1250GR奥氏体与铁素体的晶界减少,抗拉强度与屈服强度均降低。此外,双相转变过程沿着晶界发生,晶界尖角处能量较高,组织优先转化,因此1250GR组织中带有尖角的晶粒较少,应力集中减弱,1250GR塑性提高。结合图5(d)和表2可以发现,温度升高,铁素体含量增多,晶粒尺寸长大,抗拉强度降低。
将50A试样的拉伸试验结果与他人研究结果作比较,如表3所示,其中MIM–316L的数据来自美国MPIF标准,SLM–316L–1050GR的数据来自张仁奇等[14]研究的选区激光熔化制备316L试样在1050 ℃固溶处理后的力学性能,MIM–316L:430–1250GR的数据来自Sotomayor等[15]研究的注射成形316L与430混合粉末烧结件在1250 ℃固溶处理后的力学性能,3RE60力学性能来自GB/T 1220-1992。由表可知,与MIM–316L、SLM–316L、MIM–316L:430、3RE60相比,50A的DY、1150GR、1250GR试样力学性能优势明显。1250GR试样的综合力学性能优于成分相近的3RE60锻造棒材。
表 3 不同工艺试样力学性能比较Table 3. Mechanical properties of the specimens prepared by the different processes材料 抗拉强度 / MPa 屈服强度 / MPa 硬度,HV 断后伸长率 / % MIM–316L ≥517 ≥172 ≥120 ≥50 SLM–316L–1050GR[14] 673 420 — 48 MIM–316L:430–1250GR[15] 756 — — — 3RE60 ≥590 ≥390 ≤300 ≥20 50A–DY 924±5 843±4 335±6 22±1.0 50A–1150GR 994±18 444±6 320±4 16±1.5 50A–1250GR 830±14 340±6 356±7 25±3.0 50A–1350GR 598±8 375±56 253±13 21±3.0 2.3 耐腐蚀性能
图7为试样动电位极化曲线,表4为电化学试验结果,其中E为自腐蚀电位,VSCE;I为自腐蚀电流,A;Ep为点蚀电位,VSCE。因为本试验的电化学测试样面积都为1 cm2,所以可通过自腐蚀电流I的大小来评估试样的自腐蚀电流密度,自腐蚀电流密度的大小可表征腐蚀速率的快慢。由表4可知,与DY相比,1150GR、1250GR、1350GR的点蚀电位明显提高,表明50A试样经过固溶处理之后,耐点蚀性能改善。经DY的电流密度比1350GR的小,但是点蚀电位比较低,表明DY发生点蚀的倾向较大,但是腐蚀速率较慢。分析认为DY晶粒小,晶界多,晶界处的活性原子数目就多,可以加快钝化膜的形成速度,选区激光熔化工艺加热快、冷却快,有利于Cr、Ni、Mo等耐蚀元素在熔池内部均匀分布,DY熔池整体腐蚀均匀。熔池的热影响区存在成分偏析与组织偏析,会存在电位差,形成微型电池,提高腐蚀敏感性,晶界处成为点蚀形核的优先位点。因此DY试样易发生腐蚀,但腐蚀速率较慢。随着固溶温度的升高,固溶试样的自腐蚀电流升高,点蚀电位出现降低现象。经分析这是由晶粒尺寸导致的,1150GR的晶粒最小,更容易形成钝化膜,而1250GR、1350GR晶粒长大,形成钝化膜的速率较慢,因此温度为1150 ℃时,50A试样点蚀电位最高,自腐蚀电流最小,分别为−0.118 V和3.196×10−6 A,即1150GR点蚀敏感性小,腐蚀速率慢,耐腐蚀性能好。
表 4 双相不锈钢固溶处理前后电化学试验结果Table 4. Parameters of the 50A specimens before and after solution treatment in the potentio-dynamic polarization curves试样 E / VSCE I / A Ep / VSCE DY −0.402 6.983×10−6 −0.595 1150GR −0.433 3.196×10−6 −0.118 1250GR −0.460 6.145×10−6 −0.195 1350GR −0.463 7.896×10−6 −0.221 3. 结论
(1)以气雾化316L和430混合粉末为原料,采用选区激光熔化工艺制备了双相不锈钢,拓展了316L和430不锈钢粉末的应用范围。
(2)当固溶温度为1250 ℃时,铁素体与奥氏体的两相面积比为45.7:54.3,此时试样的力学性能较好,抗拉强度830 MPa,屈服强度340 MPa,硬度HV 356,断后伸长率25%。
(3)当固溶温度为1150 ℃时,试样的耐腐蚀性能较好,自腐蚀电流密度为3.196×10−6 A·cm−2,点蚀电位为−0.118 V。
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