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1、回火温度对塑料模具钢X38CrMo16的腐蚀程度的影响ChristophLerchbacher1,SilviaZinner2andHaraldLeitner1摘要：改良后的不锈钢马氏体铬钢X38CrMo16拥有良好的韧性、耐腐蚀性和耐磨性。这种钢的典型的热处理包括在更进一步的回火工艺后的硬化处理。回火会引起二次析出在升高的温度下的对提高硬度很有影响的碳化物、氮化物和碳氮化合物。实验表明，腐蚀程度的变化通常会随着这种钢的回火温度的变化范围而变化。腐蚀程度在540 C时能达到一个最低值，直到温度再次增加。为了对微观结构进行研究，将样品在较短时间和温度最低到达575C的条件下进行了测试。在该温度下，
2、值得注意的大约6nm大小范围的贫铬区能被原子探针断层扫描装置观测。在这些区域，铬浓度下降至10%。在耗尽区，存在着在回火过程中形成的较少量的氮化物和大量的碳化物。这些地方的阳极区域的存在，可以通过TEM研究来证实。关键词：马氏体不锈钢、塑料模具钢、X38CrMo16、腐蚀、原子断层探针1、介绍硬化马氏体不锈钢铬钢主要用于塑料模具的应用程序或压铸应用中。化学腐蚀的塑料加工要求有特殊性能，例如高的耐腐蚀性，良好的韧性和硬度，耐磨性和良好的加工性能及抛光性能。这些属性是由一成分稳定的组合物、高纯度和完善的热处理工艺来得到的。后者包括在多步回火过程后的淬火后形成的奥氏体化。硬化会产生一个相对脆性的马氏
3、体组织，它嵌入在不溶解于奥氏体和残留有奥氏体含量的碳化物里。回火主要会引起马氏体和二次硬化的松弛，而所要求的硬度可以通过回火温度来控制。为了达到良好的硬度和韧性，典型的回火温度范围相当于在回火图右边的最大峰值处。然而，一些调查表明，马氏体不锈钢铬钢的腐蚀行为强烈地依赖于回火温度即需硬度的选择是由腐蚀性能限制的。在随回火温度而变化的基体界面的贫铬区会对这种钢的腐蚀程度有影响。本次工作的目的是清楚的确定机制，并通过高分辨率技术证实对这种衰弱有影响的贫铬区。2、实验表1 改良的塑料模具钢X38CrMo16的定义成分表1中列出了改良的塑料模具钢X38CrMo16组合物，热处理使用了软件圆柱试样演示，式
4、样长度为15mm，直径为5mm并应用了来自Bhr Thermoanalyse公司的淬火膨胀仪Dil 805。试样被实验在1020C中大约30分钟，之后，这种材料在设置的冷却参数为0.5（）下淬火，随后，试样分别在500、525、550、575和600C下钢化两小时。盐雾实验，浸泡腐蚀和点蚀电位已经进行了测试，以评估中的腐蚀行为。盐雾实验在DIN EN ISO 9227后，已经做了6小时。浸泡实验要在20%的沸腾的乙酸中停留24小时。点蚀电位已经达到了当前的电流密度。用于腐蚀实验的试样已经经过了两次两小时的调和。TEM的调查已经对Tecnai F20进行。试样已经被蚀刻剂A2抛光。从热处理试样中
5、，已经截取了尺寸为的条块，用于原子探针样品备置。顶端已经被标准的两步法电化学抛光程序备置好。测量已被使用的激光模式上的一个飞跃3000小时CAMECA在60 K温度与0.6 新泽西的激光脉冲能量进行。从CAMECA，探测体积的重建和数据集的评估已经进行了与软件包IVAS 3.4.3。3、结论3.1 耐腐蚀性图1显示了修改后的回火马氏体不锈钢X38CrMo16图。此外，显示在有趣的回火温度范围的腐蚀电位。该图描绘的耐腐蚀性的崩溃在约540C处。图2a显示温度，B的盐雾试验结果的停留时间为6小时的样品回火分别代表图（图 1）中最小值的540C和600C。当样品在较低的回火温度时，明显的较高的化学反
6、应会发生。这种印象是由相应的浸渍试验结果证实，分别在540和60C调质条件下，显示质量损失率73和的样品。图1 回火调查x38crmo16图和相应的点蚀电位图2 在盐雾试验后对试样的回火的腐蚀外观 a）540 C；b）600 C图3 =0.5淬火样品的扫描电镜照片a）；相应的碳化物显微可视化b)3.2 微观结构高分辨率显微镜已经被应用，为了使回火温度依赖性的腐蚀行为与回火过程中的微观组织演变相联系。3.2.1 淬火条件图3a显示了在淬火条件下的试样的SEM微观形状的代表图。尺寸大小为2纳米的黑暗的球形颗粒能在马氏体结构中发现。EDX分析结果显示暗区相对应的是铬碳化物。为了获得平均相部分，已经对
7、黑暗区域进行了评估，如图3b。定量相分析已通过图像分析软件采用如图 3b显示。铬碳化物的体积分数是在硬化状态下约1 %。图4给出了 TEM显微照片对应于作为淬火状态和显示周围的不溶解的颗粒显白壳，表示强烈的蚀刻已在电解抛光区。 图4 淬火样品的TEM微观形状图5 试样的碳原子图形和大约22%的相同表面的铬进行了2小时的回火，分别在a) 500 C; b) 525 C; c) 550 C; d) 575 C; e) 600 C图6 接近直方图显示铬对应22 %铬等浓度面不同回火的样品（a）；界面区域的细节(b)3.2.2 回火腐蚀行为和TEM调查给出了假设，在回火过程中，微观结构的局部成分将发生
8、变化。原子断层探针是解决这样的浓度变化范围内的精细结构的一个非常有用的工具。图5显示了试样的碳原子图形和大约22%的相同表面的铬在不同温度下进行的2小时的回火，那么，二次硬化颗粒的结构能被识清。在探测体积铬等浓度表面22%中，形成的析出物的一个令人满意的现象指出了除碳原子形态外的所有情况。这种分析方法的细节可以在其他地方找到。试样的碳原子图形和大约22%的相同表面的铬在500C进行的2小时的制备（图5a），显示了形状不规则细碳和富集特征的铬的高浓度。此外，约50nm大富碳颗粒可以被检测到。提高回火温度会导致更多碳球和浓缩颗粒铬的形成将被发现，如图5b。试样的探测体积在550C条件下的回火定性的
9、表明，颗粒变得更加的球形状，而且这些粒子的浓度比试样在低温回火情况下要小。在颗粒的表面，最明显的现象会在回火温度为575C时发生，如图5d。检测到的碳和铬的析出物尺寸富集明显高于较低的回火温度。此外，氮和富铬颗粒尺寸小于10nm的可以发现。回火在600C似乎进一步增加的碳和铬的富集颗粒大小（图 5e）。富氮的颗粒不探测的体积内的检测。虽然探测体积小，回火包括长大和粗化在古典析出动力学是显而易见的。 图7 TEM显示的试样回火实时图像，在a) 550C; b) 575C and c) 600C温度下回火图6a显示创建22 %铬等浓度接近直方图表面记录。通过与检测到的碳化物和氮化物相关的颗粒/集体
10、，说明了铬的浓度分布。从铬含量由50%变至80%的图中可以发现在500到600C回火过程后的颗粒。应当指出的是，在颗粒铬含量的准确测量是本研究的范围之外的。图 6b显示通过详细的接口的铬浓度分布。总铬含量在基质内的范围在12 %和14%铬对不同回火样品。通过接口，铬含量的持续增加，除了形成于575C情况时的接口.如图 6b描绘的析出物的情况下，一个贫铬区，这是约6nm宽，发生在界面。在这里，铬含量降到10%。两个检测类型的沉淀物在回火阶段显示在界面有相同的行为。大的富铬碳化物以及氮化物表明小铬贫化区。同时铬浓度水平和区域的横向范围是相同的。图7 TEM显示的试样回火实时图像，从550到600C
11、温度下回火。每个样例状态的低和高的放大倍率的图像已经产生，因此，可以看清颗粒尺寸全谱。在纳米到微米大小的范围内数百次碳化物可以在样品550C（如图7a）和600C（如图7c）回火时看到。高倍率的图像显示的二次硬化析出物尺寸为20nm的约10 为550C样品（图 7a）。粒子大小的二次硬化碳化物在加热过程中增加575C（图 7b）和600C（图 7c），分别为。这三种不同回火试样的耐腐蚀性能之间的主要区别是在样品在575C，在高放大倍率的图像是明显的回火形成的沉淀物强腐蚀（图 7b）。由样品制备的来的这种化学反应表明了局部铬的贫化，证实了原子探针结果。4、讨论修改后的马氏体不锈钢钢X38CrMo
12、16表现出了对局部腐蚀有很大影响的显著微观特征。已经淬火后强腐蚀碳化物/基体界面区域进行检测（图4）。虽然，在1020C高奥氏体化温度得到的假设，铬浓度补偿可能发生在界面中溶解初生碳化物。目前还不清楚如果贫铬区奥氏体化过程中或者是在冷却时形成的已经存在。然而，耗尽区周围未溶初生碳化物影响材料整体的耐腐蚀性。Bumel et等人观测到回火诱导析出的二次碳化物和氮化物导致的腐蚀行为的变化，类似于目前的材料的行为。从贫铬理论提出了马氏体不锈钢。然而，回火参数的最大腐蚀取决于铬原子的扩散。在实验中钢显示在另一方面，温度约为540C.最大的化学攻击的腐蚀试验，相应的原子探针断层扫描数据表明没有贫铬区存在
13、回火后550C。在575C情况下的回火会导致极大的耗尽，如图6b。回火温度的变换主要会引起回火时间2小时的不同和再次可能引起温度测量的不精确。然而，耗尽区可以由原子探针断层扫描，结果清楚地测量证实了强烈腐蚀的颗粒/基体在相应的TEM照片界面如图7所示 。同样大小的贫铬区对应于10nm的小氮化物和50大纳米碳化物是相反的理论。据预测，较小的颗粒具有更小的耗尽区 2，5 。这种差异可以解释关于粒子的铬含量，如图6a中所示。小的氮化物中高铬含量必然导致更大程度上的耗尽区相比，虚构的同样大小的低铬量的碳化物。这种塑料钢回火过程中析出的铬含量的检测与著名的析出动力学不一致。碳化物转变为直到形成均衡的。这
14、种发展是随着析出的铬含量不断增加的。在图 6a中呈现增加的铬含量的合金从5075在从500到550C的回火，回火温度进一步提高，在导致百分率下降范围在55到60%的铬含量。该组合物的发展的颗粒晶体的详细调查的解释是必需的，将在未来的研究。5、结论在回火过程中，改良后的塑料模具钢X38CrMo16马氏体不锈钢的腐蚀行为是与微观组织变化相关的。1 在1020C下奥氏体化30分钟后，再对改良后的X38CrMo16钢回火两次，且每次两小时后，最小的耐蚀性会在回火温度为540C时出现。2 回火在500的温度已经导致了不规则形状的纳米颗粒的约50%的高铬含量。回火温度升到到550C使铬富集到75%以上，最
15、高的测量值内的碳化物析出物。3 高含量的铬的析出物并不一定导致贫铬区在矩阵/粒子界面中。回火在550C导致铬含量为75%的颗粒，但没有耗尽区发生，而约60%的铬碳化物明显耗尽区的界面处的575C回火后。参考文献1.Roberts, G. A.; Krauss, G.; Kennedy, R.: Tool Steels, 5th ed., ASM International, Materials Park, OH, 19982.Bumel, A.; Carl, C.: Zusammenhang zwischen Anlabehandlung und Korrosionsverhalten von
16、 hrtbaren nichtrostenden Chromsthlen, Archiv Fr Das Eisenhttenwesen, 1961, vol. 32,pp.2372493.Miller, M. K.; Cerezo, A.; Hetherington, M. G.; Smith, G. D. W.: Atom Probe Field Ion Microscopy, Clarendon Press, Oxford University Press, Oxford New York, 19964.Hellman, O. C.; Vandenbroucke, J. A.; Rsing
17、, J.; Isheim, D.; Seidman, D. N.: Analysis of Three-dimensional Atom-probe Data by the Proximity Histogram, Microscopy and Microanalysis, 2000, vol.6,pp.4374445.Berns, H.; Ehrhardt, R.: Carbon or nitrogen alloyed quenched and tempered stainless steelsa comparative study, Steel Research,1996, vol.67,
18、 pp.343349Influence of Tempering Temperature on the Corrosion Behaviour of Plastic Mould Steel X38CrMo16ChristophLerchbacher1,SilviaZinner2andHaraldLeitner1AbstractThe modified stainless martensitic chromium steel X38CrMo16 offers excellent toughness, corrosion and wear resistance. The typical heat
19、treatment of this steel consists of a hardening treatment followed by a more-step tempering procedure. Tempering causes the precipitation of secondary carbides, nitrides and carboni-trides which are responsible for the improved hardness at elevated temperatures. Experiments have shown that the behav
20、iour in corrosion resistance varies within the tem-perature range commonly used for tempering this steel.The corrosion resistance reached a minimum at 540C until it increases again. For the microstructural investiga-tions the samples have been tempered for shorter times and the minimum moved to 575C
21、. At that temperature,significant chromium depletion zones in the size range of approximately 6nm extent could be visualized by means of atom probe tomography. The chromium concentration decreased to 10 at% within these zones. Depletion zone could be shown for small nitrides as well as for larger ca
22、rbides which were formed during tempering. The exist-ence of these local anodic zones could be confirmed by TEM investigations.Keywords:Martensitic Stainless Steel, Plastic Mould Steel, X38CrMo16, Corrosion, Atom Probe Tomography1.IntroductionHardenable martensitic stainless chromium steels are main
23、ly used for plastic mould applications or for die cas-ting applications. The processing of chemically aggressive plastics demands special properties such as high corrosion resistance, well-balanced toughness and hardness, wear-resistance and good machinability and polishability. These properties are
24、 gained by a well-defined composition,high micro-cleanness and a well-defined heat treatment.The latter consists of an austenitisation with subsequent hardening followed by a multi-step tempering proce-dure. Hardening produces a relatively brittle martensitic microstructure with embedded carbides wh
25、ich were not dissolved during austenitisation and amounts of retained austenite. Tempering predominantly causes relaxation of the martensite and secondary hardening, where the requi-red hardness can be controlled by the tempering tempe-rature 1. In order to provide well-balanced hardness and toughne
26、ss properties, the typical tempering temperatures range quite on the right hand of the maximum hardening peak in the tempering diagram. However, several investi-gations showed that the corrosion behaviour of marten-sitic stainless chromium steels strongly depends on the tempering temperature which m
27、eans that the choice of required hardness is restricted by the corrosion properties.Chromium depleted zones at the particle/matrix interface which vary with tempering temperature are responsible for the corrosion behaviour in that steels 2.The aim of the present work is to clearly identify the mecha
28、nism and to evidence the chromium depletion zones which are responsible for this collapse by means of high resolution techniques.2. Experimental表1 改良的塑料模具钢X38CrMo16的定义成分The composition of the modified X38CrMo16 can be found in Table1. The heat treatment has been performed on soft-annealed cylindrica
29、l samples with a length of 15mm and a diameter of 5mm using a quenching dilatometer Dil 805A from Bhr Thermoanalyse GmbH. Austenitisation has been performed at 1,020 C for 30 minutes. Afterwards,the material has been quenched with cooling parameter set to 0.5 (6K/s). Subsequently, the samples have b
30、een tempered for 2h at 500, 525, 550, 575 and 600C.Salt spray tests, immersion tests and pitting-potential tests have been performed in order to evaluate the cor-rosion behaviour. Salt spray tests have been done for 6h following DIN EN ISO 9227. Immersion tests have been carried out in 20 % boiling
31、acetic acid with a dwell time of 24h. Pitting potentials have been recorded at a current density of 1104A/cm 2. The samples used for corrosion investigations have been tempered two times for 2h.TEM investigations have been performed on a Tecnai F20. Samples have been electropolished with etchant A2
32、on a Struers Tenupol 5.Bars of the dimension 0.30.315mm have been cut from the heat treated samples for the atom probe sample preparation. Tips have been prepared by a standard two-step electrochemical polishing procedure 3. The mea-surements have been carried out using laser mode on a LEAP 3000X HR
33、 from Cameca at a temperature of 60K and with laser pulse energy of 0.6nJ. The reconstruction of the probed volumes and the evaluation of the data sets have been carried out with the software package IVAS 3.4.3.from Cameca.3. Results3.1 Corrosion ResistanceFigure 1 shows the tempering diagram of the
34、 modified martensitic stainless steel X38CrMo16. Additionally, corro-sion potentials in the interesting tempering temperature range are displayed. The diagram depicts the collapse in corrosion resistance at a temperature of approximately 540C. Figure 2a, b show the results of the salt spray tests af
35、ter a dwell time of 6 hours on the samples tempered at 540 C which represents the minimum in the diagram.(Fig.1) and 600C, respectively. A significantly higher che-mical attack occurs in the sample with the lower tempering temperature. This impression is confirmed by the results of the corresponding
36、 immersion tests, showing mass loss rates of 73 and 15g/m2h for the samples tempered at 540 and 600C, respectively.Fig1:Tempering diagram and corresponding pitting corrosion poten-tials of the investigated X38CrMo16Fig.2:Appearance of the corrosion attack after salt-spray testing for the sample temp
37、e-red at a) 540C; b) 600CFig.3: SEM micrograph of the sample quenched with =0.5 (a); Visualization of the carbides of the corresponding micrograph (b)3.2 MicrostructureHigh resolution microscopy has been carried out in order to correlate the tempering temperature dependent cor-rosion behaviour with
38、microstructural evolution during tempering.3.2.1 As quenched conditionFigure 3a shows a representative SEM micrograph of the sample in the as-quenched condition. Dark spherical par-ticles with sizes up to 2 m can be seen within the mar-tensitic structure. EDX measurements revealed the dark phase to
39、correspond to chromium carbides.The dark phase has been evaluated for obtaining an average phase frac-tion as given in Fig. 3b. Quantitative phase analysis has been performed by means of image analysis software as demonstrated in Fig.3b. The volume fraction of chromium carbides is approximately 1% i
40、n the hardened condition.The TEM micrograph given in Fig.4 corresponds to the as-quenched condition and shows a significant white shell surrounding the undissolved particles, indicating strongly etched regions which have been produced during electropolishing.Fig.4:TEM micrograph of the as-quenched s
41、ampleFig.5:Carbon atom map and corresponding 22 at% chromium iso-surfaces of the sample tempered for 2hours at a) 500C; b) 525C; c) 550C;d) 575C; e) 600CFig.6:Proximity histograms showing chromium corresponding to the 22 at% chromium iso-concentration surfaces of the differently temperedsamples (a);
42、 Detail of the interface region (b)3.2.2 TemperingCorrosion behaviour and TEM investigations give rise to the assumption that local compositional variations within the microstructure occur during tempering. Atomprobe tomography is known to be a very useful tool to resolve such concentration variatio
43、ns within fine structu-res. Figure5 shows the carbon atom maps of the probed volumes corresponding to the samples tempered at different temperatures for 2hours, respectively. The formation of secondary hardening particles can be seen. For a satisfactory visualization of the formed precipitates withi
44、nthe probed volumes 22 at% chromium iso-concentration surfaces are shown in all cases in addition to the carbonatom maps. Details of this evaluation method can befound elsewhere 4.The carbon atom map and chromium iso-surfaces of the sample tempered at 500C for 2hours (Fig.5a) show a high density of
45、irregular shaped fine carbon and chromium enriched features. Additionally, an approximately 50 nm large carbon enriched particle could be detected.Increasing the tempering temperature to 525 C leads to the formation of more spherical carbon and chromium enriched particles as can be seen in Fig. 5b.
46、The probed volume of the sample tempered at 550C (Fig.5c) qualitatively shows that the particles become more spherical and that the number density of these particles decreases compared to the samples tempered at lower temperatures. The most significant change in the appearance of the particles occur
47、s after tempering at 575C as illustrated in Fig.5d.The size of the detected carbon and chromium enriched precipitates is significantly higher compared to the lower tempering temperatures. Additionally, nitrogen and chromium enriched particles with a size less than 10 nm can be found. Tempering at 60
48、0 C seems to further increase the size of the carbon and chromium enriched particles (Fig. 5e). Nitrogen enriched particles were not detected within the probed volume. Although the probed volumes are small, the classical precipitation kinetics during tempering including growth and coarsening are obvious.Figure 6a shows the proximity histogram recorded by creating 22 at% chromium iso-concentration surfaces.The chromium concentration profiles through the particle/matrix interface corresponding to the detected car