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便携式激光诱导击穿光谱法测定钢铁中多种元素含量
作者:苏明跃 杨金坤 孙 鑫 喻正宁 王庆祥 卜松涛 王 刚
苏明跃 杨金坤 孙 鑫 喻正宁 王庆祥 卜松涛 王 刚
Abstract The content of various elements in iron and steel was determined by portable laser-induced breakdown spectroscopy, and the performance of portable laser-induced breakdown spectroscopy was optimized. Based on the establishment of the interference algorithm model of induced content joint deduction, the content of carbon and other impurity elements in iron and steel was quantitatively analyzed, so as to provide technical support for on-site Customs inspection of iron and steel at ports. In this paper, different types of steel samples of carbon steel, medium and low alloy steel and stainless steel with uniform material are selected for 7 times of precision analysis, and the precision is good. In the correctness test with iron and steel reference materials, except that the results of some elements with content less than 0.06% are slightly different, most results are in good agreement with the marked value. The main reason is that the element content is low.
Keywords portable; laser-induced breakdown spectroscopy; iron and steel
钢铁中元素含量的测定方法有电感耦合等离子体法(ICP)、原子吸收光谱法(AAS)、X射线荧光光谱法(XRF)、直读光谱法[1-8]等。其中,AAS只能实现单一元素测定,ICP及光谱直读法虽能实现多元素的同时测定,但需要进行较为烦琐的样品前处理,检测周期长。这些技术方法都不适用于海关开展现场检测。便携式X射线荧光光谱法基于便携式X射线荧光光谱法的工作原理,虽适用于现场检测,但轻元素检测效果较差,且存在人体X射线辐射损伤风险。钢铁中的C元素作为钢铁性能的重要指标之一,是评价钢铁品质和用途的重要检测项目。但目前缺少能够在现场实现C元素检测的技术。
激光诱导击穿光谱技术(laser-induced breakdown spectroscopy,LiBS)是一种基于原子发射光谱原理的技术[9]。通过解析等离子体光谱,根据原子光谱和离子光谱的波长对应元素的不同,将光谱信号强度与对应元素的含量进行量化计算,并结合定量分析模型,得到分析样品成分的类别和含量信息。LiBS技术能实现多种元素同时检测,近年来被广泛使用。但其大型化无法实现现场检测。便携式激光诱导击穿光谱仪又称便携式LiBS,与大型LiBS具有相同的工作原理,但与大型LiBS的不同在于,具有便携性及小巧性,对于较难实现现场检测的样品分析位置,无需样品预处理,能够直接快速、多元素现场探测。便携式LiBS的锥形探头可覆盖更多拐角、接头和狭窄焊接区域,可以在狭窄复杂空间中操纵大型设备难以执行的分析任务。但早期的便携式LiBS受限于激光器及电源体积难以小型化,因此便携式LiBS测量精度较大型LiBS差[10]。现今便携式LiBS一直在对传统技术进行革新及突破[11-13],随着小型超高频脉冲激光器、复合型Czerny-Turner光谱仪光学系统、科研级CMOS传感器、内置式气体室等先进技术的应用,有效解决了早期便携式LiBS设备光谱稳定性差、元素激发难度大等问题,大大提升了小型化便携式设备定量分析的准确度和精密度。已有相关报道[14]利用便携式LiBS测定了Mn元素的定量分析方法,但对于多种元素的同时定量测定还未见报道。
本文应用便携式LiBS设备,确定仪器工作条件,建立了碳素钢、中低合金钢、不锈钢等钢铁产品的多元素快速检测方法,能够实现对C、Si、Mn、Cr、Ni、Mo、Cu、V、Co、Al、Ti元素的同时检测,为海关口岸开展钢铁的快速检测工作提供有力技术支撑。
1 实验部分
1.1 仪器和试剂
EXPEC 4100型手持式激光诱导击穿(LiBS)发射光谱仪(杭州谱育科技发展有限公司);W004534N型手持式抛光仪(WORKPRO有限公司)。
1.2 仪器工作条件
所有元素一次测定,设定测量条件。具体测量条件见表1。
表1 测量条件
Table 1 Measurement Condition
(kHz) | (μJ) | 探测器 | (L·min-1) | (nm) | (s) |
6 | 80 | CMOS | 0.2 | 190~680 | 1 |
1.3 实验方法
1.3.1 样品制备
LiBS分析过程近乎无损,为了获得准确的分析结果,要求样品表面应该平整清洁。在测量前,需要对样品表面进行适当处理,完成样品制备。有证标准样品、分析样品应在进行表面清洁处理后,需保证样品表面平整、洁净,无锈层、镀层及涂覆层,样品厚度应不小于3 mm,有效处理区域不小于1 cm2且无物理缺陷。对于需要进行表面处理的样品,可使用0.25 mm以下光谱磨样机或抛光机进行表面处理。
本方法主要在现场使用,为满足设备便携性,所用抛光设备亦为便携式。利用手持式打磨抛光笔对被测样品进行打磨抛光,除此以外,针对小样品或薄板等样品,用砂纸对表面进行打磨;对于表面有油污的样品,用酒精或其他易挥发有机溶剂对样品表面进行清洗。
1.3.2 工作曲线建立
按照在所选定的工作条件下,激发一系列光谱标准样品,每个样品至少激发3次,以每个待测元素相对强度的平均值对标准样品中该元素的浓度绘制校准曲线。每次测定前无需重新建立曲线,利用中间水平的标准样品验证工作曲线的有效性。若标准样品的检测结果超出允差范围,则利用标准化样品对工作曲线进行偏移校正或重新建立。
1.3.3 样品分析
仪器使用前应预先通电,进行开机预热,确定仪器各部件正常后使用。在相同的工作条件下进行样品分析,每个样品至少选择3个不同的点进行激发,取平均值。根据分析线对应的相对强度,从校准曲线上求出分析元素的含量。待测元素的分析结果,应在校准曲线所用的一系列标准样品的含量范围内。测定结果以质量分数(%)的形式表示,检测结果保留三位有效数字。
2 结果与讨论
2.1 诱导含量联合扣除干扰算法定量模型的建立
光谱的稳定性和基体效应是影响LiBS定量的重要问题。本方法在建立定量模型时考虑到钢铁成份的复杂性,采用诱导含量法,将基体当作未知参量,参与到定量模型的计算。但仅仅采用诱导含量法并不能得到良好线性,其主要原因在于除光谱稳定性问题外,便携LiBS检测钢铁的定量曲线光谱干扰同样会影响到定量分析。通常LiBS光谱干扰主要来自两个方面,一方面由于分辨率的原因导致相邻两峰无法完全分开,峰重叠,造成光谱干扰;另一方面由于样品中分析元素之外的其他元素含量的巨大差异造成基体效应。因此,在诱导含量算法的基础上需要扣除干扰,才能提高定量分析的准确性。本方法以诱导含量联合扣除干扰算法建立定量分析模型。先使用诱导含量法,采用I分/I内和Q分/Q内的关系绘制工作曲线,由测定的目标元素相对光强值x=I分/I内在曲线上求得诱导含量q,然后再根据内标元素的含量求出真正的百分含量Q。最后,结合扣除干扰算法进一步校正分析元素的相对光强值,得到线性极佳的工作曲线。
以中低合金钢中的Co 345为例,如图1所示,Co 345被邻近的Ni峰强烈干扰,两者的峰重叠在一起。将Ni峰扣除前后进行比对,由图2及图3可知,扣除干扰后R2达到0.99以上,扣除干扰后 的工作曲线明显比未扣除干扰前的 R 2 好。
图1 Co 345被临近的Ni峰干扰
Fig.1 Interference of Co 345 by adjacent Ni peak
2.2 激光重频的影响
激光重频即是激光器在一秒钟内产生的脉冲个数。例如7 kHz的激光器每次出光1 s,即会击打样品表面7000次,最后产生一张输出光谱。相同的激光能量条件,激光重频越高,则激发样品的时间间隔越短,单位时间样品接收激发次数越多,那么产生的等离子体的体积就会更大,同时烧蚀的样品也会更多。因此,激光重频越高,则理论上光谱的重复性会更好。如图4所示,展示了不同频率下一秒钟内产生的脉冲个数的输出光谱,很明显可以看出7 kHz的光谱曲线重频要优于3 kHz的。本方法激光器选择7 kHz。
2.3 气体氛围的选择
等离子产生在不同的气体氛围中,对等离子体的电子密度和粒子之间的相互碰撞都是有影响的。根据气体易得性及性价比角度考虑,本文选用氩气及空气作为气氛条件,对光强和光谱波动的影响进行研究。由图5及图6可知,充有氩气氛围的光谱分辨率和强度与空气气氛相比有了明显的提高,尤其是光谱强度可能提高数倍之多。因此本方法选用氩气气氛。
图4 不同频率下光谱的重复性比较
Fig.4 Comparison of spectral repeatability at different frequencies
图5 氩气气氛与空气气氛光谱分辨率比较
Fig.5 Comparison of spectral resolution between argon atmosphere and air atmosphere
图6 氩气气氛与空气气氛光谱强度比较
Fig.6 Comparison of spectral intensity between argon atmosphere and air atmosphere
2.4 精密度
选取材质均匀的钢铁样品,每个样品选取不同位置测定7次进行精密度分析,见表2。结果保留三位有效数字。结果表明,便携式LiBS法检测钢铁中多种元素,除C、Al、Cu元素部分样品RSD在10%以上外,其他结果良好。元素含量低,几近元素检测下限是RSD较高的主要原因。
表2 精密度实验
Table 2 Precision test
单位 (%)
样品 | 测量值 | 平均值 | RSD | |
1#中低合金钢 | C | 0.361,0.351,0.369,0.365,0.352,0.369,0.352 | 0.360 | 2.26 |
Si | 0.295,0.300,0.291,0.292,0.301,0.293,0.295 | 0.295 | 1.29 | |
Mn | 0.389,0.404,0.398,0.401,0.392,0.399,0.398 | 0.397 | 1.30 | |
Cr | 1.29,1.30,1.30,1.29,1.30,1.30,1.30 | 1.30 | 0.376 | |
Ni | 0.107,0.106,0.105,0.108,0.106,0.106,0.107 | 0.106 | 0.917 | |
Mo | 0.0664,0.0689,0.0723,0.0670,0.0670,0.0711,0.0710 | 0.0691 | 3.45 | |
Cu | 0.254,0.262,0.258,0.259,0.262,0.262,0.253 | 0.259 | 1.48 | |
V | 0.0616,0.0626,0.0627,0.0615,0.0627,0.0622,0.0617 | 0.0621 | 0.866 | |
Co | 0.0554,0.0494,0.0534,0.0532,0.0504,0.0532,0.0515 | 0.0524 | 3.90 | |
Al | 0.0894,0.0723,0.0829,0.0904,0.0773,0.0848,0.0829 | 0.0829 | 7.73 | |
Ti | 0.0418,0.0434,0.0444,0.0421,0.0429,0.0403,0.0439 | 0.0427 | 3.28 | |
2#中低合金钢 | C | 0.0223,0.0212,0.0262,0.0271,0.0289,0.0314,0.0334 | 0.0272 | 16.4 |
Si | 0.624,0.634,0.649,0.626,0.631,0.631,0.627 | 0.632 | 1.32 | |
Mn | 1.57,1.56,1.55,1.56,1.56,1.56,1.56 | 1.56 | 0.370 | |
Cr | 1.84,1.84,1.84,1.84,1.84,1.84,1.84 | 1.84 | 0.0205 | |
Ni | 0.120,0.126,0.122,0.125,0.123,0.127,0.128 | 0.124 | 2.21 | |
Mo | 0.883,0.908,0.926,0.921,0.901,0.894,0.899 | 0.905 | 1.66 | |
Cu | 0.0833,0.0871,0.0882,0.0848,0.0888,0.0884,0.0849 | 0.0865 | 2.49 | |
V | 0.0413,0.0369,0.0354,0.0422,0.0378,0.0398,0.0388 | 0.0389 | 6.20 | |
Co | 0.353,0.359,0.357,0.353,0.352,0.355,0.357 | 0.355 | 0.735 | |
Al | 0.0450,0.0371,0.0369,0.0394,0.0407,0.0404,0.0386 | 0.0397 | 6.93 | |
Ti | 0.0910,0.0760,0.0860,0.0793,0.0768,0.0743,0.0718 | 0.0793 | 8.60 | |
3#不锈钢 | C | 0.183,0.191,0.180,0.182,0.190,0.183,0.187 | 0.185 | 2.28 |
Si | 0.353,0.385,0.354,0.365,0.372,0.389,0.361 | 0.368 | 3.88 | |
Mn | 0.338,0.316,0.324,0.317,0.322,0.313,0.321 | 0.322 | 2.55 | |
Cr | 13.7,14.0,13.7,13.8,13.8,13.8,13.9 | 13.8 | 0.654 | |
Ni | 1.00,0.987,1.00,0.999,1.00,0.989,0.997 | 0.998 | 0.782 | |
Cu | 0.102,0.0970,0.101,0.103,0.0982,0.0990,0.102 | 0.100 | 2.28 | |
Co | 0.0384,0.0381,0.0404,0.0369,0.0410,0.0420,0.0412 | 0.0397 | 4.81 | |
Al | 0.0254,0.0194,0.0190,0.0222,0.0212,0.0163,0.0210 | 0.0206 | 13.8 | |
4#碳素钢 | C | 1.10,1.00,1.01,1.03,1.05,1.09,1.03 | 1.04 | 3.49 |
Si | 0.390,0.330,0.379,0.332,0.361,0.382,0.388 | 0.366 | 7.02 | |
Mn | 0.328,0.326,0.365,0.353,0.371,0.395,0.387 | 0.361 | 7.44 | |
Cr | 0.171,0.184,0.186,0.178,0.169,0.183,0.163 | 0.176 | 4.96 | |
Ni | 0.370,0.331,0.375,0.337,0.387,0.388,0.403 | 0.370 | 7.26 | |
Mo | 0.0534,0.0617,0.0703,0.0575,0.0690,0.0633,0.0594 | 0.0621 | 9.77 | |
Cu | 0.0690,0.0650,0.0552,0.0634,0.0524,0.0546,0.0604 | 0.0600 | 10.3 | |
V | 0.0582,0.0544,0.0630,0.0680,0.0507,0.0580,0.0631 | 0.0593 | 9.84 | |
Al | 0.0392,0.0230,0.0290,0.0418,0.0340,0.0466,0.0240 | 0.0339 | 26.7 | |
Ti | 0.156,0.171,0.162,0.133,0.152,0.162,0.172 | 0.158 | 8.36 |
2.5 方法正确度
选取不同水平及材质的标准样品4个,其中YSBS 11292-2003及YSBS 20120A-4-2001为低合金钢标准样品,YSBS 11380-2003为不锈钢标准样品,YSBS 13306-99为碳素钢标准样品。将其与标示值进行结果比对,见表3。结果表明,便携式LiBS检测钢铁标样中C、Si、Mn、Cr、Ni、Mo、Cu、V、Co、Al、Ti元素结果与标示值相比对,除部分含量在0.06%以下元素结果略有差异外,其他结果较吻合。含量低是结果有差异的主要原因。
3 结论
便携式LiBS可在狭窄复杂空间中操纵大型设备难以执行的分析任务,适合于钢铁产品,例如碳素钢、不锈钢及中低合金钢产品的现场分析检测。与同为便携设备的便携式X射线荧光光谱相比,又具有无辐射及能测量碳元素的优点。综上,本方法具有不需要样品制备、检测场景灵活、可多元素同时检测、测量速度快、非接触测量、无辐射伤害、无痕无损检测等诸多优点,有利于海关口岸的检测效率的提高。本文通过对便携式LiBS激光重频及气体氛围的性能进行优化,通过诱导含量联合扣除干扰算法除了基体效应,提高了光谱的稳定性。利用激光诱导击穿光谱法建立了钢铁中C、Si、Mn、Cr、Ni、Mo、Cu、V、Co、Al、Ti元素的分析方法,通过精密度、正确度验证,检测元素满足分析要求。
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图2 诱导含量法建立的Co曲线
Fig.2 Co curve established by induced content method
图3 诱导含量-扣除干扰法建立的Co曲线
Fig.3 Co curve established by induced content minus interference method
表2(续)
表3 正确度实验
Table 3 Correctness test
单位 (%)
标样编号 | C | Si | Mn | Cr | Ni | Mo | Cu | V | Co | Al | Ti | |
YSBS 11292-2003 | 检测值 (n=7) | 0.254 | 0.227 | 0.534 | 1.64 | 0.121 | 0.304 | 0.132 | 0.221 | / | 0.0101 | / |
标示值 | 0.262 | 0.236 | 0.545 | 1.66 | 0.125 | 0.297 | 0.145 | 0.224 | / | 0.016 | / | |
YSBS 20120A-4-2001 | 检测值 (n=7) | 0.0980 | 0.259 | 0.509 | 1.12 | 0.0490 | 0.266 | 0.0724 | 0.222 | / | 0.0254 | / |
标示值 | 0.109 | 0.247 | 0.527 | 1.04 | 0.064 | 0.269 | 0.095 | 0.203 | / | 0.037 | / | |
YSBS 11380-2003 | 检测值 (n=7) | 0.0383 | 0.728 | 1.15 | 16.9 | 12.2 | 2.70 | 0.0571 | 0.0273 | 0.0950 | 0.0393 | 0.0204 |
标示值 | 0.06 | 0.746 | 1.14 | 17.07 | 12.1 | 2.92 | 0.073 | 0.036 | 0.081 | 0.050 | 0.032 | |
YSBS 13306-99 | 检测值 (n=7) | 0.561 | 0.445 | 0.459 | 0.711 | / | 0.322 | / | 0.103 | / | / | 0.0120 |
标示值 | 0.59 | 0.47 | 0.47 | 0.74 | / | 0.32 | / | 0.12 | / | / | 0.008 |