助熔剂对碳酸钠校准高频燃烧红外吸收法测定钢铁中高含量碳的影响因素探讨

高频燃烧红外吸收法测定碳属于相对测量法,需用含碳的标准物质如碳酸钠对仪器进行校准,该校准方法的关键在于选择合适的助熔剂条件使化学试剂、钢铁样品中碳的燃烧释放率达到一致。实验重点对助熔剂影响碳酸钠校准-高频燃烧红外吸收法测定钢铁中高含量碳(质量分数为1.0%~5.0%)的因素进行了研究。实验表明,按GB/T 223.86—2009标准所述,将碳酸钠校准样置于锡囊中,采用锡-铁-钨三元助熔剂进行测定,碳酸钠中碳的结果较钢铁标样的结果偏低,且随碳含量的增加,偏低现象更为显著,据此可以判断,若采用锡-铁-钨三元助熔剂,用碳酸钠校准时,所测得的钢铁样品中碳结果会高于认定值。锡量、纯铁屑量、钨粒量的3因素3水平的正交试验表明,锡助熔剂对测定的影响最为显著,加入锡助熔剂对碳酸钠校准样测定不利,且锡助熔剂对碳酸钠校准样中碳测定的影响较生铁标样更为明显。因此,实验采用铁-钨二元助熔剂进行测定,并对助熔剂的条件进行了优化,结果表明碳酸钠校准的助熔剂条件为在样品上方依次加入1.00g纯铁粉、2.0g钨粒,钢铁样品的条件为在样品上方依次加入2.0g钨粒、0.50g纯铁粉。采用上述优化的助熔剂条件测定碳酸钠校准样,并绘制校准曲线,碳质量分数的校准范围为1.00%~5.00%,校准曲线的线性相关系数达到0.9999。采用优化的助熔剂条件对8个高碳钢铁标样中的碳含量进行测定,测定值与认定值吻合,这说明碳酸钠校准样与钢铁样品中碳的燃烧释放率一致。将上述实验方法应用于5个钢铁生产样品中碳的测定,相对标准偏差(n=5)为0.21%~0.33%,结果与管式炉燃烧-重量法基本一致。

Discussion on the influence factors of flux on determination of highcontent carbon in iron and steel by sodium carbonate calibration-highfrequency combustion infrared absorption method

The determination of carbon by high frequency combustion infrared absorption method belonged to relative measurement. The instrument should be calibrated with certified reference material containing carbon such as sodium carbonate. The key of this calibration method was to select appropriate flux conditions to make sure that the combustion release rate of carbon in chemical reagent and the sample of iron and steel was the same. The influence factors of flux on determination of high-content carbon in iron and steel (mass fraction of 1.0%-5.0%) by sodium carbonate calibration-high frequency combustion infrared absorption method was inveatigated in experiments. According to GB/T 223.86-2009, the calibration sample of sodium carbonate was placed in tin capsule. The tin-iron-tungsten ternary flux was adopted for determination. The results showed that the determination results of carbon in sodium carbonate were lower than that in certified reference material of iron and steel. Moreover, this phenomenon was more significant with the increase of carbon content. It was indicated that if the tin-iron-tungsten ternary flux was used, the determination results of carbon in iron and steel sample would be higher than the certified value when sodium carbonate was used for calibration. The orthogonal test (three factors and three levels including the amounts of tin, pure iron scrap and tungsten particle) showed that the flux of tin had most significant influence on the determination. The addition of tin flux was disadvantageous to the determination of sodium carbonate calibration sample. Moreover, the influence of tin flux on the determination of carbon in carbonate calibration sample was more obvious than that in certified reference material of pig iron. Therefore, the binary flux of iron and tungsten was used for determination and the flux conditions were optimized. The results indicated that the flux conditions in sodium carbonate calibration were listed as follows: 1.00g of pure iron powder and 2.0g of tungsten particle were added successively on the sample; the conditions for iron and steel sample were listed as follows: 2.0g of tungsten particle and 0.50g of pure iron powder were added successively on the sample. The calibration sample of sodium carbonate was determined under the above optimal flux conditions to plot the calibration curve. The calibration range for the mass fraction of carbon was 1.00%-5.00%, and the linear correlation coefficient of calibration curve was up to 0.9999. The content of carbon in eight certified reference materials of high-carbon iron and steel was determined under the optimal flux conditions. The results were consistent with the certified values, indicating that the combustion release rate of carbon in sodium carbonate calibration sample and the sample of iron and steel was consistent. The experimental method was applied for the determination of carbon in five production samples of iron and steel. The relative standard deviations (RSDs, n=5) were between 0.21% and 0.33%. The determination results were basically consistent with those obtained by tube furnace combustion-gravimetric method.