高Cr镍基合金脱氮反应热力学与动力学研究

Thermodynamic and Kinetic Study on the Nitrogen-Removing Process of High Cr Content Nickel Based Alloy

摘要:

Cr作为抗热腐蚀性元素铬在高温合金中广泛添加，因其可增大氮元素溶解度，故高铬合金的冶金脱氮难度较大。合金氮含量升高可引发氮化物析出进而影响材料组织质量与力学性能，因此，非常必要澄清高Cr合金的脱氮行为与规律。本文以高Cr镍基合金为实验对象，对其在真空感应熔炼脱氮过程的热力学与动力学进行研究。结果表明，氮在高Cr镍基合金中的热力学平衡溶解度较高且主要受真空压力控制，为实现[N]≤10 ppm的纯净度指标，合金冶炼真空压力不可高于0.1Pa。脱氮反应初期氮含量迅速下降，但反应中后期需较长时间才可接近氮平衡溶解度，提高精炼温度有助于缩短脱氮平衡反应时间。动力学分析表明高Cr镍基合金脱氮符合二级反应规律，受界面化学反应过程控制，1500℃、1550℃和1600℃条件下的脱氮表观速率常数分别为0.0184m?s-1、0.0233 m?s-1和0.0397 m?s-1，表观活化能为211.4 kJ?mol-1。

Abstract:

Nitrogen is one of the undesirable impurity elements of superalloys and it can induce the precipitation of nitrides and the formation of microporosity to worsen the mechanical properties of material. With the increasingly high demands in superalloy component quality, nowadays the nitrogen content in superalloy has been recommended to be no higher than 10×10-6 wt.%. Chromium is an important beneficial alloying element widely employed in superalloys (with an addition up to 20 wt.%) for it can enhance the anti-corrosion performance of alloys at elevated temperature. However, chromium has a high affinity to nitrogen that it could be very hard to achieve a low nitrogen content in high-Cr alloys. Therefore, it is necessary to study the features of the nitrogen-removing process of high-Cr alloys. In this work, the nitrogen-removing process during the vacuum induction melting of a high-Cr nickel based alloy was investigated through thermodynamic calculation and kinetic experiments. Results show that high-Cr nickel based alloy had a high thermodynamic equilibrium solubility of nitrogen, and it was mainly dominated by the vacuum pressure. In order to achieve the goal of [N]≤10 ppm, the melting vacuum pressure should be no higher than 0.1 Pa. To study the kinetic characteristics of nitrogen-removing process, the melted metal was held at 1500, 1550 and 1600 ℃, and sampled at 0, 30, 60, 120, 180 and 240 min at 0.1 Pa after the complete melting of alloy, respectively. Results indicate that nitrogen content dramatically decreased at the beginning of melt holding, but in the medium and later stage it took a much longer period of time to get close to the equilibrium solubility of nitrogen. Kinetic data analysis shows that the nitrogen-removing process of high-Cr nickel based alloy can be classified as the second-order reaction, which reflects that the process was controlled by the chemical reaction on melt surface. The apparent rate constants of nitrogen-removing process at 1500, 1550 and 1600 ℃ were calculated to be 0.0184, 0.0233 and 0.0397 m?s-1 , respectively, and the apparent activation energy was determined to be 211.4 kJ?mol-1.