一种新的齿轮钢的高温临界区渗碳细化晶粒技术及其机理研究

Case hardening of gears is usually fulfilled by a carburizing process in which the gears normally are held isothermally at 870-980℃ for several hours in order to permit sufficient carbon penetrate and then diffuse into surface layer in a carbon atmosphere. Elevated carburizing temperature can accelerate the penetration and diffusion of carbon and therefore improve the efficiency and reduce energy consumption. The grains tend to be coarsened with the increasing carburizing temperature. The coarsened grains deteriorate the application properties of gears including both bending fatigue of center and contact fatigue of surface layer. In this research a novel mechanism has been proposed for grain refinement of gear steel at elevated carburizing temperature, so-called as high temperature intercritical carburization. The new designed alloy would not become full austenite at the highly carburizing temperature range 930-1100℃ by adding aluminium (approximate 0.5-2 wt%), a strong ferrite stabilizer, into the conventional gear steels, where proper fraction (supposing 5-20 vol%) of ferrite coexist between austenite grain boundaries. The thermodynamically stable ferrite could inhibit austenite coarsening effectively by interrupting the carbon diffusion between austenite grains, replacing the conventional grain refining mechanism in current gear steels by pining dislocation or sub grain boundaries by micro alloy carbides or nitrides. The grain coarsening due to dissolution of carbides or nitrides with temperature enhancing could be avoided effectively by the new intercritical carburization mechanism. The Ac3 temperature at the surface layer decreases with the carbon enrichment by carburizing, which therefore become fully austenitic gradually. The carburization of surface layer in the condition of full austenite might guarantee the carburizing efficiency and sufficient depth of carburized layer. The research would investigate the coarsening kinetics of the dual phase microstructure in the center, the carbon penetration and diffusion in the surface layer and its microstructure evolution during carburizing, as well as the phase transformation in the center, the carbon distribution and microstructure constitution of the surface layer after carburized. In addition, the effect of grain refined microstructure and carbon distribution on the mechanical properties would be discussed. In order to serve and guide the industrial application, the fundamental theory system of intercritical carburization would be established ultimately in this research.

提高渗碳温度是提高齿轮渗碳效率的最有效手段，传统齿轮钢提高渗碳温度会发生晶粒粗化，恶化性能，尤其是弯曲疲劳性能不足。故本研究提出一种提高齿轮钢渗碳温度的新的细化晶粒技术：临界区渗碳处理。通过在现有齿轮钢中加入强铁素体稳定元素Al（0.5-2wt%）提高合金全奥氏体化温度，当提高渗碳温度至930-1100℃时仍有少量（5-20vol%）铁素体与奥氏体共存，利用稳定的铁素体在奥氏体晶界处限制奥氏体晶粒长大，替代普通齿轮钢中依赖微合金碳氮化物质点钉轧位错或晶界抑制奥氏体长大的机制，从而避免高温渗碳中因碳氮化物溶解而导致的晶粒长大，有效实现心部组织晶粒细化。随渗碳过程表层碳含量升高，Ac3 温度降低，因而全奥氏体化，实现全奥氏体渗碳，保证渗碳效率和渗层深度。本研究将深入研究临界区渗碳过程的晶粒长大动力学、临界区渗碳对渗碳层碳分布、组织和性能的影响的机理以及渗碳后淬火过程中的相变行为。

渗碳温度是影响渗碳效率的主要因素，齿轮渗碳温度从930℃提高到1000℃，可缩短渗碳时间50%以上，但常规齿轮钢高温渗碳时奥氏体晶粒长大，会极大降低疲劳性能。 本研究在20CrMn和20CrMnTi中加入一定量的Al，Al作为强铁素体稳定元素，能够提高合金的完全奥氏体化温度，当渗碳温度提高至930℃-1200℃时依然有少量的铁素体和奥氏体两相共存，提出利用稳定的铁素体在奥氏体晶界处抑制奥氏体晶粒的长大，替代齿轮钢中依赖微合金碳氮化物质点钉扎位错或晶界抑制奥氏体长大机制，从而避免高温渗碳中因为碳氮化物溶解而导致晶粒长大，有效实现齿轮钢心部组织晶粒细化。报告主要研究内容如下：. (1) 研究Alloy0钢880℃和930℃保温5小时和10小时的伪渗碳工艺条件下的组织演化及晶粒细化，在930℃保温最终完全奥氏体化并淬火形成全马氏体的钢进行力学性能研究，其晶粒尺寸和力学性能均达到830℃渗碳热处理的常规20CrMn钢的条件。 .(2) 研究Alloy1和Alloy3钢中添加Al元素时，在930℃、970℃、990℃、1050℃分别保温2小时、5小时的伪渗碳处理工艺下，临界区热处理的组织演化及晶粒长大行为。其在930℃和990℃保温时，因两相共存，其晶粒尺寸远小于常规20CrMnTi钢；而在1050℃保温时，已全奥氏体化，其全奥氏体化后细化晶粒主要依靠微合金碳化物TiC钉扎晶界，随着TiC的溶解，对晶界的钉扎作用消失，奥氏体晶粒迅速长大粗化，其晶粒尺寸与常规20CrMnTi钢相当。 .(3) 研究Alloy2和Alloy4钢中添加Al元素时，在930℃、970℃、990℃、1050℃分别保温2小时、5小时的伪渗碳处理工艺下，临界区热处理的组织演化及晶粒长大行为。因较高Al的添加，其始终处于临界区，铁素体相以相界面的形式比微合金碳氮化物以点的形式钉扎晶界细化晶粒效果更显著，其晶粒尺寸远远小于常规20CrMnTi钢。.(4) 研究Alloy4钢在伪渗碳处理后1200℃一次淬火过程的组织演化，其在1200℃保温0.5~5小时均没有实现完全奥氏体化，少量铁素体始终存，存在的铁素体能够很好的抑制奥氏体晶粒的长大，1200℃保温5小时奥氏体晶粒平均尺寸仍小于100μm，对比常规微合金钢晶粒细化5倍以上。