高碳铬轴承钢中碳化物演变及贝氏体相变行为
发布时间：2018-07-18 14:04
【摘要】：轴承钢的组织均匀性及组织配比等对轴承的疲劳寿命有显著的影响,因此应保证轴承钢热加工和热处理过程中组织的均匀性及热稳定性 本文以高碳铬轴承钢为研究对象,基于热力学和动力学微观组织调控理论和控制方法,以控轧控冷和热处理关键工艺为核心,定性、定量分析碳化物演变过程,探讨马氏体/贝氏体相变行为,揭示不同工艺条件下碳化物的析出和溶解规律及其对马氏体/贝氏体相变行为的影响。主要研究内容和成果如下： 轧后冷却过程先共析网状碳化物的析出对GCr15轴承钢的质量存在不利的影响,形变工艺制度、冷却速率和合金元素的添加均会影响其析出行为。在奥氏体单相区施加一定的压缩变形后,冷却过程中网状碳化物析出温度相比于静态转变有所提高,CCT曲线向左移动。Mo元素的添加及SU Mn元素含量的提高,均使得相转变区间向右移动,提高了实验钢的淬透性,对网状碳化物的析出有一定的抑制作用。 热轧后的轴承钢经球化退火后室温组织为铁素体+球状碳化物,该组织和加热速率决定了后续加热过程奥氏体化行为。鉴于JMAK模型中动力学参数k对温度的敏感性,引入加热速率对该参数的影响。运用叠加法则分别建立了连续加热过程铁素体-奥氏体和碳化物溶解动力学模型,并对模型进行了验证。 基于界面局部平衡理论,运用Thermo-Calc热力学软件的TCFE7和DICTRA动力学软件的MOBFE数据库,对860℃保温过程碳化物的溶解行为进行宏观定量计算,分析元素微观扩散机理。结果表明：碳化物的溶解过程分为两个阶段,第一阶段为保温10min前的快速溶解,C元素在奥氏体内快速均匀分布,而Cr元素只是在界面处获得了局部平衡；第二阶段为随后的缓慢溶解,Cr元素在奥氏体和碳化物内发生再分配。从微观尺度来看,碳化物的溶解依靠奥氏体/碳化物界面处平衡的破坏及两相间元素的扩散。初期碳化物的溶解机制为C原子快速扩散控制的NPLE型；当奥氏体基体内C活度与界面侧达到一致时,转变为Cr原子缓慢扩散控制的PLE型。 通过分析淬火+回火热处理和贝氏体等温处理后的微观组织形貌,发现马氏体针之间互不平行,晶粒大小不均,内部含有高密度的孪晶。回火过程中,针内碳原子聚集到孪晶部位,以碳化物的形式析出。贝氏体等温相变的鼻尖温度在300～350℃,影响其转变动力学的因素有合金元素的均匀性、过冷奥氏体的C/Cr含量、保温温度和保温时间等。贝氏体的形成与碳化物的析出同时进行,贝氏体板条间呈一定的取向生长,低的相变温度促使碳化物细小弥散的分布。贝氏体等温处理后性能与回火马氏体相比,具有更高的强度和塑性,耐磨性相当。 针对贝氏体等温处理耗时长的特点,分别研究了预应变和预淬火处理对贝氏体相变动力学的影响。预应变形成的过冷奥氏体晶内亚结构和预淬火引起的组织变化,均提高了贝氏体相变初期的转变速率。但随相变进行,最终与常规工艺归于一致,对整个贝氏体相变过程的加速作用不明显。提出了一种两步处理方法,在250℃等温处理50min后升温到270℃保温5min。残留奥氏体含量为6%,相变时间为传统工艺的27%左右,显著缩短贝氏体相变时间的同时并未明显降低力学性能和磨损性能
[Abstract]:The microstructure uniformity and microstructure of bearing steel have significant influence on the fatigue life of the bearing , so the uniformity and thermal stability of the structure during hot working and heat treatment of bearing steel should be ensured .
In this paper , high carbon chromium bearing steel is used as the research object , based on the theory and control method of thermodynamic and dynamic microstructure regulation , the key technology of controlling rolling and cooling and heat treatment is adopted as the core , qualitative and quantitative analysis of the carbide evolution process , and the influence of the precipitation and dissolution of the carbide under different process conditions and its influence on the martensite / bainite transformation behavior are revealed . The main research contents and achievements are as follows :
After a certain compressive deformation is applied to the austenite single - phase region , the precipitation temperature of the net carbide in the cooling process is improved compared with the static transition , the CCT curve moves to the left . The addition of the Mo element and the increase of the content of the SU Mn element cause the phase transition region to move to the right , the hardenability of the experimental steel is improved , and the precipitation of the reticulated carbide is inhibited .
In view of the sensitivity of kinetic parameter k to temperature in the JMAK model , the influence of heating rate on the parameter is introduced . The dynamic model of ferrite - austenite and carbide dissolution in continuous heating process is established by superposition rule , and the model is verified .
Based on the local equilibrium theory of interface , using the MOBFE database of TCFE7 and DICTRA dynamic software of Thermo - Calc thermodynamic software , the dissolution behavior of carbides at 860 鈩，
本文编号：2132133
[Abstract]:The microstructure uniformity and microstructure of bearing steel have significant influence on the fatigue life of the bearing , so the uniformity and thermal stability of the structure during hot working and heat treatment of bearing steel should be ensured .
In this paper , high carbon chromium bearing steel is used as the research object , based on the theory and control method of thermodynamic and dynamic microstructure regulation , the key technology of controlling rolling and cooling and heat treatment is adopted as the core , qualitative and quantitative analysis of the carbide evolution process , and the influence of the precipitation and dissolution of the carbide under different process conditions and its influence on the martensite / bainite transformation behavior are revealed . The main research contents and achievements are as follows :
After a certain compressive deformation is applied to the austenite single - phase region , the precipitation temperature of the net carbide in the cooling process is improved compared with the static transition , the CCT curve moves to the left . The addition of the Mo element and the increase of the content of the SU Mn element cause the phase transition region to move to the right , the hardenability of the experimental steel is improved , and the precipitation of the reticulated carbide is inhibited .
In view of the sensitivity of kinetic parameter k to temperature in the JMAK model , the influence of heating rate on the parameter is introduced . The dynamic model of ferrite - austenite and carbide dissolution in continuous heating process is established by superposition rule , and the model is verified .
Based on the local equilibrium theory of interface , using the MOBFE database of TCFE7 and DICTRA dynamic software of Thermo - Calc thermodynamic software , the dissolution behavior of carbides at 860 鈩，
本文编号：2132133
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