Effect of rare earth oxides on the morphology of carbides in hardfacing metal of high chromium cast iron

The flux cored wires with different amounts of rare earth (RE) oxides additions for hardfacing (harden-face-welding) the workpieces of high chromium cast iron were studied in this work.The morphology of carbides in hardfacing metal was observed,and the type of the carbides was determined by optical microscopy,scanning electron microscopy (SEM),energy dispersive spectrometer (EDS) and X-ray diffraction (XRD).Based on the data of effect of RE on carbides morphology,the refined reason for carbide by RE oxide was discussed with the misfit theory.The results showed that,the microstructure of hardfacing metal was composed of martensite,residual austenite and M7C3 carbides.With the increasing amount of RE oxide additions,the volume fraction and roundness of the carbides were increased,however,the area and perimeter of carbides were decreased.It indicated that carbides in hardfacing metal could be refined and spheroidized by adding RE oxides in flux cored wires.     

Measurement of Residual Stress Field of Hardfacing Metal with RE Oxide and Its Numerical Simulation

The temperature and residual stress fields of a medium-high carbon steel, welded by a cracking resistance electrode with rare earth (RE) oxide, were measured by thermo-vision analyzer and X-ray stress analyzer respectively. Meanwhile, the martensitic transformation temperatures of matrix, hard-face welding (hardfacing) metal welded by conventional hardfacing electrode and that welded by cracking resistance electrode with RE oxide were determined. According to the experimental data and the thermo-physical, mechanical parameters of materials, finite element method (FEM) of temperature and stress fields was established. In this FEM, the effect of martensitic transformation on residual stress of hardfacing metal of medium-high carbon steel was taken into account. The results show that, by adding RE oxide in the coat of hardfacing electrode, the martensitic transformation temperature can be decreased, so that the residual tensile stress on the dangerous position can be decreased. Therefore, the cracking resistance of hardfacing metal can be improved.     

Mechanism of Cracking Resistance of Hardfacing Specimens of Steel 5CrNiMo Improved by Rare Earth Oxide

The cracking morphology of the hardfacing specimens taken from steel 5CrNiMo was observed. Meanwhile, the residual stress fields were measured and simulated. Based on experiment mentioned above, the improved structure and modified inclusion in hardfacing metal with rare earth （RE） oxide were analyzed. The results show that, the hardfacing crack is initiated from the coarse dendritic crystal grain boundary, inclusions and coarse austenite grain boundary in the HAZ and propagated by the residual stress existing in the center of the hardfacing metal and HAZ. The primary columnar grain structure can be refined by adding RE oxide in the coating of the electrode. The inclusion in the hardfacing metal can be modified as well. Meanwhile, if the martensite transformation temperature is decreased, the largest value of the residual tensile stress in the dangerous region can be reduced.     

变形参数对低碳微合金钢奥氏体相变规律的影响

利用Gleeble-3800热/力模拟试验机,研究了1种低碳微合金钢的形变奥氏体连续冷却转变行为以及终轧温度、终轧变形量对相变组织形态的影响。结果表明,实验钢在研究的冷速范围内均可得到一定量的贝氏体组织。随着冷速的增加,冷速达到50℃/s时,贝氏体组织由粒状贝氏体逐渐过渡到板条状贝氏体。在较快的冷速下,随终轧温度的降低或终轧变形量的减小,贝氏体转变开始与结束温度均降低,获得的贝氏体具有不同的组织形态。     

In-situ observation on austenite grain growth of a Nb microalloyed high-carbon steel

It was reported in previous studies that the growth of austenite was inhibited by the pinning effect of Nb containing precipitates and the solute dragging effect of solute Nb. The effect of Nb on austenite grain growth of high carbon steel was investigated by laser scanning confocal microscope (LSCM). Microstructure evolution during heating process of the tested steel was observed by in situ observation. The results show that even without the pinning effect of Nb containing precipitates (at high temperatures), Nb can hinder the growth of austenite grains due to the solute dragging effect of Nb. Two models were used to fit the austenite grain growth process, and the Beck growth models of Nb microalloyed high carbon steels at different heating temperatures were established. The austenite grain growth kinetics model considering the influence of heating temperature and holding time can accurately predict the austenite grain growth process of Nb microalloyed high carbon steels.     

析出粒子对钛微合金化高强钢奥氏体晶粒长大的影响

通过析出粒子与奥氏体晶粒尺寸的定量关系,建立奥氏体晶粒长大模型,计算TiN和TiC析出粒子共同作用下钛微合金化钢奥氏体晶粒尺寸.根据析出相质点理论计算结果表明:随着加热温度的升高,析出粒子体积分数逐渐减少,粒子半径逐渐增大,TiC粒子强烈阻止奥氏体晶粒长大,TiN粒子对奥氏体晶粒长大钉扎效果一般.采用实验测试手段测量不同加热温度下保温30 min后实验钢的奥氏体晶粒尺寸,与理论计算结果吻合较好.     

钢中加入稀土对渗碳层的影响

研究了低碳钢中加入ＲＥ后对渗碳层的影响。实验结果表明，钢中加入ＲＥ以后，渗碳的有效硬化层增加，在渗碳过程中显示出钢中ＲＥ具有催渗作用。     

Effects of rare earth oxide on hardfacing metal microstructure of medium carbon steel and its refinement mechanism

The electrodes for hardfacing medium carbon steel with six additions of rare earth oxide were developed in this work. By means of optical microscopy and scanning electron microscopy, the microstructure, inclusion and the fractograph of the hardfacing metal were observed. Then, the effects of rare earth oxide on microstructure and inclusions in hardfacing metal were analyzed. The effectiveness of rare earth oxide as heterogeneous nuclei of δ-Fe was calculated with the misfit theory. The results showed that, the microstructure of hardfacing metal was composed of ferrite and small amount of pearlite. The microstructure was refined at first and then coarsened with the increase of rare earth oxide addition. The fractograph was changed from brittle to equiaxed dimples, then became quasi-cleavage and cleavage gradually. The calculated results showed that, the ferrite grain size could be refined because that LaAlO3 as heterogeneous nuclei of δ-Fe was moderately effective, and the ferrite grain size was coarsened because the misfits between Ce2O3 and δ-Fe, Ce2O2S and δ-Fe were increased with futher increase of rare earth oxide addition.     

Effect of magnesium addition in low-carbon steel part 1: behaviour of austenite grain growth

To reveal the effect of Mg addition on the austenite grain growth in low-carbon steel, the steels containing different Mg contents were refined with a vacuum induction furnace. First, the steels were subjected to the temperature range of 1000–1300°C for a holding time of 30?min. Moreover, using a confocal scanning laser microscope, the growth of austenite grains was investigated under isothermal holding conditions (1400°C), and the γ–α phase transformation was also identified after the samples were subjected to a cooling rate of 5°C?s^?1. It reveals that the grain growth is inhibited by Mg addition after increasing the temperature to 1300°C. The kinetic equations of austenite grain growth were further established by regression analysis based on the experimental results. Furthermore, a significant increase in the proportion of intra-granular ferrite takes place in 0.0026%Mg-added steel at the initial stage of γ?→?α with a cooling rate of 5°C?s^?1. This is mainly attributed to the plenty of Mg-containing inclusions, which are demonstrated to be effective nuclei for acicular ferrite, being in the Mg-added steel.     

铌对高碳钢奥氏体向珠光体转变规律的影响

 铌可以显著改善钢的组织,提高钢的性能。同时,铌在钢中还存在偏聚现象,可有效提高钢的淬透性,使共析点右移。因此,利用金相法和等温热处理试验,对高碳钢的微观组织进行分析,发现铌的析出和固溶将细化晶粒,同时铌的加入将推迟高碳钢的珠光体转变,并使其孕育期延长。利用合金价电子结构理论,构建了不含铌和含铌的两种体系的奥氏体合金价电子晶胞,从电子层次研究铌对高碳钢奥氏体的影响规律。理论计算结果表明,两种模型在碳质量分数为0.86%时的最大共价电子对数nA分别为0.972 5、0.962 4。铌的加入会改变体系中键的组成,并与碳原子结合形成新的最强键C-Nb键。而NbC的析出阻碍了碳在奥氏体中的扩散,推迟了珠光体的形成和长大,进而推迟了珠光体相变,导致添加铌元素后的CCT曲线右移。     

合金元素对曲轴用非调质钢奥氏体长大和组织细化的影响

通过研究曲轴用非调质钢奥氏体晶粒长大行为和分析工业生产热轧材的显微组织,探讨合金元素Nb、Ti和S的组织细化作用.结果表明:添加质量分数0.027%Nb和0.012%Ti,同时S从0.029%提高到0.046%,加热时间30 min时,能够把奥氏体晶粒粗化温度提高100℃,并明显细化热轧材边部组织.弥散分布、钉扎在奥氏体晶界上的未溶第二相粒子MnS和(Nb,Ti)(C,N),能够有效抑制奥氏体晶粒的长大.在热加工过程中,合金元素的组织细化作用需要适当的变形制度予以匹配,才能得以体现.     

超声波功率对高碳钢中夹杂物的影响

研究了超声波功率对加稀土高碳钢中夹杂物的作用.结果表明:超声处理可以明显弥散、细化和去除加稀土高碳钢中的夹杂物;随着超声波功率的增加,钢中总氧含量明显降低,夹杂物的平均直径明显减小,夹杂物得到一定程度的去除,但是夹杂物的当量个数明显增多.超声波功率100 W时,高碳钢中总氧的质量分数为59×10^-6,钢中夹杂物的当量个数I为134 mm^-2,平均直径d为2.91μm,小于2.31μm的夹杂物占夹杂物总量的43%以上.     

Effect of oxide film structure of grain oriented silicon steel on depth of nitriding layer

Nitriding treatment after decarburization annealing is one of the key technologies to produce low temperature high permeability silicon steel. By controlling decarburization atmosphere, different oxide film structures were obtained; and then, the effect of oxide film structure on depth of nitriding layer with approximate nitrogen content was studied. The results show that oxide film is divided into two parts: the outer granular oxide film and the inner layered oxide film. As the dew point of decarburization atmosphere increases, the oxygen content of the decarburization plate increases, the thickness of the oxide film increases, the thickness ratio of layered oxide decreases, and the depth of nitriding layer increases. When the thickness ratio of layered oxide film is relatively high, ［N］ accumulates on the extreme surface of the sample, forms a unimodal distribution of ［N］ along the thickness direction, and the nitriding layer is relatively thin. When the layered oxide film ratio is relatively low, ［N］ crosses this layer and accumulates again in the subsurface; thus, in addition to the ［N］ peak on the extreme surface, one or more ［N］ peaks form in the subsurface, and the nitriding layer is relatively thick.     

取向硅钢氧化膜结构对渗氮层深度的影响

摘要：低温板坯加热技术生产取向硅钢,其关键点之一在于脱碳退火后要进行渗氮处理。通过控制脱碳气氛,获得不同的氧化膜结构;并进一步分析在近似渗氮量条件下,氧化膜结构对渗氮层深度的影响。结果表明：氧化膜分为外层颗粒状氧化膜和内层层片状氧化膜两部分。随着脱碳气氛露点增加,脱碳板O含量增加,氧化膜厚度增厚、层片状氧化膜占氧化膜总厚度的比例下降,渗氮层深度增加。当层片状氧化膜厚度占比较高时,［N］原子积聚在试样的极表层,形成氮沿厚度方向的单峰分布,渗层较浅。当层片状氧化膜厚度占比较低时,［N］原子会越过该层,而在次表层再次发生积聚;从而除了极表层的氮峰外,在次表层形成一个或多个氮峰,渗层较深。     

奥氏体组织对低碳中锰钢冷弯性能的影响

采用冷弯直径0～60 mm,弯曲角度180°,研究了20 mm厚度低碳中锰钢的冷弯性能,冷弯后外表均无可见裂纹,判定合格。利用光学显微镜、扫描电镜、透射电镜、电子背散射衍射(EBSD)、X射线衍射仪等手段分析了显微组织,尤其是奥氏体组织在冷弯过程中对冷弯性能的影响。结果表明,冷弯前显微组织由板条马氏体和奥氏体组成,其中原始奥氏体晶界明显;冷弯直径为0 mm变形后,样品弧顶部分奥氏体的体积分数由12.3%降至1.1%,维氏硬度由295HV1增至364HV1,晶粒尺寸由4.07μm增至4.30μm。主要原因是在冷弯过程在中奥氏体组织发生塑性变形,奥氏体晶界变形消失,沿冷弯方向呈拉伸带状组织形貌,冷弯形变时奥氏体发生TRIP效应显著。     

冷却条件对EH40钢板坯表层奥氏体晶粒长大的影响

铸坯表层异常长大的奥氏体晶粒是产生横裂纹的重要原因之一,研究冷却过程对其生长行为的影响对科学制定连铸工艺、降低铸坯裂纹敏感性有重要意义。采用原创连铸坯凝固过程热模拟方法,再现了EH40低碳船板钢板坯的凝固过程,观察在传统板坯连铸条件下,2种结晶器冷却强度对铸坯表层奥氏体晶粒长大行为的影响。结果表明,在结晶器冷却阶段,热模拟坯表层5 mm的绝大多数奥氏体晶粒短轴尺寸均不超过0.5 mm,但已出现粗大晶粒,且强冷条件下奥氏体晶粒尺寸平均值和极大值均更大,分别为弱冷条件下的2.5倍和2.0倍。在足辊区到矫直点区间,表层奥氏体晶粒生长非常缓慢,平均尺寸仍未超过0.5 mm。矫直点处,结晶器强冷热模拟坯表层20 mm的晶粒短轴最大尺寸为2.2 mm,为弱冷条件下的1.7倍。综上,奥氏体晶粒在连铸不同阶段表现为不同的生长行为,且采用结晶器弱冷更有利于EH40钢板坯获得相对细小的表层奥氏体晶粒。     

Effect of pickling process on removal of oxide layer on the surface of ferritic stainless steel

Oxide layer formed on stainless steel surface can result in a decrease in surface quality. Thus, it must be removed. In this paper, we investigated the effect of different pickling processes on removal of the oxide layer. Ferritic stainless steel was used as raw material. After it received annealing treatment, the components and structure of the oxide layer formed on its surface were studied. Then, the annealed stainless steel was pickled in hydrochloric acid solution system and mixed acid solution system, respectively. Results are as follows. The oxide layer on stainless steel surface has a thickness of 15–35?μm. With increase in hydrochloric acid concentration in hydrochloric acid pickling solution, the weight loss rate of stainless steel increases and is always greater than that in mixed acid pickling system. In addition, the surface of the stainless steel pickled in hydrochloric acid is smoother than that pickled in mixed acid. Furthermore, compared with channels in the oxide layer created during mixed acid pickling, those created during hydrochloric acid pickling are more, leading to shorter removal time. These results are important for increasing the pickling efficiency and improving the surface quality of stainless steel.     

低碳含铌钢粗晶奥氏体再结晶的数值模拟

建立了基于析出、回复和再结晶之间交互作用的低碳含铌钢组织预测模型,用于模拟低碳含铌钢热轧道次间隔期间粗晶奥氏体的再结晶及析出行为,并讨论了模型在绘制低碳含铌钢的再结晶-析出-时间-温度(RPTT)图中的应用.结果表明,模型对两种低碳含铌钢在热变形道次间隔期间内粗晶奥氏体的组织演变过程的模拟结果与测试数据符合较好,可以有效预测不同低碳含铌钢在不同工艺条件下的析出和再结晶行为.     

高碳钢盘条表面氧化铁皮剥离性能研究

摘要：在马弗炉和Gleeble 3800型热模拟试验机上对高碳钢H82BJ进行了不同条件的氧化试验,采用OM、SEM等手段对氧化铁皮厚度和结构进行表征,结果表明：高碳钢氧化铁皮一般呈FeO和Fe3O4两层结构,氧化铁皮厚度和FeO的比例随温度的升高而增加;并且,随着冷速的提高,氧化铁皮厚度和FeO中共析组织Fe3O4相的比例逐渐降低。通过调整实际生产中的温度和冷却速度,得到了适合机械剥壳的氧化铁皮厚度和结构。     

热处理工艺对超级马氏体钢逆变奥氏体的影响

 试验采用光学显微镜、X射线衍射仪（XRD）、透射电镜（TEM）及能谱分析（EDS）等手段,研究不同铬含量的超级马氏体不锈钢在相同热处理工艺下逆变奥氏体含量、组织形貌及生长规律的异同。结果表明,13Cr和15Cr两试验钢经淬火＋回火后的显微组织为回火马氏体和逆变奥氏体,两试验钢中逆变奥氏体含量及尺寸均随着回火温度升高先增加再减小,且在650～700 ℃时达到最大。两试验钢内的逆变奥氏体在回溶过程中会对基体组织产生细化作用。通过对比发现,15Cr钢中的逆变奥氏体含量更多,尺寸更大,回溶时对基体的细化作用更明显。