回火马氏体中合金碳化物的3D原子探针表征Ⅰ.形核

Nb-V微合金钢在1200℃固溶0.5 h后淬火,在450℃回火不同时间,用三维原子探针(3DAP)研究了回火过程中合金碳化物的形核规律.结果显示,淬火态Nb-V微合金钢在450℃回火时合金碳化物处于形核阶段,合金元素可通过动态再分配,实现渗碳体到合金碳化物的原位转变,或者在位错等缺陷处直接与C结合,完成合金碳化物的单独成核长大,或者偏聚在残余奥氏体/基体和未溶的AlN粒子/基体界面处,实现合金碳化物的异质形核长大.     

EFFECTS OF RARE EARTH METALS ON MICROSTRUCTURAL TRANSFORMATION OF LOW OR MEDIUM CARBON Si-Mn-V STEELS DURING QUENCHING AND TEMPERING

The microstruetural transformation of steels:20SiMn2V,20SiMn2VRE,40SiMn2V and40SiMn2VRE during quenching and tempering have been examined by TEM,X-raydiffraction and dilatometer.It was shown that the addition of rare earth metals not only canrefine the austenite grains of the low or medium carbon steels and packet of lath martensiteand lath size,lower the M_s temperature,but can also raise the relative percentage of disloca-tion substructure of martensite in medium carbon steel,but there is little effect on volume frac-tion and thermal stability of retained austenite quenching and tempering structure of low ormedium carbon steels.The rare earth metals may remarkably inhibit the decomposition of lowcarbon martensite during low temperature tempering,retard the precipitation of cementiteplates in lath grains and delay the spheroidization of carbides.They may also restrain obvious-ly the precipitation and spheroidization of cementite in medium carbon martensite during hightemperature tempering.     

稀土元素对Si—Mn—V钢淬火,回火过程中组织结构转变的影响

采用透射电镜,X射线,膨胀仪等方法,对含稀土和不含稀土的4种低、中碳Si—Mn-V钢(20SiMn2V;20SiMn2VRE;40SiMn2V和40SiMn_2VRE)进行了淬火、回火过程中组织结构转变的对比试验,结果表明:稀土元素可以细化低、中碳钢的奥氏体晶粒度;降低M_s点;细化马氏体板条束和板条晶尺寸;并增加中碳钢马氏体中位错亚结构的相对比例。而对低、中碳钢淬火回火组织中残余奥氏体的体积和热稳定性影响甚微。稀土元素明显地抑制了低碳马氏体在低温回火过程中的分解,阻碍板条晶内片状渗碳体的析出,并推迟其长大球化过程。稀土元素也明显地抑制了中碳马氏体中高温回火过程中粒状渗碳体的析出和球化过程。     

马氏体温变形超细化过共析钢

利用Gleeble 1500热模拟试验机进行单轴热压缩实验, 研究了含Al和不含Al两种过共析钢马氏体温变形和等温回火过程中的组织超细球化演变及超细球化组织的力学性能. 结果表明: 与马氏体等温回火相比, 马氏体温变形加快马氏体的分解动力学, 在较短的时间 内即获得超细化 (α+θ)复相组织. 温变形过程中的组织超细化演变主要经历渗碳体粒子的析出与粗化及铁素体基体的动态回复和动态再结晶; 而在等温回火过程中, 铁素体主要发生静态回复和晶粒长大, 并没有再结晶现象发生. 合金元素Al的加入在等温回火和温变形过程中均抑制马氏体的分解, 阻碍渗碳体粒子的粗化和铁素体晶粒的长大, 导致复相组织的细化. 同时, Al的加入使马氏体温变形和等温回火后所得超细化 (α+θ) 复相组织在不降低总延伸率的前提下, 强度得以明显提高.     

55SiCr弹簧钢中的残留奥氏体与碳化物

通过扫描电镜(SEM)、电子背散射衍射(EBSD)、透射电镜(TEM)、X射线衍射仪(XRD)、热膨胀仪、洛氏硬度计等手段研究了弹簧钢55SiCr的组织和相变点以及残留奥氏体和碳化物在热处理过程中的组织演变。结果表明:55SiCr弹簧钢淬火后残留奥氏体以块状分布在基体上;随回火温度的升高,残留奥氏体减少并呈粒状和薄膜状分布;C在残留奥氏体中富集,使其稳定性增强;Si抑制了碳化物的析出,提高了残留奥氏体的稳定性。低温回火时,Si延缓了渗碳体析出;高温回火时,C原子扩散速率提高,促进渗碳体析出,引起体积的收缩。慢速加热回火时,C有足够的时间扩散,从而促进渗碳体的形成,使渗碳体的形成温度提前;快速加热回火时,C来不及扩散,抑制了渗碳体的析出。回火加热速率一样时,试验钢的硬度随回火温度的提高而下降。当回火温度为400 ℃时,硬度值最大为51 HRC;当回火温度为650 ℃时,硬度值最小为37 HRC。当加热速率为0.1 ℃/s时,硬度值最小为33 HRC;当加热速率为200 ℃/s时,硬度值最大为40 HRC。     

Decomposition of martensite by discontinuous-like precipitation reaction in an Fe–17Cr–0.5C alloy

The mechanism of martensite decomposition and the kinetics of carbide precipitation have been studied in an Fe–17 wt% Cr–0.55 wt% C alloy. The morphology of carbide precipitates formed within the decomposed regions and the crystallography of their formation were examined by means of transmission electron microscopy after tempering at 735°C for various times. The martensite decomposition starts within less than 10 s, but it is not completed even after 10 min. The reaction initiates with the nucleation of fine cementite particles preferentially at the prior austenite grain boundaries and occasionally at the martensite lath boundaries. Cementite particles are related to the ferritic matrix with the Bagaryatsky orientation relationship. The decomposition of martensite proceeds heterogeneously by the migration of a reaction front. Various carbide morphologies were observed in the region close to the reaction front: rod-like, spherical or lamellae. The kinetics of martensite decomposition changes from carbon diffusion controlled to chromium diffusion controlled. After long time tempering, the alloy carbides, M₂₃C₆ and M₇C₃, precipitate at the reaction front. The M₂₃C₆ carbides are related with respect to the ferrite by the Kurdjumov–Sachs orientation relationship. Two specific orientation relationships were found between the M₇C₃ carbide and the ferrite, which are related to each other by a rotation of 30° about their common axis of [0001]_h//[110]_α. One of them has previously been reported. The specific features of discontinuous-like precipitation in martensite are discussed and are attributed to the presence of carbon and chromium atoms, which have different mobilities. The driving forces for diffusion of carbon and chromium were qualitatively determined with the software and database ThermoCalc by assuming local equilibrium at the moving interfaces.     

Three-dimensional observations of proeutectoid cementite precipitates at short isothermal transformation times

Three-dimensional (3D) analysis techniques were used to examine a model Fe–1.34% C–13.0% Mn alloy to reveal connectivities of proeutectoid cementite precipitates whose growth was arrested at an early stage. The present results indicate that grain boundary cementite precipitates nucleate at austenite grain boundary edges and corners, then grow and spread on the grain boundary faces. No cementite precipitates were found to connect solely to austenite twin boundaries, which appear to act as barriers to precipitate growth rather than as potential nucleation sites. Cementite precipitates were all connected to austenite grain boundaries or cementite grain boundary precipitates, confirming 3D observations made earlier on a specimen transformed at a longer isothermal transformation time. Widmanstätten lath precipitates appear to emanate only from grain boundary cementite precipitates. While edges of several Widmanstätten plate precipitates were observed to intersect with areas of ‘clean’ austenite grain boundary, they may or may not nucleate directly on austenite grain boundaries.     

Characteristics of the Transformation of Retained Austenite in Tempered Austempered Ductile Iron

Controlling the amount of retained austenite is a concern in austempered ductile iron formation. Retained austenite has a strong influence on austempered ductile iron properties, such as hardness and wear resistance. In this research, the characteristics of the transformation of retained austenite were investigated as a function of the number of tempering cycles. The hardness of the austempered ductile iron samples was measured, and the specific amount of retained austenite was analyzed by x-ray diffraction (XRD). Wear tests were conducted on a ball-on-flat sliding fixture. The tempering process was found to have no effect on the hardness of the austempered ductile iron samples. This may be due to retained austenite being partially converted into brittle quenched martensite during the tempering process. However, tougher tempered martensite was also formed from existing martensite. The two effects seemed to offset each other, and no significant differences occurred in overall hardness. XRD analysis showed that under the same austempering temperature and holding time, the amount of retained austenite decreased with additional tempering cycles. Also, with the same holding time and tempering cycles, less retained austenite was contained in the matrix at higher austempering temperatures. This was due to more high carbon content austenite and needle-like ferrite being present in the austempered ductile iron matrix. In addition, tempered austempered ductile iron exhibited significantly higher wear resistance as compared to traditionally treated ductile iron.     

回火温度对1100 MPa钢组织和性能的影响

采用低碳+Nb、Ti、B、Cr、Mo、Ni成分体系,利用SEM、TEM等研究不同回火温度下试验钢的组织和力学性能变化。结果表明:220 ℃回火,马氏体发生分解,马氏体板条束变粗,束内板条逐渐合并,残留奥氏体逐渐分解,位错密度大幅度下降,淬火带来的内应力得到释放,试验钢具有最佳的综合力学性能。300 ℃回火,碳化物在奥氏体晶界或回火马氏体板条间大量聚集、长大,降低了晶界、板条间的结合力,出现了回火脆性。     

Correlations of Ni Contents,Formation of Reversed Austenite and Toughness for Ni-Containing Cryogenic Steels

It has been widely demonstrated that addition of Ni in low-carbon steels can effectively improve the cryogenic toughness,but the mechanism behind it has yet to be clarified.In the present work,the evolutions of microstructure and mechanical properties after quenching and tempering for Ni-containing cryogenic steels with different Ni contents(3.5-9 wt%) were investigated.The results showed that after quenching and tempering,the Ni-containing cryogenic steels were composed of tempered martensite and reversed austenite.The volume fraction of reversed austenite has increased from 0 up to 6.3%when the Ni content increases from 3.5%to 9%.The Charpy impact tests indicated that the lowtemperature toughness was markedly improved with the increase in Ni content,which can be correlated with the increase in reversed austenite amount.The main contribution of reversed austenite to the toughness lies in:(1) the elimination of cementite precipitates improved the plastic deformation capacity of matrix,and(2) the crack propagation is hindered through plastic deformation.     

关于60Si2MnA钢奥氏体中温转变的研究

60Si2MnA钢过冷奥氏体的中温转变产物为贝氏体铁素体和残余奥氏体,贝氏体铁索体的形成与自催化效应密切相关。在回火过程中,贝氏体中的奥氏体以扩散转变方式分解为铁素体和渗碳体。从而说明奥氏体的等温转变与回火转变是本质不同的转变。     

Effect of prior hot rolling on the microstructures and mechanical properties of duplex stainless steel containing tempered martensite and ferrite

A duplex structure of δ-ferrite and lath martensite with interlath retained austenite film is developed in this study by modifying the alloy addition. The presence of δ-ferrite can further strengthen the grain refinement of austenite during hot rolling. As a result, the amount of retained austenite is enhanced. Tempered martensite embrittlement occurs due to the decomposition of retained austenite, and grain refinement can in fact ameliorate the tempered martensite embrittlement by delaying the onset of the embrittlement to a higher temperature. The combined effect of uniform and small grains as well as a large amount of retained austenite provides a further increase in the mechanical properties. After identical tempering treatments, all mechanical properties measured in the as-rolled condition were found to be higher than those of direct quenching without rolling. After hot rolling, the increase in the hardness and tensile strength was not accompanied by a drop in the ductility and toughness.     

Effect of partitioning of Mn and Si on the growth kinetics of cementite in tempered Fe–0.6 mass% C martensite

The effects of Mn and Si addition on the growth rate of cementite in Fe–0.6 mass% C martensite have been studied by means of scanning electron microscopy, transmission electron microscopy and a three-dimensional atom probe. The growth rate of the cementite during tempering at 723 K decreases substantially with the addition of Si due to the redistribution of Si between the cementite and ferrite matrix. Mn retards cementite coarsening more effectively than Si at 923 K. In tempering at both 723 and 923 K, the Si concentration in the cementite starts to decrease from an early stage of precipitation, whereas the cementite develops initially without the redistribution of Mn, before the Mn gradually enriches into the cementite during tempering. Calculations of phase boundaries for stable equilibrium (partition local equilibrium) and metastable equilibria (para and negligible-partition local equilibriums) have revealed that there is a sufficient driving force for the formation of paracementite in the Mn-added alloys. On the other hand, paracementite is difficult to form in the Si-containing alloy because the cementite becomes unstable due to the dissolution of Si.     

P92钢的微观组织和硬度

利用布氏硬度计、光学显微镜、扫描电子显微镜和X射线衍射仪研究了不同回火温度下P92钢的微观组织和硬度变化。结果表明,P92钢正火组织主要由马氏体、残留奥氏体和M23C6析出相组成,回火后温度升高到600℃时残留奥氏体才全部完成分解。低于400℃回火时因马氏体板条结构和位错组态没有明显变化,并且M23C6析出量也较少,硬度基本保持稳定;500℃回火时因亚晶强化和细小M23C6沉淀强化,硬度有所提高;继续升高回火温度,因马氏体发生高温回复,位错密度降低,M23C6也逐渐发生熟化,强化效果减弱,导致P92钢硬度逐渐降低。     

Decomposition of Retained Austenite in a High-Speed Steel GPM A30

The effect of tempering on the decomposition of retained austenite in a powder metallurgy (PM) high-speed steel, GPM A30, has been monitored with a high-speed dilatometer. The corresponding microstructures of specimens with different tempering cycles have been investigated by a combination of scanning electron microscopy and analytical transformation electron microscopy. The as-quenched structure of the steel studied is composed of retained austenite, untempered martensite, and carbides. The results indicate that the complete transformation of retained austenite can be more nearly accomplished by double or triple tempering cycles than by a single long-time cycle. The possible transformation mechanism for the decomposition of retained austenite during multiple tempering cycles is attributed to the invariant-plane-strain of the prior martensitic transformation extending accommodation defects to the adjacent retained austenite, which favors further transformations in the subsequent tempering operations.

回火温度对工程机械用超高强钢组织及回火脆性的影响

通过SEM、TEM、-20 ℃夏比V型冲击试验等分析手段研究了回火温度对工程机械用超高强钢微观组织及回火脆性的影响,并结合断口特征及微观组织分析裂纹扩展路径。结果表明,试验钢在200~500 ℃回火时,随着回火温度的升高,马氏体分解后形成的碳化物的析出位置从马氏体板条内逐步过渡到原始奥氏体晶界和马氏体板条界,其形状由针状变为粒状,并不断粗化。回火温度为200 ℃和500 ℃时,冲击试样断口的不稳定断裂区为韧性断裂。300 ℃回火时,出现了回火脆性,其冲击试样断口的不稳定断裂区为准解理断裂,裂纹扩展路径相对平直。微观组织分析发现,在原始奥氏体晶界及马氏体板条界析出大量的针状碳化物,这些碳化物提供了裂纹形核位置,促进了裂纹扩展,导致了回火脆性的产生。     

热处理工艺对30Cr16Mo1VN钢组织和性能的影响

为探索30Cr16Mo1VN钢最佳的热处理工艺,采用冲击、拉伸试验机、洛氏硬度计、OM、SEM、XRD、TEM研究了淬、回火温度对该钢组织和力学性能的影响。结果表明:该钢最佳的淬火温度为1050 ℃,淬火后存在少量M₂₃C₆碳化物和M₂N氮化物阻碍晶界迁移,其平均晶粒尺寸为14.1 μm,而大部分碳/氮化物固溶进基体中,导致Ms点降低,残留奥氏体含量增至59.2%。经-73 ℃冷处理后,大量残留奥氏体转变成马氏体,硬度提高至57 HRC。该钢300 ℃回火时具有良好的强韧性匹配,抗拉强度达2030 MPa,断面收缩率为10.0%。回火后基体发生回复,位错密度降低,随回火温度的升高,基体上析出细小弥散的球状碳化物阻碍位错运动产生二次硬化,450 ℃回火时出现硬度峰值。回火温度低于500 ℃时,该钢的硬度值皆大于55 HRC,具有良好的回火稳定性。     

Tempering Behavior of M50 Steel with Different Austenite Proportion Induced by High Current Pulsed Electron Beam

M50 steel was irradiated by high current pulsed electron beam(HCPEB)with different pulses.The subsequent tempering was carried out between 500 and 625℃.Microstructure evolution was analyzed by scanning electron microscopy and X-ray diffraction.It is found that the HCPEB treatment could constrain martainsite transformation in the surface layer of the samples.Tempered behavior of HCPEB remelted layer strongly depends on the proportion of retained austenite.Austenite saturated more carbon and metallic elements depict higher tempering stability.During tempered process,carbides precipitates among the grain and phase boundaries.The decreased solution of the elements promotes the retained austenite into martensite.     

激光熔覆制备颗粒增强铁基复合涂层中马氏体亚结构

通过在Fe-C-Si-B粉末体系中添加强碳化物形成元素Ti的方法,激光熔覆制备了原位合成颗粒增强铁基复合涂层.用扫描电子显微镜和透射电子显微镜对其微观组织进行了研究,重点分析了激光熔覆过程中马氏体的形成和亚结构的转变.结果表明,熔覆层无气孔和裂纹、组织均匀,是一种典型的亚共晶组织,由马氏体、莱氏体、残留奥氏体和原位生成的TiC颗粒组成.涂层中的马氏体是{211},<113>系片状孪晶马氏体,二次回火效应使得马氏体中析出了大量的具有纳米尺度的渗碳体,与马氏体具有一定的取向关系.