9Cr2Mo钢中贝氏体的组织形貌

采用光学显微镜和扫描电子显微镜等技术手段观察了经1100℃奥氏体化的9Cr2Mo钢在不同温度的盐浴中等温淬火后贝氏体的组织形貌。结果表明：9Cr2Mo钢1100℃奥氏体化后,在410℃的硝盐浴中等温得到羽毛状的经典上贝氏体组织,在350℃的硝盐浴中等温得到针状（或片状）的下贝氏体组织;上贝氏体铁素体铁素体是在奥氏体晶界处形核并向晶内生长,碳化物在铁素体条间分布;下贝氏体是在奥氏体晶内形核,碳化物分布在铁素体片中间,碳化物大多数与片条的主轴方向交角排列,但角度不等。     

物理金相二级人员试题参考答案

(续上期 )　　 7.这两种组织很相似 ,都呈片状或针状 ,但是下贝氏体中有碳化物存在 ,较易侵蚀 ,在显微镜下颜色较黑 ,与高碳马氏体存在明显区别。在电子显微镜下 ,高碳马氏体为细小的孪晶组织 ,而下贝氏体内有碳化物存在 ,其形貌酷似回火的高碳马氏体 ,但在高倍下可根据碳化物的分布形态予以区别。8.贝氏体是过冷奥氏体在珠光体转变温度以下、马氏体转变温度以上的温度范围内相变产物 ,它们通常是由铁素体和碳化物所组成的非层片状组织。贝氏体组织可由中温等温得到 ,也可以在连续冷却中得到。贝氏体分上贝氏体、下贝氏体、无碳化物贝氏体、…     

正火回火14Ni3CrMoV锻钢微观组织的TEM研究

利用透射电镜 (TEM)研究了 1 4Ni3CrMoV锻钢正火 +回火后的微观组织。结果表明 ,该钢在正火过程中形成的以贝氏体为主的组织在高温回火过程中发生了明显变化。碳化物大量析出 ,分布均匀 ,大部分仍保持一定的方向性 ,显示原贝氏体铁素体板条的位向。局部存在铁素体 +球化碳化物类组织。回火后没有发现块状残余奥氏体     

热处理工艺对12Cr1MoV钢显微组织和力学性能的影响

通过热模拟试验对12Cr1MoV钢进行了不同工艺的正火+回火热处理,研究了热处理工艺对该钢显微组织及力学性能的影响。结果表明:12Cr1MoV钢正火+回火后的正常显微组织为回火贝氏体+铁素体或回火贝氏体+铁素体+珠光体或铁素体+珠光体;如果回火温度过高或正火冷却速率不足,则分别会导致钢中出现两相区组织黄块马氏体和钒的碳化物沿晶界及晶内聚集长大的情况,显著降低钢的力学性能。     

淬火冷速对转子30CrMoNiV511钢韧性的影响

使用热处理风冷炉模拟研究了大型汽轮机转子内外径处的冷却速率对转子材料30CrMoNiV511钢的组织转变及强韧性的影响。结果表明,淬火冷却速率对转子材料的强度影响不明显,而对冲击韧性的影响显著。随着淬火冷却速率从50℃/min逐渐降低至7℃/min,淬火组织由下贝氏体逐渐向上贝氏体、粒状贝氏体及其混合组织转变,冲击韧性由66 J急剧降低到16 J;当冷却速率降低到5℃/min以下时,组织中出现先共析铁素体,且上贝氏体铁素体粗大,碳化物分布极不均匀,冲击韧性极差。不同冷却速率转子钢的微观组织及碳化物析出形态和分布是决定其冲击韧性的主要原因之一。     

钢中下贝氏体相变的TEM研究

透射电镜(TEM)对40CrMnSiMoV 钢310℃下贝氏体转变研究发现,该钢下贝氏体铁素体由亚单元组成。亚单元内存在中脊,其惯习面为(0.424,0.738,0.526)f,fcc/bcc 间位向关系为(111)f//(011)b,[01],与[1]b 差3±1°。短时间等温时,组织中无碳化物析出,而存在两种不同取向分布的残余奥氏体薄膜。一定等温时间后,发现三种不同形态的碳化物同时或先后自贝氏体铁体素和奥氏体内析出。上述实验结果仅当认为下贝氏体按切变方式转变时,方能得到很好理解。     

低合金超级贝氏体钢组织形态的研究

针对贝氏体研究中的超级贝氏体组织,设计了试验用钢70MnSi2CrMo,经低温等温处理,获得贝氏体铁素体+残余奥氏体的组织,利用X射线衍射XRD、扫描电子显微镜SEM、透射电子显微镜TEM等仪器对其进行相组成和形态的检测分析.结果表明:在马氏体转变开始温度Ms点稍上的中低温区等温处理,贝氏体铁素体沿γ相晶界转变,无碳化物析出;α相转变排碳导致成分起伏,α/γ交界处过冷奥氏体稳定性增加,难以转变成马氏体;贝氏体铁素体的转变特征、过饱和的碳浓度、高密度位错、以及纳米尺寸相界面和亚结构等,影响着超级贝氏体钢的力学性能.     

冷却速度对铁素体贝氏体钢组织性能的影响

以普通低碳钢为研究对象,通过控制轧制及加速冷却实验研究了实验钢的组织及力学性能。结果表明,冷却速度达到35℃／s时,实验钢的组织由约30％的贝氏体和约70％的等轴铁素体组成,铁素体平均品粒尺寸约为8μm,贝氏体铁素体板条宽度约为0．1μm,贝氏体铁素体内有碳化物析出。具有贝氏体和铁素体组织的复相钢主要强化机制为细晶强化和贝氏体相变强化,其屈服强度达到400MPa,且具有较低的脆性转变温度。     

加速冷却生产铁素体贝氏体低碳钢的实验研究

以普通低碳钢为研究对象,通过控制轧制及加速冷却实验研究了实验钢的组织及力学性能。结果表明,冷却速度达到35℃／s时,实验钢的组织由约30％的贝氏体和约70％的等轴铁素体组成,铁素体平均晶粒尺寸约为8μm,贝氏体铁素体板条宽度约为0．1μm,贝氏体铁素体内有碳化物析出。具有贝氏体和铁素体组织的复相钢主要强化机制为细晶强化和贝氏体相变强化,其屈服强度达到400MPa,且具有较低的脆性转变温度。     

钢的热处理组织分析：第四讲 钢的热处理组织分析实例（二）

5 40Cr球化退火组织 图6为40Cr球化退火组织。从显微观察可见,该组织由两部分组成:白亮块和分布着小颗粒的白亮块。40Cr钢球化退火后只能由铁素体与渗碳体两相组成。所以白色颗粒状物为渗碳体,白亮块及分布有渗碳体颗粒的白亮块(基体)均为铁素体。从形态看也可判断它是铁素体块及分布有渗碳体的铁素体。形成这种组织完全决定于退火工艺。球化前原始组织为大块状铁素体与珠光体的混合组织,经750℃加热时,组织为铁素体块+(奥氏体和少量的渗碳体);缓冷后,铁素体块不变,而奥氏体分解为球状珠光体,致使缓冷后组织中碳化物分布极不均匀,     

Microstructural examination of two experimental high-strength bainitic low-alloy steels containing silicon

Abstract

A detailed microstructural characterization of two silicon-containing low-alloy steels, Fe–0·2C–2Si–3Mn and Fe–0·4C–2Si–4Ni (nominal wt-%), isothermally transformed in the bainitic temperature range (~ 400–250°C), has been carried out using principally electron microscopy, X-ray diffraction, and dilatometry. Upper bainite in these silicon-containing steels consists of bainitic ferrite laths and interwoven thin films of retained austenite instead of cementite. Coarser granular regions of retained austenite may also be obtained. The bainitic ferrite laths (or plates) in lower bainitic structures contain intralath carbides, but the interlath morphology of retained austenite still occurs. The variations in these microstructures with isothermal transformation temperature, and the thermal stability of the retained austenite phase is described and discussed.

MST/526     

Microstructural characterization of controlled forged HSLA-80 steel by transmission electron microscopy

In this study, transmission electron microscopy (TEM) was used to evaluate the effect of controlled forging followed by cooling at various rates on microstructure of an HSLA-80 steel. The observations demonstrate that water-quenched steel has finer multiphase constituents of lath martensite, bainite and twined martensite, whereas air-cooling has resulted in a mixture of bainitic ferrite, retained austenite or MA constituents along with some Widmanstatten ferrite. When the steel is cooled in sand, the maximum volume fraction of polygonal ferrite (PF) was produced which, in turn, increased volume fraction of MA constituents. Precipitation of fine -Cu, Nb and Ti carbides and carbonitrides was observed and identified using energy dispersive spectrometric analysis (EDS) and electron diffraction.     

A new low-carbon microalloyed steel wire in drilling rope

SAE 1065 steel wire ropes are prone to individual wire fracture due to the martensite structure with high carbon and high microhardness formed on the wire surface because of friction. A new low-carbon microalloyed steel wire composed of lath bainite with fine carbides dispersed in bainitic ferrite and lath martensite is developed in this paper. For the developed steel wire with a diameter of 1.0?mm, the tensile strength and low-cycle fatigue resistance are higher than those of commercial SAE 1065 wire. It is shown that the individual wire fracture caused by as-quenched martensite formation and severe plastic deformation can be avoided in the developed steel wire. Also, the design process and associated manufacturing process are discussed in detail.     

Strengthening behavior analysis of weld metal of laser hybrid welding for microalloyed steel

The weld metal with high strength and good toughness was obtained for medium thickness microalloyed steel by using high power laser hybrid welding technology. Mechanical properties of weld metal were evaluated by using room temperature tensile test and low temperature three-side Charpy V-notch (CVN) impact test. The results show that the yield strength and ultimate tensile strength of weld metal are up to 713 MPa and 918 MPa, respectively. Both of them are almost 1.5 times higher than those of base metal. Under the strict three sides CVN condition, the −40 °C low temperature impact absorbed energy is up to 32 J and also higher than that of base metal. Weld metal predominately consists of granular bainite and carbon-free bainite. Both of them mainly contain lath morphology bainitic ferrite. The lath morphology bainitic ferrite with fine grain size plays an important role in higher strength. Dispersive carbide and high density dislocation are found in strengthening weld metal.     

Metallography of bainitic transformation in austempered ductile iron

Abstract

An unalloyed nodular cast iron has been used to investigate the development of microstructure on heat treating in the bainite temperature region. Specimens were austenitised at 900°C for 1·5 h, then austempered for 1, 2, or 3 h at 250,300, and 350°C, respectively, and examined by light, transmission electron, and scanning electron microscopy. Experimental results indicate a microstructure consisting of a stable, highly enriched, retained austenite with one of two lower bainitic ferrite morphologies. One of these morphologies is carbide free acicular ferrite for specimens austempered at 350°C for 1 h and the other is bainitic ferrite in which carbide is distributed within the ferrite produced by different heat treatment conditions. Austempering at 350°C for 2 h and at 300°C for 1 and 2 h resulted in the formation of transition carbides in bainitic ferrite platelets. The η carbide was formed at 350°C for 2 h by precipitation from a bainitic ferrite supersaturated with carbon. By contrast, ? carbide was associated with austempering at 300°C for 1 and 2 h and precipitates either on the austenite twin/bainitic ferrite boundaries or within the bainitic ferrite. The fracture mode of tensile and impact specimens in the austempered condition was fully ductile compared with as cast specimens, which had mixed fracture characteristics.

MST/1646     

STM Study on the Surface Relief and Mechanism of Bainite Transformation

Scanning tunneling microscopy (STM ) has been first employed to study the surface relief accompanying bainite transformation in a Fe-2.17C-12.18Cr-0.31Si-0.26Mn (wt pct) steel. With the exclusive vertical resolution of STM, we observed that the surface relief associated with bainite is a group of surface reliefs related to subplates, subunits and sub-subunits. As a whole, the relief group is in a tent shape, not of invariant plane strain (IPS) type, which is obviously different from that of martensite, and implying that bainite is not formed by shear mechanism. The fine structure of bainite in Fe-1.0C-4Cr-2.0Si (wt pct) alloy has also been studied with STM and TEM. It is found that the bainite plate is composed of subplates, subunits and sub-subunits. On the basis of the fine structure inside a bainitic ferrite plate observed under STM, a sympathetic-ledgewise mechanism of bainite formation is proposed     

Effect of austenite grain size on isothermal bainite transformation in low carbon microalloyed steel

Abstract

The effect of austenite grain size on isothermal bainite transformation in a low carbon microalloyed steel was studied by means of optical microscopy, SEM and TEM. Two widely varying austenite grain sizes, a fine average grain size (～20 μm) and a coarse average grain size (～260 μm), were obtained by different maximum heating temperatures. The results showed that the morphology of isothermal microstructure changes from bainite without carbide precipitation to bainitic ferrite with a decrease in holding temperature. Coarse austenite grain can retard the kinetics of bainite transformation and increase the incubation time of bainite transformation by reducing the number of nucleation site, but it does not influence the nose temperature of the C curve of bainite start transformation, which is ～534°C.     

Tracking solute atoms during bainite reaction in a nanocrystalline steel

Abstract

The extremely slow transformation kinetics of a nanocrystalline bainitic steel makes this novel structure the perfect candidate to determine the carbon content of the bainitic ferrite away from any carbon enriched regions, such as dislocations and boundaries, as the bainite transformation progresses at extremely low temperatures. The purpose of this atom probe tomography study was to systematically track atom distributions during the bainite reaction in a nanocrystalline steel. The results will provide new experimental evidence on the explanation for the incomplete reaction phenomenon and the carbon supersaturation of the bainitic ferrite during transformation, subjects critically relevant to understanding the atomic mechanism controlling bainitic ferrite growth.     

Alloy optimisation of bainitic steel for large plastic mould

Based on the phase transformation theories, especially the T₀ concept of bainite transformation, alloy optimisation of bainitic steel with carbides has been carried out aiming at the produce of plastic mould with large cross-section. The effect of manganese and silicon on proeutectoid ferrite and bainite transformation is explored by dilatometric analysis, XRD and different microscopy techniques. The results show that after the alloy optimisation, the transformation of proeutectoid ferrite is suppressed and when the cooling rate is lower than 0·1°C?s^??1, the new lower bainite transformation appears by decreasing carbon capacity of austenite and promoting carbide precipitation. Industrial production proves that the optimised alloy SDP1 can meet the demand for the plastic mould with the thickness of 1050?mm.     

The bainite transformation behavior,the influencing factors and control strategy in ferritic‐pearlitic forged crankshafts with coarse grains

Microstructural characterization of the bainite in a ferritic–pearlitic forged crankshaft was carefully investigated. A Gleeble thermo‐mechanical simulator as well as a high resolution dilatometer were also used to analyze the effect of cooling rate on the bainite formation and the bainite transformation mechanism in steels with different austenite grain sizes. Results show that the fine structure of the bainite mainly consists of bainitic ferrite and martensite. No segregations are found where bainite forms. Bainite tends to form in the slower cooled inner part of the crankshaft with an austenite grain size exceeding 100 μm. The formation of bainite is mainly affected by the austenite grain size as well as the cooling rate in the crankshaft studied. As the austenite grain size increases, ferrite start, pearlite finish and bainite finish temperatures are decreased. More bainite forms when bainite finish temperature decreases. The critical cooling rate of bainite transformation is increased from 0.34 °C?s^‐1 to 0.44 °C?s^‐1, if the maximum austenite grain size is refined from 216 μm to 100 μm. For ferritic–pearlitic crankshafts, or other bulky products, the elimination of bainite can be achieved through austenite grain refinement.