贝氏体预相变

通过新设计的具有快速加热和淬火功能的相变内耗仪,对 Cu-Zn-Al 合金和18Cr_2Ni_4WA钢的贝氏体相变内耗进行了测量。实验表明,当在贝氏体相变区等温时,在贝氏体相变孕育阶段,存在一个随时间变化的内耗峰值,此峰值随孕育期减少而增加。Cu-Zn-Al 合金的频率和18Cr_2 Ni_4WA 钢的频率在孕育期内都较低,但前者比后者更显著。这些现象,显示了贝氏体相变在孕育期内存在某种结构变化,其实质是由点阵不稳定性或局部软化所导致的形核。     

贝氏体相变的三面观

目前，三种不同的贝氏体定义正被广泛采用。第一种是显微组织的定义：贝氏体是非层状共析分解的产物，两种共析相通过扩散形核和长大相互竞争形成。第二是相变动力学定义（ＯＲＫ）：它是用铁碳合金的术语来描术贝氏体。通常用ＴＴＴ图中两个重迭的Ｃ曲线来描述共析温度以下的奥氏体分解。温度，贝氏体相变越加不完全，最后在上限温度消失。第三种是表面浮突定义：贝氏体是在马氏体区域以上（有时也在以下）形成的片状转变产物，相变     

焊缝粒状贝氏体组织的相变过程

采用热模拟方法,研究了焊接冷却过程中 15MnVN 钢焊缝中粒状贝氏体的相变过程。结果表明,焊缝粒状贝氏体的相变过程可以划分为铁素达形核、铁素体纵向长大、铁素体横向长大、铁素体交叉分割奥氏体和残余奥氏体岛收缩五个阶段。     

Cu-Zn 合金贝氏体相变热力学

对 Cu-40at.-%Zn 合金贝氏体相变温度范围内各种可能相变的驱动力进行了计算。结果表明,由于在这温度范围内存在β→β′有序相变,因而相变驱动力ΔG~(β′→α′)及ΔG~(β′→β_1′+α)随温度下降反而增大,且ΔG~(β′→α′)>0,ΔG~(β′→β_1′+α)<0;而且各成分合金β′及α′两相平衡温度 T_0,远低于 B_s 实验值,所以贝氏体相变只能按β′→β_1′+α扩散型相变进行。     

铁基合金的贝氏体基元及其生长演化

用透射电镜研究了 Fe-C-Mo 和 Fe-C-Si-Mn 合金中温转变早期产物的组织形态,发现在整个中温转变范围非平衡先析型转变都是按贝氏体铁素体相变基元形成及其扩散生长演化方式进行,说明经生长演化一些时间后的基元难于作为判断贝氏体形核机制的形态学依据。从基元生长演化进程区分了低碳贝氏体和板条马氏体。     

Fe-Ni-C合金中珠光体、贝氏体及马氏体的相变内耗

通过研究三种 Fe-Ni-C 合金中珠光体、贝氏体及马氏体的低频相变内耗,发现马氏体相变内耗遵守相界面位错的静滞后损耗机制,珠光体和贝氏体相变内耗不遵守这种机制。在频率基本不变时,随着冷却速度的增加珠光体和贝氏体的相变内耗峰增高,峰温降低,且 Q(?)和(?)/fm 件·T_m 呈正比线性关系;频率增加时,珠光体和贝氏体的内耗峰明显降低,峰温上串。在连续冷却条下,Fe-Ni-C 合金中开始珠光体及贝氏体相变时并不存在点阵软化现象。     

8mol?O2—ZrO2中的贝氏体相变

测定８ｍｏｌ％ＣｅＯ２－ＺｒＯ２在连续冷却中变温ｔ→ｍ的Ｍｓ，在Ｍｓ经上及以下的等温相变动力学曲线张相变产物的点阵常数，等温相变产物（贝氏）的点阵常数小于变温马氏体的点阵常数，参照金属材料中的贝氏体相变，建议８ｍｏｌ％ＣｅＯ２－ＺｒＯ２的等温相变为扩散过程控制的贝氏体相变，工体可能在母相晶界形核。     

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。     

外界条件对爆轰制备纳米氧化铁的影响研究

爆轰法合成纳米氧化铁,具有工艺设备简单、成本低、操作方便等优点.不同的外界条件对生成产物的形状、性质以及晶粒大小有一定影响.本文基于相同实验材料及实验温度的基础上,使反应物在不同压力环境中爆轰反应,并利用透射电镜以及XRD衍射分析等方式,对两种情况下制得的γ-Fe2O3产物的性质、形貌以及晶粒粒径大小进行对比与分析.结果表明,外界压力变化后,反应生成的纳米氧化铁的表面形貌有所差异,如圆整度降低,分散性提高等.     

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.     

Characterisation of microstructure and mechanical properties in two different nanostructured bainitic steels

Recently, valuable combinations of mechanical properties with strength of 1.9 GPa accompanied by very decent ductility of 19% and toughness of 31 J, have been achieved in a set of nanostructured bainitic steels. However, it is necessary to elucidate the significance of various microstructural features responsible of that extraordinary mechanical response in more detail. Thus, using two steels, with different Mn, Ni and V contents, and changing the nanostructured bainite isothermal transformation temperatures (200–300°C), has led to a plethora of subtle and essential microstructural variations, necessary to explain how the mechanical response of nanostructured bainite is attained.     

Stress intensity factor calculation based on the work of external forces

The external forces method is a numerical method for K calculation based on the finite element method. It uses the work of the external forces W for the calculation of the energy release rate and is particularly advantageous when that forces are applied far from the crack front. The method was applied to a corner crack geometry with the objective of studying its accuracy. Good results were obtained for a wide range of virtual crack displacements (0.03% < Δa/a < 6%) considering 4 values of W along with a polynomial regression of order 3. For that choice of parameters the inaccuracy of K is mainly due to FEM errors. A great sensitivity of K to FEM errors was observed, however accurate values of K were obtained, with errors lower than 2 percent. So, the use of the external forces method for the calculation of K is recommended, considering its simplicity and accuracy. This revised version was published online in July 2006 with corrections to the Cover Date.     

Two-body abrasion wear mechanism of super bainitic steel

The abrasion wear resistance and wear mechanism of super bainitic steel austempered at different temperatures and time have been investigated by two-body abrasion testings, scanning electron microscopy, X-ray diffraction, transformation electron microscopy and electron backscattering diffraction. The results show that the two-body abrasion wear mechanism is predominantly micro-ploughing abrasion, and the wear resistance is decreased with increasing isothermal temperature which is attributed to the decreased hardness caused by a higher retained austenite content and the coarsening of the microstructure. The sample austempered at 230°C for 2?h with the higher amount of retained austenite and lower carbon concentration exhibits excellent wear resistance, and the transformation induced plasticity effect is observed during wear process which is beneficial for the improving of hardness and wear resistance.     

片层状贝氏体铁素体长大动力学研究

为研究合金元素含量对钢中贝氏体铁素体长大动力学的影响,采用Zener-Hillert和Bosze-Trivedi动力学模型,通过选取热力学和动力学参数,计算了合金成分不同的钢的片层状贝氏体铁素体长大速度.研究表明:Fe-0.59C、Fe-0.81C和Fe-0.478C-4.87Ni合金在贝氏体相变时,贝氏体铁素体长大速度可以用无分配局部平衡条件下的扩散模型很好地描述,Fe-0.69C-1.8Ni-0.8Mo合金贝氏体铁素体长大速度略慢于理论值,Fe-C-8.7Ni合金贝氏体铁素体的长大速度比理论值约慢2个数量级;合金元素含量高的钢的贝氏体铁素体长大速度无法用扩散控制模型很好地描述;结合对贝氏体相变机制的讨论,提出贝氏体相变机制可能与相变温度和钢的成分相关.     

Si对团球状共晶体奥氏体—贝氏体钢中贝氏体相变的影响

在热力学，动力学的基础上，从贝氏体铁素体在奥氏体贫碳区切变机制出发，研究了Ｓｉ对团球状共晶体奥－贝钢中贝氏体相变的影响，并提出Ｓｉ促使奥氏体奥碳区的形成，有利于贝氏体切变的新观点。     

Effect of microsegregation on carbide-free bainitic transformation in a high-silicon cast steel

ABSTRACT

A high silicon cast steel was studied in the as-cast condition in order to characterise its solidification macrostructure and microsegregation. The steel, poured into 32?mm-keel-block-shaped moulds, has a coarse solidification structure and marked microsegregation, containing low-alloyed areas with a total alloy content (Cr?+?Mn?+?Si) of 2.3 wt-% and high-alloyed zones of 5.3 wt-%. The bainitic transformation behaviour at 300°C was studied at different austempering times. The bainitic reaction occurs at different rates within the specimen volume, because of its chemical heterogeneity. An austempering heat treatment leads to an inhomogeneous carbide-free bainitic microstructure with different phase amounts, morphologies and sizes. The heterogeneous distribution of sizes and chemical compositions of retained austenite is speculated to benefit mechanical properties.     

Effects of hydrogen on the properties of bainitic steel crossing

The microstructures on the damaged surfaces of two served bainitic steel crossings were investigated using optical microscope and scanning electron microscope (SEM). The tensile properties of the bainitic steel with different contents of hydrogen were measured. The results showed that the plasticity of the bainitic steel, such as the reduction of area and elongation, decreases sharply with the increase of hydrogen content. There was a critical content of hydrogen without the hydrogen embrittlement for the commercial bainitic steel used for crossing, which was 7 × 10^?5 wt%. When the content of hydrogen in a bainitic steel was lower than the critical value, during the used process of the crossings, the wear failure appeared during the early stage; however, the fatigue spalling appeared in the end of the process. When the content of hydrogen was higher than the critical, brittle fracture was responsible for the failure of the crossing in a short time during use.