强磁场下冷却速率对Fe-0.76%C钢先共析铁素体组织的影响

研究了在12T强磁场下冷却速率对Fe-0.76%C钢中先共析铁素体的显微组织和性能的影响,结果表明:先共析铁索体晶粒的伸长方向与磁场方向的夹角随着冷速度的提高而增大,其原因是高速冷却时原子扩散减弱.在冷却速率相同的条件下,与非磁场热处理样品相比,强磁场热处理样品的先共析铁素体面积明显增加,宏观硬度下降,因为强磁场使Fe-0.76%C钢表现出更明显的亚共析钢特征.     

时效工艺对含铜高纯钢显微组织的影响

采用JEM-2010型高分辨电子显微镜和Quanta400型扫描电子显微镜,对经时效处理后的含铜高纯钢进行了组织观察,并研究了时效温度和保温时间对含铜高纯钢显微组织和硬度的影响,分析了时效工艺与硬度、显微组织之间的对应关系。结果表明,时效温度越高,达到时效硬度峰值所需的时间越短,且峰值硬度也随之下降。550℃时效峰处,铁素体晶粒中分布着大量的铜原子富集区,颗粒尺寸4-20nm,其结构仍为体心立方晶格。650℃时效时,富铜析出物优先在铁素体晶界处析出,随着时效时间的延长,富铜析出物不断粗化长大。     

亚微米颗粒增强6061铝基复合材料的微塑变特性

发现亚微米级Ａｌ２Ｏ３ｐ／６０６１复合材料的微屈服抗力较高，组织稳定性良好，其微屈服抗力与微塑变符合Ｂｒｏｗｎ和Ｌｕｋｅｎｓ提出的线性规律；通过与ＳｉＣｐ／６０６１和ＡｌＮｐ／６０６１的微屈服行为与组织的对比分析，认为增强颗粒形状，尺寸以及基体中位错密度，亚晶粒尺寸等因素直接影响屈服抗力，亚微米级颗粒增强复合材料对尺寸稳定性要求来说，微观组织形态较合理，是较理想的精密仪表材料。     

GH3625合金管材冷变形行为及热处理工艺研究

通过室温压缩试验、数学模型拟合、光学显微镜和洛氏硬度计等手段,并建立GH3625合金管材冷变形本构方程,研究了冷变形及热处理对GH3625合金管材组织和性能的影响。研究表明,GH3625合金管材加工硬化规律基本符合Hollomon方程,其中冷变形量是影响加工硬化的主要因素;随着冷变形量的增大,晶粒的变形程度增大,晶粒的变形均匀性逐渐改善,平均晶粒尺寸减小;合金的平均晶粒尺寸随热处理温度的升高呈现出先减小后增大的趋势,在1 100~1 250℃范围内晶粒长大激活能为180.46kJ/mol;硬度随热处理温度的升高而降低,且在晶粒长大过程中合金的硬度值与平均晶粒尺寸满足Hall-Patch关系式。     

热处理对高耐磨性NiTi基合金材料性能的影响

通过光学金相和显微硬度实验的分析方法,研究了热处理制度对高耐磨性NiTi合金材料性能的影响.实验表明,NiTi耐磨材料经过不同的热处理工艺后,材料的硬度变化明显,在800℃×30min/WQ+300℃×240min/FC热处理工艺制度下其硬度值最高;800℃固溶处理后,随着300℃时效时间的延长,材料显微组织鱼骨状晶逐渐增加,线状晶逐渐减少;黑色颗粒状质点逐渐增加,基体得到强化,材料显微硬度逐渐升高.随着冷却速度的增加,材料的晶粒得到细化.     

亚微米Al2O3P/Al-Mg-Si复合材料时效行为

为了研究亚微米级Al₂O₃颗粒对基体合金时效过程的影响规律,在 6061 铝合金基础上调整Mg、Si含量,采用压力浸渗法,制备了Al₂O₃体积分数为30%的亚微米级Al₂O_3P/Al-2.26Mg-0.63Si复合材料。通过硬度试验、SEM和TEM等手段,研究了复合材料及其基体合金的时效行为。结果表明,Al-2. 26Mg-0.63Si 合金的时效过程比较明显,随时效温度的提高峰时效时间提前,190℃时出现时效软化现象。亚微米Al₂O₃颗粒的加入强烈抑制复合材料的时效过程,时效曲线未出现明显时效峰。分析认为,由于大部分Mg 元素被消耗在界面形成MgAl₂O_4,且基体中位错稀少,不利于溶质原子扩散,因此复合材料中沉淀相在过时效阶段仍停留在GP区状态,没有充分长大,时效析出受到抑制。     

纯Al纳米晶体材料的力学性能研究

以纯铝粉(99.7%)为原料,采用不同的球磨参数(球料比、球磨时间),利用高能球磨法在室温下制备了具有纳米晶粒的纯Al体材料.通过X射线衍射(XRD)及显微硬度等手段,对不同球磨工艺参数下制备的纯Al体材料的晶粒尺寸及力学性能进行了测试和分析,并对样品的晶粒尺寸与硬度之间的关系进行了深入的探讨.研究发现,所制备的纯Al纳米晶体材料的平均晶粒尺寸在47～66nm范围内;随着球磨时间的延长,样品的显微硬度呈现出先升高后降低再升高的趋势,并在球磨时间为5h时,球磨样品的硬度达到最高值;屈服强度与晶粒尺寸之间符合正的Hall-Petch关系.     

SPS制备亚微米晶氧化铝陶瓷

以商业α-Al₂O₃粉体为原料, MgO为烧结助剂, 采用放电等离子烧结技术(SPS)制备亚微米晶氧化铝陶瓷. 系统研究了烧结温度、烧结助剂含量对亚微米晶氧化铝陶瓷的致密化过程及显微结构的影响. 分析结果表明, 1250℃以及0.05wt%分别是最佳的烧结温度和烧结助剂含量; 在此条件下获得的亚微米晶氧化铝陶瓷, 其相对密度达到99.8%TD(theoretical density),平均晶粒尺寸约0.68μm,显微硬度(HV5)达到20.75GPa,在3～5μm中红外范围内直线透过率超过83%. 当MgO掺杂量超过0.1wt%时, 第二相MgAl₂O₄形成, 引起光散射, 降低红外透过率.     

陶瓷粉末微注射成形脱脂与烧结机理相关研究

分析了粉末微注射成形亚微米级结构广阔的应用前景,以及亚微米尺度下脱脂和烧结理论研究的重要意义。综述了不同脱脂阶段的微观结构特征,亚微米尺度对不同阶段粘结剂的脱出机制,以及建立包含微结构比表面积的脱脂时间/速率数学模型的研究现状,讨论了获得脱脂过程中保形控制原理,亚微米阵列致密化过程中物质的迁移机制及晶粒长大的主要驱动力,以及坯体几何尺寸对不同烧结阶段物质扩散机制影响规律的关键作用,论述了明确陶瓷亚微米级结构力学特性的重要性。提出了亚微米级结构的脱脂和烧结机理今后的重点研究方向。     

ECAP温变形碳素钢中铁素体晶粒的细化机制

在500℃采用C方式ECAP变形制备了亚微晶45钢,用光学显微镜(OM)、透射电子显微镜(TEM)和电子衍射(SAED)等分析技术对其进行表征,研究了铁素体组织的演变特征和晶粒细化机制.结果表明:在初始变形道次后,原始的粗大铁素体晶粒因剧烈剪切变形而沿着剪切变形的方向形成剪切变形带,在其内大量的具有亚微米尺寸的板条状位...     

Effects of nanocrystalline ferrite particles on densification and magnetic properties of the NiCuZn ferrites

Effects of nanocrystalline ferrite particles addition on densification behavior and magnetic properties of the NiCuZn ferrites were investigated. It was confirmed that nanocrystalline ferrite particles enhanced densification of the samples obviously. The reason was attributed to the nanocrystalline particles, which spread around the micron-sized ferrite particles, increased contacting area and inter-diffusion of the particles. When the amount of nanocrystalline particles addition reached to 30 wt%, the samples obtained an approximate densification behavior as the 1.5 wt% Bi₂O₃ added samples. Due to relatively bigger grain size, higher sintering density and no different chemical composition sintering aids added, the sample with 30wt% nanocrystalline ferrite particles got the highest permeability and relatively high Q-factor when sintered at 900.     

Microstructural influence on the scatter in the fatigue life of steel reinforcement bars

Fatigue damage accumulation and failure of steel reinforcement bars (rebars) is a stochastic process. Scatter can be influenced by the sensitivity of the short crack growth to the microstructural features, especially near the fatigue limit. This work investigates the scatter inherent to the microscopic conditions near the fatigue limit of ferrite–pearlite and martensite microstructures found in the outer layer of rebars. An adapted Navarro–De Los Rios model within a Monte-Carlo framework is used to simulate the short crack growth in material grains. Grain size variation, grain orientation factor and multiple phases i.e., ferrite–pearlite and martensite were considered in the model. The results are compared with the scatter found in fatigue tests on hot-rolled-cold worked (HR-CW) as well as quenched and tempered (QST) rebars. It is shown that microstructural effects explains part of the observed scatter in the fatigue tests.     

STATISTICAL INVESTIGATION OF THE BEHAVIOUR OF SMALL CRACKS AND FATIGUE LIFE IN CARBON STEELS WITH DIFFERENT FERRITE GRAIN SIZES

Abstract— In order to study the relation between the scatter characteristics of small crack growth behaviour and fatigue life, rotatory bending fatigue tests of smooth specimens were carried out using 0.21% carbon steels of different ferrite grain sizes. Fifteen to eighteen specimens were fatigued at each stress amplitude, and the initiation and propagation behaviour of the cracks which led to the final fractures were examined for all the specimens. The physical basis of scatter in fatigue life was investigated, based on the successive observation of fatigue damage on the surface using the plastic replica technique, followed by an analysis of the data assuming a Weibull distribution. A statistical investigation of the physical basis of scatter in relation to the ferrite grain size was performed, i.e. the distributions for crack initiation life, crack propagation life, fatigue life and growth rate of small cracks. Finally, the fluctuation of crack growth rate was studied in relation to the application of a crack growth law for microstructurally small cracks.     

Microstructures of an Ultrafine Grained SS400 Steel in an Industrial Scale

The microstructures of a SS400 steel after thermomechanical control process（TMCP） in an industrial production were observed by optical microscope,scanning electron microscope（SEM） and transmission electron microscope（TEM）.The results indicated that the size of ferrite grains was 4-5μm,and transmission of ferrite was around 70%.The types of the ultrafine ferrite grains were analyzed and the strengthening mechanisms were discussed.The results show that the ultrafine ferrite grains came from three processes,i.e.deformation induced ferrite transformation（DIFT）.dynamic recrystallization of ferrite and accelerated cooling process.The increase in the strength of the material was mainly due to the grain refining.     

New Optimum Extrusion Parameters of 9% Cr Ferritic Steel Tubes

The mechanism of austenite grain growth prior to forging of 9% Cr ferritic steels was analyzed. Pre-forging heating temperature and holding time had an obvious effect on austenite grain size and delta ferrite morphology. Hot compression experiments were conducted by using a Gleeble 1500D thermal–mechanical simulator. A hot upsetting test was performed on an HP01-500 oil press to verify the geometric dynamic recrystallization mechanism. New extrusion technology parameters of steel tubes were optimized by the Deform 2D software, and practical tests were conducted. Results indicated that the grains grew slowly at low temperatures (i.e., 1000°C and 1050°C). The kinetic curve of grain growth was close to the “S” type at 1100°C. The grains exhibited abnormal grain growth at high temperatures (i.e., 1150°C and 1200°C). The quantity of delta ferrites markedly increased at 1250°C. In addition, grains were refined by geometric dynamic recrystallization during the formation process. Mechanical properties of 9% Cr tube met standard requirements.     

铁素体不锈钢焊接HAZ的晶粒长大模型

用物理模拟方法研究了铁素体不锈钢EB26－1焊接热影响区HAZ的晶粒长大,通过在不同规范下的等温晶粒长大物理模拟与分析,并结合焊接试验,建立了HAZ晶粒长大的模型,该模型成功地将Arrhenius公式用于热影响区晶粒长大计算,给出了焊接HAZ存在很陡的温度梯度对晶粒大长起阻碍作用的热钉扎效应,为选择合适的焊接规范和计算机模拟提供了依据。     

Effect of cooling rate on grain size of ferrite in a carbon steel

Abstract

Ferrite grain refinement by accelerated cooling has been studied in a carbon steel. The size of ferrite grains d_α formed by continuous cooling transformation from polygonal austenite has been measured as a function of cooling rate and austenite grain size d_γ. In the cooling rate range studied (q= 0·05–5 K s^?1), d_α was found to be proportional to q^?0·26d_γ^0·46. The mechanism of grain refinement by accelerated cooling is discussed, and it is shown that this occurs in the transformation where the ratio of nucleation to growth rate increases with a decrease in temperature. The austenite grain size dependence of ferrite grain size is shown to become progressively large as the nucleation mode changes from homogeneous to grain surface to edge to corner. A theoretical estimation of ferrite grain size formed by continuous cooling transformation was attempted on the basis of nucleation and growth rates. In the alloy studied, ferrite grain size was theoretically estimated to be proportional to q^?0·17d_γ^0·33. This was in close agreement with the dependence obtained in the present experiment.

MST/466     

Mesoscale simulation of ferrite transformation from deformed austenite during continuous cooling in a C–Mn steel using a cellular automaton method

A two dimensional cellular automaton (CA) method is adopted to investigate the growth kinetics of deformed austenite to ferrite transformation during continuous cooling in a C–Mn steel. During thermomechanical processing, part of the energy of deformation is stored in the austenite as the increase in the dislocation density within steels. This stored energy is incorporated into the present CA method as an additional driving force for the ferrite transformation. The modeled ferrite growth kinetics and grain size are compared with the experiments in the literature.     

022Cr25Ni7Mo4N双相不锈钢等温处理中的组织演变EI北大核心CSCD

研究经1100℃等温处理2~20 h后022Cr25Ni7Mo4N双相不锈钢的显微组织演变。观察钢中奥氏体晶粒形态变化并对其尺寸进行定量表征,测量铁素体/奥氏体两相中的元素含量变化,并探讨组织演变对实验钢中铁素体相体积分数的各向异性和低温冲击韧性的影响。结果表明:随着保温时间的延长,奥氏体晶粒发生聚集、长大、粗化现象,并伴随显著的晶粒形态变化,a/b值≥4.0时细长棒状晶粒体积分数从近20%骤降至5%以下,a/b值介于1.0~1.9的等轴晶粒体积分数显著上升的同时,尺寸≥20μm的晶粒体积分数快速增加。保温时间的延长使得Mo,Cr元素进一步向铁素体相扩散、富集,并提高铁素体相抗点蚀当量(pitting resistance equivalent number,PREN)值。细长棒状奥氏体晶粒比例的显著下降,是奥氏体体积分数各向异性改善和实验钢低温冲击韧性提高的主要原因。     

Kinetics and microstructural modeling of isothermal austenite-to-ferrite transformation in Fe-C-Mn-Si steels

During the multi-stage processing of advanced high-strength steels, the austenite-to-ferrite transformation, generally as a precursor of the formation of other non-equilibrium or metastable structures, has a severe effect on the subsequent phase transformations. Herein, a more flexible kinetic and microstructural predictive modeling for the key austenite-to-ferrite transformation of Fe-C-Mn-Si steels was developed, in combination with the classical nucleation theory, the general mixed-mode growth model based on Gibbs energy balance, the microstructural path method and the kinetic framework for grain boundary nucleation. Adopting a bounded, extended matrix space corresponding to a single ferrite grain, both soft-impingement and hard-impingement can be naturally included in the current modeling. Accordingly, this model outputs the ferrite volume fraction, the austenite/ferrite interface area per unit volume, and the average grain size of ferrite, which will serve as the input parameters for modeling the subsequent bainite or martensite transformations. Applying the model, this work successfully predicts the experiment measurement of the isothermal austenite-to-ferrite transformation in Fe-0.17C-0.91Mn-1.03Si (wt%) steel at different temperatures and explains why the final-state average grain size of ferrite has a maximum at the moderate annealing temperature. Effectiveness and advantages of the present model are discussed arising from kinetics and thermodynamics accompanied with nucleation, growth and impingement.