Observations on the morphology and crystallography of proeutectoid ferrite

Proeutectoid ferrite precipitates in Fe-0.47% C austenite partially transformed at 730°C have been studied using Kossel X-ray diffraction and detailed two-surface serial sectioning. Previous observations of habit planes near {111} in this system have been confirmed. However, there are many morphological irregularities and a minority of precipitates possess major interfaces which clearly deviate from {111} by 10–15°, which from measurements of the orientation relationship and maximum growth direction appear essentially to be of the approximate {223} type observed recently. The results are compared with various previous studies and it is concluded that the near {111} habit is not simply a pseudo-habit-plane characteristic only of longer reaction times and furthermore may only occur when a precipitate is formed under conditions of low supersaturation.     

低碳钢压缩变形时的形变不均匀性及其对铁素体转变的影响

宏观的形变不均匀性会导致相变和再结晶的差异 ,从而造成最终组织和性能的变化。本文观察并测量了单向压缩的低碳钢形变强化相变、铁素体热、冷形变时样品中心与边缘的铁素体形貌、珠光体或渗碳体分布和织构的差异。分析了不同区域相变及再结晶时的组织演变特点。讨论了形变储存能、渗碳体粒子及形变温度对铁素体相变及再结晶的影响。     

Kinetic transition during the growth of proeutectoid ferrite in Fe-C-Mn-Si quaternary steel

The kinetics of ferrite growth in Fe-0.1C-1.5Mn-0.94Si (mass pct) quaternary steel is investigated through the characterization of isothermal growth behavior, the thermodynamic prediction of kinetic boundary and the diffusional growth simulations using DICTRA. The change in microstructural evolution from slow growth to fast one is consistent with the calculated change of interface condition from the partitioning local equilibrium (PLE) to the negligible partitioning local equilibrium (NPLE). Compared with the DICTRA simulation, the observed growth kinetics of ferrite are between the calculated ones assuming local equilibrium (LE) and paraequilibrium (PE) criterions. At temperatures below the PLE/NPLE kinetic boundary, the observed growth behavior can be reasonably described by kinetic transition from PE to NPLE condition as isothermal time elapses, taking into account the critical velocity of interface at which trans-interface diffusion of subsitutional element permits the transition from PE to NPLE growth.     

药芯焊丝焊缝中先共析铁素体的数量预测

合理控制低合金高强钢药芯焊缝金属中的先共析铁素体含量,对获得理想的焊缝性能具有重要意义。将焊缝凝固过程中的偏析效应引入基于扩散控制增长的先共析铁素体生长机制,指出成份偏析效应改变了先共析铁素体生长温度和时间区间,从而影响偏析微区内先共析铁素体的体积比例,建立了预测和分析药芯焊丝焊缝中先共析铁素体体积比例的模型,与实际试验结果有很好的吻合。     

The effect of prior ferrite formation on bainite and martensite transformation kinetics in advanced high-strength steels

Most advanced high-strength steel products contain complex phases, including ferrite, bainite and martensite, which form successively during elaboration. It is essential to understand the effect of prior ferrite transformation on the subsequent bainite and martensite transformation kinetics to achieve precise control of the final microstructure. Nevertheless, the effect of the interface between the prior formed ferrite and the residual austenite (α/γ), together with the related chemical heterogeneity at the interface, on the subsequent phase transformations has been studied only rarely, and remains unclear. This study pays particular attention to the effect of the α/γ interface and its related concentration gradients on bainite and martensite transformation. It is shown that the interface and its related concentration gradients can play a very significant role on the subsequent bainite or martensite transformation kinetics: it retards bainite transformation whereas it accelerates martensite transformation. It is revealed from microprobe wavelength-dispersive spectrometry analysis and model calculations that there are both manganese and carbon gradients in front of the α/γ interface at the end of the ferrite transformation holding. The subsequent bainite transformation kinetics is controlled by the competition between the acceleration effect of the interface boundary itself and the retardation effect of the higher alloying concentration near the interface. Martensite transformation should initiate at the pre-existing dislocations in the center of the residual austenite grains, where the C and Mn contents are the lowest. A simple martensite transformation kinetics model taking into account C heterogeneity is proposed that can describe well the martensite transformation kinetics following the prior ferrite transformation.     

影响低镍718M钢先共析铁素体析出的因素

利用DIL805A型相变仪分析了不同工艺条件下大型低镍塑料模具钢718M的冷却相变过程,研究了原始组织、奥氏体化温度和保温时间对过冷奥氏体中析出先共析铁素体的影响规律。结果表明,低镍718M钢合理的成分设计使其具有高淬透性,先共析铁素体析出的临界冷速介于0.01~0.02℃.s-1,原始组织对淬透性影响显著;通过提高奥氏体化温度减少铁素体晶界形核位置来提时高淬透性的作用有限。     

Mechanical characteristics of spheroidal graphite cast irons containing Ni and Mn with mixed ferrite and bainitic ferrite microstructure

Abstract

This study aims to clarify the influence of additive elements of Ni and Mn on tensile and impact properties of three kinds of spheroidal graphite cast irons (SG irons), which are as cast, annealed and austempered samples. Spheroidal graphite cast irons with Ni (0–4˙5 mass-%) and Mn (0–0˙5 mass-%) melted by a high frequency induction furnace and cast into a Y block CO₂ mould with 30 mm in thickness. From the viewpoint of heat treatment, tensile strength and hardness of SG irons become larger in the order of ferritised<as cast<austempered ones. Matrix structures of SG irons, which are conducted to austempering treatment from α and γ mixture range, consists of bainitic ferrite with high toughness. Austempered SG iron with 3%Ni in 0˙1%Mn series is found to become higher tensile strength compound with elongation and toughness of 901 MPa, 17% and 915 kJ m^?2.     

Field-dependent neutron depolarization study of the ferrite formation in medium-carbon steels

Neutron depolarization experiments have been performed on the ferrite and pearlite phase transformations of the medium-carbon C60 and C35 steels. The interaction of the polarized neutron beam with the ferromagnetic ferrite grains gives information on the mean magnetization and the magnetic correlation length. From these parameters the ferrite fraction and the mean ferrite grain size are determined in situ as a function of time and temperature during the phase transformations. The applied magnetic field was varied periodically in order to record a full hysteresis curve of the magnetization, which gives essential information on the microstructure of the ferromagnetic ferrite grains. The field dependence of the mean particle size during the early stages of the pearlite formation is a strong indication of multi-domain behavior, which is absent in the austenite–ferrite transformation and at the end of the pearlite formation.     

The influence of impurities on the kinetics and morphology of the displacement reaction between Ni or Co and Cu2O

By use of models given by Wagner and extended by Rapp et al., a layered structure is predicted for the reaction products resulting from the displacement reaction in diffusion couples (Ni-Cu₂O or Co-Cu₂O). Experiments at 1000°C confirm these predictions. However, when Cu₂O is contaminated with chlorine, the morphology changes completely from a layered to an aggregate structure. It is shown that the resulting increase in layer thickness can be well described by assuming that the diffusion of oxygen through the Cu matrix is the rate-determining step. This behavior is compared with the phenomenon of internal oxidation of Cu-Ni alloys.     

A computational study of austenite formation kinetics in rapidly heated steels

Surface hardening of steels involves rapid austenitization and subsequent quenching of the surface. The resulting extent of hardening largely depends on the rate of austenitization of the surface under the applied high heating rates. In the present work the kinetics of austenite formation in Fe-C alloys during rapid, non-isothermal heating conditions, characterized by high heating rates and short austenitization periods, were studied by means of computational simulation. Austenitization of lamellar pearlite/proeutectoid ferrite microstructures was simulated by assuming two kinetically distinct stages: i) dissolution of lamellar pearlite followed by ii) dissolution of proeutectoid ferrite. The two stages were simulated by two corresponding 1-D diffusion models employed in series. Numerical solution of the resultant moving-boundary diffusion problems provide calculated results regarding the dependency of vol. fraction austenite on thermal cycle parameters and on initial microstructural features of the steel. Analysis of calculated results showed that the vol. fraction of pearlite transforming to austenite during pearlite dissolution depended on maximum temperature, dwell time and pearlite interlamellar spacing. A functional relationship between these variables, consisting of a thermodynamic and a kinetic term, was established. On the other hand, the total vol. fraction of austenite forming in the steel, after both stages of austenitization, was found to follow a typical sigmoidal kinetic behaviour.     

Reaction kinetics of Ni/Sn soldering reaction

Soldering between solid Ni and liquid Sn is studied, including late stages of reflow. Only during the very early stages of the process, intermetallic grains grow with a 1/3 power dependence on time. Later, kinetics clearly changes to parabolic growth, but remarkably in two subsequent regimes distinguished by different rate constants. The observed kinetics is discussed with respect to recent flux-driven ripening theory. This theory is only valid, if at all, for short reflows up to about 4 min. Transmission electron microscopy reveals the predicted scallop-like microstructure only at the very beginning. A sponge-like structure of equiaxed grains then develops, triggered by permanent nucleation of new grains at the Ni/Ni₃Sn₄ interface. Wetting of grain boundaries appears only up to a certain depth in the reaction zone. This remarkable behavior is explained by thermodynamic arguments.     

共析钢过冷奥氏体动态相变过程中先共析铁素体的生成

通过在Gleeble1500热模拟试验机上进行单轴热压缩试验,研究了共析钢过冷奥氏体在A1～Ar1之间变形时的动态相变特征.结果表明:共析钢过冷奥氏体在变形过程中动态相变与等温相变具有明显差异.动态相变过程中首先沿原始奥氏体晶界形成先共析铁素体,随后生成的珠光体与等温相变得到的珠光体相比,其片层间距和渗碳体厚度较小.提高应变速率将导致动态相变完成时先共析铁素体含量增加,珠光体片层间距减小而渗碳体厚度略有提高.由此提出过冷奥氏体变形使共析点右移的观点,并给予初步解释.     

Effect of oxides on the reaction kinetics during hot-dip galvanizing of high strength steels

Selective oxidation of the minor alloying elements during annealing prior to hot-dip galvanizing cause poor wetting with liquid zinc, resulting in bare spot defects. The effect of surface coverage, morphology (i.e., size and distribution) of the oxides on the reaction kinetics was examined. The Zn wettability has been determined by sessile drop contact angle measurement method and the Fe–Al interfacial layer was investigated by the use of a novel hot temperature spinning process. It has been found that besides the overall surface oxide coverage, size of the oxide islands is a dominant factor in affecting the reaction kinetics.