Direct observation of Cu interphase precipitation in continuous cooling transformation by atom probe tomography

Atom probe tomography (APT) combined with electron back scatter diffraction and transmission electronic microscopy (TEM) is utilized to characterize the nature of copper precipitation during austenite–ferrite transformation in a continuous cooling high-strength low-alloy steel. The copper precipitation manners in association with the austenite decomposition kinetics are studied. The prevailing microstructure of the continuous cooling steel consists of acicular ferrite (AF), which is formed at an intermediate cooling rate of 10?°C/s. Besides, a limited volume of polygonal ferrite (PF) because of fast cooling rate and a trace of retained austenite are detected. Numerous copper-rich phase is found by TEM observation both in highly dislocated AF and dislocation-free PF. Generally, the copper-rich precipitates have comparatively large sizes and are considered to be formed by interphase precipitation during austenite–ferrite transformation. A high number density of nanometre sized copper-rich clusters that are lack of diffraction contrast in conventional TEM observation are detected by APT. These smaller copper-rich clusters, which are usually located between the linear-arranged copper-rich precipitates, are considered to be formed from supersaturated solid solution after the cessation of austenite–ferrite transformation. That means an ageing reaction for Cu precipitation occurs during continuous cooling transformation. The copper-rich precipitates and clusters are both rich in nickel, manganese and iron.     

Formation of fully pearlitic microstructure in medium carbon steel

Pearlitic microstructure can be obtained when austenite of eutectoid composition is slowly cooled from high temperature to below the Ae₃ temperature. It is also possible to induce such structure in hypo-eutectoid composition of austenite through proper heat treatment as well. However, in the current work, one hypo-eutectoid steel during very slow cooling only produced completely pearlitic microstructure which was not expected from a steel with such composition. The calculations carried out considering orthoequilibrium condition actually predicted the presence of about 37% ferrite in the microstructure. Further calculations considering different equilibrium modes and kinetics of transformation indicates that the composition and thermal condition of the steel under consideration was such that proeutectoid ferrite formation could not start before the composition reaches to the Negligible Partitioning Local Equilibrium phase boundary which further coincides with the area described by Hultgren extrapolation and thus, the steel completely transforms to pearlite even with hypo-eutectoid composition during very slow cooling.     

大块非晶临界冷却速率的非等温转变计算模型

在非等温转变理论以及非稳态形核理论基础上提出了计算大块非晶合金连续冷却转变曲线和 临界冷却速率的新模型,以用于评估合金的非晶形成能力. 依据此模型对Zr基和Pd基8种合金 进行了计算，计算结果与实验值符合较好. 计算结果表明，影响临界冷却速率的主要因素为 黏度、临界形核功和临界结晶分数. 随着黏度增大，临界冷却速率降低；随临界形核功增大 ，临界冷却速率急剧降低；随临界结晶分数增大，临界冷却速率起初降低较快，达到一定程 度后下降速率趋于缓慢. 关键词： 临界冷却速率 非晶形成能力 大块非晶 连续冷却转变曲线     

Effects of N and B on continuous cooling transformation diagrams of Mo–V–Ti micro-alloyed steels

Effects of the single addition of nitrogen (N) and boron (B) and the combined addition of N and B on continuous cooling transformation (CCT) diagrams and properties of the three Mo–V–Ti micro-alloyed steels were investigated by means of a combined method of dilatometry and metallography. Microstructures observed in continuous cooled specimens were composed of pearlite (P), quasi-polygonal ferrite (QPF), granular bainite (GB), acicular ferrite (AF), lath-like bainite (LB) and martensite (M) depending on the cooling rates and transformation temperatures. Single addition of 12?ppm B effectively reduced the formation of QPF and broadened the cooling rate region for LB and M. Added N makes the action of B invalid and the QPF region was prominently broadened, and even though the cooling rate is higher than 50°C?s^?1, it cannot obtain full bainite.     

Multi-phase transformation kinetics of HSLA steels during continuous cooling: experiments and cellular automaton (CA) simulation

ABSTRACT

Kinetics of multiply ferrite/bainite phase transformation of HSLA steels is investigated by experiments and cellular automaton (CA) simulation. Peak-differentiation method to elucidate the sequential ferrite and bainite phase transformation individually, which is verified by the CA simulation. Such CA modelling executed using classic JMAK theory, but also gives an insight of microstructure evolution of the multi-phase transformation routine on different cooling rate. From that, it enables classic JMAK modelling to capture the detached phase transformation with different growth models and interface-migration mechanisms. Also, we find that the final phase constitution is sensitive to the cooling rate. With increasing the cooling rate, bainite sheaves nucleated at prior austenite boundaries and ferrite/austenite interfaces are significantly facilitated, which seriously inhibits the growth of prior ferrites. The scenario can be interpreted by the CA simulation and the influence of the cooling rate on sequential multi-phase transformation can be also obtained.     

Formation of nano-crystalline and amorphous phases on the surface of stainless steel by Nd:YAG pulsed laser irradiation

Thin surface layers consisting of nano-crystalline and amorphous phases on the surface of stainless steel have been attained under the Nd:YAG pulsed laser irradiation. The phases and microstructures were investigated by X-ray diffraction (XRD) and high resolution transmission electron microscope (HRTEM). The phase compositions of the surface determined by XRD were α-Fe (ferrite) and γ-Fe (austenite) or only γ-Fe in the near surface region on the bases of the different laser power densities. The nano-crystalline grains with sizes of 4-100 nm could result from high cooling rate and crystallization in amorphous region by homogeneous and heterogeneous nucleation. The formation of the amorphous phase was attributed to the higher cooling rates.     

Study on Ultra-fast Cooling Behaviors of Water Spray Cooled Stainless Steel Plates

Spray cooling is an effective tool to dissipate high heat fluxes from hot surfaces. This article thoroughly investigates the effect of thickness of a hot stainless steel plate on the cooling time, cooling rate, heat flux, and heat transfer coefficient under constant mass flow rate maintained at 1 MPa using water as the coolant. Cylindrical samples of stainless steel with constant diameter (D = 25 mm) and thickness (δ = 7.5, 12, 16.5, and 21 mm) were used in the present study. Critical droplet diameter to achieve an ultra-fast cooling rate of 300°C/s was estimated by using an analytical model for samples of varying thicknesses. The analytical model (one side spray cooling) showed good agreement with experimental results with a relative error of 3.2% in the plate thickness range of 1–12 mm. An increasing trend in maximum heat flux was found with increasing thickness of the plate. Maximum heat flux as high as 1,800 kW/m² was achieved for a 21-mm-thick sample. Heat transfer coefficients in the range 0.092–96.24 kW/m²K, 0.111–98.9 kW/m²K, 0.074–63.4 kW/m²K, and 0.127–55.63 kW/m²K were reported for sample of varying thicknesses in the present study. Limited published work is available with reference to water spray cooling dynamics and thickness of stainless steel plate. Therefore, the present study focuses on the correlation between the thickness of the plate and spray dynamics of water spray cooling.     

Effects of high-pressure heat treatment on the solid-state phase transformation and microstructures of Cu61.13Zn33.94Al4.93 alloys

The phase transformation activation energy of the Cu61.13Zn33.94Al4.93 alloys, which were treated at 4 GPa and 700℃ for 15 minutes, was calculated by means of differential scanning calorimetry curves obtained at various heating and cooling rates. Then, the effects of high-pressure heat treatments on the solid-state phase transformation and the microstructures of Cu61.13Zn33.94Al4.93 alloys were investigated. The results show that high-pressure heat treatments can refine the grains and can change the preferred orientation from (111) to (200) of α phase. Compared with the as-cast alloy, the sample with high-pressure heat treatment has finer grains, lower β′→β and β→β' transformation temperature and activation energy. Furthermore, we found that high cooling rate favours the formation of fine needle-like α phase in the range of 5-20℃/min.     

固体热容激光器冷却方式的数值模拟

 固体热容激光器连续运转一段时间后，需要进行冷却，以进行下一次运转，因此冷却速度直接影响着热容激光器运转的效率和实用性。从热传导方程出发，以沉积热为内热源模型，利用有限元分析方法对激光二极管阵列从4个方向对称泵浦的板条Nd:GGG激光介质的温度场和和热应力场进行了数值模拟。并对冷却阶段分别采用过冷气体、水循环、喷雾、相变冷却方式时的介质温度和应力随时间变化过程进行了模拟。结果表明在相同条件下采用相变冷却方式能在较短时间内将介质冷却到初始温度。     

高强度钢手工焊接后过程的声发射实时监测研究

本针对实际工艺中使用的高强度钢，重点对手工焊接的冷却过程利用声发射技术进行实时监测。本次试验是和先前在不同型号的钢板在手工焊接过程中所产生的声发射信号进行比较的基础上进行的，主要目的是寻找高强度钢在手工焊接的冷却中所产生的活性裂纹在冷却阶段的声信号特征。     

SPRAY COOLING OF HIGH TEMPERATURE METALS USING HIGH MASS FLUX INDUSTRIAL NOZZLES

Systematic experiments were conducted for the spray cooling of high-temperature stainless steel using three different types of industrial sprays: full cone and flat hydraulic nozzles and a flat air-mist nozzle. In the present study, a wide range of mass flux (1.5–30 kg/m²sec) is covered, which has never been thoroughly investigated before. Orientations with respect to gravity and spray angle were also explored.

The data have been analyzed and correlated. The results of this study will be important to the product quality control in industries by providing a good estimation of heat flux at different mass flux, spray types, and surface temperatures, especially for spray cooling of stainless steel and other metals with similar thermal properties.     

Jet Impingement Cooling of a Hot Moving Steel Plate: An Experimental Study

In the current study, a hot moving steel plate of 6 mm thickness with an initial temperature of 900°C has been considered for jet impingement cooling. The experiment has been designed with the help of Design of Expert software to optimize the process parameters based on the highest cooling rate. The various subsurface transient temperature histories have been measured during the cooling process. The surface heat flux and surface temperature were calculated with the help of a commercial inverse heat transfer solver called INTEMP. The experimental result has been presented in terms of cooling rate and critical heat flux.     

Effect of Oxide Layer in the Ultra Fast Cooling of a Steel Plate

This study deals with the effect of oxide layer during ultra-fast cooling of a hot plain carbon steel plate. In the current research, the hot plain carbon steel plates were cooled from an initial surface temperature of 900°C by using air atomized spray at different air pressures. The heat transfer analysis illustrates that during high pressure air atomized spray cooling, the average surface temperature is almost unaffected by the presence of an oxide layer. For better understanding, the plain carbon steel cooling data have been compared with the data obtained during the cooling of a stainless-steel plate.     

Magnetic-field-induced microstructural features in a high carbon steel during diffusional phase transformation

In this work, a high purity, high carbon steel was heat treated without and with a 12-T magnetic field. The microstructural features induced by magnetic field during its diffusion-controlled austenite decomposition were investigated by means of optical microscopy and SEM/EBSD. It is found that the magnetic field increases the amount of the abnormal structure, which is composed of proeutectoid cementite along the prior austenite boundaries and ferrite around it, because magnetic field increases the austenite grain size and promotes the transformation of carbon-depleted austenite to ferrite. No specific orientation relationship between abnormal ferrite and cementite has been found in the non-field- or the field-treated specimens. Magnetic field evidently promotes the spheroidization of pearlite, due to its effect of enhancing carbon diffusion through raising the transformation temperature and its effect of increasing the relative ferrite/cementite interface energy. As magnetic field favors the nucleation of the high magnetization phase-pearlitic ferrite, the occurrence of the P-P2 OR that corresponds to the situation that ferrite nucleates prior to cementite during pearlitic transformation is enhanced by the magnetic field.     

An In situ Experimental Study on Solidification Kinetics of Polyolefins: Effect of Cooling Conditions

The solidification kinetics of polyolefins (PO) under three cooling conditions were investigated using an in situ measurement of the temperature decay within the PO resins. The phase-change temperature range of high-density polyethylene (HDPE) was located between 110 and 120°C, and those of low-density polyethylene (LDPE) and polypropylene (PP) were 90–110°C and 100–120°C, respectively. The cooling rate of the liquid-state stage is larger than that of the crystallization stage, primarily owing to the release of the latent heat of crystallization as well as the reduced temperature difference between the sample and cooling medium; they jointly slow down the cooling rate to an extent. The time with respect to phase transformation and its lasting period have close relations to the materials' molecular characteristics (e.g., Mw, MWD, LCB, etc.). Three empirical equations were proposed, and found to be applicable for the cooling analysis of the PO molten materials at relatively low cooling rates prior to crystallization.     

考虑相变的热弹塑性本构方程及其应用

将在热加工及热处理过程中发生相变的材料考虑为热弹塑性的多相混合物,基于连续介质物理学及含内变量的不可逆热力学,推导出考虑相变的热弹塑性本构方程.根据一组不同温度水平下的高温短时拉力试验,确定本构方程中的材料参数及其随温度的变化关系.同时,提出了考虑相变的热传导方程和计及应力影响的相变动力学方程.基于这些方程编制相应有限元分析程序,用其分析计算了1Cr12WMoV不锈钢管通的焊接残余应力分布.计算所得结果与通过X射线衍射测试的实验结果具有较好的一致性. 关键词： 相变 热弹塑性 本构方程 焊接     

焊后热处理对聚变堆用奥氏体不锈钢MAG焊焊缝金属冲击韧性的影响

After heat treatment of metal active gas arc welding (MAG weld) joints of 316LN austenitic stainless steel for fusion reactor at different temperatures, Charpy impact test at liquid nitrogen temperature was carried out. The effects of heat treatment temperature on microstructure, fracture morphology and precipitates were studied with optical microscope, SEM and EDS analysis. Results indicated that the impact toughness of weld metal could be significantly improved by 873K heat treatment. However, with the rising of heat treatment temperature, the precipitates were generated gradually along the grain boundary and a gradual decrease of toughness occurred as well. All fracture was ductile. The dimple became shallow and less with the increase of temperature. Spherical and irregular precipitates were found at the bottom of the dimple. The spherical precipitates generated during welding were free from the influence of the heat treatment, and the irregular precipitates increased with the rising of temperature. High content of Mo in filler materials led to a great segregation of Mo at the grain boundary, which would promote the precipitation of σ phase. Impact toughness of weld metal decreased significantly when continuous distribution of σ phase formed at grain boundary.     

RAFM钢TIG焊和电子束焊疲劳性能实验和分析模拟

利用ANSYS 对低活化铁素体马氏体(RAFM)钢进行非熔化极气体保护焊(TIG 焊)与电子束焊的抗疲劳模拟分析，再利用SDS200 电液伺服疲劳试验机对TIG 焊和电子束焊的两种RAFM 钢试件进行实验。通过施加相同梯度负荷对TIG 焊和电子束焊试件进行焊缝的疲劳性能实验。与实验结果对比分析，结果显示电子束焊优于 TIG 焊，但在一定负载下可以用TIG 焊代替电子束焊。     

Martensitic transformation and physical properties of ‘steel–TiNi’ bimetal composite,produced by explosion welding

The aim of this work is an investigation of structure and martensitic transformation in bimetal composite ‘TiNi–stainless steel’ produced by explosion welding. The results have shown that the mixture of chemical elements is observed in very narrow intervals of 6 µm close to the joint – 2 µm from the TiNi side and 4 µm from the steel one. Micro-hardness distribution in the vicinity of the joint is non-monotonic in the interval of 60 µm. Connection of stainless steel and TiNi plates by explosion welding leads to a dramatic change of martensitic transformation kinetics. Temperatures and the temperature interval of phase transformation increase strongly and heat transformation decreases. Annealing at 500°C for 2 h of bimetal composite decreases the interval of micro-hardness variation and partially recovers kinetics of phase transitions.     

高超音速进气道的冷却研究

作为我国航天飞机研究的一个开端，本文以美国ＮＡＳＡ高超音速研究发动机ＨＲＥ的进气道几何为模型，进行了飞行马赫数为６时的进气道各部分的气动和气动热计算，并在此基础上提出了以氢燃料作冷却剂的主动冷却结构，进行了冷却计算，证明提出的冷却结构所需要的氢用量是合理的。