Dependence of heat transfer coefficient at quenching on diameter of cylindrical workpieces

Abstract

For computer simulation of a quenching process, the fundamental prerequisite is to have the relevant heat transfer coefficient (HTC) calculated as a function of the workpiece’s surface temperature and time respectively. In order to calculate the HTC experimental measurement of the temperature–time history (cooling curve) near the workpiece surface is necessary. In this investigation, cylindrical probes with diameters of 20, 50 and 80 mm are used. The cooling curve was always measured 1 mm below the surface of the probe. Special care has been taken to keep all other factors (e.g. design of the probes, temperature measurement, quenching conditions and calculation procedure), which can influence the calculated HTC, constant, in order to ensure that the only variable is the diameter of the probe. Assuming a radially symmetrical heat flow at the half length of the probe, the HTC was calculated using one-dimensional inverse heat conduction method. The unexpected striking result of this investigation is the fact that for the probe diameter (80 mm) the calculated HTC as a function of surface temperature does not show the film boiling phase. A plausible explanation for this effect is given, based on the critical heat flux density. The possibility of establishing a simple fixed relation (a correction factor) between the HTC and the diameter of cylinders is discussed.     

薄铝板淬火换热系数求解

针对薄铝板淬火过程,本文采用不同函数形式描述淬火换热系数,并借助ABAQUS有限元软件进行温度场和位移场模拟,得到换热系数和位移场的关系.基于试验所得翘曲位移反求换热系数,并对所选取的函数形式进行了优选.     

薄铝板淬火换热系数求解

针对薄铝板淬火过程,本文采用不同函数形式描述淬火换热系数,并借助ABAQUS有限元软件进行温度场和位移场模拟,得到换热系数和位移场的关系。基于试验所得翘曲位移反求换热系数,并对所选取的函数形式进行了优选。     

基于DEFORM反传热模型表面换热系数的确定

以7075铝合金厚板淬火过程为对象,研究DEFORM反传热模型中控制参数对表面换热系数计算和温度预测精度的影响规律。结果表明,当选择实测温度曲线上的拐点温度作为温度控制点,且表面换热系数初始值接近平均换热系数时,采用反传热模型确定的表面换热系数所预测的冷却曲线与实测曲线吻合较好。在此基础上选取合理的控制参数,并确定了7075铝合金厚板淬火过程的表面换热系数,经冷却曲线预测结果与实测值对比表明,采用DEFORM反传热模型确定的表面换热系数所预测的温度场有较高精度,可以满足工程应用需要。     

一种铝合金水冷界面换热系数反求方法的研究

针对水冷金属界面换热系数影响因素多,测量与求解难的问题,以温度场数学模型为基础,以实测温度曲线为基准,通过数值模拟迭代计算和自动寻优,实现了铝合金水冷界面换热系数随温度变化定量关系的反求.反求得到的铝合金换热系数结果表明:在浸入式水冷过程中,铝合金界面换热系数随表面温度由低到高呈现出先升后降的单峰形状特征,降低冷却水的温度会使换热系数的峰值点升高,但不会改变峰值点出现的温度范围,换热系数的最大值出现在200～230℃.金属与冷却水之间热交换的强度主要取决于界面温度,将界面温度控制在200～230℃会使强化传热效果达到最佳.     

Experimental determination of convective heat transfer coefficient in WEDM

An experimental determination method of the convective heat transfer coefficient in wire electro-discharge machining (WEDM) is introduced. A special device is developed to measure the average temperature increment of the wire after a period of short circuit discharges, and the thermal load imposed on the wire is also tracked and recorded in advance. Then, based on the thermal model of the wire, the convective coefficient can be calculated accurately. Some tuning experiments are carried out inside and outside a previously cut profile to examine the influence of the kerf on the convective coefficient. As soon as the wire cuts into the workpiece, the convective coefficient will decrease more than 30%. With this method, the effect of the coolant flushing pressure on the convective coefficient is also estimated. If the pressure is raised from 0.1 to 0.8 Mpa, the convective coefficient will increase more than 20%, and thus ameliorate the cooling condition of the WEDM process.     

比较厚板淬火换热系数反求的两种方法

在厚板淬火过程的数值模拟中,换热系数的正确求解是保证其温度场及应力场模拟结果与实际结果一致的前提。在实测冷却曲线的基础上建立了换热系数求解的两种数学模型,计算了换热系数随淬火时间关系曲线。基于ABAQUS模拟软件分析了两种模型在某特定区域温度场的实测与模拟误差。结果表明,换热系数随时间呈非均匀分布,在20～40 s之间出现换热系数峰值;一点法求解的换热系数优于两点法;两种方法计算的表面温度均出现温度回升现象,但一点法求解的表面温度回升较两点法的平缓。     

铝合金厚板淬火表面换热系数的离散解析求法

为了快速准确求取铝合金厚板淬火过程的换热系数,对淬火热传导过程进行分析。首先,将换热系数解析过程假设为淬火温度离散化的,并且是相邻离散点可进行迭代优化的计算过程。然后,分步解析求解了各离散温度区间的换热系数,最后完成了数据修正和仿真计算还原。结果表明,该方法获得的换热系数,可以使实验冷却曲线与计算冷却曲线较好的吻合,从而证明这种计算方法的可行性,并在文末对该方法的误差来源和特点进行了分析。     

Regularized determination of interfacial heat transfer coefficient during ZL102 solidification process

The interfacial heat transfer coefficient（IHTC） between the casting and the mould is essential to the numerical simulation as one of boundary conditions. A new inverse method was presented according to the Tikhonov regularization theory. A regularized functional was established and the regularization parameter was deduced. The functional was solved to determine the interfacial heat transfer coefficient by using the sensitivity coefficient and Newton-Raphson iteration method. The temperature measurement experiment was done to ZL102 sand mold casting, and the appropriate mathematical model of the IHTC was established. Moreover, the regularization method was used to determinate the IHTC. The results indicate that the regularization method is very efficient in overcoming the ill-posedness of the inverse heat conduction problem（IHCP）, and ensuring the accuracy and stability of the solutions.     

An experimental study of heat transfer in aluminum castings during water quenching

The influence of quenching orientation and agitation conditions on heat transfer of aluminum alloys during water quenching was experimentally investigated with a test casting. The results indicate that heat transfer in water quenching of casting aluminum alloy consists of film boiling, nucleate boiling and convection stages. The highest heat transfer coefficients (HTC) are observed in the nucleate boiling, while the lowest is in the convective cooling stage. The heat transfer coefficients on the horizontal surfaces facing down during quenching are lower than those of other surfaces regardless whether the water is agitated or not. Agitation enhances heat transfer process especially when castings are at high temperatures and heat transfer process is in the film boiling stage.     

不同构造窗户的传热系数和设计应用

窗是房屋建筑中的主要围护构件之一,是建筑物得失热的主要部位。在建筑节能的研究领域内,窗户的传热系数是一个极其重要的物理量。本文探讨了不同构造窗户的传热系数及其设计运用。     

对流换热系数对熨平板焊接变形和应力的影响

采用有限元模拟不同对流换热系数下熨平板的焊后变形和应力分布,并采用塞尺和X射线法进行试验验证。结果表明:整体变形趋势为U形板呈鼓泡状,左连接板伸出端向x正向收缩变形,两侧板自由边向内收缩变形。最大变形位置出现在连接板伸出端最上端,对流换热系数为采用曲线加载时,变形误差为34.3%,系数为0.02时,误差为17.1%;不同对流散热系数对残余应力峰值影响很小,应力分布与对流换热系数为0.02时趋势一致,误差为32.6%,采用曲线加载,不考虑低应力区结果,误差为21.1%,均满足工程应用要求,证明了模拟结果的准确性。两种加载方式均满足焊接变形预测要求,在焊缝较多、应力分布复杂的情况下,应力预测建议采用曲线加载方式,反之则采用系数为0.02加载。     

火车轮模锻成形工艺及热处理换热系数测算

为研究火车轮在模锻成形过程中内部金属流动及淬火加热、踏面淬火中的换热系数和温度变化,利用有限元软件对其模锻成形及热处理过程进行数值模拟分析,并结合黄金分割优化法对综合换热系数进行了反传热计算。结果表明:模锻过程中,辐板与上模接触区域的等效应变最大,轮辋外侧变形相对较小,踏面附近区域变形较均匀;在淬火加热过程中,换热系数随工件表面温度升高而增加,当温度在500℃以下时,换热系数随温度的升高而快速增加,500℃以后,增速缓慢,800℃时,换热系数达0.15 kW·(m^2·℃)^-1;踏面淬火时,在700℃以下,随温度下降,换热系数迅速增大,300℃时达到峰值3.1 kW·(m2·℃)^-1,在250℃以下,换热系数稍有下降,100℃时换热系数为2.5 kW·(m2·℃)^-1。     

Equivalent heat transfer coefficient at casting/Cu mould interface and temperature field simulation

1　INTRODUCTIONThesolidifiedmicrostructuresofalloysdependontheirsolidifyingprocesswhoseprimarycharacteristicsarethetemperaturedropofthesuperheatedmeltandthere leaseofthelatentheat.Sothestudyontheheattransferduringthesolidificationprocessistheessentialprobleminthesolidificationtheorystudy .Theresearchersworkingonthenumericalsimulationofthesolidificationprocessallknowthattheinterfacialheattransfercoefficientatthecasting/mouldisavariablechangingwithtime .Thusthedeterminationoftheinterfacialh…     

Determination of interfacial heat transfer coefficient and its application in high pressure die casting process

In this paper,the research progress of the interfacial heat transfer in high pressure die casting（HPDC）is reviewed.Results including determination of the interfacial heat transfer coefficient（IHTC）,influence of casting thickness,process parameters and casting alloys on the IHTC are summarized and discussed.A thermal boundary condition model was developed based on the two correlations：（a）IHTC and casting solid fraction and（b）IHTC peak value and initial die surface temperature.The boundary model was then applied during the determination of the temperature field in HPDC and excellent agreement was found.     

22MnB5钢表面微观形貌对界面换热系数的影响

为了探究冲压压强和样件表面粗糙度对22MnB5钢板的界面换热系数(Interfacial heat transfer coefficient,以下简称IHTC)的影响,自主设计了圆台试验模型,利用Beck非线性估算法,求解热成形工艺中高温样件与低温模具间界面换热系数。实验中通过调整冲压压强和改变样件表面粗糙度的方式,进而改变模具与样件间接触表面微观形貌,探究其对IHTC的影响。研究表明:压强与IHTC存在高度近似的正幂函数关系;粗糙度也会对IHTC产生影响,并且当粗糙度大于1μm时,IHTC值会随粗糙度的增大而明显减小,当粗糙度小于1μm时,由于表面氧化皮的影响,IHTC将随粗糙度值的减小而增大放缓,甚至趋于定值而不再增加。     

Measurement of temperature inside die and estimation of interfacial heat transfer coefficient in squeeze casting

As an advanced near-net shape technology, squeeze casting is an excellent method for producing high integrity castings. Numerical simulation is a very effective method to optimize squeeze casting process, and the interfacial heat transfer coefficient (IHTC) is an important boundary condition in numerical simulation. Therefore, the study of the IHTC is of great significance. In the present study, experiments were conducted and a"plate shape" aluminum alloy casting was cast in H13 steel die. In order to obtain accurate temperature readings inside the die, a special temperature sensor units (TSU) was designed. Six 1 mm wide and 1 mm deep grooves were machined in the sensor unit for the placement of the thermocouples whose tips were welded to the end wall. Each groove was machined to terminate at a particular distance (1, 3, and 6 mm) from the front end of the sensor unit. Based on the temperature measurements inside the die, the interfacial heat transfer coefficient (IHTC) at the metal-die interface was determined by applying an inverse approach. The acquired data were processed by a low pass filtering method based on Fast Fourier Transform (FFT). The feature of the IHTC at the metal-die interface was discussed.     

不同间隙下硼钢板-模具界面换热系数的反演

确定不同条件下板料-模具界面的换热系数对控制硼钢板在热成形中的局部冷却速率、获得组织性能呈区域性分布的构件具有重要意义,还可为热成形过程精确数值仿真提供可靠的热边界条件。设计了简单、有效的实验装置以模拟硼钢板热成形中不同单边间隙下板料与模具间的传热状态,其中测温点处的温度变化仅与板料-模具界面换热系数相关,从而为界面换热系数的准确反演提供了保障。建立了界面换热系数的有限元优化模型,反演获得了不同间隙下B1500HS板与H13模具钢之间的换热系数,发现其随间隙的增大呈指数函数式减小,并对反演获得的界面换热系数进行了实验验证。     

基于瞬态法的铝合金与模具钢接触换热实验研究

采用瞬态实验法研究了7050铝合金与5CrMnMo模具钢界面间接触换热过程,探讨了接触载荷与模具温度对接触换热系数的影响,对比了瞬态法与稳态法的实验结果。研究表明,接触换热系数随时间延长迅速增大到某一基本恒定值;瞬态法与稳态法的结果存在一定差异,但接触换热系数随接触载荷增大而增加的趋势是一致的;在160~340℃温度范围内,接触换热系数随模具温度升高而增大,为实际锻造过程中模具的预热提供了参考数据。     

Computation of synthetic surface heat transfer coefficient of 7B50 ultra-high-strength aluminum alloy during spray quenching

According to inverse heat transfer theory, the evolutions of synthetic surface heat transfer coefficient (SSHTC) of the quenching surface of 7B50 alloy during water-spray quenching were simulated by the ProCAST software based on accurate cooling curves measured by the modified Jominy specimen and temperature-dependent thermo-physical properties of 7B50 alloy calculated using the JMatPro software. Results show that the average cooling rate at 6 mm from the quenching surface and 420–230 °C (quench sensitive temperature range) is 45.78 °C/s. The peak-value of the SSHTC is 69 kW/(m²·K) obtained at spray quenching for 0.4 s and the corresponding temperature of the quenching surface is 160 °C. In the initial stage of spray quenching, the phenomenon called “temperature plateau” appears on the cooling curve of the quenching surface. The temperature range of this plateau is 160–170 °C with the duration about 3 s. During the temperature plateau, heat transfer mechanism of the quenching surface transforms from nucleate boiling regime to single-phase convective regime.