Finite Element Simulation and Experimental Verification of Internal Stress of Quenched AISI 4140 Cylinders

The study of internal stress in quenched AISI 4140 medium carbon steel is of importance in engineering. In this work, the finite element simulation (FES) was employed to predict the distribution of internal stress in quenched AISI 4140 cylinders with two sizes of diameter based on exponent-modified (Ex-Modified) normalized function. The results indicate that the FES based on Ex-Modified normalized function proposed is better consistent with X-ray diffraction measurements of the stress distribution than FES based on normalized function proposed by Abrassart, Desalos and Leblond, respectively, which is attributed that Ex-Modified normalized function better describes transformation plasticity. Effect of temperature distribution on the phase formation, the origin of residual stress distribution and effect of transformation plasticity function on the residual stress distribution were further discussed.

     

Modeling of Austenite Decomposition in Low Si-Mn TRIP Steel During Cooling

Transformation behavior in low carbon Si Mn TRIP steel was investigated by means of microstructural ob servation and computer modelling. A transformation model in which transformation is controlled by carbon diffusion was suggested, which well described the volume fractional change of ferrite, pearlite, and bainite during continuous cooling. The influence of Si content and austenite grain size was thoroughly investigated. The calculated results indicated that Si retards pearlite transformation, accelerates polygonal ferrite transformation, refines the austenite grain, and increases the ferrite transformation rate.     

On Various Aspects of Decomposition of Austenite in a High-Silicon Steel During Quenching and Partitioning

Using a Gleeble thermomechanical simulator, a high-silicon steel (Fe-0.2C-1.5Si-2.0Mn-0.6Cr) was laboratory hot-rolled, re-austenitized, quenched into the M _s–M _f range, retaining 15 to 40 pct austenite at the quench stop temperature (T _Q), and annealed for 10 to 1000 seconds at or above T _Q in order to better understand the mechanisms operating during partitioning. Dilatometer measurements, transmission electron microscopy, and calculations showed that besides carbon partitioning, isothermal martensite and bainite form at the partitioning temperature. While isothermal martensite formation starts almost immediately after quenching with the rate of volume expansion dropping all the time, the beginning of bainite formation is marked by a sudden increase in the rate of expansion. The extent of its formation depends on the partitioning temperature following TTT diagram predictions. At the highest partitioning temperatures martensite tempering competes with partitioning. Small fractions of bainite and high-carbon martensite formed on cooling from the partitioning temperature. The average carbon content of the austenite retained at room temperature as determined from XRD measurements was close to the carbon content estimated from the M _s temperature of the martensite formed during the final cooling.     

Effect of External Forced Flow and Boiling Film on Heat Transfer of AISI 4140 Steel Horizontal Rod During Direct Quenching

 The effects of rod falling and moving, external flow field, boiling film and radiation were investigated on fluid flow and heat transfer of AISI 4140 steel horizontal rod during direct quenching by mathematical modeling. The flow field and heat transfer in quenching tank were simulated by computational fluid dynamics (CFD) method considering falling and moving of rods during process. Therefore, modeling of flow field was done by a fixed-mesh method for general moving objects equations, and then, energy equation was solved with a numerical approach so that effect of boiling film heat flux was considered as a source term in energy equation for solid-liquid boundary. Simulated results were verified by comparing with published and experimental data and there was a good agreement between them. Also, the effects of external forced flow and film boiling were investigated on heat flux output, temperature distribution and heat transfer coefficient of rod. Also simulated results determined optimum quenching time for this process.     

Comparative Study on Austenite Decomposition and Cu Precipitation During Continuous Cooling Transformation

The nature of Cu precipitation during continuous cooling of austenite is an essential issue in thermomechanical processing of Cu-bearing high-strength low-alloyed steel. The Cu precipitation behaviors and microstructural evolution are comparatively studied at two representative cooling rates of 0.1 and 10 K/s using dilatometry, optical microscopy, transmission electron microscope, and atom probe tomography. The microstructure developed at the cooling rate of 0.1 K/s contains a variety of microconstituents of polygonal ferrite, acicular ferrite, bainite, martensite, and retained austenite, which is attributed to the varying concentrations of untransformed austenite and consequently changing austenite decomposition kinetics. The Cu precipitation occurs only in association with polygonal ferrite transformation by interphase precipitation mechanism. No Cu precipitates are detected in acicular ferrite, retained austenite, and martensite. At the cooling rate of 10 K/s, the microstructure is dominated by acicular ferrite dispersed with the C-Ni precipitates that are assumed to be formed on the retained austenite. The Cu precipitation generally occurs from aging because of the high temperature and supersaturated austenite in association with acicular ferrite transformation. The nature of Cu precipitation in different cooling rates greatly depends on the initial composition or the extent of solute enrichment in the prior untransformed austenite and the associated phase transformation mechanisms, resulting in the differences in sizes, morphology, and compositions of Cu precipitation.     

AISI1008圆钢混晶原因分析及有限元模拟和工艺改进

在棒材线轧制锻造爪机用AISI1008钢时,出现混晶现象,Φ50 mm棒材边缘铁素体晶粒度3级,1/2半径及心部铁素体晶粒度8级,通过对钢材进行金相组织及电镜检验,对轧制过程温度及等效应变利用Deform软件进行有限元模拟,发现造成圆钢边缘晶粒粗大的原因与终轧温度过低及轧制等效应变大且分布不均有关,将终轧温度由原820...     

有限元模拟技术在工字钢控冷过程中的应用

利用有限元方法对工字钢轧后控制冷却过程进行模拟，通过设定与实际冷却过程相符的初始条件和边界条件，获得轧件在冷却过程的瞬态温度场分布。有限元模拟结果与实际结果吻合较好，验证了模型的准确性。应用模拟结果将现有自然空冷的冷却工艺优化为空冷与水冷相结合的控制冷却工艺，改进后的冷却工艺显著改善了工字钢断面温度场分布。     

钢板热轧过程中轧制力的有限元模拟

轧制力是轧制过程中重要的技术参数之一。本文应用DEFORM-3D软件建立轧制模型,研究了轧制温度、轧辊转速和压下率对钢板轧制力的影响。随后通过比较第一道次模拟轧制力与钢厂实测轧制力,结果表明：在三种钢板材料中,DEFORM-3D软件模拟的轧制力均与钢厂实测轧制力较吻合,误差都在10％以内。该模拟为钢厂轧制工艺参数的制定提供了重要的参考价值。     

Hydrogen Absorption and Desorption during Heat Treatment of AISI 4140 Steel

Hydrogen plays an important role in the formation of quench cracks of structural steels. To clarify hydrogen ab- sorption and desorption during heat treatment of AISI 4140 steel, thermal desorption spectrometry （TDS） analysis was carried out for the specimens in the as-rolled, as quenched, and quenched and tempered conditions. Results show that hydrogen content increased from 0. 127 ×10 6 in the as-rolled specimen to 0. 316 × 10-6 in the as-oil-quenched specimen. After tempering at 200 ℃, the hydrogen content in the oil-quenched specimen decreased to 0. 155 × 10-6 , and the peak temperature of hydrogen desorption increased from 200 to 360 ℃. From the dependence of hydrogen content in the as-quenched specimens on austenitizing time, it can be deduced that hydrogen absorption occurs during austenitizing. The simulation of hydrogen absorption contributes to a better understanding on the distribution of hy- drogen during the heat treatment in structural steels.     

Thermal Stabilization of Austenite During Quenching and Partitioning of Austenite for Automotive Steels

Metallurgist - Published sources are reviewed devoted to use of heat treatment for high-strength automotive steels based on the phenomenon of carbon redistribution between martensite and...     

60 kg/m重轨冷却过程中的温度场有限元模拟及分析

 采用ANSYS热模拟以及对实际温度连续测量实验,研究了重轨在步进式冷床上冷却过程的温度分布。重轨表面温度、矫前温度及相对应的冷却时间计算值与实测结果基本相符。模拟结果表明,重轨断面温差最大值达到9112 ℃,而冷却到矫前温度时温度分布均匀,断面温差不超过5 ℃,断面最大温差随着换热系数的增大而增大。在重轨断面温差出现最大值之前,适当减小冷却速度,可使断面温度分布均匀,并且可通过增设风机有效提高冷床的冷却能力。     

In-Situ High-Energy X-ray Diffraction Study of Austenite Decomposition During Rapid Cooling and Isothermal Holding in Two HSLA Steels

In-situ high-energy X-ray diffraction experiments with high temporal resolution during rapid cooling (280 °C s⁻¹) and isothermal heat treatments (at 450 °C, 500 °C, and 550 °C for 30 minutes) were performed to study austenite decomposition in two commercial high-strength low-alloy steels. The rapid phase transformations occurring in these types of steels are investigated for the first time in-situ, aiding a detailed analysis of the austenite decomposition kinetics. For the low hardenability steel with main composition Fe-0.08C-1.7Mn-0.403Si-0.303Cr in weight percent, austenite decomposition to polygonal ferrite and bainite occurs already during the initial cooling. However, for the high hardenability steel with main composition Fe-0.08C-1.79Mn-0.182Si-0.757Cr-0.094Mo in weight percent, the austenite decomposition kinetics is retarded, chiefly by the Mo addition, and therefore mainly bainitic transformation occurs during isothermal holding; the bainitic transformation rate at the isothermal holding is clearly enhanced by lowered temperature from 550 °C to 500 °C and 450 °C. During prolonged isothermal holding, carbide formation leads to decreased austenite carbon content and promotes continued bainitic ferrite formation. Moreover, at prolonged isothermal holding at higher temperatures some degenerate pearlite form.

     

高碳钢线材轧后水冷换热过程的有限差分模拟

根据82B和72A钢Φ5.5～12.5mm高速线材轧后水冷传热特点,将水冷过程分成3个区域,并在水冷箱区域中根据环形喷嘴的开关状态,分别采用了水对流换热和水蒸气对流换热两种形式传热模型,建立了水冷过程按喷嘴个数分段处理的边界条件,通过有限差分模拟得出轧后水冷系统中线材断面的温度分布。通过实测温度值校正高碳钢线材水冷过程中水的对流换热系数,模拟结果表明,精轧机入口和吐丝机的预报和实测线材表面温度均值的绝对误差在±20℃以内。     

Finite Element Simulation of Flow Localization During Equal Channel Angular Pressing

The hardening or softening behavior of materials is one of the major factors affecting the deformation homogeneity in work-pieces during processing by equal channel angular pressing. In the present study, the difference between plastic flow behavior and deformation homogeneity of strain hardening and strain softening materials were investigated with the aid of finite element simulation. Results showed that localization of plastic strain occurred in the strain softening materials and fluctuations were observed in the strain distribution along work-piece. In strain hardening materials, a more uniform distribution of plastic strain was obtained compared to strain softening material.     

棒材穿水冷却温度场的仿真研究

针对棒材穿水冷却过程,采用有限元仿真软件ANSYS对其进行了温度场仿真,建立了棒材温度场预报模型。该模型的仿真结果与实测值相吻合,能够准确地预测棒材穿水冷却过程的温度分布,为棒材性能预报与优化控冷工艺提供依据。     

Influence of Strain History and Cooling Rate on the Austenite Decomposition Behavior and Phase Transformation Products in a Microalloyed Steel

The effect of simple strain path changes as well as post-deformation continuous cooling rate during thermomechanical-controlled processing of microalloyed steel was studied using laboratory physical simulation. The phase transformation characteristics were directly analyzed by dilatometry under various cooling rates. The microstructures of the transformation products were characterized quantitatively using EBSD. The results have shown that while strain path changes impose a considerable influence on the hot flow behavior of the austenite, the cooling rate following hot deformation is the determining factor of the phase transformation mechanism and behavior which establishes the final transformation products and subsequent mechanical properties.     

Combined Effects of Different Overload Ratios and Nitriding Process on Fatigue Crack Growth of AISI 4140 Steel

The combined effects of different overload ratios(OLRs)and nitriding process on fatigue crack growth(FCG)were analyzed.Steel compact tension specimens made of AISI 4140 steel were subjected to the plasma nitrification process using agas mixture of 50% N2 and 50% H2 at 500℃for 2h.Structural properties of the samples were studied by scanning electron microscopy,optical microscopy,microhardness testing and digital imaging.Plasma nitriding slowed down FCG,but the crack growth retardation caused by the overload was reduced when the OLR was 2.0and 2.5.Nitriding did not take effect when the OLR was 1.5and 3.0.     

Influence of Upset Pressure on Microstructure and Mechanical Properties of Friction-Welded AISI 4140 Low Alloy Steel Pipes

Transactions of the Indian Institute of Metals - In the present study, low alloy steel AISI 4140 pipe in reheated, quenched and tempered (RQT) condition was friction-welded using 6.4 MPa...