Verification of the Simulated Residual Stress in the Cross Section of Gray Cast Iron Stress Lattice Shape Casting via Thermal Stress Analysis

The residual stresses in the thick part of the stress lattice shape casting consist of the residual stress due to the temperature differential between the thick part and the thin part and the residual stress due to the temperature differential in the radial direction of the thick part. In this study, the gray cast iron stress lattice shape castings were cast and both types of the residual stresses were separately measured. Thermal stress analyses based on the casting experiment were conducted. Next, the measurements in this study were compared with both types of the simulated residual stresses. The thermal stress analyses estimated the residual stress due to the temperature difference in the radial direction of the thick part to be significantly higher than the measurement, although the residual stress due to the temperature difference between the thick part and the thin part was successfully predicted within a 10 pct error. Thus, this study suggested the introduction of the mechanical melting temperature, above which the very low yield stress is applied conveniently to describe the losses of the deformation resistance of the casting, to more accurately predict the residual stress due to the temperature difference in the radial direction of the thick part. From the verification of the suggested model, this study demonstrated that the conventional elasto-plastic model must introduce the mechanical melting temperature to predict the residual stress due to the temperature difference in the radial direction of the thick part and thus the overall residual stress in the stress lattice.     

The Effect of Aging on the Relaxation of Residual Stress in Cast Aluminum

Most cast aluminum-engineered components are water quenched after the solution-treatment cycle of the casting process. This rapid water quenching has the potential to induce high residual stresses in regions of the castings. Reducing the amount of residual stress can have a promising effect on the life of the component. This study was conducted to quantify how aging affects the amount of residual stress in an aluminum casting. An engineered high residual stress test sample and quenching technique has been developed, and a relaxation study has been completed. The study focused on four different temperatures: 463 K, 493 K, 513 K, and 533 K (190 °C, 220 °C, 240 °C, and 260 °C) and a range of aging times (0.3 to 336 hours). The aging data were used to verify a stress relaxation model. The results indicated that as the aging temperature increased, the amount of relaxation of the residual stress increased.     

Deformation Prediction of a Heavy Hydro Turbine Blade During the Casting Process with Consideration of Martensitic Transformation

Heavy hydro turbine castings are made of martensitic stainless steel, which undergoes martensitic transformation during the casting process. Therefore, both residual stress and deformation are affected not only by uneven cooling but also by martensitic transformation. In this paper, a coupled thermo-martensitic phase transformation–stress model was established and it was implemented by further development with ABAQUS, which also incorporated the thermal and mechanical boundaries, and the contact pair between the casting and mold. The system was applied to the analysis of a heavy hydro blade casting. Results of stress, displacement, and martensite phase fraction were obtained. It is found that martensitic transformation has a significant effect on the stress and deformation results. The displacement in the normal direction of local areas was calculated to represent deformation in the x, y, and z directions. The deformation of the blade casting occurred mainly at the two thin corners with 18 and 22 mm in opposite tendency. The simulated results were compared with the measured machining allowance, and they are basically in agreement.     

A Benchmark Study on Casting Residual Stress

A benchmark study was undertaken for casting residual stress measurements through neutron diffraction, which was subsequently used to validate the accuracy of simulation prediction. The “stress lattice” specimen geometry was designed such that subsequent castings would generate adequate residual stresses during solidification and cooling of ductile cast iron, without any cracks. The residual stresses in the cast specimen were measured using neutron diffraction. Considering the difficulty in accessing the neutron diffraction facility, these measurements can be considered as a benchmark for casting simulation validations. Simulations were performed using the identical specimen geometry and casting conditions for predictions of residual stresses. The simulation predictions were found to agree well with the experimentally measured residual stresses. The experimentally validated model can be subsequently used to predict residual stresses in different cast components. This enables incorporation of the residual stresses at the design phase along with external loads for accurate predictions of fatigue and fracture performance of the cast components.     

Effects of Recovery Behavior and Strain-Rate Dependence of Stress–Strain Curve on Prediction Accuracy of Thermal Stress Analysis During Casting

Recovery behavior (recovery) and strain-rate dependence of the stress–strain curve (strain-rate dependence) are incorporated into constitutive equations of alloys to predict residual stress and thermal stress during casting. Nevertheless, few studies have systematically investigated the effects of these metallurgical phenomena on the prediction accuracy of thermal stress in a casting. This study compares the thermal stress analysis results with in situ thermal stress measurement results of an Al-Si-Cu specimen during casting. The results underscore the importance for the alloy constitutive equation of incorporating strain-rate dependence to predict thermal stress that develops at high temperatures where the alloy shows strong strain-rate dependence of the stress–strain curve. However, the prediction accuracy of the thermal stress developed at low temperatures did not improve by considering the strain-rate dependence. Incorporating recovery into the constitutive equation improved the accuracy of the simulated thermal stress at low temperatures. Results of comparison implied that the constitutive equation should include strain-rate dependence to simulate defects that develop from thermal stress at high temperatures, such as hot tearing and hot cracking. Recovery should be incorporated into the alloy constitutive equation to predict the casting residual stress and deformation caused by the thermal stress developed mainly in the low temperature range.

     

Continuous bending and straightening technology of Q345c slab based on high-temperature creep deformation

A new continuous bending and straightening casting curve with the aim of full using of high-temperature creep deformation was proposed.The curvature of bending and straightening segment varies as sine law with arc length.The basic arc segment is shortened significantly so that the length of bending and straightening area can be extended and the time of creep behavior can be increased.The distance from solidifying front in the slab was calculated at 1 200°C by finite element method.The maximum strain rate of new casting curve at different locations inside the slab is 6.39×10~(-5) s~(-1) during the bending segment and it tends to be 3.70×10-5 s~(-1) in the straightening segment.The minimum creep strain rate is 7.45×10~(-5) s~(-1)when the stress is 14 MPa at 1 200°C.The strain rate of new casting machine can be less than the minimum creep strain rate.Thus,there is only creep deformation and no plastic deformation in the bending and straightening process of steel continuous casting.Deformation of slabs depending on creep behavior only comes true.It is helpful for the design of the new casting machine and improvement of old casting machine depending on high temperature creep property.     

厚壁冷成型钢构件残余应力分析

从理论上将厚壁冷弯残余应力形成过程模拟成大变形加载(弯曲)和弹性卸载(回弹)过程,对不同厚度、不同相对弯曲半径下的冷成型厚钢板进行了大变形弯曲和回弹分析,得到了弯曲应力场和回弹应力场,将这两种应力场相叠加,得到了残余应力场,并分析了其分布规律,最后以国外的实测结果为基础进行了比较分析,证实了该理论分析方法的正确性,从而为进一步研究残余应力对厚壁冷成型钢构件性能的影响奠定了理论基础。     

Constraint induced stresses and the hard surface-soft surface question in fatigue

Cu, 80-20 α-brass and 70-30 α-brass single crystals were cycled in a strain controlled tension-compression mode. Chemical polishing over the entire gage length of specimens in the grips at zero load and zero strain and reloading indicated no detectable difference in flow stress, or equivalently no significant difference in the resolved shear stress on the slip system. When half the gage length was electrolytically polished in specimens unloaded from tension to zero load, bending occurred in a manner indicating that the residual σ_zz stress was higher at the interior than at the surface. Unloading from compression to zero load indicated a reversal in residual σ_zz stress distribution. Finite element method (FEM) analysis, taking into account the slip behavior of a single crystal and the effect of end constraints occurring in the test, confirmed that there was little difference between the resolved shear stress on the slip system at the surface and the interior during deformation. The FEM calculations also indicated that the residual σ_zz stress was nonuniform, in agreement with the bending experiments. These results, therefore, indicate that a difference in flow stress between surface and interior is not necessary for bending to take place when half the gage length is electropolished.     

钢结构卷板机机架的应用设计

西安建筑科技大学粉体工程研究所成功地开发了一台卷板机,该卷板机机架采用全焊钢结构代替了传统的铸造结构。利用有限单元法中的板壳计算程序单元求得了该支架在使用条件下的应力和形变。数值模拟结果表明该支架完全满足生产工艺要求,这台卷板机已在工程实践中成功地应用。     

Three-dimensional modelling of thermal residual stresses and mechanical behavior of cast SiC/A1 particulate composites

Thermal residual stresses developed during casting of SiC/aluminum particulate-reinforced composites were investigated as a function of cooling rate and volume fraction of particles using thermo-elastoplastic finite element analysis. The phase change of the matrix during solidification and the temperature-dependent material properties as the composite is cooled from the liquidus temperature to room temperature were taken into account in the model. Further, the effect of thermal residual stresses on the mechanical behavior of the composites was also studied. Based on the study, it was found that the matrix undergoes significant plastic deformation during cool down and has higher residual stress distribution as the cooling rate increases. The model which does not include the solidification of the matrix tends to overestimate the residual stresses in the matrix and underestimate the tensile modulus of elasticity of the composites. In addition, the presence of thermally induced residual stresses tends to decrease the apparent modulus of elasticity and increase the yield strength of the composites compared to those without residual stresses.     

Effect of Multi-Axial Loading on Residual Strain Tensor for 12L14 Steel Alloy

Evaluating the state of residual strain or stress is critically important for structural materials and for reliable design of complex shape components that need to function in extreme environment subjected to large thermo-mechanical loading. When residual stress state is superposed to external loads, it can lead to reduction or increase in failure strength. Past diffraction studies for evaluating the residual strain state involved measuring lattice spacings in three orthogonal directions and do not often correspond to principal directions. To completely resolve the state of strain at a given location, a full strain tensor must be determined. This is especially important when characterizing materials or metallic components exposed to biaxial or complex loading. Neutron diffraction at the second Generation Neutron Residual Stress Facility (NRSF2) at Oak Ridge National Laboratory is used in this study to measure strain tensors associated with different modes of stress path. Hollow cylinder steel samples with 2 mm wall thickness are subjected to either pure axial extension or pure torsion to simulate multi-axial loading conditions. A virgin sample that is not subjected to any deformation, but subjected to identical manufacturing conditions and machining steps involved to obtain hollow cylinder geometry is used for obtaining reference d-spacing for given hkl planes at target spatial location(s). The two samples which are subjected to either pure tension or torsion are loaded to a deformation state that corresponded to equal amount of octahedral shear strain which is an invariant. This procedure is used so that a basis for comparison between the two samples can be made to isolate the stress path effects. A 2-circle Huber orienteer is used to obtain strain measurements on identical gauge volume at a series of φ and ψ values. The residual state of stress tensor corresponding to ex situ (upon unloading) conditions is presented for three lattice planes (211, 110, 200) for a bcc ferritic system exposed to tension and pure torsion.     

碳化硅晶须镁基复合材料热残余应力及其调整

结合理论分析,介绍了复合材料热残余应力的调整机制,分别采用低温处理及预变形方法调整碳化硅晶须镁基复合材料中残余应力。复合材料经过低温处理及预拉伸塑性变形后残余应力明显降低,同时其介伸屈服强度得到适当改善。     

Propagation of surface cracks on beam blank in the mould during continuous casting

ABSTRACT

We focus on crack propagation to investigate surface cracks in the mould during continuous casting, based on the crack initiation mechanism proposed in previous studies. The temperature and stress data of a solidified shell were extracted, and an extended finite element model based on the continuous damage theory of elastic–plastic materials was developed to simulate surface crack propagation. The results showed that, in the cracked area, stress concentration occurred at the crack tip, and the element split open and the crack propagated when the maximum principal stress in the stress concentration area reached the critical value. Prefabricated cracks in the fillet and web mainly developed into longitudinal cracks in the mould. The theoretical mechanism of this study was found to be the same as the crack propagation mechanism observed during the actual production of beam blanks. Thus, this study reveals the theoretical principle of crack initiation and propagation and can provide theoretical guidance for controlling surface cracks during beam blank continuous casting.     

H85黄铜带材表面点状起皮原因分析及对策

利用SEM、EDS等对H85黄铜带材表面出现起皮的原因进行分析，结果表明:因熔铸过程中炉衬中的Al元素进入熔体，与O等形成不同于基体的异物，造成基体的晶格畸变，在弹性应力场与运动位错的交互作用下，导致物料发生开裂、脱落，形成点状起皮等宏观缺陷.      

The Residual Stress Relaxation Behavior of Weldments During Cyclic Loading

Accurate measurement of residual stress is necessary to obtain reliable predictions of fatigue lifetime and enable estimation of time-to-facture for any given stress level. In this article, relaxation of welding residual stresses as a function of cyclic loading was documented on three common steels: AISI 1008, ASTM A572, and AISI 4142. Welded specimens were subjected to cyclic bending (R = 0.1) at different applied stresses, and the residual stress relaxation existing near the welds was measured as a function of cycles. The steels exhibited very different stress relaxation behaviors during cyclic loadings, which can be related to the differences in the microstructures of the specimens. A phenomenological model, which treats dislocation motion during cyclic loading as being analogous to creep of dislocations, is proposed for estimation of the residual stress relaxation.     

中厚板辊式矫直过程弯辊作用分析

 为了进一步提高中厚板产品矫直质量,采用弹塑性差分的曲率积分方法,分析了弯辊对矫直效果的影响,研究了弯辊量与压下量的关系,为建立中厚板辊式矫直过程的弯辊设定模型奠定了基础。计算结果表明,为了使板材矫直后的残余曲率和残余应力最小化,施加弯辊时要考虑入口压下量的影响。单从降低残余曲率的角度考虑,小压下量和大压下量下都可以施加负弯辊,但正弯辊只适用于小压下量。从控制宽度方向残余应力的角度考虑,正弯辊施加于小入口压下量时弯辊作用明显,为了消除正弯辊对厚度方向残余应力的不利作用,采用正弯辊时入口压下量不宜过大。施加弯辊会增加矫直力,因此设定弯辊量时要考虑矫直机能力的限制。研究发现了弯辊对塑性变形率的影响规律：负弯辊出现了使塑性变形率随着入口压下量的增加而减少的阶段,正弯辊可消除塑性变形率为零的阶段。     

The heat-transfer coefficient during the unidirectional solidification of an Al-Si alloy casting

The heat-transfer coefficient was measured during the unidirectional solidification of Al-7 wt pct Si alloy castings against a water-cooled Cu chill. Heat-transfer coefficients in the range of 2.5 to 9 kW m⁻² K⁻¹ were obtained with solidification vertically upward associated with higher values than solidification vertically downward. Horizontal solidification was associated with intermediate values. Profiles taken across the diameters of the casting surfaces at the interface with the chill showed them to be convex toward the chill by an amount which would produce a mean gap between the casting and the chill that would account for a significant proportion of the measured heat-transfer coefficient. The convex casting surfaces were attributed to the deformation of the initial casting skin by the thermal stress produced during solidification. Heat transfer during casting solidification is shown to be a complex mechanism controlled by the microscale surface roughness of the respective surfaces, mesoscale deformation of the casting skin by thermal stress, and macroscale movements of the casting and the chill due to their relative thermal expansion and contraction.     

Realistic modelling and optimisation of steel section cooling process

Abstract

The cooling process in the manufacture of long steel products generates residual stresses and bending in the section. This initial state, arising from the cooling bed, influences the final residual stresses and bending at the end of the subsequent processes. Owing to the importance of the cooling process, this paper presents realistic modelling and optimisation using computational fluid dynamics (CFD) and finite element analysis. Computational fluid dynamics rendered it possible to accurately overcome two main problems common to previous cooling models: the realistic modelling of the heat transfer coefficient (especially important when modelling outdoor cooling beds because of the implications of forced convection) and the precise view factor modelling of the different section surfaces (useful when modelling a complex section). After decoupled CFD thermo‐analysis, the temperature record of each node in the section was loaded into the finite element stress displacement model. The relevant influence of steel phase transformation was considered applying a combined methodology, involving an ABAQUS user subroutine. Accordingly, accurate residual stresses and bending were obtained. After establishing the models, several strategies were analysed for reducing the residual stresses during the cooling process. Results were successfully validated with experimental data from structural section producers.     

六辊CVC轧机接触应力与凸度控制的有限元分析

 为了深入研究六辊CVC轧机的带钢凸度控制能力,采用非线性弹塑性有限元法,利用大型非线性有限元软件MSC.Marc建立了六辊CVC轧机轧制过程三维有限元仿真模型。模型将辊系弹性变形与轧件弹塑性变形耦合在一起,进行统一建模与分析。运用该模型分别改变工作辊弯辊力、中间辊弯辊力、中间辊横移量进行有限元模拟,得到了工作辊弯辊、中间辊弯辊、中间辊横移对接触应力和带钢凸度控制的影响规律。所建模型与分析结果可为六辊CVC轧机的板形控制研究提供理论数据与参考依据。