An experimentally based thermo-kinetic phase transformation model for multi-pass laser heat treatment by using high power direct diode laser

Laser-based phase transformation hardening (LPTH), based on rapid heating and cooling cycles produces hard and wear-resistant layers only at the selective region of the components. However, the bulk mass of the material’s core property is retained. The advantages of high power direct diode laser in comparison with other high power lasers (CO₂ and Nd:YAG) have put this type of laser as a main heat source for localized heat treatment. However, a tempered zone is formed in overlapping regions of a large heat-treated area during multi-pass laser heat treatment (MPLHT) that affects the uniformity of heat-treated depth of material. This study is focused on the development of a uniform hardness distribution model to minimize the tempering effect during the MPLHT process. A tool steel AISI S7 is heat treated by using different levels of laser power (1,400–1,800 W) and scanning speeds (15–25 mm/s). An experimentally based finite element (FE) thermal model is developed to predict the cross-sectional as well as surface temperature history of the MPLHT process. The temperature-dependent material properties and phase change kinetics are taken into account in the model. The laser beam is considered as a moving rectangular-shaped heat source (12 mm?×?1 mm) with a uniform distribution (top-hat) of laser power. The temperature history acquired from the FE thermal model is coupled with thermo-kinetic (TK) equations to determine the corresponding phase transformations and hardness. The tempering effect of MPLHT is studied for different sizes of overlap (1 mm–3 mm) and lengths of scan (10 mm–35 mm). The TK model results are verified with experimental ones to optimize the processing parameters. The optimized processing parameters, including laser power, scanning speed, size of overlap, and the length of scan are used to achieve a uniform hardness distribution and an even depth of heat treatment in the MPLHT area.     

钢和铸铁激光相变硬化层的残余应力研究

对钢和铸铁中激光相变硬化引起的残余应力现象进行了研究。结果表明，激光相变硬化层中的残余压应力是普遍存在的现象。激光能量密度Ｐ／Ｄ_ｂ~ｖ对残余应力的大小有很大影响，且对具有不同原始组织的材料，影响的规律也不同。推导了激光相变硬化层的残余应力公式，并对实验结果进行了分析讨论。     

Simulation of thermal stresses due to grinding

An efficient finite element procedure has been developed to calculate the temperatures and stresses arising due to a moving source of heat. The procedure is applied to calculate the thermal stresses produced in hardened steels during grinding. The thermal load during grinding is modeled as a uniformly or triangularly distributed, 2D heat source moving across the surface of a half-space, which is insulated or subjected to convective cooling. The grinding of elastic and elastic–plastic workpiece materials has been simulated. The calculated transient stresses and temperatures in an elastic solid are found to be in good agreement with prior analytical and numerical results. In an elastic–plastic workpiece material, for which no analytical solution is available for the residual stress distributions, the finite element calculations show that the near surface residual stress is predominantly tensile and that the magnitude of this stress increases with increasing heat flux values. Based on an analysis of the effects of workpiece velocity, heat flux magnitude and convective cooling, on the residual stress distributions in an elastic–plastic solid, it is seen that the calculated thermal stress distributions are consistent with experimentally measured residual stresses on ground surfaces. Furthermore, the results explain often cited observations pertaining to thermally induced grinding stresses in metals.     

Thermal stress developed during the laser cutting process: consideration of different materials

The laser cutting of metallic substrates results in the development of thermal stresses around the cut edges. Depending on the cutting speed, laser power intensity, and material properties, stress levels reaching and exceeding the yielding limit of the substrate material can result. In the present study, the laser cutting situation is simulated and temperature as well as thermal stress fields are computed for steel, Inconel 625, and Ti-6Al-4V alloy. The cutting speed of the laser is considered to be constant and a constant temperature heat source with a focused spot diameter is assumed along the kerf surface at the cut edge, resembling the laser heat source. The equations for energy and thermal stresses are solved numerically using the finite element method (FEM). It is found that the temperature decays sharply in the vicinity of the cut edges and that the equivalent stress attains high values in this region. Inconel 625 results in the highest thermal stress levels in the vicinity of the cut edges and is then followed by steel and titanium alloy.     

Application of quenching to create highly triaxial residual stresses in type 316H stainless steels

It has been proposed that highly triaxial residual stress fields may be sufficient to promote creep damage in thermally aged components, even in the absence of in-service loads. To test this proposal, it is necessary to create test specimens containing highly triaxial residual stress fields over a significant volume of the specimen. This paper presents results from an experimental and numerical study on the generation of triaxial residual stresses in stainless steel test specimens. Spray water quenching was used to generate residual stress fields in solid cylinders and spheres made from type 316H stainless steel. A series of finite element simulations and measurements were carried out to determine how process conditions and specimen dimensions influenced the resulting residual stress distributions. The results showed that highly compressive residual stresses occurred around the surfaces of the cylinders and spheres and tensile residual stresses occurred near the centre. Surface residual stresses were measured using the incremental centre hole-drilling technique, while internal residual stresses were measured using neutron diffraction. Overall there was good agreement between the predicted and measured residual stresses. The level of triaxiality was found to be very sensitive to the heat transfer coefficient, and could be controlled by adjusting the cooling conditions and changing the dimensions of the steel samples. This differed from other processes, such as welding and shot-peening, where the magnitudes and distributions of residual stresses are ill-defined and the volume of material subjected to a triaxial residual stress state is relatively small.     

“Finite element modeling of the effect of heat input on residual stresses in dissimilar joints”

In the present study, a thermo-elastic–plastic model was developed in order to evaluate the residual stresses in dissimilar automatic tungsten inert gas (TIG) welds between plain carbon steel CK4 and a ferritic stainless steel AISI409. The effect of welding heat input on the magnitude and the distribution of residual stresses was investigated and the results of simulation were validated by X-ray diffraction measurements. It is shown that the calculated residual stresses are in good agreement with the residual stresses determined experimentally. It was found that the magnitudes of stresses at the weld center line increases with increasing the welding speed.     

304不锈钢薄壁焊管表面残余拉应力的测定及消除

利用X射线应力测定仪采用不同的条件分别测定了304不锈钢薄壁焊管的表面轴向和切向残余应力,并研究了950℃保温30min水冷热处理前后其表面残余应力的变化。结果表明:采用铬靶Kβ辐射、2mm准直管测得的残余应力数据稳定且绝对误差小;热处理前薄壁焊管表面存在较大的残余拉应力,且焊管表面切向残余应力大于轴向残余应力;热处理后,表面拉应力变为压应力。     

钛合金薄板激光焊接和TIG焊接残余应力数值模拟

基于有限元分析软件ANSYS，以激光焊接和TIG焊接温度场模拟为基础，对钛合金薄板的焊接残余应力进行了数值模拟，并分析了不同焊接工艺参数对激光焊接和TIG焊接残余应力分布的影响。数值模拟中考虑了材料参数的温度相关性，并与小孔释放法测试的焊接残余应力进行比较，结果表明：计算结果和测试结果吻合较好。     

Macroscopic and microscopic changes in the surface layers of annealed and case-hardened steel rollers due to rolling contact

Measurements of the residual stress and observations of the microstructure at the surface and in the subsurface of rollers were performed during rolling contact fatigue tests of annealed 0.45% carbon steel and case-hardened nickel-chromium steel rollers. Compressive residual stresses in annealed rollers were induced by the rolling contact. With case-hardened rollers they were induced by heat treatment prior to the rolling contact fatigue tests. After the rolling contact fatigue tests the compressive residual stresses on the surface of the annealed rollers and in the subsurface of the case-hardened rollers relaxed; a characteristic substructure was formed by the stress cycles, which caused surface failure. It was confirmed that the microcracks leading to surface failure initiate on the surfaces of annealed rollers and in the subsurface of case-hardened rollers.     

Numerical analysis of rolling-sliding contact with the frictional heat in rail

Thermal damage caused by frictional heat of rolling-sliding contact is one of the most important failure forms of wheel and rail. Many studies of wheel-rail frictional heating have been devoted to the temperature field, but few literatures focus on wheel-rail thermal stress caused by frictional heating. However, the wheel-rail creepage is one of important influencing factors of the thermal stress In this paper, a thermo-mechanical coupling model of wheel-rail rolling-sliding contact is developed using thermo-elasto-plastic finite element method. The effect of the wheel-rail elastic creepage on the distribution of heat flux is investigated using the numerical model in which the temperature-dependent material properties are taken into consideration. The moving wheel-rail contact force and the frictional heating are used to simulate the wheel rolling on the rail. The effect of the creepage on the temperature rise, thermal strain, residual stress and residual strain under wheel-rail sliding-rolling contact are investigated. The investigation results show that the thermally affected zone exists mainly in a very thin layer of material near the rail contact surface during the rolling-sliding contact. Both the temperature and thermal strain of rail increase with increasing creepage. The residual stresses induced by the frictional heat in the surface layer of rail appear to be tensile. When the creepage is large, the frictional heat has a significant influence on the residual stresses and residual strains of rail. This paper develops a thermo-meehanical coupling model of wheel-rail rolling-sliding contact, and the obtained results can help to understand the mechanism of wheel/rail frictional thermal fatigue.     

铁素体晶粒尺寸对铁素体—马氏体双相钢DP980动态变形行为影响

为了研究铁素体晶粒尺寸对铁素体—马氏体冷轧双相钢DP980动态变形行为的影响,通过连续退火试验,得到两组马氏体体积分数相同、而铁素体晶粒尺寸不同的试样。选取应变速度为1×10-4s-1和1×10-2s-1进行准静态拉伸试验;选取应变速度为500 s-1、1 000 s-1和1 750 s-1在分离式霍普金森拉杆(Split Hopkinson tensile bar,SHTB)上进行动态拉伸试验。使用不考虑晶粒尺寸影响的Johnson-Cook(J-C)率相关模型和考虑晶粒尺寸影响的修正的Khan-Huang-Liang(KHL)率相关模型分析双相钢的动态变形行为,并引入可决系数R2来判定试验结果与模型的吻合关系。分析结果得出修正的KHL模型与试验结果吻合较好,其可决系数R2达到了0.998 7,表明修正的KHL模型可以很好地描述DP980材料在低应变速度和高应变速度下的变形行为,能够反映铁素体晶粒尺寸对DP980动态变形行为的影响。     

HAZ Characterization and Mechanical Properties of QP980-DP980 Laser Welded Joints

The QP980-DP980 dissimilar steel joints were fabricated by fiber laser welding. The weld zone(WZ) was fully martensitic structure, and heat-affected zone(HAZ) contained newly-formed martensite and partially tempered martensite(TM) in both steels. The super-critical HAZ of the QP980 side had higher microhardness(～ 549.5 Hv) than that of the WZ due to the finer martensite. A softened zone was present in HAZ of QP980 and DP980, the dropped microhardness of softened zone of the QP980 and DP980 was Δ 21.8 Hv and Δ 40.9 Hv, respectively. Dislocation walls and slip bands were likely formed at the grain boundaries with the increase of strain, leading to the formation of low angle grain boundaries(LAGBs). Dislocation accumulation more easily occurred in the LAGBs than that of the HAGBs, which led to significant dislocation interaction and formation of cracks. The electron back-scattered diffraction(EBSD) results showed the fraction of LAGBs in sub-critical HAZ of DP980 side was the highest under different deformation conditions during tensile testing, resulting in the failure of joints located at the sub-critical HAZ of DP980 side. The QP980-DP980 dissimilar steel joints presented higher elongation(～ 11.21%) and ultimate tensile strength(～ 1011.53 MPa) than that of DP980-DP980 similar steel joints, because during the tensile process of the QP980-DP980 dissimilar steel joint(～ 8.2% and 991.38 MPa), the strain concentration firstly occurred on the excellent QP980 BM. Moreover, Erichsen cupping tests showed that the dissimilar welded joints had the lowest Erichsen value(～ 5.92 mm) and the peak punch force(～ 28.4 k N) due to the presence of large amount of brittle martensite in WZ and inhomogeneous deformation.     

NUMERICAL ANALYSIS OF RESIDUAL STRESSES IN TITANIUM ALLOY DURING ELECTRON BEAM LOCAL POST-WELD HEAT TREATMENT

The distributions of temperature and residual stresses in thin plates of BT20 titanium alloy are numerically analyzed by three-dimensional finite element software during electron beam welding and electron beam local post-weld heat treatment (EBLPWHT). Combined with numerical calculating results, the effects of different EBLPWHT mode and parameters, including heat treating position, heating width and heating time, on the distribution of welding residual stresses are analyzed. The results show that, the residual tensile stresses in weld center can be largely decreased when the weld is heat treated at back preface of the plate. The numerical results also indicated that the magnitude of the residual longitudinal stresses of the weld and the zone vicinity of the weld is decreased, and the range of the residual longitudinal stresses is increased along with the increase of heating width and heating time.     

激光熔覆工艺参数对铁基双层涂层组织和残余应力的影响

针对激光熔覆过程中剧烈的温度场变化伴随着应力、应变演化,进而导致零件具有高裂纹敏感性的问题,对不同激光扫描路径及工艺参数下残余应力演变规律进行研究。采用激光熔覆在Q345钢上制备了Fe基双层多道涂层,并以X射线衍射法结合电化学腐蚀剥层法测量沿涂层深度方向的残余应力分布,探究激光扫描路径、功率以及扫描速度对涂层显微组织和应力分布的影响。结果表明：涂层表面和内部为残余压应力,在涂层基体熔合线处残余应力发生突变,热影响区表现为残余拉应力;激光熔覆工艺对涂层残余应力的大小和分布规律有显著影响,当激光扫描路径为轮廓偏置式、激光功率为1.8 kW、扫描速度为0.02 m/s时,涂层具有最优的残余应力分布和成形质量;残余应力的产生主要与激光束对熔池的冲击作用以及熔覆层的非平衡凝固特性有关。     

MACHINING RESIDUAL STRESSES IN AISI 316L STEEL AND THEIR CORRELATION WITH THE CUTTING PARAMETERS

It is well known that machining results in residual stresses in the workpiece. These stresses correlate very closely with the cutting tool geometrical parameters as well as with the machining regime. This paper studies the residual stress induced in turning of AISI 316L steel. Particular attention is paid to the influence of the cutting parameters, such as the cutting speed, feed and depth of cut. In the experiments, the residual stresses have been measured using the X-ray diffraction technique (at the surface of the workpiece and in depth). The effects of cutting conditions on residual stresses are analyzed in association with the experimentally determined cutting forces. The orthogonal components of the cutting force were measured using a piezoelectric dynamometer.     

Comparative study of the tribological properties of various modified mild steels under boundary lubrication condition

AISI1045 steel was modified by laser heat-treatment and conventional heat treatment. The friction and wear behaviors of the steel specimens after various surface modifications sliding against SAE52100 steel under the lubrication of liquid paraffin containing sulphurized olefin were comparatively investigated on an Optimol SRV oscillating friction and wear tester. The worn surface morphologies of the modified steel specimens were analyzed using a scanning electron microscope. The elemental compositions and chemical states of some typical elements on the worn surfaces of the modified steel specimens were analyzed with an energy dispersive X-ray analyzer and X-ray photoelectron spectroscope, respectively. It was found that the laser heat-treated specimen showed the highest hardness and best wear-resistance. The laser heat-treated and conventionally heat treated AISI1045 steel specimens sliding against SAE52100 steel under the lubrication of liquid paraffin containing sulphurized olefin registered smaller friction coefficients than under the lubrication of liquid paraffin alone. This was partly attributed to the increased hardness of the modified specimens. The tribochemical reaction between the steel and the active elements in the additive was involved in the sliding of the modified steel specimens against SAE52100 steel ball under the boundary lubricating condition, with the formation of a surface protective film composed of various tribochemical products. This also contributed to improve the friction and wear behavior of the modified steel specimens. The steel specimens subject to different surface modifications showed differences in the wear mechanisms under the boundary lubricating condition as well. Namely, the tempered steel specimen was mainly characterized by plastic deformation and pitting, the quenched specimen by grooves and delaminating, and the laser heat-treated one by polishing and mild adhesion.     

The effect of hardness on the frictional behaviour of metals

It is shown experimentally that hard metals have lower frictional resistance than softer metals owing to stronger interatomic linking bonds. In hard metals the atomic bonds are strong and hence the resistance to adhesion is increased, providing low frictional characteristics. Surfaces hardened by heat treatment are also characterized by low friction. This is explained by metallurgical changes leading to changes in the structure by phase transformation, increase in the surface energy and the induction of internal (residual) stresses.     

A simplified analysis of the residual longitudinal stresses and strains due to the gas-cutting and welding of thin steel plate

The paper describes a simplified analysis of the residual longitudinal stresses induced in a steel plate by a moving heat source of finite width. The effects of a point source are treated as a limiting case. A linear temperature distribution, identical at every cross section and subject to the condition of zero heat loss, is shown to be an adequate model for the purpose of the analysis. The relationship of the residual stresses to the linear heat input, to the width of heat source and to the stiffness of the plate is established. The particular results for a point source agree closely with more complex published work.     

蒸汽发生器管子管板内角环焊残余应力数值模拟研究

应用ANSYS有限元软件,对核电蒸汽发生器管子管板内角环焊残余应力进行数值模拟研究。在研究中,建立三维有限元模型,基于ANSYS参数化设计语言实现带状温度热源的逐步加载和计算,得到焊接接头处残余应力分布规律,分析相邻管子先后焊接对焊接区残余应力的影响,并模拟出不同热处理温度下的残余应力。研究结果表明,管子管板焊接最大径向和环向残余应力出现在焊缝熔合区,最大轴向残余应力出现在管子内表面热影响区。相邻管子先后进行焊接时,后焊管子温度场的作用会使先焊管子焊接区域的残余应力减小。当热处理温度为600℃时,可以有效减小焊接区的残余应力。