Influence of heat treatment and surface engineering on thermal fatigue behaviour of tool steel

Abstract

Thermal fatigue (TF) is a common problem in many tool steel components. It is caused by thermal cycling in presence of internal constraints. The resulting thermomechanical stresses induce thermal cracking (heat checking). A laboratory test was developed to reproduce TF damage on a laboratory scale, under oxidizing conditions. Two different test configurations were used to induce unidirectional and bidirectional cracking. Nitriding impairs TF resistance of plain steel due to the easier propagation of cracks through the diffusion layer. The efficacy of PVD coatings is dependent on their oxidation resistance and microstructure. The higher oxidation resistance of AlCrN/AlTiN than CrN results in delayed crack initiation. TF resistance is increased if the coating is free of defects and the interface adhesion is good.     

喷丸强化的基本原理与调控正/切断裂模式的疲劳断裂抗力机制图

20世纪90年代前,汽车上使用的各种圆柱螺旋弹簧(以下称弹簧)承受较低的扭转疲劳载荷,在交变正应力作用下,绝大多数发生的是宏观正断型疲劳断裂,其形貌呈45°斜断口。因为喷丸强化弹簧表层引入的残余应力与外施交变正应力间存在交互作用,所以都利用喷丸强化工艺中的"应力强化机制"提高其疲劳断裂抗力。但21世纪伊始,随着轿车结构的迅速发展,设计者迫切要求弹簧承受的扭转疲劳载荷水平与日俱增,由此导致喷丸强化的弹簧除正断型的疲劳断裂外,时而出现纵向或横向切断型的疲劳断裂,由此引发疲劳断裂抗力发生显著下降,以往很少出现这种难以理解的现象。目前从事喷丸强化工艺技术的弹簧制造行业很少有文献关注这类问题。文中通过逻辑思维对弹簧的受力分析得出的诠释发现:在切断模式下,喷丸引入的残余应力与外施交变切应力之间不存在交互作用,表明"应力强化机制"在改善切断型疲劳断裂抗力中的强化作用已经基本消失。根据作者提出的喷丸强化工艺原理,喷丸同步引入弹簧表层的是由残余应力与循环弹塑性变形改性的组织结构组成的一对"孪生",通过分析作者发现,"孪生"中改性的显微组织结构形成的"组织结构强化机制"取代"应力强化机制"起到了改善切断型疲劳断裂抗力的作用。研究结果还表明,优化的喷丸强化工艺必须同时具备"组织结构强化机制"和"应力强化机制",才能够起到改善正断型和切断型疲劳断裂抗力的作用。     

Applicability of constant dew point corrosion tests for evaluating atmospheric corrosion of aluminium alloys

Field exposure tests of aluminium alloys were conducted at three sites in Japan. Meteorological data indicated that the dew point of the ambient atmosphere and aluminium panels remained constant for the short-term. Constant dew point corrosion tests were employed to reproduce atmospheric corrosion of aluminium alloys in the laboratory. The corrosion rates, corrosion morphology and corrosion product composition after 7 days of tests in the laboratory were similar to those formed after 3 months of exposure at coastal sites. Not only did the constant dew point corrosion test effectively reproduce the atmospheric corrosion of aluminium alloys, it also accelerated it.     

The role of creep and fatigue in determining the high-temperature behaviour of AISI H11 tempered steel for aluminium extrusion dies

The present study was aimed to analyze the effect of loading cycles on the behaviour of the AISI H11 tool steel commonly used for aluminium extrusion dies working at high temperatures and under high, cyclic stresses. A technological test method in which the specimen geometry resembled the mandrel of a hollow extrusion die was developed. Finite element analyses were performed to aid in determining specimen geometry and dimensions as well as the levels of stress to be applied to the specimen so as to replicate the conditions typically encountered by industrial hollow extrusion dies. Tests were performed on a Gleeble thermomechanical simulator by heating the specimen using Joule's effect and by applying loading for up to 6.30 h or till specimen failure. Displacements during the tests at 380, 490, 540 and 580 °C and under the average stresses of 400, 600 and 800 MPa were determined. The specimens were tested under creep (with the load held at a fixed value), fatigue (cyclic loading) and creep–fatigue (cyclic loading with a 3 min dwell-time) loading, thereby allowing a direct comparison between different deforming mechanisms. The results showed that the test could physically simulate the cyclic loading on the hollow die during aluminium extrusion and that the creep condition represented the most severe working condition. In addition, the tests could reveal the interaction between creep and fatigue mechanisms.     

Effects of continuous cast section size on torsion deformation and fatigue of induction hardened 1050 steel shafts

This study investigates the influence of continuous cast section size on the mechanical performance of induction hardened parts produced from steel bars. SAE 1050 steel from commercially produced Jumbo Blooms, Blooms, Rotary Round, and Billet were hot rolled into round bars with diameters of 37-44 mm. These bars were then normalized, machined into test specimens, the gauge sections were polished, and the specimens were case-hardened by induction hardening. Torsional monotonic and fully reversed cyclic fatigue tests were conducted to study the effect of the initial continuous cast section size on deformation and fatigue behaviors. Reduction ratios in this study ranged from a low of 20.4:1 for the Billet, up to a high of 142:1 for the Jumbo Bloom. Test results indicate that the continuous cast section size has only small effects on the torsion monotonic and cyclic deformation properties and negligible effect on the torsion fatigue performance. Small differences observed in deformation and fatigue properties between the four processes are attributed mostly to the variation in case and core hardness levels caused by small differences in chemistry, particularly carbon content. Variations in sulfur content also influence ductility and fatigue behavior. At high strains, the cracks initiated in shear as longitudinal cracks for all four materials. At low strains, the cracks initiated at the surface in tension as spiral cracks due to normal tensile stresses.     

GH4169疲劳试样加工残余应力的表征及其对低周疲劳寿命的影响

为了探究GH4169合金低周疲劳试样加工过程中表面残余应力的变化及残余应力对低周疲劳性能的影响,采用X射线衍射法对“车 磨 抛”不同加工工艺的试样表面残余应力进行表征及对成品试样进行残余应力层深度测试,通过中子衍射法对试样内部进行残余应力测试。采用电液伺服万能试验机进行了低周疲劳试验。结果表明：GH4169合金低周疲劳试样加工表面残余应力随着切削量的增加由表面残余压应力变为残余拉应力。根据残余应力层深度测试,机加工对试样表面造成残余应力层深度小于0.01 mm,内部残余拉应力减小;根据疲劳试验结果,GH4169合金疲劳寿命与表面轴向残余压应力呈正相关关系。     

GH4169疲劳试样加工残余应力的表征及其对低周疲劳寿命的影响

为了探究GH4169合金低周疲劳试样加工过程中表面残余应力的变化及残余应力对低周疲劳性能的影响，采用X射线衍射法对“车 磨 抛”不同加工工艺的试样表面残余应力进行表征及对成品试样进行残余应力层深度测试，通过中子衍射法对试样内部进行残余应力测试。采用电液伺服万能试验机进行了低周疲劳试验。结果表明：GH4169合金低周疲劳试样加工表面残余应力随着切削量的增加由表面残余压应力变为残余拉应力。根据残余应力层深度测试，机加工对试样表面造成残余应力层深度小于0.01 mm，内部残余拉应力减小；根据疲劳试验结果，GH4169合金疲劳寿命与表面轴向残余压应力呈正相关关系。     

Application of the local approach to the fatigue assessment for welded joints

The fatigue behavior of welded structures is currently determined by means of recommendations defined in terms of S-N curve corresponding to the detail classes of welded joints without taking account of the actual geometry of the weld. A new fatigue strength assessment method based on Dang Van multiaxial fatigue limit criterion was introduced, which is named the local approach and presented by lnstitut de Soudure recently. The local approach has advantages in taking welding residual stresses and the geometry of the weld toe and weld root into consideration. The application of the local approach to the fatigue strength assessment of low carbon steel Q235B welded joints was studied. The fatigue tests and finite element analysis results show that the local approach parameters recommended by lnstitut de Soudure were incorrectly for low carbon steel Q235B welded joints. With aluminum alloy welded joints being used widely, the parameters of the local approach used for aluminum alloy welded joints were obtained and verified on bases of the fatigue tests and finite element analysis.     

车体振动模态对疲劳强度的影响分析

铁路运输车辆不断朝着高速以及轻量化方向发展,车体承受着复杂的交变载荷,极大地增加了车体结构疲劳断裂的风险。为探究车体结构振动对车辆结构安全性的影响,利用ANSYS Workbench进行车体模态仿真计算。结合服役环境下动车组车体运行模态测试数据,提取出车体1阶菱形(8~9 Hz)、1阶垂弯(12~13 Hz)、1阶横弯(15~16 Hz)及1阶扭转(17~18 Hz)模态频率,对车体有限元模型进行对比修正。利用雨流计数法对部分实测载荷谱数据谱进行处理,得到载荷谱雨流计数矩阵。在模态分析的基础上进行谐响应分析,得到上述不同模态频段范围内的频率响应函数,结合nCode疲劳仿真软件对车体疲劳强度进行仿真计算,得到不同频段范围内车体的疲劳损伤;采用Miner线性累积疲劳损伤理论对仿真计算结果进行疲劳损伤评估,结果表明:车体1阶菱形模态(8~9 Hz)附近频段对车体造成的损伤最大,其损伤位置在车钩与枕梁交接处附近。     

Optimization of the Scratch Test for specific coating designs

The proper design of wear resistant coatings applied to cutting tools comprises the optimization of the mechanical properties (Young's modulus, yield strength, adhesion, intrinsic stresses, fracture, fretting etc.) of the coating-tool system. The goal is to find material and structural solutions which keep the resulting stress-strain field under typical application conditions below the stability limits of the system. Based on nanoindentation measurements obtained from the coating-tool system which should be optimized, a scratch test is dimensioned with respect to load range and indenter geometry. The measured data from this “Physical Scratch Test” are used to simulate spatial stress profiles and to calculate the von Mises stress characteristics and the maximum normal stresses in the scratch direction. In a further step, the simulations are used to suggest scratch parameters for a “Fine Tuned Scratch Test” which increase the sensitivity of the test for specific depth regions in the coating-tool architecture and allow improved and more sensitive investigations of critical interfaces, transition layers and surface-near substrate regions. The tests were performed at PVD coated inserts (nitrides and oxides) and compared with the results obtained from cutting tests.     

Characterization of Thermal Spray Coatings for Cylinder Running Surfaces of Diesel Engines

Future demands of diesel engines are about low-friction and wear-resistant materials in order to increase the efficiency and achieve environmentally sound solutions. Thus, thermally sprayed Fe-base coatings are investigated for application as cylinder running surfaces in cast aluminum crankcases. They should allow the desired combination of structural, productional, and topographical properties required in Diesel engines. To understand the influence of the characteristic microstructures on the integrity of the composites the coatings have to be examined in laboratory tests in terms of different loading situations. Cavitation tests were carried out where the tribological stability of these coatings and their ability to resist high-frequency cyclic impact stresses are revealed. Composite samples (base material and coating) were investigated in terms of crack initiation in a scanning electron microscope with an in situ 3-point-bending test. The endurance under cyclic mechanical stresses was tested with a 4-point-bending stress controlled test.     

Experimental test results from an environmental protection agency test method for determination of vapor suppressant effectiveness

The results obtained from laboratory experiments conducted using Environmental Protection Agency (EPA) subpart WWWW of 40 Code of Federal Regulations (CFR) part 63 (1)-test method are discussed in this article. The original test method was developed to measure the effectiveness of wax suppressants used to reduce hazardous air pollutant (HAP) emissions from unsaturated polyester (UP)/vinyl ester resins. Wax additions of ∼1.5% by weight to commercial UP resins suppress HAP emissions through the formation of surface barrier films. However, the tests performed in this study included the use of limestone and an adjunct, organic fiber reinforcement, rather than the wax. The addition of either commercial product to the UP formulations tested in this study was also shown to reduce HAP emissions. Suppression was a combination of absorption and an increased diffusion path barrier for the volatile organic carbon (VOC) components. Based on the limited data obtained, it was shown that the oil absorption characteristics of the two adjunct products could be used to estimate the expected level of vapor suppression for a specific resin formulation. Values reported in the literature for the oil adsorption characteristics of the adjunct limestone and the commercial biomass fiber were used in the laboratory tests. Although the oil adsorption characteristic of any ingredient added to a base resin formulation is indicative of its potential for emissions reduction, the EPA test protocol is still required to be performed for validation. Such screening tests will always be needed due to the variability associated with commercial UP resins and the evolution of customized UP/fiberglass composite formulations developed by custom molding shops.     

汽车传动系总成冲击耐久试验台检测系统的研究

阐述了研究汽车传动系总成冲击耐久性试验台检测系统的重要性,设计了试验台的结构,研究并建立了模拟传动系工况的数学模型,为了进行试验台控制系统的开发,提出了在试验台上用简化的试验工况,模拟汽车传动系实际使用产生大的扭转冲击载荷工况的思想,设计了试验台的检测系统.经验证,试验台达到了预期的效果,相比传统汽车传动系冲击耐久性试验周期缩短了十几倍,大大提高了生产效率.     

Blistering and spalling of thin hard coatings submitted to repeated impacts

The endurance life of thin DLC coatings, obtained by plasma assisted chemical vapour deposition (PACVD), has been evaluated under repeated impact conditions. The observations of the damaged surfaces have permitted to identify several failure mechanisms. Impact tests, interrupted after various test durations, enable us to clarify the damage origin and to follow the failure evolution up to the complete coating failure. In particular, a blistering phenomenon has clearly been revealed. Some hypotheses for this unusual behaviour are proposed in reference to the film mechanical properties and residual stresses level.     

扭杆弹簧的早期断裂和预防措施

45Cr NiMoVA钢扭杆弹簧发生早期断裂。通过断口分析、化学成分、金相检验和硬度检测分析了扭杆弹簧断裂的原因。结果表明,扭杆弹簧的花键齿表面存在脱碳现象,使断裂韧度和疲劳强度降低,在交变载荷作用下,在花键齿根萌生疲劳源,最终导致扭杆弹簧早期疲劳断裂。预防扭杆弹簧早期断裂的措施是避免扭杆花键齿表面脱碳和适当提高淬火后的回火温度。     

The Correlation of the TBC Lifetimes in Burner Cycling Test with Thermal Gradient and Furnace Isothermal Cycling Test by TGO Effects

Two types of typical thermal cycling tests are used for the evaluation of thermal cycling lifetime of thermal barrier coatings. Those are the burner cycling test with a thermal gradient and the isothermal furnace cycling test. There are diverse explanations to test results up to now. Although certain correlations should exist between the results obtained by two types of the tests, no evident parameters in two tests were directly related, possibly due to large range of difference test conditions. In this investigation, a series of TBC samples with carefully prepared Al₂O₃-based TGO of different thicknesses were used for both the burner cycling and the furnace cycling tests. The relationships between thermal cycling lifetime and TGO thickness were obtained for two types of the tests. It was found that TGO thickness presents the same influence tendency despite of different types of thermal cycling test. The results reveal the existence of the critical TGO thickness by which the transition of failure mode takes place. Moreover, the values of the critical TGO thickness for two tests are comparable. The results evidently suggest that the lifetimes during different thermal cycling tests can be correlated by TGO effects on failure behavior. However, it is clear that the apparent dominant driving factors to TBC failure are different in two types of tests. Accordingly, the burner cycling test could be used for optimizing the durability of ceramic top coat by separating the effect of individual factors through test condition design, while the furnace cycling test results represent the integrated TBC durable performance of the bond coat and top ceramic coating.     

Forecasting method of fatigue life test data for metal materials

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Constitutive modeling of a directionally solidified nickel-based superalloy DZ125 subjected to thermal mechanical creep fatigue loadings

A transversely isotropic continuum elasto-viscoplasticity model,which was developed from Chaboche's unified constitutive model,was formulated to capture the thermal mechanical creep fatigue deformation behavior of a directionally solidified nickel-based superalloy.A fourthorder tensor was introduced to model material anisotropy.In order to model the tertiary creep behavior,the Kachanov damage evolution equation was coupled into the stress tensor.Based on the test results of uniaxial tensile,fatigue,and creep loadings at isothermal temperature conditions,the material parameters are obtained.Thermal mechanical fatigue(TMF) and creep-fatigue interaction test results were used to verify the robustness of the model.Additionally,strain-temperature-dependent stress-strain responses under TMF loadings were analyzed using the present model.Under strain-controlled conditions,both of the stress ranges and mean stresses are strongly influenced by the strain-temperature phases,a key parameter for TMF tests.     

齿轮低温化学热处理渗层抗疲劳抗胶合特性

对分别比１１种低温化学热处理试样的渗层进行了Ｆａｌｅｘ抗咬合试验和ＪＰＭ接触疲劳快速试验，筛选几种工艺进行了齿轮的接触疲劳、弯曲疲劳及胶合试验，取得了一些实用数据。探讨了硬化层深度、表面相结构及Ｎ－Ｓ复合渗工艺对齿轮强度及抗胶合性的影响规律，从而对上述工艺在工业上的应用，提出了适用范围。     

The behavior of welded dissimilar metals structures under rotating reversed cyclic bending

A cantilever type rotating bending testing machine was used to test several welded specimens, these consisted of welding of steel ST60 to stainless steel SS304, and welding of steel ST60 to stainless steel SS316, using different filler types of stainless steel SS308, SS309 and SS310. The effect on the heat affected zone (HAZ) was investigated. Fracture surface examinations were also carried out. The numerical analysis adopted the finite element package ABAQUS^® to model the welded specimens. Elasto-plastic non-linear analysis was conducted for stress distribution contours. von Mises stresses were obtained as well as crack initiation fatigue life. Both numerical and experimental studies were carried out at four different levels of applied stress from 0.6S_y to 1.2S_y at steps of 0.2S_y. The developed maximum stresses exhibited linear behavior followed by non-linear behavior with further increase of applied stress. The filler type had a pronounced effect on the developed stress such that higher stresses were obtained for filler SS308 and lower stresses for SS309 and for SS310, respectively. The crack initiation fatigue life decreased non-linearly with increasing the applied stress and it was greatly affected by the type of filler material; filler SS310 had higher fatigue life cycles than SS309 while SS308 had the lowest fatigue life cycles. The crack initiation fatigue life (N_i) and failure life (N_f) exponentially decreased with the increase in the applied stress. The experimental and numerical results were in good agreement. The SS310 filler, which has the highest Cr%, Mn% and Ni% is the best filler for welding ST60 to SS304 and also for welding ST60 to SS304, and it is recommended for such welding applications.