Residual stresses in high-velocity oxy-fuel metallic coatings

X-ray based residual stress measurements were made on type 316 stainless steel and Fe₃Al coatings that were high-velocity oxy-fuel (HVOF) sprayed onto low-carbon and stainless steel substrates. Nominal coating thicknesses varied from 250 to 1500 μm. The effect of HVOF spray particle velocity on residual stress and deposition efficiency was assessed by preparing coatings at three different torch chamber pressures. The effect of substrate thickness on residual stress was determined by spraying coatings onto thick (6.4 mm) and thin (1.4 mm) substrates. Residual stresses were compressive for both coating materials and increased in magnitude with spray velocity. For coatings applied to thick substrates, near-surface residual stresses were essentially constant with increasing coating thickness. Differences in thermal expansion coefficient between low-carbon and stainless steels led to a 180 MPa difference in residual stress for Fe₃Al coatings. Deposition efficiency for both materials is maximized at an intermediate (∼600 m/s) velocity. Considerations for X-ray measurement of residual stresses in HVOF coatings are also presented.     

Fatigue crack propagation in carburized high alloy bearing steels

Fatigue cracks were propagated through carburized cases in M-50NiL (0.1 C,4 Mo, 4 Cr, 1.3 V, 3.5 Ni) and CBS-1000M (0.1 C, 4.5 Mo, 1 Cr, 0.5 V, 3 Ni) steels at constant stress intensity ranges, ΔK, and at a constant cyclic peak load. Residual compressive stresses of the order of 140 MPa (20 Ksi) were developed in the M-50NiL cases, and in tests carried out at constant ΔK values it was observed that the fatigue crack propagation rates,da/dN, slowed significantly. In some tests, at constant peak loads, cracks were stopped in regions with high compressive stresses. The residual stresses in the cases in CBS-1000M steel were predominantly tensile, probably because of the presence of high retained austenite contents, andda/dN was accelerated in these cases. The effects of residual stress on the fatigue crack propagation rates are interpreted in terms of a pinched clothespin model in which the residual stresses introduce an internal stress intensity, K_i where K_i, = σ_id _i ^1/2 (σ_i = internal stress, d_i = characteristic distance associated with the internal stress distribution). The effective stress intensity becomes K_e = K_a + K_i where K_a is the applied stress intensity. Values of K_i were calculated as a function of distance from the surface using experimental measurements of σ_i and a value of d_i = 11 mm (0.43 inch). The resultant values of K_e were taken to be equivalent to effective ΔK values, andda/dN was determined at each point from experimental measurements of fatigue crack propagation obtained separately for the case and core materials. A reasonably good fit was obtained with data for crack growth at a constant ΔK and at a constant cyclic peak load. The carburized case depths were approximately 4 mm, and the possible effects associated with the propagation of short cracks were considered. The major effects were observed at crack lengths of about 2 mm, but the contributions of short crack phenomena were considered to be small in these experiments, since the two steels were at high strength levels, and short cracks would be expected to be of the order of 10 μm. Also, the two other steels behaved differently and in a way which followed the residual stress patterns. Both M-50NiL and CBS-1000M have a high fracture toughness, with K_lc = 50 MPa · m^1/2 (45 Ksi · in^1/2), and the carburized cases exhibit excellent resistance to rolling contact fatigue. Thus, M-50NiL, carburized, may be useful for bearings where high tensile hoop stresses are developed, since fatigue cracks are slowed in the case by the residual compressive stresses, and fracture is resisted by the relatively tough core.     

Thermal residual stresses in functionally graded and layered 6061 Al/SiC materials

The thermal residual stresses that develop in spray atomized and codeposited functionally graded and layered 6061 Al/SiC metal-matrix composites (MMCs) during cooling from the codeposition temperature to ambient temperature were studied using thermo-elastoplastic finite element analysis. In an effort to investigate the effect of layered and graded structures on the residual stress distribution, the composites with homogeneous distribution of SiC particulates were also analyzed. The effect of SiC volume fraction in the SiC-rich layers and the effect of SiC-rich layer thickness on the residual stresses were investigated. Based on the present study, it was found that the residual stress distribution is very distinct for the aluminum and the SiC-rich layers in the layered materials. As the volume fraction of SiC increases in the SiC-rich layer, the magnitude of residual stresses also increases. The radial stress was found to be tensile in the aluminum layers and compressive in the SiC-rich layers. It was also found that, as the thickness of the SiC-rich layer increases, the magnitude of radial stress in the aluminum layers increases, and that in the SiC-rich layers decreases. In the graded material, the lower region of each layer exhibits tensile radial stress, and the upper region of each layer shows compressive radial stress in order to maintain continuity between layers during cooldown. In general, the layered and the graded materials have greater residual stresses and more complicated stress distribution, as compared with those in the composite materials with homogeneous distribution of SiC particulates.     

Hydrogen Effect on Nanomechanical Properties of the Nitrided Steel

In situ electrochemical nanoindentation is used to examine the effect of electrochemically charged hydrogen on mechanical properties of the nitride layer on low-alloy 2.25Cr-1Mo martensitic structural steel. By application of this method, we were able to trace the changes in the mechanical properties due to the absorption of atomic hydrogen to different depths within the compound and diffusion layers. The results clearly show that the hydrogen charging of the nitriding layer can soften the layer and reduce the hardness within both the compound and the diffusion layers. The effect is completely reversible and by removal of the hydrogen, the hardness recovers to its original value. The reduction in hardness of the nitride layer does not correlate to the nitrogen concentration, but it seems to be influenced by the microstructure and residual stress within the compound and diffusion layers. Findings show that nitriding can be a promising way to control the hydrogen embrittlement of the tempered martensitic steels.     

拉压不同应力对磁记忆信号的影响及机理

通过对Q235钢退磁试件的拉伸、压缩试验,利用磁记忆在线监测系统实时跟踪记录了不同拉压应力作用下试件表面的磁信号变化特征. 结果表明:拉伸载荷对合成磁场的影响是先减小后增加的,在接近材料屈服强度的0.3倍左右后趋于稳定不变;而压应力引起的合成磁场初期快速下降,之后处于上下波动变化. 通过引入拉压应力所产生的不同应力退磁项,对J-A磁机械效应模型进行了改进,模拟结果与试验数据具有较好的的一致性,可用于拉压不同应力致磁机理的理论解释.      

Formation of residual stress in steels and titanium alloys during ion nitriding

An X-ray diffraction method is developed for determining residual stress in the surface layers of metal samples characterized by a variable chemical composition across the layer depth. The method includes the determination of interplanar spacings for different diffraction reflections and the separation of the contributions to the interplanar spacing from elastic stresses and a solid-solution composition. The separation is based on the elastic anisotropies of the crystal lattices of metals with cubic and hexagonal structures. The efficiency of this method is demonstrated for EI961 and 12Kh18N10T steels and PT7M titanium alloy upon ion nitriding.     

Influence of Cryoprocessing on Mechanism of Carbide Development in Cobalt-Bearing High-Speed Steel (M35)

Cryogenic treatment has been used commonly to high-speed tool steels to enhance the wear resistance of the materials. In the current research study, specimens of complex alloyed high-speed tool steel (M35) were hardened at 1473 K (1200 °C), triple tempered at 673 K (400 °C) and then cryogenically treated at 88 K (?185 °C) for varying lengths of period starting from 16 to 48 hours of cryosoaking followed by soft tempering at 373 K (100 °C). These treated specimens were studied as a function of cryosoaking period for their electrical resistivity, residual compressive stress, and its correlation with carbide density was established. TEM analysis indicated carbide size 0.156 to 1 μm, which confirms that the cryogenic treatment enhances the precipitation of finer carbides. Lower residual stresses in the higher carbide density regimes identified in 2D contour map were explained by the stress relaxation in the matrix through precipitation of incoherent carbides.     

Residual stresses and interface strengths in MoSi2/stainless steel 316L joints produced by metallic-glass brazing

The residual stresses in molybdenum disilicide (MoSi₂)/stainless steel 316 L joints were evaluated using X-ray diffraction (XRD) and instrumented indentation techniques. These joints were produced by high-temperature brazing using a cobalt-based metallic glass (METGLAS 2714A). Joining was completed in two different ways, either by feeding excess braze into the braze gap upon heating or by constraining the MoSi₂/stainless steel assembly with an alumina (Al₂O₃) fixture during the heating cycle. These steps were necessary to ensure the production of a high-quality void free joint. The indentation results show higher tensile residual stresses in the stainless steel for the joint with the external constraint, in comparison to the unconstrained state. In contrast, the compressive residual stresses in the MoSi₂ (as measured by XRD) were lower in the constrained state relative to the unconstrained state. These results and a lack of residual stress balance indicate that the stress state in the braze is significantly different under the two joining conditions and the volume of the braze plays an important role in the development of the residual stresses. Push-out tests carried out on these joints gave higher joint strengths in the unconstrained as compared to the constrained condition. The results of this study have important implications on the selection of the appropriate joining process (use of constraint vs extra braze).     

The application of fractography to demonstrate the presence of crack tip residual stresses during programmed fatigue crack propagation in mild steel

Constant amplitude and programmed loading fatigue crack propagation tests were carried out on a low carbon steel in three point bending. The Palmgren-Miner summation(Σ _Nⁿ) was found to be consistent and always slightly greater than unity irrespective of the load program. It is suggested that this effect is caused by crack tip compressive stresses opposing propagation. Fracture surface observations using a scanning electron microscope showed changes in fracture mode immediately after a load drop from which it was possible to make estimates of the levels of residual stress present.     

The application of fractography to demonstrate the presence of crack tip residual stresses during programmed fatigue crack propagation in mild steel

Constant amplitude and programmed loading fatigue crack propagation tests were carried out on a low carbon steel in three point bending. The Palmgren-Miner summation(Σ _N ⁿ ) was found to be consistent and always slightly greater than unity irrespective of the load program. It is suggested that this effect is caused by crack tip compressive stresses opposing propagation. Fracture surface observations using a scanning electron microscope showed changes in fracture mode immediately after a load drop from which it was possible to make estimates of the levels of residual stress present.      

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MS980 is one of advanced high-strength steels, which has a great potential for producing square tubes. Residual stress plays a significant role in determining roll-formed members?? behavior and strength. An experimental study on transverse residual stress of roll-formed shape with square section was conducted via X-ray diffraction method. The distribution of transverse residual stresses at different position in sections was measured and studied, and influence of cold-rolled-sheet and hot-rolled-sheet, different fillet radius, different processes, and different pickling time on circumferential residual stress distribution for square section was investigated. The experimental results show the transverse residual stresses are compressive stress in the corner portion and the tensile stress in the straight edge. Fillet radius, sheet, and process have a significant impact on the distribution of residual stress, but the conventional pickling hardly affects residual stress.     

Plasma Nitriding of 722M24 Steel with Pure Lanthanum, Cerium and Neodymium as Sputter Sources

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Fatigue crack propagation in carburized X-2M steel

The growth rates of fatigue cracks propagating through the case and into the core have been studied for carburized X-2M steel (0.14 C, 4.91 Cr, 1.31 Mo, 1.34 W, 0.42 V). Fatigue cracks were propagated at constant stress intensities, ΔK, and also at a constant cyclic peak load, and the crack growth rates were observed to pass through a minimum value as the crack traversed the carburized case. The reduction in the crack propagation rates is ascribed to the compressive stresses which were developed in the case, and a pinched clothespin model is used to make an approximate calculation of the effects of internal stress on the crack propagation rates. We define an effective stress intensity, K_e = K_a + K_i, where K_a is the applied stress intensity, K_i = σ_id _i ^1/2 , σ_i is the internal stress, and d_i is a characteristic distance associated with the depth of the internal stress field. In our work, a value of d_i = 11 mm (0.43 inch) fits the data quite well. A good combination of resistance to fatigue crack propagation in the case and fracture toughness in the core can be achieved in carburized X-2M steel, suggesting that this material will be useful in heavy duty gears and in aircraft gas turbine mainshaft bearings operating under high hoop stresses.     

Formability of sandwich sheet materials in plane strain compression and rolling

The stress states developed during room temperature, plane strain compression modes of deformation of stainless steel clad aluminum and aluminum clad strainless steel sheets have been investigated in order to gain insight into the formability of bonded ductile sandwich sheet materials in primary metalworking processes. Assuming uniform, isostrain deformation in the component layers, sandwich compression stress-strain curves were predicted to be rule of mixtures averages of component compression stress-strain curves. These predictions showed good agreement with experimental data when friction and in-homogeneous deformation were taken into account. Since the through-thickness applied pressure can be assumed to be the same in both components of thin sandwich sheet materials, in-plane stresses which are tensile in the harder component and compressive in the softer component of a clad sheet are developed in order to satisfy the yield conditions. The nature of these in-plane stresses was confirmed by measurements of residual stress distributions in rolled clad sheet specimens, and it was shown how the tensile stress in the harder component may lead to unstable flow and failure of this component during forming. The observed failures were similar in both plane-strain indentation and rolling tests. Although the initiation of instability in symmetric clad sheet metals appears to be independent of the arrangement of the component layers, the process of final localization leading to fracture was observed to depend heavily on the layer arrangement. S. L. SEMIATIN, formerly Graduate Student, Department of Metallurgy and Materials Science, Carnegie-Mellon University.     

Effect of Shot Blasting and Shot Peening Parameters on Residual Stresses Induced in Connecting Rod

This paper investigates the effect of shot blasting and shot peening parameters on residual stresses induced in connecting rod. Compressive residual stresses are induced using shot peening to increase fatigue life of connecting rod. Shot peening is also responsible for increase in surface roughness. Surface roughness is detrimental for fatigue life of the connecting rod. This necessitates shot blasting to reduce surface roughness. Shot peening and shot blasting processes are analysed to find optimum process parameters which will induce required value of compressive residual stress on the surface of connecting rod. Compressive residual stresses induced in the connecting rod specimen have been experimentally measured using X-ray stress analyser. The experimental results have been analysed using grey relational analysis to find optimum values of process parameters for target value of compressive residual stress and surface roughness. The experimental investigation and the analysis of it have resulted in achieving the desired value of compressive residual stress, which is 10.5% higher over the existing connecting rod. Surface roughness also decreases to 3.84 Ra which is 8.5% lesser than specified value to achieve better fatigue life.     

硬质合金基体上金刚石膜的XRD研究

近年来CVD金刚石涂层硬质合金刀片的制备与工业化应用已取得进展,对金刚石薄膜的沉积质量、应力状态、粘附性能进行无损表征具有显著的实用价值。本文主要借助XRD分析方法对硬质合金刀片上沉积的金刚石薄膜的微观结构参量、残余应力状态、粘附性能之间的相互关系进行了初步研究,探讨了CVD金刚石膜中残余应力的形成及其对金刚石膜粘附性能的影响。     

The development of nitrogen concentration profiles on nitriding iron

On nitriding iron specimens nitrogen concentration profiles within the specimens are built up. A numerical method for the calculation of such concentration profiles was developed. The results calculated were compared with experimental data. It was found that during nitriding the nitrogen surface concentration approached relatively slowly the equilibrium value. This effect strongly influenced the development of the nitrogen concentration profile. The model predicted correctly the incubation time for compound(i. e., iron nitride) layer formation at the surface. If the fatigue resistance is strongly dependent on the (compressive) residual surface stress, the present treatment allows calculation of anoptimum nitriding time by determining when the maximum (compressive) residual surface stress occurs.     

Characterization of Tribological and Mechanical Properties of the Si3N4 Coating Fabricated by Duplex Surface Treatment of Pack Siliconizing and Plasma Nitriding on AISI D2 Tool Steel

Silicon nitride (Si₃N₄) coating was deposited on AISI D2 tool steel through employing duplex surface treatments—pack siliconizing followed by plasma nitriding. Pack cementation was performed at 650 °C, 800 °C, and 950 °C for 2 and 3 hours by using various mixtures to realize the silicon coating. X-ray diffraction analyses and scanning electron microscopy observations were employed for demonstrating the optimal process conditions leading to high coating adhesion, uniform thickness, and composition. The optimized conditions belonging to siliconizing were employed to produce samples to be further processed via plasma nitriding. This treatment was performed with a gas mixture of 75 pct H₂-25 pct N₂, at the temperature of 550 °C for 7 hours. The results showed that different nitride phases such as Si₃N₄-β, Si₃N₄-γ, Fe₄N, and Fe₃N can be recognized as coatings reinforcements. It was demonstrated that the described composite coating procedure allowed to obtain a remarkable increase in hardness (80 pct higher with respect to the substrate) and wear resistance (30 pct decrease of weight loss) of the tool steel.

     

Effect of the residual near-surface stresses of the working surface on the contact resistance of rolls

The results of contact resistance tests of roll steel samples with various levels of near-surface residual compressive stresses are analyzed. A relation between these stresses and the contact resistance is found. Recommendations are made for the formation of rational near-surface residual stresses in the active surfaces of rolls in order to increase their resistance.     

Consequences of the HeterogeneousNitriding of α-lron: Dislocation Production and Oriented Precipitation

In commercial practice nitriding of a surface layer of workpieces of steel is employed to improve the mechanical properties, such as the fatigue resistance. To study the effects of such a heterogeneous nitriding treatment on microstructure, relatively thin and relatively thick specimens of α-iron have been nitrided heterogeneously at 833 K in gas mixtures composed of NH₃ and H₂. Transmission electron microscopy was applied to investigate the microstructure as a function of depth below the surface. Electron transparent foils parallel to the surface were taken at predefined depths in the nitrogen diffusion zone employing a special preparation technique. On nitriding dislocations were produced in the diffusion zone. The dislocation density varied with location in the zone. After aging following nitriding, near the surface α-Fe₁₆N₂ precipitates in the form of discs aligned along (100) planes of the iron matrix making the smallest angle with the surface at those places where no appreciable dislocation production had occurred. For larger penetration depths these precipitates aligned along (100) planes of the iron matrix making the smallest angle with the diffusion direction (perpendicular to the surface). These effects could be related to the diffusion-induced state of stress in the specimen. Student at the Laboratory of Metallurgy of the Delft University of Technology