Applied mechanics in grinding—VI. Residual stresses and surface hardening by coupled thermo-plasticity and phase transformation

The present study aims to investigate the residual stresses and surface hardening in ground components caused by the coupled effect of thermo-plasticity and phase transformation. A feasible numerical method was developed to accommodate appropriately the phase transformation in a workpiece experiencing critical temperature variation during grinding. The change of material properties was modelled as a function of temperature history. It found that if material properties are temperature-independent, the residual stresses in both the grinding direction and that perpendicular to it are tensile. The maximum residual stress in this case does not vary with the further increment of grinding heat. When material properties are temperature-dependent, however, the rise of grinding temperature promotes surface hardening and increases the maximum residual stress. The study showed that the volume growth associated with phase change plays an important role in the formation and nature transition of residual stresses. Nevertheless, residual stresses in the no-martensite zone is nearly unaffected by surface hardening and volume change. This paper offers insight into the understanding of surface hardening mechanism introduced by grinding.     

Residual stress and softening in welded high-strength low-alloy steel with a buffering layer

Welding residual stresses, Vickers hardness and microstructure of welded high-strength low-allow steel, known for high strength and low carbon content, were studied under the following conditions: as-received high-strength low-alloy steel, welded high-strength low-alloy steel without a buffering layer, and welded high-strength low-allow steel with various thickness buffering layer. A soft buffering layer with a modest thickness between the parent metal and the weld metal could be incorporated to welded high-strength low-alloy steel to effectively reduce the widths of tensile residual stress zone and welding softening zone, to change the residual stress (in y-direction) nature at the weld root from tensile to compressive, and to refine the grains of the welded high-strength low-alloy steel. The width of the tensile residual stress (in x-direction) zone was approximately equivalently to that of the welding softening zone for those welded high-strength low-alloy steel with and without buffering layers.     

Superficial severe plastic deformation of 316 LVM stainless steel through grit blasting: Effects on its microstructure and subsurface mechanical properties

The microstructure and mechanical properties of austenitic stainless steel 316 LVM (Low Vacuum Melting) blasted with either small and rounded ZrO₂ particles or larger and angular shaped Al₂O₃ particles are analysed through magnetic force microscopy, synchrotron radiation diffraction and ultramicroindentation techniques. It is shown that blasting causes a severe plastic deformation that roughens the surface and produces a significant subsurface grain refinement and work hardening. The gradient in the plastic deformation and the volume increase associated with the deformation induced α′-martensite account for the development of compressive residual stresses with a maximum value close to the surface. All these features yield a gradient in hardness with a maximum value beneath the surface. Compared with the Al₂O₃ particles, the ZrO₂ particles cause a higher value of compressive residual stress and a lower increase in hardness. Also, the Al₂O₃ particles lead to more α′-martensite formation at deeper regions from the surface than the ZrO₂ particles. The different results are related with the specific morphology of the particles and their specific role in the blasting process.     

模具液氮冷却对铝型材出口温度及挤压速度的影响

通过现场生产试验,研究了6063铝合金型材挤压过程中模具液氮冷却技术对型材出口温度及挤压速度的影响。结果表明,采用对模垫的液氮冷却技术能有效起到冷却作用。液氮的开度大小直接影响到型材出口温度,当液氮开度值较小时,对型材出口温度影响较小。当液氮开度值达到70%时,型材出口温度显著下降。采用模具液氮冷却,铝合金型材挤压速度由0.2 m/s提高到0.3 m/s,大大提高了生产效率。     

退火对液氮轧制CrCoNi中熵合金组织与性能的影响

针对CrCoNi中熵合金具有优异的液氮性能,但其液氮轧制后塑性较差（<8%）所带来的难加工问题,以电弧熔炼铸态CrCoNi中熵合金为研究对象,在液氮（77 K）下对CrCoNi合金进行3道次轧制变形,总变形量为50%。随后,在650、700、800℃下保温30min进行退火。结果表明,CrCoNi中熵合金经过液氮轧制及随后退火,未发生物相结构改变。退火后,变形晶粒发生再结晶而细化,并产生∑3退火孪晶。随退火温度的升高,再结晶程度增加,延伸率提高。通过液氮轧制与中温短时间退火进行性能调控,可获得强度与韧性匹配的性能,并提高了热处理工艺效率。     

Using simulation for heat treatment process design: matching quenching process with steel grade and product geometry

Abstract

The performance of steel parts is heavily dependent on the heat treatment process applied. The alloy content of the steel establishes the steel hardenability. The severity of the quenching establishes the local temperature history throughout the body of the part. In combination, the steel hardenability and the quenching process determine the final microstructure, mechanical properties, residual stress state and the performance of the part. The residual stress state, especially the surface stress state, is a significant factor in affecting fatigue life of the part. The steel hardenability and quenching practice can be adjusted to enhance residual surface compression and improve the fatigue life of a component. Computer simulation of the heat treatment process that includes calculation of the metallurgical phase transformations during the heating and cooling processes offers a method for scientifically designing the heat treatment process and selecting the steel alloy to optimise the performance of a particular product. In this paper, the DANTE heat treatment simulation software will be used to demonstrate this design methodology for a spur gear.     

Thermally-induced formation of hexagonal AlN in AlCrN hard coatings on sapphire: Orientation relationships and residual stresses

Cubic AlCrN coatings were epitaxially grown onto Al₂O₃(00.1) substrates by reactive magnetron sputtering at 500°C from Al/Cr targets with an atomic ratio of 70/30. The coatings were vacuum annealed at 1000°C for 2 hours in order to induce formation of wurtzite-type AlN. The as-deposited and annealed coatings were characterized using X-ray diffraction techniques. Pole figure measurements revealed orientation relationships of the cubic AlCrN phase with respect to the substrate. Residual stress characterization indicated compressive stresses of -1246 MPa in the as-deposited cubic AlCrN phase. After annealing, the residual stresses in the hexagonal wurtzite-type Al(Cr)N and the Al-depleted cubic Cr(Al)N phase are -132 and 346 MPa, respectively. The stress changes can be interpreted as a consequence of point defect recovery at temperatures above deposition temperature and Al(Cr)N formation in the annealed coating.     

Thermodynamically consistent phase field approach to phase transformations with interface stresses

Thermodynamically consistent phase field theory for multivariant martensitic transformations is developed with the main focus on introducing correct interface stresses (tension). The nontrivial point is that the interface tension (physical phenomenon) is introduced with the help of some geometric nonlinearities, even when strains are infinitesimal. Total stress at the diffuse interface consists of elastic and dissipative parts which are determined by the solution of the coupled system of phase field and viscoelasticity equations and the introduced interface stresses. An explicit expression for the free energy is derived that results in the desired expression for the interface stresses consistent with the sharp interface for the propagating nonequilibrium interface. Analytical expressions for nonequilibrium interface energy, width, entropy excess, as well as distribution of the interface tension are derived and parametrically studied. Interface stress tensor distribution is also obtained and analyzed for a critical martensitic nucleus. The possibility of extending the developed approach to other phenomena and more general models is discussed.     

室温及液氮控温对轧制态Al-Sc合金力学性能及织构的影响

通过显微硬度、拉伸性能测试、显微组织分析、扫描电镜分析以及背散射电子衍射分析,研究了室温与液氮控温80%轧制变形对Al-Sc合金组织及力学性能的影响。结果表明:室温轧制与液氮控温轧制后合金的硬度分别为105 HV0.3和162 HV0.3,抗拉强度、屈服强度、伸长率分别为335 MPa、296 MPa、5.5%和443 MPa、415 MPa、6.7%;轧制后合金中多为小角度晶界,室温与液氮控温轧制后平均晶粒尺寸分别为40 μm和1 μm;由于层错能的影响,合金液氮控温轧制之后的主要织构类型为Brass织构{110}<112>、S织构{123}<634>和 Copper织构{112}<111>。     

Effect of tin addition on Nb–Si-based in situ composites. Part II: Oxidation behaviour

This paper reports the effects of adding from 2 to 8 at.% tin on the oxidation behaviour of Nb/Nb₅Si₃ composites at 815 °C and at higher temperatures (1100 and 1200 °C). The role of tin in the elimination of pesting and in the oxidation process at high temperatures was established. The consumption of elements with a higher affinity for oxygen than Sn induces the accumulation of tin at the oxide/internal oxidation zone boundary. Low melting point phases (NbSn₂ and/or pure Sn) form at 815 °C, whereas a layer of M₅Si₃ and Nb₅SiSn₂ forms at 1100 and 1200 °C. Once these products are formed, they generate an oxygen diffusion barrier and allow the elimination of pesting. However, for long oxidation processes at 1100 °C, the oxidation rate of Nb/Nb₅Si₃ composites containing tin should be higher than that for tin-free composites. Moreover, some oxidation results have suggested that the presence of A15-(Nb,Ti)₃(Sn,Ti) in the microstructure of composites with at.%Sn > 2 can severely impact the low temperature fracture toughness of these composites.     

液氮淬火法制备高性能锂离子电池正极材料LiFePO4/C

以FePO4、Li2CO3和葡萄糖为原料,用液氮急速淬火法制备单一橄榄石结构的锂离子电池正极材料LiFePO4/C。结果表明:淬火使得LiFePO4晶格中产生Li空位,有利于提高其电子导电性。淬火样品的一次颗粒细小(100~500 nm),无明显团聚,并形成多孔结构;该样品在1C、2C和4C倍率下的首次放电比容量分别为151.4、138.0和116.7 mA.h/g,循环100次后的容量保持率高达99.3%、98.6%和94.5%。     

Rapidly solidified non-equilibrium microstructure and phase transformation of plasma cladding Fe-based alloy coating

The rapidly solidified microstructural and compositional features, the precipitation and transformation of carbides during aging of Fe-based alloy coating prepared by plasma cladding have been investigated. The clad coating materials, whose powder mixture of Fe, Cr, Ni, B, Si and C with a weight ratio of 54.5:35:5:1:2:2.5, is processed using a non-transferred plasma arc. The clad coating adheres with low carbon steel in a good metallurgical bonding and the rate of dilution is 15-20%. Microstructural studies demonstrate that the coating possesses the metastable microstructure comprising the primary dendritic γ-austenite which is a non-equilibrium phase with an extended solid solution of alloying elements and interdendritic eutectic consisting of γ-austenite and (Cr,Fe)₇C₃ carbides. During the high temperature aging at 1253 K for 2 h, the fine spherical (Cr,Fe)₂₃C₆ carbide nucleates within (Cr,Fe)₇C₃ carbide and austenite matrix, and some martensite (α) also forms during cooling. The solidification and evolution sequence of the phase can be represented as follows: L → γ + L → γ + (γ + (Cr,Fe)₇C₃) → (γ + (Cr,Fe)₂₃C₆ + α). Due to the precipitation of (Cr,Fe)₂₃C₆ carbide and uniform distribution of carbide in the as-aged coating, the average hardness becomes higher than that of the as-clad coating.     

Hot workability of a NiTiCu shape memory alloy with acicular martensite phase based on processing maps

Hot deformation behavior and processing maps of Ni₄₅Ti₅₀Cu₅ (at.%) shape memory alloy (SMA), which possesses acicular martensite phase at room temperature, were investigated systematically based on isothermal compression tests at the temperatures ranging from 600 to 1000 °C and the strain rates ranging from 0.0005 to 0.5 s⁻¹. The flow stresses of the studied alloy were found to be dependent on the strain rate as well as the temperature. Processing maps at strains of 0.3, 0.6 and 0.9 were constructed on the basis of dynamic material model (DMM) theory by using the flow stresses obtained from compression tests. According to the processing maps, flow instability was found to mainly occur in the region with high strain rate and the scope of instability region was found to increase with the increase in the true strain. Based on the combination of processing map and microstructural observations, microstructural defects were found in the alloy samples deformed in the instability regions and the parameters suitable for hot working of the studied alloy were determined as the temperatures ranging from 750 to 850 °C and the strain rates ranging from 0.0005 to 0.001s⁻¹.     

Fatigue response of a grain refined TiAl alloy Ti-44Al-5Nb-1W-1B with varied surface quality and thermal exposure history

Fatigue specimens with four types of designed surface (EDM plane-sited, EDM notched, shot peened, electropolished) were assessed under three exposure conditions (no exposure, block exposure, individual exposure-oxidation at 700 °C for 10000 h) to quantify the effects of surface roughness, stress concentration, oxidation and inner microstructural embrittlement on fatigue behaviour of a grain refined TiAl alloy Ti-44Al-5Nb-1W-1B. With the yield strength of 568 MPa, fatigue is found to occur under a loading condition of σ_max<σ_0.1. Local plastic deformation is difficult to occur. The alloy becomes sensitive to surface damages but not to V-notch because the small surface area sampling the highest stress significantly reduces the EDM impact. Electropolishing rather than shot peening is found to be more effective in improving fatigue strength for the high strength alloy. When subjected to block exposure, both annealing effect (beneficial) and microstructural embrittlement (detrimental) occurred on all the surfaces, and the latter was dominant in governing fatigue behaviour except for EDM surfaces. After individual exposure-oxidation, fatigue performance deteriorated significantly for the shot peened and moderately for the electropolished but not for EDM surfaces. The mechanism for specific fatigue behaviour is discussed individually based upon whether or not the beneficial effects outweigh the detrimental effects.     

中间包覆盖渣中B2O3和Al2O3对钢液氮含量的影响

常压下,选用CaO-SiO2-Al2O3碱性渣系作为基础渣系,通过改变渣中B2O3和Al2O3含量,来分析其对钢液氮含量的影响作用.通过对实验结果的分析得知,虽然渣中B2O3和Al2O3对钢液脱氮作用不太明显,但是,熔渣中B2O3和Al2O3对钢液都有较好的保护作用,可以较好地阻止大气中的氮气进入钢液中.     

Lateral gradients of phases,residual stress and hardness in a laser heated Ti0.52Al0.48N coating on hard metal

The influence of a local thermal treatment on the properties of Ti–Al–N coatings is not understood. In the present work, a Ti_0.52Al_0.48N coating on a WC–Co substrate was heated with a diode laser up to 900 °C for 30 s and radially symmetric lateral gradients of phases, residual stress and hardness were characterized ex-situ using position-resolved synchrotron X-ray diffraction, Raman spectroscopy, transmission electron microscopy and nanoindentation. The results reveal (i) a residual stress relaxation at the edge of the irradiated area and (ii) a compressive stress increase of few GPa in the irradiated area center due to the Ti–Al–N decomposition, in particular due to the formation of small wurtzite (w) AlN domains. The coating hardness increased from 35 to 47 GPa towards the center of the heated spot. In the underlying heated substrate, a residual stress change from about − 200 to 500 MPa down to a depth of 6 μm is observed. Complementary, in-situ high-temperature X-ray diffraction analysis of stresses in a homogeneously heated Ti_0.52Al_0.48N coating on a WC–Co substrate was performed in the range of 25–1003 °C. The in-situ experiment revealed the origin of the observed thermally-activated residual stress oscillation across the laser heated spot. Finally, it is demonstrated that the coupling of laser heating to produce lateral thermal gradients and position-resolved experimental techniques opens the possibility to perform fast screening of structure–property relationships in complex materials.     

Oxidation Behavior of a Cast Polycrystalline Ni-Base Superalloy in Air: At 900 and 1000 °C

The oxidation behavior of a cast polycrystalline Ni-base superalloy was studied at 900 and 1000 °C and analyzed by thermogravimetric analysis (TGA), X-ray diffraction (XRD) and SEM. The results indicate that the cast Ni-base superalloy exhibits subparabolic oxidation kinetics, which are controlled by the growth of the inner Al-rich layer. A mixed scale forms on the alloy after prolonged oxidation. The oxide scale formed on the γ matrix is composed of an outer layer of spinel and a uniform inner, compact Al-rich layer. The oxidation behavior on the region overlying MC carbide precipitates is slightly different from that in the γ matrix. The difference is explained by the big difference in composition between MC carbide and γ matrix.     

Residual stress and surface roughness when facing age hardened Inconel 718 with CBN and ceramic cutting tools

The demand for increasing productivity when machining heat resistant super alloys has resulted in the use of advanced cutting tools such as ceramics and cubic boron nitride (CBN). However, the effects of these tools on the surface integrity, especially the residual stresses created, in the high speed facing operation of Inconel 718 has not been dealt with. In this paper, the residual stresses and the surface roughness when facing age hardened Inconel 718 using CBN and mixed ceramic cutting tools at their respective optimum performance based on productivity has been investigated. The residual stress and surface finish generated during facing with CBN cutting tools have been investigated as a function of speed, depth of cut, coolant, tool geometry and nature of the tool coating. In addition, mixed ceramic cutting tools have been investigated for comparison. The results show that mixed ceramic cutting tools induce tensile residual stresses with a much higher magnitude than CBN cutting tools. The residual stresses and the surface roughness generated by CBN cutting tools are more sensitive to cutting speeds than depth of cut. The use of coolant results in either compressive residual stresses or lowers the magnitude of the tensile residual stresses, whereas dry cutting always resulted in tensile residual stresses. From this investigation, it is suggested that round CBN cutting tools should be used at slow cutting speeds (150 m/min) and small depths of cut (0.05 mm) and with the use of coolant to achieve compressive or minimal tensile residual stresses and good surface finish.     

Numerical simulation and experimental investigation of temperature and residual stresses in GTAW with a heat sink process of Monel 400 plates

A 3D thermo-mechanical simulation model was developed to predict distributions of temperature and residual stresses during the gas tungsten arc welding (GTAW) process with a heat sink for Monel 400 plates using finite element method. The model was validated against the experimental measurements of both temperature and released strain in the welded plates. Effects of heat input, pipe diameter and water flow rate in the heat sink welding process were investigated. The results showed that in the GTAW process with a heat sink, the high temperature region was only limited to the vicinity of heat source and the maximum temperature of the sample was much lower than that of conventional GTAW process. This resulted in a lower residual stresses and even compressive stresses near the weld zone.