Level Of Tensile Residual Stresses Research Articles

Effects of PWHT on the Residual Stress and Microstructure of Bisalloy 80 Steel Welds

Quenched and tempered (Q & T) steels have numerous applications, particularly in the defence industry with welding as the main fabrication route. Since welding imparts stresses due to thermal gradients development during welding, plus the fact that the Q & T fabricated structures are expected to function in a complex loading environment, it is critically important to relax the welding stresses before exposing the parts to service conditions. The present study reports on the generated residual stresses when Bisalloy 80 is welded by pulsed gas metal arc welding (GMAW-P) and verifies the effects of post-weld heat treatment (PWHT) on the microstructural changes, removal or reduction of residual stresses and the resulting mechanical properties of the welded Q & T steel joints. Neutron diffraction was utilized to measure the residual stresses in the as-welded and after PWHT of the Bisalloy 80 steel weldments. High levels of tensile residual stresses reaching to the yield strength of the weld metal were present (642 ± 24 MPa) in the as-welded joints but were substantially reduced after PWHT (145 MPa ± 21 MPa, which is ~23% of the yield strength of the weld metal). PWHT led to microstructural changes in different regions of the parent and weld metals, including the formation of coarsened polygonal ferrite grains and bainitic ferrite laths. This finding is in line with hardness measurements, where hardness reductions were evident in the heat-affected zone (HAZ) and the weld metal (WM) of the heat-treated specimens.

Experimental Characterisation and Numerical Modelling of Residual Stresses in a Nuclear Safe-End Dissimilar Metal Weld Joint

In this study, a mock-up of a nuclear safe-end dissimilar metal weld (DMW) joint (SA508-3/316L) was manufactured. The manufacturing process involved cladding and buttering of the ferritic steel tube (SA508-3). It was then subjected to a stress relief heat treatment before being girth welded together with the stainless steel tube (316L). The finished mock-up was subsequently machined to its final dimension. The weld residual stresses were thoroughly characterised using neutron diffraction and the contour method. A detailed finite element (FE) modelling exercise was also carried out for the prediction of the weld residual stresses resulting from the manufacturing processes of the DMW joint. Both the experimental and numerical results showed high levels of tensile residual stresses predominantly in the hoop direction of the weld joint in its final machined condition, tending towards the OD surface. The maximum hoop residual stress determined by the contour method was 500 MPa, which compared very well with the FE prediction of 467.7 Mpa. Along the neutron scan line at the OD subsurface across the weld joint, both the contour method and the FE modelling gave maximum hoop residual stress near the weld fusion line on the 316L side at 388.2 and 453.2 Mpa respectively, whereas the neutron diffraction measured a similar value of 480.6 Mpa in the buttering zone near the SA508-3 side. The results of this research thus demonstrated the reasonable consistency of the three techniques employed in revealing the level and distribution of the residual stresses in the DMW joint for nuclear applications.

Mathematical model and numerical analysis method for dynamic fracture in a residual stress field.

Residual stress field is a self-equilibrium state of stress in the bulk solid material with the inhomogeneous field of the inelastic deformations. The high level of tensile residual stress often leads to dynamic fracture resulting in the instantaneous and catastrophic destruction of the materials because the cracks are fed with the strain energy initially stored in the bulk materials due to the residual stress. The dissipation of the strain energy with crack growth results in the release and the redistribution of the residual stress. In this paper, we propose an effective mathematical model and a numerical analysis method for dynamic fracture in residual stress field. We formulate the dynamic behavior of solid continuum with residual stress field in the context of particle discretization scheme finite element method. This formulation enables the appropriate evaluation of (i) release and redistribution of residual stress due to dynamic propagation of the cracks and (ii) the effect of the elastic wave on crack propagation, which are the most substantial problems on dynamic fracture in residual stress field. We perform the experiments and the simulations of dynamic fracture process in chemically tempered glass sheets with residual stress field to validate the proposed numerical analysis method. The simulation results show remarkable agreement with the experiments of the catastrophic failure of the glass sheets with residual stress field in all aspects of crack behavior. These results indicate that the proposed model and method can rigorously evaluate the release and the autonomous redistribution of the residual stress in the dynamic fracture process.

Redistribution of residual stresses in girth weld of a pipe of strength class K60 after ultrasonic impact treatment

When applying technological methods of post-treatment of welded joints, it is necessary to control residual stress fields. The degree of redistribution of residual stresses mainly depends on the strength characteristics of steel, the geometric dimensions of the joint, the initial stress state, and treatment modes. The paper studies the redistribution of residual stresses in the ring joint of a pipe with a diameter of 530 mm made of steel (0.10%C-2%Mn-1%V-1%Nb-1%Al) of strength class K60. The pipes are connected by arc welding. Residual stresses were measured by the X-ray method based on measurement of microstrain of the crystal lattice of the material under its influence. Ultrasonic impact treatment (UIT) was done using a special technological complex. It consisted of an ultrasonic generator UZGT 0.5/27 and an ultrasonic impact tool BOMBUS. It is shown that after arc welding tensile residual stresses in the girth weld are formed in the inner side of the pipe. It was revealed that the UIT of the girth weld at a speed of 0.06 m/min from the inner side of the pipe wall contribute to the redistribution of residual welding stresses up to -100... 200 MPa, which provides the formation of compression stresses on the inner surface of the pipe. It’s shown that ultrasonic impact treatment reduces the level of tensile residual stresses and transforms them into compressive residual stresses. This can have a positive effect on the fatigue strength of the pipe girth weld.

Effects of SiC content on the densification, microstructure, and mechanical properties of HfB2–SiC composites

AbstractTo investigate the effects of SiC on microstructure, hardness, and fracture toughness, 0, 10, 20, and 30 vol% SiC were added to HfB2 and sintered by SPS. Upon adding SiC to 30 vol%, relative density increased about 4%; but HfB2 grain growth had a minimum at 20 vol% SiC. This may be due to grain boundary silicate glass, responsible for surface oxide wash out, enriched in SiO2 with higher fraction of SiC. By SiO2 enrichment, the glass viscosity increased and higher HfO2 remained unsolved which subsequently lead to higher grain growth. Hardness has increased from about 13 to 15 GPa by SiC introduction with no sensible variation with SiC increase. Residual stress measurements by Rietveld method indicated high levels of tensile residual stresses in the HfB2 Matrix. Despite the peak residual stress value at 20 vol% SiC, fracture toughness of this sample was the highest (6.43 MPa m0.5) which implied that fracture toughness is mainly a grain size function. Tracking crack trajectory showed a mainly trans‐granular fracture, but grain boundaries imposed a partial deflection on the crack pathway. SiC had a higher percentage in fracture surface images than the cross‐section which implied a weak crack deflection.

Investigation of the distribution of residual stresses in dissimilar thickness girth welds

Investigation of the distribution of residual stresses in girth welds of pipes is a rather laborious process. It is not always possible to predict how residual stress fields will be formed after welding. The result depends on steel grade, technological process of welding (heat input welding, welding speeds), temperature of environment, geometrical parameters of pipes. Distribution of residual stresses in a dissimilar thickness girth butt weld of pipes with a diameter of 530 mm of low alloyed (0.9%C-2%Mn-1%Si) steel was studied. Residual stresses were measured by the X-ray method based on measurement of microdeformation of a crystal lattice of a material under its influence. The research results showed that tensile residual stresses in the girth weld were formed at the inner side of the pipe after welding. It was revealed that the magnitude and distribution of residual stresses depend on the wall thickness of the pipes. In a thinner pipe, the level of tensile residual stresses is 40-50% higher than that of a thicker pipe. Investigation of the distribution of residual stresses in welded joints can be useful for developing methods to reduce the high residual stresses that occur after welding and also at deciding of constructive and technological task in the process of welding works.

A Study on the Effect of Surface Grinding on Chloride Induced Pitting Initiation of Austenitic Stainless Steel 304 under Thermal Insulation at Ambient Temperature

The investigation was carried out to study the effect of surface grinding on chloride induced stress corrosion cracking (CISCC) of austenitic stainless steels 304 at ambient temperature condition. The U-bend tests with thickness 13 mm as per ASTM G 30 were used to investigate the effect of surface grinding on chloride induced stress corrosion cracking (CISCC) of austenitic stainless steel 304 in a corrosive atmosphere containing sodium chloride at ambient temperature. At a high level of tensile residual stress had developed the pits on U-bend specimen surface at ambient temperature with the presence of low to high chloride concentration level. The experimental results recommend that develop a proper metallurgical fabrication criteria, specification, and procedures for pressure vessels to avoid a recurrence in future.

Effect of Process Parameters on Distortion and Residual Stress of High-Pressure Die-Cast AZ91D Components

This paper presents a study of distortion and residual stress within a high-pressure die-cast AZ91D component, cast under different processing conditions. The influence of process parameters, i.e., die temperature, cooling time, intensification pressure and first-phase injection speeds, was examined. Distortions were measured using the in-house standard analog quality control fixture. Residual stress depth profiles were measured using a prism hole-drilling method. It was found that the most important process parameter affecting the distortion was intensification pressure and the second most important was temperature difference between the two die halves (fixed and moving side). Tensile residual stresses were found very near the surface. Increasing the intensification pressure resulted in an increased level of tensile residual stresses.

Measurement of residual stresses induced by sequential weld buttering and cladding operations involving a 2.25Cr-1Mo substrate material

Dissimilar metal welds are necessary in high-pressure subsea systems and in cases where forged components must be welded to pipelines. F22 (2.25Cr-1Mo) steel is often used in such forged steel components and, since this steel cannot enter service without undergoing post-weld heat treatment (PWHT), the components are usually prepared for field welds through the application of a buttering layer. Furthermore, a weld overlay is deposited for the purpose of mitigating corrosion. This combination of multiple welding tasks and dissimilar materials leads to the possibility of developing substantial residual stresses. This study aims to provide insights to the evolution of residual stresses at each stage of the welding operation. The assessment has been undertaken on laboratory-scale weld mock-ups using the contour method for residual stress measurement, and incremental centre hole drilling. It was found that both buttering and cladding introduce near-yield levels of tensile residual stresses, but that these stresses are successfully relieved upon PWHT.

Effect of surface grinding on chloride induced SCC of 304L

The effect of surface grinding on the stress corrosion cracking (SCC) behavior of 304L austenitic stainless steel in boiling magnesium chloride has been investigated. SCC tests were conducted both without external loading and with varied levels of four-point bend loading for as-delivered material and for specimens which had been ground parallel or perpendicular to the loading direction. Residual stresses due to the grinding operation were measured using the X-ray diffraction technique. In addition, surface stress measurements under applied load were performed before exposure to evaluate the deviation between actual applied loading and calculated values according to ASTM G39. Micro-cracks initiated by a high level of tensile residual stress in the surface layer were observed for all the ground specimens but not those in the as-delivered condition. Grinding along the loading direction increased the susceptibility to chloride induced SCC; while grinding perpendicular to the loading direction improved SCC resistance. Surface tensile residual stresses were largely relieved after the initiation of cracks.

Application with Simultaneous Electrocontact Heating Stretching to Reduce Residual Stresses in the Heat Exchange Tube

The main reason of appearance and growth of corrosion damages of the nuclear steam generator heat exchanger tubes is the process of stress-corrosion cracking of metal under the influence of residual tensile stress.The article contain the suggestion of the method that combines the operation of heat processing and straightening of pipes by stretching. The approbation of the method was carried out in the laboratory conditions. There is described the construction and working principle of a trial installation. During the experiments different parameters of electrocontact heating with stretching were tested. Approbation showed that the proposed method can reduce the level of tensile residual stresses or get compressive residual stresses in the wall of the tubes, which increases their resistance to corrosion.Keywords:Steam generator, heat exchange tubes, stress corrosion cracking, intercrystalline corrosion, residual stresses

Analysis of Thermal Effect on Residual Stresses of Broached Inconel 718

Inconel 718 is a nickel based superalloy that is widely used as a turbine disc material in gas turbine industries. This study details the effect of thermal exposure on the residual stresses produced when broaching Inconel 718. The chosen parameters for broaching in this study are similar to those used when manufacturing turbine discs. The broaching operation produced a high level of tensile residual stresses at the broached surface. A layer with tensile residual stresses was formed in the sub-surface region, followed by a layer several times thicker with compressive residual stresses. Thermal exposure was conducted at 550 °C. The depth distributions of residual stresses after thermal exposure are presented and discussed in this paper. Complete relaxation of the surface tensile residual stresses was observed after 30 h thermal exposure, whereas the 3000 h thermal exposure influenced both the surface and sub-surface residual stress states.

Analysis of Retained Austenite and Residual Stress Distribution in Ni-Cr Type High Strength Steel Weld by Neutron Diffraction

Prevention of weld cracking is necessary for ensuring the reliability of high strength steel structures. Tensile residual stress in the weld metal is one of the major factors causing the weld cracking, therefore, it is important to clarify the residual stress distribution in the weld metal. Conventional stress measurement, the stress relief method using strain gauges and the X-ray diffraction technique, can only provide the stress information in the surface region of the steel weld. The neutron diffraction is the only non-destructive method that can measure the residual stress distribution inside the steel weld [1-3]. The neutron stress measurement was applied for the 980MPa class high strength steel weld and it was revealed that high level of tensile residual stress can affect the weld cracking to a significant degree [4-5]. Recently, it was reported that Ni-Cr type steel weld exhibit higher resistance to the weld cracking compared with conventional low alloy type weld. Increase of tensile residual stress is prevented by lower transformation temperature of the Ni-Cr type weld metal and retained austenite phase is dispersed in the martensite microstructure. It is considered that lower level of tensile residual stress and the existence of retained austenite may prevent hydrogen accumulation in the weld metal [6]. However, retained austenite and the residual stress conditions in the Ni-Cr type high strength steel weld is not well understood. In this study, neutron diffraction analysis was conducted on the Ni-Cr type steel weld joint with the tensile strength level of 980MPa in order to investigate the effect of the retained austenite and the residual stress distribution on the weld cracking.

Comparison of rotational turning and hard turning regarding surface generation

In this paper, two different turning processes namely rotational turning and hard turning are compared to each other regarding surface generation aspects. By experiments it is shown that, with higher feed rates, rotational turning yields same surface quality as hard turning. Feed rates can be chosen six times higher in rotational turning than in conventional hard turning without losses in the surface roughness quality. Also experiments reveal that the tool wear in rotational turning has a beneficial effect on the surface roughness. A corresponding explanation model is thereby presented which takes the specific tool/work piece engagement in rotational turning into account. Furthermore, it is shown that rotational turning has negative effects on the surface integrity. The phase transformation zones (“white layers”) are thicker in rotational turned parts than in hard turned parts. Also the level of tensile residual stress in rotational turning is higher than in hard turning. Both effects are probably caused by high thermal material loads in rotational turning due to increased friction. However, the results of this paper show that rotational turning has a high potential to become an efficient alternative to hard turning, especially when it comes to large scale production of simple shaped parts.

Estudo das tensões residuais de juntas soldadas de aço API 5L X80

Este trabalho teve por objetivo determinar os níveis de tensões residuais decorrentes de processos de soldagem em juntas soldadas de um aço API 5L X80. Inicialmente foram executados os procedimentos de soldagem das juntas utilizando os processos de soldagem eletrodo revestido - Shielded Metal Arc Welding (SMAW) e eletrodo de tungstênio com proteção gasosa - Gas Tungsten Arc Welding (GTAW), variando-se os consumíveis utilizados e empregando-se uma temperatura de interpasse de 175ºC, obtendo-se, desta maneira, três juntas soldadas. Foram avaliadas as regiões superiores e inferiores das juntas, determinando-se os níveis de tensões residuais transversais e longitudinais. As medidas de tensão residual foram realizadas por difratometria de raios-x através de um difratômetro portátil e respectivo software para análise dos resultados. Para a região superior das juntas soldadas, foi observado que quanto mais elevado for o aporte térmico mais compressivas foram os níveis de tensões residuais transversais, e mais deslocadas ficaram os picos de tensões trativas. Na região inferior das juntas soldadas, foi verificado que para maiores aportes térmicos, maiores foram os níveis de tensões residuais trativas na região do Metal de Solda (MS) e menor foi o nível de tensões residuais trativas na Zona Termicamente Afetada (ZTA).

水素侵入による980MPa級溶接金属の低温割れ限界条件

Prevention of weld cracking is primely important for ensuring the integrity of steel construction. Especially, for high strength steel weld, which is susceptible to hydrogen cold cracking, it is important to clarify the critical condition for cracking in order for expanding use of high strength steel applications. In this study, critical conditions of hydrogen content and residual stress in the high strength steel weld with tensile strength level of over 980MPa were investigated. Cold cracking test was conducted using Y-grooved constraint weld joint by intentionally introducing hydrogen gas, and critical hydrogen content in the weld metal to cold cracking was determined. Cold cracking occurred in the weld metal from the root, bottom portion in the weld metal, when hydrogen content exceeds about 2ppm, and fracture surface was typical grain boundary fracture caused by hydrogen embrittlement. Then, residual stress distribution in the weld meal was measured by neutron diffraction technique. Same Y-grooved weld joint with no cold cracking was used for stress measurement. Neutron beam was filtered by 2mm×2mm slit to measure the residual stress in the small region. Reference sample without residual stress was prepared by introducing multiple saw cuts with small intervals in the same weld joint, and lattice spacing of stress free condition was first measured. Then, lattice spacing in the three different directions along the centerline of the weld metal were measured to calculate residual stress in the three directions. All portions in the weld metal center showed tensile residual stress. The root portion showed highest tensile stress of over 1110MPa in the transverse direction, which is the same as the crack opening direction. Therefore, it is considered that high level of tensile residual stress is the key factor for cold cracking. Hydrogen concentration to the root region by pressure stress was discussed in terms of local hydrogen content condition for cracking.

An investigation of the residual stress characterization and relaxation in peened friction stir welded aluminum–lithium alloy joints

In this investigation the residual stresses generated from friction stir welded (FSW) 2195 aluminum–lithium alloy joints were characterized. The results derived from this research revealed significant levels of tensile residual stresses at the surface and throughout the thickness of the FSW samples. Furthermore, residual stress relaxation at the surface and throughout the thickness of the samples was assessed for laser peened friction stir welded aluminum–lithium joints. To do so the samples were cycled several times at a constant amplitude load. The results indicated that most of the relaxation for the surface residual stresses took place during the first cycle of loading. Also, residual stresses relaxation throughout the thickness of the welded region of unpeened samples significantly exceeded the relaxation exhibited by the laser peened samples.

Residual Stresses in Orthogonal Cutting of Metals: The Effect of Thermomechanical Coupling Parameters and of Friction

The generation of residual stresses in orthogonal machining is analysed by using an Arbitrary Lagrangian Eulerian (ALE) finite element approach. It is shown that a substantial level of tensile residual stresses can be obtained in the vicinity of the machined surface without any contribution of thermal effects. This motivates the development of a parametric study to analyse the effects of the thermomechanical coupling parameters on residual stresses. The roles of thermal expansion, of thermal softening and of the Taylor–Quinney coefficient (controlling the heat generated by plastic flow) are considered separately. The influence of friction is also analysed by assuming dry cutting conditions and a Coulomb friction law. The friction coefficient has a complex effect by controlling heat generation (frictional heating) along the tool rake and clearance faces and the propensity for the chip to stick to the tool. Geometrical effects such as the tool rake angle and the tool edge radius are also discussed.

Effects of abrasive type cooling mode and peripheral grinding wheel speed on the AISI D2 steel ground surface integrity

In this work the AISI D2 steel ground surface integrity was evaluated at different grinding conditions. Two sets of experiments were conducted. The first and the second sets are referred to as conventional grinding tests (CGTs) and high-speed grinding tests (HSGTs), respectively. For the CGTs two different types of abrasives (aluminium oxide and sol–gel alumina) and two cooling modes (oil-based grinding fluid and cryogenic cooling) were tested at a peripheral wheel speed v s =22 m/s. For the HSGTs, an electroplated cBN grinding wheel was used at a peripheral wheel speed ranging from 60 to 260 m/s. Experimental results show that the grindability and the surface integrity of the AISI D2 steel could be substantially improved by using the sol–gel grinding wheel and cooling by liquid nitrogen comparatively with conditions using aluminium oxide and cooling with oil-based grinding fluid. These conditions reduce the grinding force components, lower the level of tensile residual stresses and expand the range of stock removals rate for which compressive residual stresses can be obtained. In this case the stock removal rate could be increased 7 times and still having compressive residual stresses. These experimental results were further explored and it was possible to establish linear relationships between the specific energy and the amplitudes of the surface residual stresses when grinding under the CGTs conditions. This relationship is very useful for process control and optimization. Results of the HSGTs show that the highest levels of the compressive surface residual stresses were obtained at a peripheral wheel speed v s =120 m/s ( σ ∥=−520 MPa, σ ⊥=−770 MPa). These values are much higher than those measured at the ground surface generated using the sol–gel grinding wheel and cooling by liquid nitrogen ( σ ∥=−295 MPa, σ ⊥=−250 MPa). Further, the CGTs and HSGTs results were used to establish the burn-free and burn conditions in relation with the specific grinding energy. It was shown that condition using the sol–gel grinding wheel and cooling with liquid nitrogen and condition using cBN grinding wheel at peripheral wheel speed up to 180 m/s generate specific grinding energy that are significantly lower than the burning threshold energy.

Surface integrity and tool life when turning Inconel 718 using ultra-high pressure and flood coolant systems

The use of high-pressure cutting fluids has been shown to increase tool life or enable higher cutting speeds to be used when machining advanced aerospace materials such as Inconel 718. Cutting fluids act to provide cooling and to reduce friction between the tool and the chip. When supplied at high pressure, greater cooling and penetration of the fluid into the interface between the tool and the chip occur, when compared to conventional flood supply. Both these factors have the potential to alter the resulting surface integrity of the machined component. Following a brief review of the general machinability of Inconel 718, a series of experiments examining the effects of varying the cutting fluid supply pressure and direction of the jet are detailed when finish turning. Surprisingly, no increase in tool life was observed when using up to 450 bar pressure. When the high-pressure jet was directed at the flank face, reduced flank wear occurred but the level of notching was not affected, resulting in a similar life. Effective chip-breaking occurred at 150 bar; below this, continuous tubular-type chips were produced. Cutting fluid pressure and direction had relatively little influence on the level of surface integrity obtained. However, the use of ultra-high-pressure coolant provided a clear reduction in the level of tensile residual stresses developed in the near-surface layer. The lowest level of residual stress was induced with 450 bar coolant supplied to the flank face. Flood coolant induced a higher tensile stress in the near-surface layer and a larger compressive stress zone beneath this layer.