Neutron diffraction measurements of residual stresses in a 50 mm thick weld

Residual stresses were determined through the thickness of a 50 mm thick ferrite steel weld plate using neutron diffraction. Whereas the limiting penetration depth for iron-based alloys is about 25 mm in the most typical neutron diffractometers, we significantly enhanced the penetration depth up to 50 mm with 2 mm spatial resolution by using the neutron wavelength of 2.39 Å. The selected wavelength minimizes the total neutron cross-section and beam attenuation, thereby, maximizes the neutron fluxes at depth. Two-dimensional mapping of the residual stresses shows that significant amounts of the tensile longitudinal stresses (over 90% of yield strength) were developed along the heat-affected zone of the weld.     

Residual stresses and fracture mechanics analysis of a crack in welds of high strength steels

This study presented the characteristics of residual stresses in welds of high strength steels (POSTEN60, POSTEN80) whose tensile strengths were 600 MPa and 800 MPa, respectively. Three-dimensional thermal elastic-plastic analyses were conducted to investigate the characteristics of welding residual stresses in welds of high strength steels through the thermal and mechanical properties at high temperatures obtained from the elevated temperature tensile tests. A finite element analysis method which can calculate the J-integral for a crack in a residual stress field was developed to evaluate the J-integral for a centre crack when mechanical stresses were applied in conjunction with residual stresses.The results show that the volumetric changes associated with the austenite to martensite phase transformation during rapid cooling after welding of high strength steels significantly influence on the development of residual stresses in the weld fusion zone and heat-affected zone. For a centre crack in welds of high strength steels where only residual stresses are present, increased tensile strength of the steel, increased the J-integral values. The values of the J-integral for the case when mechanical stresses are applied in conjunction with residual stresses are larger than those for the case when only residual stresses are present.     

Fatigue strength of laser beam welded automotive components made of thin steel sheets considering size effects

The applicability of and the quality of assessment using the nominal stress, structural stress and notch stress approaches for calculating the fatigue strength of laserbeam welded components made of thin steel sheets has been investigated. For this purpose, the fatigue lives of a longitudinal carrier, an injector and two tube-flange specimens have been determined by tests under constant amplitude loading. Fatigue cracks initiated at sharp root notches on all of these components. While the nominal stress is derived by theory of structural mechanics, the determination of structural and notch stresses is performed using 3D finite element models and solid elements. The structural stress is derived by an extrapolation of surface stress to the fatigue critical notch and the notch stresses by rounding the sharp root notch with a reference radius of r_ref = 0.05 mm. For all of the concepts used, the endurable stresses have been compared to the design SN-curves recommended by the International Institute of Welding (IIW).On comparing the quality of assessment of the different concepts, the notch stress approach shows the highest scatter. The highest endurable notch stresses occur in specimens with crack initiation at weld ends. These specimens have a very small highly loaded weld length. The lowest endurable stresses are determined for specimens with a long, equally loaded weld. The reason for these findings can be explained by statistical size effects. For an improved fatigue assessment, an easily applicable method is introduced, which takes into account the highly stressed weld length.     

Nondestructive Evaluation of Welding Residual Stresses in Dissimilar Welded Pipes

This paper investigates the nondestructive capability of ultrasonic waves in residual stress evaluation of dissimilar welded pipes. Longitudinal critically refracted (L _CR ) waves are employed to measure the residual stresses in a pipe-pipe joint of stainless steel 304 and carbon steel A106. Measuring the acoustoelastic constant is usually accomplished through the tensile test which needs cutting the tested material to extract tensile test specimens. However, cutting the tested pipe to complement the nondestructive ultrasonic measurement was not considered here. Instead, a dissimilar welded plate with the same welding specification, joint geometry, thickness and the same dissimilar materials is used to extract tensile test samples. The measured acoustoelastic constant of the plate along with the measured time of flight of the L _CR wave on the pipe, are utilized for ultrasonic stress measurement. A finite element model of welding process validated by hole-drilling method is used to verify the ultrasonic results. The results show good agreement between finite element and ultrasonic measurements in the pipe measured without any destructive process.     

Effects of deposition conditions of the Al film in Al/glass specimens and annealing conditions on internal stresses and hillock formations

In the present study, 25 kinds of specimen with five Al-film thicknesses were prepared to investigate the relation between the internal stress formed during the annealing process and the hillocks. In the preparation of specimens, the governing factors including deposition conditions, annealing temperature, and annealing time, were arranged following the orthogonal table of five-level and six-factorial (L₂₅(5⁶)) design. Stoney's formula is applied to describe the internal stresses before and after annealing (σ₀ and σ_f), respectively. The internal stress arising during the annealing process (σ_an) is evaluated using the model developed by Flinn et al. [1]. Then, the response surface methodology (RSM) is used to express the three stress parameters in terms of influential factors. The incipient σ_an value for hillocks appearing in the specimens was found to be between − 28.7 MPa and − 32 MPa in a compressive form. The annealing temperature, time, and Al-film thickness are the three major factors, affecting internal stress σ_an. An increase in the annealing time reduces the tensile stress or increases the compressive stress, or both. The tensile stress decreases and the compressive stress increases during the annealing process with increasing Al film thickness and annealing temperature. The number of hillocks formed in a unit of area is linearly proportional to both σ_an and (σ_f − σ_an).     

Characterization of laser welds in Al–10 wt.%Si coated ferritic stainless steel

409L stainless steel hot-dipped with Al–10 wt.%Si was welded using CO₂ laser and the microstructure and hardness of the weld were investigated. When the specimen was welded with laser power of 5 kW and welding speed of 5 m/min, full-penetrated sound weld was obtained. With that specimen, the relationship between the microstructure and hardness of the weld was examined. The hardness of the weld was the highest in the fusion zone (FZ) and decreased to the base metal (BM) via heat affected zone (HAZ). The hardness of the HAZ near bond line was also higher than that near the base metal. The maximum hardness in the fusion zone could be explained by the existence of the precipitates, that is, TiN, Ti(C,N), Al₂O₃ and Al₂O₃ + TiN mixed compounds with the size of 500 nm, and solution strengthening due to the elements Al and Si dissolved from the coating layer to the fusion zone. There were subgrains within the HAZ and more in the area near the bond line. In addition, fine TiC particles with the size under 50 nm was precipitated in the sub-grain boundaries. The formation of sub-grain boundaries and the particles precipitated in the boundaries might contributed to the high hardness in the HAZ.     

Analysis of residual stresses in thick aluminum friction stir welded butt joints

Plates of aluminum alloys 2219-T62 were joined in a butt joint by friction stir welding. The residual stresses on the top and bottom surfaces were measured using the hole-drilling strain-gauge method. In the test specimen, it was found that the residual stresses on the top surface peaked at about 171 MPa, while the value reached 243 MPa for the weld with tunnel defect and had the conventional “M” profile with tensile stress peaks in the heat-affected zone. Those attached on the bottom surface had the inverted “V” profile with tensile stress peaks in the weld centre and the corresponding value was 99.4 MPa. Meanwhile, with the increase of rotary speed, the longitudinal residual stress decreased on the top surface, but increased on the bottom surface.     

Inclusion-controlled high cycle fatigue behavior of a high V alloyed powder metallurgy cold-working tool steel

Axial loading fatigue tests were carried out to study the influence of inclusion on high cycle fatigue behavior of a high V alloyed powder metallurgy cold-working tool steel (AISI 11). The fatigue strength of 1538 MPa with endurance life of 10⁷ cycles were obtained by stair-case method. The fatigue specimens were also subjected to a constant maximum stress of 1650 MPa to investigate the relationship among inclusion origin size (10-30 μm), fish-eye size (70-130 μm) and fatigue life (10⁵-10⁷ cycles). The fatigue life was found to be dependent on the inclusion size and the crack propagating length. A compressive residual stress of 300-450 MPa turned out to be present at the specimen surface, and finally induced the interior failure mode. Further investigation into the correlation between stress intensity factors of inclusion origin and corresponding stages of fatigue crack growth and fatigue life revealed that the high cycle fatigue behavior was controlled by crack propagation. According to the fractographic investigation, two distinct zones were observed in fish-eye, representing Paris-Law and fast fatigue crack growth stage, respectively. Threshold stress intensity for crack propagation of 3.9 MPa√m was obtained from the well correlated line on the ΔK_I-log N_? graph. The fracture toughness can also be estimated by the mean value of stress intensity factor ranges for fish-eye.     

Fracture toughness of grade D ship steel

Results from fracture mechanics tests on 15 mm thick grade D ship steel and weld are organised into a toughness distribution indexed to the Charpy 27 joule temperature, T_27J. The tests are carried out at 300 MPa√m/s to simulate the strain rate appropriate to a long (≈1 m) through thickness crack in the deck of a ship under storm conditions. Most of the data are in the brittle to ductile transition region and end in cleavage fracture. A best fit to the data is found using the exponential curve fit (ECF) method. Lack of censoring of invalid results means that the trend line is not a true ‘plane strain’ fit. It is argued that inclusion of ‘plane stress’ data makes the resultant toughness distribution more relevant to ship fracture predictions. Equations are presented which allow the toughness to be plotted at any chosen probability level as a function of temperature relative to T_27J. A safe lower bound to the data is given by the 0.1% probability trend assuming that T_27J for grade D plate and weld is no higher than −20 °C. The data are also used to propose that it is impossible to generate an elastic ductile tearing instability in ship steel with Charpy upper shelf values of 100 J or more.     

Ultrasonic inspection of a welded stainless steel pipe to evaluate residual stresses through thickness

This study investigates ultrasonic method in axial and hoop stress measurement through thickness of an austenitic stainless steel pipe. Longitudinal critically refracted (L_CR) waves are employed to measure the welding residual stresses while outer and inner surfaces of the pipe are inspected by using different frequency range of ultrasonic transducers. The acoustoelastic constant is measured on a plate with the same material and thickness of the investigated pipe to keep the pipe intact. Welding process of the pipe is simulated by a 3D finite element (FE) model which is validated by hole-drilling method performed on 25 points. The residual stresses calculated by FE simulation are then compared with those obtained from the ultrasonic measurement while a good agreement is observed. It is demonstrated that the residual stresses through thickness of the stainless steel pipe can be evaluated by combining FE and L_CR method (known as FEL_CR method).     

Investigations on the mechanical properties and microstructure of dissimilar cp-titanium and AISI 316L austenitic stainless steel continuous friction welds

Friction welding process is a solid state joining process that produces a weld under the compressive force contact of one rotating and one stationary work piece. In this study, the friction welding of dissimilar joints of AISI 316L stainless steel and cp-titanium is considered. The optical, scanning electron microscopy studies of the weld were carried out. Moreover, the X-ray diffraction analysis was performed. The integrity of welds was achieved by the micro hardness and tensile tests. The fracture surface was examined by the scanning electron microscopy. The study showed that the magnitude of tensile strength of the dissimilar welded specimen was below that of the titanium base material if preheating was not applied at the interface. The high weld tensile strength was achieved by preheating the 316L stainless steel material to 700 °C, smoothing and cleaning of the contact surfaces. Results illustrated that in dissimilar joints, different phases and intermetallic compounds such as FeTi, Fe₂Ti, Fe₂Ti₄O, Cr₂Ti and sigma titanium phase were produced at the interface. The presence of brittle intermetallic compounds at the interface resulted in degradation of mechanical strength which in turn led to premature failure of joint interface in the service condition. Preheating caused to produce oxide layer at the interface which was harmful for bonding. The oxide layer could be eliminated by applying pressure and smoothing the surface. Results of hardness tests illustrated that the high hardness was occurred in the titanium side adjacent to the joint interface. Moreover, the optimum operational parameters were obtained in order to achieve the weld tensile strength greater than the weak titanium material.     

Fracture resistance testing of dissimilar nickel–chromium girth welds for clad line pipes

This work presents an experimental investigation of the ductile tearing properties for the girth weld of a typical C–Mn pipe internally clad with ASTM UNS N06625 Alloy 625 using measured crack growth resistance curves (\(J{-}\Delta a\) and \(\mathrm {CTOD}{-}\Delta a\) curves). Here, the material of the external pipe is a typical API 5L Grade X65 pipeline steel whereas the inner clad layer is made of a nickel–chromium corrosion resistant alloy steel. Testing of the girth weld employed side-grooved, clamped SE(T) specimens with a weld centerline notch to determine the crack growth resistance curves based upon the unloading compliance method using a single specimen technique. This experimental characterization provides additional toughness data which serve to evaluate the effectiveness of current procedures in determining accurate experimentally measured R-curves for this class of material, including the effects of weld strength mismatch.     

Effect of surface oxidation on thermal shock resistance of the ZrB2-SiC-ZrC ceramic

The isothermal oxidation of the ZrB₂-SiC-ZrC ceramic was carried out in static air at a constant temperature of 1000 ± 15 °C, 1200 ± 15 °C and 1400 ± 15 °C for 30 min, respectively. Compared with the original strength of 580 MPa, the strength for the specimen oxidized at 1000 °C, 1200 °C and 1400 °C for 30 min increased to 609 MPa, 656 MPa and 660 MPa, respectively, because the flaws in the surface of the specimen were sealed by the oxide layer. The thermal shock resistance of the specimens before and after the oxidation was measured by the water quenching. The measured ΔT_crit for the specimen oxidized at 1000 °C, 1200 °C and 1400 °C were 352 °C, 453 °C and 623 °C, respectively, which was obviously higher than 270 °C for the unoxidized specimen. The improvement in the thermal shock resistance was attributed to the formation the oxide layer on the surface of the specimen. The results here pointed to a promising method for improving strength and thermal shock resistance of ZrB₂-based ceramics.     

Compressive and impression creep behavior of a cast Mg–Al–Zn–Si alloy

Creep behavior of an Mg–6Al–1Zn–0.7Si cast alloy was investigated by compression and impression creep test methods in order to evaluate the correspondence of impression creep results and creep mechanisms with conventional compression test. All creep tests were carried out in the temperature range 423–523 K and under normal stresses in the range 50–300 MPa for the compression creep and 150–650 MPa for impression creep tests. The microstructure of the AZ61–0.7Si alloy consists of β-Mg₁₇Al₁₂ and Mg₂Si intermetallic phases in the α-Mg matrix. The softening of the former at high temperatures is compensated by the strengthening effect of the latter, which acts as a barrier opposing recovery processes. The impression results were in good agreement with those of the conventional compressive creep tests. The creep behavior can be divided into two stress regimes, with a change from the low-stress regime to the high-stress regime occurring, depending on the test temperature, around 0.009 < (σ/G) < 0.015 and 0.021 < (σ_imp/G) < 0.033 for the compressive and impression creep tests, respectively. Based on the steady-state power-law creep relationship, the stress exponents of about 4–5 and 10–12 were obtained at low and high stresses, respectively. The low-stress regime activation energies of about 90 kJ mol⁻¹, which are close to that for dislocation pipe diffusion in the Mg, and stress exponents in the range of 4–5 suggest that the operative creep mechanism is pipe-diffusion-controlled dislocation viscous glide. This behavior is in contrast to the high-stress regime, in which the stress exponents of 10–12 and activation energies of about 141 kJ mol⁻¹ are indicative of a dislocation climb mechanism similar to those noted in dispersion strengthening mechanisms.     

Effect of quenching mode on the mechanical properties of a Zr64Al10Ni15Cu11 metallic glass

This paper studied the effect of quenching mode on the mechanical properties of a Zr₆₄Al₁₀Ni₁₅Cu₁₁ metallic glass. It has been shown that the specimen through annealing and water quenching treatments has a higher strength of 1698 MPa than the specimen without annealing treatment (with a strength of 1572 MPa) due to the presence of nanocrystals in the amorphous matrix, which act as obstacles to the shear bands movement during deformation and thus improve the strength. It has also been shown that the specimen through annealing and air quenching treatments has a lower strength of 1119 MPa than the specimen without annealing treatment (with a strength of 1572 MPa) due to the heterogeneous distribution of the internal stress, which induces the crack initiations and catastrophic failure at a rather low external applied stress.     

Microstructure and mechanical performance of depositing CuSi3 Cu alloy onto 30CrMnSi steel plate by the novel consumable and non-consumable electrodes indirect arc welding

A novel consumable and non-consumable electrodes indirect arc welding (CNC-IAW) with low heat input was successfully applied in depositing CuSi₃ Cu alloy onto 30CrMnSi steel plate. The indirect arc was generated between the consumable and non-consumable welding torch. The microstructure of the deposited weld was analyzed by means of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and optical microscopy (OM). The results showed that the dilution ratio of the bead-on-plate weld was controlled no higher than 5% and the deleterious iron picking up was effectively restrained. The deposited metal mainly consisted of ε-Cu solid solution and a small amount of Fe₂Si phase. In the interfacial zone between the deposited metal and base metal, the thickness of the zone changed from thick to thin and the microstructure changed from complex to simple from the middle to both sides. In the middle of the interfacial zone, the microstructure presented three sub-layers consisting of Fe₃Si (L)/Fe₃Si (S) + ε-Cu/α-Fe. In the both sides of the interfacial zone, the microstructure presented single α-Fe layer. The formation mechanism of the interfacial zone could be successfully explained by the formation of the Fe liquid–solid phase zone adjacent to the Fe base metal and the interfusion between Fe and Si. The average compressive shear strength reached 321 MPa and its fracture morphology mainly belonged to ductile fracture.     

Effect of loading type on fatigue properties of high strength bearing steel in very high cycle regime

Very high cycle fatigue tests under axial loading at frequencies of 95 Hz and 20 kHz were performed to clarify the effect of loading type on fatigue properties of a high strength bearing steel in combination with experimental result of this steel under rotating bending. As a result, this steel represents the single P-S-N (probabilistic-stress-life) curve characteristics for surface-induced fracture and interior inclusion-induced fracture, just like that under rotating bending. However, fatigue strength is lower, where the run-out stress at 10⁹ cycles is evaluated to be 588 MPa, less than that under rotating bending with about 858 MPa. Occurrence probability of larger and deeper inclusion-induced fracture is much higher than that under rotating bending. Furthermore, the formation process of fine granular area (FGA) is independent of the type and frequency of loading, which is very slow and is explained as the crack nucleation process under the special dislocation mechanism. The stress intensity factor range at the front of FGA, ΔK_FGA, is approximately regarded as the threshold value controlling the stable propagation of interior crack. For the control volume of specimen under axial loading, the estimated value of fatigue limit by FGA is similar to experimental run-out stress value at 10⁹ cycles, but that by inclusion is larger. However, the corresponding estimated results under rotating bending are all conservative.     

Chromizing of plasma nitrided AISI 1045 steel

In this study AISI 1045 steel specimens were plasma nitrided at 803 K for 5 h, in a gas mixture of 75% N₂ + 25% H₂. The specimens were then chromized in powder mixtures consisting of ferrochromium, ammonium chloride and alumina at 1273 K for 5 h. Scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis and Vickers micro-hardness test were used as characterizing techniques. The thickness of white nitrided layer was around 5 μm, which was mainly composed of iron nitrides and its hardness was around 740 HV. Chromizing of nitride layer resulted in formation of Cr₂N chromium nitride and Fe₃N iron nitrides. A significant increase was observed in hardness after chromizing of the nitrided layer. Despite its higher hardness, the post chromised specimen showed higher wear rate than single plasma nitrided specimen.     

Dissimilar lap joining of 304 stainless steel to CP-Ti employing friction stir welding

A 304 stainless steel plate was lap joined to a CP-Ti one by friction stir welding technique. Stainless steel was selected as the top member. Sound dissimilar joints were achieved using an advancing speed of 50 mm/min and rotation speeds in the range of 700–1100 rpm. A region of vortices of bimetallic weld of 304 stainless steel and CP-Ti was formed in the lap joint fabricated using the highest applied tool rotation speed; this was associated with plasticizing of both members with the aid of a double-shoulder tool. In addition, due to complex material flow, mechanical interlock features were shaped that consists of extruded stainless steel into the plasticized titanium region. A maximum shear strength value of ∼119 MPa was achieved; this was found to be close to that of CP-Ti. The lap joint was strengthened by the formation of vortices of bimetallic weld of 304 stainless steel and CP-Ti and mechanical interlock features at joint interface due to complex materials flow.     

Diffusion bonding of Fe-28Al(Cr) alloy with low-carbon steel in vacuum

Fe-28Al(Cr) alloy and low-carbon steel were diffusion bonded in a vacuum of 10⁻⁴-10⁻⁵ Pa. The relationship of the bond parameters and shear strength at the interface was discussed. Microstructure characteristics and the reaction products at the interface were investigated by scanning electron microscopy (SEM) and X-ray diffractometry (XRD). The thickness of the diffusion reaction layer was measured with electron probe microanalysis (EPMA). The results indicated that controlling bonding temperature 1333 K for 3.6 ks, shear strength at the interface can be up to 112 MPa. Three kinds of reaction products were observed to have formed during the vacuum diffusion bonding, namely FeAl, Fe₃Al and α-Fe (Al) solid solution. The thickness (X) of the diffusion reaction layer increases with bonding time (t) according to a parabolic law X²=6.4×10³ exp(−104.1/RT)(t-t₀) (μm²).