Effects of temperature on tensile and impact behavior of dissimilar welds of rotor steels

Tensile and impact behavior of dissimilar weld joints of newly developed rotor steels 23CrMoNiWV88 and 26NiCrMoV145 were conducted at various temperatures below 350 °C. Inhomogeneous microstructures and asymmetrical micro-hardness along the dissimilar welding joint were observed. With the increase of temperature, strength decreased which was associated with the increased plasticity, and fracture location changed from weld metal (WM) to intermediate pressure (IP) base metal (BM) at around 300 °C. Compared to the homogeneous impact specimen with two fracture zones at fracture surface, a combined quasi-cleavage and ductile fracture mode with three zones is observed at the fracture surface of the dissimilar weld joint when the testing temperature is in the range of 0–40 °C. The occurrence of separated zones are mainly ascribed to the multi-layer welding process and thus improved the impact toughness of the welding joint.     

Improvement of weld temperature distribution and mechanical properties of 7050 aluminum alloy butt joints by submerged friction stir welding

Submerged friction stir welding (FSW) in cold and hot water, as well as in air, was carried out for 7050 aluminum alloys. The weld thermal cycles and transverse distributions of the microhardness of the weld joints were measured, and their tensile properties were tested. The fracture surfaces of the tensile specimens were observed, and the microstructures at the fracture region were investigated. The results show that the peak temperature during welding in air was up to 380 °C, while the peak temperatures during welding in cold and hot water were about 220 and 300 °C, respectively. The temperature at the retreated side of the joint was higher than that at the advanced side for all weld joints. The distributions of microhardness exhibited a typical “W” shape. The width of the low hardness zone varied with the weld ambient conditions. The minimum hardness zone was located at the heat affected zone (HAZ) of the weld joints. Better tensile properties were achieved for joint welded in hot water, and the strength ratio of the weld joint to the base metal was up to 92%. The tensile fracture position was located at the low hardness zone of the weld joints. The fracture surfaces exhibited a mixture of dimples and quasi-cleavage planes for the joints welded in cold and hot water, and only dimples for the joint welded in air.     

Results of a Japanese round robin program for creep crack growth using Gr.92 steel welds

High-Cr ferritic heat-resistant steels are commonly used for boiler components in ultra-super critical thermal power plants operated at about 600 °C. In the welded joints of these steels, Type-IV cracks initiate in the fine-grained HAZ during long-term use at high temperatures, causing their creep strength to decrease. To assist the standardization of the testing and evaluation method for creep crack growth (CCG) in the welded components, we conducted round robin tests (RRT) using 9Cr-0.5Mo-1.8 W-V-Nb steel (ASME Grade 92 steel) welded joint as part of the VAMAS TWA31 collaboration. The CCG tests were carried out using the CT specimen and the circumferentially-notched round bar specimen for both the base metal and welded joint of Gr.92 steel. Testing was performed at four different laboratories. The effects of specimen configuration, temperature, load, and stress triaxiality conditions on the crack growth rate and fracture life were investigated.     

Effect of NiCrAlY platelets inclusion on the mechanical and thermal shock properties of glass matrix composites

NiCrAlY platelets modified glass matrix composites were prepared. Their microstructures were characterized, their Young's modulus, fracture strength in bending, Vickers hardness, and indentation toughness were measured, and their thermal shock resistance was studied using quenching-strength and indentation-quench methods. With increasing NiCrAlY content, evident enhancements of the Young's modulus and indentation toughness were obtained. The NiCrAlY alloy inclusion could exert significant influences on the retained bending strength of the samples after quench tests, from 9.6 MPa for NiCrAlY-free glass to 32.0 MPa for 30 wt.% NiCrAlY-containing composites. The indentation-quench tests showed that NiCrAlY alloy inclusion elevated the critical quenching temperatures for propagation of pre-crack, from 150 °C for NiCrAlY-free glass to 225 °C for 30 wt.% NiCrAlY-containing composites. Inclusion debonding and intersection, crack deflection and bridging were observed, and are likely the micromechanisms accounted for the improvement of fracture resistance.     

High temperature crack initiation and defect assessment of P22 steel weldments using time dependent failure assessment method

High temperature deformation and crack resistance of low alloy ferritic grade P22 steel weldments applied in power plants are reported. The creep crack initiation (CCI) and creep crack growth (CCG) data were determined using compact type (C(T)) and C-Shape (CS(T)) fracture mechanics specimens at 550 °C. The deformation and crack growth behaviour of similar weldment zones and significance of CCI and CCG in defect assessment of components were addressed. The weldments with industrially relevant properties were produced in butt welded pipe joint from which test specimens are sampled. The studied material covers a spectrum of microstructures and ductility over the weldment zones to give representative for a welded component. The emphasis is placed on the measurement and particularly analysis of crack initiation for failure assessment in P22 steel weldments. The particular importance of construction of isochronous curves for time dependent failure assessment diagram (TDFAD) method is reported. It is aimed to contribute to establishing guidelines for acceptable methodologies for testing, analysis and assessment of welded components using TDFAD for high temperature service.     

Influence of peak temperature during simulation and real thermal cycles on microstructure and fracture properties of the reheated zones

The objective of this paper is to study the influence of the second peak temperature during real and simulated welding on properties of the subcritically (S), intercritically (IC) and supercritically (SC) reheated coarse grained heat affected (CGHAZ) zones. The X80 high strength pipeline microalloyed steel was subject to processing in a double-pass tandem submerged arc welding process with total heat input of 6.98 kJ/mm and thermal cycles to simulate microstructure of reheated CGHAZ zones. This involved heating to a first peak temperature (T_P1) of 1400 °C, then reheating to different second peak temperatures (T_P2) of 700, 800 and 900 °C with a constant cooling rate of 3.75 °C/s. Toughness of the simulated reheated CGHAZ regions were assessed using Charpy impact testing at 0 °C, −25 °C and −50 °C. The microstructure of the real and simulated reheated CGHAZ regions was investigated using an optical microscope and field emission scanning electron microscope. Morphology of the martensite/austenite (MA) constituent was obtained by the use of a field emission scanning electron microscope. The blocky and connected MA particles, along prior-austenite grain boundaries, act as a brittle phase for the initiation site of the brittle fracture. Charpy impact results indicated that IC CGHAZ had less absorbed energy with higher transition temperature and hardness. The SC CGHAZ region showed higher absorbed impact energy with lower hardness. Design of multipass weld joints with less IC CGHAZ regions can result in a higher toughness property.     

Tensile and impact-toughness behaviour of cryorolled Al 7075 alloy

The effects of cryorolling and optimum heat treatment (short annealing + ageing) on tensile and impact-toughness behaviour of Al 7075 alloy have been investigated in the present work. The Al 7075 alloy was rolled for different thickness reductions (40% and 70%) at cryogenic (liquid nitrogen) temperature and its mechanical properties were studied by using tensile testing, hardness, and Charpy impact testing. The microstructural characterization of the alloy was carried out by using field emission scanning electron microscopy (FE-SEM). The cryorolled Al alloy after 70% thickness reduction exhibits ultrafine grain structure as observed from its FE-SEM micrographs. It is observed that the yield strength and impact toughness of the cryorolled material up to 70% thickness reduction have increased by 108% and 60% respectively compared to the starting material. The improved tensile strength and impact toughness of the cryorolled Al alloy is due to grain refinement, grain fragments with high angle boundaries, and ultrafine grain formation by multiple cryorolling passes. Scanning electron microscopy (SEM) analysis of the fracture surfaces of impact testing carried out on the samples in the temperature range of −200 to 100 °C exhibits ductile to brittle transition. cryorolled samples were subjected to short annealing for 5 min at, 170 °C, and 150 °C followed by ageing at 140 °C and 120 °C for both 40% and 70% reduced samples. The combined effect of short annealing and ageing, improved the strength and ductility of cryorolled samples, which is due to precipitation hardening and subgrain coarsening mechanism respectively. On the otherhand, impact strength of the cryorolled Al alloy has decreased due to high strain rate involved during impact loading.     

The effect of solution temperature on the microstructure and tensile properties of Al–15%Mg2Si composite

The effect of different solution temperatures has been investigated on the microstructure and tensile properties of in situ Al–Mg₂Si composite specimens were subjected to solutionizing at different temperatures of 300 °C, 350 °C, 400 °C, 450 °C, 500 °C, 550 °C and 580 °C for holding time of 4 h followed by quenching. The microstructural studies of the polished and etched samples by scanning electron microscopy (SEM) in the solution condition indicated that the increase in the temperature changes the morphology of both the primary and secondary Mg₂Si phases. Solutionizing led to the dissolution of the Mg₂Si particles and changed their morphology. Tensile test results indicated that ultimate tensile strength (UTS) gradually decreased upon solutionizing from 300 to 550 °C while further increase in the temperature followed by a sharp decrease in UTS up to 580 °C solutionizing temperature. It was found that the elongation has become three times greater in comparison to the as-cast state. Elongation results showed an increase up to 500 °C and then reduced temperatures of 550 and 580 °C. Fractographic analysis revealed a cellular nature for the fracture surface. On the cellular fracture surface, the features of both brittle and ductile fracture were present simultaneously. As a result of solution treatment the potential sites for stress concentration and crack initiation areas were reduced due to softening of the sharp corners and break up of eutectic network respectively, while increase in the number of fine dimples rendered the nature of fracture to ductile and also increased elongation.     

Correlation of austenite stability and ductile-to-brittle transition behavior of high-nitrogen 18Cr-10Mn austenitic steels

Ductile-to-brittle transition behavior of high-nitrogen 18Cr-10Mn austenitic steels containing different contents of Ni, Mo, Cu as well as nitrogen is discussed in terms of austenite stability and associated deformation-induced martensitic transformation (DIMT). Electron back-scattered diffraction and transmission electron microscopy analyses of cross-sectional area of the Charpy impact specimens fractured at −196 °C indicated that the brittle fracture planes were almost parallel to one of {1 1 1} slip planes and some metastable austenites near the fracture surface were transformed to α′-martensite by localized plastic deformation occurring during crack propagation. Quantitative evaluation of deformation-induced martensite together with characteristics of true stress-strain and load-displacement curves obtained from tensile and Charpy impact tests, respectively, supported that DIMT might take place in high-nitrogen austenitic steels with relatively low austenite stability. The occurrence of DIMT decreased low-temperature toughness and thus increased largely ductile-to-brittle transition temperature (DBTT), as compared to that predicted by empirical equations strongly depending on nitrogen content. As a result, the increased DBTT could be reasonably correlated with austenite stability against DIMT.     

Effect of welding procedure on wear behaviour of a modified martensitic tool steel hardfacing deposit

In the last years hardfacing became an issue of intense development related to wear resistant applications. Welding deposits can functionalize surfaces and reclaim components extending their service life. Tool steels are widely used in hardfacing deposits to provide improved wear properties. Nevertheless systematic studies of wear behaviour of new alloys deposited by hardfacing, under different service conditions are scarce. In this work the effects of shielding gas, heat input and post-weld heat treatment on the microstructural evolution and wear resistance of a modified AISI H13 martensitic tool steel deposited by semi-automatic gas shielded arc welding process using a tubular metal-cored wire, were studied. Four coupons were welded with different welding parameters. The shielding gases used were Ar–2% CO₂ and Ar–20% CO₂ mixtures and two levels of heat input were selected: 2 and 3 kJ/mm. The as welded and 550 °C–2 h post-weld heat treated conditions were considered. From these coupons, samples were extracted for testing metal–metal wear under condition of pure sliding with a load of 500 N. Chemical compositions were determined; microstructure and microhardness were assessed. It was found that content of retained austenite in the microstructure varied with the welding condition and that heat-treated samples showed secondary hardening, associated with precipitation phenomena. Nevertheless, as welded samples showed higher wear resistance than heat treated specimens. Under these test conditions post-weld heat treatment led to a reduction in wear resistance. The best wear behaviour was observed in samples welded with low heat input and under the lowest oxygen potential shielding gas used here, in the as welded condition. The intervening mechanism was mild oxidative. These results were explained in terms of the relative oxidation resistance stemming from different welding conditions.     

Strengthening behavior analysis of weld metal of laser hybrid welding for microalloyed steel

The weld metal with high strength and good toughness was obtained for medium thickness microalloyed steel by using high power laser hybrid welding technology. Mechanical properties of weld metal were evaluated by using room temperature tensile test and low temperature three-side Charpy V-notch (CVN) impact test. The results show that the yield strength and ultimate tensile strength of weld metal are up to 713 MPa and 918 MPa, respectively. Both of them are almost 1.5 times higher than those of base metal. Under the strict three sides CVN condition, the −40 °C low temperature impact absorbed energy is up to 32 J and also higher than that of base metal. Weld metal predominately consists of granular bainite and carbon-free bainite. Both of them mainly contain lath morphology bainitic ferrite. The lath morphology bainitic ferrite with fine grain size plays an important role in higher strength. Dispersive carbide and high density dislocation are found in strengthening weld metal.     

A comparative study of laser beam welding and laser-MIG hybrid welding of Ti-Al-Zr-Fe titanium alloy

Ti-Al-Zr-Fe titanium alloy sheets with thickness of 4 mm were welded using laser beam welding (LBW) and laser-MIG hybrid welding (LAMIG) methods. To investigate the influence of the methods difference on the joint properties, optical microscope observation, microhardness measurement and mechanical tests were conducted. Experimental results show that the sheets can be welded at a high speed of 1.8 m/min and power of 8 kW, with no defects such as, surface oxidation, porosity, cracks and lack of penetration in the welding seam. In addition, all tensile test specimens fractured at the parent metal. Compared with the LBW, the LAMIG welding method can produce joints with higher ductility, due to the improvement of seam formation and lower microhardness by employing a low strength TA-10 welding wire. It can be concluded that LAMIG is much more feasible for welding the Ti-Al-Zr-Fe titanium alloy sheets.     

The microstructure and mechanical strength of Al-6061-T6 GMA welds obtained with the modified indirect electric arc joint

Plates of Al-6061-T6 with a thickness of 12.7 mm were welded with a non-heat treatable ER4043 electrode in four welding passes using the typical single V groove butt joint (at room temperature) and in one pass using the novel modified indirect electric arc (MIEA) joint (at 50, 100 and 150 °C). Microstructural characterization of the weld metal revealed that the MIEA joint design yielded microstructures in which columnar growth was restricted and a grain refining effect occurred without the addition of inoculants, even with preheating of the joints. The self-refining effect in the MIEA joint was discussed in terms of the cooling behavior and the large dilution of base material. Vickers microhardness profiles revealed that the high dilution rates obtained with the MIEA joint caused alloying of the weld pool with magnesium producing a weld metal composition sensible to heat treating due to the precipitation of Mg₂Si precipitates. The grain self-refining effect of the MIEA joint along with alloying of the weld metal were found to have a profound impact on the mechanical properties of the welded joints.     

Investigation of the strength and cyclic fracture toughness of BT-16 titanium alloy subjected to thermal cycling simulating diffusion welding

The effect of the thermal cycle of diffusion welding on the service characteristics of BT-16 titanium alloy are considered. Using standard test equipment the mechanical properties of the alloy after thermal cycling to 860 or 960°C were studied at test temperatures of 960, 860, 300 and 20°C and the fatigue strength and fracture toughness were studied at room temperature.

It was shown that although specimens welded at 960°C displayed superior plastic properties, these were only partially effective in arresting propagating cracks. A greater fatigue crack resistance was seen with material welded at 860°C and it is this temperature which is recommended for diffusion bonding BT-16 titanium alloy components.     

Fracture behaviour of specimens with surface notch tip in the heat affected zone (HAZ) of strength mis-matched welded joints

The generally accepted conditions for the strength overmatched welded joints of high strength steel are not clearly defined. In this paper, the fracture mechanics analysis of specimens, with surface notch tips completely embedded in the heat affected zones was conducted. The results showed that the strength of mismatching of a welded joint caused a redirection of the crack propagation towards the low strength region of the welded joint. This redirection of the crack propagation affected the values of the critical CTOD. In the cases of the overmatched welded joints containing a soft root layer it is possible to achieve a comparable fracture behaviour related to the homogeneous overmatched welded joint if the impact toughness of the soft root layer is higher than the impact toughness of the overmatched weld metal. Such a type of welded joint is therefore preferable for the welding of high strength low alloy steels, because it enables the manufacturing of a welded joint without preheating.     

On the change of fracture mechanism with test temperature

Three point bending (3PB) tests of precracked specimens were carried out for coarse grain C-Mn steel at three low temperatures. Details of fracture surfaces in the specimens were microscopically observed and cleavage initiation sites were located. Calculations of local critical parameters and simulations of fracture behavior were made using finite element method (FEM). The results reveal that at very low temperature (−196 °C), the critical event controlling cleavage fracture is the nucleation of crack at the precrack tip in ferrite. The critical event moves to the initiation and propagation of a second phase particle crack at moderately low temperature (−110 °C). At higher temperature (−30 °C), the critical event for cleavage fracture after a fibrous crack extends is the propagation of a grain-sized crack.     

Transparent, amorphous and organics-free ZnO thin films produced by chemical solution deposition at 150 °C

We have studied the low-temperature processing of ZnO by chemical solution deposition. A transparent, stable precursor solution prepared from zinc acetate dihydrate dissolved in 2-methoxyethanol was spin-coated on SiO_x/Si, soda-lime glass and polymer substrates and heated at 150 °C. Selected thin films deposited on SiO_x/Si were additionally heated at 450 °C.Microstructural and chemical analyses showed that the thin films heated at 150 °C in air were amorphous, contained no organic residues and had a root mean square roughness of 0.7 nm. The films deposited on SiO_x/Si and heated at 450 °C were crystallised and consisted of randomly oriented grains with a diameter of about 20 nm. All thin films were transparent, exhibiting a transmission of over 80% in the visible range. The resistivity of the 120-nm thick ZnO films processed at 150 °C was 57 MΩ cm and upon heating at 450 °C it decreased to 1.9 kΩ cm.     

The effect of post-weld heat treatment on the mechanical properties of 2024-T4 friction stir-welded joints

In this study, the effect of post-weld heat treatment (PWHT) on the mechanical properties of friction stir-welded 2024 aluminum alloys in the T4 temper state was investigated. Solution heat treatment and various ageing treatments were given to the welded joints. The PWHT procedures caused abnormal coarsening of the grains in the weld zone, which resulted in a drop in micro-hardness at the weld zone compared to the base material of the joints. T6 (190 °C – 10 h) ageing treatment after welding was found to be more beneficial than the other heat treatments in enhancing the mechanical properties of the 2024-T4 joints. However, the T6 (190 °C – 10 h) heat treatment led to significant ductility deterioration in the joint.     

Processing of multi-sheet structures of an aluminum alloy by laser welding/superplastic forming

Multi-sheet structures of an aluminum alloy were fabricated through laser welding combined with superplastic forming technique in this paper. The novel welding design and bonding have been developed to improve the formation quality of the structure for the aluminum alloy. The laser welding microstructure of the fine-grained 5083 Al alloy was studied. The fine equiaxed grains with 1.5 μm in center zone of weld were observed by transmission electron microscope. Tensile tests show that ultimate tensile strength of the welded specimen is about 91% that of the base metal at 500 °C. The distribution in thickness within the formed structure was investigated. It reveals homogeneous deformation and well-bonding property of the structure. The present study verifies the feasibility of the processing procedures for the multi-sheet structures of an aluminum alloy.     

Evaluation of interfacial bonding in dissimilar materials of YSZ-alumina composites to 6061 aluminium alloy using friction welding

The interfacial microstructures characteristics of alumina ceramic body reinforced with yttria stabilized zirconia (YSZ) was evaluated after friction welding to 6061 aluminum alloy using optical and electron microscopy. Alumina rods containing 25 and 50 wt% yttria stabilized zirconia were fabricated by slip casting in plaster of Paris (POP) molds and subsequently sintered at 1600 °C. On the other hand, aluminum rods were machine down to the required dimension using a lathe machine. The diameter of the ceramic and the metal rods was 16 mm. Rotational speeds for the friction welding were varied between 900 and 1800 rpm. The friction pressure was maintained at 7 MPa for a friction time of 30 s. Optical and scanning electron microscopy was used to analyze the microstructure of the resultant joints, particularly at the interface. The joints were also examined with EDX line (energy dispersive X-ray) in order to determine the phases formed during the welding. The mechanical properties of the friction welded YSZ-Al₂O₃ composite to 6061 alloy were determined with a four-point bend test and Vickers microhardness. The experimental results showed the degree of deformation varied significantly for the 6061 Al alloy than the ceramic composite part. The mechanical strength of friction-welded ceramic composite/6061 Al alloy components were obviously affected by joining rotational speed selected which decreases in strength with increasing rotational speed.