Austenitic stainless steel is welded as a cladding on the inner surface of a reactor pressure vessel (RPV) made of low alloy steel. In order to assess the structural integrity of the RPV precisely, the residual stress distribution caused by weld-overlay cladding and post-weld heat treatment (PWHT) is evaluated. Since the cladding layer is very thin compared to vessel wall, it is necessary to evaluate the residual stress distribution around the weld fusion line can be very steep. In this study, cladded specimens were fabricated using different welding methods. Residual stress measurements using both sectioning and deep hole drilling (DHD) methods were then performed to evaluate the residual stress distributions through the weld fusion line. Three-dimensional thermal-elastic-plastic-creep analyses based on finite element method were also conducted to evaluate the residual stress caused by weld-overlay cladding and PWHT. It was shown that analytical results provided reasonable agreements on weld residual stress with experimental results. It was also clarified that the main cause of residual stress due to welding and PWHT was the difference of thermal expansion between weld and base metals.
In recent years, it has become much more necessary to control or reduce weld distortion, which has negative influences on structural integrity, without loss of manufacturing efficiency. Higher accurate prediction of weld distortion brings in one of contributions for improvement of manufacturing efficiency. Commonly, angular distortion, which is controlled by temperature distribution along direction of plate thickness, is controlled by heat input parameter (Qnet / h2). However, it is also known that there are several concerns against applicability of conventional heat input parameter for prediction of angular distortion. One of concerns is the effect of welding speed. This is because that the conventional heat input parameter is derived from welding thermal conduction theory without considering moving of weld heat source. Actually, pre-study by experiment about effect of welding conditions, including welding speed, on angular distortion has shown that angular distortion is not always controlled by heat input parameter in case of different welding speed. Then, in order to clarify the effect of welding speed on that, more detailed study is further required. In this paper, numerical study on the effect of welding speed on relationship between heat input parameter and angular distortion are performed based on the past experimental results. In addition, the reason why the relationship between heat input parameter and angular distortion is affected by welding speed is discussed from a viewpoint of distribution of the maximum temperature rise and residual plastic strain. It is concluded that the effect of welding speed on relationship between heat input parameter and angular distortion is understood by the difference in distribution of mechanical melting temperature, which identifies the distribution of residual plastic strain for angular distortion, in plate thickness section.
In order to elucidate distortion and residual stress generated by welding of high strength steel (HT780) by laser beam, a series of experiments and analyses were carried out. The angular distortion generated by bead-on-plate welding was V-shape and its magnitude was about 2mm. The longitudinal bending distortion was extremely small. On the surface of plate, the residual stress component in welding direction was tensile and it was smaller than yield stress in the weld metal. The residual stress was almost zero outside the weld metal. The phase transformation range in cooling stage and the temperature dependency of mechanical properties were obtained. The mechanical properties in the phase transformation range in cooling stage could not be specified due to transformation expansion. Therefore, they were idealized by considering transformation expansion and transformation superplasticity. The validity of the idealized mechanical properties was verified by simulating the experiment by the thermal elastic-plastic analysis. It was elucidated that the welding out-of-plane distortion and tensile residual stress were largely controlled by phase transformation in cooling stage although the bead width of laser beam welding was extremely narrow.
High tensile strength 950MPa steel, HT950, which is steel used in penstocks, was developed to provide two vital properties: fracture arrest to stop brittle fracture, and high weldability. This steel has been already used for penstocks in some hydropower plants in Japan. To widely apply high tensile strength steels in penstocks in the future, fewer restrictions against their welding conditions such as preheat- and postheat- temperature controls are required. One proposed solution is to undermatch the strength of the filler metal to that of the base material. This allows less pre-heating, or no preheating at all, and use of conventional rod and process management is easier. Previous studies have shown that there are softening conditions under which the strength of the joint can be considered as that of the base material. However, The shape and distribution of the soft region is assumed to be ideal. In this study, the method for calculating the change of the strength in heat affected zone (HAZ) during welding process is discussed. Then, the influence of the strength distribution of HAZ and welded zone to the strength of the joint is investigated by wide plate test in both experiment and elastic-plastic analysis. Applicability of under-matched joints to penstock fabrication is considered by these discussions. As the results, it is concluded that the Vickers hardness distribution in HAZ can be estimated by the method which is proposed in this study and the strength of the under-matched joints are enough high in both experiment and analysis in which the Vickers hardness distribution is considered. By these conclusions, applicability of under-matched joints whose weld metal is HT570 class to penstock fabrication is conformed.
Recently, high strength steel plate such as 950MPa class strength is focused as material of hydropower plant due to its effect on cost reduction and it has been already used to penstocks in Kanna-gawa and Omaru-gawa hydropower plants in Japan. However, it comes to issue that welding restrictions for preventing welding cracks and keeping strength are much severer than ordinary strength steel, for example, preheat condition or inter-pass-temperature range. If under-matching (lower strength in weld metal than base plate) is applied, it is possible that high strength steel plate becomes easier to use because restrictions in welding process may decrease by application of softer welding materials. By applying softer welding, it is assumed that mechanical performances of the structure changes due to existence of heterogeneous region. However, quantitative effect of application of under-matching for high strength steel on deformation behavior and fracture assessment is not certain. In this report, in order to evaluate the effect of under-matching design on brittle fracture behavior, standard CTOD tests and wide plate tests with through-thickness notch by the welded joint using actual combination of 950MPa steel plate for penstock and various welding materials (950-570MPa class), were performed. As a result, welded joint with softer welding materials showed higher resistance to brittle fracture. Furthermore, taking into consideration of allowable stress, sufficient load capacity was observed even in case of application of 570MPa class welding consumables.
In cold spray, feedstock powders are accelerated by supersonic jet with solid phase and deposited onto substrate. Compared with the conventional thermal spray, the dense coatings can be prepared without oxidation and decomposition of original powders. In this study, two types of Ni-Al mixture powders with different mixture form were used to fabricate Ni-Al composite coatings by cold spraying. Subsequently the coatings were annealed to synthesize NiAl intermetallic compounds. As a result, a dense NiAl intermetallic compounds coating was fabricated by using the Ni-Al mechanical milling powder. Moreover, the adhesive strength and micro vickers hardness of this coating is found to be higher than that formed by Ni-Al blended powder.
Experimental study has been conducted on erosion of hafnium electrodes for oxygen plasma arc cutting. Hafnium (Hf) is used as a refractory cathode material for thermionic emission at high temperature region. Erosion of Hf electrode was investigated by measuring the concavity depth on Hf electrodes after operations. The measurements were done on several operating conditions. The erosion properties have been found to be separated to two portions. One of the Hf erosion happens in transient period including start-up and shut-off phases of plasma arc when arc current is increasing or decreasing. The other erosion happens in the stable arc phase when arc current is kept at operating current. Spectroscopic measurements of the arc plasma jet coming out from a nozzle have been conducted in order to study the behavior of erosion of Hf electrode. An atm line of Hf at λ=340 nm was observed by a monochromator. Characteristic intermittent radiations of Hf in addition to background continuum were observed in transient period when the arc current or gas pressure changes. It is considered that the intermittent radiation comes from droplets of Hf ejected from the melted electrode surface. And the ejection is considered to be induced by a break of balance of fluid dynamic on the melted electrode.
This paper describes the joint properties and its improvement of high tensile strength steel joint by an autocompleting friction welding method, that method was developed by authors. The base metal was 800MPa class high tensile strength steel. The weld faying surface of the fixed specimen had a 10mm diameter, and the effect of the thickness and that at the bottom of the grooves (groove bottom thickness) for the insert piece on the joining phenomena and joint properties were investigated. The value of a circumferential shear fracture (CSF value) was defined and calculated by the ratio between the theoretical and the actual generated friction torques. When the CSF value was lower than 1, the insert piece had the circumferential shear fracture before the friction torque reached the initial peak. Also, when the CSF value was larger than 1, the insert piece had the circumferential shear fracture after the friction torque reached the initial peak. When the joint was made at the insert thickness of 5mm with the CSF value of nearly 1, that had 100% joint efficiency although it had the softened region at the near of the weld interfaces. The joint had cracks at the weld interface when it was made with friction pressures of 36 and 120MPa. However, the joint had no crack at the weld interface when it was made with a friction pressure of 90MPa. When the joint was made at the insert thickness of 4mm with the CSF value of nearly 1, that had also 100% joint efficiency although it had the softened region at the near of the weld interfaces. However, the softened region at the weld interface of the joint with the insert thickness of 4mm was lower than that with 5mm. Also, this joint had 90 degrees bend ductility with no crack at the weld interface. In conclusion, a joint with no cracks for high tensile strength steel was able to make by an autocompleting friction welding method.
It is well known that the strain rate affects tensile behavior; for example, it affects the tensile strength and fracture elongation of metals. When examining the tensile behavior of steel, it is important to pay attention to the heat-affected zone (HAZ) as well as the base metal. The effects of the strain rate on the tensile strength were investigated by using specimens with stress concentrations; in particular, the tensile strength of SM400, SM490, and HT780 were obtained from HAZs by performing tensile tests at strain rates of 0.085%/s, 85%/s, and 1000%/s. The HAZs were simulated by using a thermal/mechanical simulator. The main results are summarized as follows: 1) Regardless of the grade of steel, the tensile strength of specimens that underwent ductile fracture increase at high strain rates, whereas the tensile strength of specimens that underwent brittle fracture did not increase. 2) Regardless of the grade of steel and regardless of whether welding heat was supplied or not, the rate of increase in the tensile strength as a result of the high strain rate can be predicted from the tensile strength, which can be determined by static testing.
Displacement during welding provides important information to understand the mechanisms of welding deformation and residual stress. In particular, if welding deformation can be measured sequentially and the displacement distribution over full-field can be measured such as the results obtained by finite element analysis, they can be valuable information. Therefore, in this study, 3-dimensional deformation (in-plane and out-of-plane deformation) measurement method is developed using a digital camera, which requires no special equipment. This method is a non-contact method and it can sequentially measure over the entire photographed image. Furthermore, since image analysis is based on the technique of image matching, the method is applicable even when measuring deformation is large. In addition, since it is possible to use all pixels as measuring points, the number of available measuring points at one time is the same as the number of effective pixels of the camera. This is currently more than 15 million points, and the measuring precision is expected to increase as the camera pixel resolution continues to increase. Therefore, this method is expected to have future potential. In this study, proposed method is applied to the sequential measurement of displacement under the strong lighting levels in arc welding. By comparing the time history of transverse shrinkage, longitudinal shrinkage and angular distortion with the results of FEM thermo-elastic plastic analysis, the qualitative validity of the proposed method is verified. To investigate the measurement precision and usefulness of the method, a 3-dimensional shape measurement system (LAT-3D) using a laser displacement gauge and digital caliper are used. The distributions of residual transverse shrinkage and residual angular distortion are measured by the proposed method, LAT-3D and digital caliper. Through the comparison of the results measured by these methods, quantitative validity of the proposed method is also verified.
The influence of threaded and wear simulated (half-thread) tools on the mechanical properties of dissimilar Al-alloy friction stir spot welds is investigated. The failure load properties of dissimilar A5754/A6111 lap joints produced using wear simulated (half-thread) tools were much poorer in joints made using tool rotational speeds of 1000 and 1500 RPM. Similar failure load properties were produced using these tools, when the tool rotational speed was increased to 2250 RPM. In joints made using a tool rotational speed of 3000 RPM, the failure load properties of joints made using the half-thread tool were higher than those of joints made using the threaded tool. Similar failure load properties were found in A5052/A6061 lap and butt joints made using threaded, half threaded and no thread tools and a tool rotational speed of 1000RPM. Also, the failure load properties of lap joints made using the different tools corresponded with their bonded widths and Y values. The area of the stir zone on the bonded cross-section corresponded with the actual bonded region on the fracture surface of A5052/A6061 butt joints. However, the weld profiles were different from those in A5052/A6061 lap joints. It is therefore suggested that the failure load properties of lap joints are similar when the bonded widths and Y values are similar. In addition, the thread on the cylindrical probe has negligible influence on the mechanical properties of lap joints. The major role played by the thread involves stirring in the vertical direction. Stirring in the horizontal direction is unchanged in joints made using the different tools.