鋼材の溶接遅れ割れの研究 ＩＩ 拘束多層継手の熱影響部割れの防止

Preheating is carried out to avoid cold cracking in steel welding. The occurrence of cold cracking is considered to be governed by accumulating diffusible hydrogen, welding residual stresses and hardness at the crack initiation site. The hydrogen accumulating at the crack site depends on the initial diffusible hydrogen content in weld metal, the weld heat input and the wall thickness. The local residual stress is governed by the weld metal yield strength, the joint restraint and the notch stress concentration factor. The HAZ hardness is influenced by the steel chemical composition, the weld heat input and the plate (wall) thickness.These influential factors affect cold cracking independenly or in an interacted manner. It must be, thus, difficult to predict the necessary preheat temperature by a theoretical method or simple formulae. The method presently proposed is completely based on the empirical results by y-groove weld cracking tests. The present method determines the necessary preheat temperature through the charts describing the following respective effects : 1) steel composition ; 2) diffusible hydrogen content of weld metal ; 3) weld heat input ; 4) wall thickness ; 5) weld metal yield strength ; 6) joint restraint. As to the steel composition, this method uses CEN carbon equivalent that preferably assesses weldability of a wide variety of steels. Also, this method considers a logarithmic dependence of the weld metal hydrogen and the analysis of hydrogen diffusion in a weld has proved that the hydrogen effect on cold cracking must be logarithmic.

When welding stainless steel components, particularly in plant engineering applications, it may be necessary to join unalloyed or low alloyed ferritic steels to austenitic chromium-nickel-(molybdenum) stainless steels. Such dissimilar joints are also called “black-and-white joints”. With regard to corrosion resistance, there are normally no particular requirements demanded from this type of joint, because the low alloyed steel is normally not corrosion resistant. Because of the different structure between the types of steels to be joined, there may be certain difficulties experienced during welding. Low alloyed steels, for example, normally require controlled preheat and interpass temperatures during welding, slow cooling after welding and often a final tempering or stress relieving heat treatment. Austenitic steels on the other hand should be welded “cold”, the welds should cool rapidly and post weld heat treatments should be avoided wherever possible.

The 12CrNi2 alloy steel powder studied in the present paper is mainly used to manufacture camshafts for nuclear power emergency diesel engines. Laser cladding deposition is of great significance for the manufacture of nuclear power emergency diesel camshafts, which has the advantages of reducing material cost and shortening the manufacturing cycle. However, due to the extremely uneven heating of the components during the deposition process, a complex residual stress field occurs, resulting in crack defects and residual deformation of the components. In the present paper, 12CrNi2 bulk specimens were prepared on the Q460E high-strength structural steel substrate at different preheating temperatures by laser cladding deposition technique, and a finite element residual stress analysis model was established to investigate the effects of different preheating temperatures on the microstructure, properties, and residual stress of the specimens. The results of the experiments and finite element simulations show that with the increase of preheating temperature, the content of martensite/bainite in the deposited layer decreases, and the ferrite content increases. The proper preheating temperature (150 °C) has good mechanical properties. The residual stress on the surface of each specimen decreases with the increase of the preheating temperature. The longitudinal stress is greater at the rear-end deposition part, and the lateral residual stress is greater on both sides along the scanning direction.

In this study, the analysis of the lowest spatial resolution for measuring weld cracks based on the magnetic flux leakage (MFL) test method was developed. MFL detection is a widely used method of pipeline nondestructive testing. The spatial resolution of the MFL sensors influences the data amount and detection accuracy. The magneto-optical detection technique can be used to solve the problem of insufficient spatial resolution, but the amount of data is very large and redundant. Using the lowest spatial resolution with ensuring accuracy can reduce the amount of data and decrease the time cost of analyzing data. A girth weld crack is a common and complex defect that occurs in pipelines, and high spatial resolution is required to ensure its detection accuracy. In this study, the MFL of weld cracks and its influencing factors were investigated. Based on the frequency domain analysis, the lowest spatial resolution was obtained, which was verified by experiments.

Microstructure and mechanical properties of welded joints between nickel base alloy and 10Ni5CrMoV steel by MIG welding

Fields of application for quenched and tempered advanced high-strength steels are mainly ground-moving, mining equipment, commercial vehicles, and truck cranes. One of the main aspects determining range of applicability for the high strength steels is the possibility to obtain welded joints with the given fatigue strength. Information about fatigue behaviour of the weld joints strongly increases the operational safety of the structures. Factors influencing mechanical properties of the joints can be related to the welding parameters, i.e. welding current, voltage, and interpass and preheat temperatures. Thermo-mechanical process (fusion welding) also causes the diversification of the microstructure in the weld and heat affected zones. Microstructure changes induce residual stress, influencing the fatigue behaviour of the weld joint. The main topic of the paper are results of the experimental fatigue tests performed for S1100QL butt welds produced with two different technological parameter setups. Technological parameters configurations were selected on the basis of experimental tests carried out for sheets with a thickness of 12 mm. The low and high welding parameters differed due to energy introduced into the system. The energy was controlled by current parameters and the preheating of joined elements. In both configurations, it was possible to make welded joints that meet regulatory requirements but differ in the level of introduced internal stresses and microstructural properties in heat affected zone sizes. The joints were additionally subjected to standard strength and microstructural tests.

The initiation and propagation characteristics of the lamellar tearing are studied by the AE technique. MIG welding process in a chamber charged with argon is used to eliminate the noises started from the self-crushing of slag.From the results of the small type through thickness restraint, cracking test, three mechanisms to the occurrence of lamellar tearing are proposed which correspond with the metallurgical factors, such as chemical analysis and non-metallic inclusions, of the rolled plate. As the conventional type of the lamellar, tear, the cracking is originated by the decohesion of the non-metallic inclusions from the matrix and extended to fibrous fractures right after completing the weld deposition prior to the individual element of the crack linking up intermittently. This intermittent behavior reveals to be lasted for several hours, and the propagation of the crack is of the typical hydrogen induced cracking.For the steel which has sufficiently poor through thickness ductility, the lamellar tear can be initiated and propagated by only shear-mode. For the steel which has high susceptibility to hydrogen embrittlement, the lamellar tear is apt to follow the root or toe cracks in the pattern of parallel to the surface of the plate.

It is essential to adopt the welding fabrication which assures high level of soundness of the welded joint in welding high performance structures such as submersible vessel made of steels with both high toughness and high tensile strength. In that case, it is important to prevent weld cracking which affects the soundness of welded joint.Therefore, the authors carried out various weld cracking tests for butt joint and fillet joint of steels with high toughness and high tensile strength to be used for pressure hull of submersible vessel, and investigated the effect of water content in flux of electrode and ambient humidity on the weld cracking behavior from the viewpoint of welding fabrication. On the basis of the investigation, the conditions for preventing weld cracking were established.

A number of articles (including a series by Electric Light & Power) were published in the 1970's on the subject of toe-cracking in base plate weldments. Despite nearly 30 years of experience, toe-cracks in base/flange plate weldments are still being discovered. This paper will review a Six-Sigma study undertaken by Valmont Industries to better identify the causes and the variables that we feel are essential to minimize the risk of toe-crack occurrences.

To study the individual and correlated effects of mechanical and thermal restraints on weld cracking behaviours of steels and electrodes, two types of dynamic cracking testers have been used in the present paper, that is, a Murex type bending cracking tester and a new type tensile cracking tester. In the latter, one piece in a butt weld specimen is tensile and can be displaced at any constant speed against the other stationary piece during or after welding at an arbitrary instant.Dynamic cracking tests of several mild steel coated electrodes showed that these two testers were satisfactory for classifying cracking susceptibility. Among the tested electrodes, the low hyd-rogen type showed the least, the high titania type the greatest and the high cellulose and illmenite types medium. susceptibility to both hot and cold cracking of weld metal.Stainless steel coated electrodes could be satisfactorily evaluated with this dynamic cracking tester concerning the suitability for the first pass welding of a clad steel.Comparing the two cracking test methods, the tensile restraint cracking tester was found to be more sensitive and reproducible than the Murex type bending restraint cracking tester. An instant of weld cracking in the former tester can be determined very successfully by observing a corres-ponding discontinuous change in the tensile load across the test weld. Further studies are being continued with the tensile restraint cracking tester.

This report summarizes work performed by Argonne National Laboratory on fatigue and environmentally assisted cracking (EAC) in light water reactors (LWRs) from January to December 2002. Topics that have been investigated include: (a) environmental effects on fatigue crack initiation in carbon and low-alloy steels and austenitic stainless steels (SSs), (b) irradiation-assisted stress corrosion cracking (IASCC) of austenitic SSs in BWRs, (c) evaluation of causes and mechanisms of irradiation-assisted cracking of austenitic SS in PWRs, and (d) cracking in Ni-alloys and welds. A critical review of the ASME Code fatigue design margins and an assessment of the conservation in the current choice of design margins are presented. The existing fatigue {var_epsilon}-N data have been evaluated to define the effects of key material, loading, and environmental parameters on the fatigue lives of carbon and low-alloy steels and austenitic SSs. Experimental data are presented on the effects of surface roughness on fatigue crack initiation in these materials in air and LWR environments. Crack growth tests were performed in BWR environments on SSs irradiated to 0.9 and 2.0 x 10{sup 21} n x cm{sup -2}. The crack growth rates (CGRs) of the irradiated steels are a factor of {approx}5 higher than the disposition curve proposed in NUREG-0313 for thermally sensitized materials. The CGRs decreased by an order of magnitude in low-dissolved oxygen (DO) environments. Slow-strain-rate tensile (SSRT) tests were conducted in high-purity 289 C water on steels irradiated to {approx}3 dpa. The bulk S content correlated well with the susceptibility to intergranular SCC in 289 C water. The IASCC susceptibility of SSs that contain >0.003 wt. % S increased drastically. bend tests in inert environments at 23 C were conducted on broken pieces of SSRT specimens and on unirradiated specimens of the same materials after hydrogen charging. The results of the tests and a review of other data in the literature indicate that IASCC in 289 C water is dominated by a crack-tip grain-boundary process that involves S. An initial IASCC model has been proposed. A crack growth test was completed on mill annealed Alloy 600 in high-purity water at 289 C and 320 C under various environmental and loading conditions. The results from this test are compared with data obtained earlier on several other heats of Alloy 600.

With advances in metallurgy and steel-making technology, structural steel with yield strengths up to 690N/mm2 is produced regularly in many modern steel mills in different countries, such as China, France, Germany, India and Japan. The yield strength of S690 steel is almost twice that of the commonly used structural S355 steel. High-strength steels are used in a wide range of structures, such as heavy-duty lifting equipment in mines and ports, frames and members in wind turbines, as well as container trailers and passenger trains. Use of high-strength steels is highly advantageous for these moving structures because they can carry increased payloads and hence use less energy during operation. For civil engineering and building structures, the use of S690 steel allows a significant reduction in the self-weight of a structure and also a reduction in the loads to be imposed on supporting structures and foundations. In recent years, the cost per tonne of S690 steel in many parts of the world has decreased steadily, which has improved its competitive position with respect to S355 steel. In China, the unit cost of S690 steel has varied typically from 1.25 to 1.35 times that of S355 steel over the past five years. It may be argued that owing to the increased strength of S690 steel, only half of the steel tonnages are needed, compared with S355 steel. Hence, the steel material cost becomes roughly 1.3 times 0.5, or 0.65, of that of S355 steel, leading to a significant saving in material costs. Research work on mechanical properties and structural behaviour of welded sections made from S690 steel conducted at the Chinese National Engineering Research Centre for Steel Construction (Hong Kong Branch) has shown that provided the welding processes are properly controlled, it is possible to minimise or even eliminate any reduction in strength and ductility which might occur due to microstructural change. Extensive research and development work have also been conducted to promote application of high-strength steel for construction. A steel bridge with two arches using 4400 tonnes of S690 steel in Hong Kong, and a steel bridge with mega trusses using 1625 tonnes of Q690 steel in Macau are briefly described. In addition, the initiative of the Development Bureau of the Government of Hong Kong SAR in promoting the use of high-strength S690 steel is highlighted.

High-strength structural steels are used in machine, steel, and crane construction with yield strength up to 960 MPa. However, welding of these steels requires profound knowledge of three factors in terms of avoidance of hydrogen-assisted cracking (HAC): the interaction of microstructure, local stress/strain, and local hydrogen concentration. In addition to the three main factors, the used arc process is also important for the performance of the welded joint. In the past, the conventional transitional arc process (Conv. A) was mainly used for welding of high-strength steel grades. In the past decade, the so-called modified spray arc process (Mod. SA) has been increasingly used for welding production. This modified process enables reduced seam opening angles with increased deposition rates compared with the Conv. A. Economic benefits of using this arc type are a reduction of necessary weld beads and required filler material. In the present study, the susceptibility to HAC in the heat-affected zone (HAZ) of the high-strength structural steel S960QL was investigated with the externally loaded implant test. For that purpose, both Conv. A and Mod. SA were used with same heat input at different deposition rates. Both conducted test series showed same embrittlement index “EI” of 0.21 at diffusible hydrogen concentrations of 1.3 to 1.6 ml/100 g of arc weld metal. The fracture occurred in the HAZ or in the weld metal (WM). However, the test series with Mod. SA showed a significant extension of the time to failure of several hours compared with tests carried out with Conv. A.

Lamellar tearing of opening type and root cracking occasionally occur in multipass welded corner joints.In this paper, the influence of welding methods such as submerged arc welding (SAW) and covered arc welding (SMAW) is investigated on the occurrence of these weld cracks. The main results obtained in this study are as follows.(1) When SAW is applied, the transverse welding residual stress (σx) is less on the edge surface of the vertical plate and the lamellar tearing is less probable to occur in comparison with SMAW.(2) By SAW, σx, induced at the root is less and root cracking is less probable to occur against SMAW.Accordingly, in order to prevent occurrence of the weld cracks in multipass welded corner joints, it is effective to use SAW whose deposited metal weight per unit welding pass is large and which enables to fill up a given groove with smaller number of passes.

In this study, a metal/polymer tri-layer composite was produced by direct adhesion (without adhesive), and mechanical locks were created using the warm rolling process. The effect of the process parameters including preheating temperature, rolling speed, thickness reduction, surface roughness, and the orientation of the surface scratches on the bond strength between layers was investigated. The results indicated that the suitable polymer fluidity and penetration, to provide stronger mechanical locks and higher bond strengths, could be achieved at an optimum preheating temperature and a rolling speed of 240°C and 36 r/min, respectively. In addition, the most appropriate surface pretreatment was obtained in the wire brush in the rolling direction mode with the surface roughness of 0.65 µm, so that the failure mechanism in this case was cohesive and the optimum thickness reduction was achieved at 40%. Furthermore, the mechanical properties of the sandwich sheet with highest bonding strength were evaluated.