Yukito Hagihara

Plastic Rotational Factor Calculation for Shallow-Notched SE(B) Specimens

The plastic part of crack tip opening displacement (CTOD) is derived from the plastic hinge model for deep-notched single edge-notch bend (SE(B)) specimens in BS, WES and ISO CTOD testing standards, and a typical plastic rotational factor is given by a constant value of 0.4. This value is appropriate for deep-notched SE(B) specimens, but is not suitable for shallow-notched SE(B) specimens. In this study, a new equation of calculating the plastic rotational factor was obtained by using the Electric Power Research Institute (EPRI) scheme. The equation shows the effect of crack length and strain hardening on the plastic rotational factor, and is useful for evaluating CTOD in shallow-notched SE(B) specimens.

DIFFERENCE BETWEEN ASTM E1290 AND BS 7448 CTOD ESTIMATION PROCEDURES

Since the British Standards Institution (BSI) standardized BS 5762 in 1979, a popular way of calculating the Crack Tip Opening Displacement (CTOD) has been the use of the plastic hinge model with an assumed rotational centre. The American Society for Testing and Materials (ASTM) previously accepted the plastic hinge model and standardized E1290 in 1989. However, ASTM revised E1290 in 2002, and has proposed a fracture parameter conversion from the J-integral to CTOD. These two different CTOD calculations probably lead to confusion for Fitness-for-Service (FFS). The Fracture Toughness Study Committee of the Japan Welding Engineering Society (JWES) organized a working group, and the effects of CTOD testing methodology on CTOD values were investigated. In this paper, the results of CTOD round-robin tests in the working group are summarized, and the difference between the critical CTOD values obtained by ASTM E1290–02 and those by BS 7448, which involves the plastic hinge model, is described.

The existing state of hydrogen in electrochemically charged commercial purity aluminum and its effect on the tensile properties

Hydrogen was introduced in commercial-purity (99%) aluminum by electrochemical charging to study the existing state of hydrogen and its effects on the mechanical properties of aluminum. Electrochemical charging was conducted in an aqueous H2SO4 solution with 0.1% NH4SCN as a hydrogen recombination poison. The potential and pH during the charging were determined from the immune, passive, and corrosive regions in the Pourbaix diagram to determine the optimum conditions for the charging. The maximum amount of hydrogen absorbed was obtained in the immune region. The amount of hydrogen and its existing state were examined using hydrogen desorption curves, which were obtained by thermal desorption spectroscopy. The curves showed distinctive peaks corresponding to trapping sites of hydrogen in the material. One of the peaks was observed at approximately 100 � C, and it corresponds to vacancies and dislocations in the material; another peak was observed at approximately 400 � C and it corresponds to molecular hydrogen in blisters. It was presumed that charged hydrogen diffuses into the bulk of the material to form hydrogen-vacancy pairs, and then these pairs cluster to form blisters. The fracture strain of charged aluminum in the immune region decreased with decreasing strain rate, showing an inverse dependence on the fracture strain of the uncharged material. This phenomenon was considered to be caused by hydrogen transport by dislocations through the interaction between hydrogen and dislocations. The phenomenon was further confirmed by the observation of hydrogen release during tensile deformation, where the amount of hydrogen was high in the strain rate range where the interaction between dislocations and hydrogen was prominent. [doi:10.2320/matertrans.M2011035]

Development of 7%Ni-TMCP Steel Plate for LNG Storage Tanks

Demand of natural gas continues to increase in the recent years due to the rise of environmental issue and the drastic increase of crude oil price. These events led to the increase of constructions of Liquefied Natural Gas (LNG) storage tanks worldwide. The inner tank material for above ground LNG storage tanks have mostly been made of a 9% nickel steel plate over the last 50 years as it has excellent mechanical properties under the cryogenic temperature of −160deg-C. During this period, the LNG storage tanks made of 9%Ni steel plate have been operated safely at the many LNG export and import terminals in the world. Meanwhile, technologies of steel making, refinement, design, analysis, welding and inspection have been improved significantly and enabled enlarging volumetric capacity of the tank 2–3 times. There was a tendency for nickel price to increase in recent years. In such a circumstance lowering Ni content has focused attention on the 9%Ni steel as nickel is an expensive and valuable rare metal and a 7%Ni steel plate was eventually researched and developed by optimizing the chemical compositions and applying Thermo-Mechanical Controlled Process (TMCP). As a result, it was demonstrated that 7%Ni-TMCP steel plate had excellent physical and mechanical properties equivalent to those of 9%Ni steel plate. In order to evaluate fitness of the 7%Ni-TMCP steel plate and its weld for LNG storage tanks a series of testing was conducted. Several different plate thicknesses, i.e. 6,10,25,40 and 50 mm, were chosen to run large scale fracture toughness tests including duplex ESSO tests, cruciform wide plate tests as well as small scale tests. It was concluded that the 7%Ni-TMCP steel plate warrants serious consideration for use in LNG storage tanks. This paper reports details of the research and development of the 7%Ni-TMCP steel plate.Copyright

Recent Developments in Japanese Flaw Assessment Methods of WES 2805

The standard for the method of assessment for flaws in the welded joints of WES 2805 was first published in 1976 and was revised in 1980 and 1997. A further revision has been carried out by the technical committee of FTS in the Japan Welding Engineering Society and the revision was completed in 2007. The standard of WES 2805 is based on a CTOD (crack tip opening displacement) design curve approach for brittle fracture, and is used for the assessment of the significance of flaws in a stress concentrated region, where large scale yielding takes place. Main topics for the recent developments for flaw assessment methods are described in this paper. These are the interaction criterion of multiple flaws, fatigue crack growth laws, determination of equivalent crack length and strain due to stress concentration, estimation method of the critical CTOD from Charpy energy and proposal of partial safety factors. In order to examine the effectiveness of the standard, extensive 2-D and 3-D FE analyses are performed for various welded joints such as a load-carrying fillet welded joint, a non-load-carrying fillet welded joint and a box welded joint. Some of them are introduced in this paper. Their analytical results indicate that the present CTOD design curve method gives a reasonable evaluation.Copyright

An Evaluation Method for Laser Weld Metal Toughness by Side-Notched Charpy Test

Ultra-fine grained (UFG) steel has been developed on a laboratory scale, which has both high strength and superior toughness. When welded, however, grain growth takes place at the heat-affected zone (HAZ), and consequently HAZ softening occurs. To avoid the detrimental effect of HAZ softening, laser welding is one of the promising methods. The hardness distribution of laser welded joints is so steep in a narrow region that fracture path deviation (FPD) is often observed in the standard Charpy V-notch (STD-Cv) test in and above the transition temperature region. Side-notch Charpy (SN-Cv) test is widely used to prevent FPD. The SN-Cv test gives a higher transition temperature and lower absorbed energy as compared with the STD-Cv test. Therefore, it is necessary to evaluate quantitatively the difference between the STD-Cv and the SN-Cv test. In this paper, STD-Cv and SN-Cv tests were carried out on laser weld metal for UFG steel and on base plates of conventional steels from mild steel to 800 MPa high strength steel. The fracture surface of Cv test was categorized into three types, that is, fibrous fracture at the notch tip and at the edge of the specimen, brittle fracture, and slant fracture at both sides. The area of each fracture surface was measured for all specimens tested and each unit fracture energy was determined by using the regression analysis method. Then, the difference in the formation of each fracture surface between STD-Cv and SN-Cv tests was discussed, and finally a method to estimate STD-Cv energy from the SN-Cv test results was proposed.

Main factors causing intergranular and quasi-cleavage fractures at hydrogen-induced cracking in tempered martensitic steels

Though intergranular (IG) and quasi-cleavage (QC) fractures have been widely recognized as typical fracture modes of the hydrogen-induced cracking in high-strength steels, the main factor has been unclarified yet. In the present study, the hydrogen content dependence on the main factor causing hydrogen-induced cracking has been examined through the fracture mode transition from QC to IG at the crack initiation site in the tempered martensitic steels. Two kinds of tempered martensitic steels were prepared to change the cohesive force due to the different precipitation states of Fe3C on the prior γ grain boundaries. A high amount of Si (H-Si) steel has a small amount of Fe3C on the prior austenite grain boundaries. Whereas, a low amount of Si (L-Si) steel has a large amount of Fe3C sheets on the grain boundaries. The fracture modes and initiations were observed using FE-SEM (Field Emission-Scanning Electron Microscope). The crack initiation sites of the H-Si steel were QC fracture at the notch tip under various hydrogen contents. While the crack initiation of the L-Si steel change from QC fracture at the notch tip to QC and IG fractures from approximately 10 µm ahead of the notch tip as increasing in hydrogen content. For L-Si steels, two possibilities are considered that the QC or IG fracture occurred firstly, or the QC and IG fractures occurred simultaneously. Furthermore, the principal stress and equivalent plastic strain distributions near the notch tip were calculated with FEM (Finite Element Method) analysis. The plastic strain was the maximum at the notch tip and the principle stress was the maximum at approximately 10 µm from the notch tip. The position of the initiation of QC and IG fracture observed using FE-SEM corresponds to the position of maximum strain and stress obtained with FEM, respectively. These findings indicate that the main factors causing hydrogen-induced cracking are different between QC and IG fractures.Though intergranular (IG) and quasi-cleavage (QC) fractures have been widely recognized as typical fracture modes of the hydrogen-induced cracking in high-strength steels, the main factor has been unclarified yet. In the present study, the hydrogen content dependence on the main factor causing hydrogen-induced cracking has been examined through the fracture mode transition from QC to IG at the crack initiation site in the tempered martensitic steels. Two kinds of tempered martensitic steels were prepared to change the cohesive force due to the different precipitation states of Fe3C on the prior γ grain boundaries. A high amount of Si (H-Si) steel has a small amount of Fe3C on the prior austenite grain boundaries. Whereas, a low amount of Si (L-Si) steel has a large amount of Fe3C sheets on the grain boundaries. The fracture modes and initiations were observed using FE-SEM (Field Emission-Scanning Electron Microscope). The crack initiation sites of the H-Si steel were QC fracture at the notch tip under var...

Application of 7%Ni-TMCP Steel Plate to Large Scale LNG Storage Tank

Increase of natural gas demand has led the increase of constructions of Liquefied Natural Gas (LNG) storage tanks worldwide. The inner tank material for above ground LNG storage tanks has mostly been made of 9% nickel steel plate over the last 50 years as it has excellent mechanical properties under the cryogenic temperature of −160deg.C. During this period, the LNG storage tanks made of 9%Ni steel plate have safely been operated. Meanwhile, technologies of steel making, welding, design, construction and inspection of 9%Ni steel plate have been improved significantly and enable us to achieve enlarging volumetric capacity of the tank to 2–3 times.It is known that nickel is an expensive and valuable rare metal and recent tendency of increase in nickel price has influenced the price of 9%Ni steel plate. In such a circumstance lowering the Ni content would save cost and this has led to an extensive research and development resulting in the 7%Ni-TMCP steel plate. This 7%Ni-TMCP steel plate developed is characterized by adjustment of chemical compositions and application of suitable Thermo-Mechanical Controlled Process (TMCP).In order to evaluate fitness of the 7%Ni-TMCP steel plate and its weld for LNG storage tanks, a series of testing was conducted. Several different kinds of plate thicknesses from 6 to 50 mm were chosen to run large scale fracture toughness tests such as duplex ESSO tests, cruciform wide plate tests as well as small scale tests. As a result it was demonstrated that the 7%Ni-TMCP steel plate had a quite excellent resistance to brittle fracture of the inner tank steel and its welds exposed to the cryogenic temperature of LNG.Furthermore, other various issues such as extent of applicable heat input, repair weld, difference of physical constants and so on were investigated and found acceptable.In Japan Ministry of Economy, Trade and Industry (METI) approved to apply this steel to new LNG storage tanks of Osaka Gas. After that Osaka Gas decided to construct a new full containment LNG storage tank applying 7%Ni-TMCP steel in Senboku terminal 1. The capacity is 230,000m3, which is larger than present largest scale of 180,000m3 in Japan. The construction is planned to start in September 2012 and be completed by November 2015.Copyright

Fracture toughness and microstructure of 780 MPa class high-tensile strength steel plates: improvement of steel structural performance by laser beam welding for mid-thick section steel plate

Fracture toughness and microstructure of laser weld metal of 780 MPa class steels are investigated and compared with those of SM490A and SM570Q. In SM490A and SM570Q, Charpy energy transition temperature of laser weld metals is 60–90°C higher than that of base metal, and upper bainite microstructures are observed in the laser weld metals. In 780 MPa class steels, difference of Charpy energy transition temperatures between laser weld metal and base metal are only 10–30°C, and no upper bainite microstructures are observed in the laser weld metals. Hardness of the laser weld metals of 780 MPa class steels is identical to that of martensite microstructure. A superior toughness of the laser weld metal of 780 MPa class steels would be owing to the martensite microstructure resulted from a high carbon equivalent.

Improvement of CTOD Calculation in Japanese Flaw Assessment Methods of WES2805

The relationship between the average elastic stress concentration factor, K t and the stress intensity magnification factor, MK , was theoretically indicated, and the applicability of MK to the WES 2805 CTOD design curve was investigated by using the results of finite element analysis for four welded joints. In addition, the average local strain by area around an assumed crack was calculated by the analysis, and the strain was used as the local strain, e, in the WES 2805 CTOD design curve. It was demonstrated that MK was helpful in calculating accurate K t , which contributes to a good CTOD estimation using the WES 2805 CTOD design curve. The average local strain also contributes to a good CTOD estimation, especially for a toe crack in the high strain gradient region. These two options, the use of MK and the average local strain, were approved by the Fracture Toughness Study Committee of the Japan Welding Engineering Society, and were adapted for WES 2805 revised in 2007.Copyright