In the present investigation, molten aluminium corrosion tests have been performed on boronised s... more In the present investigation, molten aluminium corrosion tests have been performed on boronised samples of duplex 22Cr -5Ni stainless steel, hot work steel (AISI H13) and nodular cast iron, and on a non-boronised g-TiAl sample as a reference. The experimental results show that the corrosion rate, as expressed in terms of volume loss per square centimetre of the specimen per hour, was reduced with boronising by a factor of 16 and 20 for the nodular cast iron and the duplex stainless steel, respectively. Apparently, the corrosion resistance becomes better than that of TiAl under the prevailing conditions. For the AISI H13 tool steel, the corrosion rate was reduced by a factor of 3. The corrosion mechanisms involved are discussed in terms of diffusion rate controlling elements. D
ABSTRACT Local brittle zones, i.e., martensite–austenite (M–A) islands, are formed within the coa... more ABSTRACT Local brittle zones, i.e., martensite–austenite (M–A) islands, are formed within the coarse-grained heat-affected zone (CGHAZ) and the intercritically reheated CGHAZ (ICCGHAZ) during welding of many HSLA steels. In the current study, the M–A constituents in the microstructure of simulated ICCGHAZ of an API X80 pipeline steel were investigated using transmission electron microscopy and scanning electron microscopy. The focused ion beam technique was applied to make TEM specimens of M–A constituents that were located in the initiation sites of cleavage cracks. The main purpose of the study was to identify crack-initiation sites of cleavage fracture in ICCGHAZ and to prove the presence of M–A constituents in such initiation sites. Twinned martensite was detected in all local brittle zones that were investigated in the current study, demonstrating that they are M–A constituents. It was also demonstrated that the fracture initiation occurred preferentially at M–A constituents by a debonding mechanism rather than cracking of the M–A constituents.
In the present report, the joining of shape memory alloys to themselves or to other metals is dis... more In the present report, the joining of shape memory alloys to themselves or to other metals is discussed. The joining processes included are arc welding, beam welding, brazing, adhesive bonding and some other techniques like friction welding, resistance welding, explosive welding, transient liquid phase diffusion bonding and soldering. Welding of SMA by arc welding or beam welding processes is possible but tends to produce brittle joints. Brittleness can be reduced by stress relieving, but degradation of the shape memory effect may occur due to the high temperatures that are required. Welding should also be carried under inert gases to prevent passive oxide build-up. Welding is seen to change the phase transformation temperatures, which may reduce the application window. Post welding training cycles may enhance the as welded shape memory effect and superelasticity. Laser welding tend to give narrow welds, and may thus be preferable to arc welding due to possible finer microstructure ...
Ternary alloying of MoSi 2 with adding a series of transition elements was investigated by X-ray ... more Ternary alloying of MoSi 2 with adding a series of transition elements was investigated by X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy (TEM), and energy dispersive spectroscopy (EDS). Iron, Co, Ni, Cr, V, Ti, and Nb were chosen as alloying elements according to the AB 2 structure map or the atomic size factor. The studied MoSi 2 base alloys were prepared by the arc melting process from high-purity metals. The EDS analysis showed that Fe, Co, and Ni have no solid solubility in as-cast MoSi 2 , while Cr, V, Ti, and Nb exhibit limited solid solubilities, which were determined to be 1.4 ע 0.7, 1.4 ע 0.4, 0.4 ע 0.1, and 0.8 ע 0.1. Microstructural characterization indicated that Mo-Si-M VIII (M VIII ϭ Fe, Co, Ni) and Mo-Si-Cr alloys have a two-phase as-cast microstructure, i.e., MoSi 2 matrix and the second-phase FeSi 2 , CoSi, NiSi 2 , and CrSi 2 , respectively. In as-cast Mo-Si-V, Mo-Si-Ti, and Mo-Si-Nb alloys, besides MoSi 2 and C40 phases, the third phases were observed, which have been identified to be (
Diffusion bonding of ceramics to ceramics and to metals is reviewed, with primary emphasis on the... more Diffusion bonding of ceramics to ceramics and to metals is reviewed, with primary emphasis on the effects of operational variables on joint strength. These include principal bonding parameters such as temperature, time and pressure. In addition, the influence of atmosphere, mismatch in coefficient of thermal expansion between the joint members, interlayers and surface structure are discussed. The mechanisms involved, i.e. plastic deformation, various forms of diffusion and power law creep, imply that temperature is the most important process parameter. Finally, a survey of variables employed in bonding of different ceramic-metal and ceramic-ceramic joints is included as a guideline for selection of materials and parameters.
The main parameters in direct brazing of ceramics to ceramics and to metals are reviewed, with pr... more The main parameters in direct brazing of ceramics to ceramics and to metals are reviewed, with primary emphasis on those influencing wetting of solid ceramics by liquid filler metals. In general, wetting of ceramics by conventional brazing alloys has been regarded as difficult. As a consequence, premetallization of the faying surfaces is frequently used to facilitate the brazing of ceramics. However, it is evident from the literature that recent developments in filler metals, based on active metal (e.g. Ti) additions (the amount depending on alloy composition and type of ceramic), have provided a basis for a substantial reduction of the contact angle. This favourable effect is caused by their reactivity, resulting in the formation of oxides when joining oxide ceramics (e.g. AI203), and nitrides or carbides and silicides in the case of nonoxide ceramics (e.g. Si3N 4 or SIC). In addition to insufficient wetting, the mismatch in thermal expansion between the joint members may give rise to a low strength level due to the formation of high residual stresses on cooling. These stresses may limit the maximum allowable flaw size in ceramics to a few micrometres, i.e. of a similar size to that of pores.
In this paper, the metallurgical aspects with respect to the shape memory alloys used in seismic ... more In this paper, the metallurgical aspects with respect to the shape memory alloys used in seismic dampers are addressed and the various factors which affect the mechanical properties of the shape memory alloys, such as alloy composition are briefly discussed. A concise overview on the latest development of constitutive models suitable for seismic damper application is presented and a damper design using different types of shape memory alloys in parallel is proposed.
The authors discuss how the H{sub 2}S stress corrosion cracking resistance of welded low-carbon l... more The authors discuss how the H{sub 2}S stress corrosion cracking resistance of welded low-carbon low-alloy steels is strongly dependent on the microstructure. The threshold stress, below which cracking does not occur, is highest in the case of a fine-grained polygonal ferrite (grain-refined HAZ) and lowest in the presence of martensite (rain-coarsened HAZ). This implies that extra-low-carbon steels are most resistant to stress corrosion cracking in the as-welded condition. Empirical equations have been developed to predict the threshold stress. It has been shown that the threshold stress passes through a maximum of 360 MPa at yield strength levels between 500 and 750 MPa. At higher R{sub p0.2} values, the degradation in {sigma}{sub th} is associated with formation of more susceptible microstructures such as martensite, while at lower yield strengths, the reduction is caused by a general decrease in HAZ strength level.
The deepwater hyperbaric GMA welding procedures of sub-sea reparation are evaluated with respect ... more The deepwater hyperbaric GMA welding procedures of sub-sea reparation are evaluated with respect to its behavior under shallow water conditions. In this study, the welding thermal cycles at three different pressure levels were investigated through experiments and simulations. Conventional thermocouple measurement was utilized along with a 2D heat flow model to identify the effect of weld pool shape on the thermal cycle of the weld. A new approach was assumed for considering the effect of third dimension. It was found that increasing the pressure results in minor changes of characteristic weld thermal cycle features. Initially, these variations were considered purely dependent upon the weld pool shape and size. It was found that a number of parameters are competing in different ways, reducing the direct effect of weld pool geometry. Good agreement was observed between simulated and experimental results.
The degradation of the fracture toughness of high strength low alloy steels is attributed to the ... more The degradation of the fracture toughness of high strength low alloy steels is attributed to the formation of ‘local brittle zones’ in the welded joint. These local brittle zones are mainly located within the coarse grained heat affected zone (CGHAZ) and the intercritically reheated CGHAZ (ICCGHAZ). Cracking and debonding of M-A constituents from the surrounding matrix are generally accepted as initiation events of fracture in the ICCGHAZ. In the present work, the low temperature fracture toughness of X80 pipeline steel was examined. The main purposes were (i) to evaluate possible crack initiation sites of cleavage fracture and (ii) to identify the mechanism by which M-A constituents deteriorate the ICCGHAZ toughness. The results revealed that the microstructure of ICCGHAZ contained blocky M-A constituents along prior austenite grain boundaries. Finally, it was shown that fracture initiation occurred preferentially at M-A constituents by a debonding mechanism rather than cracking of...
Electrochemical hydrogen permeation measurements have been performed on pre-strained pipeline ste... more Electrochemical hydrogen permeation measurements have been performed on pre-strained pipeline steel base metal and weld simulated heat affected zone specimens. The level of plastic pre-strain was 0-3%. The effective diffusion coefficient decreased with higher pre-strain. At higher pre-strain levels the effect of plastic deformation on hydrogen diffusion is dominating the influence of microstructure. Sub-surface hydrogen concentration was found to be unaffected by plastic strain. The calculated number of traps was clearly higher than results available for alpha ferrite in literature.
In this investigation, we investigate the effects of solid state phase transformation on residual... more In this investigation, we investigate the effects of solid state phase transformation on residual stresses in welding supermartensitic stainless steels, using the Satoh experimental test approach. The results obtained clearly demonstrate that the austenite-to-martensite phase transformation has remarkable effects on the final tensile residual stress level, and stresses as low as 70 to 170 MPa were found, depending on the weld thermal program. These results may imply that unnecessary conservatism is often used when assuming residual stresses to be of the same order as the base metal yield strength.
24th International Conference on Offshore Mechanics and Arctic Engineering: Volume 3, 2005
Supermartensitic 13% Cr stainless steels have been in use in offshore satellite flowlines for sev... more Supermartensitic 13% Cr stainless steels have been in use in offshore satellite flowlines for several years. Since they contain microstructure that is susceptible to hydrogen cracking, the pick up of hydrogen in welding with subsequent transport to critical areas may be very important, also with respect to hydrogen embrittlement when hydrogen is coming from other sources than welding (e.g., cathodic protection). In the present investigation the pick up of hydrogen has been assessed using mechanized TIG welding with superduplex 25% Cr wire. The WM and HAZ hydrogen levels were analyzed. With addition of hydrogen in the shielding gas in multipass welding, the mean WM hydrogen contents were found to be approximately 10 and 6 ppm in the cap layer and root pass, respectively. The corresponding mean HAZ concentration was 3.1 ppm (scatter between 1.3 and 4.8) immediately after welding. Post weld hydrogen diffusion heat treatment showed that hydrogen diffusion was retarded at room temperature, even for 1 month storage. Limited diffusion took place at 90°C, particularly for the cap region. The results indicate that superduplex weld metal with high hydrogen content (6-10ppm) will act as a hydrogen reservoir supplying H to the 13% Cr HAZ as long as 2-3 years after welding.
ABSTRACT The grain coarsened tensile properties in the heat affected zone (HAZ) of medium strengt... more ABSTRACT The grain coarsened tensile properties in the heat affected zone (HAZ) of medium strength (Rp0·2⋍330–470 MN m−2) low carbon microalloyed, C–Mn, and 2·25Cr–1Mo steels have been examined using the weld simulation technique. Very high values of yield and tensile strength (Rp0·2>900 MN m−2; Rm>1100 MN m−2) can be obtained for low heat input welding (E < 1 MJ m−1; ∆t8/5< 5 s). For high heat input welding (E > 2 MJ m−1; ∆t8/5> 10 s), however, a substantial reduction in the values of Rp0·2 and Rm occurs, with the exception of the 2·25Cr–IMo steel. Based on the empirical equations developed, and the Suzuki BL70 hardness formula, it is shown that the grain coarsened HAZ yield and tensile strength can be calculated to an accuracy of ±50 MN m−2, while elongation at fracture and reduction in area can be predicted to an accuracy of ±3% and ±7%, respectively.MST/I077
In the present investigation, molten aluminium corrosion tests have been performed on boronised s... more In the present investigation, molten aluminium corrosion tests have been performed on boronised samples of duplex 22Cr -5Ni stainless steel, hot work steel (AISI H13) and nodular cast iron, and on a non-boronised g-TiAl sample as a reference. The experimental results show that the corrosion rate, as expressed in terms of volume loss per square centimetre of the specimen per hour, was reduced with boronising by a factor of 16 and 20 for the nodular cast iron and the duplex stainless steel, respectively. Apparently, the corrosion resistance becomes better than that of TiAl under the prevailing conditions. For the AISI H13 tool steel, the corrosion rate was reduced by a factor of 3. The corrosion mechanisms involved are discussed in terms of diffusion rate controlling elements. D
ABSTRACT Local brittle zones, i.e., martensite–austenite (M–A) islands, are formed within the coa... more ABSTRACT Local brittle zones, i.e., martensite–austenite (M–A) islands, are formed within the coarse-grained heat-affected zone (CGHAZ) and the intercritically reheated CGHAZ (ICCGHAZ) during welding of many HSLA steels. In the current study, the M–A constituents in the microstructure of simulated ICCGHAZ of an API X80 pipeline steel were investigated using transmission electron microscopy and scanning electron microscopy. The focused ion beam technique was applied to make TEM specimens of M–A constituents that were located in the initiation sites of cleavage cracks. The main purpose of the study was to identify crack-initiation sites of cleavage fracture in ICCGHAZ and to prove the presence of M–A constituents in such initiation sites. Twinned martensite was detected in all local brittle zones that were investigated in the current study, demonstrating that they are M–A constituents. It was also demonstrated that the fracture initiation occurred preferentially at M–A constituents by a debonding mechanism rather than cracking of the M–A constituents.
In the present report, the joining of shape memory alloys to themselves or to other metals is dis... more In the present report, the joining of shape memory alloys to themselves or to other metals is discussed. The joining processes included are arc welding, beam welding, brazing, adhesive bonding and some other techniques like friction welding, resistance welding, explosive welding, transient liquid phase diffusion bonding and soldering. Welding of SMA by arc welding or beam welding processes is possible but tends to produce brittle joints. Brittleness can be reduced by stress relieving, but degradation of the shape memory effect may occur due to the high temperatures that are required. Welding should also be carried under inert gases to prevent passive oxide build-up. Welding is seen to change the phase transformation temperatures, which may reduce the application window. Post welding training cycles may enhance the as welded shape memory effect and superelasticity. Laser welding tend to give narrow welds, and may thus be preferable to arc welding due to possible finer microstructure ...
Ternary alloying of MoSi 2 with adding a series of transition elements was investigated by X-ray ... more Ternary alloying of MoSi 2 with adding a series of transition elements was investigated by X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy (TEM), and energy dispersive spectroscopy (EDS). Iron, Co, Ni, Cr, V, Ti, and Nb were chosen as alloying elements according to the AB 2 structure map or the atomic size factor. The studied MoSi 2 base alloys were prepared by the arc melting process from high-purity metals. The EDS analysis showed that Fe, Co, and Ni have no solid solubility in as-cast MoSi 2 , while Cr, V, Ti, and Nb exhibit limited solid solubilities, which were determined to be 1.4 ע 0.7, 1.4 ע 0.4, 0.4 ע 0.1, and 0.8 ע 0.1. Microstructural characterization indicated that Mo-Si-M VIII (M VIII ϭ Fe, Co, Ni) and Mo-Si-Cr alloys have a two-phase as-cast microstructure, i.e., MoSi 2 matrix and the second-phase FeSi 2 , CoSi, NiSi 2 , and CrSi 2 , respectively. In as-cast Mo-Si-V, Mo-Si-Ti, and Mo-Si-Nb alloys, besides MoSi 2 and C40 phases, the third phases were observed, which have been identified to be (
Diffusion bonding of ceramics to ceramics and to metals is reviewed, with primary emphasis on the... more Diffusion bonding of ceramics to ceramics and to metals is reviewed, with primary emphasis on the effects of operational variables on joint strength. These include principal bonding parameters such as temperature, time and pressure. In addition, the influence of atmosphere, mismatch in coefficient of thermal expansion between the joint members, interlayers and surface structure are discussed. The mechanisms involved, i.e. plastic deformation, various forms of diffusion and power law creep, imply that temperature is the most important process parameter. Finally, a survey of variables employed in bonding of different ceramic-metal and ceramic-ceramic joints is included as a guideline for selection of materials and parameters.
The main parameters in direct brazing of ceramics to ceramics and to metals are reviewed, with pr... more The main parameters in direct brazing of ceramics to ceramics and to metals are reviewed, with primary emphasis on those influencing wetting of solid ceramics by liquid filler metals. In general, wetting of ceramics by conventional brazing alloys has been regarded as difficult. As a consequence, premetallization of the faying surfaces is frequently used to facilitate the brazing of ceramics. However, it is evident from the literature that recent developments in filler metals, based on active metal (e.g. Ti) additions (the amount depending on alloy composition and type of ceramic), have provided a basis for a substantial reduction of the contact angle. This favourable effect is caused by their reactivity, resulting in the formation of oxides when joining oxide ceramics (e.g. AI203), and nitrides or carbides and silicides in the case of nonoxide ceramics (e.g. Si3N 4 or SIC). In addition to insufficient wetting, the mismatch in thermal expansion between the joint members may give rise to a low strength level due to the formation of high residual stresses on cooling. These stresses may limit the maximum allowable flaw size in ceramics to a few micrometres, i.e. of a similar size to that of pores.
In this paper, the metallurgical aspects with respect to the shape memory alloys used in seismic ... more In this paper, the metallurgical aspects with respect to the shape memory alloys used in seismic dampers are addressed and the various factors which affect the mechanical properties of the shape memory alloys, such as alloy composition are briefly discussed. A concise overview on the latest development of constitutive models suitable for seismic damper application is presented and a damper design using different types of shape memory alloys in parallel is proposed.
The authors discuss how the H{sub 2}S stress corrosion cracking resistance of welded low-carbon l... more The authors discuss how the H{sub 2}S stress corrosion cracking resistance of welded low-carbon low-alloy steels is strongly dependent on the microstructure. The threshold stress, below which cracking does not occur, is highest in the case of a fine-grained polygonal ferrite (grain-refined HAZ) and lowest in the presence of martensite (rain-coarsened HAZ). This implies that extra-low-carbon steels are most resistant to stress corrosion cracking in the as-welded condition. Empirical equations have been developed to predict the threshold stress. It has been shown that the threshold stress passes through a maximum of 360 MPa at yield strength levels between 500 and 750 MPa. At higher R{sub p0.2} values, the degradation in {sigma}{sub th} is associated with formation of more susceptible microstructures such as martensite, while at lower yield strengths, the reduction is caused by a general decrease in HAZ strength level.
The deepwater hyperbaric GMA welding procedures of sub-sea reparation are evaluated with respect ... more The deepwater hyperbaric GMA welding procedures of sub-sea reparation are evaluated with respect to its behavior under shallow water conditions. In this study, the welding thermal cycles at three different pressure levels were investigated through experiments and simulations. Conventional thermocouple measurement was utilized along with a 2D heat flow model to identify the effect of weld pool shape on the thermal cycle of the weld. A new approach was assumed for considering the effect of third dimension. It was found that increasing the pressure results in minor changes of characteristic weld thermal cycle features. Initially, these variations were considered purely dependent upon the weld pool shape and size. It was found that a number of parameters are competing in different ways, reducing the direct effect of weld pool geometry. Good agreement was observed between simulated and experimental results.
The degradation of the fracture toughness of high strength low alloy steels is attributed to the ... more The degradation of the fracture toughness of high strength low alloy steels is attributed to the formation of ‘local brittle zones’ in the welded joint. These local brittle zones are mainly located within the coarse grained heat affected zone (CGHAZ) and the intercritically reheated CGHAZ (ICCGHAZ). Cracking and debonding of M-A constituents from the surrounding matrix are generally accepted as initiation events of fracture in the ICCGHAZ. In the present work, the low temperature fracture toughness of X80 pipeline steel was examined. The main purposes were (i) to evaluate possible crack initiation sites of cleavage fracture and (ii) to identify the mechanism by which M-A constituents deteriorate the ICCGHAZ toughness. The results revealed that the microstructure of ICCGHAZ contained blocky M-A constituents along prior austenite grain boundaries. Finally, it was shown that fracture initiation occurred preferentially at M-A constituents by a debonding mechanism rather than cracking of...
Electrochemical hydrogen permeation measurements have been performed on pre-strained pipeline ste... more Electrochemical hydrogen permeation measurements have been performed on pre-strained pipeline steel base metal and weld simulated heat affected zone specimens. The level of plastic pre-strain was 0-3%. The effective diffusion coefficient decreased with higher pre-strain. At higher pre-strain levels the effect of plastic deformation on hydrogen diffusion is dominating the influence of microstructure. Sub-surface hydrogen concentration was found to be unaffected by plastic strain. The calculated number of traps was clearly higher than results available for alpha ferrite in literature.
In this investigation, we investigate the effects of solid state phase transformation on residual... more In this investigation, we investigate the effects of solid state phase transformation on residual stresses in welding supermartensitic stainless steels, using the Satoh experimental test approach. The results obtained clearly demonstrate that the austenite-to-martensite phase transformation has remarkable effects on the final tensile residual stress level, and stresses as low as 70 to 170 MPa were found, depending on the weld thermal program. These results may imply that unnecessary conservatism is often used when assuming residual stresses to be of the same order as the base metal yield strength.
24th International Conference on Offshore Mechanics and Arctic Engineering: Volume 3, 2005
Supermartensitic 13% Cr stainless steels have been in use in offshore satellite flowlines for sev... more Supermartensitic 13% Cr stainless steels have been in use in offshore satellite flowlines for several years. Since they contain microstructure that is susceptible to hydrogen cracking, the pick up of hydrogen in welding with subsequent transport to critical areas may be very important, also with respect to hydrogen embrittlement when hydrogen is coming from other sources than welding (e.g., cathodic protection). In the present investigation the pick up of hydrogen has been assessed using mechanized TIG welding with superduplex 25% Cr wire. The WM and HAZ hydrogen levels were analyzed. With addition of hydrogen in the shielding gas in multipass welding, the mean WM hydrogen contents were found to be approximately 10 and 6 ppm in the cap layer and root pass, respectively. The corresponding mean HAZ concentration was 3.1 ppm (scatter between 1.3 and 4.8) immediately after welding. Post weld hydrogen diffusion heat treatment showed that hydrogen diffusion was retarded at room temperature, even for 1 month storage. Limited diffusion took place at 90°C, particularly for the cap region. The results indicate that superduplex weld metal with high hydrogen content (6-10ppm) will act as a hydrogen reservoir supplying H to the 13% Cr HAZ as long as 2-3 years after welding.
ABSTRACT The grain coarsened tensile properties in the heat affected zone (HAZ) of medium strengt... more ABSTRACT The grain coarsened tensile properties in the heat affected zone (HAZ) of medium strength (Rp0·2⋍330–470 MN m−2) low carbon microalloyed, C–Mn, and 2·25Cr–1Mo steels have been examined using the weld simulation technique. Very high values of yield and tensile strength (Rp0·2>900 MN m−2; Rm>1100 MN m−2) can be obtained for low heat input welding (E < 1 MJ m−1; ∆t8/5< 5 s). For high heat input welding (E > 2 MJ m−1; ∆t8/5> 10 s), however, a substantial reduction in the values of Rp0·2 and Rm occurs, with the exception of the 2·25Cr–IMo steel. Based on the empirical equations developed, and the Suzuki BL70 hardness formula, it is shown that the grain coarsened HAZ yield and tensile strength can be calculated to an accuracy of ±50 MN m−2, while elongation at fracture and reduction in area can be predicted to an accuracy of ±3% and ±7%, respectively.MST/I077
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