Layus, Pavel (Lappeenranta University of Technology) | Kah, Paul (Lappeenranta University of Technology) | Parshin, Sergey (Peter the Great St. Petersburg Polytechnic University) | Dmitriev, Vitaly (Peter the Great St. Petersburg Polytechnic University) | Belinga, Eric Mvola (Lappeenranta University of Technology)
Development of innovative consumables for GMAW could bring sufficient improvements in the weld quality. LaB6 (Lanthanum Boride) is a compound of rare earth metal element Lanthanum and nonmetal element Boron. LaB6 has an excellent electrical conductivity, high melting point, considerable strength, and high hardness. Nanoparticles added to the surface of the flux cored wire can improve the stability of the arc and the nanoparticles are eventually transported into the weld pool to improve its properties. Currently, LaB6 finds its application only in the electron beam welding process. Current work proposes the application of nanocoated lanthanum boride welding wire for GMAW of S960QC steel plate of 5 mm thickness. The study compares conventional welding wire Union X96 without the coating and with nanocoating, which was made by electrolysis. The analysis of welding results was conducted and tensile strength, impact test, hardness test, and macrostructure evaluation were performed. Experiments show that the arc stability is affected by introducing nanoparticles, voltage changes within 7 volts range, and current changes within 50 amperes. Hardness tests show an increase in hardness in the weld, and it gradually reduces at the fusion line. The results of this work can be useful for academia, as it provides explanations of nanoparticles effects on the welds; and to filler wire manufacturers, as it provides a novel idea on a nanocoating modification for conventional welding wire.
The recent rapid development of nanotechnology has shown significant benefits in various applications, especially in the field of mechanical engineering. A large amount of scientific research has been carried out in recent years, and the interest towards this field is increasing. Nanotechnology plays a key role in the development of the field of mechanical engineering and finds many applications, such as improving and adding new properties to materials and manufacturing processes, such as the development of lightweight and high-strength materials (National Nanotechnology Initiative, 2017). Figure 1 shows the number of scientific papers published on the topic of nanotechnology in the field of engineering over recent years. It can be seen that currently about 25000 scientific papers are published annually, which clearly indicates the strong research interest to this field.
Kano, Satotu (Welding Business, Kobe Steel, Ltd.) | Kitagawa, Yoshihiko (Welding Business, Kobe Steel, Ltd.) | Sasakura, Shuji (Welding Business, Kobe Steel, Ltd.) | Inomoto, Masahiro (Materials Research Laboratory, Kobe Steel, Ltd.) | Nako, Hidenori (Materials Research Laboratory, Kobe Steel, Ltd.) | Okazaki, Yoshitomi (Materials Research Laboratory, Kobe Steel, Ltd.)
Postweld heat treatment (PWHT) is commonly applied to welded steel constructions to relieve the residual stresses raised in the welds, thereby improving the fatigue strength and fracture toughness of the welded joints. On the other hand, PWHT deteriorates the impact notch toughness of high strength low alloy welds of conventional rutile-type flux-cored wires. This is because the heat of PWHT promotes the precipitation of fine carbides in the weld metal by combining carbon with small amounts of niobium and vanadium, which is known as precipitation hardening. It has been already verified that the microalloying with niobium and vanadium is unfavorable for the Charpy impact value of 550MPa class weld metal. However, there were still unclear points in the said phenomenon for the higher strength steel (>90ksi class) welds. For the purpose of understanding the PWHT embrittlement of the high strength low alloy weld by the flux cored arc welding process, the microstructure and morphology of the carbide have been investigated in detail after PWHT at 620°C. As a result, the size of the cementite precipitated at the prior-austenite grain boundary (PAGB) has been clarified to be the dominant factor of the notch toughness of high strength low alloy welds; this is supposedly because the precipitates act as the origin of the intergranular fracture. It was effective for the refinement of the PAGB cementite to control the carbon content and the carbide formative elements such as chromium and molybdenum. Based on these results, an advanced 90ksi class flux- cored wire has been developed, which offers an excellent balance of strength and low temperature notch toughness at −40°C.
In the construction of structures such as spherical tanks and pressure vessels, the weldment is subjected, after welding was finished, to postweld heat treatment (PWHT) for reducing the residual stresses induced by welding and for improving the fracture toughness and fatigue properties of the weld. For these structures, there is a tendency to be built with a larger size and to be operated at a higher pressure in tandem with the recent growing in energy demand, and thus the steel materials used are strengthened increasingly. In association with the strengthening of steel material, the welding consumables for such applications are also desired to possess higher strengths; however, for the flux-cored wires (FCWs) of rutile type for TS90ksi or higher class steel materials, there was the problem of remarkable deterioration in the impact toughness of weld metal after PWHT.
In this study, we developed a numerical simulation method for the NRL (Naval Research Lab.) drop-weight test using three-dimensional dynamic finite element analysis. Prior to the simulation, material parameters used for the simulation were obtained from mechanical tests for base metal, embrittled weld metal, and a heat-affected zone (HAZ) induced by welding. A quenched and tempered high-strength steel plate with a thickness of 28 mm was used for the base metal. A welding consumable of NRL-S was used for the embrittled weld metal. Specimens for the HAZ were produced by a simulated thermal cycle method. Constitutive laws of the materials were determined from the results of tensile tests conducted under several temperatures and strain rates. Brittle crack propagation was simulated by a cohesive zone technique with a concept of dissipated fracture surface energy (Gc) during crack propagation. A concept of local critical stress was employed for determining brittle crack propagation. The local critical stresses of the materials were determined from the local critical stresses at the crack tip in static crack tip opening displacement (CTOD) tests estimated by finite element analysis. In order to validate the proposed simulation, NRL drop-weight tests were conducted at several temperatures and the results were compared with those from the simulation. It was confirmed that the developed simulation method accurately predicted the results of the NRL drop-weight test at different temperatures.
The NRL (Naval Research Lab.) drop-weight test is a testing method used to determine the nil-ductility transition temperature (NDTT) of materials. Since the NDTT has been found to have a good correlation with brittle crack arrest toughness, the NRL drop-weight test has been utilized as a small-scale testing method alternative to large-scale crack arrest tests (Smedley, 1989; Wiesner et al., 1992; Wiesner, 1995; Otani et al., 2003; Fukui et al., 2003; Ishikawa et al., 2012; Funatsu et al., 2012; Okawa et al., 2017). However, the NRL drop-weight test is generally regarded as an empirical testing method rather than a fracture mechanics testing method. Investigations based on fracture mechanics regarding the NRL drop-weight test are limited, and therefore, a physical meaning of the NDTT may not be fully understood. It may be possible to accurately estimate brittle crack arrest toughness (Kca) of materials if brittle crack propagation and arrest behavior in an NRL drop-weight test can be simulated by finite element analysis.
This paper presents results from a recently completed program that evaluated numerous commercially available flux-cored arc welding and submerged arc welding consumables designed for use with structural steels having nominal yield strengths of 70 to 80 ksi (483 to 552 MPa). The program characterized tensile and toughness properties of test welds as a function of welding process and parameters. The paper provides guidance on the optimum welding conditions that will help maintain minimum weld metal strength and toughness properties for high-strength steels used in critical offshore structures.
As the oil and gas industry continues to develop reservoirs in deep water, the growing complexity and desire for weight limitations in deep water installations is driving the need for use of higher-strength plate steel (>65 ksi (450 MPa) yield strength (YS)). Steel manufacturers are able to produce quality base materials with appropriate strength and toughness levels to meet the current need. However, structural fabricators that have traditionally built this type of infrastructure have not had much experience in working with steels in this strength level. These fabricators often work with materials not exceeding 50 or 60 ksi (345 or 414 MPa) in YS and some have, consequently, expressed concern about taking on fabrication projects requiring higher-strength materials.
The concerns fabricators have identified include:
Without adequate guidance offered to the industry, expanded use of high-strength steels (HSS) could be constrained due to these perceived technology gaps in welding of these materials. As a consequence, the design and weight saving advantages that HSS offers for deep water installations may not be fully realized without prudent guidance on optimized welding parameters.
To aid in addressing some of these issues, a program was funded by the Oil & Gas Strategic Technology Committee, coordinated by EWI, to characterize typical mechanical and metallurgical properties that can be achieved using commercially available submerged arc welding (SAW) and flux-cored arc welding (FCAW) consumables having nominal yield strengths of 70 and 80 ksi (483 and552 MPa). The results of this program will aid in establishing weld procedure specification requirements for HSS.
UNS N07022 alloy, with a nominal composition of Ni-21Cr-17Mo (wt.%), has found success in demanding applications requiring excellent corrosion resistance and high strength. It has been incorporated in the NACE MR0175 / ISO 15156 and NACE MR0103 / ISO 17945 standards at the highest test levels. The focus of this paper is the welding metallurgy and weld metal properties of N07022 alloy. Evaluation of the N07022 weld metal microstructure will be discussed. Sour gas testing of N07022 weld overlay material will be highlighted, including slow strain rate tensile test results in the NACE Level VII environment of 25% NaCl + 500 psi (3.45 MPa) CO2 + 500 psi (3.45 MPa) H2S, at temperatures as high as 550°F (288°C). In addition, weld mechanical properties will be presented, including transverse weld tensile and all-weld-metal tensile test results. The overall results demonstrate the suitability of N07022 alloy for demanding welded applications.
UNS N07022 alloy, known commercially as HASTELLOY† C-22HS† alloy, is a Ni-21Cr-17Mo (wt. %) alloy that exhibits an excellent combination of material properties.1 The nominal chemical composition of N07022 alloy is provided in Table 1. Similar to other Ni-Cr-Mo ”C-type” alloys, which contain high levels of both Cr and Mo, N07022 alloy exhibits excellent resistance to uniform and localized corrosion in both oxidizing and reducing acids. In addition, N07022 alloy provides very high strength in the cold-worked (CW) condition. Room temperature (RT) yield strength levels of 180 ksi are commonly achieved in CW bar and tubular material.2,3
N07022 alloy has been incorporated in the NACE MR0175 / ISO 15156 and NACE MR0103 / ISO 17945 standards at the highest test levels. It has also exhibited resistance to the NACE Level VII environment with 5 g/L elemental sulfur at 401°F (205°C) and to the demanding conditions of 25% NaCl + 1,000 psi (6.9 MPa) CO2 + 1,000 psi (6.9 MPa) H2S at 550°F (288°C).2 This superior sour gas resistance, in combination with excellent mechanical properties, makes CW N07022 alloy well-suited for demanding oil and gas applications.4
Barson, R. (Intertek P & IA - Manchester Technology Centre) | Korwin-Kochanowski, D. D. (Intertek P & IA - Manchester Technology Centre) | Turgoose, S. (Intertek P & IA - Manchester Technology Centre) | Economopoulos, G. (Intertek P & IA - Manchester Technology Centre) | Dicken, G. E. (Intertek P & IA - Manchester Technology Centre)
The composition of the weld consumable or the post welding microstructure of the carbon steel parent pipe can contribute towards the attack. PWC can be prevented or mitigated by selection of a suitable corrosion inhibitor. However, uniform inhibitor filming cannot be guaranteed and sensitivity to weld metal composition is possible with some chemicals. Weld metal sensitivity can induce or exacerbate the problem. The work conducted produced conditions where preferential weld corrosion initiated in uninhibited conditions and was sustained throughout inhibition. Two types of weld metallurgies were evaluated; the principle difference being that one weld consumable contained nominally 1% nickel whilst the second was welded with a parent pipe matching consumable. The performance of two generic corrosion inhibitor formulations was assessed; a sulphur-containing chemical - a quaternary amine with thioglycolic acid synergist, and a non-sulphur containing chemical, consisting of imidazoline and a phosphate ester. The influence of the functionality of the compounds versus PWC mitigation was examined. In addition, the effect of the precorrosion period on the concentration of corrosion inhibitor required to mitigate corrosion was investigated.
An extensive inspection campaign was executed in a West European refinery on circuits potentially subjected to high temperature low silicon carbon steel sulfidation (<0.1wt.%). This effort came on the top of the existing refinery sulfidation inspection plan for carbon steel piping. Ultrasonic testing (UT) measurements showed no excessive corrosion on any inspected components.
However, after these inspections were completed, a pinhole leak occurred on a weld in the Crude Distillation Unit (CDU) Heavy Gasoil (HGO) circuit. This circuit operates at 350°C (660°F), the HGO sulfur concentration ranges between 1.5 wt. % & 2.5 wt.%, and the Total Acid Number (TAN) is below 0.15 mg KOH/g. The affected piping spool was sent for metallurgical investigation, results indicating severe weld preferential corrosion whereas the surrounding base metals were not corroded.
Additional inspections were performed and results showed that 60% of this circuit circumferential welds were affected, with 30% below the retirement thickness and 30% that would have reached the retirement thickness before the next Turn-Around. Very different corrosion behavior was observed between adjacent welds, and even sometimes on the same weld, changing from no corrosion to heavy corrosion.
This paper describes the investigation that was performed to identify the corrosion mechanism, specifically trying to understand the influence of filler metal composition and welding parameters used on the corroded parts.
High temperature sulfidation is one of the most common degradation mechanisms in petroleum refinery units (e.g., CDU, Vacuum Distillation Unit (VDU), Visbreaking Unit (VBU), Fluid Catalytic Cracking (FCC), Hydrodesulfurisation unit (HDS))1,2. Its first documented occurrence was back in the late 19th century, and so is one of the oldest petroleum refinery corrosion mechanisms described in the literature3. However, despite multiple studies, it continues to be a significant cause of failures in the refining industry1.
Whereas the corrosion mechanism seems straightforward ”metal reacting with sulfur compounds at high temperature resulting in a wall thinning”, multiple parameters have to be taken into account to define the rate and the form of this corrosion4.
Metallurgical investigations were carried out on a welded sample of 25% Cr super duplex stainless steel (UNS S32750) taken from a vessel that had been accidentally operated above 300-350°C (570-660°F) for at least six months, which had resulted in brittle fracture. As a result of microstructure observations, no abnormalities were found in the ferrite/austenite balance and no intermetallic phases were observed in the sample, while Charpy impact tests at room temperature revealed quite a low absorbed energy in both, the base metal and the weld metal. These results indicated the occurrence of 475°C (885°F) embrittlement in the steel. De-embrittlement heat treatment trials were also performed to find out the effective temperature range. The aim of this paper is to draw attention to the risk of inservice embrittlement of duplex stainless steels, highlighting the difference in susceptibility to 475°C (885°F) embrittlement of the base metal and the weld metal.
Duplex stainless steels have a two-phase microstructure, approximately 50% ferrite (α) and 50% austenite (γ), and are now widely used for various industry applications because of their high strength and excellent corrosion resistance compared to 300 series stainless steels. However, these steels are prone to embrittlement when they are exposed to temperatures in the range of 315 – 540°C (600 – 1000°F) for longer than a certain period of time.1 Since this phenomenon most rapidly occurs at about 475°C (885°F), it is called “475°C (885°F) embrittlement.” This embrittlement results from decomposition of the original ferrite (α) phase in a ferrite (α)-austenite (γ) mixture to an Fe-rich α’ phase and a Cr-rich α” phase.
Figure 1 2 shows typical time-temperature embrittlement curves of duplex stainless steel base metals. The lower nose (or “bay”) indicates the occurrence of 475°C (885°F) embrittlement. 25% Cr super duplex stainless steel (UNS S32750, marked “2507” in the figure) is more susceptible to this embrittlement than 22% Cr duplex stainless steel (UNS S32205, marked “2205” in the figure) is. It is known that 475°C (885°F) embrittlement tends to occur more rapidly in duplex stainless steel weld metals than in the base metals; however the embrittlement behavior of the weld metals has not been investigated in such detail as that of the base metals has been.
The application of corrosion inhibitors to mitigate carbon dioxide corrosion in offshore oil and gas pipelines has been practiced for several tens of years. In many instances, the performance of the corrosion inhibitor has achieved the mitigation required and as such, the use of such products has become commonplace. However, the depletion of relatively easily accessible reserves has focused attention on deeper, hotter and frequently more corrosive fields. The concept that the injection of a corrosion inhibitor demonstrated to achieve a corrosion rate of less than 0.1 mm/y on a representative piece of carbon steel in a static or rotating autoclave is a panacea for aqueous carbon dioxide corrosion is becoming less evident in upstream operations. Considerations regarding the impact of the active inhibitor molecules preferentially partitioning into either the water phase or the liquid hydrocarbon phase and the availability of the inhibitor molecules at top of line locations appear less frequent and the requirement to conduct corrosion inhibitor testing in representative conditions using representative fluids and representative materials is seen as a nuisance rather than a fundamental. Typical test requirements for the assessment of a carbon dioxide corrosion inhibitor, for application in a subsea pipeline, and user expectations are identified and discussed together with some general considerations relating to the production of more aggressive fluids.
Basic FCAW electrodes of the EXXXT-5 designation offer excellent toughness, however, the operation of most T-5 electrodes is very harsh with high levels of spatter. Furthermore, the weld pool tends to be too fluid for out-of-position welding. Rutile-based FCAW electrodes, such as AWS EXXXT-1, offer smooth operation and good all-positional capabilities. Yet, because the weld deposits tend to be relatively high in oxygen, impact toughness will typically be much lower than what can be achieved with a basic slag system. Furthermore, as weld metal tensile strength increases, it is common to see a drop-off in impact toughness. Consequently, many users will opt for EXXX18 SMAW electrodes to achieve good toughness and all-positional capabilities, thereby sacrificing productivity. This paper details a new approach for producing flux-cored electrodes that provide excellent toughness and exceptionally low weld-metal oxygen, with good operability and out-of-position welding capabilities.