Toughness Of Welded Joints Research Articles

Evaluation of metallurgical and mechanical properties of 12 mm thick aerospace grade MDN 250 steel by plasma ARC welding technique

Carbon-free nickel alloy maraging steel has high demand in manufacturing aerospace vehicles and hypersonic missiles. Due to the absence of carbon and the strengthening of intermetallic precipitates, maraging steel can attain high strength and toughness with ductility. This material has enormous weldability in both annealed and post-weld heat-treated (PWHT) conditions. During high-thickness sections, the heat flow rate and the number of passes were found to be redundant. The existence of reverted austenite (RA) has been identified because prolonged aging also has an impact on the mechanical characteristics of the weldment. The present research demonstrates the characterization of plasma arc welded (PAW) maraging steel MDN 250 in welded and PWHT conditions. The weldments were subjected to three types of heat treatment, namely, (i) Subsequent Aging (TA), (ii) Solution treating + Subsequent Aging (STA), and (iii) Homogenizing + Solution treating + Subsequent Aging (HTA). This study also emphasizes the importance of precipitates and RA in affecting the strength and toughness of welded joints. The HTA treatment solely helps to eliminate the RA from the weldment. The microstructural study depicts the formation of reverted austenite in the TA condition. The reverted austenite has not been found in the microstructure under HTA conditions. The EBSD (Electron backscatter diffraction) analysis has resulted that the RA has been eliminated. The EDS (Electron Dispersive Spectroscopy) analysis shows that segregation of the enriched alloys has been effectively lowered in HTA condition. Compared to the other weldments’ mechanical characterization, the HTA condition gives supremacy results. The average ultimate tensile strength of 1830 MPa has been achieved in HTA condition, with a yield strength of 1809 MPa. The average fracture toughness results of 88.3 MPa√ m was obtained in HTA condition. The work display of PAW (Plasma Arc Welding) is capable of meeting all of the requirements for welding maraging steel in high-thickness sections.

SELECTION OF TECHNOLOGICAL PARAMETERS OF MODES FOR MANUAL ARC WELDING OF SHUT-OFF VALVES MADE OF STEEL ALLOYED WITH 3–4 % NICKEL

Body of ball valves can be manufactured from A350-LF3 steel according to the US standard ASTM A350, alloyed with nickel up to 4 %. This steel has a tendency to harden in the heat affected zone during welding. The results of computer simulation showed that cooling rates during shielded metal arc welding exceed the recommended range. This can lead to the formation of hardening structures, the development of cold cracks and a decrease in the performance properties of welded joints of ball valves made of A350-LF3 steel.
 To prevent the formation of cold cracks, the preheating and concurrent heating temperatures were calculated. According to the results of computer simulation, it was found that pre-heating and concurrent heating provide the cooling rate of heat affected zone in the recommended range.
 The technology has been developed for welding ball valves made of A350-LF3 steel with mounting nozzles. This technology prevents the formation of cold cracks and provides a high level of strength and toughness of welded joints. The high level of mechanical properties of welded joints is confirmed by results of mechanical tests and metallographic studies.

Effect of Postweld Heat Treatment on Microstructure and Properties of Thick S11306 Ferritic Stainless Welded Joints

This study investigated the evolution of microstructure and mechanical properties of 25 mm thick S11306 ferritic stainless steel welded joints during Post weld heat treatment (PWHT) by a series of tests, including the optical microscope observation, hardness test, tensile test, bending test, and scanning electron microscope tests. The experimental results show that at as-welded and 870 °C, the toughness of welded joints was poor; and when the heat treatment temperature is between 770 and 820 °C, the content of the composite phase of lower bainite and martensite (LB/M) in the weld zone increased to 43.9~47.6%, and the mean values of tensile strength and yield strength were 476 MPa and 309 MPa, the elongation was close to 40%, and the welded joints show good toughness. The bending test of the welded joints under several groups of PWHT was qualified. Heat preservation of welded joints was conducted at 820 °C for 30 s~1 h and with the increase of holding times, the content of the LB/M in weld zone showed an upward trend, and the recrystallization of base metal zone was basically completed at 15 min. In addition, a recrystallization kinetic model of the base metal zone was also established by measuring the hardness of the base metal zone at 820 °C for different holding times, which provides reference opinions for obtaining the favorable microstructure of steel grades during PWHT in the engineering.

Failure analysis on weld joint of centrifugal pump diffuser for oil and gas pipeline transportation

The failure mechanism of weld joint in centrifugal pump diffuser was analyzed through scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and electron back-scattering diffraction (EBSD). Meanwhile, the strength and toughness of weld joint, heat-affected zone (HAZ), and matrix are quantitatively compared by geometrically necessary dislocation density (ρGND). The result indicated that the strength and toughness of HAZ are the highest, followed by matrix, and weld joint is the lowest. Defects such as slag inclusion and residual austenite create initial conditions for the corrosion of weld joint. Severe chloride ion corrosion resulted in micro-cracks on the weld surface. Meanwhile, corrosion reduces the surface energy of the micro-cracks. Therefore, the concentrated driving force makes the micro-cracks with reduced surface energy spread rapidly, eventually causing the rapid failure of diffuser weld joint. This study may provide a reference for petroleum engineers and centrifugal pump manufacturers.

Investigation on mechanical and microstructure characteristics of nickel based C-2000 super alloy using laser beam welding

The main aim of the research is to investigate the microstructure and mechanical properties of advanced corrosion resistant alloy C-2000 welded with Laser Beam Welding (LBW) process. Laser welding is a high-power-density fusion-welding process that produces high aspect ratio welds with a relatively low heat input compared with other arc-welding processes. Dinolite macro analyzer was used to determine the aspect ratio and other defects in the weld joint. The micro structural characteristics of the weldment are analyzed with Optical and Scanning Electron Microscope (SEM). The micro segregation of alloying element in interdendritic regions is quantified by Energy dispersive X-ray spectroscopy (EDS). Microstructure examination shows that mild copper microsegregation is observed near the interdendritic region. The strength, ductility, hardness and toughness of weld joints are evaluated by tensile, Vicker’s hardness and Charpy impact test respectively. The result outcome shows that mechanical properties are slightly lower than base metal due to the combined existence of columnar and equiaxed grains in the weld fusion zone.

Improving Impact Toughness for Welding Joints of Steel by Adding TiO2 Nanoparticales

The using of nanotechnology in the welding process has great importance in developing the mechanical properties of welding joints for metals. This paper investigated the effect of addition TiO2 NPs to welding joints to improving the Impact toughness property of the welding joints. The cold spray coating method of the nanoparticles are used to adding the TiO2 NPs to the welding joints during the welding process. Three weight fraction are used for TiO2 NPs (0.75%, 1.5%, and 2%). The testing samples were prepared for Impact test, and microstructure by SEM. The results show the increasing in Impact toughness of welded joints with an increase of TiO2 NPs concentrations. The average Impact toughness for the welded sample without TiO2 NPs. was (162.4 J), while the average Impact toughness for the welded sample with (1.5)% TiO2 NPs. was (231.2 J) with improving rate of (42.36%). The Microstructure images by SEM show the adding of TiO2 NPs decreases the grain size and homogenous region in welded joints cross-section compared with a sample without adding TiO2 NPs. The EDS analysis show that increase Ti contain and decrease (Mn and Si) contain with increase TiO2 NPs.

Microstructural Evolution and the Effect on Hardness and Impact Toughness of Sanicro 25 Welded Joints After Aging at 973 K

Sanicro 25 tubes were welded using Alloy 617 mod. as the filler material, and the welded joints were aged at 973 K up to 10,000 hours. The hardness and impact toughness of welded joints were measured, and the microstructural evolution after aging was systematically investigated by means of OM, SEM, and TEM. After aging, the predominant precipitates in the base metal are secondary NbCrN, Cu-rich particles, and M23C6. The great increase in the hardness of base metal at the early stage of aging is attributed to the precipitation of secondary NbCrN, Cu-rich particles, and M23C6. The calculation of pinning force implies that secondary NbCrN possesses the strongest precipitation strengthening because it is extremely fine and stable. Also, Cu-rich particles are the key precipitates owing to the significant effect on the hardness change of base metal during aging. Aging results in the precipitation of needle-like M23C6 and spherical γ′ phase as well as large M23C6 particles at the grain boundaries in the weld metal. Hence, the hardness of weld metal increases. With the increasing aging time, the growths of M23C6 and γ′ phase decrease the hardness. Some cuboids of M23C6 are observed after long-term aging. The significant decrease in the impact toughness of the aged welded joints mainly results from the growth of M23C6 at the grain boundaries and the interdendritic regions. Needle-like and cuboids of M23C6 further decrease the impact toughness.

Experimental Study on the Fracture Toughness of Welded Joints of API X90 High-grade Pipeline Steels Using Single-Edge-Notched Tension Specimens

The fracture toughness and R-curves of welded joints of API X90 pipeline steels were experimentally studied. Single-edge- notched tension (SENT) specimens and a normalization method were adopted. Joints welded by using three technologies (A, B, and C) were investigated. The results show that the SENT specimen could be used to test the fracture toughness and R-curves of the welded joint; the welded joint of welding method B presented the poorest fracture toughness; and the R-curves and fracture toughness of the heat-affected zone were seriously affected by the weld zone and base metal. A new idea to test the fracture toughness and R-curves of the heat-affected zone was also proposed.

Electromagnetic nature of the movement of liquid metal in the weld pool

The electromagnetic nature of movement of liquid metal in a weld pool is discussed. When increasing the welding speed as a result of decreasing the induction of the magnetic field at the back of the arc and increasing the electromagnetic force directed forward and reducing the size of the active arc spot, the speed of movement of the liquid metal increases in welding in the direction from the current supply and rapidly in welding towards the current supply. This confirms the electromagnetic nature of formation of undercutting. A method is described for high-speed one-sided welding with a compound electrode in which the active spot of the arc becomes larger, the arc and liquid metal of the pool move forward, the electromagnetic forces directed downwards in the area of the side edges of the pool decrease, and the resultant welded joint is of high quality without undercutting, the reverse bead forms on a flux cushion and the impact toughness of welded joints increases.

Effect of welding technology on the mechanical properties of welded joints in pipes for deep water offshore gas pipelines

Results of investigations into the dependence of mechanical properties of welded joints of pipes for offshore pipelines on welding conditions are presented. In particular, the results show that the mechanical properties of pipes of the strength categories K60 (X70) and K56 (X65) are influenced mainly by the heat input, the fracture toughness of welded joints, and pipes of a high strength category are lower by 30–35% than in the case of the K56 (X65) pipes. The hardness of the weld metal in the upper layers is 15–17% higher for automatic submerged arc welding and in the lower layers for gas-shielded automatic arc welding. In inspecting the mechanical properties of the welded joints, special attention must be given to the longitudinal welded joints in the pipes.

Experimental study on the welding procedure of the central groove of armored face conveyor

The central groove is a key component of armored face conveyor. Failure of welded joints in the central groove often occurs in engineering. In this article, the welding procedure parameters were analyzed to the improve strength and toughness of the dissimilar steel weld joint in the central groove. First, four types of super-high wear-resistant steel and two types of high-strength welding wire were chosen as the welding materials. Carbon equivalent method, cold crack sensitivity index method, and maximum hardness of heat-affected zone test were applied to analyze their welding properties. Second, to improve their weldability, the proper preheating temperature was determined using Y-groove cracking test and mechanical property tests. Finally, a semi-automatic CO2welding method was adopted, and a second mechanical property test was conducted to determine the proper welding wire. Results show that the chosen steels have poor weldability and cold cracking sensitivity. Preheating must be used to prevent cold cracking. When the preheating temperature is higher than 80°C, the cold crack rate can be controlled to under 20%, which can satisfy application requirements. Welding wire YMGR76 is appropriate. Using YMGR76, the preheating temperature drops to 120°C, and the strength and toughness of the weld joint are improved to 813 MPa and 112 KV2/J, respectively. This study provides a welding procedure to improve the reliability of the central groove.

Magnetic field in electric arc welding

It is shown that the longitudinal magnetic field of welding current in welding of two sheets transforms to the field of a single sheet and this confirms the electromagnetic nature of the interatomic binding forces. At a gap of 0.002 m and as a result of the separation of the lines of force, the induction of the longitudinal magnetic field of welding current rapidly increases and the arc starts to rotate. The electromagnetic force and magnetic pressure directed forward and to the side edges of the weld pool then appear. This results in the movement of liquid metal to the front part of the crater and to the side edges, formation of welded joints without undercutting, and confirms the electromagnetic nature of the formation of undercutting in one-sided high-speed welding. A method of one-sided high-speed welding with a compound electrode on a flux cushion with a gap of 0.002 m has been developed. The method improves the stability of the process and impact toughness of welded joints.

Microstructural characterization and wide temperature range mechanical properties of NiCrMoV steel welded joint with heavy section

Abstract

Fracture toughness of welded joints of a steam turbine low pressure rotor

In order to ensure safety and reliability of a steam turbine welded rotor, the present investigation focused on evaluation of the crack resistance parameter of the base metal (BM), weld metal (WM), and heat affected zone (HAZ) of a turbine rotor welded joint constituent at both room temperature and 300℃. The property of crack resistance was evaluated in terms of fracture energy JIc or plane strain fracture toughness KJIc. Three-point bend experiments were performed to obtain a J-integral based crack resistance curve ( J–R curve) from a single specimen in accordance with ASTM E1820, from which the fracture toughness was determined. To construct a J–R curve, the corresponding crack extension was calculated by incorporating the crack opening displacement measured by a clip-gage, in a compliance method. From the experimental results, the WM was found to have the lowest KJIc values whereas the HAZ was found to have the highest KJIc values at both room and high temperature although the HAZ had little or no crack extension during the experiments. It was also observed that the HAZ samples fractured with crack jumped toward the fusion line. Microstructural root-cause analysis of the abnormal HAZ behavior was performed and discussed.

Effect of chemical composition on microstructure and mechanical properties of laser weld metal of high-tensile-strength steel

Laser beam welding (LBW) and laser-arc hybrid welding are expected to be applied to welding of high-strength steel plates over 780 MPa, because they have advantages such as low distortion and a narrow heat-affected zone. This research examined the relationship between the chemical composition of high-strength steel plates and the microstructure and mechanical properties of the weld metal with the aim of improving the toughness of welded joints of high-strength steels by the LBW process. The results revealed that weld metal toughness is strongly dependent on the carbon (C) content of the steel plates, and excellent toughness can be obtained in weld metals with low C contents having an almost fully lower bainite structure without formation of slender martensite austenite constituent (MA). Based on these results, a guideline for appropriate alloying element design for high-strength steels for LBW was proposed, targeting mechanical properties of tensile strength ≥980 MPa and toughness vE−40 °C ≥ 47 J.

Properties of X80 Pipeline Girth Welds for Different Welding Procedures

The X80 girth welds were produced by solid-wire gas metal arc welding (GMAW) and shield metal arc welding (SMAW) using two welding consumables respectively, which contained different mass fraction of C, Mo and Ni. The tensile properties, notch toughness, hardness, and microstructures of welded joints were evaluated. The results indicate that high strength and good toughness of welded joints can be achieved. But the tensile properties of all weld metal of GMAW and SMAW process were evidently different because of the difference of mass fraction of C, Mo, Ni. The strength reduced slightly in softening zone of HAZ. Using welding consumable which contain higher Mo additions, the microstructure in filler weld and fusion zones were IAF+GB and GB+M respectively. Furthermore, the mechanical properties of X80 pipeline welded by single wire welding and double wire welding respectively have been compared. The double wire welds exhibited lower yield strength but higher toughness compared to the corresponding single wire welds.

Development of additions to welding fluxes in welding of low-alloy steels

It is shown that the oxygen content of metal of welded joints in automatic submerged arc welding can be reduced by adding additions to the flux. The content of the non-metallic inclusions is reduced, and the mechanical properties and impact toughness of welded joints at sub-zero temperatures increase. The results also show that when adding up to 6% of a carbon-containing addition to the AN-60 flux, the carbon content of the weld metal in welding a low-alloy steel does not exceed the permissible value.

Physical Simulation for Evaluating Heat-Affected Zone Toughness of High and Ultra-High Strength Steels

Physical simulation of the most critical sub-zones of the heat-affected zone is a useful tool for the evaluation of the toughness of welded joints in high-strength and ultra-high-strength steels. In two high-strength offshore steels with the yield strength of 500 MPa, the coarse grained, intercritical and intercritically reheated coarse grained zones were simulated using the cooling times from 800 to 500 °C (t8/5) 5 s and 30 s. Impact and CTOD tests as well as microstructural investigations were carried out in order to evaluate the weldability of the steels without the need for expensive welding tests. The test results showed that the intercritically reheated coarse grained zone with the longer cooling time t8/5=30 s was the most critical sub-zone in the HAZ due to the M-A constituents and coarse ferritic-bainitic microstructure. In 6 mm thick ultra-high-strength steel Optim 960 QC, the coarse grained and intercritically reheated coarse grained zones were simulated using the cooling times t8/5 of 5, 10, 15 and 20s and the intercritical zone using the cooling times t8/5 of 5 and 10 s in order to select the suitable heat input for welding. The impact test results from the simulated zones fulfilled the impact energy requirement of 14 J (5x10 mm specimen) at -40 °C for the cooling times, t8/5, from 5 to 15 s, which correspond to the heat input range 0.4-0.7 kJ/mm (for a 6 mm thickness).

CTOD Evaluation for HAZ Toughness of Offshore Structure Steels with Heavy Thickness Based on API RP 2Z

The CTOD (Crack Tip Opening Displacement) evaluation and control for HAZ (Heat Affected Zone) toughness of welded joints are the most effective measures to the security of steel structures. This paper address the problem of the newly developed offshore structure steel API 2W 50Z with heavy thickness of 100mm. In accordance with the American Petroleum Institute standard Recommended Practice 2Z (API PR 2Z), the CTOD tests of two critical positions of HAZ in three kinds of heat inputs were carried out in low temperature (-10°C). All of these CTOD testing and assessments are finished under the witness of American Bureau of Shipping (ABS). The result shows that all of the CTOD values are greater than the required value (0.38mm) in API RP 2Z. This indicates that the newly developed offshore structure steel (API 2W 50Z) meet the requirement of the toughness. Further, the CTOD testing and assessments in several international standards about weld joints of steel plates are also compared and discussed in this paper.

Thermal conduction theoretical analysis of temperature distribution during multiple-electrode submerged arc welding

In recent years, in order to reduce the costs of transportation and construction of pipelines, which are often constructed using multiple-electrode submerged arc welding (SAW), higher joint performance is required. Therefore, there has arisen the need to understand theoretically and control appropriately metallurgical and mechanical characteristics in heat-affected zone (HAZ), which has a significant influence on the strength and toughness of welded joints. Commonly, metallurgical phenomena in HAZ are evaluated based on the highest temperature and the cooling rate. Therefore, in order to control metallurgical and mechanical characteristics in HAZ by means of the welding conditions, evaluating the temperature distribution and the temperature history near the melted zone is essential. However, a detailed investigation of the temperature distribution for multiple-electrode SAW has not yet been carried out enough. In this study, in order to investigate the temperature distribution and histories during multiple-electrode SAW, the experimental results are compared with the theoretical results. In the theoretical analysis, the temperature rise equation in multiple heat sources welding is developed using the method of summation. Furthermore, on temperature distribution during welding, the effects of multiple heat sources, such as the number of heat sources and the distance between each electrodes, are considered quantitatively through the thermal conduction theoretical analysis. As the result, the distance between lead heat source and final heat source primally influences the area with the difference between a single heat source welding and multiple heat sources welding. Based on the results, it is expected to control temperature distribution near melted zone by more appropriate heat input characteristics, which is depended on heat source arrangement.