Specific features of the influence of hydrogen on the properties and mechanism of fracture of the metal of welded joints of steam pipelines at thermal power plants

We study the standard tensile mechanical properties of a live-steam pipeline operated for about 2 · 10⁵ h and model (repair) welded joints. As a result of the degradation of weld metal under service conditions, its strength becomes lower than that of base metal. We show that the unregulated orientation of specimens along the axis of a welded joint is the most acceptable for detecting the degradation of weld metal. Cathode polarization facilitates evaluating the state of the degraded weld metal and that of the base metal. We have also disclosed the specific features of the influence of internal (preliminary hydrogenation) and external (electrolytic hydrogenation in the course of tests) hydrogen on the properties and mechanisms of the fracture of weld metal. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 42, No. 4, pp. 26–35, July–August, 2006.     

Degradation of Welded Joints of Steam Pipelines of Thermal Electric Power Plants in Hydrogenating Media

The mechanical properties of a welded joint of a steam pipeline of live steam after operation for ∼2 · 10⁵ h are compared with the properties of a model (repair) welded joint. In the process of operation, the properties of the metal suffer degradation in all zones of the welded joint. The short-term crack resistance proves to be the most sensitive indicator of degradation. The decrease in the crack resistance is fractographically detected in the form of low-energy intergranular or cleavage fracture. The hardness of the metal is quite sensitive to the changes in the state solely of the weld metal, especially susceptible to degradation in the process of operation. The plasticity of the operating welded joint abruptly decreases as a result of hydrogenation.__________Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 40, No. 6, pp. 105–110, November–December, 2004.     

Comparison of creep crack growth rate in heat affected zone of welded joint for 9%Cr ferritic heat resistant steel based on C, dδ/dt, K and Q parameters

Creep crack growth behavior is very sensitive to the materials’ micro-structures such as the heat affected zone of a weld joint. This is a main issue to be clarified for 9%Cr ferritic heat resistant steel for their application in structural components. In this paper, high temperature creep crack growth tests were conducted on CT specimens with cracks in the heat affected zone of weld joints of W added 9%Cr ferritic heat resistant steel, ASME grade P92. The creep crack growth behavior in the heat affected zone of welded joint was investigated using the Q^∗ concept following which the algorithm of predicting the life of creep crack growth has been proposed. Furthermore, three-dimensional elastic-plastic creep FEM analyses were conducted and the effect of stress multiaxiality of welded joint on creep crack growth rate was discussed as compared with that of base metal.     

Crack resistance of the near-weld region of welded joints of titanium and aluminum alloys

We studied the crack resistance K_c (K_Ic) of the near-weld region of welded joints of VT3-1 and VT14 titanium alloys and AMg6NPP, 12401, and 1420 high-strength aluminum alloys under fatigue and static loading. By the methods of fracture mechanics and according to the results of testing cylindrical specimens with external circular cracks initiated in the weld metal, fusion zone, zone of thermal influence, and parent material, we established that the minimum crack resistance is observed in the fusion zone and the maximum crack resistance in the parent material. The weld metal and the zone of thermal influence are characterized by intermediate values of crack resistance both under fatigue and static loading. By using 1402 high-strength aluminum alloy as an example, we studied the influence of the texture, procedure of welding, modes of thermal hardening, and scaling factor on the character of brittle fracture of the metal in the near-weld region. To enhance the characteristics of crack resistance of the near-weld region of welded joints in its weakest zones (weld and fusion zone), we propose efficient methods for welding these alloys, e.g., electron-beam welding together with the efficient choice of the modes of thermal hardening instead of argon-arc welding. This enables one to improve the purity and homogeneity of the structure of the metal in the molten-metal bath in the process of welding by decreasing its porosity and the number of inclusions and cracks appearing in these zones. “L’vivs’ka Politeknika” State University, L’viv, Ukraine. Translated from Problemy Prochnosti, No. 5, pp. 89–99, September–October, 1998.     

Effect of mechanical mismatching on the ductile-to-brittle transition curve of welded joints

The effect of mechanical mismatching (ratio between the yield strength of base and weld metal) on the toughness of welded joints at different temperatures was analysed and the ductile-to-brittle transition curves of these welded joints were experimentally obtained. The filler metal of the joints was always the same, varying the base metal and the width of the welded zone. Two base metals were selected, one with a higher strength than the filler metal (undermatched joint) and the other with a lower strength than the filler metal (overmatched joint). In addition, the joints were made using two different weld widths, 20 and 10 mm.The fracture behaviour of the joints were determined at different temperatures using SE(B) specimens provided with short cracks (a/W = 0.22). Besides, long crack specimens (a/W = 0.5) were also used for comparison. In the case of overmatched joints, the J-values for ductile crack growth are larger than for the undermatched joints. In addition, the ductile-to-brittle transition curve is displaced towards lower-temperatures and higher-toughness values and the toughness for cleavage fracture is also larger for overmatching than for undermatching. All these effects are more significant as the weld width decreases and have been explained in terms of constraint modifications.     

Applicability of C parameter in assessing Type IV creep cracking in Mod. 9Cr-1Mo steel welded joint

A welded joint of Mod. 9Cr-1Mo steel, whose Type IV cracking behavior is an important issue to be assessed, was subjected to a series of creep crack propagation experiments in order to clarify the applicability of existing standard ASTM E1457-98. Standard 1T-C(T) specimens made of base metal (BM) and welded joint (WJ), in which the heat affected zone (HAZ) was set to be the crack plane, were subjected to the experiments under 600, 650, and 700 °C, and with a few load level conditions. While the crack planes of BM specimens were fairly flat, those in WJ specimens showed bumpy surfaces following the shape of multi-path weld beads. The cracks in WJ specimens were of typical Type IV cracking, and their crack passed through nearby the interface of BM and weld metal. There are the fairly good relationships between the creep crack propagation rate (da/dt) and C^* parameter. All the BM and WJ data fallen in each one C^*-da/dt relationship for BM and WJ, respectively, regardless of the temperature and load level. The C^* parameter used here is defined for the homogeneous material and does not give a physically correct C^* for WJ, nevertheless all the WJ data still tends to gather each other on single C^*-da/dt relationship. This fact suggests that the geometrical limitations of E1457-98 standard also can act well as the limitation for the inhomogeneity of weld structure and may eliminate the effect of large scale inhomogeneity due to the combination of BM and weld metal. The da/dt of WJ were about 3-10 times faster than that of BM for the same C^* value. This difference can be attributed as the effect of difference in triaxiality, the relative constraint between the weld metal and the base metal, or the difference in resistance for creep crack propagation in HAZ material.     

Improvement of the mechanical characteristics and corrosion resistance of welded joints of maraging steels

We show that the specific character of a structural state of welded joints of Kh11N10M2T steel as a sequence of structural changes in the course of accelerated heating of maraging steels causes a regular inhomogeneity of their mechanical characteristics and low corrosion resistance. We develop a procedure of thermal treatment which guarantees homogeneity of welded joints in strength, plasticity, and toughness. The mechanical characteristics and corrosion resistance of these joints correspond to those of the base metal that is reached due to both removal of interdendritic liquating inhomogeneity of the weld metal and equalization of the structure and phase composition in all zones of a welded joint. Institute of Mechanics, Ukrainian Academy of Sciences, Kiev. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 35, No. 1, pp. 47–54, January–February, 1999.     

Research on fatigue behavior and residual stress of large-scale cruciform welding joint with groove

Fatigue fracture behavior of the 30 mm thick Q460C-Z steel cruciform welded joint with groove was investigated. The fatigue test results indicated that fatigue strength of 30 mm thick Q460C-Z steel cruciform welded joint with groove can reach fatigue level of 80 MPa (FAT80). Fatigue crack source of the failure specimen initiated from weld toe. Meanwhile, the microcrack was also found in the fusion zones of the fatigue failure specimen, which was caused by weld quality and weld metal integrity resulting from the multi-pass welds. Two-dimensional map of the longitudinal residual stress of 30 mm thick Q460C-Z steel cruciform welded joint with groove was obtained by using the contour method. The stress nephogram of Two-dimensional map indicated that longitudinal residual stress in the welding center is the largest.     

Crack resistance of homogeneous and inhomogeneous butt welded joints

We describe the methods used for the evaluation of strength of butt welded joints according to their crack resistance both by the axial tension of small-sized cylindrical specimens with circular cracks and three-point bending of beam specimens with lateral cracks induced in low-or medium-strength surfacings welded into the high-strength bases of these specimens. We perform the comprehensive investigation of the near-weld zone of butt welded joints, where the lowest crack resistance is exhibited by the metal of the zone of fusion and the highest crack resistance is detected for the source material independently of the type of tested specimens. Metals of the weld and the heat-affected zone are characterized by higher values of crack resistance than the zone of fusion and lower values of crack resistance than the source material. By analyzing, as an example, the case of axial tension of a tubular specimen with internal circular crack in the surfacing, we study the crack resistance of seven types of inhomogeneous welded joints depending on the mode of thermal treatment of 35 and St. 3 steels after normalization and 09G2S, 20Kh, and 30KhGSA steels after hardening, medium tempering, and normalization. __________ Translated from Problemy Prochnosti, No. 4, pp. 62–76, July–August, 2007.     

On the influence of wet underwater welding on CTOD-δ5

The influence of wet underwater welding on the crack tip opening displacement CTOD-δ₅ is investigated in this paper. The welding process is numerically simulated. The instationary temperature field and welding residual stresses are calculated using the finite element method (FEM). The crack tip opening displacement for a sharp, stationary crack on the surface of a bead-on-plate weld under bending is determined. The results for an underwater wet welded specimen and a dry welded specimen are compared. The welding residual stresses are considered in the 3D fracture mechanics FE calculation as well as the material heterogeneity due to the different material properties of weld metal, heat-affected zone and base metal. This revised version was published online in July 2006 with corrections to the Cover Date.     

Interfacial ultrafine-grained structures on aluminum alloy 6061 joint and copper alloy 110 joint fabricated by magnetic pulse welding

Magnetic pulse welding is a solid state impact welding process, similar to explosive welding, which produces metallurgical bond by oblique high-speed impact between two metal bodies. This violent impact removes the metal surface oxide layers and then joins the two atomic level clean metal surfaces together by the incidental compression pressure. The impact velocity is at 200–400 m/s and the being welded metal surface undergoes severe plastic deformation with strain rate in the order of 10⁶–10⁷ s⁻¹. The ultrafine-grained structure was observed on the welded interface. This article studied two types of similar material lap joint interfaces and the base metals were aluminum alloy 6061 and copper alloy 110. Nano-indentation testing shows that the welded interfaces have significantly greater hardness than the base metals. The interface microstructure was studied by optical microscopy, electron backscatter diffraction microscopy, and transmission electron microscopy. The welded aluminum alloy 6061 interface exhibits extremely fine grains and an extremely high dislocation density. The impact welded copper alloy 110 interface presents nano-scale lamellar band structure and deformation twins. The interface hardness increasing was attributed to this impact-induced microstructural refinement.     

Effect of joint gap on the quality of laser beam welded near-β Ti-5553 alloy with the addition of Ti–6Al–4V filler wire

Ti–5Al–5V–5Mo–3Cr (Ti-5553) sheets were welded using a Nd: YAG laser system and Ti–6Al–4V filler wire. The effect of joint gap on weld geometry, defects, microstructure, and hardness was investigated. Fully penetrated welds up to a joint gap of 0.5 mm were produced. The two main defects observed were porosity and underfill. The addition of filler wire reduced underfill but increased porosity, especially at large joint gaps. The fusion zone (FZ) microstructure at low joint gaps consisted of retained β with a dendritic morphology. At a joint gap of 0.3 mm, regions of orthorhombic α″ martensite were observed in the weld zone which increased in proportion as the joint gap increased from a volume percentage of 4.9% at 0.3 mm to a volume percentage of 44% at 0.5 mm. Despite the differences in microstructure with increasing joint gap, the FZ hardness remained relatively constant for all joint gaps evaluated.     

组合焊缝对中碳合金钢焊接裂纹的影响

采用Ｙ形坡口裂纹试验和显微分析方法，研究了三种不同的组织焊缝对中碳合金钢焊接裂纹的影响，结果表明，底层焊道熔俣区裂纹为氢致延迟裂纹；底层焊条的合金系统对该裂纹的生成有一定的影响，底层焊道熔合区的马氏体和氢的富集以及大的拘束应力，是裂纹生成的必要和充分条件；提出了防止裂纹产生的用工艺。     

Effect of fracture path on the toughness of weld metal

The crack propagation direction may affect weld metal fracture behavior. This fracture behavior has been investigated using two sets of single edge notched bend (SENB) specimens; one with a crack propagating in the welding direction (B×2B) and the other with a crack propagating from the top in the root direction (B×B) of a welded joint. Two different weld metals were used, one with low and one with high toughness values. For Weld Metal A, two specimen types have been used (B×B and B×2B) both with deep cracks. The weld metal A (with high toughness values) has reasonably uniform properties between weld root and cap. The resulting J-R curves show little effect of the specimen type, are ductile to the extent that the toughness exceeds the maximum J_max, value allowed by validity limits and testing is in the large –scale yielding regime. In the case of weld metal B (with low toughness values) with two specimen types (B×B and B×2B) the B×B specimen has shallow cracks while the B×2B specimen has deep cracks. Both resulting J-R curves show unstable behavior despite the fact that the types of specimen and their constraints are different. The analysis has shown that crack propagation direction is most influential for a weldment with low toughness in the small scale yielding regime, whereas its influence diminishes due to ductile tearing during stable crack growth and large scale yielding. The results have shown that these effects are different in both the crack initiation phase and during stable crack growth, indicating a dependence on weld metal toughness and the microstructure of the weld metal. It can be concluded that, if resistance curves during stable crack growth do not show differences in both notch orientations, the fracture toughness values of the whole weld metal can be treated as uniform.     

The effect of microstructures on fatigue crack growth in Q345 steel welded joint

It is a traditional that the fatigue crack growth behavior is sensitive to microstructure in threshold regime, while it is sensitive to R‐ratio in Paris regime. Fatigue test is carried out for welded joints of a Q345 steel where the compact tension specimens with 3.8 and 12.5 mm thickness are used, and comparisons of fatigue crack growth behavior between base metal and a few different locations in the welded joint are considered in Paris regime. Welding residual stresses are removed by heat treatment to focus the study on the microstructural effect. It is shown that fatigue crack growth rate (FCGR) in the base metal is not sensitive to R‐ratio, but the FCGR increases in the overheated zone, the fusion zone and the weld metal zone with R‐ratio increasing. To the low R‐ratio, FCGR in the three zones is smaller than that in the base metal, but they approximate the same with base metal under the high R‐ratio. The mechanism of fatigue crack growth is analyzed through crack path in microstructures and SEM fractograph. The coarse‐grained ferrite in the base metal is of benefit to relaxation of the average stress at the crack tip, and the fatigue crack growth predicts branching and deflection within above different locations in the welded joint. These tortuous crack paths with crack branching and deflection will promote crack closure as well as crack‐tip stress shielding and then resulted in higher crack growth resistance.     

BT20钛合金激光焊接接头的断裂韧性研究

对BT20钛合金及其激光焊接接头的断裂韧性进行了研究.同时分析了合金及激光焊接接头的硬度分布及显微组织.断裂实验表明,除了一个焊接接头紧凑拉伸(CT)试样是脆性启裂外,其它CT试样均在裂纹延性启裂并缓慢扩展后,发生脆性失稳断裂.母材的断裂韧性明显高于焊接接头,轧制方向对母材断裂韧性的影响不明显.焊接热影响区的断裂韧性介于母材和焊缝金属之间.本研究采用的焊后热处理没有改善焊接接头的断裂韧性,还有进一步恶化的趋势.添加活性剂对焊缝金属的断裂韧性没有明显作用,但对延性裂纹扩展长度有所改善.     

Effect of cooling to 4.2 K on the strength and cracking resistance of the parent metal in welded joints in 07Kh13N4AG20 steel

The results are presented of experimental examination of short-term strength and cracking resistance of the parent metal and welded joints in 07Kh13N4AG20 steel in the temperature range 293–4.2 K. The mechanical characteristics of the welded joint in the examined steel are compared with the characteristics of 12Kh18N10T and 03Kh20N16AG6 steels. The results show that the mechanical properties and crack propagation resistance of the weld metal in 07Kh13N4AG20 steel are similar to those of the parent metal.Translated from Problemy Prochnosti, No. 9, pp. 45–48, September, 1990.     

Mechanical heterogeneity and validity of J-dominance in welded joints

Fracture criterion of the J-integral finds wide application in the integrity evaluation of welded components, but there exist some confused problems such as the dependence of the fracture toughness on the strength mis-matching and specimen geometry which need to be clarified. It is rough and unsuitable to attribute the variation of J-integral fracture parameter simply to the effect of mechanical heterogeneity. In the present paper, a two-dimensional finite element method is employed to analyze the distribution and variation of crack tip field of welded joints with different strength mis-matching in four kinds of specimen geometry, and then the validity of J-dominance in welded joints is investigated. It is found that the crack tip field of mis-matched joint is different from that of either the weld metal or base metal of which the joint is composed, but it is situated between those of weld metal and base metal. Under the plane strain, there is obvious difference in stress triaxiality for different strength mis-matched joints. The validity of J-dominance in welded joint can not be obtained by comparing whether the stress triaxiality meets that required by the HRR solution because of the existence of mechanical inhomogeneity. By ascertaining if the stress triaxiality of welded joint near the crack tip is dependent of specimen geometry, the conclusion can be arrived at: for plane stress the validity of J-dominance is valid, whilst for plane strain the validity of J-dominance is lost. Based on the above, attempt has been made to point out that the influence of mechanical heterogeneity on the fracture toughness of weldment arises from the variation of constraint intensity-crack tip stress triaxiality. Compared with the effect of mechanical heterogeneity on the stress triaxiality, the losing of validity of J-dominance in mis-matched joint under plane strain may play a more critical role in the variation of J-integral fracture parameter of weldment.     

Abnormal manifestation of the high-temperature degradation of the weld metal of a low-alloy steel welded joint

We have shown that hardness, impact toughness, mechanical properties in tension, and the local parameters of fracture mechanics (static and cyclic crack resistance) are sensitive to the operating degradation of weld metal of steam pipelines of thermal power plants made of 15Kh1M1F steel. The simultaneous decrease in the resistance to brittle and plastic fracture (hardness, strength, and impact toughness) represents a phenomenon of the operating degradation of weld metal. We have established a specific correlation between the characteristics of plasticity and other mechanical parameters of operated metal: the increase in °₅ of operated weld metal is in good agreement with the decrease in its strength, whereas the reduction of Ψ correlates with the lowering of resistance to brittle fracture. Electrolytic hydrogenation decreases the characteristics of strength and plasticity of operated weld metal much stronger than in the initial state. The absence of ferritic edgings on the boundaries of primary austenitic grains makes for a low resistance to brittle fracture, and the change in acicular ferrite deteriorates the mechanical properties. The ductile fracture of nonoperated metal is replaced by brittle intercrystalline failure in operated metal. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 1, pp. 73–79, January–February, 2007.     

Failure analysis of a natural gas pipeline

In the present work, the failure investigation of a 30 in. diameter gas transmission pipeline (API 5L X-60 grade steel) has been described. The failure was due to a longitudinal crack developing in the centerline of longitudinal weld joint. Mechanisms and morphology of crack initiation and propagation were studied through different tests including: thickness measurement, chemical composition analysis, metallographic inspection, mechanical property testing, scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS). The experiments resulted to the conclusion that some factors associated with Sulphide Stress Cracking (SSC) and metallurgical defects cause the failure of weld joint pipe. Detailed examination revealed that these factors are inappropriate welding parameters, pitting corrosion on longitudinal weld, and hydrogen permeation to the weld metal.