Effect of Boron Addition and Initial Heat-Treatment Temperature on Microstructure and Mechanical Properties of Modified 9Cr-1Mo Steels Under Different Heat-Treatment Conditions

The effect of initial heat treatment on microstructure and mechanical properties of boron-free and boron-containing modified 9Cr-1Mo steel (P91 and P91B, respectively) has been studied under different heat-treatment conditions. The prior austenite grains evolved in P91 steel, having different prior austenite grain sizes, were found to be similar in size after heat treatment in the range of 1073 K to 1448 K (800 °C to 1175 °C) for 5 minutes. The microstructural evolution in P91B steel having different prior austenite grain sizes appeared to be uniform when subjected to different heat-treatment temperatures with the prior austenite grain size being similar to that of initial grain size. Lath martensite was observed in P91B steel after all heat treatments. On the other hand, lath martensite was observed in P91 steel only when subjected to high-temperature heat treatment, whereas subgrain/substructure as well as coarse precipitates were observed after a lower temperature heat treatment. Large differences in the hardness/strength values between different microstructures corresponding to coarse-grained heat-affected zone (CGHAZ) and intercritical HAZ (ICHAZ) of P91 steel weldment were due to the distinct difference in these microstructures. The difference in hardness/strength values between the CGHAZ and ICHAZ was found to be insignificant in P91B steel under similar heat-treatment conditions.     

Role of microstructural degradation in the heat-affected zone of 2.25Cr-1Mo steel weldments on subscale features during steam oxidation and their role in weld failures

Microstructural degradations in the base metal adjacent to the weld pool, i.e., the heat-affected zone (HAZ), caused during welding of 2.25Cr-1Mo steel, were characterized by electron and optical microscopy of different regions of the weldments. In order to study the influence of the microstructural degradations on scaling kinetics in steam and the resulting subscale features, samples of the base metal, the HAZ, and weld metal specimens were extracted from the weldment and oxidized in an environment of 35 pct steam+nitrogen at 873 K for 10 hours. Oxide scales formed in the three regions and the underlying subscales were characterized using scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). Influence of the “free” chromium content in the three weldment regions on protective scale formation and on the subscale features has been investigated. As the principal achievement, this study has clearly shown the occurrence of oxidation-induced void formation in the subscale zone and grain boundary cavitation in the neighboring area during steam oxidation of the HAZ. This article also discusses the possible role of oxidation-induced void formation and grain boundary cavitation in the inferior service life of welds in 2.25Cr-1Mo steel components.     

Investigation of precipitation behavior in a weld deposit of 11Cr-2W ferritic steel

This article is aimed at investigating the difference in precipitation behavior in the fine-grained heat-affected zone (FGHAZ), coarse-grained heat-affected zone (CGHAZ), and base metal for the welded joint of high Cr ferritic heat-resistant steel (11Cr-0.4Mo-2W-1Cu-V-Nb, normalized 1323 K×1 h and tempered 1033 K×1 h). Simulated HAZ (SHAZ) specimens were used, whose thermal cycles were controlled to be the same as those in the actual welded joint with peak temperatures of 1523 and 1173 K to represent CGHAZ and FGHAZ, respectively. Based on scanning electron microscopy (SEM) observation, it looks that the precipitates in FGHAZ specimens (1173 K) were fewer and larger than those in CGHAZ (1523 K) specimens and base metal specimens. This phenomenon implied that the growth and coarsening of precipitates in FGHAZ may play a role in the deterioration of creep properties and type IV cracking, which was observed in previous creep tests. X-ray diffraction analysis for the electrolytic extraction showed that the types of precipitates are the same for the 1173 K specimens and base metal specimens, including M₂₃C₆, MX, Laves phase, and μ phase. Further, the elemental analysis of the extraction showed that the mass percentages of Cr, W, and Mo in the precipitates to specimen mass were higher in the FGHAZ specimen than those in the base metal specimen, especially during the period between 600 and 2464 hours. Finally, a two-dimensional (2-D) model was proposed to simulate the precipitation behavior of the Laves phase.     

Type IV Creep Damage Behavior in Gr.91 Steel Welded Joints

Modified 9Cr-1Mo steel (ASME Grade 91 steel) is used as a key structural material for boiler components in ultra-supercritical (USC) thermal power plants at approximately 873 K (600 °C). The creep strength of welded joints of this steel decreases as a result of Type IV creep cracking that forms in the heat-affected zone (HAZ) under long-term use at high temperatures. The current article aims to elucidate the damage processes and microstructural degradations that take place in the HAZ of these welded joints. Long-term creep tests for base metal, simulated HAZ, and welded joints were conducted at 823 K, 873 K, and 923 K (550 °C, 600 °C, and 650 °C). Furthermore, creep tests of thick welded joint specimens were interrupted at several time steps at 873 K (600 °C) and 90 MPa, after which the distribution and evolution of creep damage inside the plates were measured quantitatively. It was found that creep voids are initiated in the early stages (0.2 of life) of creep rupture life, which coalesce to form a crack at a later stage (0.8 of life). In a fine-grained HAZ, creep damage is concentrated chiefly in an area approximately 20 pct below the surface of the plate. The experimental creep damage distributions coincide closely with the computed results obtained by damage mechanics analysis using the creep properties of a simulated fine-grained HAZ. Both the concentration of creep strain and the high multiaxial stress conditions in the fine-grained HAZ influence the distribution of Type IV creep damage.     

An assessment of creep deformation and fracture behavior of 2.25Cr-1Mo similar and dissimilar weld joints

The evaluation of the creep deformation and fracture behavior of a 2.25Cr-1Mo steel base metal, a 2.25Cr-1Mo/2.25Cr-1Mo similar weld joint, and a 2.25Cr-1Mo/Alloy 800 dissimilar weld joint at 823 K over a stress range of 90 to 250 MPa has been carried out. The specimens for creep testing were taken from single-V weld pads fabricated by a shielded metal arc-welding process using 2.25Cr-1Mo steel (for similar-joint) and INCONEL 182 (for dissimilar-joint) electrodes. The weld pads were subsequently given a postweld heat treatment (PWHT) of 973 K for 1 hour. The microstructure and microhardness of the weld joints were evaluated in the as-welded, postweld heat-treated, and creep-tested conditions. The heat-affected zone (HAZ) of similar weld joint consisted of bainite in the coarse-prior-austenitic-grain (CPAG) region near the fusion line, followed by bainite in the fine-prior-austenitic-grain (FPAG) and intercritical regions merging with the unaffected base metal. In addition to the HAZ structures in the 2.25Cr-1Mo steel, the dissimilar weld joint displayed a definite INCONEL/2.25Cr-1Mo weld interface structure present either as a sharp line or as a diffuse region. A hardness trough was observed in the intercritical region of the HAZ in both weld joints, while a maxima in hardness was seen at the weld interface of the dissimilar weld joint. Both weld joints exhibited significantly lower rupture lives compared to the 2.25Cr-1Mo base metal. The dissimilar weld joint exhibited poor rupture life compared to the similar weld joint, at applied stresses lower than 130 MPa. In both weld joints, the strain distribution across the specimen gage length during creep testing varied significantly. During creep testing, localization of deformation occurred in the intercritical HAZ. In the similar weld joint, at all stress levels investigated, and in the dissimilar weld joint, at stresses ≥150 MPa, the creep failure occurred in the intercritical HAZ. The fracture occurred by transgranular mode with a large number of dimples. At stresses below 150 MPa, the failure in the dissimilar weld joint occurred in the CPAG HAZ near to the weld interface. The failure occurred by extensive intergranular creep cavity formation.     

Microstructural Evolution and Mechanical Properties of CSEF/M P92 Steel Weldments Welded Using Different Filler Compositions

In the present investigation, P92 steel weld joints were prepared using a shielded metal arc welding (SMAW) process for two different fillers, E911 and P92. A comparative study was performed on the microstructural evolution, tensile strength, microhardness, and Charpy toughness across the P92 steel weldments in the as-welded and post-weld heat-treated (PWHT) conditions. The PWHT was performed at 760 °C for 2 hours. To study the effect of the different filler metals and PWHT on the mechanical properties, longitudinal and transverse tensile tests were carried out at room temperature for a constant cross-head speed of 1 mm/min. In the longitudinal direction, the tensile strength of the P92 steel welds was measured as 958 ± 35 and 1359 ± 38 MPa for the E911 and P92 filler, respectively. In the as-welded condition, the transverse tensile specimens were fractured from the fine-grained heat-affected zone or inter-critical heat-affected zone (FGHAZ/ICHAZ) and, after PWHT, the fracture location was shifted to over-tempered base metal from the FGHAZ/ICHAZ. After the PWHT, the tempering reaction resulted in lowering of the hardness throughout the weldment. After PWHT, the Charpy toughness of the weld fusion zone and heat-affected zone (HAZ) of the E911 filler weldments was measured as 66 ± 5 and 142 ± 8 J, respectively. The minimum required Charpy toughness of 47 J (EN1557: 1997) was achieved after the PWHT for both E911 and P92 filler.

     

Influence of microstructural variations in the weldment on the high-temperature corrosion of 2.25Cr-1Mo steel

In order to study the influence of microstructural variation on the oxidation of the weldment of 2.25Cr-1Mo steel, regions with different microstructures were identified by optical microscopy. The weld metal, the base metal, and the heat-affected zone (HAZ), as well as the subzones within the HAZ, i.e., the intercritical (ICR), the fine-grain bainite (FGB), and the coarse-grain bainite (CGB) regions were separated from the weldment by precise steps of metallography. Transmission electron microscopic examinations for the identification of the secondary phases in microstructurally different regions and subzones have suggested that M₂₃C₆ and M₇C₃ pre-cipitates form predominantly in the subzones of HAZ, whereas the Mo₂C type of carbide forms exclusively in the weld-metal and base-metal regions of the weldment. However, population and distribution of the secondary phases were different in the three subzones of the HAZ. In order to understand the influence of these microstructural variations on the oxidation behavior, the various regions and subzones were oxidized at 773 and 873 K. The HAZ and its constituents were found to oxidize at much higher rates than the weld metal and the base metal. Relative compositions and morphologies of the scales were compared by scanning electron microscopy with energy-dispersive analyses of X-rays (SEM/EDX), and secondary ion mass spectrometry (SIMS). Scale formed over the weld metal shows a greater tendency for spallation, as suggested by tests monitoring acoustic emission. X-ray diffraction (XRD) patterns of the scales over these specimens were taken. Results of the SEM/EDX, SIMS, and XRD investigations suggest for-mation of inner scales with less Cr(i.e., less protective) over the HAZ than over the weld-metal and the base-metal regions. Variation in the Cr contents of the scales formed over the various regions is proposed to arise from the difference in microstructural features in different regions of the weldments. Formerly with the Metallurgy Division, Indira Gandhi Centre for Atomic Research, Kalpakkan, India     

Relevance of high-temperature oxidation in life assessment and microstructural degradation of Cr-Mo steel weldments

Measurement of the thickness of oxide scales that develop over high-temperature components has found an innovative application in life assessment of steam generation and handling systems. The present study is an investigation of the high-temperature corrosion and scale thickness across the weldments of a “chromium-molybdenum” steel, and reviews its possible relevance to the life assessment of the welded high-temperature components by scale thickness measurement. Results are presented of the recent investigations on the combined roles of the oxidizing environment and secondary carbide precipitation on the extent of void formation in the microstructurally different regions of weldments of the chromium-molybdenum steel. Specimens of the weld metal, heat-affected zone (HAZ), and base metal of a 2.25Cr-1Mo steel weldment were oxidized in steam. Extensive internal oxidation and oxidation-induced void formation (with a much greater intensity in the case of the HAZ) is discussed. The greater intensity of oxidation-induced void formation in the HAZ may facilitate preferential cracking in this region of the weldments and, hence, is proposed to be an important parameter in the context of the recently developed codes for life assessment of aging high-temperature components.     

Effect of Application of Short and Long Holds on Fatigue Life of Modified 9Cr-1Mo Steel Weld Joint

Modified 9Cr-1Mo steel is a heat-treatable steel and hence the microstructure is temperature sensitive. During welding, the weld joint (WJ) is exposed to various temperatures resulting in a complex heterogeneous microstructure across the weld joint, such as the weld metal, heat-affected zone (HAZ) (consisting of coarse-grained HAZ, fine-grained HAZ, and intercritical HAZ), and the unaffected base metal of varying mechanical properties. The overall creep–fatigue interaction (CFI) response of the WJ is hence due to a complex interplay between various factors such as surface oxides and stress relaxation (SR) occurring in each microstructural zone. It has been demonstrated that SR occurring during application of hold in a CFI cycle is an important parameter that controls fatigue life. Creep–fatigue damage in a cavitation-resistant material such as modified 9Cr-1Mo steel base metal is accommodated in the form of microstructural degradation. However, due to the complex heterogeneous microstructure across the weld joint, SR will be different in different microstructural zones. Hence, the damage is accommodated in the form of preferential coarsening of the substructure, cavity formation around the coarsened carbides, and new surface formation such as cracks in the soft heat-affected zone.     

A Comparison of Creep Rupture Strength of Ferritic/Austenitic Dissimilar Weld Joints of Different Grades of Cr-Mo Ferritic Steels

Evaluations of creep rupture properties of dissimilar weld joints of 2.25Cr-1Mo, 9Cr-1Mo, and 9Cr-1MoVNb steels with Alloy 800 at 823 K were carried out. The joints were fabricated by a fusion welding process employing an INCONEL 182 weld electrode. All the joints displayed lower creep rupture strength than their respective ferritic steel base metals, and the strength reduction was greater in the 2.25Cr-1Mo steel joint and less in the 9Cr-1Mo steel joint. Failure location in the joints was found to shift from the ferritic steel base metal to the intercritical region of the heat-affected zone (HAZ) of the ferritic steel (type IV cracking) with the decrease in stress. At still lower stresses, the failure in the joints occurred at the ferritic/austenitic weld interface. The stress-life variation of the joints showed two-slope behavior and the slope change coincided with the occurrence of ferritic/austenitic weld interface cracking. Preferential creep cavitation in the soft intercritical HAZ induced type IV failure, whereas creep cavitation at the interfacial particles induced ferritic/austenitic weld interface cracking. Micromechanisms of the type IV failure and the ferritic/austenitic interface cracking in the dissimilar weld joint of the ferritic steels and relative cracking susceptibility of the joints are discussed based on microstructural investigation, mechanical testing, and finite element analysis (FEA) of the stress state across the joint.     

Characterization of Microstructures across the Heat-Affected Zone of the Modified 9Cr-1Mo Weld Joint to Understand Its Role in Promoting Type IV Cracking

In the postweld heat-treated (PWHT) fusion welded modified 9Cr-1Mo steel joint, a soft zone was identified at the outer edge of the heat-affected zone (HAZ) of the base metal adjacent to the deposited weld metal. Hardness and tensile tests were performed on the base metal subjected to soaking for 5 minutes at temperatures below Ac₁ to above Ac₃ and tempering at the PWHT condition. These tests indicated that the soft zone in the weld joint corresponds to the intercritical region of HAZ. Creep tests were conducted on the base metal and cross weld joint. At relatively lower stresses and higher test temperatures, the weld joint possessed lower creep rupture life than the base metal, and the difference in creep rupture life increased with the decrease in stress and increase in temperature. Preferential accumulation of creep deformation coupled with extensive creep cavitation in the intercritical region of HAZ led to the premature failure of the weld joint in the intercritical region of the HAZ, commonly known as type IV cracking. The microstructures across the HAZ of the weld joint have been characterized to understand the role of microstructure in promoting type IV cracking. Strength reduction in the intercritical HAZ of the joint resulted from the combined effects of coarsening of dislocation substructures and precipitates. Constrained deformation of the soft intercritical HAZ sandwich between relatively stronger constitutes of the joint induced creep cavitation in the soft zone resulting in premature failure.     

Fracture toughness behavior of ex-service 2-1/4Cr-1Mo steels from a 22-year-old fossil power plant

Elevated-temperature fracture toughness properties were developed on ex-service 2-l/4Cr-1Mo steel weldments. Fracture toughness was measured on both base and heat-affected zone (HAZ) metals. A composite specimen consisting of base, HAZ, and weld metals was used to develop fracture toughness properties in the HAZ area. It was observed that the J-R curve of the HAZ was significantly lower than that of the base metal. Increasing crack extension increased the difference between theJ-R curves of the base metal and the HAZ. Dimpled fracture was the prime fracture mode in the base metal specimen, and a mixed-mode (ductile and “granular”) fracture was found in the HAZ specimens. Scanning transmission electron microscopy (STEM) examination revealed significant intergranular carbide precipitation and agglomeration within the HAZ. The lower fracture toughness of the HAZ, as compared to the base metal, was attributed to the large accumulation of carbides in the grain boundaries of the HAZ, which weakened the grain boundaries and caused “granular” fracture.     

Role of gaseous environment and secondary precipitation in microstructural degradation of Cr-Mo steel weldments at high temperatures

This study is an attempt to understand the combined role of variations in oxidizing environment and secondary precipitation, in the microstructurally different regions of a standard Cr-Mo steel weldment, on the intensity of internal oxidation during high-temperature oxidation in air and steam environments. Samples of the weld-metal, heat-affected zone (HAZ), and base-metal regions were separated from the weldment of 2.25Cr-1 Mo steel and oxidized in the environments of air and steam at 873 K. The oxide scales and underlying subscales were characterized using scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) analysis, and electron probe microanalysis (EPMA). Extensive internal oxidation and oxidation-induced void formation in the subscale zone and grain-boundary cavitation in the neighboring region were found to occur during oxidation in the steam environment. However, the internal oxidation and void formation were much more extensive in the subscale regions of the HAZ than in the subscales of the weld-metal and base-metal regions. As a result, the alloy matrix in the area neighboring the subscale region of the HAZ specimen suffered extensive grain-boundary cavitation. This behavior has been attributed to a rather specific combination and complex interplay of the environment, alloy microstructure, oxidizing temperature, and nature of the resulting external scale in causing and sustaining internal oxidation. The article also discusses the role of internal oxidation-assisted microstructural degradation in deteriorating the service life of components of 2.25Cr-1Mo steel.     

Hydrogen attack behavior of the heat affected zone of a 2.25Cr-1Mo steel weldment

The kinetics of hydrogen attack (HA) has been studied in the heat affected zone (HAZ) in a 2.25Cr-1Mo steel weld to determine the relative rates of attack and bubble nucleation in the HAZ, base metal, and weld metal. The HAZ was found to suffer hydrogen attack at nearly twice the rate of the base metal, but not as rapidly as the weld metal. Nucleation of bubbles does not occur during HA of the HAZ of a 2.25Cr-1Mo steel, on exposure to hydrogen pressure of 20.5 MPa or less, but does occur at higher pressures up to 31.5 MPa (4500 psi) at 550 °C, or up to 27.5 MPa (4000 psi) at 580 °C. Such nucleation results in enhancement of the HA rate by a factor of six. The weak dependence of nucleation effects on hydrogen pressure and the saturation of the nucleation effects in a short time suggest some thermally activated nucleation of fresh bubbles. Formerly with The Ohio State University.     

Fracture toughness of modified P91 weldments

There are efforts to develop modified P91 steel (9Cr-1Mo-V) consumables to optimize strength and fracture toughness in weldments for similar and dissimilar welding of 9Cr-1Mo (modified P91) for both new construction and replacement of serviced components. Fracture toughness is an important consideration which plays a vital role in determining the performance and life of the materials under the given service conditions. Toughness characterization was done by the Crack Tip Opening Displacement (CTOD) method. Welding results in a variety of non-equilibrium microstructures in the HAZ of 9Cr-lMo-V, modified P91 steel. These variations of microstructures from wrought base material through transformed HAZ to cast weld metal, may give rise to considerable inhomogeneity with respect to tensile & creep strength and ductility across the weld joints. However the mechanical properties of the individual regions of HAZ are difficult to obtain because of the small extent over which each region exists. Welded joints are used as structural parts of boilers and pressure vessels working at high temperatures, hence the main requirement is creep resistance. In the present investigation, the fracture toughness characteristics of base metal and weld metal have been evaluated by CTOD method as per the standard BS 7448. The fracture surfaces of the CTOD tested specimens were examined under Scanning Electron Microscope (SEM). Fractographic studies revealed the mode of failure and the characteristics of the fracture surface.     

Welding-Induced Microstructure Evolution of a Cu-Bearing High-Strength Blast-Resistant Steel

A new high strength, high toughness steel containing Cu for precipitation strengthening was recently developed for naval, blast-resistant structural applications. This steel, known as BlastAlloy160 (BA-160), is of nominal composition Fe-0.05C-3.65Cu-6.5Ni-1.84Cr-0.6Mo-0.1V (wt pct). The evident solidification substructure of an autogenous gas tungsten arc (GTA) weld suggested fcc austenite as the primary solidification phase. The heat-affected zone (HAZ) hardness ranged from a minimum of 353 HV in the coarse-grained HAZ (CGHAZ) to a maximum of 448 HV in the intercritical HAZ (ICHAZ). After postweld heat treatment (PWHT) of the spot weld, hardness increases were observed in the fusion zone (FZ), CGHAZ, and fine-grained HAZ (FGHAZ) regions. Phase transformation and metallographic analyses of simulated single-pass HAZ regions revealed lath martensite to be the only austenitic transformation product in the HAZ. Single-pass HAZ simulations revealed a similar hardness profile for low heat-input (LHI) and high heat-input (HHI) conditions, with higher hardness values being measured for the LHI samples. The measured hardness values were in good agreement with those from the GTA weld. Single-pass HAZ regions exhibited higher Charpy V-notch impact toughness than the BM at both test temperatures of 293 K and 223 K (20 °C and –50 °C). Hardness increases were observed for multipass HAZ simulations employing an initial CGHAZ simulation.     

Prediction of type IV cracking behaviour of 2.25Cr-1Mo steel weld joint based on finite element analysis

Creep tests were carried out on 2.25Cr-1Mo ferritic steel base metal and its fusion welded joint at 823 K over a stress range of 100–240 MPa. The weld joint possessed lower creep rupture strength than the base metal and the reduction was more at lower applied stresses. The failure occurred in the intercritical region of heat-affected zone (HAZ) of the joint, commonly known as Type IV cracking. Type IV cracking in the joint was manifested as pronounced localization of creep deformation in the soft intercritical region of HAZ coupled with preferential creep cavitation. The creep cavitation in intercritical HAZ was found to initiate at the central region of the creep specimen and propagate outwards to the surface. To explain the above observations, the stress and strain distributions across the weld joint during creep exposure were estimated by using finite element analysis. For this purpose creep tests were also carried out on the deposited weld metal and simulated HAZ structures (viz. coarse-grain structure, fine-grain structure, and intercritically annealed structure) of the joint. Creep rupture strength of different constituents of joint were in the increasing order of intercritical HAZ, fine-grain HAZ, base metal, weld metal and coarse-grain HAZ. Localized preferential creep straining in the intercritical HAZ of weld joint as observed experimentally was supported by the finite element analysis. Estimated higher principal stress at the interior regions of intercritical HAZ explained the pronounced creep cavitation at these regions leading to Type IV failure of the joint.     

Improving the creep properties of 9Cr-3W-3Co-NbV steels and their weld joints by the addition of boron

New ferritic steels with a controlled addition of boron have been developed recently for ultrasuper-critical fossil power plants. These steels possess excellent creep resistance compared to conventional steels like P91, P92, P122, etc., and this has been attributed to the delay in coarsening of the carbides during creep owing to partial replacement of carbon by boron in these carbides. However, the susceptibility of the weld joints of the boron-containing ferritic steels to type IV cracking, which significantly brings down the rupture life of the weld joints, has not been investigated so far. In the present work, the creep properties of recently developed 9Cr-3W-3Co-NbV steels with boron contents varying from 47 to 180 ppm and of their weld joints have been studied. Creep tests were carried out at 923 K in the stress range of 140 to 80 MPa. Specimens were examined for particle coarsening using field-emission scanning electron microscopy, and the boron content in the precipitates was estimated using field-emission auger electron spectroscopy (FE-AES). The grain size of the parent metal and the heat-affected zone (HAZ) were estimated using electron backscattered pattern (EBSP) imaging. Results showed that the creep properties of the steels with 90 and 130 ppm boron and of their weld joints are superior to those of the P92 steels and its weld joints. Further, no weld joints exhibited type IV cracking. No significant coarsening of the carbides was observed, not only in the parent metal but also in the HAZ of the steels with ≥90 ppm of boron. In addition to the delay in carbide coarsening, the large prior-austenite grain size of the parent metal and the absence of a conventional fine-grained HAZ (FGHAZ) in the weld joints also seem to have a beneficial effect on improving the creep properties of these steels and their weld joints.