9Cr-1Mo Steel Weld Research Articles

Effect of thickness on mechanical properties of modified 9Cr 1Mo steel welds made by narrow gap hot wire gas tungsten arc welding process

Hot wire Gas Tungsten Arc Welding (GTAW) process is selected for fabrication of thick section modified 9Cr 1Mo steel components of Steam Generators (SGs) for 500MWe Prototype Fast Breeder Reactor (PFBR) at Kalpakkam, India. During the welding procedure qualification for fabrication of SGs, the 30 mm and 62 mm thick modified 9Cr 1Mo steel welds were subjected to chemical analysis, visual examination, liquid penetrant examination, radiography examination, micro-hardness tests, longitudinal and transverse tensile tests (at ambient temperature and 525 °C), bend tests, drop weight tests and impact tests (at 0 °C and +18 °C). Literature reveals both positive and negative view on the welding of thick section steel components. Limited information is available in open literature on the effect of thickness on material properties of modified 9Cr 1Mo steel welds made by hot wire GTAW method. In view of the above, authors attempted to systematically investigate the influence of thickness on mechanical properties of modified 9Cr 1Mo steel welds made by hot wire GTAW process. In this paper, the chemical compositions, hardness, tensile strength, bending strength and impact properties of 30 mm and 62 mm thick welds produced by narrow gap hot wire GTAW process are characterized and compared. It is found that the mechanical properties of 30 mm and 62 mm thick welds are comparable except the impact properties at 0 °C. The impact strength at 0 °C for 62 mm thick weld was found inferior in comparison with 30 mm thick weld.

Tensile Deformation Study on Heat Affected Zone of Mod. 9Cr-1Mo Steel Weld

Tensile Deformation Study on Heat Affected Zone of Mod. 9Cr-1Mo Steel Weld

Microstructure anomaly upon high temperature exposure and its influence on the mechanical properties of a modified 9Cr-1Mo steel weld

The present work evaluates the microstructure stability of modified 9Cr-1Mo steel welds when subjected to high temperature exposure (HTE) at 775 °C for different times (5, 100, 500, and 1000 h). Prior to HTE, a post weld heat treatment (PWHT) at 760 °C for 2 h was performed. Upon HTE, the individual weld zones viz., weld metal (WM), coarse-grain heat affected zone (CGHAZ) and fine-grain heat affected zone (FGHAZ) demonstrate distinct microstructural variations in terms of precipitate size, grain size and substructure (lath/block/packet boundaries) fractions. The least- and the highest-microstructure stability are respectively seen in FGHAZ and WM after HTE. A detailed investigation involving electron backscattered diffraction (EBSD) and high-resolution transmission electron microscopy (HR-TEM) confirm that a pronounced recovery of the substructure in FGHAZ happens due to enhanced diffusion along the higher fraction of prior-austenite grain boundaries (PAGBs), leading to M23C6 precipitate coarsening and substantial grain growth. The microstructure degradation upon HTE for 1000 h had eventually resulted in deterioration of mechanical properties, where the hardness reduced the most for FGHAZ (~34%), while the least for WM (~7%). Similarly, after HTE, the tensile strength and toughness had deteriorated by ~67% and ~57%, respectively, from the PWHT condition. Fractography further confirms the pronounced degradation after HTE as they result in failure from FGHAZ.

Impact properties of modified 9Cr 1Mo steel welds: Comparison between cold wire and hot wire gas tungsten arc welding processes

Modified 9Cr 1Mo steel is used for the manufacture of Steam Generators (SG) for Prototype Fast Breeder Reactor at Kalpakkam, India. The hot wire Gas Tungsten Arc Welding (GTAW) process was selected for the manufacture of SGs wherever feasible in addition to the conventional cold wire GTAW process due to various advantages. Drop weight tests on modified 9Cr 1Mo steel welds produced by hot wire and cold wire GTAW processes were carried out in accordance with ASTM E208. It is found that the Nil Ductility Transition (NDT) behavior of the hot wire and cold wire GTA welds were similar during drop weight tests performed up to −10 °C. Subsequently, Charpy V notch impact tests were carried out at +18 °C and 0 °C in accordance with ASME Boiler and Pressure Vessel Code. The impact strength at 0 °C of welds produced by cold wire GTAW process was found to be superior compared to the welds of hot wire GTAW process. Subsequently, the analysis of impact strength was carried out using a two-sided students t-test. With a 95% confidence level, the results of the t-test revealed that there is a genuine difference in impact strength at 0 °C between the welds of hot wire and cold wire GTAW processes.

Type IV creep crack initiation and propagation in mod.9Cr-1Mo steel welds

ABSTRACT The ratio of crack initiation and crack propagation life is important from the viewpoint of residual life prediction of mod. 9Cr-1Mo steel welds under Type IV creep damage. Both the creep void density distribution and the failure process with crack initiation/propagation/final failure are predicted for three types of welds in pipes and nozzles, which are compared with experimental results at 650°C. The ratio of crack initiation and final failure life depends on weld type and loading conditions; the ratio is about 0.95 in the cases where redistribution of stress occurs only in the thickness; the ratio is 0.59 ~ 0.65 in the cases where redistribution of stress occurs in both the thickness and circumferential directions of the pipe. Influence of stress ratio (axial vs. circumferential stress) is also predicted for circumferential welds subject to internal pressure/axial load and is compared with experimental results at 650°C.

Characterization of heterogeneous creep deformation in vanadium-modified 2.25Cr1Mo steel weldments by digital image correlation

Low alloy ferritic steels are widely used to manufacture critical components that operating at elevated temperatures in the process industry, but a better understanding of the creep responses of their weldments are still needed to provide guidance for the design of high-temperature welded structures. In the present paper, the heterogeneous creep behavior of vanadium-modified 2.25Cr1Mo ferritic steel weldments was investigated by using the digital image correlation (DIC) technique. Three-dimensional plot of creep strain across the weld joint was constructed to intuitively represent the spatio-temporal evolution of creep strain for the base metal (BM), weld metal (WM) and the heat affected zone (HAZ). Combining the analysis of strain contour evolution during creep with the microstructural examination, it is found that creep strain was primarily concentrated in the HAZ, resulting in the creep cavitation at grain boundaries of the coarse-grain bainitic HAZ. In addition, BM exhibited a faster deformation rate than WM during creep. In order to quantitatively describe the heterogeneous creep deformation, a sub-region extensometer method based on the construction of virtual extensometers was proposed to obtain the representative creep strain data of individual regions of weldments. Furthermore, a parameter identification procedure combining a global optimization method (i.e. genetic algorithm) with an initial value determination scheme, was developed to determine the model parameters of BM, WM and HAZ for the hyperbolic-sine creep constitutive model. The results show that the heterogenous creep behavior of the V-mod 2.25Cr1Mo steel weldment can be well characterized by combining the sub-region extensometer method, the hyperbolic-sine creep constitutive model and its genetic algorithm-based parameter identification method.

Experimental investigation of creep crack growth behavior in the heat affected zone of boron added modified 9Cr–1Mo steel weld

Abstract This study aims to quantify the improvement in the creep crack growth (CCG) resistance in the heat-affected zone (HAZ) of boron added modified 9Cr–1Mo (P91B) steel weld at 898 K. The crack growth rates in the HAZ of P91B weld have been compared with the crack growth rates in HAZ of without boron P91 steel weld. The creep crack growth rates are estimated as per ASTM E1457 using 7 mm thick compact tension specimens with a notch in the HAZ and test duration 248h-11,000 h. The CCG rate ( a ˙ ) vs. C* correlation, commonly expressed as a ˙ = D ( C ∗ ) ϕ , has been established. In comparison, the crack growth rate in the HAZ of P91 weld is found to be threefold as compared to HAZ of P91B weld; with the difference in CCG rate increasing towards a higher C* range. Creep damage assessment study has been done in the vicinity of creep crack in HAZ of a P91B weld specimen tested for 11,000 h. The extent of cavitation, cavities link-up, and micro-crack formation in HAZ of P91B weld is found to be relatively lower than HAZ of P91 weld. Most of the cavities appear to be discrete in nature. This testifies improvement in the creep crack growth resistance in the HAZ of P91B weld due to controlled boron addition of about 100 ppm to modified 9Cr–1Mo steel.

Characterisation of hydrogen assisted cracking in modified 9Cr-1Mo steel welds using acoustic emission non destructive technique

ABSTRACT This study aims a systematic experimental investigation using acoustic emission (AE) non-destructive technique for online monitoring of hydrogen assisted cracking (HAC) in modified 9Cr-1Mo steel (P91 steel) welds during Gap-Bead on Plate (G-BOP) and implant tests. Welds made without preheating, with different preheating and combined pre and post heating were tested using G-BOP test. AE results of G-BOP tests have shown that time duration in which HAC active varies with temperature of preheating and combined pre and post heating. Reduction of AE activity in welds made with pre and post heating compared to those only made with preheating revealed a beneficial effect of the former in reducing HAC. In the case of implant tests, crack initiation and propagation are identified for different applied loads from the AE analysis. An attempt has been made to compare HAC during G-BOP tests and implant tests using AE frequency analysis. The dominant frequency of AE signals characteristic of HAC has been identified. This study shows the potential of using AE frequency analysis for online monitoring of hydrogen assisted cracking (HAC) in welds.

Effect of preheating and post heating in reducing diffusible hydrogen content and hydrogen assisted cracking susceptibility of modified 9Cr–1Mo steel

The hydrogen assisted cracking susceptibility of Modified 9Cr–1Mo steel weld is evaluated by implant test by determining lower critical stress for different preheating and combination of pre, post heating conditions. The diffusible hydrogen present in the implant specimens for different test conditions is estimated. Residual stress distribution in the weld for different heating condition is estimated using SYSWELD software. For a combination of pre + post heating at 200°C, the diffusible hydrogen content of the weld comes down to 1.17 from 4.7 mL/100g and the lower critical stress of the implant specimen increases from 250 to 370 MPa. Preheating + post heating also brings down the peak tensile residual stress level in the weld joints and lowers the cracking susceptibility.

Influence of Oxide Particles and Hardness on the Toughness of Modified 9Cr-1Mo Steel Weld Metals

The main welding processes used for modified 9Cr-1Mo steel for the main steam pipes in ultra-supercritical thermal power plants are shielded metal arc welding (SMAW), submerged arc welding (SAW), and gas tungsten arc welding (GTAW). However, welding is rarely performed with the understanding that the toughness of the weld metals differs depending on the welding process. We investigated the dominant factors that determine the toughness of weld metals for these welding processes with different post-weld heat treatment (PWHT) temperatures. Both SMAW and SAW weld metals with high oxygen content possessed low toughness in their as-welded state, with many oxide particles (MnSiO3) observed on fracture surfaces. In contrast, the GTAW metal did not have any oxide particles and exhibited high toughness. Therefore, the presence of oxide particles facilitated crack formation at the interface between the oxide particles and the matrix, leading to low toughness. The toughness of SMAW and SAW metals was improved by PWHT, which was accompanied by a reduction in the oxide particle density on the fracture surface. The hardness is decreased by a decrease in the carbon content in the matrix with a decrease in the solid solution strengthening of carbon by precipitation of carbides. This decrease was reflected by the lower impact force measured during the Charpy test on the interface between the oxide particles and the matrix. Therefore, the dominant factors determining the toughness are the density of the oxide particles and the hardness of the matrix. Carbides precipitated by PWHT did not affect toughness. From the above, the toughness properties of modified 9Cr-1Mo steel weld metals were clarified from a metallographic point of view.

Evaluation of hydrogen-assisted cracking susceptibility in modified 9cr-1mo steel welds

The hydrogen-assisted cracking susceptibility of modified 9Cr-1Mo steel weld is evaluated by Y-groove and gapped bead-on-plate test. It is found that a combination of pre + post heat is able to prevent cracking. However, the pre + post heat temperature combination determined from G-BOP test to prevent cracking (200 °C) is found to be higher than that in Y-groove test (100 °C). This is attributed to the higher weld restraint experienced in G-BOP than Y-groove test. For both the tests, the weld metal is found to be more susceptible to cracking than the heat affected zone. In addition to the diffusible hydrogen content of the welding consumables, diffusible hydrogen present in the G-BOP specimens prepared with different preheating and combinations of pre + post heat is also estimated by maintaining identical cooling conditions. The diffusible hydrogen in the weld for which no cracking is reported for G-BOP test (200 °C pre + post heat) is less than 1 ml/100 g. Considering the high restraint conditions present in the G-BOP test, it can be concluded that to prevent HAC in P91 steel, the choice of welding parameters (hydrogen content in welding consumable, heat input, pre/post-heating) should be such that diffusible hydrogen in the weld is brought down to below 1 ml/100 g.

Effect of PWHT Conditions on Toughness and Creep Rupture Strength in Modified 9Cr-1Mo Steel Welds

Abstract We clarified the effect of post weld heat treatment (PWHT) conditions on the toughness and creep rupture strength of modified 9Cr–1Mo steel weldments used for high temperature components of ultra-supercritical power plants. Fracture appearance transition temperature (FATT) decreased as PWHT temperature increased, and for all of the weld metals of tungsten inert gas welding, submerged arc welding and shielded metal arc welding, FATTs were lower than 293 K when the PWHT temperature was higher than 1,008 K. In contrast, in the uniaxial creep test with a loaded stress of 108 MPa, the creep rupture strength of the specimen on which PWHT was carried out for a holding time of 7.2 ks was significantly decreased when the PWHT temperature was more than 1,033 K. Therefore, the appropriate PWHT temperature range to maintain the toughness and creep fracture strength was 1,008 K ≤ T ≤ 1,033 K.

Zone-wise investigation of creep behaviour 9Cr–1Mo steel weld joints

ABSTRACTDistinct regions such as weld metal, heat-affected zone (HAZ) and base metal of P9 steel weld joints fabricated by various welding processes were investigated using impression creep testing. Smaller prior austenitic grain size, lower density of precipitates and dislocations resulted in faster recovery and higher creep rate of HAZ in comparison to the weld and base metal. Compared to base metal, shielded metal arc weld (SMAW) and activated tungsten inert gas (A-TIG) weld of the P9 steel weld joints exhibited better resistance to creep and displayed higher activation energy due to their coarser prior austenite grain size. A-TIG HAZ exhibited superior creep properties compared to the SMAW and TIG HAZ due to the presence of higher number density of precipitates.

Results of tensile, hardness and bend tests of modified 9Cr 1Mo steel welds: Comparison between cold wire and hot wire gas tungsten arc welding (GTAW) processes

Hot wire Gas Tungsten Arc Welding (GTAW) process is considered in addition to conventional cold wire GTAW process for the manufacture of Steam Generators (SG) for India's Prototype Fast Breeder Reactor (PFBR). During manufacture, it is an obligatory requirement to weld one test coupon for every 20 m of weld length on the actual SG for ascertaining the properties during fabrication. Weld pads thus prepared were used for characterization of hardness, bend and tensile properties of modified 9Cr 1Mo steel welds produced by hot wire and cold wire GTAW processes and the test results were compared. The results revealed that hardness, bend and tensile properties of modified 9Cr 1Mo steel welds of hot wire and cold wire GTAW processes are similar and no genuine difference exist between these two welding processes. Further, the advantage of high quality with excellent weld profile and high productivity has been realized using hot wire GTAW process.

Influence of joint thickness on Type IV cracking behaviour of modified 9Cr-1Mo steel weld joint

ABSTRACTEffect of joint thickness on Type IV cracking behaviour of modified 9Cr-1Mo steel weld joint has been investigated. Creep tests on multi-pass double-V cross weld joint flat specimens of the steel having thicknesses in the range 1–17 mm have been carried out at 923 K (650°C) and 50 MPa stress. Creep rupture life of the weld joint was found to increase with thickness and reached a maximum value around 10 mm of thickness followed by decrease with further increase in thickness. Failure in the weld joints occurred in the soft intercritical region of the heat-affected zone (HAZ). Creep strain localisation was observed at the fractured location and was more in the thinner weld joints than in the thicker weld joint. Creep cavitation in the intercritical region of HAZ close to the unaffected base metal was more extensive at the mid-location of the weld pass, where the HAZ width was relatively larger and hardness was lowest. The type IV cavitation in intercritical HAZ was more extensive in thicker joint, whereas creep strain concentration in the intercritical HAZ was more in thinner weld joint. Creep cavitation in the joint was more pronounced at near mid-thickness locations than those beneath the specimen surface. Joints of intermediate thickness possessed higher creep rupture life because of relatively less accumulation of creep deformation coupled with lower creep cavitation in the intercritical region of HAZ.

Characterizations of the Microstructure of 9Cr-1Mo Steel Weld Joint After Long-Term Service in a Supercritical Fossil Power Plant

In the present work, the microstructure, micro-hardness, and chemical composition of 9Cr-1Mo weld joint after long-term service in a supercritical power plant were analyzed by using various analytical techniques. The hardness profile showed a maximum at the fusion boundary and a minimum in the region of the fine grain heat-affected zone (FGHAZ). Many cavities along the grain boundaries and in grains were also observed in the FGHAZ. Ferrite zones with lower hardness and larger grain sizes were observed in the weld metal. The formation mechanisms of the cavities and ferrite zones were discussed based on the results.

An improvement in creep strength of thermo-mechanical treated modified 9Cr-1Mo steel weld joint

Modified 9Cr-1Mo steel weld joints generally experience the type IV premature failure in the intercritical region (ICR) of HAZ under long term creep exposure at high temperature. Possibility of improving the resistance of this joint to type IV cracking through thermo-mechanical treatment (TMT) of the steel has been explored. Weld joints have been fabricated from the TMT and conventional normalized and tempered (NT) steels using electron beam (EB) welding process. Creep tests have been carried out on NT and TMT steels joint at 923 K (650°C) and 110–100 MPa applied stress. Creep rupture life of the TMT weld joint was significantly higher than the NT steel weld joint. Significant variations of microstructural constituents such as M23C6 precipitate; lath structure and hardness across the joint have been examined in both the joints. The coarser M23C6 precipitate and lath, and subgrain formation in the ICR resulted in the soft zone formation and was predominant in the ICR of NT steel joint. The enhanced MX precipitation through TMT processing and reduction in coarsening of M23C6 precipitate under thermal cycle resulted in improved creep rupture strength of TMT steel weld joint.

Effect of Welding Processes on the Microstructure, Mechanical Properties and Residual Stresses of Plain 9Cr-1Mo Steel Weld Joints

9Cr-1Mo steel designated as P9 is widely used in the construction of power plants and high-temperature applications. It is chosen for fabricating hexcan fuel subassembly wrapper components of fast breeder reactors. Arc welding processes are generally used for fabricating 9Cr-1Mo steel weld joints. A-TIG welding process is increasingly being adopted by the industries. In the present study, shielded metal arc (SMA), tungsten inert gas (TIG) and A-TIG welding processes are used for fabricating the 9Cr-1Mo steel weld joints of 10 mm thickness. Effect of the above welding processes on the microstructure evolution, mechanical properties and residual stresses of the weld joints has been studied in detail. All the three weld joints exhibited comparable strength and ductility values. 9Cr-1Mo steel weld joint fabricated by SMAW process exhibited lower impact toughness values caused by coarser grain size and inclusions. 9Cr-1Mo steel weld joint fabricated by TIG welding exhibited higher toughness due to finer grain size, while the weld joint fabricated by A-TIG welding process exhibited adequate toughness values. SMA steel weld joint exhibited compressive residual stresses in the weld metal and HAZ, while TIG and A-TIG weld joint exhibited tensile residual stresses in the weld metal and HAZ.

Evaluation of strength property variations across 9Cr-1Mo steel weld joints using automated ball indentation (ABI) technique

The variations of strength properties across 9Cr-1Mo steel weld joints fabricated by different arc welding processes such as shielded metal arc welding (SMAW), tungsten inert gas (TIG) and activated tungsten inert gas (A-TIG) have been evaluated employing automatic ball indentation (ABI) technique. ABI tests were conducted at 298K across various zones of the weld joints comprising of base metal, weld metal, heat affected zone (HAZ) and intercritical HAZ (ICHAZ) regions. The flow curves obtained from ABI tests were correlated with corresponding conventional tensile test results. In general, the tensile strength decreased systematically across the weld joint from weld metal to base metal. Inter critical HAZ exhibited the least strength implying that it is the weakest zone. The incomplete phase transformation in the ICHAZ during weld thermal cycle caused the softening. The A-TIG weld metal exhibited higher UTS and strain hardening values due to higher carbon in the martensite. The strain hardening exponent exhibited only slight variation across the various regions of the weld joints. A-TIG weld joint exhibited higher weld metal and HAZ strength, marginally higher UTS to YS ratio in the weld metal and HAZ compared to that of the other two processes. Hence, among the three welding processes chosen, A-TIG welding process is found to be superior in producing a 9Cr-1Mo steel weld joint with better strength properties.

Influence of electron beam welding parameters on microstructure and Charpy impact properties of boron-added modified 9Cr-1Mo steel weld

The effect of electron beam welding parameters on microstructure and ductile-to-brittle transition temperature (DBTT) of a boron-added modified 9Cr-1Mo steel weld is presented in this paper. Compared to the base metal, for the weld, the upper shelf energy is lower and the DBTT is significantly lower. While the influence of welding parameters on the upper shelf energy of the weld is insignificant, its influence on the transition region and the lower shelf region is quite significant due to the presence of delta-ferrite in the weld. High welding speed reduces the time available for transformation of delta-ferrite to austenite and results in retention of delta-ferrite in weld. Lower welding speed promotes completion of delta-ferrite to austenite transformation as cooling rate reduces which improves lower shelf energy.