9Cr-1Mo Weld Research Articles

Assessment of Impact Strength of Welds Produced by Cold Wire and Hot Wire Gas Tungsten Arc Welding (GTAW) Processes

Modified 9Cr 1Mo material is used as principal material of construction of Steam Generators (SG) for 500MWe Prototype Fast Breeder Nuclear Reactor (PFBR), at Kalpakkam, India. One of the major problems associated with welding of modified 9Cr 1Mo steel is poor impact strength of weld metal produced from the processes that involves the use of flux. Due to this reason, 100% Gas Tungsten Arc Welding (GTAW) process is chosen for fabrication of PFBR Steam Generators.The major limitation of conventional cold wire GTAW process is low deposition rate which makes it highly uneconomical especially for welding component with large section thickness. Therefore, hot wire GTAW process is selected for welding of PFBR Steam Generators wherever possible in addition to the conventional cold wire GTAW process. The impact properties of modified 9Cr 1Mo welds produced by hot wire GTAW process are compared with impact properties of cold wire GTAW welds. The impact strength of welds produced by hot wire GTAW process is found to be inferior compared to that of welds produced by cold wire GTAW process. Nevertheless, hot wire GTAW system has the benefit of high weld deposition rate with excellent profile of weld with reduced dilution and porosities

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.

Effect of Trace Yttrium Oxide on High and Low Temperature Properties of 9Cr-1Mo Weld Joint for Super Critical Unit

Abstract The effects of trace yttrium oxidation on the properties and the microstructures of 9Cr-1Mo steel welds for super critical generation units and USC units have been studied. Yttrium has been added into weld metal by coating transition (e. g. molten droplet method). The yttrium content for optimized properties and structures has been determined. Comparison tests including room temperature and high temperature tensile tests, serials impact test, metallographic analysis, diffusion hydrogen measuring, phase transformation point, SEM, XDR etc. have been performed on the welds with and without yttrium. It is found that second phase particles make an important contribution to improving the properties of the weld metal, especially low temperature impact performance.

Evaluation of post-weld heat treatment behavior in modified 9Cr–1Mo steel weldment by magnetic Barkhausen emission

The butt joint of two sight tubes of modified 9Cr–1Mo steel is welded by TIG technique. Subsequently, the post-weld heat treatment (PWHT) of the tubes has been carried out at 650, 700 and 750 °C temperatures for 3 h. Magnetic Barkhausen emission (MBE) measurements are performed in fusion zone, heat affected zone (HAZ) and base metal region of as-welded and all the post-weld heat treated samples. In each weldment, the variation of the MBE signal emanated from fusion zone, HAZ and base metal has been examined. It is found that the RMS voltage of MBE signal increases with PWHT temperature. The conversion of retained austenite to alpha ferrite during PWHT makes the fusion region as a softer zone in the weldment. The variation of magnetic parameters with different PWHT is correlated with hardness and microstructures. This study can be used as an effective tool in evaluation of PWHT parameters in modified 9Cr–1Mo weld joint.

Evaluation of Mechanical Properties of Modified 9Cr-1Mo Welds Produced by Narrow Gap Hot Wire and Cold Wire Gas Tungsten Arc Welding Processes for 500MWe PFBR Steam Generators

Modified 9Cr-1Mo material is selected as principal material of construction of Steam Generators (SG) for India's first of its kind 500MWe Prototype Fast Breeder Reactor (PFBR). The fabrication of PFBR Steam Generators involves welding of 12mm, 30mm and 90mm thick modified 9Cr-1Mo components which were carried out by combination of hot wire and cold wire Gas Tungsten Arc Welding processes. Hot wire Gas Tungsten Arc Welding (GTAW) is a relatively new process and limited machines/facilities are available in India for welding with this technique. The most important benefit from the use of a hot wire GTAW system is high weld deposition rate and reduced dilution & porosities from the weld deposits. Eventhough welds were meeting the specification requirements during welding procedure qualification, limited information available on hot wire GTAW process in open literature motivated authors for characterization of welds produced by this specialized technique. During welding procedure qualification, it was found that impact test results of weld metal produced by cold wire GTAW process is superior than weld metal produced by hot wire GTAW process. In light of this, detailed investigation & micro-structural analysis is carried out to find out the root cause for variation in impact properties. This paper details the systematic evaluation and interesting observations during characterization of microstructure & mechanical properties of modified 9Cr-1Mo welds produced by hot wire and cold wire GTAW processes.

Evaluation of Mechanical Properties of Modified 9Cr-1Mo Welds Produced by Narrow Gap Hot Wire and Cold Wire Gas Tungsten Arc Welding Processes for 500MWe PFBR Steam Generators

Modified 9Cr-1Mo material is selected as principal material of construction of Steam Generators (SG) for India's first of its kind 500MWe Prototype Fast Breeder Reactor (PFBR). The fabrication of PFBR Steam Generators involves welding of 12mm, 30mm and 90mm thick modified 9Cr-1Mo components which were carried out by combination of hot wire and cold wire Gas Tungsten Arc Welding processes. Hot wire Gas Tungsten Arc Welding (GTAW) is a relatively new process and limited machines/facilities are available in India for welding with this technique. The most important benefit from the use of a hot wire GTAW system is high weld deposition rate and reduced dilution & porosities from the weld deposits. Eventhough welds were meeting the specification requirements during welding procedure qualification, limited information available on hot wire GTAW process in open literature motivated authors for characterization of welds produced by this specialized technique. During welding procedure qualification, it was found that impact test results of weld metal produced by cold wire GTAW process is superior than weld metal produced by hot wire GTAW process. In light of this, detailed investigation & micro-structural analysis is carried out to find out the ischolar_main cause for variation in impact properties. This paper details the systematic evaluation and interesting observations during characterization of microstructure & mechanical properties of modified 9Cr-1Mo welds produced by hot wire and cold wire GTAW processes.

Comparison of Microstructure and Properties of Modified 9Cr-1Mo Welds Produced by Narrow Gap Hot Wire and Cold Wire Gas Tungsten Arc Welding Processes

Modified 9Cr-1Mo material is used as principal material of construction of Steam Generators (SG) for India's first of its kind 500MWe Prototype Fast Breeder Reactor (PFBR).The fabrication of Steam Generators is carried out by combination of hot wire and cold wire Gas Tungsten Arc Welding (GTAW) processes. Eventhough welds were meeting the specification requirements, limited information available on hot wire GTAW process in open literature motivated authors for characterization of welds produced using this specialized technique. The impact properties at 0°Cof modified 9Cr-1Mo welds produced by hot wire GTAW processis found inferior compared to welds produced by cold wire GTAW process. The properties such as tensile strength, hardness, impact toughness at +18°C were found almost comparable for the welds produced by both the welding processes. This paper characterizes the microstructure and various mechanical properties of modified 9Cr-1Mo welds produced by narrow gap hot wire and cold wire GTAWprocesses.

Creep Crack Growth Behavior of a P91 Steel Weld

Modified 9Cr-1Mo steel (P91) weld joints operating at elevated temperatures are well known to be prone to premature failure due to cracking in the heat affected zone because of the gradients in microstructure, popularly referred to as Type IV cracking. A campaign was undertaken to study the creep crack growth behaviour of modified 9Cr-1Mo weld joints. Creep crack growth (CCG) tests were carried out on compact tension (CT) specimens machined from P91 weld joints prepared using multipass shielded manual metal arc welding procedure. Specimens with two notch locations have been employed, (i) within the weld metal, between the centreline and the fusion line and (ii) in the heat affected zone. Constant load CCG tests were carried out at different applied loads at 798 and 898 K. The C* - da/dt correlations (da/dt =A C*m) were established for both notch locations. At 898 K, a higher A (0.064) and lower m (0.533) for the case with notch placed in the HAZ, compared to those for the case of notch in the weld (0.0399 and 0.75 respectively) were observed indicating the higher creep crack growth in HAZ, confirming type IV cracking. The difference increases at lower C* levels which correspond to long term behaviour. Heavy creep damage was observed in the HAZ region even for the sample with notch in the weld, whereas the weld metal regions showed relatively less damage. As the crack grew, a change in its course to follow the HAZ region was observed.

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.

Comparison of Creep Behavior of 2.25Cr-1.6W/Mod. 9Cr-1Mo Dissimilar Weld Joint with Each Matching Filler Metal

This paper describes the evaluation of the creep behavior with two types of matching filler metals for 2.25Cr-1.6W(T23)/Mod. 9Cr-1Mo(T91) dissimilar weld joint. Through the welding procedure qualification tests prior to the creep tests, optimum PWHT holding times at 745±5oC were determined as 30minutes for T23 matching filler metal and 60minutes for T91 matching filler metal. It was also confirmed that carbon migration across the weld interfaces and the softened area at HAZ occurred during PWHT. Creep rupture tests were carried out at 575-650oC and 70-220MPa for two dissimilar metal welds manufactured by an optimum PWHT condition. From the comparison of creep rupture strength, it was shown that the creep strength of the welded joint including T23 matching filler metal was similar to that of T23 base material and somewhat higher than that of the welded joint including T91 matching filler metal. From metallurgical study on the crept specimen, the creep damage of T23/T91 dissimilar weld joints could occur at T23 side IC HAZ and near the T23/T91 weld interface simultaneously. However, the final failure locations seemed to depend on the filler metal and the cause of this phenomenon might be regarded as the additional effect of carbon migration across the weld interfaces. It is thought that the dissimilar weld including T91 filler metal which failures at T23 side CG HAZ subject to both metallurgical notch and carbon migration become more susceptible to creep damage than that including T23 filler metal in which the carbon depletion occurs at the T23 filler metal with higher creep strength than T23 HAZ. T23 filler metal can therefore be proposed for matching filler metal of T23/T91 dissimilar weld joints

Mechanical properties of modified 9Cr–1Mo steel welds with notches

Notched specimens of a modified 9Cr–1Mo steel and its laser welds were extensively evaluated by various methods. Impact, notched tensile and fatigue crack growth tests were employed on specimens tempered at different temperatures. Notched tensile specimens were also tested in gaseous hydrogen, in addition to laboratory air. The results indicated that the deterioration in impact toughness of the weld metal was pronounced for tempering below 680 °C, in contrast to a narrow tempering range around 540 °C for the base metal. The coarse-grained weld metal was more susceptible to hydrogen embrittlement (HE) than the fine-grained base metal for specimens tempered at 250 and 540 °C as verified by the loss of notched tensile strength (NTS) in hydrogen. For specimens with low impact toughness and high NTS losses, the unstable crack growth which was characterized by quasi-cleavage on fracture surfaces and abrupt change in FCGRs was observed for Δ K beyond certain values. In general, the Paris gradient of the weld metal was steeper than that of the base metal with the same tempering treatment, but the discrepancy became insignificant for specimens tempered at 750 °C. For specimens tempered at 750 °C, not only the impact energy but also the HE resistance could be significantly increased. It was suggested that modified 9Cr–1Mo welds should be tempered in the neighborhood of 750 °C for improved mechanical properties.

Toughness and austenite stability of modified 9Cr–1Mo welds after tempering

The influence of tempering treatments on toughness and austenite stability was conducted on modified 9Cr–1Mo welds. Deteriorated impact toughness was noticed for the welds tempered between 450 and 610°C for 1 h, in which the peak hardness and the lowest impact energy occurred at 540°C. The impact toughness of the welds increased rapidly at tempering temperatures above 680°C, and the associated fracture surface exhibited mainly dimple fractures for 750°C/1 h tempered welds, in contrast to quasi-cleavage for 540°C/1 h welds. Retained austenite at martensite interlath boundaries was found to be mechanically stabilized by transformation stresses in the welds tempered at lower temperatures. It could also be transformed to untempered martensite upon cooling if the stresses were relieved to a certain degree by proper selection of temperature and time combinations in tempering. For welds tempered at higher temperatures, e.g. 750°C, the elimination of transformation stresses and the isothermal transformation of retained austenite to ferrite proceeded simultaneously. As a result, no martensitic transformation upon cooling could be found for such welds. The transformation of austenite to untempered martensite would certainly contribute to the minimum toughness of the weld after being tempered at 540°C/1 h.

A Comparison of Creep Rupture Behaviour of 2.25Cr-1Mo and 9Cr-1Mo Steels and Their Weld Joints

Creep rupture behaviour of 2.25Cr-1Mo and 9Cr-1Mo base metals and weld joints has been studied at 823 K over a stress range of 100-250 MPa. In the stress range examined, 2.25Cr-1Mo steel showed better creep rupture strength compared to 9Cr-1Mo steel. Both the weld joints displayed a creep rupture strength inferior to those of their respective base metals. Failure occurred in the respective intercritical region of the heat affected zone (HAZ) in both the steel weld joints. At a given applied stress, the difference in rupture life of base and weld joint was more pronounced in 2.25Cr-1Mo steel than in 9Cr-lMo steel and this difference increased with decrease in applied stress. As the presence of Mo 2 C precipitate imparts creep resistance to 2.25Cr-1Mo steel, its absence in the intercritical HAZ region of the joint is held responsible for its poor creep life. On the other hand, 9Cr-1Mo steel which is strengthened by solid solution hardening and dislocation substructure, is less prone to degradation in the intercritical region. Also, the presence of δ-ferrite in the coarse grain region of HAZ of 9Cr-1Mo steel reduces its grain growth tendency and also its susceptibility to intergranular creep cavitation compared to 2.25Cr-1Mo steel.

Dynamic fracture toughness properties of a 9Cr1Mo weld from instrumented impact and drop-weight tests

This paper reports the RT NDT and K Id results obtained from instrumented impact and drop-weight tests of 9Cr1Mo welds. RT NDT results for welds prepared using electrodes of diameter 2–5, 3–15 and 4 mm agree within 5K, the respective values being 264, 269 and 268K. For all the three welds, 269K can be taken as the conservative RT NDT. The procedures presented in this paper enable estimation of reasonably conservative values of K Id from instrumented impact test of unprecracked Charpy V-notch specimens. For the range of temperatures 193–303K, the lower bound K Id estimates obtained for the 9Cr1Mo welds are higher than the ASME K IR curve; hence, the ASME K IR curve will give conservative K Id values for the welds of this 9Cr1Mo steel in the above temperature range; at higher temperatures, the applicability of the ASME K IR curve to the welds of the present 9Cr1Mo steel needs verification/validation. Microcleavage fracture stress or cleavage fracture strength, σ f, estimated for the 9Cr1Mo welds is about 2160 MPa and is much lower than the 2550–2850 MPa reported for a normalised and tempered 9Cr1Mo plate material.

Creep Deformation and Rupture of 9Cr-1Mo and 2.25 Cr-1 Mo Weld Joints

Creep rupture behaviour of 2.25Cr-1Mo and 9Cr-1Mo base metals and weld joints has been studied at 823 K over a stress range of 100 - 250 MPa. In the stress range examined, 2.25Cr-1Mo showed better creep rupture strength than that of 9Cr-1Mo steel. Both the weld joints displayed inferior creep rupture strength than their respective base metals. Failures occurred in ths intercritical region of heat affected zobe (HAZ) for both the weld joints. At a given applied stress, the difference in rupture life of base and weld joint was more pronounced for 2.25Cr-1Mo than for 9Cr-1Mo steel and this difference increased with decrease in applied stress. As presence of MOj,C precipitates imparts creep resistance to 2.25 Cr-1 Mo steel, its absence in intercritical region of joints is responsible for poor creep life. On the other hand, 9Cr-1Mo, which is strengthened by solid solution hardening and dislocation substructure, is less prone to degradation due to heating in intercritical region. Also presence of 5-ferrite in the coarse-grained region of HAZ of 9Cr-1Mo reduces its grain growth tendency and also its susceptibility to intergranular creep cavitation compared to 2.25Cr-1Mo steel.