Study of creep rupture strength in heat affected zone of 9Cr‐1Mo‐V‐Nb‐N steel by welding thermal cycle simulation
TL;DR: In this paper, the authors describe an investigation of the creep rupture strength (CRS) of 9Cr•1Mo•V•Nb•N steel welded joints by welding thermal cycle simulation.
Abstract: Summary This paper describes an investigation of the creep rupture strength (CRS) of 9Cr‐1Mo‐V‐Nb‐N steel welded joints by welding thermal cycle simulation. The reduction in the CRS of the welded joints is successfully reproduced, and the reasons for this reduction are discussed from the perspective of fine precipitates in the matrix. The CRS of the simulated HAZ (heat affected zone) varies with the peak temperature of welding thermal cycle simulation (PT). It falls sharply beyond the Ac1 temperature, gradually reaching the minimum at the Ac3 temperature of 925 °C. At any higher temperature than Ac3, it rises steadily, showing almost the same CRS as the base metal at 1100°C. The weakest CRS of the simulated HAZ lies at the lowest limit of the CRS data band of various welded joints prepared by GTA (TIG), MMA, and submerged‐arc (SA) welding with the same base metal. The simulated HAZ is useful for evaluation and analysis of the CRS reduction of welded joints. Heating to the Ac3 temperature by welding change...
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TL;DR: In this article, the authors conducted long-term creep rupture tests for 9Cr-1Mo-V-Nb (P91) steel welded joints and examined the relationship between microstructural changes and crack nucleation site and propagation path.
165 citations
TL;DR: In this article, microstructure evolution in P91 steel and their weldments are reviewed in as-virgin and heat treatment and creep exposure condition, and the role of grain coarsening, Cr/Fe ratio, lath widening and dislocation density on creep rupture life of base metal and weldments is discussed.
155 citations
TL;DR: In this paper, 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, leading to premature failure of the weld joint in the intercritical region of the HAZ, commonly known as type IV cracking.
Abstract: 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.
153 citations
TL;DR: In this article, a plate of modified 9Cr-1Mo steel was welded with three different filler materials: standard 9Cr 1Mo, modified 9 Cr 1Mo and nickel-base alloy Inconel 182 post-weld heat treatment (PWHT) for periods of 2 and 6 hours.
Abstract: Modified 9Cr-1Mo steel finds increasing application in power plant construction because of its excellent high-temperature properties While it has been shown to be weldable and resistant to all types of cracking in the weld metal and heat-affected zone (HAZ), the achievement of optimum weld metal properties has often caused concern The design of appropriate welding consumables is important in this regard In the present work, plates of modified 9Cr-1Mo steel were welded with three different filler materials: standard 9Cr-1Mo steel, modified 9Cr-1Mo, and nickel-base alloy Inconel 182 Post-weld heat treatment (PWHT) was carried out at 730 and 760 °C for periods of 2 and 6 h The joints were characterized in detail by metallography Hardness, tensile properties, and Charpy toughness were evaluated Among the three filler materials used, although Inconel 182 resulted in high weld metal toughness, the strength properties were too low Between modified and standard 9Cr-1Mo, the former led to superior hardness and strength in all conditions However, with modified 9Cr-1Mo, fusion zone toughness was low and an acceptable value could be obtained only after PWHT for 6 h at 760 °C The relatively poor toughness was correlated to the occurrence of local regions of untransformed ferrite in the microstructure
75 citations
TL;DR: In this article, the type-IV cracking susceptibility in 2.25Cr-1Mo, 9Cr- 1Mo, and 9Cr 1MoVNb ferritic steel weld joint has been assessed.
Abstract: Relative type-IV cracking susceptibility in 2.25Cr-1Mo, 9Cr-1Mo, and 9Cr-1MoVNb ferritic steel weld joint has been assessed. The type-IV cracking was manifested as preferential accumulation of creep deformation and cavitation in the relatively soft intercritical region of heat affected zone of the weld joint. The type-IV cracking susceptibility has been defined as the reduction in creep-rupture strength of weld joint compared to its base metal. The 2.25Cr-1Mo steel exhibited more susceptibility to type-IV cracking at relatively lower temperatures; whereas, at higher temperatures, 9Cr-1MoVNb steel was more susceptible. The relative susceptibility to type-IV cracking in the weld joint of the Cr-Mo steels has been rationalized on the basis of creep-strengthening mechanisms operating in the steels and their venerability to change on intercritical heating during weld thermal cycle, subsequent postweld heat treatment, and creep exposure.
63 citations
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TL;DR: In this article, a theory of the yield stress of dispersion-hardened single crystals as a function of temperature and strain rate is described, and a computer program which simulates a dislocation going through a random array of point obstacles some of which are by-passed by climb is described.
Abstract: A theory of the yield stress of dispersion-hardened single crystals as a function of temperature and strain rate is described. This theory assumes that the dislocations remain on their slip planes, except at some particles which the dislocations by-pass by climbing locally over these particles. The number of particles by-passed by local climb depends on the strain rate and temperature. A computer programme which simulates a dislocation going through a random array of point obstacles some of which are by-passed by climb is described. The relation between yield stress, temperature and strain rate obtained using this programme agrees reasonably well with experiment. The most important result of the calculations is that the dislocation velocity can depend exponentially on the applied stress even though the time required for the dislocation to overcome an individual obstacle depends only weakly on the stress.
203 citations
TL;DR: In this article, the morphology of the complex carbonitride of Nb and V in 9% Cr ferritic steels was investigated and the role of the carbonitrate on the creep strength was discussed.
Abstract: It has been considered that small additions of Nb and V have striking effects on the creep strength of high Cr ferritic steels which are used for elevated temperature services such as boilers, steam generators etc. Although Nb and V are thought to form complex precipitates which may act as obstacles for the dislocation glide, the distribution and morphology of the precipitates have not been clarified yet. Several examples of simple precipitates of V in low alloy steels were only reported. In this paper, the morphology of the complex carbonitride of Nb and V in 9% Cr ferritic steels was investigated and the role of the carbonitride on the creep strength was discussed.
34 citations
TL;DR: In this paper, the effects of nitrogen contents and tempering treatments on hardness and creep rupture properties of 9Cr-1Mo-V-Nb steel welded joints change with chemical compositions and heat treatments.
Abstract: Synopsis : Creep rupture properties of 9Cr-1Mo-V-Nb steel welded joints change with chemical compositions and heat treatments. Therefore, simulated HAZ heating below or over Act temperature of 830•Ž was conducted in order to investigate microstructural changes of welded joints of the steel. Effects of nitrogen contents and tempering treatments on hardness and creep rupture properties were discussed. The results are summarized as follows. ( 1 ) Hardness and creep rupture strength of the steel heated at 850•Ž ( simulated HAZ heating) decreased significantly after post-weld heating at 740•Ž for 5 h. It was confirmed that recovery and softening of martensitic structure were accelerated by partially ƒÁ-transformed heating just over Ac1 temperature. (2) On the above simulated HAZ heating, finely dispersed Vanadium Nitride ( VN) , which increased in amount by addition of nitrogen, was observed. These precipitates were confirmed to retard recovery, and to improve creep rupture strength effectively. (3) In case of relatively low temperature tempering at 620•Ž, excess free carbon and nitrogen precipitated as fine VN and M23C6 during simulated HAZ heating and post-weld heat treatment. These precipitates were confirmed to improve creep rupture strength.
23 citations
01 Jan 1990
TL;DR: In this paper, the authors describe new alloys for pressure vessels and piping applications, such as Ni-Cr-W-Mo alloys and Ni-Ni-Si alloys, and HAZ liquation cracking in lean 316 stainless steels.
Abstract: This book describes new alloys for pressure vessels and piping applications. Topics include: Cr-Mo-Si alloys, HAZ liquation cracking in lean 316 stainless steels, copper bearing stainless steels, and Ni-Cr-W-Mo alloys.
16 citations