The effect of tempering temperature on microstructure and mechanical properties was studied in a low-nitrogen, high-boron, 9%Cr steel. After normalizing and low-temperature tempering, cementite platelets precipitated within the martensitic matrix. This phase transformation has no distinct effect on mechanical properties. After tempering at 500 °C, M23C6 carbides appeared in the form of layers and particles with irregular shapes along the high-angle boundaries. Approximately, 6% of the retained austenite was observed after normalizing, which reduced to 2% after tempering at 550 °C. This is accompanied by reduction in toughness from 40 J/cm2 to 8.5 J/cm2. Further increase of the tempering temperature led to spheroidization and coagulation of M23C6 particles that is followed by a significant increase in toughness to 250 J/cm2 at 750 °C. Three-phase separations of M(C,N) carbonitrides to particles enriched with V, Nb and Ti were detected after high-temperature tempering.

/pdf/tempering-behavior-of-a-low-nitrogen-boron-added-9-cr-steel-1idx135e3r.pdf

Tempering behavior of a low nitrogen boron-added 9%Cr steel

A laser welding process using a 30-kW fiber laser with scanning mode optics is investigated in the paper. Welding is conducted in two ways: constant laser beam trajectory and wobbling trajectory with the use of lower speed and power. The main goal was to investigate the influence of the second wobbling laser welding pass on microstructure and mechanical properties of structural steel. The following parameters were monitored: visual control and mechanical properties (microhardness, three-point bend, and Charpy impact V-notch test); metallographic analysis and 2D and 3D computer tomography (CT) were also done. The results show that after the second welding pass, with wobbling trajectory of laser beam, middle and cap parts of the seam have a lower microhardness, in relation to the root part. It can be explained by annealing influence of the second wobbling pass at weld metal.

The effect of laser beam wobbling mode in welding process for structural steels

The fatigue crack growth behavior and the related microstructure of the dissimilar welded joint made from advanced 9Cr and CrMoV steels were systematically investigated in this paper. The dissimilar materials of 9Cr and CrMoV with heavy section were welded by narrow-gap submerged arc welding (NG-SAW) via multi-layer and multi-pass technology. The fatigue crack growth behavior was comparatively studied for different regions including advanced 9Cr and CrMoV base metals (9Cr-BM, CrMoV-BM), heat affected zones of both sides (9Cr-HAZ, CrMoV-HAZ) and weld metal (WM). The results indicated that a lower fatigue crack growth rate ( da / dN ) as well as a higher fatigue crack growth threshold (Δ K th ) for WM was gained compared to the BMs and HAZs, while a larger da / dN and lower Δ K th for 9Cr-HAZ were obtained under the same stress ratio. The WM showed a better fatigue crack growth resistance than others, while the 9Cr-HAZ with narrow size became the weaker part of the whole welded joint. Microstructure was revealed that the different microstructure character such as tempered martensite and tempered bainite could account for this diversity in properties. In addition, the observation of fatigue crack growth path displayed that the obstacles of coarse columnar grains and the ductile equiaxed grains in WM caused the higher resistance of fatigue crack growth, while the tempered martensite with and without lath structure led to the higher and lower fatigue crack growth resistance for 9Cr-BM and 9Cr-HAZ, respectively. However, the fatigue crack growth behavior of 9Cr-HAZ would not great influence the whole welded joint for its narrow size.

Microstructure correlation and fatigue crack growth behavior in dissimilar 9Cr/CrMoV welded joint

In this paper, in-situ SEM images with both low and high resolution near fusion line of a Cr-Ni-Mo-V steel welded joint were taken during interrupted monotonic tensile tests, with which macro- and micro-strain were measured based on digital image correlation technique and correlated with microstructures. An error analysis of strain measurement indicated sufficient resolution for image correlation with intrinsic microstructures as speckles. Results from digital image correlation showed that base metal, heat-affected zone and weld metal of the welds exhibited different responses to external tensile loading with mismatched strain distribution. A transfer of strain localisation from tempered martensites at base metal to lathy bainites at weld metal was observed around the fusion zone, due to varied hardening capabilities among non-homogeneous microstructures. Micro-strain could be several times higher than macro-strain at the same strained area, implying severe non-uniform deformation at microstructural scale. The most straining zone around the fusion line was found to be localized at blocks of soft δ-ferrites.

Correlation of local strain with microstructures around fusion zone of a Cr-Ni-Mo-V steel welded joint

Advanced 9% Cr and CrMoV steels chosen as candidate materials are first welded by narrow-gap submerged arc welding (NG-SAW) to fabricate the heavy section rotor. The present work focuses on studying the high-cycle fatigue (HCF) behavior of advanced 9% Cr/CrMoV dissimilarly welded joint at different temperatures. Conditional fatigue strength of this dissimilarly welded joint was obtained by HCF tests at room temperature (RT), 400 °C and 470 °C. It was observed that the failure occurred at the side of CrMoV base metal (BM), weld metal (WM) and heat affected zone (HAZ) of CrMoV side over 5×107 cycles for the specimens tested at RT, 400 °C and 470 °C. The detailed microstructures of BMs, WMs and HAZs as well as fracture appearance were observed by optical microscopy (OM) and scanning electron microscopy (SEM). Precipitation and aggregation of carbides along the grain boundaries were clearly detected with the increase of temperature, which brought a negative effect on the fatigue properties. It is interesting to note that the inclusion size leading to crack initiation became smaller for the HCF test at higher temperature. Therefore, reduction in the inclusion size in a welded joint helps to improve the HCF performance at high temperature.

Microstructure characteristics and temperature-dependent high cycle fatigue behavior of advanced 9% Cr/CrMoV dissimilarly welded joint

The invention discloses a method for measuring the fracture toughness J of each micro area of a heat affected zone of a welded joint, which is applicable to measurement on fracture toughness of various welded joints. According to the method, by forming a V-shaped groove in a coarse grain zone or a fine grain zone in the welded heat affected zone on a three-point bending test sample, the direction of crack development is effectively controlled to ensure the crack development in the coarse grain zone or the fine grain zone in the heat affected zone, and thus the fracture toughness of the different micro areas in the heat affected zone of the welded joint can be measured. The method is simple, is low in cost, ensures real and reliable measurement results, and has great significance for evaluating the mechanical property of each micro area of the welding joint, confirming a weak section of the welded joint and studying the fracture toughness of the heat affected zone of the welded joint.

Method for measuring fracture toughness J of heat affected zone of welded joint

The invention discloses a method for measuring the length of a heat-affected zone of 617 nickel-based alloy. The method comprises the steps that a metallographic specimen is prepared, and the length of the heat-affected zone of the 617 nickel-based alloy is measured by observing the metallographic specimen under a microscope; the boundary line of the heat-affected zone and a 617 nickel-based alloy basic material is determined by observing the morphologic changes of carbides in a near weld zone of a welding joint of the metallographic specimen through the microscope, that is to say, by combining the characteristic changes around the carbides, and then the distance between the boundary line and a fusion line is obtained, that is to say, the length of the heat-affected zone is obtained. According to the method, the problem that the length of the heat-affected zone of the 617 nickel-based alloy is difficult to measure is solved; meanwhile, a simpler and more accurate measurement method is provided, and help is provided for scientific research in the field.

Method for measuring length of heat-affected zone of 617 nickel-based alloy

Dramatically enhanced impact toughness in welded 10%Cr rotor steel by high temperature post-weld heat treatment

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Papers

Microstructure characteristics and temperature-dependent high cycle fatigue behavior of advanced 9% Cr/CrMoV dissimilarly welded joint

Method for measuring fracture toughness J of heat affected zone of welded joint

Method for measuring length of heat-affected zone of 617 nickel-based alloy

Dramatically enhanced impact toughness in welded 10%Cr rotor steel by high temperature post-weld heat treatment