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Part 1: Studies Of Local Differences In Material Creep Properties On Weldments: Finite Element Case Study of Local Stress Effects in Long Seam Welded Piping

Creep damage in weldments has led to loss of life, property, operating time and, of course, profit. Only relatively recently has it been appreciated that the life expectancy of a weld may be a small fraction of that of the base metals being joined. The problem is not an inherent deficiency in the weld metal. Rather it is the local interaction of metallurgical structures with differing creep properties that leads to stress intensification and associated increases in the triaxial or hydrostatic stresses in the neighborhood of the joint. Hydrostatic tensions serve to reduce the creep rates of less creep resistant areas but accelerate the rate of cavity growth. The failures and damage reported in service show cavitation, microcracking and then, rapid macrocracking. Furthermore, it has been concluded from postmortems that creep damage initiates primarily at subsurface locations, and in long seam welded piping the creep damage progresses to an advanced state without any apparent OD surface evidence.

The Pressure Vessel Research Council (PVRC) of WRC and its sister organization, The Materials Properties Council (MPC), have responded to the concerns of industry with a variety of studies ranging from NDE performance demonstration, to development of life assessment tools. One of these tools consisted of the software discussed herein. The software incorporated estimates of the stress intensification associated with local variations in weld deposit or heat affected zone creep properties and with geometrical factors such as weld joint geometry and pipe peaking. When combined with the Omega creep damage model, also developed under the auspices of MPC, and a cavity growth algorithm, the software explained quantitatively the observed reductions on operating life for both normalized and subcritically heat treated weldments.

Failures of the latter group are often described as Type IV failures, and have only recently been observed in the United States. The significant creep rate mismatch expected in the heat affected zones of high strength ferritic steels increases the tendency towards such failures as described in the two papers. The work by Lundin et al represents a first step toward systematically gathering data needed to improve the analytical models. It takes advantage of a novel, long "singular" simulated heat affected zone specimen that facilitates measurement of creep rates on other aspects of creep performance of microstructures of interest. The works reported here should be viewed as the first systematic steps aimed at developing tools to be used for design and life assessment and even alloy development or optimization.

Part 2: Studies Of Local Differences In Material Creep Properties On Weldments: Determination of Creep Behavior of Singular HAZ Regions to Model the Behavior of the Entire HAZ of Cr-Mo Steels in Elevated Temp Service

Creep behavior of the singular CG, FG, IC HAZ regions and base metal for 1.25Cr-0.5Mo (ASME T/P11), modified 9Cr-1Mo (ASME T/P91), NF616 (ASME T/P92), and HCM12A (ASME T/P122) steels was investigated. Creep tests were implemented for Gleeble simulated singular HAZ regions (CG, FG, IC HAZs) and base metals. It was found that the creep rates of the FGHAZ and ICHAZ regions are significantly greater than those of the CGHAZ and base metal for 1.25Cr-0.5Mo (ASME T/P11), modified 9Cr-1Mo (ASME T/P91), NF616 (ASME T/P92), and HCM12A (ASME T/P122) steels.

Metallorgraphic assessment was implemented on the creep rupture specimens for the modified 9Cr-1Mo FGHAZ and ICHAZ. Profiles of the cavity size, cavity density, and the reduction in area were determined from the rupture surface to the end of the gage length. Microstructural characterization and hardness measurements were accomplished for the CG, FG, IC HAZs and base metal (modified 9Cr-1Mo) in the PWHTed, N&T and long term ages conditions. Carbide evolution in the CG, FG, IC HAZs and the base metal upon PWHT was studied using TEM and EDS.

The microstructure of the CG, FG, IC HAZs and base metal of 1.25Cr-0.5Mo steel was investigated in both the as simulated and creep tested condition. Carbide morphology in each HAZ region and base metal was examined using SEM. In addition, the microstructure the CG, FG, IC HAZs and base metals of NF616 and HCM12A were examined. The grain size and microhardness were determined for the CG, FG, IC HAZs and base metals of NF616 and HCM12A.

In accord with creep testing results and metallographic evaluation, a mechanism of Type IV creep damage in terms of the increased creep rate for the FG and IC HAZs is proposed for the tested materials. See more