Kent Coleman

EPRI Guidelines for Fabrication of Components Manufactured From Grade 91 Steel

Over the last 10 years EPRI has been researching critical information on the factors affecting the performance of creep strength enhanced ferritic steels in general and Grade 91 steel in particular. This work has resulted in a major new report which provides recommended guidelines for fabrication and the associated quality assurance to ensure that component properties meet or exceed the minimum expectations of ASME design approaches. The present paper outlines the recommendations in the report and provides technical background for specific aspects of the guide.Copyright

Assessment of the Tempering Behavior of Grade 91 Steel

At any point in the fabrication and installation of Grade 91 components, undesirable metallurgical conditions can develop as a result of incorrect heat treatment. Industry techniques for locating and evaluating these microstructures using nondestructive methods are necessary. Field hardness testing is commonly utilized to characterize Grade 91 steel installed in plants. However, it is apparent that there are numerous variables that can affect the results of field hardness testing. Thus, the results can vary significantly depending on the equipment and procedures used. The present study has been undertaken to identify the accuracy of hardness measurements made under controlled laboratory conditions. Measurements were performed on samples that had been subjected to a range of different tempering conditions. The trend in these data provides information relevant to understanding the metallurgical behavior of this steel. Comparison of the data also indicates how measurements made using different hardness testers and methods can be used to assess component condition.

Sub-Surface Cracking in Circumferential Seam Welds: The New Face of an Old Problem? Results of the Metallurgical Analysis of Damaged Welds

Samples removed from several cracked piping and header circumferential seam welds have been examined destructively as part of an effort to understand why in the last few years there have been an increasing number of such seam weld failures at US power plants. To assist in this effort the information available regarding the design and operating history of a number of those welds has been evaluated where it was available; in addition, a limited review has been made of literature pertaining to cracking at seam welds in elevated temperature piping and headers. Certain features of the failures, such as the fact that the cracks in many cases have initiated sub-surface and that the vast majority of the cracks have been detected in shop-fabricated welds, have raised concerns that these failures represent a new and unique damage mechanism for which no satisfactory technical explanation currently exists. The information obtained to date does not support those concerns. Instead, the results of the destructive analyses of damaged girth welds are consistent with the general understanding of failures at welds in pressure parts operating at elevated temperatures: that is, failure in such welds can occur at times far shorter than the expected rupture time for unaffected base metal for the following reasons: • Welds are heterogeneous structures consisting of multiple discrete regions, the properties of which can differ significantly. In some of these regions the elevated temperature strength and/or ductility of the material can be substantially inferior to the “average” base metal, so that under certain loading conditions the rate at which damage will accumulate in weaker, less ductile regions can be orders of magnitude higher than in the stronger, more ductile regions. • The stress state at circumferential seam welds in elevated temperature piping and headers can be a complex admixture of mechanical and thermally-induced stresses imposed on the metallurgically varied structure. The extensive experience with Type IV or Type IIIa cracking in the CrMoV piping in Europe has demonstrated that under the influence of “normal” operating loads premature failure at weakened regions of the weld are to be expected. That experience also has shown that, in the absence of a very high bending load related to failure of the component support, sub-surface cracking is not unexpected. • The damage in the welds examined as part of this study was in all cases concentrated in portions of the weld that previous research has indicated can be either creep weak and/or creep brittle; other areas of the weld showed no signs of visible damage.Copyright