Doctoral Dissertations

Gang Zhou

Date of Award

8-1997

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Metallurgical Engineering

Major Professor

Carl D. Lundin

Committee Members

Charlie R. Brooks, Raymond A. Buchanan, David C. Joy

Abstract

A continuous occurrence of catastrophic failures, leaks and cracks of the Cr-Mo steam piping has created widespread utility concern for the integrity and serviceability of the seam-welded piping systems in power plants across USA. Cr-Mo steels are the materials widely used for elevated temperature service in fossil-fired generating stations. A large percentage of the power plant units with the Cr-Mo seam-welded steam piping have been in operation for a long duration such that the critical components of the units have been employed beyond the design life (30 or 40 years). This percentage will increase even more significantly in the near future. There is a strong desire to extend and thus there is a need to assess the remaining life of these units. Thus, understanding of the metallurgical causes for the failures and damage in the Cr-Mo seam-welded piping plays a major role in estimating possible life-extension and decision making on whether to operate, repair or replace.

In this study, an optical metallographic method and a Cryo-Crack fractographic method have been developed for characterization and quantification of the damage in seam-welded steam piping. More than 500 metallographic assessments, from more than 25 power plants, have been accomplished using the optical metallographic method, and more than 200 fractographic specimens from 10 power plants have been evaluated using the "Cryo-Crack" fractographic technique. For comparison, "virgin" SA welds were fabricated using the Mohave welding procedure with re-N&T Mohave base metal with both "acid" and "basic" fluxes. The damage mechanism, damage distribution pattern, damage classification, correlation of the damage with the microstructural features of these SA welds and the impurity segregation patterns have been determined. A physical model of cavitation (leading to failure) in Cr-Mo SA weld metals and evaluation methodologies for high energy piping are proposed.

The failures and damage in the seam-welded steam piping initiate and propagate by a creep-related mechanism, which is directly related to the SA welds regardless of PWHT. It has been found that preferential damage locations exist in seam-welded steam piping dependent on weld geometry and PWHT. Creep damage initiates heterogeneously at subsurface locations through entire wall thickness of the piping in either the weld metal or the HAZ. A through-wall damage gradient is often observed from a preferential damage location through other locations. Creep damage is typically cumulative at all locations.

Cavitation is the basic mechanism of creep damage in seam-welded piping. Cavity initiation, growth and coalescence dominants the creep behavior of the seam-welded piping, and no measurable plastic deformation accompanies cavity initiation and growth. Based on metallographic studies, cavitation is characterized by four stages; isolated cavities, aligned cavities, microcracks and macrocracks. In general, cavities initiate on grain boundaries and are normally associated with grain boundary particles (inclusions and/or carbides) or grain boundary triple points. The cavitated grain boundaries are aligned in a direction perpendicular to the principal service stress. The optical metallographic results show that cavities are angular in their isolated and aligned cavity stages, and then become spherical as coalescence causes microcracks. It is believed that the change in shape of the cavities through creep life is controlled by diffusion and minimum surface energy principles. From the optical metallographic evaluation results, a linear correlation between a "Linear Damage Factor" (creep damage level) and measured average cavity size was determined.

For SA welds with an N&T prior to service, cavities initiate in the weld metal adjacent to the fusion boundary along segregation bands. The segregation bands are basically parallel to the fusion boundary and more "densely packed" along the fusion boundary than at other locations in the weld. However, for SA welds with a subcritical PWHT prior to service, cavitation preferentially occurs in the base metal FG/ICHAZ and/or in the weld metal FGHAZ. The majority of the cavities in the base metal FG/ICHAZ and/or in the weld metal FGHAZ initiate preferentially at grain boundary triple points. Particles on grain boundaries, such as inclusions, are another favored location for cavitation; especially in the weld metal.

The "Cryo-Crack" technique induces a fracture in a service exposed SA weld under cryogenic conditions, which defines the weak-link path created by cavitated grain boundaries and intergranular damage. This method reveals cavities and particles associated with cavities in a "3D" appearance under the SEM's high resolution and enhanced depth of field. The true morphology of creep damage is revealed since, in the Cryo-Crack method, plastic deformation does not occur. In addition, EDS evaluation provides chemical information regarding cavities and particles within the cavities. For the service exposed SA welds, with an N&T prior to service, the fractographic morphology of the "Cryo- Cracked" weld metal includes intergranularly separated and cavitated grain boundaries, cavities and cleavage facet surfaces. Two types of intergranularly separated grain boundaries have been observed in terms of "smooth" grain boundaries with cavities and "rough" grain boundaries decorated with carbides and cavities. It is believed that the "smooth" grain boundaries are ferrite/ferrite boundaries and the "rough" grain boundaries are bainite/ferrite or bainite/bainite boundaries. Auger scanning spectroscopic analysis, using the "Cryo-Cracked" specimens, shows that antimony (Sb) is the major impurity segregated to the cavity surfaces. A strong segregation of phosphorus (P) has been determined on the intergranularly separated and cavitated grain boundary surfaces. In addition, sulfur (S) is the major impurity on the inclusion surface, and phosphorus (P) is also the major impurity on carbide surfaces. However, no impurity segregation was detected on the cleavage facets.

For the service exposed SA welds, with a subcritically PWHT prior to service, the fractographic morphology of the "Cryo-Cracked" base metal FG/ICHAZ is generally cleavage over almost entire fracture surface except for the creep cavitated grain boundaries. Cavities appear on grain boundaries intersecting with cleavage facets, especially at grain boundary triple points. It has been determined that antimony (Sb) is the major impurity on the cavity surfaces. In the weld metal FGHAZ cavities have been found on the intergranularly separated grain boundaries in addition to cleavage facets, which is identical to the fractographic morphology of service exposed SA weld metal with an N&T prior to service. No impurities have been found on cleavage facets.

Cavities are normally polyhedral (multi-faceted) with a negative side wall curvature (concave) for both isolated and aligned cavities in the service exposed condition regardless of location either in the base metal FG/ICHAZ or the weld metal. On the microcrack surfaces, cavities generally become rounded/spherical with single or multiple particle(s) in each cavity and multiple cavities cover the entire grain boundary.

Re-normalizing (1700°F/1 hour) has a significant influence on the fractographic morphology of the "Cryo-Cracked" appearance and impurity distribution. Intergranular type fracture is totally absent on the fracture surface produced by "Cryo-Cracking" after re-normalizing, and cleavage is the dominant fracture morphology. In the re-normalized condition cavities are still present and located on grain boundaries intersecting the cleavage facets. However, the shape of the cavities is essentially smooth and rounded instead of polyhedral shape because of the extensive diffusion, which occurs during re-normalizing.

The typical "Cryo-Crack" fracture morphology for "newly" fabricated SA welds (virgin) is cleavage in the weld metal, HAZ and base metal. Cavities and intergranular separation are absent in both the as-welded and N&T condition. Inclusions are found tightly bound to the matrix within the grains and on the grain boundaries in addition to cleavage facets.

In addition, a linear correlation between Areal Damage Factor (creep damage level) and true average cavity size has been determined using "Cryo- Crack" fractographic method.

Microstructural evaluation has been conducted on the service exposed SA welds in both the service exposed and re-N&T condition and the "newly" fabricated SA weld (made with "acid" flux) in both the as-welded and N&T condition. The microstructure of the base metal and the weld metal is ferrite plus carbides in the service exposed condition with a hardness of 70-80 HRB for 2 1/4Cr-1/2Mo SA welds and 70-76 HRB for 1 1/4Cr-1/2Mo SA welds. After re-N&T, the microstructure of the base metal and weld metal is tempered bainite plus ferrite with a 75-80 HRB hardness independent of the materials.

For the "newly" fabricated SA welds in the as-welded condition, the principal microstructural constituents are bainite plus polygonal ferrite for the base metal with a hardness of 80-85 HRB and bainite plus lath ferrite for the weld metal with a hardness of 95-100 HRB. In the N&T condition, the microstructure of the base metal and weld metal consist of tempered bainite plus ferrite with a hardness of 75-80 HRB.

The solidification nature of the SA weld metal produces the segregation bands. These segregation bands are basically parallel to the fusion boundary and more "densely packed" along the fusion boundary than at the center of the weld metal because of the high speed, high current nature of the welding conditions. It is to be noted that the creep damage has been found to preferentially occur in these segregation bands. In general, the segregation bands have a width of approximately 0.05-0.2 mm and a center-to-center spacing of 0.1-0.2 mm. In the service exposed condition, the hardness of the bands is 75-85 HRB in comparison with 65-75 HRB for the weld metal adjacent to the bands. A higher inclusion density and higher bainite content is found in the segregation bands compared to the weld metal adjacent to the bands as well as at the center of the weld. Formation of the segregation bands is influenced by welding procedure and the flux used. Significantly coarser segregation banding is observed in the SA weld made with "acid" flux than in the SA weld made with "basic" flux. However, the segregation bands are minimized in the SA welds made with a "basic" flux after N&T (1700°F/1 hour + 1350°F/2 hours). However, they still remain in the SA welds made with a "acid" Flux.

Inclusions play a leading role in cavitation due to the non-coherent interface with the matrix. In the SA welds, inclusions are significantly more numerous in the segregation bands. The inclusion density is also significantly greater in the segregation bands than in the weld metal adjacent to the segregation bands or at the center of the weld. Inclusions on grain boundaries act as the pre-existing nuclei for cavities (The inclusions in the grain matrix are not active). In addition, the inclusion density and distribution are influenced by the type of flux used in the SA welding. It has been found that the inclusion density is significantly higher in the SA weld made with "acid" flux than in the SA weld made with "basic" flux. N&T PWHT have strong influences on the inclusion density on the grain boundaries as compared to the as-welded condition.

Carbides in the Cr-Mo SA welds are not only important with regard to creep strength but also with regard to the occurrence of cavitation. Carbides initiate cavities when decoherence occurs at the carbide/matrix interface, especially when impurities are present. Carbides have been found in cavities using Cryo-Crack fractographic method. Regardless of service exposure, the as-welded or the N&T condition, coarse and stable carbides, such as M6C, M23C6 or M7C3, preferentially precipitate along grain boundaries while fine M2C carbides are mainly formed in the grain matrix. The type of flux used in SA welds not only influences formation of inclusions but also affects the precipitation of carbides through its influence on the carbon and oxygen content in the weld metal. A higher linear density of grain boundary carbides has been determined in the SA welds made with an "acid" flux than in the SA weld made with a "basic" flux. This result indicates that a higher carbon content does not indicate a higher carbide density along grain boundaries. In contrast, a coarser carbide size either along the grain boundary or in the grain matrix corresponds to a higher carbon content, which is characteristics of SA welds made with a "basic" flux.

Based on the results generated in this study, a physical model for cavitation in the weld metal has been proposed to understand the effect of the metallurgical factors on the creep cavitation behavior of the seam-welded piping. In this model, cavity nucleation, growth and coalescence associated with grain boundary particles under service conditions is defined in detail as well as the effect of welding procedure and PWHT. In addition, through extensive application of the optical metallographic and "Cryo-Crack" fractographic methods to the evaluation of the service exposed seam-welded high energy piping, a correlation between creep damage level (Damage Factors or average cavity size) and estimated creep life expended is postulated based on the creep damage classification, physical model of cavitation and correlation between cavity size and Damage Factors. Accordingly, evaluation methodologies for assessment and quantification of creep damage in high energy piping are proposed. The methods include detailed evaluation procedures for both optical metallographic and "Cryo-Crack" fractographic techniques as well as the evaluation criteria based on average cavity size or Damage Factor values. Considerations for run-repair-replacement of damaged piping are suggested.

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