Masters Theses

Date of Award

12-1988

Degree Type

Thesis

Degree Name

Master of Science

Major

Metallurgical Engineering

Major Professor

Carl D. Lundin

Committee Members

Dominic Canonico, Charles Brooks

Abstract

Following a number of premature service failures in SA seam weldments on critical power plant components, a test program was initiated to assess the influence of the SA flux composition on the elevated temperature properties of the weld metal. The interest in the flux was based on evidence that extensive creep damage in the failed SA weld metal was intimately associated with the numerous non-metallic inclusions present in the deposit.

Pursuant to that end, a series of 2-1/4Cr--1Mo test weldments were fabricated using three different fluxes, each representing a different basicity index value that corresponded to an "acid," a "neutral" and a "basic" flux composition. For each flux a different oxygen content, and, thereby, a different inclusion content was developed in the weld deposit. Weldments from each of the composition groups were given one of two production—type heat treatments, either a normalize and temper, or a subcritical post weld heat treatment. These test welds then were evaluated by chemical analysis, mechanical properties testing, creep rupture testing and metallography.

Chemical analyses of the test welds showed that the major elements affected by the flux were manganese, silicon and oxygen, which increased with decreasing flux basicity, and carbon, which decreased in the lower basicity flux weldments. In view of the possible influence of the loss of carbon on rupture strength, a set of test welds was fabricated using the "acid" flux with the carbon content artificially raised to a level approximating that of the "basic" flux weldments.

The tensile and yield strengths of the weld metals were, in general, consistent with the predicted level of hardenability for the weld materials based on carbon content and the levels of manganese and silicon. Values for elongation and reduction in area were affected by inclusion content, with both properties showing substantial reductions in the low basicity flux weldments.

A marked effect of the inclusions was observed in Charpy V-Notch tests, with the higher inclusion content "acid" flux weldments exhibiting significantly lower upper shelf values, and much broader transition ranges than the "basic" flux weldments.

Overtemperature creep rupture tests of all weld metal specimens showed a consistent deterioration in elevated temperature properties in the "acid" and "neutral" flux weldments. When compared to unexposed 2-1/4Cr--1Mo base material, rupture strength values for the "neutral" flux weld metal were at the minimum of the base metal scatterband, while the rupture strength of the "acid" flux weld metal was well below minimum base metal properties. It was noted that the slope of the log time-temperature curve for all of the weld metals was more shallow than the slope of the base metal curve, indicating that, when extrapolated to service-like temperatures, the weld metal rupture values would deviate from base metal properties to an even greater extent than was indicated by the higher temperature tests. There was no significant variation in rupture properties observed with respect to the type of heat treatment given the weld material.

The lower rupture strengths of the "acid" and "neutral" flux weldments were accompanied by reduced rupture ductility values, an anomaly in normal creep rupture behavior that is believed due to the influence of the number and type of grain boundary inclusions in the weld deposit.

Optical microscopy established that the "acid" flux weld metal contained a significantly higher number of inclusions than were present in the "basic" flux weld metal; this was consistent with the higher oxygen content of the "acid" flux weld metal. The number and size of inclusion particles contained in the "neutral" flux weld metal were intermediate between the "acid" flux and the "basic" flux weld metals. It was noted, as well, that within a given deposit there was an unequal distribution in the number of particles adjacent to the fusion boundary as compared to the center of the weld: at the fusion boundary the number of inclusions was substantially greater due to the orientation of segregation bands that developed during solidification of the weld deposit.

SEM analysis of the inclusions showed that the particles contained in the "acid" flux weld metal were the glassy manganese-silicates, intermixed with manganese-sulfides. Examination of creep-damaged test specimens demonstrated that, where the nucleating agent could be identified, there was a one-to-one correlation between inclusion particles and cavities.

A theory has been proposed to explain the reduction in weld metal rupture properties which involves both the large number of inclusion particles present in the SA weld deposit and the nature of the bond that exists between certain of the particles and the weld matrix. Where the composition of the inclusion particles is such that the bond between the matrix and the particle is non-coherent, then very small levels of strain are sufficient to rupture the bond. Because many of the inclusions are located at grain boundaries, this, in effect, creates "instantaneously" a large population of stable cavity nuclei. The rupture life of the weld metal is reduced, therefore, by an amount corresponding to the reduction in the time normally required for cavity nucleation. Where the rupture strength of the weld metal, itself, is low, the cavity growth process is accelerated due to the relative ease with which the matrix is able to creep; however, the rupture ductility remains low due to the influence of the large population of grain boundary cavities on strain accumulation.

SEM analysis confirmed that the inclusion particles present in the "acid" flux weld metal were predominantly MnO+MnO-SiO2+FeO+MnS. In the "basic" flux weld metal many of these same constituents were present, but with the addition of Al2O3 in all particles.

In stress rupture tests of large composite specimens, all but one of the normalized and tempered weldment samples failed in the weld metal, with only one sample showing a definite influence of the interface structure. The absence of interface failures in the test specimens is believed due to material effects associated with test acceleration. The PWHT weldment samples all failed in the fine-grained region of the HAZ, indicating that at the test temperatures the fine-grained HAZ exhibits the poorest rupture properties. This has been attributed to a grain size effect related to the loss of grain boundary rupture strength above the equicohesive temperature.

Based on the results of these tests, it is apparent that considerable variations in the elevated temperature properties of SA weld metal may exist depending upon the nature of the flux used for fabrication. As indicated by the tests, the flux composition influences the properties through its effect on the oxygen, and thereby, the inclusion content of the weld deposit. Since increased oxygen/inclusion content has been shown to have a deleterious effect on weld metal rupture properties, any SA seam welds fabricated for elevated temperature service particularly where the seam is oriented in an unfavorable direction relative to the principal stress, should be made with a flux that will produce a weld deposit low in total oxygen content, and already in service, future inspections in which the remaining life of the component is to be assessed should include a determination of the weld metal oxygen content and the type of non-metallic inclusions present in the weld deposit. These results will provide at least a qualitative indication of the type of rupture properties that may be expected from the weld metal, based on a comparison with anticipated base metal properties.

In addition, since the test results suggest an even greater divergence between base metal and weld metal properties at service-like temperatures, and this for the "basic" flux weld metal as well as the "acid" flux weld metal, it has been recommended that research be conducted to determine if the addition of small amounts of elements that form more stable carbides at the anticipated service temperatures will improve the properties of the low oxygen SA weld metal to a level equalling or exceeding those of the base metal.

Download
Files over 3MB may be slow to open. For best results, right-click and select "save as..."

Share

COinS