The welding of dissimilar metal joints in new and retrofit power plant boiler tubing has long proved challenging. New plants designed to operate at higher temperatures and pressures require advanced alloys and a filler metal that produces reliable welds. EPRI recently developed and sponsored the commercialization of a new filler metal. Its first application is the fabrication of boiler tubes for American Electric Power’s ultrasupercritical John J. Turk, Jr. Power Plant.
Boiler tubing is made of different types of steel. For superheater and reheater sections that operate at higher temperatures, components are manufactured from austenitic stainless steel due to its properties of high creep strength and good corrosion resistance. However, because austenitic stainless steel is expensive, tubing in the earlier boiler stages, where design temperatures are lower, can be made of less-costly ferritic alloys such as Grade 22 steel, which contains chromium and molybdenum, and is commonly known as a Cr-Mo steel. Unfortunately, at some point, the austenitic steel and the ferritic alloy have to be welded together, with the result that, among the thousands of tubing joints in a typical boiler, many are transition joints, where the two metals have to be joined by dissimilar metal welds (DMWs).
Historically, DMWs have proven to be a weak location where premature failures may occur. If not properly fabricated, these welds can result in inferior properties and substantially reduce component life. Careful selection of welding filler material, preheat temperature, and postweld heat treatment temperature are paramount for dissimilar welds to avoid poor reliability.
Why Dissimilar Metal Welds Fail
In the 1980s, research conducted by the Electric Power Research Institute (EPRI) and others indicated that a number of the issues associated with DMW failures are related to the composition of the welds’ filler metal — the metal added in the making of a joint during the welding process (Figure 1). Research also showed that conventional 309 stainless steel filler metal resulted in the shortest life and that nickel-based filler metals resulted in three to four times that life.
1. Filler metal. Cross section of filler metal weld. Courtesy: EPRI
Research further indicated that DMW failures are caused by two key mechanisms. One mechanism is a result of the difference in the rates of thermal expansion among different alloys and filler metals. Thermal expansion of an alloy is the amount that the material expands upon heating and shrinks during cooling, and that property is unique to a given material. When two alloys with different thermal expansion rates are joined, stress develops at the fusion line between the alloys as temperature changes. This differential expansion mismatch can contribute to creep fatigue damage.
Research also showed that premature failures of DMWs are caused by a mechanism called carbon migration. One of the factors that give the Cr-Mo alloys their creep strength is that they form carbides by the combination of carbon and other elements, including chromium. When two materials with different levels of chromium are joined together, the carbon migrates during elevated temperature service from the lower-chromium-containing alloy to the higher-chromium alloy. As the temperature rises, the rate of carbon migration increases. This migration results in an area of depleted carbon, called a "carbon-denuded zone," in the lower-alloyed material and results in lower creep strength due to there being less carbon available to form carbides (Figure 2).
2. Carbon migration. In dissimilar metal welds using conventional filler metals, carbon can migrate, under increased temperature, from the low-alloy base metal to the high-alloy filler metal, creating a weak, carbon-denuded zone in the base metal next to the fusion line. Courtesy: EPRI
In the 1990s, based on this research, EPRI developed a new filler metal, called HFS6, that was intended to solve these problems. The high nickel content of the filler metal resulted in thermal expansion similar to that of low-alloy ferritic tube materials. HFS6 also contained a low chromium content that would result in a smaller carbon-denuded zone than was possible with available nickel-based and austenitic fillers, thereby eliminating carbon migration. HFS6 was never commercialized, however, because of its tendency to develop microscopic cracks, called microfissures, which resulted in lower service life.
Email
Comments
Print
Reuse
