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Premature creep-rupture swelling of AISI 321 austenitic stainless steel superheater tubes resulted in failures in the middle 1950s and promoted an extensive investigation. These failures in large utility boilers operating at pressures above 1500 psi and temperatures above 1000 F seriously challenged the economics and reliability of high pressure and high temperature. Failures were initially attributed to an extremely fine grain size occasioned by a low temperature solution heat treatment. Reheat treatment at higher temperatures to produce a coarser grain structure reduced the incidence of failure.
Subsequent results of a research project provided evidence that the final solution heat treatment and not the grain size was the most significant factor. Control of the carbon content was also found to be an important factor. A new grade called the “H” grade was introduced which incorporated specific heat treatments and control of carbon. This was applied to the 300 series steel in all product forms. Service experience has been successful with the H grades.
Recent investigations of Type 304 austenitic stainless steel indicate that a controlled nitrogen addition significantly enhanced the rupture life of this material, thereby increasing reliability. While the quest for higher temperature steam fossil units has been blunted with the advent of nuclear power, the fact still exists that higher pressures and temperatures result in significant increases in efficiency, and eventually will be pursued.
nitrogen, creep rupture, failures, austenitic stainless steel, solution heat treatment, grain size, high pressure, high temperature
Assistant to chief mechanical engineer, Public Service Electric and Gas Co., Newark, N.J.