Residual and trace element effects on the high-temperature creep strength of austenitic stainless steels
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I.
INTRODUCTION
AUSTENITIC stainless steels were developed primarily for corrosion resistance, rather than mechanical strength, and early alloys contained only sufficient alloying element content to stabilize austenite at chromium levels of 18 pct and higher. Today a wide variety of special-purpose austenitic stainless steels are available whose properties and compositions have been tailored for specific applications. However, in the high-temperature pressure vessel applications, the most commonly used stainless steels are types 304, 316, 347, 321, and 310. These steels, some of the first to be developed, have very few specified alloying elements. 2 Further, a fairly wide range of compositions and thermomechanical heat treatments are allowed within the specification limits. As a result, the high-temperature strength and ductility of these alloys vary widely from heat to heat. 3.4 For purposes of discussion it is convenient to outline the factors that contribute to the variation in strength and ductility in the following way: I. Composition A. Alloying elements B. Residual elements C. Trace elements II. Thermomechanical Treatment A. Solution temperature B. Cold or warm work C. Aging R.W. SWINDEMAN, Metallurgist, V.K. SIKKA, Member, Senior Research Staff, and R.L. KLUEH, Research Metallurgist, are all with Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830. This paper is based on a presentation made at the symposium "The Role of Trace Elements and Interfaces in Creep Failure" held at the annual meeting of The Metallurgical Society of AIME, Dallas, Texas, February 14-18, 1982, under the sponsorship of The Mechanical Metallurgy Committee of TMS-AIME.
METALLURGICAL TRANSACTIONS A
III. Testing Methods A. Sampling procedures B. Basis for comparison For type 304 stainless steel, nickel, chromium, carbon, and manganese are considered to be alloying elements while molybdenum, niobium, and titanium are typical residual elements. Clearly, molybdenum is an alloying element in type 316 stainless steel, and similarly niobium in type 347 or titanium in type 321 stainless steel. We regard sulfur and phosphorus to be important trace elements in all of these steels, although phosphorus is sometimes added to weld metal to improve properties, 5 and some specialty steels contain high phosphorus. 1.6 Two other very important elements are nitrogen and boron. Historically, neither is specified in the alloys mentioned above, although both are recognized to impart high-temperature strength or ductility. 7's'9 As a consequence, new alloys are being developed that include these elements in the compositional specifications. 2'1~ At typical solution treating temperatures (e.g., 1100 ~ austenites can accommodate copious quantities of most alloying, residual, and trace elements. Further, after rapid cooling, a great number of these elements contribute to solid solution hardening. This hardening effect has been studied in great detail by Picketing et a1,12:3 who have used regression analysis techniques to develop c
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