Mechanical properties of a 29 Pct Cr-4 Pct Mo-2 Pct Ni ferritic stainless steel
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t i c stainless s t e e l s possessing excellent resistance t o pitting corrosion have recently been developed. Streicher 1 demonstrated that F e - 2 8 pct Cr-4 pct Mo and Fe-28 pct Cr-4 pct Mo-2 pct Ni alloys were resistant t o s e v e r e laboratory tests under pitting and c r e v i c e corrosion conditions. In addition, by limiting the m a x i m u m carbon and nitrogen content (i.e., 100 ppm m a x i m u m carbon and 200 ppm m a x i mum nitrogen w i t h total C + N less than 250 ppm) the alloys demonstrated good ductility and were not susceptible t o intergranular corrosion a f t e r welding. The 2 pct nickel addition rendered the 28 pct Cr-4 pct Mo alloy susceptible t o s t r e s s corrosion cracking in some s e v e r e media, but increased the g e n e r a l corrosion resistance t o boiling sulfuric and hydrochloric acids. This paper will describe the mechanical properties of the new ferritic stainless s t e e l consisting of 29 pct Cr-4 pct Mo-2 pct Ni (29-4-2). Although the alloy was developed primarily for pitting corrosion resistance, it also possesses excellent ductility as determined by bend tests. As a r e s u l t of the low interstitial content (C and N), this s t e e l also has excellent room temperature toughness as m e a s u r e d by impact tests. The work of Binder and Spendelow2 and Baerlecken 3 e t al clearly established that a low l e v e l of interstitial elements is a necessary condition for excellent impact resistance in f e r r i t i c stainless steels. Binder and Spendelow,2 u s i n g alloys containing up t o 1500 ppm of carbon and nitrogen and from 12 to 40 pct chromium, demonstrafed that the carbon and nitrogen limits necessary for good room temperature toughness d e c r e a s e d as T . J. NICHOL is Specialist, Market Development, Market and Product Development, Allegheny Ludlum Steel Corporation Research Center, Brackenridge, P A 1 5 0 t 4 . Manuscript submitted July 1 9 , 1976. METALLURGICAL
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the chromium content increased, e.g., from 375 ppm at 20 pet chromium to 200 ppm at 30 pet chromium. The room temperature toughness of f e r r i t i c stainless steels is also influenced by the cooling rate from elevated temperatures. A r e c e n t study4 showed that t h e r e are two temperature zones through which slow cooling can r e s u l t in embrittlement of high chromium a n d / o r molybdenum ferritic stainless steels. A h i g h e r temperature zone, 760 to 927°C (1400 t o 1700°F) w h e r e phases such as s i g m a a n d / o r chi5 precipitate and a lower temperature zone, 427 t o 510°C (800 t o 950°F) commonly r e f e r r e d to as 475°C (885°F) embrittlement w h e r e a chromium rich phase6 (a') precipitates. Exposure t o these temperature r a n g e s manifests itself in the degradation of room temperature ductility and toughness of ferritic stainless s t e e l s upon subsequent cooling from the embrittlement temperature t o room temperature. Both f o r m s of embrittlement can be removed by heat treatment to resolution the precipitates followed by r a p i d cooling t o room
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