STAINLESS STEELS. Experience has shown that, at least for stainless. for Corrosion Resistance

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2 STAINLESS STEELS for Corrosion Resistance Newly developed stainless steels provide increased corrosion resistance, improved mechanical properties, easier forming and fabrication, and/or improved economy. This report is from Stainless Steels for the New Millennium, a paper presented at the NACE Corrosion 2001 conference in March in Houston, Texas. James D. Fritz,* John F. Grubb,* Ronald E. Polinski* Allegheny Ludlum Corp., an Allegheny Technologies Company Brackenridge, Pennsylvania Experience has shown that, at least for stainless steels, commercially successful alloy developments do not spring from laboratory breakthroughs, but arise instead from the recognition of an unmet customer need. The development of new processes facilitates the alloy development process. One such new process was AOD refining (coupled with improved, rapid instrumental analysis techniques) that allows rapid production of highly alloyed materials with low carbon and controlled nitrogen contents. New knowledge also aids alloy development. In recent years the use of the ThermoCalc software system has helped elucidate the interaction among alloying elements and reduced the number of experimental iterations in several alloy developments. *Member ASM International Pipe flanges and fittings for the petro-chemical industry and other corrosive environments are frequently made of AL 6XN stainless steel alloy. Producers and others continue to invent new alloys and launch them into the marketplace. The following is an overview of recent developments and is intended to illustrate some important trends. It is not exhaustive. Heat-resistant stainless steels Several heat-resistant stainless steels have been introduced recently. Alloy AC 66, UNS S33228, a higher chromium variant of alloy 800, is being selected for a variety of high-temperature applications. Special Metals Incoloy 864 alloy, UNS S35135, was developed to fill a gap between UNS N06625 (alloy 625) and UNS S31635 (type 316Ti). The target application is light-wall, welded, flexible connectors for auto exhaust systems. Among ferritic stainless steels, the UNS S43932 and UNS S46800 alloys were introduced as improved versions of type 439 (S43035). They promise to provide smoother surface quality, improved weldability, higher mechanical properties, and enhanced corrosion resistance. High chromium stainless steels Chromium is the basic source of the corrosion resistance of stainless steels, and higher levels of chromium promote increased resistance to corrosion in a variety of environments. VDM has commercialized the UNS R20033 alloy. This 33% chromium alloy exhibits resistance to highly oxidizing environments, including concentrated sulfuric acid. High Mn+N stainless steels Although traditionally classified as an austenite stabilizer, the efficacy of manganese in suppressing the formation of delta-ferrite has been questioned repeatedly. It is abundantly clear, however, that manganese increases the solubility of nitrogen in stainless steels. This increased nitrogen solubility allows the incorporation of higher levels of nitrogen into manganese-bearing alloys. Nitrogen additions increase strength, stabilize austenite, increase corrosion resistance, and allow reduction of nickel content. Several Mn+N alloys have been introduced recently: S32654 (Avesta), S34565 (VDM), S21000, and S20430 (Carpenter). The first is a 7% Mo superaustenitic, designed to compete with nickelbased corrosion-resistant alloys. The relative re- 36 ADVANCED MATERIALS &PROCESSES/JUNE2001

3 sistance of the S32654 alloy is shown in Table 1, which summarizes the results of coupon testing in three municipal waste incineration plants. The second is a 5% Mo superaustenitic stainless steel designed to provide corrosion resistance comparable to the 6% Mo superaustenitic stainless steels, along with higher strength. The S34565 alloy has been reported to have a critical pitting temperature, as determined by the ASTM G48 test procedure, at 85 C. The third alloy is a high-strength, low-magneticpermeability alloy developed for drill collars. The last alloy is a low work-hardening alloy designed to mimic the formability of S30400 while containing much less of the expensive nickel. High PREN stainless steels The Pitting Resistance Equivalent, PRE, was initially a correlation between pitting resistance in ferric chloride test solutions and the alloy composition. The original correlation was expressed as: PRE = Cr * Mo Later, after the beneficial influence of nitrogen was recognized, this expression was modified to include the contribution of nitrogen. However, some controversy has developed about the exact equivalence factor for nitrogen. The most generally accepted correlation is PREN = Cr * Mo + 16 * N. Others have proposed higher multipliers, where PREN = Cr * Mo + 30 * N would be the highest widely accepted value. In either case, it is generally agreed that, all else being equal, alloys with higher PREN should exhibit greater resistance to pitting and crevice corrosion. Some customers have begun demanding that the alloys they buy for specified purposes provide guaranteed minimum PREN values. In response, several producers have designed higher PREN alloys. New, higher PREN, 6% Mo alloys include VDM 31 (N08031), Uranus B66 (S31266), and SR50A (S32050) alloys. The AL-6XN PLUS (N08367) alloy represents another route to higher PREN in 6% Mo alloys. It represents not a completely new alloy, but rather is an enhanced version of the existing AL- 6XN (N08367) alloy, which offers a higher guaranteed minimum PREN of 50.0 (using PREN = Cr * Mo + 30 * N). The increased corrosion resistance possible with this alloy modification is summarized in Table 2. High-silicon stainless steels Silicon additions promote resistance to corrosion in highly oxidizing environments, such as nitric acid or concentrated sulfuric acid. UNS S30600 and S30601 Fe-Cr-Ni-Si alloys have been handling hot nitric acid, while S32615 alloy has been handling hot, concentrated sulfuric acid. A summary of the corrosion performance of the S30600 and S30601 alloys in concentrated nitric acid is presented in Table 1 Weight loss, pitting, crevice corrosion Weight loss, Pit depth, Crevice depth, Alloy mg/m 2 mm mm N S S S N N N Attacks from exposures in three waste incineration plants. 1Single pit in weld Table 2 Critical corrosion temperatures of N08367 Test Test Typical Enhanced Test method solution surface N08367 N08367 Alloy ASTM G48 Acidified Pickled CCCT = CCCT = Practice B ferric chloride mill surface 43 C 55 C ASTM G48 Acidified Pickled CPT = CPT >_ 90 C Practice C ferric chloride mill surface 75 C ASTM G48 Acidified Pickled CCCT = CCCT = Practice D ferric chloride mill surface 35 C 45 C ASTM G 150 1M NaCl Ground ECPT = ECPT>_90 C surface 78 C Critical crevice corrosion temperature. Critical pitting temperature. Electrochemical pitting temperature Table 3 Corrosion of S30600 and S30601* Average corrosion rate Test duration, Test temperature, Alloy hours C ( F) mils/year mm/year S (68) S (100) S (122) S (180) S (60) S (100) S (122) S (140) *In 98.5 wt% nitric acid Table 3. Recently, two new high-silicon alloys were developed. UNS S70003, with 7% Si, has the highest silicon content in iron-base alloys. UNS S38815, with 6% Si, was developed as a more economical material for handling hot, concentrated sulfuric acid. Low-chromium stainless steels ASTM has recognized that type 409 stainless steel may not be stabilized against intergranular corrosion. ASTM has added three subtypes of type 409 stainless steel that provide increased resistance to intergranular corrosion. All three subtypes S40910, S40920, and S40930 qualify as S The benefits of increased titanium and niobium (columbium) additions on the intergranular corrosion resistance of welded 409 stainless steels have been investigated, and the resulting stabilization plot is shown in Fig. 3. Additional variants of type 409 have been created to provide enhanced capabilities, including S41045 for applications involving brazing, and S40975 for enhanced toughness. Several low-cost, low-chromium alloys may be considered as upgrades for carbon steels. These alloys, sometimes called Utility Stainless Steels, in- ADVANCEDMATERIALS&PROCESSES/JUNE

4 Table 4 Compositions of selected stainless steel alloys in weight percent UNS C Cr Ni N Mo Ti Cb (Nb) Si Mn Cu Other (440 mod) V (465) (108) N Bal Fe, 0.4 Al N N08367 * R S S min S S S W S x (C+N) S S S S Ce S S S Al S40900 # x C S x (C+N) S x (C+N) S x (C+N)< (Ti+Cb) <0.75 S x(C+N) S x (C+N) S S S C S ; x (C+N) min. S S *Composition is restricted range of UNS N #Obsolete, replaced by S40910, S40920, or S Values are maxima unless a range is indicated. clude S40977 (EN ) and S Although originally different, recent specification changes have made these alloys essentially identical. While they do not provide the levels of corrosion resistance normally associated with stainless steels, they do provide a significant improvement over the classical weathering steels. Martensitic stainless steels Recently introduced martensitic stainless steels include 440-HX and 465. Alloy 440-HX contains 1.6% C for wear resistance, and illustrates how the martensitic stainless steels merge seamlessly with tool steels. Alloy 465 is an age hardenable alloy that can be heat-treated to strength levels of over 2000 MPa (290 ksi). Nickel-free stainless steels Health concerns have led to a demand, especially in Europe and parts of Asia, for nickel-free austenitic stainless steels. These are for surgical, dental, jewelry, and food handling applications. One recently developed nickel-free austenitic stainless steel is Carpenter BioDur 108. The austenitic structure of this alloy is maintained by a high nitrogen content of approximately one percent. Besides austenitic stability, the high nitrogen content also contributes to high levels of corrosion resistance and strength. For more information: James D. Fritz, John F. Grubb, Ronald E. Polinski, Allegheny Ludlum Corp., Technical Center, Brackenridge, PA 15014; jgrubb@alleghenyludlum.com; tel:724/ ; fax: 724/ How useful did you find the information presented in this article? Very useful, Circle 317 Of general interest, Circle 318 Not useful, Circle ADVANCED MATERIALS &PROCESSES/JUNE2001

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