Sigma Pha Characterization in AISI 316 Stainless Steel榛蘑的做法
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Xiaoli Tang* * Swagelok Company, 29500 Solon Road, Solon, OH 44139, USA Sigma pha (σ) is a chromium/molybdenum-rich intermetallic pha found in the Fe-Cr-Mo system, which occurs when the material dwells or slow cools through the temperature range of 550-1050°C. Thermodynamically, it is possible for this pha to form in standard AISI 316 stainless steel, although in practice it has rarely been en. With the “lean” chemical composition of AISI 316 stainless steel, (that is its relatively low chromium and molybdenum contents), the kinetics of σ pha formation are slow. In addition, in the types of lean stainless steels, the general recognized upper temperature limit for σ existence is around 1050°C. As most austenitic stainless steels are supplied with solution annealing (usually at or above 1050°C) as the final heat treatment to achieve optimum corrosion resistance, it is not surprising that the existence and impact of σ pha in AISI 316 stainless steel have been largely overlooked. However, the incread u of the continuous casting method of stainless steel manufacturing is resulting in a wider occurrence of σ pha in AISI 316 stainless steel. As compared to static ingot cast stainless steels, continuously cast billets experience more chemical gregation at the center, resulting in localized areas enriched in Cr and Mo. As the result, σ pha is more prone to form and at a much fast kinetics than historically thought. The detrimental effects of σ pha on mechanical properties and
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corrosion resistance are well known. Every measure should be taken to prevent this pha from existing in stainless steels. However, revealing the prence of this pha in AISI 316 stainless steel requires carefully applied metallographic techniques. In this study, some successful characterization methods are discusd. Material ud in this study is standard AISI 316 stainless steel manufactured by electric arc furnace melting with argon-oxygen decarburization (AOD) process and continuously cast into billets. Specimens were taken from a round bar of over 50 mm in diameter and solution annealed at 1065°C. The ction parallel to the longitudinal direction of the bar was chon for the characterization becau of the good visibility of elongated phas on this ction. Although common etchants for stainless steels, such as Kalling’s, sufficiently revealed the prence of σ pha from the austenitic matrix by outlining the interfaces between austenite matrix and σ pha, it also revealed retained ferrite, a common feature of AISI 316 stainless steel, in the same fashion. This made it difficult to parate the two phas, and may have contributed to the lack of recognition of σ pha in this material. More effective methods usually involve tint etching. The tint etching method successfully reveals σ pha and differentiates it from other phas, such as retained ferrite and MnS inclusions. A routinely ud tint etchant is Groesbeck’s solution [1]. Although the recommended etching condition was 60-90°C for 1-10 minutes, in this study it was found that in the temperature range 60-70°C, 3-4 minute submerging etching gave very good results
银河补习班评价for AISI 316 stainless steel. After such etching, σ pha exhibited an orange-brown color, while the retained ferrite showed a light brown color and MnS the dark gray feature. Too high a temperature or too long a time could cau over etching, which would either darken the σ pha too much and make it difficult to be differentiated from MnS, or stain the ferrite dark brown and made it similar to the σ pha. The appearances of the three phas after Groesbeck’s solution etching are shown in Figure 1. In sufficient quantity, σ pha was usually prent in stringer form along the rolling direction of the bar. Ferrite also usually existed in elongated shape on the longitudinal ction of bars. Clearly, the tint etching made the two phas more distinguishable from each other. In addition, σ stringers and ferrite were also different from each other in morphology. As shown in Figure 1, σ pha usually exhibited a “lace” type pattern. On the other hand, ferrite stringers were more bulky. Groesbeck’s solution was effective in revealing σ pha and differentiating it from other phas in AISI 316 stainless steel. However, maintaining appropriate temperature and the length of etching DOI: 10.1017/S143192760550374X Copyright 2005 Microscopy Society of America Microsc Microanal 11(Suppl 2), 200578春泥歌词
time made this method less convenient, especially when the characterization was conducted in a large volume of specimens. In duplex stainless steels, due to the high Cr and Mo contents, the pote微信公众号管理
ntial prence of σ pha is a cloly monitored feature. An electrolytic etching method described in ASTM A923, method A [2] proved very convenient for revealing σ pha in AISI 316 stainless steel. This method is especially easy to u becau it is a voltage controlled etching method, instead of a current proportional one. Therefore, no surface area calculation of the specimen is needed. In 40% NaOH solution with around 2V voltage at room temperature, 10 conds of electrolytic etching gave a similar result as the tint etching using Groesbeck’s solution, as shown in Figure 2. The σ pha exhibited the similar orange-brown tint described earlier. Ferrite appeared either light brown or bluish gray. In summary, two effective etching methods, tint etching and electrolytic etching, successfully revealed σ pha and differentiated it from other phas in AISI 316 stainless steel. The method bad on ASTM A923 method A was faster and easier to control than tint etching with Groesbeck’s solution, while prenting similar results. References [1] George Vander Voort, Metallography Principles and Practice, McGraw-Hill, Inc., 1984 [2] ASTM A923-01, Standard Test Methods for Detecting Intermetallic Pha in Wrought Duplex Austenitic/Ferritic Stainless steels
σB. MnS of dark gray and ferrite of light brown.
σA B
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A B Microsc Microanal 11(Suppl 2), 200579怎样煮鸡蛋
