Sri Krishna Chimbli 2012-12-08 02:51:40
Carbide Solution Treatment, or Solution Annealing, is familiar to those in the fastener industry. Steel grades AISI 304 and AISI 316 are very commonly used bolting materials that are austenitic grades specified in consensus standards such as ASTM A 193 (B8, B8M), ASTM A 320 (B8, B8M), ASTM F 593 (Alloy Group-1 & 2), ASTM F 738 (A1, A4), ISO 3506-1, EN10269, and MIL-DTL-1222. This article will provide information about carbide solution treatment and the test required to ensure the passivity in stainless steel-headed products is maintained. What are austenitic steels? They are steels that primarily contain austenite phase or austenite structure in room temperature. Austenite is an interstitial solid solution of carbon in Face Centered Cubic (FCC) iron or gamma iron. In plain carbon and low alloy steels, austenite is stable only at high temperature, and quenching results in the formation of martensite. The addition of alloying elements like nickel and other austenite stabilizers to the molten steel produces austenite stability at room temperature. Austenite is named after Sir William Chandler Roberts-Austen, (March 3, 1843 in Kennington–November 22, 1902, London), who was a metallurgist noted for his research on the physical properties of metals and their alloys. He also published the first iron-carbon phase diagram1. Austenitic stainless steels are non-magnetic and cannot be hardened by heat treatment, but they harden by cold working. Carbide solution treatment, or solution annealing, on austenitic steel bolts is performed after the heading operation. The process called “carbide solution treatment,” “solution annealing,” or “quench annealing” means the bolts or fasteners are heated to a temperature of 1900ºF to 2050ºF with a 1.5- hour holding time, then water-quenched or gas fan-quenched. When austenitic stainless steel fasteners are heated to 1900ºF for sufficient time, the free carbides are driven into solid solution, and rapid quenching will freeze them. When using other methods of cooling, like gas fan-quenching, it may be necessary to test and verify the rate of cooling is rapid enough to prevent grain boundary carbide precipitation2. Stainless steels are stainless as the chromium in the steels reacts with oxygen, forming protective stable chromium-rich oxide layer on surface. Slower cooling of the bolts will result in the formation of chromium carbides at grain boundaries, which will lead to intergranular corrosion and is technically termed as sensitization. Grain boundaries are the preferred regions of segregation. Carbide precipitation is noted to occur at temperatures 800°F and 1650°F3. In simple terms, chromium carbides are formed at grain boundaries, then chromium at grain boundaries is depleted and may not be available to react with oxygen for forming the protective surface chromium oxide film, thus leading to intergranular corrosion. Hence, corrosion resistance of the fasteners will be decreased by precipitation of chromium carbides. Sensitization can be readily detected by performing any one of the tests listed in ASTM A 262 standard specification. ASTM A 262 is a standard practice for detecting susceptibility to intergranular attack in austenitic stainless steels4. An oxalic acid etch test, as defined in ASTM A 262 practice A, is used to determine susceptibility to inter granular corrosion due to chromium carbide precipitation. Fastener manufacturers may test one sample from every batch after solution annealing to ensure the produced fasteners are not susceptible to corrosion5. The basic steps in oxalic acid etch testing are: cutting; polishing; etching with oxalic acid; and analyzing. The results of the analysis can be cate - gorized into three structures: step structure, dual structure, and ditch structure—as presented in Figures 1–5. The step structure and dual structure in microstructures, Figure 1 and Figure 2, are acceptable etch structures indicating that fasteners are not susceptible to intergranular corrosion. Ditch structure is not an acceptable structure as shown in microstructure (Figure 3) , and the lot may be further tested to ASTM A 262 practice E, Copper-copper sulfate—16 percent sulfuric acid test for determining susceptibility to intergranular attack. Figures 4 and 5 are representative of oxalic acid etch structures on AISI 316 bolts. CONCLUSIONS 1 Solution annealing is a heat treatment process that involves quenching in water or rapid cooling, unlike the normal annealing in steels with furnace cooling. 2 Carbide precipitation may occur at the grain boundaries if austenitic steel fasteners are not rapidly cooled. 3 Post carbide solution treatment, one sample from each lot, shall be tested to ASTM A 262 to determine the presence of carbide precipitation. 4 Step structures and dual structures are acceptable etch structures indicating the fasteners are free from carbide precipitation. 5 Ditch structure shall be further tested to ASTM A 262 practice E for acceptance or rejection. QUESTIONS? Email firstname.lastname@example.org REFERENCES 1 The International Metallographic Society, Volume 35, Issue 2. 2 MIL-H-6875H, Heat Treatment of Steel. 3 Metals Handbook Volume 2, 8th Edition, Heat Treating of Stainless Steel. 4 ASTM Standard A262, 2010 “Standard Practices for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels,” ASTM International, West Conshohocken, PA, 2010, DOI: 10.1520/ A0262-10, www.astm.org. 5 ASTM F593, 2002(2008)e1 “Standard Specification for Stainless Steel Bolts, Hex Cap Screws, and Studs,” ASTM International, West Conshohocken, PA, 2008, DO: 10.1520/F0593-02R08E01, www.astm.org. SRI KRISHNA CHIMBLI Sri Krishna Chimbli has a Masters degree in Materials Engineering & Sciences (SDSM&T) and a Bachelors degree in Metallurgy & Materials Technology (JNTU). He is an active member of ASTM committees A01 & F16, and a member of ASQ. He is employed with Alloy & Stainless Fasteners Inc., supporting quality, forging, rolling, heat treatment, coating and metallurgical departments. He is also a Principle Auditor recognized by RABQSA in auditing quality management systems, ISO 9001 and a lead auditor.
Published by American Fastener Journal. View All Articles.
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