TECHNICAL
S2 R2 C3 or better
Do you know your bolts must meet macroetch requirements? 4.F11 By Sri Krishna Chimbli, Alloy & Stainless Fasteners Inc, www.goasf.com On several occasions fastener engineers and fastener buyers have approached the author with questions about macroetch testing and what S2 R2 C3 means. This article will provide information about macroetch testing, test results, acceptance criteria, and reporting.
A
merican Society for Testing & Materials (ASTM) A962 is a standard specification on common requirements for bolting intended for use at any temperature from cryogenic to the creep range [1]. The consensus standards on bolting referred to ASTM A962 are ASTM A193, ASTM A194, ASTM A320, ASTM A437, ASTM A453, and ASTM A540. The general requirements covered in ASTM A962 include the steel producer to test the steel per ASTM E381, which is a standard method of macroetch testing steel bars, billets, blooms, and forgings [2]. The product specifications such as ASTM A193 require the certification to include the results of macroetch examination for carbon and alloy steels [3]. Macroetch test will provide information about the homogeneity of the steel sample. S2, R2, and C3 are conditions to a certain degree of graded macroetch photographs of alloy and killed carbon steels. S stands for subsurface conditions, R for random conditions, and C for center segregation (listed in ASTM E381 Plate-1 standard method for rating macro etched steel). Each condition has five graded series, based on the order of severity that may be acceptable or not, depending on product specification or as agreed by the purchaser. MIL-STD-430, “Macrograph standards for steel bars, billets, and blooms,” was the first published macrostructure rating chart in 1957. This chart had four conditions with eight macroetch structures [4]. The four conditions are A: center defects, B: subsurface defects, C: ring defects, D: miscellaneous defects. Later the MIL-STD-430 was revised to show three conditions of macroetch structures with two charts; these charts were adopted by ASTM E381 in 1961. Macroetch testing according to ASTM E381 involves sampling the alloy bar/billet,
polishing, etching, and rating the structure. Sample is a disc taken from the end of the bar (after sufficient discard) by cutting the specimen transversely with saws or abrasive cut off wheels. The specimen is then polished to a smooth surface and cleaned to remove any residue or oil. To reveal the macroetch structure the specimen is hot etched using etchant, 1:1 mixture of hydrochloric acid (HCL) by volume and water as reagent. The etchant is maintained at a temperature of 160ºF to 180ºF and the specimen is etched for 15 to 30 minutes depending on the specimen size and composition. There are other etchants covered in ASTM E381 that may be used for etching as part of the procedure [2]. Specimen macroetch structure need to be examined visually or at low magnifications to rate the structure. The rating of the structure varies depending on the casting process employed to produce the steel. Reference standard charts and conditions vary for Ingot cast or Strand cast. Continuously cast products with a reduction ratio of more than 3:1 (wrought product form) can be rated as Ingot cast products using the same reference standard charts. There are three reference standard rating charts namely Plate-I, Plate-II and Plate-III. Plate-I and Plate-II reference standard charts are used on Ingot cast products. Plate-III is used for continuously cast products. In Plate-I subsurface conditions are graded S1 to S5 with S1 being the lowest severity and S5 being the highest severity. Similarly, random conditions are graded R1 to R5 and Center segregations are graded C1 to C5. Plate-II has ungraded structures representing pattern, flute cracks, gassy, butt tears, splash, and flakes. Pattern is acceptable in any degree. Structures with flute cracks, gassy, butt tears, splash, and flakes are not acceptable. Plate-III has three types of conditions
106 Fastener + Fixing Magazine • Issue 79 January 2013
that may be observed in macroetch structures: linear conditions, circular conditions, and other conditions. A linear condition consists of surface cracks, diagonal crack, subsurface crack, mid-radius crack, and center crack. Circular conditions include pipe or center void, star crack, center unsoundness, and dark center. Other conditions are grouped as pin holes, scattered porosity, white band, chill structure, strong dendritic structure, weak dendritic structure, equiaxed structure, columnar structure, mold slag, and pattern. The acceptance criteria for macroetch testing are based on the product specifications or as agreed upon by the purchaser. Few conditions like hydrogen flakes, cokey center segregation, inclusion pits, pattern, entrapped gas, heavy segregation, voids, dendrite structure, star pattern, centre porosity that are found in macroetch structures are shown in pictures 1 to 8 (Reprinted with permission of ASM International. All rights reserved. www.asminternational.org) [4].
About Krishna Chimbli Sri Krishna Chimbli has a Masters degree in Materials Engineering and Sciences (SDSM&T) and a Bachelors degree in Metallurgy and Materials Technology (JNTU). He is an active member of ASTM committees A01 and 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 recognised by RABQSA in Auditing Quality Management Systems, ISO 9001 and a lead auditor. email: kchimbli@goasf.com