Figure 1: Hot-acid etching of this disc from a bearing steel billet revealed broken corners (circled in red), a lap (upper left, circled in red), several small seams, and freckle-type segregation. Figure 2: Hot-acid etching of 9-in square disc of AISI 4142 alloy steel revealed ingot pattern and inclusion pits.
Figure 3: Hot-acid etching of disc from AISI 4140 alloy steel billet revealed entrapped gas, heavy segregation, voids, and ingot pattern.
Figure 4: Hot-acid etching of disc from an AISI 1050 billet revealed a “cokey” center, inclusion pits, and a dendritic pattern.
Figure 5: Hot-acid etching of disc from an AISI 4145 modified alloy steel billet revealed hydrogen flakes (circled in red).
Figure 6: Hot-acid etching of a transverse disc from continuously cast AISI 1045 carbon steel revealed coarser dendrites (circled in red) at top compared to bottom, light center segregation, and halfway cracks. Figure 7: Hot-acid etching of a transverse disc from continuously cast AISI 1008 carbon steel revealed a star-pattern open condition. (Courtesy of M. Schmidt, Bethlehem Steel Corp) Figure 8: Hot-acid etching of this transverse section of continuously cast AISI 4140 revealed a dendritic structure, center porosity, and a band (arrow) from induction stirring. (Courtesy of B. L. Bramfitt, Bethlehem Steel Corp)
Fastener manufacturers producing alloy steel fasteners and carbon steel fasteners in compliance to standards listed in ASTM A962 are required to produce fasteners with bars that have been macroetch tested to ASTM E381 and accepted based on the acceptance criteria in ASTM A962. The steel producer shall report the macroetch test results on the steel mill test report or mill certificate, which may subsequently be transferred to the fastener manufacturing test report. The fastener manufacturer may elect to do the macroetch testing on alloy and killed carbon steel bars or forgings independently for the purpose of compliance or verification.
Acceptance criteria: a) Ingot cast product: ASTM A962 requires the macroetch structures to be better than grade S2, R2, and C3 in Plate-I for ingot cast product. Etch structures with conditions of flute crack, gassy, butt tears, splash, and flakes, are not acceptable. b) Continuous cast product: Per ASTM A962 macroetch structures with conditions of surface crack, diagonal crack, sub-surface crack, mid-radius crack, center crack, white band, chill structure, strong dendritic structure, weak dendritic structure, equiaxed structure, columnar structure, and mold slag are not acceptable. Conditions such as pipe or center void, center unsoundness, dark center, pin holes, and scattered porosity shall be acceptable if they are within the acceptance criteria of an ingot cast product S2, R2, and C3 or better. Certain conditions as stated above in ingot cast and continuous cast are acceptable as hot working and sub-sequent reduction process or forging shall improve the macrostructure. Bars with unacceptable macroetch structures per ASTM A962 shall not be used to produce fasteners.
Conclusions: 1. Compliance to ASTM A962 requires fastener manufacturers to perform macroetch testing on alloy steel and killed carbon steel bars. 2. Macroetch test results shall be accepted or rejected based on the acceptance criteria specified in ASTM A962. 3. The test determines the internal quality or homogeneity of the steel. 4. Fastener manufacturers should report the results of the macroetch test in the fastener manufacturing certificate. 5. Fastener manufacturers can purchase the wrought steel or steel bars with results of macroetch testing as part of their purchasing contract / agreement. 6. S2, R2 and C3 are graded series in Plate-I referring to sub-surface conditions, random conditions, and center segregation.
References: 1. ASTM standard A962 “Specification for Common Requirements for Bolting Intended for Use at Any Temperature from Cryogenic to the Creep Range” ASTM International, DOI: 10.1520/A0962_A0962M-12, www.astm.org 2. ASTM standard E381 “Standard Method of Macroetch Testing Steel Bars, Billets, Blooms, and Forgings” ASTM International, DOI: 10.1520/E0381-01R12, www.astm.org 3. ASTM standard A193, 2012 B “Standard Specification for Alloy-Steel and Stainless Steel Bolting for High Temperature or High Pressure Service and Other Special Purpose Applications”, ASTM International, DOI: 10.1520/A0193_A0193M-12B, www.astm.org 4. Metallography: Principles and Practice, George F. Vander Voort, ASM International, 1984, ISBN: 978-0-87170-672-0. www.asminternational.org
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