ASM Handbook, Volume 9: Metallography and Microstructures Copyright © 2004 ASM International® G.F. Vander Voort, editor, p820–859 All rights reserved. DOI: 10.1361/asmhba0003737 www.asminternational.org Metallography and Microstructures of Heat-Resistant Alloys George F. Vander Voort and Gabriel M. Lucas, Buehler Ltd. Elena P. Manilova, Polzunov Central Boiler and Turbine Institute, St. Petersburg, Russia HEAT-RESISTANT ALLOYS cover a wide nickel-base, and cobalt-base. The metallographic The procedures used to prepare metallo- range of chemical compositions, microstructural methods discussed also are suitable for preparing graphic specimens of cast or wrought heat-resis- constituents, and mechanical properties. This ar- both cast and wrought heat-resistant alloys; mi- tant grades are quite similar to those for iron- ticle summarizes metallographic techniques and crostructural constituents are quite similar ex- base alloys, especially stainless steels (see the microstructural constituents for three types of cept for obvious differences in homogeneity and Section “Metallographic Techniques” in this cast and wrought heat-resistant alloys: iron-base, porosity. Volume). Aspects particularly significant to the Table 1 Compositions of Fe-Cr-Ni heat-resistant casting alloys Composition, % Grade Type C Mn, max Si, max P, max S, max Cr Ni Mo, max (a) HF 19 chromium, 9 nickel 0.20–0.40 2.00 2.00 0.04 0.04 18.0–23.0 8.0–12.0 0.50 HH 25 chromium, 12 nickel 0.20–0.50 2.00 2.00 0.04 0.04 24.0–28.0 11.0–14.0 0.50 HI 28 chromium, 15 nickel 0.20–0.50 2.00 2.00 0.04 0.04 26.0–30.0 14.0–18.0 0.50 HK 25 chromium, 20 nickel 0.20–0.60 2.00 2.00 0.04 0.04 24.0–28.0 18.0–22.0 0.50 HE 29 chromium, 9 nickel 0.20–0.50 2.00 2.00 0.04 0.04 26.0–30.0 8.0–11.0 0.50 HT 15 chromium, 35 nickel 0.35–0.75 2.00 2.50 0.04 0.04 17.0–21.0 33.0–37.0 0.50 HU 19 chromium, 39 nickel 0.35–0.75 2.00 2.50 0.04 0.04 17.0–21.0 37.0–41.0 0.50 HW 12 chromium, 60 nickel 0.35–0.75 2.00 2.50 0.04 0.04 10.0–14.0 58.0–62.0 0.50 HX 17 chromium, 66 nickel 0.35–0.75 2.00 2.50 0.04 0.04 10.0–14.0 58.0–62.0 0.50 HC 28 chromium 0.50 max 1.00 2.00 0.04 0.04 26.0–30.0 4.0 max 0.50 HD 28 chromium, 5 nickel 0.50 max 1.50 2.00 0.04 0.04 26.0–30.0 4.0–7.0 0.50 HL 29 chromium, 20 nickel 0.20–0.60 2.00 2.00 0.04 0.04 28.0–32.0 18.0–22.0 0.50 HN 20 chromium, 25 nickel 0.20–0.50 2.00 2.00 0.04 0.04 19.0–23.0 23.0–27.0 0.50 HP 26 chromium, 35 nickel 0.35–0.75 2.00 2.50 0.04 0.04 24.0–28.0 33.0–37.0 0.50 (a) Castings having a specified molybdenum range agreed on by the manufacturer and the purchaser may also be furnished under these specifications. Source: ASTM A 297 Table 2 Nominal compositions of nickel- and cobalt-base heat-resistant casting alloys Composition, % Alloy C Cr Mo Nb Ti Al B Zr Fe W V Ta Re Hf Ni Co Nickel-base alloys Alloy 713 C 0.12 12.5 4.2 2 0.80 6.1 0.012 0.10 2.5 . bal . Ta 0.90 0.50 . 18.5 . bal ם Alloy 718 0.04 19 3.05 5.13 B-1900 0.10 8 6 0.1 max 1 6 0.015 0.08 0.35 0.1 . 4.3 . bal 10 Hastelloy B 0.10 0.6 28 . 5 . 0.30 . bal 2.5 max Hastelloy C 0.07 16 17 . 5 4 . bal 2.5 max IN-100 0.15 10 3 . 4.7 5.5 0.015 0.06 . 1 . bal 15 IN-738 0.17 16 1.75 0.9 3.4 3.4 0.01 0.10 0.50 max 2.6 . 1.75 . bal 8.5 MAR-M 246 0.15 9 2.5 . 1.5 5.5 0.015 0.05 . 10 . 1.5 . bal 10 TRW-NASA VI A 0.13 6.1 2 0.5 1 5.4 0.02 0.13 333 5.5 333 9 0.2 0.43 bal 7.5 U-700 0.15 max 15 5.2 . 3.5 4.25 0.05 max . 1 max . bal 18.5 Cobalt-base alloys Haynes 21 0.25 27 5 . 1 . 3 bal Haynes 31 0.50 25.5 . 0.01 . 2 7.5 . 10.5 bal Haynes 151 0.50 20 . 0.05 . 12.7 . bal MAR-M 302 0.85 21.5 . 0.005 0.20 . 10 . 9 . bal MAR-M 509 0.30 24 . 0.20 . 0.50 . 7 . 3.5 . 10 bal Wi-52 0.45 21 . 2 . 2 11 . 1.0 max bal Metallography and Microstructures of Heat-Resistant Alloys / 821 preparation of cast or wrought heat-resistant al- tenitic Fe-Ni-Cr heat-resistant alloys are the of ring patterns remains obscure. Examination loys are emphasized. Tables 1 to 3 list the nom- same as those recommended for cast and has revealed little difference between ring and inal compositions of Fe-Ni-Cr Alloy Casting In- wrought austenitic stainless steels (see the article matrix areas and no measurable influence on me- stitute H-series alloys and other iron-nickel, “Metallography and Microstructures of Stainless chanical properties. nickel-, and cobalt-base cast and wrought heat- Steels” in this Volume). Macroetchants for cast resistant alloys, respectively. and wrought iron-nickel-, nickel-, and cobalt- base heat-resistant alloys are given in Table 4. Specimen Preparation Features observed on the macroetched cast Macroetching disks depict the results of solidification. Near the Sectioning. Various sectioning devices have surface, the grain structure will be finer than been used with heat-resistant alloys. The usual Cast and wrought heat-resistant alloys are ex- elsewhere. Dendrites will be visible, with the pri- precautions regarding excessive heating should amined for macrostructure in the same manner mary axis in the solidification direction. Segre- be followed. For austenitic grades, which are as tool steels and stainless steels. Macroetching gation and shrinkage cavities may also be ob- sensitive to deformation, the more vigorous sec- of cast specimens is generally performed as a served. Porosity due to gas evolution is unlikely tioning techniques, such as band sawing or research tool to study the solidification charac- to be seen. Macroetch features on disks from power hacksawing, will introduce excessive dis- teristics; macroetching of wrought specimens consumable-electrode remelted superalloys are tortion or work hardening, depending on the al- can be performed as part of a research study but different from those observed in ingot cast steels. loy and heat treatment condition. Such methods is more commonly performed as a required test Unique macroetch features observed in these re- are suitable for initial sectioning of large pieces, by product specifications and is, therefore, a rou- melted alloys include freckles, radial segrega- but final cutting of specimens should be per- tine production test. To evaluate castings, the tion, and ring patterns (Ref. 1–7). Freckles are formed using abrasive cutoff machines with disk is generally removed so that the etched sur- dark-etching spots due to localized segregation abrasive blades designed for metallographic face (ground smooth before etching for best re- or to enrichment in carbides or Laves phase. work (blades designed for production cutting in- sults) is parallel to the solidification direction. They are detrimental to material quality. The mi- troduce too much damage and should not be used For wrought alloys, disks are cut (usually from crostructure of a freckle in Rene´41 is shown in for metallography). Heavy deformation intro- wrought billet samples) at representative loca- Fig. 1. Radial segregation appears as dark-etch- duced by sawing may be difficult, if not impos- tions, such as the top and bottom of ingots or ing elongated spots in a radial or spiral pattern. sible, to remove by subsequent grinding and pol- remelted (electroslag remelted or vacuum arc re- Ring patterns are concentric rings that etch ishing steps. Abrasive cutoff wheels used are melted) stock, and are ground before macroetch- lighter (usually) or darker than the matrix. They usually the consumable type. Coolant flow must ing. Macroetchants for the cast or wrought aus- are revealed only by macroetching. The nature be adequate and uniformly distributed to mini- Table 3 Nominal compositions of wrought heat-resistant alloys Composition, % Alloy C Fe Ni Co Cr Ti Mo Others Iron-nickel-base alloys A-286 (AISI 660) 0.05 bal 26.0 ... 15.0 2.15 1.25 0.3 V, 0.2 Al, 0.003 B Greek Ascoloy 0.18 bal 2.0 ... 13.0 ... 0.50 max 3.0 W Moly Ascoloy 0.12 bal 2.5 ... 12.0 ... 1.75 0.70 Mn, 0.32 V, 0.025 N Incoloy 800 0.05 bal 32.5 ... 21.0 0.38 ... 0.38 Al Incoloy 901 0.05 bal 42.7 ..