Iron and Steel

Examples of Phase Decoration

Depending upon the steel composition, a variety of procedures can be used to decorate the prior-austenite grain boundaries –if the steel can be heat treated. Examples include ferrite or cementite decorating the grain boundaries after slow cooling or after carburizing, or using the oxidation method. If the specimen can not be heat treated, as in failure studies, we can only use etchants to bring out the prior-austenite grain boundaries.

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Wrought Iron

Microstructure of wrought iron (Fe – 0.07% C – 0.06% Mn – 0.2% Si – 0.16% P – 0.02% S) etched with 2% nital revealing slag stringers and ferrite (alpha) grains and grain boundaries. Original at 500X.

Electrical Iron

Electrical iron (<0.02% C) etched with 2% nital revealing a ferrite grain structure.
Motor Lamination Steel – Surface of electrical steel specimen showing large sub-surface ferrite grains. Tint etched with Klemm’s I. The grain orientations are random (note the random distribution of colored ferrite grains). Original at 100X. Viewed with crossed polars + sensitive tint.

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Microstructure of austempered ductile iron tint etched with Beraha’s CdS reagent containing large graphite nodules (arrow), bainite (blue and brown), also called “ausferrite” and retained austenite (white) when viewed with polarized light plus sensitive tint. Original at 500X.
Microstructure of austempered ductile iron containing “ausferrite” needles and retained austenite. The specimen was etched with aqueous 10% sodium metabisulfite and the magnification bar is 10 µm long.

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Carburizing has been a popular method for obtaining a unique balance of properties in steels for many years. Steels with low carbon contents are characterized by good toughness and good ductility, but low strength and low wear resistance. By locally increasing the carbon content at the surface, followed by heat treatment to produce a very high hardness in this layer while maintaining a lower hardness in the core, the “best of both worlds” can be obtained a hard, wear-resistant surface with good fatigue strength and a tough, ductile core to inhibit the growth of any cracks that might form at the surface. Such a steel will be tougher than a through-hardened medium carbon content steel with the same surface hardness.

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Preparation of Cast Iron Specimens

• Grind with 220-320 grit SiC (P240-P400), 240-300 rpm, 6 lbs/specimen, complementary rotation, until all samples in holder are co-planar
• Polish with 9-µm Metadi Supreme Diamond on an Ultra-Pol psa cloth, at 150 rpm, comp. rotation, 5 minutes
• Polish with 3-µm Metadi Supreme Diamond on a Texmet 1000 psacloth at 150 pm, comp. rotation, 4 minutes
• (Optional) Polish with 1-µm Metadi Supreme Diamond on a Trident psacloth at 150 rpm, comp. rotation, for 3 minutes
• Final polish with 0.05-µm Masterpolish alumina suspension on Microcloth psa pad, 120-150 rpm, contra rotation, for 2 minutes

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7-Mo PLUS

PLUSFerrite-austenite grain structure of hot-rolled and annealed 7-Mo PLUS duplex stainless steel revealed using (left) 15% HClin ethanol and (right) 10% CrO3in water, 6 V dc, 10 s.

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The mechanical properties of heat-treated alloy steels are strongly influenced by the grain size of the parent austenite phase before quenching. But revealing the prior-austenite grain boundaries (PgGBs or PAGBs) can be quite difficult depending upon the alloy and its microstructure.

There are a number of well-established (see ASTM E 112, for example) procedures that are used to decorate the PgGBs during a heat-treatment cycle, e.g., the McQuaid-Ehn carburizing test (Fig. 1) and the oxidation test. In some medium-carbon steels, at a specific cooling rate, proeutectoid ferrite will precipitate at the PgGBs, while in high-carbon steels (generally hypereutectoid tool steels), proeutectoid cementite will precipitate on the PgGBs upon slow cooling from elevated temperatures. These conditions are often seen in as-cast or as-rolled steels, as shown in Figures 2 and 3. But these methods cannot be applied to determine the prior-austenite grain size of a steel part or component that has already been heat treated, as these methods will produce a different grain size. For this problem – and this is a common situation in failure analysis – one can only use an etching technique to reveal the PgGBs. By George F. Vander Voort