Methods for determining austenite grain size

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01 Method for determining austenite grain size — Carburization method. The carburization method is suitable for determining the austenite grain size of carburized steel. The specific steps are as follows: first, the specimen is placed in a container containing carburizing agent and then sent into a furnace to be heated to 930±10°C, where it is held at this temperature for 6 hours, so that a carburized layer of more than 1 mm thick forms on the specimen, and its surface layer acquires a hypereutectoid composition. Then slow cooling is carried out, to a degree sufficient for cementite networks to precipitate at the austenite grain boundaries in the hypereutectic region of the carburized layer. After cooling, the specimen was ground and etched, revealing the morphology of austenite grains in the hypereutectic region. Samples for grain size testing are usually etched using one of three types of etchants: a 3%–4% nitric acid-alcohol solution, a 5% picric acid-alcohol solution, or a boiling alkaline sodium picrate solution (2 g of picric acid, 25 g of sodium hydroxide, 100 mL of water). 02 Method for determining austenite grain size – oxidation method. The oxidation method takes advantage of the fact that oxygen atoms diffuse into the crystal grains at high temperatures, causing the grain boundaries to oxidize preferentially, thereby revealing the size of the austenite grains. Oxidation methods are further divided into atmospheric oxidation and molten salt oxidation etching. The atmosphere oxidation method is generally used. After polishing, the sample was placed with its polished surface facing up in an air heating furnace and heated for 1 hour, after which it was cooled in water. Depending on the degree of oxidation, the specimen can be tilted appropriately by 10°–15° for grinding and polishing, so that an oxide layer remains in certain areas of the specimen’s testing surface, allowing the austenite grain size to be determined directly under a microscope. For better visibility, it can be etched using a 15% hydrochloric acid alcohol solution. Additionally, the polished specimen can also be heated in a vacuum and held at that temperature, after which it is cooled in air or slowly cooled to oxidize the grain boundaries; the austenite grain size is then measured after performing the aforementioned treatments, and this method is known as the vacuum method. 03 Method for determining austenite grain size – reticular ferrite method. The reticular ferrite method is suitable for carbon steels with a carbon mass fraction of 0.25%~0.60% and alloy steels with a carbon mass fraction of 0.25%~0.50%. Unless otherwise specified, the heating temperature is chosen as in the oxidation method; the material should be held at that temperature for at least 30 minutes, after which it is cooled either by air or by water. After grinding and etching, a ferrite network distributed along the original austenite grain boundaries becomes visible. For carbon steel specimens with a high carbon content and alloy steel specimens with a carbon content exceeding 0.40%, the cooling method needs to be adjusted in order to facilitate the precipitation of a distinct ferrite network at the austenite grain boundaries. At this point, it is recommended to keep the sample at the quenching temperature for at least 30 minutes, then lower the temperature to 730±10°C and hold it there for 10 minutes, before quenching it in oil or water. After further grinding and etching of the specimen, the ferrite network distributed along the original austenite grain boundaries can be revealed. Generally, etching can be carried out using one of the following corrosives: a 3%~4% nitric acid alcohol solution and a 5% picric acid alcohol solution. 04 Method for determining austenite grain size – reticular carbide method. Applicable to hypereutectoid steels (with a carbon mass fraction generally higher than 1.0%). Unless otherwise specified, all specimens are heated to 820±10°C and held at that temperature for at least 30 minutes, after which they are cooled slowly in the furnace to a temperature below the lower critical temperature, so as to allow the precipitation of cementite networks on the austenite grain boundaries. After grinding and etching, the specimen reveals the morphology of the original austenite grains with cementite networks precipitated along the grain boundaries. At this time, etching can also be carried out using one of the aforementioned etchants. 05 Method for determining austenite grain size – reticular pearlite (troostite) method. For eutectoid steels that are difficult to analyze using other methods, rod-shaped specimens of appropriate size can be used for local quenching; that is, one end of the heated specimen is plunged into cold water for cooling while the other end remains exposed to air, resulting in a small area that is not fully hardened. In this region, the original austenite grain boundaries will exhibit a network of small pearlitic particles (clustered bainite), revealing the shape of the original austenite grains. This method can be used for certain steel grades that have a composition slightly higher or slightly lower than the eutectoid composition. The corrosive agent for the etched specimen is the same as the one mentioned above. 06 Method for determining austenite grain size — Grain boundary etching method. For steels hardened by direct quenching, this method can be used to reveal the original austenite grains. Unless otherwise specified, the heating temperature and holding time are the same as those for the oxidation method. After heating and holding, quenching is carried out at a cooling rate sufficient to achieve complete hardening, resulting in a martensitic structure. After grinding and etching, the specimen reveals the morphology of the original austenite grains that have been fully hardened into martensite. To clearly display the grain boundaries, the specimen can be tempered at 550±10°C for 1 hour before etching. A commonly used etchant is a saturated picric acid aqueous solution with a small amount of ethylene oxide polymer.