Metallographic Corrosion Principle of Carburizing Heat Treatment Workpiece

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After the carburizing heat treatment of the workpiece, the heat treatment structures that need to be checked are: internal oxidation, non-marine structure, martensite, retained austenite, carbide, and core ferrite. How to corrode the sample correctly will directly affect the judgment of the carburizing heat treatment structure.

For carburized heat-treated workpieces, the commonly used corrosion method is chemical corrosion, which is to corrode the polished surface of the sample in a chemical corrosive for a certain period of time, thereby showing the structure of the sample. 4% nitric acid alcohol is the most commonly used chemical corrosive reagent to show the metallographic structure of carburizing heat treatment. The operation method is: rinse the polished sample with water or wipe off the remaining dirt on the surface with alcohol, and then use a rubber tip dropper to absorb the corrosive agent, and quickly drop it on the polished surface of the carburized sample, and the polished surface That is, it gradually loses its luster; after the sample is corroded properly, rinse it with alcohol immediately, use a hair dryer to dry the surface of the sample, and then place it under a microscope for observation.

The degree of corrosion of carburized heat-treated specimens is mainly determined according to the structure to be observed. The corrosion of carburized heat-treated specimens follows the following principles: observe internal oxidation and directly metallographic measurement after polishing; surface non-horse tissue corrosion for 3-5 seconds; observe Martensite, austenite, and ferrite corrode for 7-15 seconds; observe the carbide for heavy corrosion for 15-25 seconds.

Principle of chemical corrosion: The corrosion of single-phase structure is a process of chemical dissolution. Due to the irregular arrangement of atoms on the grain boundary and high free energy, the grain boundary is susceptible to corrosion and presents concave grooves, which makes the structure visible. Polygonal crystal grains can be seen under the microscope. The corrosion of two-phase alloy is mainly an electrochemical corrosion process. The two constituent phases have different electrode potentials, and in the corrosive agent, very many tiny local cells are formed. The phase with a higher negative potential becomes the anode, which is dissolved into the electrolyte and gradually sinks; the other phase with a higher positive potential is the cathode, maintaining the original plane height. Thus, the structure of each phase can be clearly displayed under the microscope.

Surface organization of heat-treated workpiece 500×           Core organization of carburizing heat treatment workpiece 500×

In the organization inspection after carburizing heat treatment, the first thing to do is to polish and corrode the sample. If the operation is improper, the sample will cause false phases and misjudge the product. A qualified heat treatment metallographic inspector must practice this basic skill.

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