engleski

Prediction of Properties of Gas-Quenched Work Pieces Based on the Modified Hardenability Test

The hardenability of steel is usually characterized by the Jominy end-quenched test. Cooling rates produced at this test range from 270 K/s at 1.5 mm distance from the quenched end to 5.6 K/s at 25 mm distances. For steel grades of high hardenability, having low values of critical cooling rate, there is no decrease of hardness in the mentioned region on the Jominy specimen, i.e. the Jominy test is not enough selective and sensitive for these steels. To establish hardenability for these steels, when they are gas quenched with different pressure and gas flow velocities (usually milder quenching intensity compared to water or oil), lower cooling rates at the quenched end of the specimen and more gradual decrease of cooling rates along the specimen is required. In order to make hardenability test more selective with a broader range of lower cooling rates a modified test specimen and special device should be used, as described in the paper. This new hardenability testing method with the modified Jominy test specimen has been proposed by the collaborative research project established among the Stiftung Institut für Werkstofftechnik, Bremen (IWT), the company Ipsen International, and the Faculty of Mechanical Engineering and Naval Arhitecture (FMENA), the University of Zagreb. The modified hardenability testing is based on the high pressure gas end-quenching of cylindrical specimen with variations of kind of gas, pressure and its velocity to achieve more gradual decrease of cooling rates along the specimen and to make the hardenability test more selective with a broader range of lower cooling rates which are characteristic for gas quenching in vacuum furnaces. The hardenability curves obtained by this new test were used for prediction of microstructure and hardness over cross section of gas-quenched cylinders made of steel grade EN 90MnCrV8. The following three models has been compared: Creusot-Loire calculations ; Liscic-Filetin method with finding an equidistant points of Jominy curve and work piece and method with calculation cooling time from 800 to 500 C at discrete points in work piece and finding their correspond hardness from hardenability curve. The simulated and experimental results of the hardness distribution and portions of microstructure phases of particular model have been compared and discussed.

high pressure gas quenching; steel hardenability

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