New process for quenching metal materials
In the long-term production practice and scientific experiments, people's understanding of the internal state of metal structure changes, especially since the 1960s, the application of transmission electron microscopy and electron diffraction technology, the continuous improvement of various testing techniques, in the study of horses The morphology, substructure and its relationship with mechanical properties, the conditions of martensite with different morphology and substructure, the influence of the morphology, size, number and distribution of the second phase on mechanical properties have been greatly improved. The progress, new quenching processes based on these foundations are also emerging.
(1) Cyclic rapid heating and quenching
The strength of quenched and tempered steel is related to the austenite grain size. The finer the grain and the higher the strength, so how to obtain ultrafine grain with higher than 10 grain size is one of the important ways to improve the strength of steel. . The steel undergoes recrystallization by α→γ→α multiple phase transformation to refine the grains; increasing the heating rate and increasing the crystal center can also refine the grains. Cyclic rapid heating quenching is a new process for obtaining ultra-fine grains according to this principle to achieve strengthening. For example, 45 steel, repeated heating and quenching in a lead bath at 815 ° C for 4-5 times, can make austenite grains from 6 to 12 to 15; and as 20CrNi9Mo steel, with 3000 Hz 200 kW medium frequency induction The heating device is heated to 760 ° C at a rate of 11 ° C / s, then water quenched, increasing the σs from 960 MN / m 2 to 1215 MN / m 2 , the gas from 1107 MN / m 2 , increased to 1274 MN / m 2 , and the elongation remains unchanged , It is 18%.
(2) High temperature quenching
Here, the high temperature is relative to the normal quenching heating temperature. If the low carbon steel and the medium carbon steel use a higher quenching temperature, the lath martensite can be obtained, or the number of lath martensite can be increased, thereby obtaining a good Comprehensive performance.
Experiments from the relationship between the carbon content of austenite and the morphology of martensite prove that the steel obtained by quenching the carbon content of less than 0.3% is all lath martensite. However, ordinary low carbon steel has extremely poor hardenability. To obtain martensite, in addition to alloying to improve the stability of supercooled austenite, only the austenitizing temperature and the quenching cooling can be improved. For example, a five-plow plow arm made of 16Mn steel is quenched in a 10% NaOH aqueous solution at 940 ° C and low tempered, and good results are obtained.
The medium carbon steel is quenched by high temperature to make the austenite composition uniform: more stripe martensite is obtained to improve its comprehensive performance. For example, AISl4340 steel, after quenching at 870 °C, tempered at 200 °C, its σs is 1621 MN / m2, the fracture toughness Kc is 67.6MN / m, and heated at 1200 ° C, pre-cooled to 870 ° C after quenching, 200 ° C tempered, Σs was 1586 MN/m 2 and the fracture toughness Kc was 81.8 MN/m. If compared in the quenched state, the fracture toughness of high temperature quenching is almost double that of ordinary quenching. Metallographic analysis showed that high temperature quenching avoided the appearance of flaky martensite (twisted martensite) and all obtained lath martensite. In addition, a layer of 100-200-pound retained austenite is placed on the outside of the martensite lath, which can buffer the stress concentration at the crack tip and thus improve the fracture toughness.
