In the cooling or quenching process, the forming tool remains closed for a certain amount of time until the part is cooled down from approximately 900 °C to below 400 °C. Cooling enables the steel material to harden so that the finished sheet metal part has a final strength of up to 1500 MPa. It is not possible to achieve similar strengths using conventional forming methods.
In quenching, the hot metal sheet is quenched through contact with the cooled forming tools. This is made possible by cool water which flows through the cooling canals integrated within the tools. Since the tools are colder than the formed metal sheet, the sheet is cooled down and as a result the material properties are changed. Before the sheet is cooled, the high temperatures soften it, making it easy to form: This is known as the austenitic structure of the steel material.
The heat flow and phase transformation must be taken into account during quenching. Forming and quenching of hot parts after austenitization leads to a martensitic microstructure.
In order to achieve the desired strength, it is necessary to reach a cooling rate of more than 27 K/s during forming, but especially directly after forming and in the closed tool. A sufficiently rapid rate of cooling is required for the structure to change from austenite into martensite; after this transformation, the material is hardened and tempered.
The most important factor influencing the rate of cooling is the direct contact between the cooled tool surface and hot formed sheet metal. Since an air gap between the sheet and tool acts as an insulator, the forming tools must be built so that when they are closed, they are in contact over all of the formed sheet metal.
Another factor influencing the quenching process is the tool material. New developments have produced tool steels which have particularly high thermal conductivity and are therefore especially well suited for the quenching process. In general, however, a compromise in terms of wear resistance must be made.
In summary, the cooling or quenching process is a part of the press hardening process. The quenching process ensures that the sheet metal part obtains its desired strength; the part is hardened.
Further information on quenching die stamping at AutoForm:
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