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Heat forge is performed within a certain temperature range. The forging temperature of steel refers to the temperature range between the start forging temperature (initial forging temperature) and the end forging temperature (final forging temperature).
Ensure that the steel has higher plasticity, lower deformation resistance, and obtain high-quality forged products. At the same time, the forging temperature range is as wide as possible to reduce heating times and improve forging productivity.
(1) Based on the steel balance diagram, and then refer to the steel plasticity diagram, resistance diagram and recrystallization diagram, make a comprehensive analysis from three aspects of plasticity, quality and deformation resistance, so as to determine the initial forging temperature and the final forging temperature.
(2) Generally, the forging temperature range of carbon steel can be directly determined according to the iron-carbon balance diagram. The forging temperature range of most alloy structural steels can be considered with reference to carbon steels with the same carbon content. However, for high-alloy steels with low plasticity and steels that do not undergo phase transformation (such as austenitic steels, pure ferritic steels), tests must be passed to obtain a reasonable forging temperature range.
(1) To determine the initial forging temperature of steel, heat forge must first ensure that there is no overburning of the steel. Therefore, for carbon steel, the initial forging temperature should be lower than the initial melting line of the iron-carbon balance diagram of 150 to 250°C. In addition, factors such as rough structure, forging method and deformation process should also be considered.
(2) The final forging temperature of heat forge. When determining the final forging temperature for heat forge, it is necessary to ensure that the steel has sufficient plasticity before the final forging, and to enable the forging to obtain good structural properties. Therefore, the final forging temperature of the steel should be higher than the recrystallization temperature to ensure complete recrystallization after forging and to obtain fine grain structure for the forging.
(3) For carbon steel, the final forging temperature cannot be lower than the specified line. Otherwise, the plasticity is significantly reduced, the deformation resistance is increased, the work hardening phenomenon is serious, and forging cracks are prone to occur.
(4) For hypoeutectoid steel, the final forging temperature should be 15 to 50°C above the specified line, because it is located in the single-phase austenite zone, the structure is uniform and the plasticity is good. However, for low carbon steel (carbon content is less than 0.3%), the final forging temperature can be lowered below the specified line. Although it is in the (γ+α) dual phase zone, it still has sufficient plasticity, and the deformation resistance is not too high. And the forging temperature range is also expanded.
(5) For hypereutectoid steel, the final forging temperature of heat forge should be below the Acm line and 50 to 100℃ above the specified line. This is because if the final forging temperature is selected above the Acm line, the secondary network cementite will precipitate along the grain boundary during the cooling process after forging, which will greatly reduce the mechanical properties of the heat forge. For example, when forging between the Acm wire and the carbon steel A1 wire, due to the mechanical crushing effect of plastic deformation, the precipitated secondary cementite can be dispersed.
It must also be pointed out that the final forging temperature of hot-forged steel is also related to the structure, forging process and subsequent processes of the steel. For non-phase-transformed steels, since heat treatment cannot be used to refine the grains, forging can only be used to control the grain size. In order to obtain fine grains for forgings, the final forging temperature of this type of steel is generally low. When the waste heat treatment of forgings is performed immediately after forging, the final forging temperature should meet the requirements of waste heat treatment. If the material of the heat forge is low carbon steel, the final forging temperature is slightly higher than the A line.
