After the forging process is completed, the large forgings should be treated immediately after forging.
The following principles should be adhered to in the formulation of the heat treatment process after forging: 1. Make forgings as soon as possible, fully converted from austenite to ferrite one carbide tissue. This is not only beneficial to the dissolution and diffusion of hydrogen, but also helps to adjust and refine the grain microstructure of steel by the phase change recrystallization.
The process parameters such as the cooling speed, supercooled temperature and heat preservation time of the forgings should be reasonably determined according to the stability of the supercooled austenite of the steel and the inhomogeneity of the chemical composition and the internal microstructure. 2. The hydrogen in the forging is reduced to the point of no white spot or hydrogen embrittlement limit hydrogen content by the hydrogen-removing annealing, and the distribution is even, in order to avoid the harm of white spots and hydrogen embrittlement. For most large forgings, this is the first task of heat treatment after forging, must be completed.
The key technological parameters of hydrogen-annealing are: Annealing temperature: usually take 650±10℃.
Therefore, the temperature and the high temperature tempering of steel, so often the hydrogen-expanding annealing and high-temperature tempering combined.
Holding time: Refer to the typical process curve approximate determination, or according to the actual forging of the fixed hydrogen results, through the forging of the hydrogen expansion calculation determined. Cooling speed: Should be slow enough to prevent the transient stress in the cooling process when the white spots, and minimize the residual stress in forgings.
The cooling process is usually divided into two phases: above 400 ℃, because the steel is in the plasticity is good, the brittleness low temperature range, the cold speed may be slightly quicker; under 400 ℃, because the steel has entered the temperature range of chilled and brittle, in order to avoid cracking and reduce instantaneous stress, should adopt more slow cooling speed. 3. After one or more recrystallization, the grain refinement, tissue improvement and performance of steel are improved. For most carbon steel forgings and some low alloy forgings, after forging heat treatment is its final heat treatment, should be in the process of heat treatment after the hot and high-temperature tempering, so that it can obtain the necessary organization and performance. For forgings with more alloying elements and higher performance requirements, even though the final heat treatment is still required, it is necessary to arrange a recrystallization process in the heat treatment after forging, in order to improve the microstructure and performance of forgings, prepare the necessary tissue conditions for final heat treatment, and improve the ultrasonic flaw detection performance of forgings.