For all kinds of important forgings, the first consideration must be to prevent and eliminate the white spot problem when making the heat treatment process after forging. Therefore, it is necessary to know the results of hydrogen sampling at the risers of the large ingot of the forging, which can be used as the data of the average content in the steel, and then determine the necessary dehydrogenation annealing time by hydrogen expansion calculation of the large forgings to ensure that there is no white spot defect in the forging, and arrange it in the post-forging heat treatment process. This is the most important and must be solved first in the formulation of large forgings after heat treatment process, must be done.
In order to make the steel forgings have better mechanical properties and machinability, and to prevent white spots, dehydrogenation annealing is adopted.
Hydrogen in the forgings is reduced to below the limit hydrogen content of steel without white spot or hydrogen embrittlement by dehydrogen annealing, and its distribution is uniform to avoid the harm of white spot and hydrogen embrittlement. For most large forgings, this is the primary task of post forging heat treatment and must be completed.
The key process parameters of dehydrogenation annealing are:
1. Annealing temperature: usually 650+/-10℃. Therefore, the temperature is similar to the high temperature tempering of steel, so dehydrogenation annealing and high temperature tempering are often combined. Take 650℃ for annealing temperature of forgings.
2. Heat preservation time: according to the actual results of the workpiece, it needs to be determined by the calculation of hydrogen expansion of the forging.
3. Cooling speed: should be slow enough to prevent white spots due to excessive instantaneous stress in the cooling process, and minimize the residual stress in the forging. Generally, the cooling process is divided into two stages: above 400℃, because the steel is in the temperature range of good plasticity and low brittleness, the cooling rate can be slightly faster; Below 400℃, because the steel has entered the cold hard and brittle temperature range, in order to avoid cracking and reduce instantaneous stress, a slower cooling rate should be adopted.