Effect of increasing nickel and molybdenum content in austenitic steel on thermal deformation conditions for achieving the peak strain

Obtaining the required grain size in steel forgings with austenitic metal structure is a problem, especially for the forged thick-walled blanks made by forging on hydraulic presses. For such steels, there is no possibility of grain refinement during heat treatment like for steels of the bainitic and martensitic classes which undergo polymorphic transformation. In this case, refinement to the required grain size is possible only at the stage of hot plastic deformation as a result of recrystallization. Formation of required grain size happens in the process of forging and this grain size is one of the control parameters when blanks acceptance is carried out. The rate of nucleation of new grains increases sharply during dynamic recrystallization at the peak strain (ер) and depends on temperature, deformation rate, initial grain size and chemical composition. The article presents the results of measurements of ер and peak stress (σр) in austenitic steels with a nickel content of ~10, 20, 25 % and a molybdenum content of ~0.01, 2.5 % under thermodeformation conditions typical for forging large-sized blanks on a hydraulic press. It is shown that an increase in the nickel content up to 25 % in combination with alloying with molybdenum, in the temperature range 900 –1200°C, has the most significant effect on the level of ер and σр values during forging at the 10−1 s−1 deformation rate. At deformation rates of 10−2 and 10−3 s−1, the values of ер and σр of the studied steel grades are practically the same.