Optimization of Precipitates in High Strength Steels
Some steels like AF1410 utilize a fine dispersion of M2C precipitates to achieve high strength and toughness. This is sometimes referred to as secondary hardening, as the hardness increases during a tempering treatment as the M2C precipitates form. To achieve the best properties, it is desirable to have both a high volume fraction and a fine dispersion of precipitates. This can be achieved by creating an alloy with a high driving force for that precipitate to form at the heat treatment temperature. However, if the precipitates coarsen too much, the strength will start to decrease (sometimes called overaging). In thick part sections it may be difficult to perform short heat treatments, so it is desirable that the coarsening rate of the precipitates is low, so that we can maintain a wide heat treatment process window.
To visually optimize these inherently competing properties, a cross plot can be created using the Property Model Calculator and a suitable database, in this case the Thermo-Calc Steel and Fe-Alloys Database, TCFE. Below is a cross plot for the driving force and coarsening rate of M2C vs V and Mo content for a fixed Fe-10Ni-14Co-0.15C (wt%) chemistry. In this case – increasing both V and Mo is beneficial as Mo has a strong effect on slowing down the coarsening rate, and both elements work to increase the driving force. To choose an optimum chemistry from this, we might set the Mo content to be 4 wt.%, since it has the strongest effect on lowering the coarsening rate. With Mo fixed at 4 wt%, adding some V will increase the driving force, but the effect is not very strong above about 0.5 wt%. So one possible optimum chemistry to maximize strength and processability would be Fe-10Ni-14Co-0.15C-4Mo-0.5V (wt%).
