Christian Doppler Laboratory

          Early Stages of Precipitation

Christian Doppler Laboratory

          Early Stages of Precipitation

 

 

Project manager: Rene Radis

 

Microalloyed high strength low alloy steels (HSLA) owe their superior mechanical properties to a high density of second phase precipitates, e.g. nitrides, carbides or complex carbonitrides. These particles are made responsible for two very important strengthening mechanisms in the development of steel. On the one hand a huge amount of fine dispersed distributed nitrides and carbides lead to an enhancement of the yield strength due to precipitation hardening. On the other hand, additionally to the precipitation hardening, some precipitates are made responsible for grain size control due to the favoured precipitation of nitrides and carbides at grain boundaries. Important mechanical properties like toughness, deep drawability or weldability are directly related to the achievement of small grains, thus to the precipitation of these particles. Especially Aluminium is known to precipitate predominantly at grain boundaries, particularly in the austenitic phase field. Therefore it delivers a huge contribution to the mechanism of fine grain hardening.

For a better understanding of the precipitation behaviour of these carbides and nitrides as well as its effects on the mechanical properties of steel, it is essential to understand their precipitation kinetics. Therefore the present project deals with the numerical simulation of the precipitation process of these second phase particles as well as their kinetic interactions. Using the software package MatCalc it is possible to predict important microstructure parameters like phase fraction, particle size and number density of precipitation phases. Figure 1 shows exemplarily a calculated time-temperature-precipitation diagram for AlN in a typically microalloyed steel, containing 0.05 % Al and 0.005 % N. The lines represent 5 %, 50 % and 95 % of the equilibrium phase fraction at each temperature. Using this simulation tool, new heat treatments and/or alloys can be developed or existing ones can be optimized. Thereby time and costs, regarding the development of new alloys or optimization of the production process, can be reduced, which leads to a couple of advantages.

 

Figure 1 : Calculated time-temperature-precipitation (TTP) diagram for AlN

in a steel containing 0.05 % Al and 0.005 % N