Transformation Induced Plasticity (Trip) in a maraging steel
Résumé
An extensive experimental program on a 9 % Ni, 12% Cr, 2% Mo steel maraging steel is introduced. For the specific purpose of studying the physical phenomenon of transformation induced plasticity this material exhibits some very desirable features: The martensite start temperature (M s ) can be found around 150°C; the martensitic finish temperature (M f ) is at ca. 80°C. The material has a yield stress of about 200 MPa at ca. 200°C (fully austenitic state) and of ca. 900 MPa at room temperature (fully martensitic state). Plastification is accompanied by rather small hardening up to a strain of roughly 8 %. Since the martensitic transformation takes place at a low temperature level upon cooling on air only, this material can be considered as an ideal testing material with practically no creep effects during and after transformation. First of all dilatation tests are demonstrated for two types of specimens: longitudinal, also called axial specimens with a fiber texture and radial specimens, as shown in Figure lb. A material characterization using M s and M f lines, i.e. martensite start and finish temperature monitored for different loading conditions and stress levels reveals the influence of the type of loading (tension, shear, compression). Consequently two different loading histories are tested:
- loading above M s and then cooling, accompanied by partial or full unloading,
- loading at a constant temperature in the transformation temperature interval.