The accurate determination of the volume fraction of retained austenite is of great importance for the optimization of transformation induced plasticity (TRIP) steels. In this work, two aluminium-containing TRIP steels are studied by means of magnetization and X-ray diffraction (XRD) measurements. By fitting the field dependence of the approach to saturation in the magnetization curves, the saturation magnetization is determined, which is linearly related to the volume fraction of retained austenite. Moreover, information with respect to the microstructure can be obtained from the fitting parameters and the demagnetizing factor for the magnetization curve. The volume fractions obtained from the magnetization measurements are compared with data from XRD measurements. A discussion of the data suggests that magnetization measurements lead to more reliable results and a more sensitive detection of the retained austenite than XRD measurements.

Magnetic and X-ray diffraction measurements for the determination of retained austenite in TRIP steels

Dual-phase and transformation-induced plasticity (TRIP)-assisted multiphase steels are related families of high-strength formable steels exhibiting excellent mechanical characteristics. This study shows how a ferrite-bainite-martensite microstructure containing retained austenite can improve the mechanical properties of a cold-rolled low-carbon, low-silicon steel. Such a multiphased microstructure is obtained by a heat treatment involving intercritical annealing followed by a bainite transformation tempering. Depending on the heat-treatment parameters, the samples present a variety of microstructures. Due to the presence of retained austenite, some samples exhibit a TRIP effect not anticipated with such a low silicon content. A composite strengthening effect also results from the simultaneous presence of a ductile ferrite matrix with bainite and martensite as hard second phases. A true stress at maximum load of 800 MPa and a true uniform strain of 0.18 can be obtained by forming a ferrite-bainite-martensite microstructure containing up to 10 pct of retained austenite. These properties correspond to a favorable evolution of work hardening during plastic deformation.

Enhancement of the mechanical properties of a low-carbon, low-silicon steel by formation of a multiphased microstructure containing retained austenite

The mechanical behavior of transformation induced plasticity (TRIP) steels depends on the martensitic transformation of the retained austenite. The studied material is a three-phase TRIP steel with ferrite base containing less than 1% of bainite and 8% of retained austenite. Tensile tests were performed at various temperatures ranging from −60 to +120 °C. X-ray diffraction was used to determine the kinetics of transformation at various temperatures. First results show that at low temperatures, the austenite transforms into martensite very rapidly, whereas the kinetics of transformation are much slower at high temperatures. The stress state in the austenite at room temperature was also obtained by X-ray diffraction. After pre-strain, it is noticed that retained austenite is in tension probably due to the carbon content which is more important in austenite than in ferrite.

Analysis of the martensitic transformation at various scales in TRIP steel

Modeling of precipitation hardening during the aging and overaging of 18Ni–Co–Mo–Ti maraging 300 steel

In this paper, quantifications of the austenitic phase in a maraging 300 steel heat treated at different temperatures and periods of time were carried out using the direct comparison method by X-ray diffraction. The influence of taking into account the chemical compositions of austenite and martensite phases in the results by the direct comparison method was evaluated. In order to analyze the instability of austenite under plastic deformation, the quantifications were carried out with and without previous grinding of the samples. The behavior of the austenite volume fraction against aging time at 560°C, 600°C and 650°C were determined. The variation of the martensite lattice parameter with aging time was also analyzed. The results show an increase of the austenite content with aging time at 560°C and 600°C. At 650°C, however, the austenite content present at room temperature decreases and the martensite parameter increases with the aging time above 1 h.

Study of the austenite quantification by X-ray diffraction in the 18Ni-Co-Mo-Ti maraging 300 steel

Magnetic properties in annealed and cold rolled conditions have been investigated for maraging steel grade 18%Ni-2400. The austenite content, coercive field, saturation magnetisation and remanence were determined after ageing for 1 h in the temperature range from 400 to 800°C. The results show that the degree of deformation imparted to martensite influences both the amount of reverted austenite and the magnetic properties obtained following ageing. Transmission electron microscopy was carried out in order to investigate the structural changes taking place during reversion of austenite.

Magnetic properties of maraging steel in relation to deformation and structural phase transformations

Bainite forms by a displacive transformation mechanism and exhibits an incomplete reaction phenomenon. The kinetics and mechanism of bainite transformation have been studied in high- strength Fe-C-Si-Mn and Fe-C-Si-Ni steels using high-speed dilatometry. The new experiments reported here confirm the incomplete reaction phenomenon, with the bainite transformation stop- ping well before paraequilibrium is achieved. These results show that bainite probably grows without diffusion, but soon afterwards, excess carbon is partitioned into the residual austenite.

A.Q. Khan

Papers

Magnetic properties of maraging steel in relation to deformation and structural phase transformations

Incomplete reaction phenomenon

Phase transformations in rapidly solidified CuAlNi β phase alloys