A. Dimatteo

Microstructural Features Affecting Tempering Behavior of 16Cr-5Ni Supermartensitic Steel

In industrial production processes, the respect of hardness and UTS maximum values of 16Cr5Ni steel is of utmost importance and a careful control of chemical composition and thermo-mechanical treatments is a common practice. Nevertheless, some scatter of properties is often observed with consequent rejection of final components. To better understand the role played by different factors, two heats of 16Cr-5Ni supermartensitic stainless steels with very close chemical compositions but different thermal behavior during tempering have been studied by means of TEM observations, X-ray diffraction measurements, dilatometry, and thermo-mechanical simulations. It has been found that Ms–Mf temperature range can extend below the room temperature and the relative amount of retained austenite in as-quenched conditions plays a significant role in determining the thermal behavior. When present, the γ-phase increases the amount of reversed austenite formed during tempering and accelerates the process kinetics of martensite recovery. Moreover, increasing amounts of retained austenite after quenching lower the critical temperature for austenite destabilization and influence the optimum temperature–time combination to be adopted for controlling final mechanical properties. In the studied cases, the very close chemical composition of the heats was not a sufficient condition to guarantee the same as-quenched structure in terms of retained austenite amount. This was proven to be related to solute segregation effects during solidification of original heats.

Microstructural Evolution during Tempering of 16Cr-5Ni Stainless Steel: Effects on Final Mechanical Properties

In this work, the structural behaviour during tempering of two different heats of 16Cr-5Ni supermartensitic stainless steel has been studied by means of dilatometry, transmission electron microscopy and X-ray diffraction. A thermomechanical simulator (Gleeble 3800) has been also used to characterize the effects on final mechanical properties of different tempering temperatures in the range 600 °C to 700 °C and the influence of sub-zero cooling on industrial double tempering treatments. It has been found that the pre-existence of retained austenite in as-quenched conditions can induce significant differences in the microstructural evolution during tempering and on the final mechanical properties of industrial components, thus inducing problems in controlling final maximum hardness allowable by normative requirements.

Improving the estimation of mean flow stress within hot rolling of steel by means of different artificial intelligence techniques

Abstract Prediction of mean flow stress within the hot rolling of steel products can lead to significant improvement in the manufacturing. In this paper this problem is dealt on data coming from a real industrial plant and it is shown how inadequate well known literature models are. Alternative models are then proposed by improving existing formulae by means of genetic algorithms based optimization and by the development of artificial neural networks based models. The performance of the proposed methods are compared and put into evidence the advantages of the use of artificial intelligence techniques within the handled industrial problem.

Diagnosis of the instability of the cooling behaviour of flat steel products through parametric characterisation, neural networks and statistics

This paper presents a mathematical model developed by means of an analytical function whose shape depends on the values of a few parameters for the run-out table cooling which is used in hot strip mills. The system relies on a first-order differential equation for describing the temperature loss along the run-out table. Neural networks have been applied in order to find correlations between the model parameters and the steel and process variables. Then, traditional statistical techniques have been applied in order to evaluate the stability of the cooling behaviour. Numerical results obtained on an industrial database are presented and discussed.