Research On Hot Deformation Behavior Of Nodular Cast Iron And Its Application

Nodular cast iron is widely used as a structural material in various fields, such as automotive industry, agricultural machinery, machine tools and metallurgy and so on, because of its several advantages over steel including wear resistance, shock absorption, machining, casting abilities, especially from the economical point of view. At present, almost all of the nodular cast iron is used directly as cast or after simply machining. Although its mechanical properties can be improved via alloying and heat treatments usully, alloying increases the opration costs and heat treatments leads to inefficient production, energy dissipation and environmental pollution. Compared with these two former processes, plastic deformaiton method is a more efficient processing technology which leads to a product with optimal properties, not only for dimensional precision and appearance, but also with respect to the mechanical and physical properties. So the applications of nodular cast iron will be extended significantly when the plastic deformation method is employed.In the present paper, the hot deformation behaviors of nodular cast iron were studied systematically by physical simulation. According to the analysis of the graphite particles morphology evolution and phase transition during the plastic deformation process, it can be seen that the final microstructure had a close relationship with deformation parameters, such as deformation temperature, strain rate and strain. In addition, the die forging and hot rolling processes parameters were optimized. A new quenching process for nodular cast iron (QT450-10) named forge-quenching heat treatment was developed, and preliminary process specification was proposed. Main research details and results are as follows:In order to study the hot deformation behaviors of nodular cast iron (QT450-10), isothermal compression tests were carried out with a Gleeble 1500 over a range of temperature from 650 to 950℃and constant strain rates from 10^-2 to 10 s^-1. The flow curves are marked by a peak stress and softening which decline as temperature rises. The peak stress diminishes with the increase of temperature and the decrease of strain rate while the peak strain presents a maximum with rise in temperature. The activation energy is 391.52 kJ/mol for nodular cast iron in peak stress. The flow stress (σ) of nodular cast iron was described well by the Zener-Hollomon parameter (Z) Where, Z =εexp(?)The relationship of critical strain for dynamic recrystallization (DRX) initiation and peak strain was described byε_c=0.61ε_pMicrostructure analysis showed that the phase transition occurs during the compression process of nodular cast iron. The transformation start temperature of ferrite and pearlite are raised 150℃and 43℃, respectively. In addition, the polygonal type ferrite emerges, and the pearlite spheroidizes, that is to say, the microstructure consists of ferrite and partially spheroidized cementite. The average size of ferrite grain is about 1.16μm and the granular cementite distributed at the ferrite grain boundaries is only 338 nm.The graphite particles morphology evolution is investigated. The results show that both of the volume fraction of graphite and shape ratio (major axis/minor axis) decrease while the space between two graphite particles increase as the temperature increases; when the strain rate increases the space between two adjacent graphite particles becomes smaller slightly, and the volume fraction of graphite and shape ratio are invariant; the space and the volume fraction of graphite decreases significantly while the volume fraction of graphite is constant with the increase of the strain.The results of mechanical calculation show that the stress concentration of nodular cast iron caused by graphite particles can be weakened when the direction of applied stress is parallel to the major axis of elliptic graphite compared with the nodular graphite particles. Because of the friction between the matrix and the graphite, the graphite becomes elongated on the direction of stress until the graphite occurs fracture when the strain is higher than the minimum strain (ε>ε_min).The optimized hot working temperatures of QT450-10 are determined by successive approximation method. In comparison with the interlamellar spacing (0.46μm) of pearlite in nodular cast iron after common normalizing, the finer pearlite whose interlamellar spacing is < 0.319μm is obtained during the die forging process. Although the elongation of forged nodular cast iron decreases 14%, the tensile strength and the bending strength are increased 61.6% and 61.2%, respectively. Nodular cast iron also showed good workability since the rolled nodular cast iron still kept plate without any shatter with the reduction of 89% on the compression direction. Besides, the nodular cast iron paltes show anisotropic mechanical properties. For example, the tensile strength and the elongation of samples parallel to the rolling direction are higher 18.9% and 13.3% than these values of samples perpendicular to the rolling direction, respectively, as compared with the samples perpendicular to the rolling direction.At last, the forge-quenching treatment test of nodular cast iron was designed. The results show that an existence of finer acicular martensitic structure and prolate ellipsoids graphite particles. The hardness and impact tests results show that the forge-quenched nodular cast iron displays superior mechanical properties to conventional quenched nodular cast iron. The wear rates of forge-quenched nodular cast iron are lower 53.1% and 51.1% than conventional quenched nodular cast iron and white cast iron, respectively. Meanwhile, the friction coefficients are also lower 7.4% and 9.6% than conventional quenched nodular cast iron and white cast iron, respectively.