STRUCTURE-ANALYSIS, FATIGUE TESTING, AND LIFETIME PREDICTION OF COMPOSITE STEELS

Composite steels prepared by technology of powder metallurgy are widel y used as low cost parts with good resistance to wear, fracture, and c orrosion. The development of powder composite steels is directly relat ed to strength under vibration, fatigue stabilizing, and accurate life time prediction for actual composite topology. The fatigue behavior of powder steels was studied by experimental and numerical methods of co mposite mechanics and materials science. The chemical composition of c omposite steel is a pure iron powder as the base material and a handfu l of carbon, chromium, nickel, or phosphorus powders. The powder multi -component mixture is compacted by cold isostatic pressing to a rectan gular form. The compactants are sintered in protective atmosphere. The microscale examination of the composite structure included an METAM-R V-21 metallographic optic microscope with a high-resolution ScanNexIIc scanner and an image processing package on the PC platform. The phase composition of powder steels has complex disordered topology with irr egular ferrite/austenite grains, iron carbide inclusions, and pores. T he microstructure images are treated according to the theory of stocha stic processes as ergodic probability functions; statistical moments a nd a structural covariance function of the composite steels are given. The microscale stress-strain state of the composite steel is analyzed by finite element methods. The stiffness matrix of the composite stee l is also presented together with stiffness matrices of ferrite/austen ite grains, iron carbide inclusions, and gores as zero matrices. Endur ance limits of the microstructural components are described by the Bas quin or Coffin-Manson laws, respectively, as high and low cycle fatigu e; cumulative microdamage in loading with a variable amplitude is take n from the Palmgren-Miner rule. Planar specimens were tested by consol e bending. Symmetric fatigue cycling was performed at a stable frequen cy of 20 Hz with endurance limits up to 5.10(6) cycles. The probabilis tic S-N curves were studied for various types of the composite steels. The fatigue properties of: the structural components such as ferrite/ austenite grains and carbide particles were defined by the microscale stress-strain modeling. Structural impact on the fatigue lifetime was computed; the probabilistic fatigue curves of the composite steels of various phase compositions tare given. The prediction of cyclic lifeti me and fatigue testing show good agreement for the powder composite st eels studied.