Phosphorus as sintering activator in powder metallurgical steels: characterization of the distribution and its technological impact
Powder metallurgy is a highly developed method of manufacturing reliable ferrous parts. The main processing steps in a powder metallurgical line are pressing and sintering. Sintering can be strongly enhanced by the formation of a liquid phase during the sintering process when using phosphorus as sintering activator. In this work the distribution (effect) of phosphorus was investigated by means of secondary ion mass spectrometry (SIMS) supported by Auger electron spectroscopy (AES) and electron probe micro analysis (EPMA). To verify the influence of the process conditions (phosphorus content, sintering atmosphere, time) on the mechanical properties, additional measurements of the microstructure (pore shape) and of impact energy were performed. Analysis of fracture surfaces was performed by means of scanning electron microscopy (SEM). The concentration of phosphorus differs in the samples from 0 to 1% (w/w). Samples with higher phosphorus concentrations (1% (w/w) and above) are also measurable by EPMA, whereas the distributions of P at technically relevant concentrations and the distribution of possible impurities are only detectable (visible) by means of SIMS. The influence of the sintering time on the phosphorus distribution will be demonstrated. In addition the grain boundary segregation of P was measured by AES at the surface of in-situ broken samples. It will be shown that the distribution of phosphorus depends also on the concentration of carbon in the samples.
Powder Metallurgically Produced Carbide Free Bainite
Steels with carbide free bainitic (CFB) microstructures show excellent strength, toughness and wear resistance. Cast or wrought products produced by conventional metallurgy have become gradually introduced in manufacturing of numerous machine components. The required silicon addition of more than 1.5wt% in CFB-steels limits the possibilities to produce components of these steels by P/M methods. The aim of this work has been to investigate the possibilites to produce CFB-steels by pressing and sintering. Four different powder mixtures based on Distaloy DC powder have been pressed to a relative density of 90 % and sintered in a N2-H2 atmosphere at 1150 °C. The sintered components were then austenitized followed by austempering at a temperature above the martensite start temperature. Tensile and impact testing together with microhardness measurements have been performed. The microstructures were studied by optical microscopy as well as SEM and XRD-methods. The tensile strength values achieved varied from 313 to 737 MPa, the elongation after fracture were between 0.1 and 0.2%. The impact toughness values varied between 4 and 11 J. The hardness of the bainite after short sintering time varied between 630 and 710 HV and the hardness of the CFB was 350 HV after short sintering time but reached 573 after prolonged sintering. The microstructure consisted mainly of bainite, small amount of CFB mixed with austenite but also of ferrite and retained austenite after short sintering time. A longer sintering time created a structure consiting of mainly CFB with bainite and a small amount of ferrite. The most interesting applications for P/M produced CFB-containing steels should be components subjected to sliding or rolling-sliding wear loads, as gears. The hardness and strenght values achieved in the present work indicate that P/M produced CFB-steels can prove superior to conventional P/M steels in many applications.
Recent advances in ferrous powder metallurgy
Ferrous Powder Metallurgy (P/M) has advanced significantly over the past thirty years in providing opportunity for a parts designer and producer to make high strength net shape parts. Currently, for example, transmission gears in U.S. built cars use P/M parts. This review addresses recent advances in the area of ferrous powder for the P/M industry. Development of molybdenum prealloyed powder for increased density, Cr-Mn containing powder for through hardenability and improved wear resistance, binder treated powders for higher control of alloying ingredients and increasing compaction rates are presented. Two major innovations, namely achievement of higher compaction densities through warm compaction and improved magnetic properties by dielectric coating on iron powders are presented.