Zusammenhang zwischen der Porenmorphologie und der Schwingfestigkeit von Sinterstahl und Sintereisen

Within this work the relationship between the pore morphology and the fatigue properties of sintered pure iron and steel has been investigated. The mechanical properties of P/M are controlled by their microstructure and the porosity. Many parameters are influencing the microstructure and the porosity. A few of these parameters were considered in the investigation, which will have a significant influence on the performance characteristics of the materials. These parameters are the density, the kind of the powder, the sinter temperature and technique of alloying as well as the composition. The density acts as main parameter because it includes all other parameters. Therefore, the density has the maximum potential for optimizing the properties of the sintered components. The above mentioned parameters influence the morphology of the pores. Due to this, the characterizing of the pore characteristics is very important. The mechanical properties and the porosity have been investigated by varying the density, the sintering temperature and basic powder for sintered pure iron and alloyed steel. The investigated materials were Distaloy AE and SE, pure iron ABC 100.30 and MH 65.17. Basic powder was water atomized and pure broken sponge iron. The sintering conditions were 1120°C and 1280°C in an atmosphere of 70% N2 and 30% H2 in industrial equipment. If the relationship between the geometrical features of the pores and the influencing parameters is known, it is possible to optimize the morphology of the pores by the influencing factors. The base characteristics are pore area and perimeter, convex perimeter, maximum and minimum pore dimensions and the intercepts or distances between the pores. From the primary data a huge variety of secondary quantities can be derived which are used to describe the pore size, the pore shape, and pore interpore space. The pore morphology was quantified by image analysis. According to the results of the image analysis, the majority of all pores is small and rather round. Only a few large pores which are automatically also very irregular can act as potential fracture initiation sites. For this reason 98% of all pores were excluded from the further processing. Only 2% of the remaining larger and most irregular pores were evaluated. Pore irregularities are commonly characterized by a shape factor. The bending fatigue strength was investigated for a varying density by different sintering temperatures and basic powders. For each experimental condition 60 specimens were used in fully reversed flat bending tests at 6 stress levels of which 3 occurred in fracture. The endurance limits were determined for 50% probability at 107 cycles. Many properties of porous sintered materials are related to the pore microstructure. An attempt was made to correlate the fatigue behavior of industrially manufactured sintered iron and steels with quantitative pore characteristics. Crack initiation in fatigue tests was restricted to very few large irregular pores. All efforts to correlate average pore size or shape factors with fatigue strength values yield at best tendencies. Powder laws to describe the relationships between the strength, the density, and the shape factor the coefficients were calculated by regression analysis.