Comparison - By nanoindentation - Among PM steels obtained from diffusion-bonded powders (nominally equivalent)

In principle, powdered raw materials, produced by a given process and having equal chemical com-position, are supposed to be equivalent. In previous research works, the differences among response to sintering behaviour have been investigated on P/M steels based on four diffusionbonded powders (Fe + Ni + Cu + Mo), with atomized iron base, at the same alloy contents. Two levels of carbon (0.3 and 0.6 wt%) and two sintering conditions have been investigated, in industrial manufacturing conditions. However the comparison of dimensional changes, carbon content, and hardness reveals only modest differences. The comparison of microhardness distribution, fractal analysis, pore features, microstructure patterns show some clear differences, sometimes not univocal. The previously achieved results show that the claimed equivalence is not adequately and completely confirmed by closer and more complete comparative evaluation of experimental data. Only accurate analyses of microhardness and microstructure distributions seem powerful tools to rate, with modest sensitivity, the real similarity of the so claimed equivalent ironbase powders. Clearer differences have been shown by dilatometric analysis. To verify these observed differences, nanoindentation tests have been carried out. Each material has been characterized by nanoindentation measurement, for any process condition. The measured values, plotted on normal distribution graphs, only partially confirm the differences revealed by dilatometry. Some different ranking of diffusion level results from nanoindentation. This recently developed method, suitable to characterize very small volumes of material, appears suitable to find even minor differences among materials that could appear equivalent if evaluated by simpler and easier test methods.