The fatigue behaviour of SAE 5160 steel was evaluated before and after applying a shot peening process by using different Almen intensities and surface coverings (uncovered, partial coverage and total coverage). In the high-cycle fatigue tests, maximum stresses of 0.8 Sut, 0.7 Sut and 0.6 Sut were applied in the three-point bending test on an Instron 8872 servo-hydraulic machine at a frequency of 10 Hz and a constant stress ratio of Smin/ Smax = 0.2 for all tests. The fatigue tests were performed based on the ASTM E8, the specimens were classified into six groups for each stress evaluated, and each group consisted of three standardised specimens (ASTM E466). Also, yield strength, ultimate strength, hardness and microhardness were obtained. The Wilcoxon’s non-parametric test was used to statistically compare all the mechanical properties obtained from the base material with those obtained after the application of the shot peening, for the different surface coverings and Almen intensities. The results showed that the shot peening process significantly increases the fatigue strength of the material, with a 94% increase in fatigue strength of the fully coated specimens. However, no significant increase in fatigue strength was found due to a change in the Almen intensity value. A high correlation factor was found between the increase in the ultimate resistance and the increase in Almen intensity; however, for the yield stress the correlation was medium and inverse. For hardness and microhardness, the correlation factor was very low. Finally, the microhardness values revealed a 3% increase in Vickers microhardness of the shot peening specimens compared to the untreated specimens.
CrN/ZrN (1, 8, 15, and 30) bilayers were deposited onto AISI 420 steel substrates at
250 °C and 6.6x10-3 mbar with gas ratio Ar/N2 50:3.0 as gas mixture and bias -60V were applied.
AFM analysis presented different morphologies, showing that the coatings with 15 bilayers had an
average grain size of 49 nm; while the 30-bilayer coating exhibited grain sizes of 99 nm. Coating
thicknesses were 3 μm, approximately. The Vickers Test revealed that coatings with 8, 15, and 30
bilayers bore better impact resistance than coatings with 1 bilayer. This result is considered,
bearing in mind that in many bilayers propagation of fissures is slower, because the presence of
layer inter-phases leads to fissures straying in other directions. Slight corrosion specks are present,
but mass loss was around 40 mg. in one bilayer, a higher value than for the coatings with 15
bilayers that was near 18 mg. Homogeneity, grain size, fracture resistance, and corrosion resistance
of the coatings with 15 and 30 bilayers are suitable for mechanical applications of these types of
coatings, as shown in mechanical measurements. These results indicate that for engineering
applications under corrosive environments, the use of these types of bilayer coatings on AISI 420
stainless steel is highly recommended.
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