Issue 7

S. Bagheri Fard et alii, Frattura ed Integrità Strutturale, 7 (2009) 3-16 ; DOI: 10.3221/IGF-ESIS.07.01 11 Thermal stability In order to investigate thermal stability of nanostructured surface layer, NC layer was fabricated on silicon steel Fe-3.29Si (and also on an ultra low carbon steel) by means of air blast shot peening with different conditions: (shot size and material: SUS304 /0.3 mm- Fe0.8C /0.8 mm- Fe1.0C /0.05 mm, air pressure (MPa) and speed (ms -1 ): 0.4, 50–100, 0.8, 50–100, 0.5, 150–200). After annealing, the nanolayer showed good thermal stability up to 873 K and also a sharp boundary to the underlying work-hardened area which was completely recrystallized. The microstructure of samples with subsequent annealing is shown in Fig. 10. It can be seen that the typical recrystallization occurred in the former work hardening region. On the contrary, no obvious change can be detected in nano region by SEM. The experience indicates that only slight grain growth may be possible in nanograins. However, the grain coalescence due to grain rotation might be the responsible mechanism to slight grain growth in nanocrystallite [19] . Figure 10 . Microstructure of shot peened Fe-3.29Si after annealing at 873 K for 3.6 ks: (a) 3,000%, (b) 10,000% [19]. Hardness Some experiments have been conducted in order to investigate the influence of NC process via SP methods on hardness of different materials [73, 80, 93] . All results indicate that the hardness near the treated surface significantly increases, by the SP process, and that the increase of hardness from the bulk to the surface seems to follow the grain refinement as observed by TEM. The variation in hardness with depth agrees well with the structural and compositional analysis result s [88, 93]. Published results also demonstrate that the hardness increment from the bulk to the surface cannot be explained by the existence of residual stresses but it is certainly due to another mechanism related to the grain size diminution as the increase of dislocation density and deformation bands [73] . Technological potential of the SNH process has become apparent in preliminary studies where hardness increases dramatically with respect to untreated components. Hardness tests were conducted on C-2000 alloy SNH treated samples (five tungsten carbide and cobalt (94%WC+ 6%Co, in wt%) balls with a diameter of 7.9mm for the duration times of 30, 60, 90, and 180 min). the results showed that compared with the as-received sample, the hardness of the treated sample has been increased substantially [80]. Figure 11 : Hardness profile of the samples [80] . 200 250 300 350 400 450 500 0 100 200 300 400 500 600 700 800 900 1000 1100 Distance from the surface (micrometer) Vickers's Hardness (Hv) 30 min-SNH treated 60 min-SNH treated 180 min-SNH treated As recieved Load: 300gf Error: +/- 10%