The availability of high-purity metals and high-purity based alloys has recently allowed us to better understand the various processes of dynamic recrystallization (DRX) that take place during hot working. According to the mobility of dislocations and the subsequent efficiency of dynamic recovery (which mainly depends on stacking-fault energy), continuous (CDRX) or discontinuous (DDRX) dynamic recrystallization may take place. In aluminium, recent work has shown that a transition from CDRX to DDRX occurs with increasing purity, whereas in austenitic stainless steels close to the AISI 304L grade, DDRX is observed whatever the purity. A simple theoretical criterion for this transition is proposed and discussed. The key parameter is the grain boundary migration rate, whose temperature dependence is analyzed first. For the influence of purity, an indirect approach is then proposed on the basis of a model recently developed for CDRX.
The effect of post-oxidizing treatment on the characteristics of modified surface layers produced by salt bath nitrocarburizing on the industrial American Iron and Steel Institute (AISI) 02 tool steel types is investigated. Nitrocarburizing treatment is performed for 6 h and 8 h at 570• C and post-oxidizing treatment for 30, 60 and 90 min at 520 • C, using argon-steam mixture. Formed layers are characterized by their basic properties such as thickness layer, depth, surface hardness and wear resistance. Detailed estimation of the modified metal surface quality, in terms of chemical composition, formed phases, microstructures and diffusion mechanisms are performed by metallographic techniques, EDX, X-ray diffraction, scanning electron microscopy (SEM) and glow discharge optical electron spectroscopy (GDOES). The corrosion resistance was investigated in 0.4 M H 2 SO 4 solutions, using steady-state electrochemical polarization methods. The obtained results revealed the existence of a superficial oxide layer which consists of magnetite (Fe 3 O 4 ) and hematite (Fe 2 O 3 ) and the presence of an ε-phase associated with a small amount of γ -phase. Important improvements in wear, microhardness and corrosion resistance occur after these treatments and it is specifically concluded that the sole application of a nitrocarburizing treatment does not significantly ameliorate the corrosion resistance of the as-received steel. In fact, post-oxidation treatment contributes to increase corrosion resistance by forming a dense magnetite layer and at the same time, it partially covers the compound layer pores.
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