PSI - Issue 13

Tatiana Oršulová et al. / Procedia Structural Integrity 13 (2018) 1689–1694 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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b)

a)

Fig. 1 Microstructures of the AISI 304 a) initial state; b) after solution annealing

Fig. 2 Vickers hardness test of investigated materials

For verification of these structural changes in the tested material, a hardness test after Vickers was used (Fig. 2). During the heat treatment the values of hardness decreased. After the recrystallization annealing of deformation reinforced steel, a gradual decrease in hardness occurred. Decrease of hardness was related to a gradual change of α' - martensite to austenite. The structure was gradually changed to the equilibrium by the heating process. In the recrystallization annealing with a 15-minute time-out, there was a slight increase in the hardness value. This may be caused by refining of the austenitic grain. For longer durations at T = 850°C, the hardness value again decreased. In this case, it may have occurred that longer holding time caused an increase and thickening of the austenitic grain. At the dissolution annealing temperature, the lowest hardness values were reached. A homogenization of structure has been achieved by the rapid cooling in water, thus there were no carbides, oxides and other inclusions produced. These structure elements would affect the resulting austenitic structure of the steel, if they were present. A similar case was also found in work of Tukur et al., 2014, where the influence of temperature on mechanical properties, such as hardness, strength and ductility, was evaluated. Sensitized sample of AISI 304 grade has the highest hardness value (41 HRC) compared to the sample at baseline (36 HRC) and after the annealing (20.4 HRC). Increased hardness was attributed to carbide formation along the grain boundaries of the sensitized specimen. The excluded carbides impeded the dislocation movement and reduced the deficiencies within the grit of the sensitized steel (Tukur et al., 2014).

3. Experimental procedure

Configuration for realization of the experiments is introduced in this section. The given experimental material is the conductive stainless steel specimens that are nondestructively inspected. Three different austenitic steel grades were evaluated: AISI 304, 316L and 316Ti grade. The specimens had initially the brick shape, with initial