Evaluation of the Austenitic Grain Growth by Thermoelectric Power Measurements

The influence of the deformation grade on the recrystallised grain size has been studied in the AISI 304 stainless steel. Therefore, cold rolled samples of this material with reductions varying between 30% and 80% were annealed at different temperatures and subsequently quenched. The mean austenitic grain sizes were measured and compared. Moreover, the correlation between the variation of the thermoelectric power and the grain growth was investigated for each degree of prior deformation. Introduction Austenitic stainless steels AISI 304 are widely used steels combining high corrosion-oxidation resistance, good heat resisting properties and a relative low cost. However, one of the main drawbacks of these steels is their low mechanical resistance obtained in standard production processes. Therefore, in recent years an increasing interest can be noted in the development of new processing routes. One of the key issues in this field is the study of the microstructural evolution of the cold-rolling material at different annealing temperatures. In this context, the aim of the work presented in this paper was to characterise the recrystallised grain growth of the AISI 304 steels annealed at temperatures between 1000 and 1200 oC and for different rolling reductions, using thermoelectic power (TEP) measurements. Experimental Procedure The chemical composition of the studied steel in wt% is as follows: 0.039% C, 0.22% Si, 0.001% S, 0.029% P, 1.15% Mn, 18.0% Cr, 8.52% Ni, 0.24% Mo, 0.25% Cu, 0.0005% B, and 0.04% N. The material was obtained from a hot-rolled coil and was subsequently cold rolled down to 30%, 50%, 70% and 80% reduction. In order to study the evolution of the recrystallised grain growth, samples 30 mm in length, 2 mm in width and 0.8 mm in thickness were annealed at different temperatures (1000, 1050, 1100, 1150 and 1200 °C). After 180 seconds of holding time, specimens were rapidly cooled to room temperature. These heat treatments were appropriately chosen in order to obtain fully recrystallised samples in each case. The samples were etched with a solution containing 60% hydrochloric acid, 20% nitric acid and 20% methanol. Fig. 1 shows the optical micrographs of the annealed microstructures are shown for the case of 50% deformation. The micrographs were then analysed with the Image Tool 2.0 software. Fig. 1. Optical micrographs of the AISI 304 steel with 50% rolling reduction (a) in the as-cold rolled condition and after heat treatment at (b) 1000 oC, (c) 1050 oC, (d) 1100 oC, (e) 1150 oC and (f) 1200 oC. A schematic representation of the TEP apparatus is given elsewhere [1]. The experimental procedure of the TEP measurement is the following: the sample is pressed between two blocks of a reference metal (in this case, pure copper). One of the blocks is at 15 °C, while the other is at 25 °C to obtain a temperature difference T. A potential difference V is generated at the reference metal contacts. The apparatus does not give the absolute the TEP value of the sample (S), but a relative TEP (S) in comparison to the TEP of pure copper (S0) at 20 °C. S is given by the following relation: * * 0 . V S S S T ∆ = − = ∆