Subsequent recrystallizations technique of local plastic strain in 304 stainless steel

Abstract

The recrystallization technique has been successfully used to observe and measure local plastic zone and plastic strain in engineering materials: mild steel [1], high-tensile strength steel [2], austenitic stainless steel [3, 4], heat-resistant alloys [5], etc. Plastic strain within the plastic zone is to be determined from the recrystallized grain size by use of the relationship between strain and the grain size. However,. the accuracy of the measured strain is not good because of the large scatter of the grain size for a given strain and the small variation of the grain size on strain, especially in the high-strain region. To avoid these ambiguities and to reveal plastic zones with different amounts of strain more clearly, a multiple recrystallizations technique was developed in the present study; the recrystallization temperature is subsequently increased stepwise in order to observe at first the plastic zone with high plastic strain, next the plastic zone with lower plastic strain, and so on. The present experiment was carried out on 304 stainless steel, a commercially available 304 stainless steel plate (5.8 mm thick). To find the suitable recrystallization conditions, the indentation technique [5] was adopted, i.e. the polished surface was indented with a conical indentor with angle of 60 ° under a compression load of 1470 N. The indented surface was polished carefully using waterproof emery papers to #1500. The specimens were annealed in a vacuum furnace for 86.4 ks, first at 1023 K, secondly at 1173 K and finally at 1223 K. Conventional recrystallization annealing (only at one temperature) was also carried out to check whether the multiple heat treatments affect the recrystallization response. Fig 1. shows the recrystallization response. In the annealing at 1023 K and 1123 K, thermal etching was poor. The specimens were therefore polished and etched in a etchant (C2HsOH:HCI:HNO 3 = 100:15:30). In the annealing at 1173 and 1223 K, thermal etching during annealing was good. The micrographs of the thermal etched surface are shown in Fig. 1. It is seen that the recrystallized zone increases with increasing temperature. This means that the plastic zone with and above a critical plastic strain, eCR, can be observed (eCR being the critical strain for the occurrence of recrystallization). In the annealing at 1023K (Fig. lb), the recrystallized zone is only near the indentation edge, about 30-40/xm. The darkly etched zone around the recrystallized zone would be due to the carbide precipitation in the strained grains. In the annealing