Keywords:

1 Introduction

Cross-rolling reduces the asymmetry of the mechanical properties along rolling (RD) and transverse direction (TD) by changing the slip systems and the Schmid factor [1]. Narayanswamy et al. [2] observed that cross-rolling strengthens the {100}<110> texture of α fiber in the deformed sheets. Austempering (AUST) near the Ms temperature is reported to generate fine lower bainite and retained austenite (RA) having ultra-high strength (>2 GPa) and considerable toughness [3, 4]. The previous studies revealed that the original microstructure of the low-temperature bainite is retained during tempering [5, 6]. This phenomenon is more distinct for high-carbon steels [6]. The rotary tool-like circular saws used for guided sawing applications should have excellent isotropic mechanical properties and perfect levelness [7]. The present study investigates the feasibility of manufacturing high-performance circular saws through cross-rolling and austempering-tempering (AUST-T).

2 Materials and Methods

High-carbon low-alloy steel plates (Fe- 0.70C- 0.60Si- 0.54Mn- 0.40Cr- 0.73Ni- 0.069Mo- 0.28Al (wt.%)) were cold-rolled to reduce their thickness from 5.3 to 2 mm (62% reduction). For one series of specimens, unidirectional-rolling (UR) was performed, while for the second one, cross-rolling (CR) was achieved by rotating the specimens by 90° after each pass. The Ms and Bs temperatures were calculated at 222 and 397 °C, respectively, using the MUCG83 software [8]. The sheets were austenitized at 860 °C for 30 minutes, followed by austempering at 250 °C for 1 hour. The austempered materials were clamp tempered at 400 °C for 2 hours. The microstructure and texture evolution were characterized through OM, FESEM, XRD, EDS, and EBSD. The hardness, tensile, and flexural properties were also measured and compared to a commercial saw manufactured from the same material through the quench and tempering (Q-T) process.

3 Results and Discussion

Figure 1 shows the microstructure of the CR AUST and AUST-T materials. AUST develops lath-like bainite and filmy RA (Fig. 1a). Tempering causes the coarsening of the microstructure and decomposition of RA to bainitic ferrite and carbide. The remaining RA is characterized by blocky and filmy morphology (Fig. 1b). Transformation-induced plasticity (TRIP) after AUST results in an ultra-high strength in the materials (Table 1). The CR samples showed higher resistance to softening than unidirectionally UR sheets during tempering (Table 1).

Fig. 1.
Two FESEM micrographs of 10 micro meter magnification depict the C R A U S T and A U S T-T sheets etched with Nital.

FESEM micrographs of the CR AUST (a) and AUST-T (b) sheets etched with Nital.

Full size image
Table 1. Mechanical properties of the AUST, AUST-T, and Q-T specimens.
Full size table

UR developed the <110>//RD and <111>//ND fibers and CR induced distinct {100}<110> texture of α and ε fibers (Fig. 2). The {100}<110> component of CR samples retained after AUST-T (Fig. 2). These components locate on the high-density 110 planes resulting in uniform tensile behavior along perpendicular directions. However, UR sheets only have ductile behavior along RD. The intense texture of CR samples showed higher resistance to softening during tempering (Table 1).

Fig. 2.
A set of four thermal images depict O D Fs of the rolled and A U S T-T sheets

φ2 = 45° ODFs of the rolled and AUST-T sheets.

Full size image

4 Conclusions

Based on the above research, it can be concluded that: (1) AUST-T results in a bainitic microstructure with an excellent strength/ductility balance. (2) CR generates distinct uniform {100}<110> texture, increasing material resistance to softening. (3) Production of thinner rotary tools with the same material is possible with CR and AUST-T.