Differential scanning calorimetry study of constrained groove pressed low carbon steel: Recovery, recrystallisation and ferrite to austenite phase transformation

Abstract

Low carbon steel sheets are subjected to severe plastic deformation (SPD) via constrained groove pressing (CGP) up to five passes. As a result of this process, strain magnitude up to 5·8 is imposed to the sheets, which leads to grain size of 225 nm. These nanostructured steel sheets, due to their high dislocation density and ultrafine microstructure, are very sensitive to heating. In the present study, recovery, recrystallisation and ferrite to austenite phase transformation phenomena for the SPD steel are investigated using differential scanning calorimetry method. The results show that with increasing the strain in steel sheets, the deformed stored energy (released through recovery and recrystallisation) and enthalpy of ferrite to austenite phase transformation are significantly increased and varied in 38·5–85·8 and 109–156·1 MJ m−3 ranges respectively. In addition, transformation temperature is decreased from 761 to 750°C after five CGP passes. However, recovery stored energy, recovery and recrystallisation peak temperatures are not changed, considerably. Experimental data show that with increasing the hardness, the stored energy is increased. One empirical equation is developed for relationship between hardness and stored energy of severely deformed low carbon steel. In addition, using the dislocation model, this mentioned relationship is justified.

Nanostructured Low Carbon Steels Obtained from the Martensitic State via Severe Plastic Deformation, Precipitation, Recovery, and Recrystallization

“…Furthermore, the heavily deformed structure both facilitates nucleation processes and promotes a higher driving force for recrystallization, which results in the shift of this process toward lower temperatures. This observation is in agreement with the analogous shift of recrystallization temperature reported for a 0.05 wt% C (non‐martensitic) steel after SPD and a 0.2 wt% C martensite after rolling . The large variation of plastic strain from the center to the edge of a HPT disk is a possible explanation for the large broadening of the DSC peak.…”

Synergistic Strengthening by Severe Plastic Deformation and Post‐Heat Treatment of a Low‐Carbon Steel

“…In higher number of passes, the decrease in UTS and elongation is observed in all treated samples and the rate of strength improvement is also decreased for the as‐deformed samples. These may be attributed to dislocation annihilation in deformation stage, appearance of microcracks, microstructural evolution during the annealing stage, and the effect of strain amount on the shift of transformation regions in Fe − C phase diagram …”

“…Furthermore, Khodabakhshi et al showed that for the samples deformed more than 1 pass of CGP, dislocation density gradually decreased, so it is expected that in our experiment, less routes are available for carbon to diffuse and redistribute in the earlier stages of annealing when the samples experienced higher strains. Additionally, as the gradual transition of low‐angle grain boundaries to high‐angle grain boundaries may take place through deformation process, grain growth can follow the recrystallization and phase transformation processes during the annealing stage in samples processed by 2 and 3 CGP passes, while the recrystallization temperature itself is shown to remain almost constant at higher amounts of deformation . Consequently, it is expected that larger ferrite grain size in combination with higher concentration of carbides along the grain boundaries is obtained through annealing after higher strains.…”

Twin parallel channel angular extrusion as a development of ECAE in parallel channels

“…This ultra-grain refinement brings about exceptional mechanical properties based on the Hall-Petch theory [7]. So far, special processing techniques of severe plastic deformation have been developed and used in order to fabricate UFG metals and alloys, including equal channel angular extrusion (ECAE) [8][9][10], high pressure torsion (HPT) [11][12][13], accumulative roll bonding (ARB) [14][15][16], twist extrusion (TE) [17][18][19], simple shear extrusion [20], severe forward extrusion (SFE) [21], and constrained groove pressing (CGP) [22][23][24]. ECAE is one of the most well developed SPD processing techniques in which the material is deformed through a die with two channels intersecting at a certain angle, which is described completely in previous reports [25,26].…”