Time filter

Source Type

Sadeghpour S.,Isfahan University of Technology | Kermanpur A.,Isfahan University of Technology | Najafizadeh A.,Fould Institute of Technology
Materials Science and Engineering A | Year: 2014

The formation of strain-induced martensite (SIM) during nanoindentation experiments was investigated in a Ti-bearing high-Mn stainless steel with three different grain sizes, e.g. coarse-grained, ultrafine-grained and nano-grained structures. In the nanoindentation load-displacement curves, there was clear distinction in the number of pop-ins from five in the coarse-grained structure to one in the nano-grained structure representing the difference in the susceptibility to formation of SIM. It was found that grain refining down to the nano scale leads to a significant restriction in the formation of SIM. This can be attributed to the increased stability of austenite as a result of increase in the stacking fault energy by reducing grain size. © 2014 Elsevier B.V.


Behjati P.,Isfahan University of Technology | Kermanpur A.,Isfahan University of Technology | Najafizadeh A.,Isfahan University of Technology | Najafizadeh A.,Fould Institute of Technology | And 5 more authors.
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science | Year: 2014

Martensite reversion treatment was utilized to obtain ultrafine grain size in Fe-18Cr-12Mn-N stainless steels containing 0 to 0.44 wt pct N. This was achieved by cold rolling to 80 pct reduction followed by reversion annealing at temperatures between 973 K and 1173 K (700 °C and 900 °C) for 1 to 104seconds. The microstructural evolution was characterized using both transmission and scanning electron microscopes, and mechanical properties were evaluated using hardness and tensile tests. The steel without nitrogen had a duplex ferritic-austenitic structure and the grain size refinement remained inefficient. The finest austenitic microstructure was achieved in the steels with 0.25 and 0.36 wt pct N following annealing at 1173 K (900 °C) for 100 seconds, resulting in average grain sizes of about 0.240 ± 0.117 and 0.217 ± 0.73 µm, respectively. Nano-size Cr2N precipitates observed in the microstructure were responsible for retarding the grain growth. The reversion mechanism was found to be diffusion controlled in the N-free steel and shear controlled in the N-containing steels. Due to a low fraction of strain-induced martensite in cold rolled condition, the 0.44 wt pct N steel displayed relatively non-uniform, micron-scale grain structure after the same reversion treatment, but it still exhibited superior mechanical properties with a yield strength of 1324 MPa, tensile strength of 1467 MPa, and total elongation of 17 pct. While the high yield strength can be attributed to strengthening by nitrogen alloying, dislocation hardening, and slight grain refinement, the moderate strain-induced martensitic transformation taking place during tensile straining was responsible for enhancement in tensile strength and elongation. © 2014, The Minerals, Metals & Materials Society and ASM International.


Samaei Baghbadorani H.,Isfahan University of Technology | Kermanpur A.,Isfahan University of Technology | Najafizadeh A.,Isfahan University of Technology | Najafizadeh A.,Fould Institute of Technology | And 3 more authors.
Materials Science and Engineering A | Year: 2015

The present study was aimed to investigate the mechanical properties of a nano/ultrafine grained Nb-containing 201 austenitic stainless steel. For this purpose, 90% cold rolled sheets with fully martensitic microstructure were isothermally annealed at 900. °C for different times of 1 to 1800. s, leading to the reversion of strain- induced α'-martensite to austenite and significant grain refinement. Ferritescopy, X-ray diffractometery and optical/electron microscopy techniques along with hardness measurements and tensile tests were used to study the evolution in microstructure and mechanical properties in the course of annealing. It was found that heavy cold-rolling promoted formation of Nb-rich carbonitrides which effectively retarded the growth of fine reverted austenite grains. The obtained results showed that the complete transformation of martensite to austenite took about 60. s with the corresponding austenite grain size of about 90. nm. This sample had an ultrahigh yield strength of 1170. MPa, which was almost four times higher than that of the raw material and outstanding elongation of 37%. Further, the true stress-strain curves of the reversion annealed samples revealed two distinct uniform elongation stages (stage I and stage II), whereas, the onset of stage II was concurrent with pronounced strain hardening. This was related to the sharp increase in the formation of α'-martensite upon tensile straining. © 2015 Elsevier B.V.


Baghbadorani H.S.,Isfahan University of Technology | Kermanpur A.,Isfahan University of Technology | Najafizadeh A.,Isfahan University of Technology | Najafizadeh A.,Fould Institute of Technology | And 3 more authors.
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science | Year: 2015

In this study, influence of Nb-microalloying on formation of nano/ultrafined grain microstructure and mechanical properties during martensite reversion process in a 201-type austenitic stainless steel microalloyed with Nb was investigated. For this purpose, the 90 pct cold-rolled samples with almost fully martensitic microstructure were reversion annealed at 1023 K to 1173 K (750 °C to 900 °C) for 5 to 1800 seconds. The microstructural evolution was characterized using X-ray diffractometer, Ferritescope, optical microscope, scanning, and transmission electron microscopes. Mechanical properties were evaluated using hardness and tensile tests. The reversion mechanism was found to be diffusion controlled. In comparison with other types of 201 steel, the kinetics of grain growth at 1173 K (900 °C) was much slower in the Nb-bearing steel, being related to the rapid precipitation of nano-sized Nb-rich carbonitrides during reversion process. At this temperature, the finest austenitic microstructure was achieved in the specimen reversion annealed for 60 seconds, possessing a microstructure composed of nano and ultrafined grains with an average grain size of 93 nm. This specimen exhibited an excellent combination of ultrahigh strength (yield strength of 1 GPa and tensile strength of 1.5 GPa) and good ductility (tensile elongation of 35 pct). © 2015, The Minerals, Metals & Materials Society and ASM International.


Sadeghpour S.,Isfahan University of Technology | Kermanpur A.,Isfahan University of Technology | Najafizadeh A.,Fould Institute of Technology
Materials Science and Engineering A | Year: 2013

The martensite thermomechanical treatment was used for the formation of nano/ultrafine grain structure in the 201L austenitic stainless steel containing 0.12. wt% Ti microalloying element. The initial microstructure was provided through homogenizing, hot rolling and solution annealing of the as-cast ingots. The specimens were then cold rolled between 5% and 90% thickness reduction and subsequently annealed at 750-900. °C for various times. The results showed a promoting effect of Ti on the formation of strain-induced martensite (SIM). A nanocrystalline austenitic structure with average grain size of 45. nm was achieved by annealing at 900. °C for 60. s through a diffusional transformation mechanism. It was found that precipitation of nanosized TiC particles during the reversion annealing could retard the reversion process and suppressed grain growth in further annealing times. The tensile testing of the thermomechanically processed specimens showed a good combination of high yield strength (~1000. MPa, six times higher than that of the initial coarse-grained steel) and excellent ductility (42% total elongation) for the Ti microalloyed 201L steel due to the SIM formation during the deformation and impressions of the nanosized Ti carbides distributed within the nano/ultrafine grain structure. © 2013 .


Moallemi M.,Isfahan University of Technology | Kermanpur A.,Isfahan University of Technology | Najafizadeh A.,Isfahan University of Technology | Najafizadeh A.,Fould Institute of Technology | And 3 more authors.
Materials Science and Engineering A | Year: 2016

The present study deals with the correlation of stacking fault energy's synergy and driving force in the formation of deformation-induced martensitic transformation in a 201 austenitic stainless steel. The fraction of deformation-induced martensite was characterized by means of X-ray diffraction and magnetic induction techniques. The kinetics of the martensite formation versus applied strain was evaluated through the sigmoidal model. It was shown that the volume fraction of α-martensite is closely related to the driving force/SFE ratio of the alloy. The results also showed that the martensite content is similar in both XRD and magnetic methods and the applied sigmoidal model was consistent with the obtained experimental data. © 2015 Elsevier B.V.

Loading Fould Institute of Technology collaborators
Loading Fould Institute of Technology collaborators