Papers by Keyword: Carbide-Free Bainite

Abstract: The mechanical properties and microstructure of low-carbon high-silicon alloy steel were examined under various tempering temperatures using the quenching, partitioning and tempering (Q–P–T) process. The performance changed with the variation in tempering temperature. The results show that the microstructure of low carbon high silicon alloy steel treated by Q-P-T process was mainly ferrite, martensite, carbide-free bainite and film-like retained austenite. This alloys exhibited good mechanical properties at tempering temperature of 300 °C. The product of strength and elongation were 33.7 GPa%. Specifically, the Ultimate tensile strength were 1508 MPa, the yield strength were 1048 MPa, and the elongation were 22.4%. At this temperature of 300 °C, the volume fraction of retained austenite reached 10.4%.

Abstract: A new low-alloy high-carbon steel was designed to obtain the carbide-free bainite under low-temperature austempering. The microstructure, wear resistance under dry sliding friction and the wear mechanism were comparatively studied between samples austempered and low-temperature tempered. Results show that the carbide-free bainitic microstructure composed of bainitic ferrite laths with mean thickness of 120 nm and retained austenite films can be achieved by austempering at 220 °C for 120 h in salt bath after austenitizing at 1000 °C. The wear resistance under dry sliding friction of austempered sample is 19% more than that of low-temperature tempered sample. The wear mechanism of the low-temperature austempered sample is determined as the adhesive wear.

Abstract: We elaborated two carbide-free bainitic steels with different microstructures through specific heat treatments and alloy design. EBSD analysis was used to point out major differences in these microstructures. In-situ characterizations of the bainitic transformation were performed by high energy synchrotron diffraction to go further into the study of each phase characteristics. The elaborated microstructures exhibited various phase fractions of bainitic ferrite, retained austenite and blocks of martensite and retained austenite. Moreover, the volume fraction of retained austenite increased with higher austempering temperatures. On the other hand, the austempering temperatures showed a strong influence on the kinetics of the bainitic transformation. Isothermal transformation under Ms showed a two stage transformation which led first to the formation of self-tempered martensite and then to bainitic ferrite. Furthermore, the evolution of the austenitic cell parameter showed enrichment in carbon ruled by diffusional mechanisms.

Abstract: A novel carbide free bainite/martensite (CFB/M) high strength steel of 1500MPa grade is micro-alloyed with Nb in order to refine the microstructure and improve the toughness. After the steel containing Nb was forged, coarse microstructure and strong transmissibility of structure were found. It is suggested that the large numbers of precipitates of Nb(CN) in the steel makes the structure transmissibility more serious. After solution treatment at 1200°C, both complete annealing twice and isothermal treatment at 600°C twice can eliminate the structure transmissibility and refine the structure. Investigation on refinement of cycle phase transformation shows that excellent effect of the grain refinement is obtained when the heating velocity exceeds some threshold. In addition, the temperature of austenitizing is of little influences on the size of refined austenite grain.

Abstract: The air cooling rate of the Low Carbon Mn-Si-Cr steel bar with different diameter after austenitizing at 910oC and 960oC was simulated by Formaster-F Phase transforming instrument and Gleeble-1500 thermal /mechanical simulating machine. Microstructure of the specimen was observed by OLYMPUS PME3 optical microscope and FEI QUANTA200F scanning electron microscope. The hardness and impact toughness of the steel was tested by HBRV-187.5 hardness tester and JCSJ300-I instrumented Charpy impact tester. The experimental result showed that with the amount of CFB in CFB+M mixed microstructure increasing the combination of strength and toughness of the steel was improved. The higher the austenitizing temperature of the steel, the wider the air-cooling rate range obtaining CFB+M mixed microstructure. However, the steel produces mixed grain after austenitizing at 960 oC. For obtaining fine prior austenite grain size, Ti and Nb alloying element need to be added.

Abstract: An experimental study of Fe-C-Mn-Cr low-carbon steels with varying Cr content is presented. Pronounced bay and a near-stasis behavior were shown in the alloy with 1.1% Cr. Isothermal transformation at temperatures roughly between 500 and 600°C reveals two pronounced stages. SEM examination of the microstructures showed that the rapid first stage is controlled by carbide-free bainite formation while the slow second stage is controlled by a eutectoid-type reaction. Some microstructure features of the transformation were noted and discussed. Based on experimental observations, a model of austenite decomposition was developed to account for the main features of transformation kinetics in the alloys with varying Cr content.