Papers by Keyword: Bainitic Steel

Abstract: Problems with corrosion in boilers and their parts can be solved by the application of different nickel alloys like 622, 625 or 686 by a variety of welding processes. This solution is used mostly in Waste-to-Energy plants or biomass stations burning waste wood, but it can also be found in the recovery boilers used in the paper industry. The most common material grades include low alloyed boiler steels like 16Mo3, 13CrMo4-5 and 10CrMo9-10. When there is a need to increase steam temperature and pressure more complicated alloys to become a natural choice. This paper focuses on the fabrication experience of welding of 7CrMoVTiB10-10 base tube with Inconel 625 weld overlay and presents a welding solution of matching filler metal used for root pass and S Ni 6625 filler metal for a fill-up and the cap performing a full strength weld without the need of overlay peeling and manual tie-in overlay. This method of welding saves a lot of time during execution in the workshop and on-site during installation and assures much better quality in the end. All examinations were performed to allow welding procedure qualification according to ASME and EN standards.

Abstract: Multiphase steels consisting of retained austenite and martensite/bainite microstructures such as TRIP, low-temperature-bainite, and Q&P steels are attractive candidates for the new-generation of AHSS. These steels exhibit a remarkable combination of strength and toughness which is essential to meet the objective of weight reduction of engineering-components, while maintaining the compromise of tough-safety requirements. Such good mechanical properties are due to the enhanced work hardening rate caused by austenite-to-martensite transformation during deformation and the strengthening contribution of martensite/bainite. The retained austenite can thermally decompose into more thermodynamically stable phases as a consequence of temperature changes, which is referred to as the thermal stability of retained austenite. TRIP-aided steel is an effective candidate for automotive parts because of safety and weight reduction requirements. The strength–ductility balance of high strength steel sheets can be remarkably improved by using transformation induced plasticity behavior of retained austenite. In manufacturing hot rolled TRIP-aided sheet steels, austenite transforms into bainite during the coiling process. Because black hot coils cool slowly after the coiling process, they are exposed at about 350–450°C for a few hours or days. Therefore, the metastable residual austenite can be decomposed into other phases. This decomposition of residual austenite can produce serious deteriorate of mechanical properties in hot rolled TRIP-aided sheet steels. The present work identified the decomposition behavior and study the thermal stability of retained austenite in the TRIP-aided steel with bainitic/ferrite matrix depending on coiling temperatures and holding times by means of DSC and XRD analysis.

Abstract: This work focuses on the study of the solid state transformations that take place during the austempering of high silicon carbide-free bainitic cast steels with different chemical composition. In order to get this objective three cast steel melts with different chemical compositions were produced, evaluating the influence of Cr, Mn, Ni, Al and Co. For each of these steels, samples were subjected to an austempering heat treatment at 340 oC varying the austempering time from 5 sec to 120 sec. The results show that small regions of free ferrite were obtained during continuous cooling from the austenitising to the austempering temperatures in unalloyed high silicon cast steels. At short austempering time (5 sec), the presence of a small fraction of ausferrite was observed. Austempering for 60 sec showed a larger amount of ausferrite. However, the ausferritic reaction is incomplete, and some martensite also was present, mainly located in last to freeze (LTF) zones. After an austempering of 120 sec, a fully ausferritic matrix was obtained. The addition of Cr and Mo avoided the initial precipitation of free ferrite, and lowered the isothermal transformation kinetics. On the other hand the use of Al and Co increase the presence of ferrite formed during continuous cooling and accelerates the kinetics of the ausferritic reaction.

Abstract: The thermo-simulation test and transmission electron microscopy (TEM) were applied to investigate the evolution of dislocation configuration and strain induced precipitation behavior during relaxation at 850°C in a deformed Fe-40Ni-Ti alloy. The stress relaxation curve can be divided into three stages, namely, the process of incubation, nucleation and growth, and the coarsening of strain-induced precipitates. The highly dense and twisted dislocations formed during the deformation develop into dislocation cells and finally, the sub-grains can be observed when relaxing to 1000s. The strain induced precipitates occur both onto the dispersed dislocations and dislocation cells. The precipitates pin the dislocations which results in retarding the progress of dislocation configuration evolution. As precipitates start to coarsen, the pinning effect weakens and the dislocations get rid of the pinning though bypassing mechanism. Adopting the same simulation test to bainitic steel, the optimum refinement could be obtained at 60-200s during relaxation processing, corresponding to the perfect dislocation cells formation of Fe-40Ni-Ti alloy.

Abstract: The influence of Ti/N ratio on the effective boron and mechanical properties was investigated by analyzing data from low carbon boron alloyed bainitic steel plates. The result shows Ti/N ratio varies with effective boron value. Less than 50% effective boron was obtained when Ti/N ratio is below 3.3, nearly 90% effective boron is obtained when ratio Ti/N is more than 4; Adding enough Titanium is an effective and economic way to improve qualified ratio of bainitic steel plate. The Ti content between 0.010% and 0.030% does not have obvious effect on the toughness of the bainitic steel;

Abstract: In order to research the cold deformation work hardening characteristic of new type low carbon bainitic steel, this article studies the effect of different degrees of cold deformation (elongation and compression) and different tempering temperatures on microstructure and mechanical properties of 15SiMn2Mo low carbon beinaitic steel. The results showed that with the tempering temperature increasing after 10% pre-tension deformation, the tensile strength and yield strength of the test material increased first and then decreased, and reached its peak value at 300°C, roughly the same as the strength of hot-rolling and 300°C tempering. With the compression deformation degree rising, the hardness of test material increased and showed the test material has good work hardening performance. Streamline and "z" shape ferrite banding appeared in microstructure. With the tempering temperature increasing, the microstructure of compressed deformation steel recoveried and recrystallized, the tendency of ferrite along the streamline was weakened, the new refining granular phase was enhanced and uniformity of microstructure was improved. The microstructure refinement was significantly increased with the compressive deformation degree rising.

Abstract: Industrial design is important for industry, especially in the circumstance concerning the human security, of which the material selection and design basing on serious experiments, especially mechanical-tests have drawn more and more attentions of many artists and scientists. In this study, by tension, impact, hardness and scanning electron microscopy (SEM), the mechanical property of the flash butt welding joint of U71Mn rail steel to bainitic alloy steel used for railway frog is studied. The results indicate the distribution of hardness in the alloy steel side is uniform, and the hardness at the HAZ (hot affected zone) of the rail steel is higher than that of the base material. The tension property is the same as the U71Mn steel, and both the tension and impact property meet the corresponding requirements of Railways Ministry, which indicates the feasible of the bainitic alloy steel to the rail steel joint by flash butt welding.

Abstract: The influence of deep cryogenic treatment (DCT) on the microstructure of a bainitic steel is investigated by means of internal friction and transmission electron microscopy (TEM). Two relaxation peaks (P_c1 and P_c2) are observed during cooling and one relaxation peak (P_h) during heating from 100 to 320K. Peak P_c1 may be related to dislocation pinning. Peak P_h is attributed to dislocation-carbon atoms interaction. The decreasing of peak P_h after cycles deep cryogenic cooling indicates that soaking time under the deep cryogenic temperature is not contributed to the precipitation of carbides, while the cycles cryogenic treatment lead to more fine carbides precipitation.

Abstract: The contemporary protective structures are now critical elements of the floating units, military, public and other objects. Their task is to weaken the effects of a terrorist action against selected areas with the use of explosives or projectiles. The knowledge of material behaviour at high strain rates is essential for designing the ballistic shields. The hull external shells of the Polish Navy ships now in service do not meet the requirements of ballistic anti-terrorist shields against small arms shooting or the rocket and shell splinters. This is a significant problem in the case of defending a ship against terrorist attack threats. Therefore, there is a tendency for building internal and external ballistic shields in the ship hull structure to protect the critical compartments and action stations. The paper presents investigations of materials subjected to high strain rates, with the 10GHMBA E-620 bainitic steel as an example. The experiments were carried out on the MTS 810.12 testing machine, rotary impact testing hammer, the Taylor test stand and on the ballistic pendulum. In the final part of the paper, some numerical simulations are presented allowing to analyse the fast-changing processes arising during loading the structure by an impact of high velocity bodies.

Abstract: The microstructure of a bainitic steel after different cooling rates has been investigated by transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD). The effect of cooling rate on the intermediate transformation microstructure was studied. The results showed that the final microstructure contained complex mixture of bainitic ferrite, granular bainite and polygonal ferrite. There was mainly lath-like bainitic ferrite at fast cooling rate (20Ks^-1), while microstructure in samples cooled with intermediate rates (8~15 Ks^-1) contained bainitic ferrite and granular bainite. When cooling rate decreased to less than 5Ks^-1, polygonal ferrite occurred.