Papers by Keyword: δ-ferrite

Abstract: The features of the structure evolution under thermal effect of Cr, Cr-Ni, Cr-Ni-Mn, and Cr-Mn-austenitic stainless steels with high nitrogen content and a total high content of carbon and nitrogen are analyzed. When studying the structure, we used light and electron microscopy, X-ray diffraction, dilatometry analysis and electrical resistance measurements. Fine structure and aging processes of austenite, nature and morphology of excess phases, as well as character of phase transformations and their relationship with the properties of steels have been studied. It is shown that Cr-Mn-steels with a high content of (C + N), having a homogeneous structure of austenite without excess phases, surpass Cr-Ni austenitic steels in mechanical and corrosion properties, have higher process ability than Cr-Mn-N-steel and are comparable with them in mechanical properties.

Abstract: M₆C carbides were found to form during short-term creep tests at 600 °C for 1100 h in an 11Cr ferritic/martensitic (F/M) steel with the normalized and tempered condition. The M₆C carbides have a face-centered cubic crystal structure, and a metallic element composition of 41-45Fe, 30-33W, 19-21Cr, 3Co/5Ta in atomic pct. The M₆C carbides were a dominant phase in the crept steel. δ-ferrite in high-Cr F/M steels may lead to a premature formation of large-sized M₆X precipitates during high temperature creep.

Abstract: The phase transformation of high Cr ferritic heat-resistant steel has been investigated by using differential scanning calorimeter and predicted by Thermo-calc calculation. The steel specimens were hot rolled and followed by air cooling, and then heated from room temperature up to different austenitization temperature as 800°C, 900°C, 1000°C and 1100°C. The DSC curves during heating process showed that the magnetic transition temperature and the Ac1 temperature are 744.9°C and 850.9°C, respectively. The austenitization range was about 58°C. The onset and offset temperature of martensite transformation both increase with the increase of austenitization temperature. The experimental results and the Thermo-calc calculated results both displayed that M23C6 carbides precipitated at around 950°C, and δ-ferrite started to form at about 1020°C.

Abstract: The microstructure and carbide precipitate of simulated coarse grain heat affected zone(CGHAZ) in modified high Cr ferritic heat-resistant steel at different cooling rates have been investigated by means of thermal simulator, optical microscope, SEM and TEM . It was found that the microstructure of CGHAZ of testing steel was mainly lath martensite and δ-ferrite under the different welding thermal cycles. However, the prior austenite grain size reduced with increasing the cooling rate. Furthermore, with increasing the cooling rate, the amount of carbide precipitate inside laths of martensite increased, and the size and morphology of precipitates have changed from elongated and coarse to needlelike and fine.

Abstract: The microstructures and mechanical properties of 17-4 PH stainless steel at each steps of heat treatment, such as homogenizing, solid solution treatment followed by aging treatment, longterm aging at 400 °C, and recovery treatment, in order to obtain a better understanding of the embrittlement phenomena on aging, was investigated. As the homogenizing treatment time increased, the length of δ-ferrite decreased and elongated shape of δ-ferrite turned to sphere shape with the decrease of volume fraction. The solution treated specimen mainly consists of lath martensite with a small fraction of elongated δ-ferrite. The spherical particles existed a little in the martensite matrix, while no precipitates were found in the δ-ferrite at the solution treated specimen. As the aging treatment temperature increased, the strength decreased while the toughness increased. The fcc Cu-rich particles precipitated in the δ-ferrite during the long-term aging at 400 °C after the solution heat treatment. This precipitation causes the aged hardening after long-term aging accompanied by decreases in elongation and charpy V-notch energy absorption. The strength and elongation was restored after recovery treatment and the fcc-Cu precipitated were almost dissolved into the δ-ferrite matrix.

Abstract: Low hot ductility of steel at the straightening stage of the continuous casting process is a problem found in steels containing microalloying and/or certain alloying additions. The thermal schedule undergone by the billet surface in the mill has a significant effect on the hot ductility. In this work, thermomechanical processing was employed to alleviate the problem of hot ductility in the Nb-microalloyed steel. Specimens were melted in situ and subjected to the billet surface thermal schedule in order to generate a microstructure similar to that present at the straightening stage of the continuous casting process. Some deformation schedules were incorporated with the thermal schedule at very high temperatures, specifically during solidification, within the d-ferrite region, and during the d®g transformation, and the hot ductility was subsequently evaluated at the end of the thermal schedule where the straightening operation is performed. After the thermal schedule alone, the steel exhibited a very low hot ductility at the straightening stage. It was found that deformation at very high temperatures prior to the straightening stage had a considerable effect on the hot ductility, either detrimental or beneficial, depending on the region in which the deformation has been executed. The mechanisms leading to loss and improvement of hot ductility are explained in this paper.

Abstract: The embrittlement fracture mechanism caused by microstructural evolution of 17-4 PH stainless steel at long term aging was studied using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The solution treated specimen consists largely of lath martensite with a small fraction of elongated δ-ferrite. The spherical particles existed a little in the martensite matrix, while no precipitates were present in the δ-ferrite at the solution treated specimen as non-aging. The precipitation of Fe-Cu in the δ-ferrite causes the aged hardening after long term aging accormpanied by decreases in elongation and charpy V-notch energy absorption. The increased fraction of brittle fracture on the fractured surface by impact and tensile test reveals that the embrittlement of the 17-4 PH alloys during long term aging is mainly caused by the precipitation hardening in the δ-ferrite matrix.