Analysis of Recrystallization and Strain-Induced Precipitation on High Nb- and N-Bearing Austenitic Stainless Steel

The mechanical properties and corrosion resistance of stainless steels are due to the combined effect of chemical composition and thermomechanical processing. The objective of this study was to investigate the interaction precipitation-recrystallization of an austenitic steel with high additions of nitrogen and niobium through continuous-cooling multiple deformation hot-torsion tests. Samples were heated up to a soaking temperature of 1250oC and kept at this temperature for 5 minutes, and then deformed during cooling. The deformation pass was 0.3 with a strain rate of 1 s-1and interpass times of 20 or 50 s. The evolution of the microstructure was investigated by optical, EBSD and transmission electron microscopy, using thin foils and carbon extraction replica samples. The results showed that some precipitates were not dissolved after reheating and the presence of niobium-and chromium-rich particles after processing was confirmed. The strain accumulation with the interpass time of 20 s yielded finer precipitation and improved grain refinement than observed after 50 s. Some interaction of the precipitates with dislocations and grain boundary could be evidenced.

Effect of Niobium on Microstructure of X80 Grade Pipe Line Steels

Microstructure of high strength pipeline steels containing different Nb contents was investigated using optical microscopy and transmission electron microscopy. Second phase particles were analyzed by extraction replica method. The results show that microstructure of the low Nb steel comprises granule bainite and the low bainite with a few of martensite-austenite (M-A) constituents. However, microstructure is consisted of acicular ferrite with M-A constituents in high Nb content steel. Moreover, M-A constituent is consisted of twinning martensite, lath martensite and retained austenite. In the low Nb steel precipitates are a large of square TiN particles. Second particles of high Nb content are mainly the large size duplex type (Nb,Ti)C and small NbC precipitates.

The Study on the Precipitation Behavior of the Second Phase Particles for Fine Grain and High Strength IF Steel

The new type SFG HSS (super fine grain, high strength steel sheet) has been developed by adding solid-solution strengthening elements to conventional IF steel such as Si, Mn. The precipitation behavior of the second phase particles was observed and studied by a carbon extraction replica technique used transmission electron microscopy (TEM). The shape of the small second particles is similar to spherical and ellipse, the sizes of which are 10~30nm. It is seen that the particles are dispersed on the matrix. The precipitate composition of small particles is Nb (CN) and the precipitate composition of large particles is NbC examined by energy dispersive X-ray (EDX). It is noted that the yield strength is low as well as the tensile strength is high by the PFZ which is free of precipitate called precipitated free zone on the one side of the grain boundary. The results of microstructure shows that the second phase particles pinned on grain boundary not only can inhibit the grain growth, but also the grain can be fined.

Carbo-Nitride Precipitation in Tempered Martensite - Computer Simulation and Experiment

In the present work, the precipitation behavior of a V-microalloyed, quenched and tempered steel with 0.3wt % C is investigated experimentally and by computer simulation. The specimens are analyzed by means of transmission electron microscopy using selected area diffraction (SAD) and energy dispersive x-ray spectroscopy (EDX). The analysis is done on electropolished foils and on extraction replica. The numerical simulation is performed with the thermokinetic software package MatCalc, where the precipitation kinetics is examined for the experimentally applied thermo-mechanical cycles. Good agreement between experiment and simulation is obtained and the experimentally observed precipitate microstructure can be well explained on the basis of these simulations.

The Study on the Precipitation Behavior of the Second Phase Particles for As-Hot Rolled Dual-Phase Nb-Bearing Steel

The precipitation behavior of the second phase particles was observed and studied by a carbon extraction replica technique used transmission electron microscopy (TEM) for Nb-bearing dual-phase steel. It is found that there are more second phase particles on the surface than that of in the center of sample. The mainly smaller particle which sizes of is below 20nm show the shape of spherical and ellipse. The sizes of larger particles are range of 20nm~40nm and the shape of them present rectangle or ellipse. The particles are dispersed on the matrix and the particle boundary is clear. The precipitate composition of particles are all Nb(C、N) examined by energy dispersive X-ray (EDX). The results of microstructure shows that the second phase particles pinned on grain boundary not only can inhibited the grain growth, but also the grain can be fined during the heating and cooling course.

Refinement of Cementite in High Strength Steel Plates by Rapid Heating and Tempering

The precipitation behavior of cementite in low carbon steels at various heating rates from 0.3 to 100 K/s has been studied using a high-frequency induction heating apparatus. The materials used in this study were steel platesfor welded structures: 610 and 780 MPa class steel plates with a mixed microstructure of bainite and martensite.Cementite was observed using a carbon extraction replica method and the hardness and toughness were also examined. When heated at the conventional slow rate of 0.3 K/s, relatively large cementite particles with an average diameter of 72 nm precipitated at the lath boundaries, whereas when heated at a rapid rate over 3.0 K/s, cementite precipitated both within the laths and at the lath boundaries, and the cementite was refined down to an average diameter of 54 nm. With such refinement of the cementite, the toughness was improved. On the other hand, the hardness was irrespective of the heating rate and was dependent on the tempering parameter. TEM observations of the cementite precipitation behavior during the rapid heating process revealed that cementite begins to precipitate at the lath boundaries at about 773 K and within the laths at about 873 K. It is concluded that rapid heating especially from 773 to 873 K contributes to the cementite refinement and consequently the improvement in toughness. The effect of alloying elements such as chromium, molybdenum or silicon on the cementite growth during the rapid heating and tempering treatment is also discussed.

Observations on the precipitates in steels by extraction replica technique using the potentiostatic etching method with a non-aqueous electrolyte

Quantitative analysis of the precipitates of the steel is necessary in order to improve mechanical properties of the steel, because the precipitates in steels play an important role on the grain growth, recrystallization, and precipitation hardening. Recently, in-situ scanning electron microscope(SEM) observations on the precipitates with potentiostatic etching method in non-aqueous electrolyte have been reported. We applied the potentiostatic etching method for the preparation of the extraction replica.