Papers by Keyword: Ferritic Stainless Steel (FSS)

Abstract: Laser surface remelting and alloying of sintered stainless steel type 410L with FeNi and Ni have been studied for improvement of corrosion resistance and hardness increase. The influences of high power diode laser (HPDL) processing conditions, laser power in range 0.7-2.1 kW on the microstructure and properties of alloyed surface layer have been evaluated. The FeNi alloyed layer shows microstructure composed of austenite and martensite formed, due to high cooling rate in laser remelting process, with average Ni content in range of 39 to 8% depending on laser processing conditions. The Ni alloyed layer was composed of austenitic microstructure with Ni content from 65% to 33%. The improvement in microhardness was achieved by laser surface alloying and remelting. Excellent corrosion properties were observed for such remelted and alloyed layers in salt spray test.

Abstract: By using a thermal simulator this work investigated grain growth behaviour of a Ti+Nb stabilized 12%Cr ferritic stainless steel at high temperature. The results showed that the grain growth rate was less than 1.8μm/s at the temperature of 1200°C, but it suddenly became very high and reached about 50μm/s when the temperature was 1250°C. Analysis results indicate that grain growth of this steel is affected by the small particles on the grain boundaries, and grain growth exponent is about 3.3. Moreover, the activation energy of the grain growth is when the temperature is above 1250°C. Compared with a 27Cr ferritic stainless steel containing only 0.14%Nb, the grain growth exponent of this steel is greater, and grain initial rapid growth temperature is higher.

Abstract: In this work, the preliminary result on the effect of cryogenic cooling on grain growth in weld is reported. Ferritic stainless steel weld produced under TIG torch in argon environment is cooled in liquid nitrogen. The weld structure is characterized using LOM, SEM and EDX spectroscopy. The results suggest that cryogenic cooling reduced the weld width within 2% to 5% and HAZ to 39% relative to those cooled in normal condition. This ensures that the area of the base metal affected and exposed to the weld thermal cycle is reduced and hence probably generates less metallurgical distortion. The cryogenic cooling also generated 14% to 36% grain refinement compared to welds cooled in normal condition.

Abstract: Intergranular corrosion in heat-affected zone of low carbon 12Cr-Ni ferritic stainless steel was investigated by the method of practice Z in ASTM A 763-93. The microstructure of welded joint was characterized using an optical microscope and a scanning electronic microscope. Severe intergranular corrosion (IGC) was only observed in low temperature heat-affected zone (LTHAZ) adjacent to base metals at the back of the welded joints in unstabilized steel. On the other hand, IGC was not observed in the welded joints of stabilized steels. According to the analysis of microstructure, the severe IGC is attributed to both the precipitated phase along the grain boundaries inducing the presence of chromium-depleted zone and the welding residual tensile stress promoting the corrosion. Therefore, stabilized elements addition such as Ti and Nb is required in order to avoid the IGC in welded joint of these steels. Relief residual tensile stress by post weld heat treatment would also be an effective method to avoid the IGC in the welded joints of these steels.

Abstract: The high-temperature strength and thermal fatigue properties of Fe-Cr-Nb-Mo ferritic stainless steel (FSSNEW) developed for automobile exhaust system were investigated. The results show that the high-temperature tensile strength and yield strength of FSSNEW are better than or equal to those of the presently applied ferritic stainless steels. The thermal fatigue cracks nucleate at the V-notch. The inclusions along grain boundaries become prior regions for initiation of the cracks. The inclusions distributed at the defects make the formation of cracks in the materials easily through the effects of cycle thermal stress and thermal strain. The length and propagated rate of thermal fatigue cracks increase with the maximum tested temperature increasing. When the maximum temperature arrives at 900°C, the high-temperature oxidation is serious along the grain boundaries, which aggravates the cracks propagating along the grain boundaries. The principle mechanism of stress assisted grain boundary oxygen (SAGBO) embrittlement can be applied to illustrate the effects of external stress on aggravating the damage caused by environmental factors. Therefore, the high-temperature oxidation is the main reason for the propagation of thermal fatigue cracks. The FSSNEW is satisfied for the applied requirement of high-temperature strength in the hot side of the automobile exhaust system.

Abstract: The influence of cold working and grain size on the pitting corrosion resistance of Fe-Cr-Nb-Mo ferritic stainless steel is investigated using optical microscope and electrochemical methods. The pitting corrosion resistance firstly decreases with increasing the cold-rolling reduction from 0% to 30% due to the number of nucleation site increasing. With increasing the cold-rolling reduction from 40% to 60%, the disappearance of grain boundaries, stacked dislocation and uniform microstructure results in the pitting corrosion resistance of the steel. With prolonging the annealing time, the grain size of the steel grows, and the pitting potential of the steel decrease. The smaller grain size promotes the formation of compact passive film and improves the pitting corrosion resistance.

Abstract: In this paper, the texture and micro-orientation of FSS (Ferritic Stainless Steel) during hot rolling, cold rolling and annealing were studied by using x-rays and EBSD technology. The results shown that as cast ingot of FSS was compose of random orientations equiaxed grains and <001>∥ND columnar grains; after hot rolling, the preferred orientation of <001>∥ND columnar grains of as cast ingot obtained inherits, the fiber texture was obvious stronger than that of equiaxed grains; during cold rolling, the initial <001>∥ND columnar grains turn to  fiber texture,  fiber texture can not be found.

Abstract: The influence of the texture on ridging and forming properties of transformable 16mass-%Cr steel was studied for two different specific processing routes. In the first route (HACA), hot strips were annealed prior to cold rolling, whereas in the second route (HCA), un-annealed hot strips were directly subjected to cold rolling. Results indicate that compared to HACA, HCA results in an improved surface smoothness, i.e. reduced roping, but a lower mean r-value, as a result of strengthening of the α-fiber texture components. Differences in the roping and forming properties could also be achieved by compositional differences resulting in differences in the fraction of austenite at hot rolling temperatures.

Abstract: Ti stabilized AISI 439 type 18% Cr ferritic stainless steel offers an attractive alternative for replacing Ni-containing austenitic steels. The influence of the cold rolling reduction on texture development and the planar anisotropy of Ti-stabilized 18% Cr ferritic stainless steel have been studied in order to find an optimized processing route to obtain improved formability properties. In the present study, both annealed and un-annealed hot strips were given different cold rolling reductions up to 86%, and recrystallization annealed. The recrystallization texture was analyzed by means of XRD. Tensile tests were carried out on cold rolled and annealed samples oriented in seven different directions to the rolling direction to evaluate the planar anisotropy in detail. The mean normal anisotropy rm increased with increasing cold reduction for both annealed hot band and un-annealed hot band. The planar anisotropy revealed a behavior related to the development of the recrystallization texture component in the annealed hot band and the texture component in the un-annealed hot band, with increasing cold reduction.

Abstract: Ferritic stainless steels (FSSs) have excellent corrosion resistance and good mechanical properties. Applications include heaters, houseware, and automotive exhaust systems. Alloying, even in small amounts, affects the recrystallization behavior of FSSs by selective dragging or pinning effects. In the present study, we present the main results regarding the recrystallization of a coarse-grained Nb-containing AISI 430 ferritic stainless steel. The material was processed by hot rolling and further annealed at 1250oC for 2 h to promote secondary recrystallization. Following, the material was cold rolled to a 80% reduction in thickness and annealed at 400-1000oC for 15 min. Scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) were used to characterize the microstructure. Recrystallization of this steel begins at 700oC. Important orientation effects were observed in both as-rolled and annealed conditions. Recrystallization kinetics was strongly dependent on the initial orientation of the coarse grains. Results show that grain boundaries, transition bands and coarse Nb(C,N) particles are preferential sites for nucleation at moderate annealing temperatures.