• Journal of the Korean Society for Nondestructive Testing
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• v.27 no.5
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• pp.417-425
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• 2007

The ferritic 9Cr-1Mo-V-Nb heat-resisting steel was experimentally studied in order to characterize its microstructural evolution during creep-fatigue by coercivity measurement. The creep-fatigue test was conducted at $550\;^{\circ}C$ with the tensile holding time of 60s and 600s, respectively. The coercivity decreased until the failure and the hardness monotonously decreased for the whole fatigue life. As the life fraction of creep-fatigue increased, the $M_{23}C_6$ carbide coarsened following the Ostwald ripening mechanism. However, the MX carbonitrides did not grow during creep-fatigue due to so stable at $550\;^{\circ}C$. The width of martensite lath increased because of the dislocation recovery at the lath boundaries. The magnetic coercivity has an influence on the microstructural properties such as dislocation, precipitates and martensite lath boundaries, which interpreted in relation to microstructural changes. Consequently, this study proposes a magnetic coercivity to quantify the level of damage and microstructural change during the creep-fatigue of ferritic 9Cr-1Mo-V-Nb steel.

• Journal of the Korean Institute of Gas
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• v.2 no.3
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• pp.78-87
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• 1998

The mechanical properties, heat treatment and the history for the development of Ni-bearing ferritic steels for the application at low temperatures are reviewed. Ni-bearing ferritic steels are classified into $2-3\%$ Ni steels, $5.5\%$ Ni steel, $9\%$ Ni steel and $13\%$ Ni steel., of which $9\%$ Ni steels are most widely used for the large LNG storage tanks owing to their encellent fracture toughness up to $-196^{\circ}C$. The effect of retained austenite on the tensile properties and toughness was precisely discussed. As the size of LNG storage tank increases, thicker plates are needed. Thus, the recent efforts for the improvement of low temperature toughness and homogeneity are also introduced.

• Korean Journal of Metals and Materials
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• v.56 no.11
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• pp.779-786
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• 2018

There have been previous attempts to observe the occurrence of dynamic ferritic transformation at temperatures even above $Ae_3$ in a low-carbon steel, and not only in steels, but recently also in titanium alloys. In this study, a new approach is proposed that involves treating true stress-true strain curves in uniaxial compression tests at various temperatures, and different strain rates in 0.1C-6Ni steel, which is a model alloy used to decelerate the kinetics of ferrite transformation from austenite. The initial flow stress up to peak stress was used to analyze the change in dynamic softening phenomena, such as dynamic recovery, dynamic recrystallization, and dynamic transformation. It is worth mentioning that for predicting the occurrence of dynamic transformation, flow stress before reaching peak stress is much more sensitive to the change in the dynamic softening rate due to dynamic transformation, compared to peak stress. It was found that the occurrence of dynamic ferritic transformation could be successfully obtained even at temperatures above $Ae_3$ once the deformation condition was satisfied. This deformation condition is a function of both the strain rate and the deformation temperature, which can be described as the Zener - Hollomon parameter. In addition, the driving force of dynamic ferritic transformation might be much less than that of the dynamic recrystallization of austenite at a given deformation condition. By applying this technique, it is possible to predict the occurrence of dynamic transformation more sensitively compared with the previous analysis method using peak stress during deformation.

• Journal of Welding and Joining
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• v.27 no.2
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• pp.27-31
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• 2009

Ferritic stainless steels have a good heat resistance and economic advantage. They are used for applications such as automotive exhaust systems where resistance to general corrosion is superior to carbon steels. However, there are not enough research for ferritic stainless steels on weldability mainly used as automotive exhaust manifolds. In this study, mechanical and microstructure properties of as-welded STS 429L and STS 444 ferrite stainless steels were confirmed by tensile, bending, hardness test, optical microscopy and scanning electron microscopy. Tensile strength of the STS 444 is higher than the STS 429L when it is a raw material. In contrast to this fact, STS 429L indicated higher tensile strength after butt welded. In addition, the hardness have a increasing tendency as getting down on the bead.

• Journal of Welding and Joining
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• v.32 no.2
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• pp.14-17
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• 2014

Ferritic stainless steels are widely used in the construction industry and in exhaust manifolds due to their low cost and relatively superior stress corrosion cracking resistance and pitting corrosion resistance compared to austenite stainless steels. Ferritic stainless steels are currently welded by various welding process including gas tungsten arc welding (GTAW), electron resistance welding (ERW) and laser beam welding. However, when these stainless steels are welded by fusion welding, some problems occur in the fusion zone (FZ) and heat affected zone (HAZ). First, the ductility of the weld is reduced due to the grain growth in the FZ and HAZ. Second, as its HAZ is frequently sensitized during welding, corrosion resistance deteriorates in this region due to the Cr depletion zone. To prevent these problems, it is recommended that ferritic stainless steels be welded with a low heat input. In this study, recent researches in the view of friction stir welded ferritic stainless steels are briefly reviewed.

• Korean Journal of Metals and Materials
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• v.46 no.11
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• pp.737-746
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• 2008

Mechanisms of sticking phenomena occurring during hot rolling of a modified STS 430J1L ferritic stainless steel have been investigated in this study by using a pilot-plant-scale rolling machine. As the rolling pass proceeds, the Fe-Cr oxide layer formed in a reheating furnace is destroyed, and the destroyed oxides penetrate into the rolled steel to form a thin oxide layer on the surface region. The sticking does not occur on the surface region containing oxides, whereas it occurs on the surface region without oxides by the separation of the rolled steel at high temperatures. This indicates that the resistance to sticking increases by the increase in the surface hardness when a considerable amount of oxides are formed on the surface region, and that the sticking can be evaluated by the volume fraction and distribution of oxides formed on the surface region. The lubrication and the increase of the rolling speed and rolling temperature beneficially affect to the resistance to sticking because they accelerate the formation of oxides on the steel surface region. In order to prevent or minimize the sticking, thus, it is suggested to increase the thickness of the oxide layer formed in the reheating furnace and to homogeneously distribute oxides along the surface region by controlling the hot-rolling process.

• Journal of Welding and Joining
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• v.28 no.3
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• pp.92-98
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• 2010

Ferritic stainless steels, which have relatively small thermal expansion coefficient and excellent corrosion resistance, are increasingly being used in vehicle manufacturing, in order to increase the lifetime of exhaust manifold parts. But, there are limits on use because of the problem related to cosmetic resistance, corrosions of condensation and high temperature salt etc. So, Aluminum-coated stainless steel instead of ferritic stainless steel are utilized in these parts due to the improved properties. In this investigation, Al-8wt% Si alloy coated 409L ferritic stainless steel was used as the base metal during Gas Tungsten Arc(GTA) welding. The effects of coated layer on the microstructure and hardness were investigated. Full penetration was obtained, when the welding current was higher than 90A and the welding speed was lower than 0.52m/min. Grain size was the largest in fusion zone and decreased from near HAZ to base metal. As welding speed increased, grain size of fusion zone decreased, and there was no big change in HAZ. Hardness had a peak value in the fusion zone and decreased from the bond line to the base metal. The highest hardness in the fusion zone resulted from the fine re-precipitation of the coarse TiN and Ti(C, N) existed in the base metal during melting and solidification process and the presence of fine $Al_2O_3$ and $SiO_2$ formed by the migration of the elements, Al and Si, from the melted coating layer into the fusion zone.

• Transactions of the Korean Society of Pressure Vessels and Piping
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• v.11 no.1
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• pp.12-19
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• 2015

As a measure of improving the mechanical properties of a diffusion bonded joint of a ferritic/martensitic steel (FMS), the post-bonding heat treatment (PBHT) is applied. In the temperature range of normalizing condition ($950-1,050^{\circ}C$), diffusion bonding is employed with compressive stress (6 MPa). Due to the martensite structure distributed in the matrix, Vicker's hardness values of the as-bonded are much higher than those of the as-received. Through the PBHT for 1 h at $720^{\circ}C$, hardness values are recovered to as low as those of the as-received condition. Also, tensile properties of PBHT are similar to those of the as-received at up to the test temperature of $550^{\circ}C$, when the diffusion bonding is carried out over $1,000^{\circ}C$. Based on the creep-rupture testing performed at $650^{\circ}C$ in air environment, the joint efficiency of the PBHTed specimens is about 80% in, which is higher than that of the as-bonded specimens.

• Journal of Welding and Joining
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• v.28 no.5
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• pp.93-98
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• 2010

The properties of GTA weld for ferritic stainless steel have been studied with different $O_2$ contents in Ar shielding gas at the constant welding speed. A small amount of $O_2$ (0.01~1.0%) was mixed in Ar shielding gas in order to improve the weld penetration. The fully penetrated GTA weld was acquired at 160A weld current shielded by pure Ar gas. Addition of oxygen larger than 0.1% made a full penetration at lower weld current than 160A. The small addition of $O_2$ in Ar shielding gas improved the penetration properties of GTA weld because the $O_2$ in the molten pool accelerated the flow of molten pool and changed the flow pattern from outward to inward direction. The impact energy and DBTT (Ductile- Brittle- Transition-Temperature) of the GTA weld shielded by Ar+$O_2$ (less 0.3%) was similar and the corrosion properties of GTA weld was slightly inferior to those of GTA weld shielded by pure Ar gas.

• Transactions of Materials Processing
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• v.30 no.6
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• pp.301-305
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• 2021

The role of Nb addition in the tension and corrosion responses of ferrite-based stainless-steel sheets processed by cold-rolling and annealing treatment at 1173 °K was studied to evaluate the possibility for commercialization. For this purpose, the grain sizes in the samples with and without Nb content were controlled to be reasonably identical by utilizing different heat treatments within the specific range for 60 seconds. The corrosion properties of both samples were evaluated based on the analysis of polarization curves. Room-temperature tension testing showed that the strength of the sample with Nb was much higher than that without Nb whereas the elongation of the sample with Nb was slightly lower than that without Nb. The polarization curves revealed that the addition of Nb contributed to the significant improvement in protective capabilities. We thought that such combinational behavior was mainly due to the formation of niobium carbides found in the ferritic matrix.