• 한국압력기기공학회 논문집
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• 제20권1호
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• pp.56-65
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• 2024

Cast austenitic stainless steels (CASS) and austenitic stainless steel weldments with a ferrite-austenite duplex structure are widely used in nuclear power plants, incorporating ferrite phase to enhance strength, stress relief, and corrosion resistance. Thermal aging at 290-325℃ can induce embrittlement, primarily due to spinodal decomposition and G-phase precipitation in the ferrite phase. This study evaluates the effects of thermal aging by collecting and analyzing various mechanical properties, such as Charpy impact energy, ferrite microhardness, and tensile strength, from various literature sources. Different model expressions, including hyperbolic tangent and phase transformation equations, are applied to calculate activation energy (Q) of room-temperature impact energies, and the results are compared. Additionally, predictive models for Q based on material composition are evaluated, and the potential of machine learning techniques for improving prediction accuracy is explored. The study also examines the use of ferrite microhardness and tensile strength in calculating Q and assessing thermal embrittlement. The findings provide insights for developing advanced prediction models for the thermal embrittlement behavior of CASS and the weldments of austenitic steels, contributing to the safety and reliability of nuclear power plant components.

• Journal of Welding and Joining
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• 제18권2호
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• pp.49-56
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• 2000

Recently developed Austenite stainless steel, 309L was used to overlay on 3Cr-1Mo-V-Ti-B steels, using Electroslag welding process, which wide electrodes were adopted. Transition region in welding interlayer relating to disbonding crack was investigated. Also, the effect of welding condition on the width of transition region and coarsening grains of the austenite were studied. 1) With increasing welding speed the width of martensite at transient region was increased, but the amount of delta ferrite in weld metal was reduced, being fine grained. 2) The form of martensite at the transition region was occurred by reversible transformation during cooling since the interdiffusion of Cr and Ni from weld metal and Fe and C from base metals at the transition region, causes to lowering the concentration of Cr and Ni at the transition region, leading to increasing Ms point. 3) With increasing welding speed, the grain of austenite formed at the welding interface was finer. With increasing welding current under the same welding speed, the grain size of the austenite was finer. At high current, original grain size of the austenite is coarse, but the austenite has fine grains because the austenite was transformed to martensite during cooling. 4) In the case of high welding speed, the width of martensite at the welding interface was increased, but the grain size of austenite at the welding interface was finer. This indicates that the inhibition of disbonding crack may be achieved through dispersening fine carbides in the gain boundary.

• 한국기계연구소 소보
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• 통권19호
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• pp.53-60
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• 1989

Hot upsetting experiments were carried out on presintered steel powder preforms in the temperature range 700- $950^{\circ}C$ to examine the hot deformation behavior. Following conclusions were drawn on the basis of the present study. -The flow stress during hot deformation is directly related to $\alpha$- $\gamma$ phase trasformation - The flow stress of ferrite is lower than that of austenite in the moderate temperature range 800- $900^{\circ}C$ for most alloys used in the present study - Major restoration behavior during hot deformation in the ferrite range is dynamic recovery.

• Journal of Welding and Joining
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• 제31권6호
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• pp.8-16
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• 2013

As the energy consumption increases rapidly, power generation needs the high energy efficiency continuously. To achieve the high efficiency of power generation, the materials used have to endure the higher temperature and pressure. The 9-12%Cr steels possess good mechanical properties, corrosion resistance, and creep strength in high temperature due to high Cr contents. Therefore, the 9-12%Cr steels are widely used for the high-temperature components in power plants. Even though the steels usually have a fully martensitic microstructure, they are susceptible to the formation of ${\delta}$-ferrite specifically during the welding process. The formation of ${\delta}$-ferrite has several detrimental effects on creep, ductility and toughness. Therefore, it is necessary to avoid its formation. As the volume fraction of ${\delta}$-ferrite is less than 2% in microstructure, it has the isolated island morphology and causes no significant degradation on mechanical properties. For ${\delta}$-ferrite above 2%, it has a polygonal shape affecting the detrimental influence on the mechanical properties. The formation of ${\delta}$-ferrite is affected by two factors: a chemical composition and a welding heat input. The most effective ways to get a fully martensite microstructure are to reduce the chromium equivalent less than 13.5, to keep the difference between the chromium and nickel equivalent less than 8, and to reduce the welding heat input.

• 열처리공학회지
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• 제10권1호
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• pp.1-9
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• 1997

The variation of the mechanical properties and the formation of retained austenite with heat treatment conditions in austempered Si bearing carbon steels has been investigated. In the case of a steel containing 0.35C-1.48Si-0.95Mn, it has been found that a feather shape bainite structure of lath are obtained under a isothermal treated condition at just below the Ms temperature, and the martensite, bainitic ferrite and retained austenite of second phase particles on the ferrite matrix for a isothermal treated steels after intercritical annealing are precipitated in a linked shape. The retained austenite with $2{\mu}m$ size induced as TRIP is found to increase with increasing the formation rate of retained austenite for the intercritical annealing and high Si containing steels. The tensile strength is increased as austempering temperature increases in all isothermal treatment temperature, whereas the elongation is shown to roughly decrease as the tensile strength increases. The values of tensile strength-elongation balance have showed a marked dependence upon the elongation rather than the tensile stregth, and their values are increased for high Si containing steels and intercritical annealing condition. The most optimum result has been shown to be the tensile stregth-elongation balance of $2882.4kgf/mm^2.%$ and the elongation of 33.3% for a "B" steel in the heat treating temperature range of $780{\sim}370^{\circ}C$.

• Corrosion Science and Technology
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• 제6권4호
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• pp.164-169
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• 2007

Hydrogen induced cracking (HIC) was studied phenomenologically and the effect of microstructure on HIC was discussed for the steels having two different levels of nonmetallic inclusions. Steels having different microstructures were produced by thermomechanically controlled processes (TMCP) from two different heats which had the different level of nonmetallic inclusions. Ferrite/pearlite (F/P), ferrite/acicular ferrite (F/AF), ferrite/bainite (F/B) were three representative microstructures for all tested steels. For the steels with higher level of inclusions, permissible inclusion level for HIC not to develop was different according to steelmicrostructure. On the contrary, HIC occurred also at the martensite/austenite (M/A) constituents regardless of steel microstructure when they accumulated to a certain degree. It was proved that M/A constituents were easily embrittled by hydrogen atoms. Steels having F/AF is resistant to HIC at a given actual service condition since they covers a wide range of diffusible hydrogen content without developing HIC.

• 대한용접접합학회:학술대회논문집
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• 대한용접접합학회 1998년도 특별강연 및 춘계학술발표 개요집
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• pp.150-154
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• 1998

The purpose of this paper is to investigate optimum welding conditions of STS 304 thin sheet by GTA welding and control 6 $\delta$--fenite which is harmful in mechanical processing, corrosion problem and can be formed brittle a phase in using long term at high temperature. One series of automatic welds was made using argon plus 10, 20, 30 % nitrogen to ensure a fully austenite deposit. Results obtained were summarized as follows: 1) 6 $\sigma$ferrite content in the weld metals is influenced largely by the nitrogen content. 2) Additions of nitrogen to the shielding gas can significantly reduce the amount of retained delta ferrite and result in an increase in hot cracking. 3) Bead width was increased when Ar + $N_2$ shielding gas was used and travel speed was increased. 4) Ar+$N_2$ shielding gas made weld metal ductile and reduce 6 -$\delta$-ferrite.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2007년도 추계학술대회 논문집
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• pp.154-157
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• 2007

The austenite phase observed in low carbon HSLA steels is well known to be decomposed to various bainitic microstructures, such as granular bainite, acicular ferrite and bainitic ferrite during continuous cooling process. These bainitic microstructures have been usually identified by using either scanning electron microscope (SEM) or transmission electron microscope (TEM). However, SEM and TEM images do no exactly coincide, because of the quite different sample preparation method in SEM and TEM observations. These conventional analysis method is, thus, not suitable for characterization of the complex bainitic microstructure. In this study, focused ion beam (FIB) technique was applied to make site-specific TEM specimens and to identify the 3-dimensional grain morphologies of the bainitic microstructure. The morphological feature and grain boundary characteristics of each bainitic microstructure were exactly identified.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2008년도 추계학술대회 논문집
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• pp.273-276
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• 2008

In this study, microstructure and distribution of alloy elements were investigated in thermo-mechanically processed C-Mn-Si transformation induced plasticity (TRIP) steels. The microstructures of TRIP steels were investigated by using advanced analysis techniques, such as three dimensional atom probe tomography (3D-APT). At first, the microstructure was observed by using TEM. TEM results revealed that microstructure of C-Mn-Si TRIP steel was composed of ferrite, bainte, and retained austenite. 3D-APT was used to characterize atomic-scale partitioning of added elements at the phase interface. In the retained austenite phase, Ti and B were enriched with C. However, there was no fluctuation of Mn and Si concentration across the interface. Through these analysis techniques, the advanced characteristics of constituent microstructure in C-Mn-Si TRIP steels were identified.

• 열처리공학회지
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• 제11권4호
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• pp.333-341
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• 1998

The study was investigated on the effect of austenitizing and austempering conditions on retained austenite amount and carbon contents in retained austenite and simultaneously the effect of these variation on hardness, tensile and impact properties. A material of as-cast condition is composed of bull's eye structure with ferrite surrounding spheroidized graphite having about $5-10{\mu}m$ size and matrix structure of pearlite. Then, the contents of spheroidized graphite was about 5%. The retained austenite and carbon contents in the retained austenite were increased with the increasing of austenitizing and austempering temperatures, while the retained austenite showed the peak value and is decreased with increasing of austempering time. With increasing of austenitizing temperature, tensile strength, elongation and impact absorb energy increased and hardness was almost not changed, while with increasing of austempering temperature, tensile strength and hardness decreased, whereas elongation and impact absorb energy was increased. With increasing of retained austenite amount, the tensile strength is slowly decreased but elongation was increased with direct proportion. Also, Impact absorb energy is shown identity value untile about 18%, but rapidly increased above it. Elongation and Impact absorb energy are strongly controlled by the amount of retained austenite, but tensile strength is affected with various factors such as retained austenite amount and bainitic morphology.