Byoungchul Hwang

Effect of carbide distribution on the fracture toughness in the transition temperature region of an SA 508 steel

Abstract An investigation was conducted into the effect of carbide distribution on fracture toughness in the ductile–brittle transition temperature region of an SA 508 steel used for nuclear reactor pressure vessels. Tensile properties and elastic–plastic cleavage fracture toughness were measured in the transition temperature region, and the fracture toughness data were interpreted by using a simple fracture model containing carbide size distribution. This modeling study indicated that the critical nearest-neighbor distance between coarse carbides was an important microstructural factor affecting elastic–plastic fracture toughness, since it satisfied a linear relationship with the critical distance between a crack tip to a cleavage initiation site. These findings suggested that reducing the total number of carbides, particularly the number of M3C carbides larger than the critical size, and homogeneously distributing fine M2C carbides, were useful ways to improve fracture toughness in the transition temperature region.

Improvement of Wear Resistance of Plasma-Sprayed Molybdenum Blend Coatings

The wear resistance of plasma sprayed molybdenum blend coatings applicable to synchronizer rings or piston rings was investigated in this study. Four spray powders, one of which was pure molybdenum and the others blended powders of bronze and aluminum-silicon alloy powders mixed with molybdenum powders, were sprayed on a low-carbon steel substrate by atmospheric plasma spraying. Microstructural analysis of the coatings showed that the phases formed during spraying were relatively homogeneously distributed in the molybdenum matrix. The wear test results revealed that the wear rate of all the coatings increased with increasing wear load and that the blended coatings exhibited better wear resistance than the pure molybdenum coating, although the hardness was lower. In the pure molybdenum coatings, splats were readily fractured, or cracks were initiated between splats under high wear loads, thereby leading to the decrease in wear resistance. On the other hand, the molybdenum coating blended with bronze and aluminum-silicon alloy powders exhibited excellent wear resistance because hard phases such as CuAl2 and Cu9Al4 formed inside the coating.

Effect of oxides on wear resistance and surface roughness of ferrous coated layers fabricated by atmospheric plasma spraying

Abstract The objective of this study is to investigate the correlation of microstructure, wear resistance, and surface roughness in ferrous coated layers applicable to cylinder bores. Four kinds of ferrous spray powders, two of which were STS 316 steel powders and the others were blend powders of ferrous powders mixed with 20 wt.% Al 2 O 3 –ZrO 2 powders, were sprayed on a low-carbon steel substrate by atmospheric plasma spraying. Microstructural analysis of the coated layers showed that iron oxides were formed in the austenitic matrix by oxidation during spraying for the STS 316 coated layers, while Al 2 O 3 –ZrO 2 oxides were mainly formed in the martensitic matrix for the blend coated layers. The wear test results revealed that the blend coated layers showed the better wear resistance than the STS 316 coated layers because they contained a number of hard Al 2 O 3 –ZrO 2 oxides. However, they had rough worn surfaces because of the preferential removal of the matrix and the cracking of oxides during the wear process. The STS 316 coated layers showed the slightly worse wear resistance than the blend coated layers, but they showed the excellent surface roughness resulting from homogeneous wear in oxides and matrix due to the smaller hardness difference between them. In order to improve the overall wear properties with consideration of the wear resistance of a counterpart material and the surface roughness, the hardness difference between oxides and matrix should be minimized, while the hardness should be maintained up to a certain level by forming an appropriate amount of oxides.

Effects of Cooling Conditions on Microstructure, Tensile Properties, and Charpy Impact Toughness of Low-Carbon High-Strength Bainitic Steels

In this study, four low-carbon high-strength bainitic steel specimens were fabricated by varying finish cooling temperatures and cooling rates, and their tensile and Charpy impact properties were investigated. All the bainitic steel specimens consisted of acicular ferrite, granular bainite, bainitic ferrite, and martensite-austenite constituents. The specimens fabricated with higher finish cooling temperature had a lower volume fraction of martensite-austenite constituent than the specimens fabricated with lower finish cooling temperature. The fast-cooled specimens had twice the volume fraction of bainitic ferrite and consequently higher yield and tensile strengths than the slow-cooled specimens. The energy transition temperature tended to increase with increasing effective grain size or with increasing volume fraction of granular bainite. The fast-cooled specimen fabricated with high finish cooling temperature and fast cooling rate showed the lowest energy transition temperature among the four specimens because of the lowest content of coarse granular bainite. These findings indicated that Charpy impact properties as well as strength could be improved by suppressing the formation of granular bainite, despite the presence of some hard microstructural constituents such as bainitic ferrite and martensite-austenite.

Isothermal Decomposition of Ferrite in a High-Nitrogen, Nickel-Free Duplex Stainless Steel

The formation and crystallography of second phases during isothermal decomposition of ferrite (α) in a high-nitrogen, nickel-free duplex stainless steel was examined by means of transmission electron microscopy (TEM). At an early stage of aging, the decomposition of α started along the α/γ phase boundaries where sigma (σ) phase and secondary austenite (γ2) precipitated in the form of an alternating lamellar structure. The combined analyses based on the simulation of diffraction patterns and stereographic projection have shown that most of the σ phase was related to the γ2 by the following relation:

Tensile Deformation Behavior and Phase Transformation in the Weld Coarse-Grained Heat-Affected Zone of Metastable High-Nitrogen Fe-18Cr-10Mn-N Stainless Steel

(111)_{\gamma } \parallel (001)_{\sigma }

Effects of C and Si on strain aging of strain-based API X60 pipeline steels

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