Kae-Myung Kang

Evaluation of Hydrogen Embrittlement of High Strength Steel for Automobiles by Small Punch Test

The hydrogen embrittlement of high strength steel for automobiles was evaluated by small punch (SP) test. The test specimens were fabricated to be 5 series, having various chemical compositions according to the processes of heat treatment and working. Hydrogen charging was electrochemically conducted for each specimen with varying of current density and charging time. It was shown that the SP energy and the maximum load decreased with increasing hydrogen charging time in every specimen. SEM investigation results for the hydrogen containing samples showed that the fracture behavior was a mixed fracture mode having 50% dimples and 50% cleavages. However, the fracture mode of specimens with charging hydrogen changed gradually to the brittle fracture mode, compared to the mode of other materials. All sizes and numbers of dimples decreased with increasing hydrogen charging time. These results indicate that hydrogen embrittlement is the major cause of fracture for high strength steels for automobiles; also, it is shown that the small punch test is a valuable test method for hydrogen embrittlement of high strength sheet steels for automobiles.

Hydrogen Embrittlement Evaluation of Subsurface Zone in 590DP Steel by Micro-Vickers Hardness Measurement

This study describes a hydrogen embrittlement evaluation of the subsurface zone in 590DP steel by micro-Vickers hardness measurement. The 590DP steel was designed to use in high-strength thin steel sheets as automotive materials. The test specimens were fabricated to 5 series varying the chemical composition through the process of casting and rolling. Electrochemical hydrogen charging was conducted on each specimen with varying current densities and charging times. The relationship between the embrittlement and hydrogen charging conditions was established by investigating the metallography. The micro-Vickers hardness was measured to evaluate the hydrogen embrittlement of the subsurface zone in addition to the microscopic investigation. The micro-Vickers hardness increased with the charging time at the surface. However, the changing ratio and maximum variation of hardness with depth were nearly the same value for each test specimen under the current density of 150 mA/ and charging time of 50 hours. Consequently, it appears that hydrogen embrittlement in 590DP steel can be evaluated by micro-Vickers hardness measurement.

Micro-Hardnesses and Microstructural Characteristics of Surface Layer of 590MPa DP Steels According to Hydrogen Charging

【High strength sheet steels for automobile are seriously compromised by hydrogen embrittlement. This issue has been continuously studied, but the field of interest, which lies between microstructural characteristics and hydrogen behavior with hydrogen charging, has not yet been thoroughly investigated. This study was done to investigate the behavior of hydrogen according to the hydrogen volume fraction on 590MPa grade DP steels, which are developed under hydrogen charging conditions as high strength sheet steels for automobiles. The penetration depths and the mechanical properties, according to charging conditions, were investigated through the distribution of micro-hardness and the microstructural observation of the subsurface zone. It was found that the amount of hydrogen trapping in 590MPa DP steels was related to the austenite volume fraction. It was confirmed that the distribution of micro-hardnesses according to the depth of the subsurface zone under the free surface showed the relationship of the depth of the hydrogen saturation between the charging conditions.】

The Effect of Hydrogen on the Variation of Properties at the Surface Layers of 590 MPa DP Steels Charged with Hydrogen

It was investigated that the effects of hydrogen charging on the properties of 590 MPa Dual Phase(DP) steels at the surface layers. The hydrogen-charging time was changed from 5 to 50 hours and current densities from 100, 150, and 200 , respectively. It was found that the hydrogen content in the specimen was increased with as the charging time and the current density. The microvickers hardness of the subsurface zone was increased from 215.3 HV to 239.5 HV due to the increase in current density and charging time. The comparison of the absorbed energies tested by a small-punch (SP) test showed that the absorbed energy of the specimen was greatly reduced from 436 to 283 because of hydrogen embrittlement. It was confirmed that bulb aspects of fracture surface became more brittle with increasing hydrogen content.

A Behavior of Embrittlement at the Subsurface Zones of Multiphase Steels Charged with Hydrogen

In the present work, it was investigated a behavior of hydrogen embrittlement at the subsurface zones of 590 DP steels by using the micro-Vickers hardness test. The micro-Vickers hardnessess of DP steels were measured to evaluate the degree of embrittlement as the effective hardening depths of subsurface zones with hydrogen charging conditions. The results showed that the distributions of micro-Vickers hardness in width varied from maximum hardness 239.5 Hv to minimum hardness 174 Hv, while the depth of effective hardening layer at the subsurface zones of DP steels was from to with hydrogen charging conditions, respectively. It was proposed that the distribution of microhardness be used as the evaluation index of the degree of embrittlement. But the variations of martensite volume fractions were not affected along depth of hardening at the same changing time, hydrogen charging times were appeared as an effective factor of the degree of embrittlement. Therefore, the micro-Vickers hardness test is an attractive tool for evaluation of hydrogen embrittlement at the subsurface zones of these DP steels.

Hydrogen Delayed Fracture of TRIP Steel by Small Punch Test

The strain-induced phase transformation from austenite to martensite is responsible for the high strength and ductility of TRIP steels. However high strength steels are susceptible to hydrogen embrittlement. This study aimed to evaluate the effects of hydrogen on the behavior of hydrogen delayed fracture in TRIP steel with hydrogen charging conditions. The electrochemical hydrogen charging was conducted at each specimen with varying current density and charging time. The relationship between hydrogen concentration and mechanical properties of TRIP steel was established by SP test and SEM fractography. The maximum loads and displacements of the TRIP steel in SP test decreased with increasing hydrogen charging time. The results of SEM fractography investigation revealed typical brittle mode of failure. Thus it was concluded that hydrogen delayed fracture in TRIP steel result from the diffusion of hydrogen through the ` phase.

The Influence of Hydrogen Charging with the Volume Fraction of Phases in Dual Phase Steels

A study on microstructure control of multi-phase steel have been implemented to higher strength with improved formability. However, it is well known that the high strength of steel are susceptible to hydrogen embrittlement. The mechanisms of hydrogen embrittlement is caused by complex interactions. In this paper, the test specimens were fabricated to 5 type of 590DP steels at different levels of volume faction. The hydrogen charging was conducted by electrochemical hydrogen-charge method with varying charging time. The relationship between hydrogen concentration and volume fraction of 590DP steel was established by SP test and SEM-fractography. It was shown that the hydrogen amounts charged in 590DP steels increased with increasing the volume faction of austenite. The maximum loads of the 590DP steels in SP test were sharply decreased with increasing hydrogen charging time. The results of SEM-fractography investigation showed typical brittle-fracture surfaces for hydrogen-charged 590DP steels.

Growth Characteristics of SnO 2 Thin Film for Gas Sensor with Annealing Treatment

Relationships between the electrical resistivity and the growth characteristic of thin films were investigated. thin films with thickness from 64 nm to 91 nm were made by controlling the RF deposition energy from 80 to 150 W. These thin films were annealed at temperature range of interval in the gas condition. After annealing treatments, the microstructures of the thin films were changed mixed structure(amorphous & crystalline) to lamina columnar crystalline structure. Both the film thickness and the grain size were increased with increasing the local crystallization of microstructure of thin films by annealing treatment. Their electrical resistivity increased up to the annealing temperature of , and then slowly decreased.

Development of Electroconductive Paints for Electric-Shock on Human Body Using Carbon Black

)Abstract For development of a human body model for electric shock, electroconductive paints with carbonblack as a filler material were developed. The characteristics of the volume resistivities of thin films fabricatedusing the electroconductive paints were investigated as a function of the particle sizes and content of carbonblack. With a carbon black particle size over 80µm, agglomeration of carbon black powders was observed. Thevolume resistivity of the particles increased as the porosity increased and as the amount of carbon blackdecreased due to the agglomeration of carbon black powders. With a particle size of 4µm and 20µm,agglomeration of carbon black powders was not observed and their porosities were measured as 0.86 % and1.12 % with volume resistivities of 20Ω·cm and 80Ω·cm, respectively. A carbon black particle size of lessthan 20µm is considered to be suitable as a type of electric-shock electroconductive paint for a human bodymodel. Keywords electroconductive paint, carbon black(CB), volume resistivity, electric shock.

Microstructural Characteristics of Al 2 O 3 /Cu Nanocomposites Depending on Fabrication Process

The microstructural characteristics of /Cu composites hot-pressed at different temperatures for atmosphere switching from to Ar have been studied. When the composite atmosphere was switched at it led to more homogeneous microstructure than when the atmosphere was switched at . The strong sensitivity of Cu to atmosphere, especially the oxygen content in the atmosphere, was found to be responsible for the observed change, based upon the interfacial phenomena related to the formation of . The practical implication of these results is that an optimum processing condition for the design of homogeneous microstructure and stable properties can be established.