• 동력기계공학회지
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• 제20권4호
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• pp.5-11
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• 2016

This study analyzed the damage to a slitting knife after cutting steel sheets. Damages to the structure were observed and wear tests were conducted. In addition, the degradation on the damaged and undamaged parts was compared with a micro Vickers hardness test. Weibull statistical analysis was carried out in order to evaluate the reliability of the micro Vickers hardness measured data. Spalling of the edge portion occurred by degradation during use over a long period. Rough parts in the specimens were caused by damage because the slitting knife was used for 1 year. The friction coefficient and wear loss at the damaged parts of the knife edge were slightly larger from shock due to repetitive cutting operation. The micro Vickers hardness followed a two-parameter Weibull probability distribution.

• Tribology and Lubricants
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• 제29권6호
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• pp.360-365
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• 2013

In this study, sliding wear tests were conducted to investigate the effects of tempered hardness on the sliding wear behavior of bearing steel. At a sliding speed of 0.3 m/s, the wear resistance of bearing steel with a tempered hardness of HRC 54 was superior to that with HRC 62. It was found that bearing steel with HRC 54 showed a strong tendency for the occurrence of oxidation wear at that speed, compared to that with HRC 62. This would be due to the troostitic structure of bearing steel with HRC 54, which is highly susceptible to corrosion. In this context, it is considered that sliding wear behavior could be affected by the corrosion resistance of the material.

• 한국공작기계학회:학술대회논문집
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• 한국공작기계학회 2004년도 추계학술대회 논문집
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• pp.34-39
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• 2004

The cutting tests of aluminum alloy for various hardnesses were carried out using CNC milling machine. The surface roughness{Ra, Rmax) of cut surface and cutting forces are measured at various cutting conditions such as spindle speed, feed speed and hardness. In the CNC end-milling, the surface roughness increases as feed speed increases and decreases as spindle speed increases. However, the bulit-up edge has occurred on in case of low hardness and low feed speed. In experimental conditions, as the hardness of aluminum alloy increases, the surface roughness(Ra, Rmax) decreases

• 열처리공학회지
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• 제31권1호
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• pp.1-5
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• 2018

This work demonstrated the application of an artificial neural network model for predicting the Jominy hardness curve by considering 13 alloying elements in low alloy steels. End-quench Jominy tests were carried out according to ASTM A255 standard method for 1197 samples. The hardness values of Jominy sample were measured at different points from the quenched end. The developed artificial neural network model predicted the Jominy curve with high accuracy ($R^2=0.9969$ for training and $R^2=0.9956$ for verification). In addition, the model was used to investigate the average sensitivity of input variables to hardness change.

• Corrosion Science and Technology
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• 제7권3호
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• pp.145-151
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• 2008

Material degradation such as high temperature oxidation of metallic material is a severe problem in energy generation systems or manufacturing industries. The metallic materials are oxidized to form oxide films in high temperature environments. The oxide films act as diffusion barriers of oxygen and metal ions and thereafter decrease oxidation rates of metals. The metal oxidation is, however, accelerated by mechanical fracture and spalling of the oxide films caused by thermal stresses by repetition of temperature change, vibration and by the impact of solid particles. It is therefore very important to investigate mechanical properties and adhesion of oxide films in high temperature environments, as well as the properties in a room temperature environment. The oxidation tests were conducted for Ni and Ni-Co alloy under high temperature corrosive environments. The hardness distributions against the indentation depth from the top surface were examined at room temperature. Dynamic indentation tests were performed on Ni oxide films formed on Ni surfaces at room and high temperature to observe fractures or cracks generated around impact craters. As a result, it was found that the mechanical property as hardness of the oxide films were different between Ni and Ni-Co alloy, and between room and high temperatures, and that the adhesion of Ni oxide films was relatively stronger than that of Co oxide films.

• 한국정밀공학회지
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• 제18권12호
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• pp.167-176
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• 2001

The effects of heat treatments and testing temperatures on the mechanical properties of Invar materials were investigated through experiments, which call influence the formability in metal forming fields. Annealing temperatures were changed from $900^{\circ}C$ to $1200^{\circ}C$ with an increment of $100^{\circ}C$ under two different furnace atmosphere(vacuum and H$_2$gas). Microstructure and hardness tests were performed for annealed specimens at room temperature(RT) and tensile tests were also performed by changing annealing temperatures as well as testing temperatures from RT to $300^{\circ}C$. The grain size of annealed materials increased with increasing annealing temperature, while micro-hardness distributions showed almost same hardness values regardless of annealing temperatures. Strength ratio (tensile/yield strength), which influences the forming characteristics of sheet metal, remained almost constant for various experimental conditions in case of unannealed specimens. However, it showed increasing tendency with increasing both annealing and testing temperatures, particularly at the testing temperature higher than $200^{\circ}C$. Therefore it can be concluded that press formability of fully-annealed Invar material can be improved by warm forming technique.

• International Journal of Korean Welding Society
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• 제2권2호
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• pp.7-13
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• 2002

A comparison of the welding performance of ship hull structural steels has been made. The weldability of steels especially designed for laser processing was compared to that of conventional hull and structural steels with plate thicknesses up to 12 mm. Autogenous laser beam welding was used to weld butt joints as well as skid and stake welded T-joints. The welds were assessed in accordance with the document "The Classification Societies" Requirements for Approval of $CO_2$ Laser Welding Procedures" Small imperfections in the weld only grew slightly in root bend tests and they only had a minor influence on the fatigue properties of laser fillet welded joints. In Charpy impact tests, the 27 J transition temperature of the weld metal and HAZ ranged from below -60 to $-50^{\circ}C$. The amount of martensite in the weld metal depended on the carbon equivalent of the steel with the highest amounts and highest hardness levels in conventional EH 36 (389 HV 5). Thermomechanically rolled steels contained less martensite and showed a correspondingly lower maximum hardness.ximum hardness.

• International Journal of Automotive Technology
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• 제6권1호
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• pp.45-52
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• 2005

Interrupted creep tests for investigating the structural degradation during creep were conducted for a Mod.9Cr-1Mo steel in the range of stress from 71 to 167 MPa and temperature from 873 to 923 K. The change of hardness and tempered martensitic lath width was measured in grip and gauge parts of interrupted creep specimens. The lath structure was thermally stable in static conditions. However, it was not stable during creep, and the structural change was enhanced by creep strain. The relation between the change in lath width and creep strain was described quantitatively. The change in Vickers hardness was expressed by a single valued function of creep LCR(life consumption ratio). Based on the empirical relation between strain and lath width, a model was proposed to describe the relation between change in hardness and creep LCR. The comparison of the model with the empirical relation suggests that about 65% of hardness loss is due to the decrease of dislocation density accompanied by the movement of lath boundaries. The role of precipitates on subboundaries was discussed in connection with the abnormal subgrain growth appearing in low stress regime.

• 한국기계가공학회지
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• 제10권1호
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• pp.67-72
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• 2011

The main objective of this paper is obtaining the optimal carburizing hours which impacts geometric tolerances, OPD(Over Pin Diameter), runout, hardness and fatigure strength of SCM415 spur gear. In order to observe the deformation of the gear, the circularity, squareness, OPD(Over Pin Diameter) and runout were measured at 3hour, 4hour and 5hour respectively. As the result, the 3hour situation is the best, which very similar with the 4hour one. Afterwards, with the purpose of getting the result of gear hardness, the surface hardness, maximum hardness and interior hardness were measured. The result is 5h situation is the best, and it's similar with 4hour one. At last, the fatigue tests have been done which receive the result that the 4h situation showed 9~12% fatigue strength improvement compared with the 3h and 5h ones. To sum up the results, the 4hour situation shows the best performance in accuracy, hardness and fatigue strength.

• 소성∙가공
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• 제32권4호
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• pp.180-190
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• 2023

This study deals with residual stress prediction and hardness evaluation within cross ball grooved inner race fabricated by cold upsetting process consisted of upsetting and ejection steps. A raw workpiece material of AISI 5120H (SCr420H) is first spheroidized and annealed, then phosphophyllite coated to form solid lubricant layer on its outer surface. To investigate influences of the heat treatment, uni-axial compression tests and Vickers micro-hardness measurements are conducted. Three-dimensional elasto-plastic FE simulations on the upsetting step and the ejection one are performed to visualize the residual stress and the ductile (plastic deformation) damage. External feature of the fabricated inner race is fully captured by using an optical 3D scanner, and the micro-hardness is measured on internal cross-sections. Consequently, the dimensional compatibility between the simulated inner race and the fabricated one is ensured with a difference of under 0.243mm that satisfied permissible error range of ±0.50mm on the grooved surface, and the predicted residual stress is verified to have similar distribution tendency with the measured Vickers micro-hardness.