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Effect of Laser Preheating AISI 4140 Specimens for Micro-Forging

Jung C., Lee M. G., Jeon Y.

Archives of Metallurgy and Materials

2017

Vol. 62, iss. 2B

1209--1213

Many high performance and permanent service parts require suitable material characteristics-high fatigue strength is one of the most important characteristics. For this reason, surface treatment processes are essential to increase the material performance and avoid the use of costly ineffective material. There exist various surface treatment processes for various applications. Each process has advantages and disadvantages and hybridization can solve various problems. The micro-forging process delivers a controlled and uniform surface hardness, but the depth of the forged surface is limited. On the other hand, laser heat treatment can increase the hardness drastically, but the surface may become brittle, which reduces the fatigue life. Laser-assisted micro-forging is a novel hybrid process of laser heat treatment and micro-forging that has the potential to increase the forging depth and relax the stress caused by the high temperature of the forging process. This study examines the effect of laser preheating in the micro-forging of AISI 4140. The processes were varied as follows: no treatment, micro-forging only, and laser-assisted micro-forging. The fatigue strength of the specimens was examined by means of an ultrasonic fatigue tester and then compared. The microstructural changes were investigated with respect to the processes by using scanning electron microscopy. In conclusion, it was confirmed that the laser preheating auxiliary forging affects the fatigue life. It was confirmed that the fatigue life was the mostly increased in 550°C temperature laser preheating micro forging process and the temperature was identified as the most important factor.

Search for Variable Stars in Two Old Open Clusters: NGC 2506 and NGC 2420

Kim S.-L., Chum M.-Y., Park B.-G., Kim S. C., Lee S. H., Lee M. G., Ann H. B., Sung H., Jeon Y.-B., Yuk I.-S.

Acta Astronomica

2001

Vol. 51, No. 1

49--63

We present results of photometric observations of two old open clusters: NGC 2506 and NGC 2420. V-band time-series and UBVI absolute CCD photometric observations were carried out to search for variable stars and to investigate their physical properties. From the UBVI photometry of NGC 2506, we obtain interstellar reddening of E(B-V)=0.04±0.03 mag, distance modulus (V-M_V)0=12.5±0.1 mag, and age log(t/yr)=9.25. From monitoring of 590 stars with 304 CCD frames taken for ten nights, three δ Sct stars and one eclipsing binary star are discovered in NGC 2506. For the other cluster, NGC 2420, we have examined light variations of 505 stars using 347 time-series data obtained for five nights, finding no variable star. It should be noted that we have found no γ Dor star among main-sequence stars near F0 in these two old open clusters, which is consistent with the suggestion that γ Dor-type phenomenon occurs in stars younger than log(t/yr)=8.4. On the other hand, it has been known that color-magnitude diagrams of these two clusters show well-established binary sequences, implying high incidence of binary systems: ≥20% for NGC 2506 and ≈50% for NGC 2420. However, only one eclipsing binary star was found in these two clusters.