• 동력기계공학회지
• /
• 제14권2호
• /
• pp.71-77
• /
• 2010

This paper was studied on microstructure, mechanical properties and corrosion characteristics of 316L stainless steel pipe welds was fabricated by orbital welding process. S-Ar specimen was fabricated by using Ar purge gas and S-$N_2$ specimen was fabricated by using $N_2$ purge gas. Ferrite was not detected in weld metal of S-$N_2$ specimen but the order of 0.13 Ferrite number(FN) was detected in weld metal of S-Ar specimen. Oxygen and Nitrogen concentration of S-$N_2$ specimen was higher than S-Ar specimen on HAZ and inner bead. The welds microstructural characteristics of S-Ar and S-$N_2$ specimens are similar. The microvickers hardness values of S-Ar and S-$N_2$ specimens welds were similar and average values of each regions were in the range of 174~194. The microstructures of S-Ar and S-$N_2$ weld metal were full austenite by primary austenite solidification. The Solidification structures of S-Ar and S-$N_2$ weld metal were formed directional dendrite toward bead center. The potentiodynamic polarization curve of STS 316L pipe welds exhibited typical active, passive, transpassive behaviour. Corrosion current density$(I_{corr.})$ and corrosion rate values of S-Ar specimen in 0.1M HCl solution were $0.95{\mu}A/cm^2$ and $0.31{\mu}A$/year respectively. The values of S-$N_2$ specimen were $1.4{\mu}A/cm^2$ and $0.45{\mu}m$/year.

• Journal of Welding and Joining
• /
• 제32권2호
• /
• pp.29-36
• /
• 2014

Recently, in order to enhance the function and usefulness of products, cladding of dissimilar materials that maximizes the performance of the material is being widely used in all areas of industry as an important process. Clad steel plate, produced by cladding stainless steel plate, an anticorrosive material, on carbon steel plate, is being used to produce pressure vessels. Stainless steel plate has good corrosion resistance, and carbon steel plate has good rigidity and strength; clad steel can satisfy all of these qualities at once. This study aims to find the ${\delta}$-ferrite behavior, mechanical properties, structure change, integrity and reliability of clad steel weld on hot rolled steel plates. For this purpose, multi-layer welding, repair welding and post weld heat treatment were implemented according to welding procedure specifications (WPS). In order to observe the mechanical properties and toughness of clad steel weld zone, post weld heat treatment was carried out according to ASME Sec. VIII Div.1 UW-40 procedure for post weld heat treatment. With heat treatment at $625^{\circ}C$, the hold time was used as the process variable, increased by intervals that were doubled each time, from 80 to 1,280 min. The structure of weld part was typical cast structure; localized primary austenite areas appeared near central vermicular ferrite and fusion line. The heat affected zone showed rough austenite structure created by the weld heat input. Due to annealing effects of heat treatment, the mechanical properties (tensile strength, hardness, impact value) of the heat affected area tended to decrease. From the results of this study, it is possible to conclude the integrity of clad steel welds is not affected much in field welding, repair welding, multi-layer welding, post weld heat treatment, etc.

• 한국재료학회지
• /
• 제12권5호
• /
• pp.414-422
• /
• 2002

Three different multi-component white cast irons alloyed with Cr, V, Mo and W were prepared in order to study their as-cast and solidification structures. Three combinations of the alloying elements were selected so as to obtain the different types of carbides and matrix structures : 3%C-10%Cr-5%Mo-5%W(alloy No.1), 3%C-10%V-5% Mo-5%W(alloy No. 2) and 3%C-17%Cr-3% V(alloy No.3). The as-cast microstructures were investigated with optical and scanning electron microscopes. There existed two different types of carbides, $M_7C_3$ carbide with rod-like morphology and $M_6C$ carbide with fishbone-like one, and matrix in the alloy No. 1. The alloy No. 2 consisted of MC carbide with chunky and flaky type and needle-like $M_2C$ carbide, and matrix. The chunky type referred to primary MC carbide and the flaky one to eutectic MC carbide. The morphology of the alloy No. 3 represented a typical hypo-eutectic high chromium white cast iron composed of rod-like $M_7C_3$ carbide which is very sensitive to heat flow direction and matrix. To clarify the solidification sequence, each iron(50g) was remelted at 1723K in an alumina crucible using a silicon carbide resistance furnace under argon atmosphere. The molten iron was cooled at the rate of 10K/min and quenched into water at several temperatures during thermal analysis. The solidification structures of the specimen were found to consist of austenite dendrite(${\gamma}$), $ ({\gamma}+ M_7C_3)$ eutectic and $({\gamma}+ M_6C)$ eutectic in the alloy No. 1, proeutectic MC, austenite dendrite(${\gamma}$), (${\gamma}$+MC) eutectic and $({\gamma}+ M_2C)$ eutectic in the alloy No. 2, and proeutectic $M_7C_3$ and $ ({\gamma}+ M_7C_3)$ eutectic in the alloy No 3. respectively.

• 열처리공학회지
• /
• 제4권1호
• /
• pp.1-12
• /
• 1991

This study has been performed to find out the effect of austenitizing temperature, austempering temperature and its holding time, and tempering cycle on the mechanical properties such as impact resistance, hardness etc. of AISI $M_2$ Mo series high speed tool steel austempered or tempered after austempering treatment. The results obtained from the experiment are as follows ; (1) Optical micrograph has revealed that the transformation rate of bainite is delayed as the austenitizing temperature increases and that bainite is most apparently transformed at an austempering temperature of $290^{\circ}C$. (2) The amount of retained austenite during austempering has been analysed to be increased by the X-ray diffraction technique as the transformation product of bainite is increased. It has also been shown that the longer the holding time of austempering, the more the transformation quantity of bainite is formed, exhibiting, however, that the rate of bainitic transformation is considerably retarded after a certain period of holding time elapses. (3) Hardness measurement has shown that hardness values obtained after austempering increase with decreasing the amount of retained austenite. (4) The austempering and then tempering cycle has been formed to give hardness values which are more greatly improved as austenitizing temperature is increased. (5) The mechanical property of the specimen primary-tempered for 1 hour at $550^{\circ}C$ after austempering for 2 hours at $290^{\circ}C$ from the austenitizing temperature range of $1180^{\circ}C$ to $1210^{\circ}C$ have been estimated to be good values.

• 열처리공학회지
• /
• 제8권4호
• /
• pp.279-288
• /
• 1995

To investigate the characteristics of nitrocarburizing for high carbon chromium bearing steel, it was undertaken 4 hours holding at $850^{\circ}C$ in the atmosphere containing 60% endothermic gas and 40% ammonia. The microstructure of nitrocarburized surface consists of ${\varepsilon}-Fe_{2-3}N$, ${\gamma}^{\prime}-Fe_4N$, $Fe_3C$ and $Fe_3$(C,N), and the ${\varepsilon}$-nitride was rich in the surface-internal part. The nitrocarburized surface contains a larger volume fraction of primary carbonitrides and has more retained austenite and is slightly harder than the interior.

• 한국주조공학회지
• /
• 제5권3호
• /
• pp.13-18
• /
• 1985

Decarburization phenomena were investigated at $800^{\circ}C$ by the $PH_2O/PH_2$ + Ar gas mixture in the case iron range which contains Si, Mn and Cr as an alloying elements. Dissociation of cementite in a matrix which contains graphitizer as Si begins at the carbon rich cementite dendrite arms. Several primary austenite $({\gamma})$ skeletons are surrounded by those nucleated graphite nodules, and that forms a limited area of nucleation region. Decarburization reactions at $800^{\circ}C$ in Fe-C, Fe-Mn-C and Fe-Cr-C alloy are followed by parabolic rate law under the gas mixture of $PH_2O/PH_2=0.01$ and the modified rate const. ${\kappa}$ were in the range of $1{\sim}6{\times}10^{-10}cm^2/s$.

• 한국분말재료학회지
• /
• 제4권3호
• /
• pp.222-229
• /
• 1997

A P/M high speed steel of ASP 30 grade was austenitized, gas quenched and tempered at various conditional. The mechanical properties such as hardness, bend strength and fracture toughness were evaluated after heat treatment. The microstructure and the type and volume fraction of carbides were analyzed by an optical microscope, image analyzer and XRD. The primary carbides after the heat treatment were MC and $M_6C$ type. The volume of the total carbide varied from 10 to 15% depending on the austenitizing and tempering temperature. The tempering temperature for maximum hardness was at around 52$0^{\circ}C$. But the maximum bend strength was obtained at about 55$0^{\circ}C$. The fracture toughness was largely affected by the presence of retained austenite after gas quenching and secondary hardening during tempering.

• 한국전기전자재료학회:학술대회논문집
• /
• 한국전기전자재료학회 2000년도 하계학술대회 논문집
• /
• pp.950-952
• /
• 2000

Considering the effective distribution coefficient of Ni in Fe-Ni-Co invar alloy containing a little amount of carbon, we investigated on the thermal expansion coefficient(${\alpha}$). Fe-Ni-Co invar alloy had a large thermal expansion coefficient in as-casted compared with solution treated. The thermal expansion coefficient of Fe-Ni-Co alloy increased with the carbon content in both state of as-casted and solution treated. The effective distribution coefficient(Ke$\^$Ni/) of Ni was smaller than unity in alloy of not containing carbon, but is way larger than unity in alloy of containing carbon. It was considered that the homogeneity of Ni in primary austenite affected thermal expansion coefficient.

• 열처리공학회지
• /
• 제28권4호
• /
• pp.181-189
• /
• 2015

Two types of advanced cold-work tool steels were characterized and compared. A higher-alloyed ledeburitic steel with primary carbides (denoted as 9Cr) and a lower-alloyed steel without primary carbides (5Cr) were fabricated by vacuum induction melting and subsequent hot forging. They were spheroidizing-annealed at $870^{\circ}C$, quenched at $1030^{\circ}C$ and tempered at 180 or $520^{\circ}C$. Their machinability after annealing and hardness, impact toughness, wear resistance after tempering were compared and interpreted in association with their characteristic microstructures. After annealing, 5Cr showed higher resistance to machining due to higher ductility and toughness in spite of lower strength and smaller carbide volume. Owing to smaller carbide volume fraction and the absence of coarse primary carbides, 5Cr showed even better impact toughness although the hardness was lower. The improved toughness of 5Cr resulted in excellent wear resistance, while smaller volume fraction of retained austenite also contributed to it.

• Journal of Welding and Joining
• /
• 제34권5호
• /
• pp.77-82
• /
• 2016

In this study, Microstructure and hardness of 1st layer with crystallographic orientation were investigated about solidification structure in multipass weld using high Mn-Ni flux cored wire. Microstructure of solidification consisted of austenite matrix and a little ${\varepsilon}-phase$ in grain boundaries. Orientation of grains was usually (001), (101), (111). According to crystallographic orientation, morphology of primary dendrite was different. The depletion of Fe and the segregation of Mn, C, Ni, Si, Cu, Cr, O were found along the grain boundaries. The area of segregation was wide with an order of (001), (101), (111) grains. And hardness of grains with crystallographic orientation increased with an order of (001), (101), (111) grains because of the segregation along dendrite boundary.