PTA 공법에 의한 Nimonic 80A 초합금재 Overlay층의 열처리에 따른 마찰 마모 특성에 관한 연구

Abstract

Inlet and exhaust valve of the main vessel engine were made from Nimonic 80A, a kind of Ni super heat resist super alloy that works under rigorous condition. When the valve which exceeds limit of use due to abrasion and corrosion while working is repeated, Plasma Transferred Arc Process (PTA Process) is used.

Use of PTA process was begun to improve surface property of base metal by attaching materials having wear resistance, corrosion resistance, or heat resistance etc. Welding surface by PTA is better and superior condition than other old welding methods.

It is known that overlay welding by Nimonic 80A is difficult in this study due to its low melting point, and liquation crack might occur inside and cause crack growth due to low melting point after PTA process.

High temperature crack is mainly caused by segregation of trace element at low melting point and local melting of process extract at low melting point. Moreover, it is occurred when carbide is formed with flat or rod type at Heat Affected Zone (HAZ). Caution is needed to prevent high temperature crack by no subsiding atoms having low melting

In this study, Nickel (Ni) alloys such as Inconel 625 and Inconel 718 the same as Nimonic 80A base and Cobalt (Co) alloy Stellite 6 were overlayed using PTA process under all heat treated condition of filler metal, and then anti-wear property, hardness, and systemetic properties were analyzed.

Stellite 6 showed lower abrasion rate compared to Inconel625 and Inconel718 at the anti-wear test of Pin-on-Disk style, and also Inconel625 and Inconel 718 showed low abrasion rate because its anti-wear property is related to heat treated condition.

Hardness at overlay layer was also increased by heat treated condition in comparison with non-heat treatment, but hardness at HAZ was decreased.

Liquation crack, a kind of high temperature crack formed by welding extract of Nimonic 80A, was found a little in the base HAZ because of low melting metal. Thus it is considered that preheat treatment is necessary for diffusion of metal having low melting point and elimination of stress that might remain inside of base metal before welding.