Hardfacing Overlay Welding by RP Plasma Arc

Study on Hardfacing Process Using Extremely Hard Material (2nd Report)

Abstract

In the previous report, the test results of the newly developed hardfacing process with transition metal carbide powder by the conventional DCSP plasma torch were described. In this study, bead-on-plate and multilayer overlay welding tests by the trial manufactured DCRP plasma torch have been carried out of improve weldability of this process.
The results obtained are summarized as follows:
1) It is necessary to add a small amount of oxidizing gas (CO₂ or O₂) into the carrier gas in order, to stabilize the arc. By the addition of oxidizing gas, the weldability is improved due to increase of the wettability of overlay weld metal, though the penetration of overlay weld is increased.
2) The deposition efficiency is also improved, because the powder feeding rate can be increased exceedingly even at the lower heat input level than the case of DCSP welding.
3) In the case of high carbide content, the cathode flame is apt to occur due to fixing cathode spot of oxidizing gas is not added. Once the cathode flame occur, it will become impossible to'carry out favorable overlay welding, because the shielding is disturbed and the cleaning action is gone out.
4) The occurrence of the cathode flame can be prevented by an addition of 0.20.4% CO₂ or O₂ in the one-layer overlay welding, but this preventing effect. is not. observed at the second and upper layers in the multilayer overlay welding. Therefore, in the multilayer overlay welding of carbide content above 80%, it is hardly to obtain the sound overlay weld without any welding defects such as blowhole and lack of fusion.