The semitransferred arc option means that dilution of the hardfacing alloy into the base metal can be reduced to an absolute minimum even on very thick substrates with single passes at high deposition rate.
In the PTA process, the surfacing alloy is deposited and welded to the base metal in only one step. When used in the semitransferred mode, both the pilot and
transferred arc can be varied independently and precisely so that only just enough deposition and welding currents are used to preheat the surfacing alloy and weld it to the base metal.
Because the powder being preheated (see Fig. 2) is conveyed by a stream of inert gas and deposition is carried out within a protective gas shroud, it is not essential to build in self-fluxing
properties to the surfacing alloy. Thus Co-Cr-W-C alloys, used for instance on valve seats, can be easily applied to areas vulnerable to wear and the upper service
temperature levels are increased with alloys produced most readily as powders.  PLASMA TEAM PTA unit runs in either semitransferred or transferred arc mode or a combination of both. As the size of workpiece increases, the power
input of the transferred arc is increased.
Fig. 2 - Powder Grain before entering the fusion bath (A) -
inside the fusion bath (B) The semitransferred arc mode reduces the heat transferred to the base metal
and its importance increases as component size reduces, as in engine valves.This technique has proved of advantage also in surfacing hardnable materials,
as the lower base metal temperature eliminates the Austenitic/Martensitic cycle which otherwise needs preheat and postheat treatments, adding costs and complexity to the process.
Besides the low and controlled dilution, deposits are very uniform and smooth, so finish machining is reduced to a minimum.
The semitransferred technique permits by its versatility of utilizations to reduce and control the dilution level (results obtained on exhaust valves, 2% of Iron). |
