Plasma Transferred Arc (PTA) Hardfacing

The application of hardfacing consumables in the powder form has been known for a long time. The PTA principle was developed and used for the hardfacing of components in the early 60's. Since that time and especially the last two decades PTA hardfacing is successfully used world wide to protect working faces of high responsibility components with special materials, which are resistant to intensive wear, corrosion, thermal and percussive loading.

 

Principle of PTA-process

PTA is an arc welding process and is considered as further development of the tungsten inert gas (TIG) process. With PTA the pilot arc is ignited between non-consumable tungsten electrode and, so called, plasma forming nozzle with the help of high voltage generator (figure 1). The pilot arc is blown out of this nozzle by a gas stream (plasma gas) and initiates in its turn the transferred arc between the tungsten electrode and treated component. The transferred arc is constricted by the plasma forming nozzle, getting higher temperature up to 20000 C, concentration and stability. Similar to the plasma spraying the consumable (additive material) is used in a form of powder. The powder is fed into the arc column by so called transport gas. Shielding gas is used to protect the welding zone from the influence of the atmosphere.

 

Figure 1 - PTA Diagram

 

Figure 1: PTA Process Principle
The transferred arc melts the powder and surface of the component. It is possible to regulate process conditions so, that the whole amount of powder and only thin film of component surface under the arc will melt. As a result, a common weld pool appears on a treated surface. Because of relative motion of the plasma torch and the component new portions of the powder and substrate material are involved into the process of melting in front of the weld pool and crystallise in the rear part of the weld pool. During solidification a metallurgical bond between deposit and substrate is provided with the minimum dilution of the detail material (3-10%). In spite of some similarity to the plasma spraying one can see decisive difference: by PTA the deposit has a cast homogeneous structure and is completely fused (metallurgically bonded) to a substrate has opposed to mechanical bonding with plasma spray.

Working gas Ar as well as He in special cases with additions of H2, O2 or CO2 are used for an arc plasma supply, a powder transport and a shielding of a high temperature zone. Because of high operating temperatures a plasma torch has to be water cooled.

Layers of 0.5 to 6 mm thickness and 3 to 50 mm width can be obtained in a single pass. To clad wide layers a cross oscillation of a plasma torch has to be performed. PTA process productivity of 2 - 8 kg/h depends on the application case and plasma torch type.

Advantages of PTA:

  • low dilution of deposit by substrate material
  • high deposition rate
  • less dependency on operator's skill
  • local heat input
  • relatively small component deformation
  • reduced costs for machining
  • low losses of powder
  • smooth deposit
  • high reproducibility
  • easy to mechanize and to automate

 

Deloro Stellite PTA Machines

 

Manipulator Systems
Starweld® PTA systems can be supplied with a variety of torch manipulators and part positioning devices for semi automatic welding. A wide range of devices can be provided to suit for almost any size, weight or configuration of work piece. These can be equipped torches with a range of sizes suitable for either inside or outside diameter work.

Beam and Carriage Systems
For parts with long lengths requiring support at both ends beams and moving weld carriages are most appropriate. These can be supplied with lathe type positioners or combined with tilting positioners for increased versatility. Almost any length and weight capacity can be accommodated.

Valvestar Systems
Valvestar systems are specifically designed for high volume production of gas or diesel engine valves. They are integrated with automated load and unload systems and can be installed directly in automated production lines.

Robotic Systems
For the ultimate in flexibility and automation Starweld® PTA units can be integrated into robotic cells allowing complete control of the part positioning and weld process directly through the robot controller.