African Fusion March 2015

AFRICAN FUSION

March 2015

INDEX TO

ADVERTISERS

Afrox ......................................................................8

Air Liquide...........................................................20

Air Products ........................................................23

BED Holdings: Fronius........................................10

Crown Publications.......................................... IBC

ESAB....................................................................22

Lincoln Electric.......................................OFC, OBC

Probraze Metals..................................................15

Renttech .............................................................14

SAIW ......................................................................2

SKS Welding Systems.........................................13

Smarter Welding Africa .....................................IFC

voestalpine Böhler Welding SA..........................34

Yaskawa Motoman SA ........................................30

Today’s technology

Refurbishment using the PTA process

Journals and pot roll scraper blades for the zinc

galvanising process, for example, are coated with a

cobalt-based layer for high temperature wear and

corrosion resistance.

he plasma transferred arc (PTA)

welding and hardfacing process

from thermal spray and surface en-

gineering coating specialist, Ther-

maspray, provides a versatilemethod of

depositing high quality, metallurgically

fused deposits on relatively low-cost

substrates.

First introduced in 1964 to better

control lower current arc welding, the

PTA process is complimentary to both

thermal spray and conventional weld-

ing and is mainly used on components

that are subjected to severe corrosion or

abrasion, thermal shock, slurry erosion

or extreme impact forces. The process

offers the necessary protection to the

substratebyproviding a coating that can

withstand these conditions. PTA can be

applied in practically every case where

hardfacing is needed.

According to Shaik Hoosain, metal-

lurgical engineer at Thermaspray, a

wide variety of materials, ranging from

soft to medium hardness (e.g., stain-

less steel) to very hard, wear resistant

materials (e.g., carbide composites), can

be deposited using the PTA process to

achieve diverse surface properties such

asmechanical strength, wear, corrosion

and creep resistance. Bead thicknesses

range from1.2 to 2.5 mmandmulti-pass

runs depend on the application.

Hoosain explains that the plasma

arc welding process uses a plasma to

transfer an electric arc to theworkpiece,

a process which he says is similar to gas

tungsten arcwelding (GTAW). “To create

plasma, a gas, typically argon, is heated

to a sufficiently high temperature and

ionised so that it becomes electrically

conductive. The plasma welding torch

contains a negatively charged non-

consumable tungsten electrode within

a positively charged copper nozzle with

a small orifice at the tip,” continues

Hoosain.

“A pilot arc is struck between the

torch electrode and the nozzle tip and

the arc is transferred to the metal to

be welded. By constricting the plasma

gas and the arc through the orifice, the

torch delivers a high concentration of

heat to a small area, which leads to high

quality welds.” For hardfacing, an alloy

powder froma hopper is introduced, via

a carrier gas, into the plasma stream.

This forms a fused layer on the surface

being coated, with the required alloy

properties.

The plasma transferred arc process

offers a number of important advan-

tages over conventional arc welding,

including:

•

It is easily automated and offers a

high degree of reproducibility.

•

Precisemetering ofmetallic powder

feedstock allows lessmaterial to be

used compared to traditional weld-

ing processes.

•

Precise control of the welding pa-

rameters also offers a high degree

of consistency.

•

Controlled heat input, less than

conventional arcweldingprocesses,

ensures weld dilution can be con-

trolled to between 5.0 and 7.0 %.

•

The deposit of a specific alloy

is tougher and more corrosion

resistant than those producedusing

GTAW or oxy-fuel processes.

•

PTA weld deposits have character-

istically less oxides, inclusions and

discontinuities.

•

Post weld machining is minimised

due to the smooth deposit created

through the PTA process.

Typical PTAapplications successfully un-

dertakenby Thermaspray include the re-

furbishment of the process rolls used in

the steel industry. Journals and pot roll

scraper blades for the zinc galvanising

process, for example, are coated with a

cobalt-based layer for high temperature

wear and corrosion resistance. “Scraper

rings formanufacturing gearbox compo-

nents are also PTA-coated with cobalt-

base alloys, as are valve seats used for

high temperature steam valve compo-

nents. And components used in glass

manufacturing are PTA-coated with a

nickel-basedalloy toprovide impact and

wear resistance at high temperatures,”

adds Hoosain.

In closing, Hoosain says that be-

cause the PTA welding process has

not received as much attention in

South Africa as conventional arc weld-

ing processes such as GTAW, it is an

under-utilised welding process in the

local market. “PTA is a versatile, high

temperature, wear and impact resistant

welding process that is ideally suited to

a wide range of applications, ensuring

highqualitywelddepositswith excellent

reproducibility frombatch to batch. It is

ideal for severe impact, wear and cor-

rosive environments.”

In addition to PTA welding, Ther-

maspray also provides a comprehensive

range of support coating finishing tech-

nologies including machining, grinding

and burnishing from its state-of-the

art facilities in Olifantsfontein, Johan-

nesburg.

www.thermaspray.co.za