African Fusion August 2017

AFRICAN FUSION

August 2017

Hardfacing: the ten steps

Hardfacing consumables organised in terms of their suitability for use against abrasion and impact

wear combinations.

To determine preheat required, Laurent suggests

that the Seferian Diagram be used. For a 50 mm

thick 42CD4/42CrMo4 plate with a carbon equivalent

of 0.63, the preheating temperature will be above

220 °C, for example.

and Supramangan consumables tend to work

best,” Laurent says.

Temperature and corrosion resistance are

taken into account as secondary criteria, and

Oerlikon has developed a similar temperature

versus corrosion grid to assist operators to

choose appropriate consumables.

In terms of the welding process, Laurent

says that each has its advantages and disad-

vantages. SMAW (shielded metal arc welding) is

easy to implement both indoors and onsite and

a comprehensive range of consumables is avail-

able, covering every segment. “Some of our best

sellers include Abracito 62S, Supradur R 600 and

Supramangan,” he notes. The only downside for

SMAW is that the productivity is lower than other

more automated processes.

Solid GMAW wires offer higher deposition

rates of – up to 6.0 kg/h – and are well suited to

automatic or semi-automatic applications. But

they require shielding gas and this can

prevent themfrombeing usedoutdoors.

A large range of impact resistance wires

exist, most notably, Carbofil A350, Car-

bofil A600 and Inertfil 430.

For the flux-cored process, which

offers deposition rates of up to 8.0 kg/h,

bothopenarc (gasless) andgas-shielded

wires are available in the Fluxodur and

Fluxofil ranges respectively.

For higher deposition welding, al-

beit limited to the flat (1G) position and

indoors, submerged arc welding (SAW)

offers high deposition rates of up to

12 kg/husing consumable combinations

such as OP 122 flux with Fluxocord 52

wire.

Strip Cladding, which offers dilution

rates as low as 25% at deposition rates

as high as 20 kg/h is also an option for

thosewhose cladding requirements can

justify the equipment expense.

Step 4: Prepare the surface

Critical to a coatings success is the

cleanliness of the surface prior to weld-

ing. All traces of dirt, grease, oil and

paint needs to be removed. In addition,

previous hardfacing layers usually need

to be removed, especially if the deposit

composition is unknown.

“Before rebuilding manganese

steels, about 2.0 mm of the work-

hardened surface is usually removed.

Failure todo somight result inweldbead

spalling,” Laurent notes.

Step 5: Preheat

Preheating and interpass temperature

control needs to be done while hardfac-

ing to avoid cold cracking; allow diffus-

ible hydrogen to escape from the weld

metal; and to reduce shrinkage stress.

To determine preheat required, Lau-

rent suggests that the Seferian Diagram

be used, which relies on the carbon

equivalent (C

) formula of the base

metal and the thickness of the material

to determine the preheat required.

Bywayofanexample,Laurentshows

that, for a 50 mm thick 42CD4/42CrMo4

plate with a carbon equivalent of 0.63,

the preheating temperature will be

above 220 °C.

“In the case of manganese (Mn)

steels, however, preheating is forbidden

and, instead, the interpass temperature

must be restricted to less than 150 °C

during the hard-surfacing operation.

Step 6: Rebuild

Forworncomponents, it is always neces-

sary to restore the original shape of the

base meal surface before hardfacing.

To do so, a deposit as close as possible

to the original base metal composition

should be sought.

Step 7: Establish a buffer layer

Once restored to its original shape, a

buffer layer is often applied. Its main

purpose is to prevent cracks fromtravel-

ling fromthe hardfaced surface deposits

into the base metal. This step is always

necessary between surface layers con-

taining nitrides or carbides.

“The buffer layer also ensures good

bonding with base material, prevents

the surface layer fromsinkingunder high

load conditions and helps overcome

dilution issues,“ says Lauren, adding

that it is important to avoid having a

ductile deposit on top of hard metal.

“The harder material should always be

on top,” he advises.

Austenitic-type consumables, gen-

erally a 307L or 312L are commonly

applied for the buffer layer.

Step 8: Hard surface

“The key issue with respect to the weld

deposit of the hard-surfaced layer is low

dilution,” says Laurent. It is important to

minimise the percentage of hardening

constituents lost to the buffer layer or

to the base metal. This is to ensure that

the top surface of the hard layer is to the

exact composition required to achieve

long wear life.

Target dilution should be as low

as possible and is controlled using the

welding parameters, such as welding

speed, current and polarity settings.

Parameters should be set to achieve a

minimum plate penetration, which is

most often associatedwith highwelding