African Fusion August 2017

Hardfacing: the ten steps

rent suggests that the Seferian Diagram be used, which relies on the carbon equivalent (C eq ) 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

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

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-

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

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.

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August 2017

AFRICAN FUSION