African Fusion June 2015

electrodes distortion and a lowering of the risk of burn-through. “These tubular elec- trodes can be welded at low operating currents, which allows less expensive power sources to be used and, most importantly, it allows thin sections such as the flights of screw conveyors to be hardfaced successfullywithout destroy- ing the basic geometry,” he suggests, adding that because of the spiral shape of conveyor screws, these products are not so easy to hardface using more au- tomated processes. While WA’s 6.0 mm tubular elec- trodes have a steel stub, the larger diameters use analuminiumstub, which enables the stubdiameter tobe retained at 6.0 mm, regardless of the electrode size. This makes the entire range com-

WA tubular hardfacing electrodes (TEs), while designed to be used in the same way and with the same power sources as coated shielded metal arc electrodes, are constructed like a cored wire electrode. The alloying constituents, which are mostly metal powders rather than flux, are contained inside the hollow electrode by an outer metal sheath.

Offering a 50% higher deposition rate than coated SMAWhardfacing elec- trodes, Zylstra points out that a further niche advantage is the positional capa- bility of tubular electrodes. “Because the electrodes canbeweldedat currents as low as 80 A (6.0 mm), our tubular consumables can be used to hardface in the vertical-up position (3G/3F),” he assures. “This makes the range very versatile, with down hand welding be- ing possible at 120-130 A for the 6.0 mm electrode and as low as 150‑160 amps and 200‑220 A for the 8.0 and 12 mm electrodes, respectively. The product range At the starting point of Welding Alloy’s tubular electrode (TE) hardfacing range are the primary chromium carbide coatings (HC-TE and HC40-TE). These electrodes produce a microstructure of chromiumcarbides in an austeniticma- trix. “Thesearehighchromiumcast irons andwill produce stress relief cracks. This is normal,” Zylstra suggests. “If no cracks are produced, then something is wrong with the hardfacing composition,” he points out. Typical uses include palmoil extrud- ers, screw conveyors, gyratory crusher cones and mantles, dredge pump bod- ies, mining and earth moving equip- ment, agricultural equipment, wear plates, sugar mill crusher hammers, drag line components, quarry and sand handling equipment, and brick manu- facturing equipment. One step up in terms of metallurgi- cal sophistication are the CN-TE and the CNV-TE electrodes that are used to deposit complex chromium carbide layers that offer better wear resistance at higher temperatures. The CNV-TE al- loy, for example, maintains its abrasion resistance at up to 600°C and is suitable

for use in boilers, sinter plant or for blast furnace components. A third alloy group, STEELCARBW25- TE and STEELCARBW45-TE produce lay- ers of tungsten carbide (WC) andFe-Cr-W carbides in an austenitic matrix. “Many will have seen tungsten carbideparticles being dropped into a melted pool to produce layers such as these,” Zylstra recalls. “But the powders used in these tubular electrodes are so fine that they can’t be seen.” Both these grades have very high abrasive resistance, with the W25 grade having slightly better impact resistance than the harder W45 grade. Typical ap- plication include materials handling components in contact with medium to fine ores, coal crusher blades, conveyor screens, concrete mixer components, woodchippingor crushingmachinecom- ponents, and brick and clay mill augers. Concluding, Zylstra says that tubular hardfacing electrodes are ideal for use on irregular shapes where out of posi- tion welding is sometimes required, for low volume applications or for applying surface coatings on many small parts. “They are typically suited to repairs of worn coatings and for use whenever the advantages of continuous welding with cored wires cannot be achieved,” he suggests.

patible with typi- cal MMA electrode holders. The electrodes are

highlyalloyedanddense- ly packed, with a thin steel sheath, which requires low amperage tomelt andsustain an arc. “A key differentiating feature for us is the green outer coating, which is designed to offer ex- ceptional moisture resistance,” Zylstra believes. “This means that baking is no longer required as the core ingredients are not susceptible to moisture pickup at all. This saves energy, processing time and, therefore, money. It also makes the electrodes ideal for use in humid, outdoor environments,” he adds. Describing the feel while welding, Zylstra says tubular electrodes are “feather-light”, which takes somegetting used to, but the “weldability is excel- lent”. Spatter is almost non-existent and, because the core is mostly made up of metal powders rather than flux, slag lev- els in the deposit are low, requiring no slag removal or grindingbetween layers.

Performance

Tubular Excellent Extra Low

Coated

Weldability Heat Input

Average/Poor

High

Spatter

Insignificant High

Slag

Negligible

High

Slag Removal

Not required Essential

Deposition Efficiency High

Average/Low

Dilution Baking

Low

High

Not required Necessary

A comparison of the welding performance of tubular and coated hardfacing electrodes.

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June 2015

AFRICAN FUSION