African Fusion June 2016

Out of position Stellite cladding

Cobalt-based cladding: a local success story

In this article, Wiehan Zylstra (right), technical manager of Welding Alloys South Africa (WASA), presents a case study on the use of Welding Alloys’ cobalt-based cladding material, STELLOY 6-G, whichwas welded out of position using Fronius’ TPS synergic pulsed-GMAW equipment.

“ I am going to share a break-through applica- tion with you today, which involved collaboration between the petrochemical client, Sasol; piping contractor, Petrochemical Piping Services; equipment OEM, Fronius and its local distributor BED; and Welding Al- loys South Africa, WASA,” Zylstra begins. As with all application related- successes, the story startswith the client need. “Sasol have a requirement for a cobalt-based alloy for high-temperature erosion resistance – and erosion in- volves the combinationof corrosion and abrasion, so both wear and corrosion resistance were needed,” he explains, adding that a cobalt-based rather than nickel-based alloy had to be used because of the need for hot corrosion and sulphidation resistance from the clad layer. Cladding had to be applied in- situ and out of position, while time constraints drove the client towards exploringhigher deposition ratewelding processes. “Theweldingwas previously applied using SMAW or stick electrode welding with some manual TIG weld- ing using consumable rods. Both these processes have productivity issues, so an additional requirement was speci- fied for a procedure to be developed that reducedwelding time,” Zylstra says. Summarising the need, he says that suitable cobalt consumables with good out-of-position weldability were required, along with a process and as-

sociated equipment to enable higher productivity. A history of cobalt-based alloys After playing the Beach Boys song, Ko- komo, Zylstra says that Elwood Haynes developed the basic metallurgy of cobalt-chromium (Co-Cr) alloys in the early 1900s – in a town near Kokomo, Indiana, USA. “And now that you have heard that song, youwon’t forget where Haynes came from.” Haynes contributed significantly to our knowledge of martensitic stainless steels and, in 1905, he designed amotor- car and founded theHaynes Automobile Company. Turning attention back to the cobalt alloys Haynes invented, Zylstra says that he derived the name for his alloy from the Latin word ‘stella’, because of its ‘star-like lustre’. Haynes formed the Haynes Stellite Company and reg- istered the trade name ‘Stellite’. His company becameUnionCarbide in 1920 and Deloro Stellite in 1922. “Haynes’ cobalt-chromiumalloys for weldoverlay cladding have been known for over 100 years and many different brand names have emerged,” Zylstra informs us, add- ing that the ‘Stellite’ trade name is still owned by Deloro Stellite today. Moving onto the history of the Welding Alloys Group, he says that the company was started in 1966 in Fowl- mere, a town near Cambridge in the UK. Originally, WAwas a hardfacing business founded by Jan Stekly to solve abrasive wear problems. Today, the company has

10 factories producing flux-cored wires, one of which is here in Roodepoort, where hardfacing flux-cored wires are produced – “98% of which are chrome- carbide (CrC) consumables”. As well as wire production, the busi- ness still has production units all over theworld. “In South Africa, for example, 52 t/a of our ownwires are used tomake CrCoverlay plate for a range ofmaterials handling applications for mining equip- ment such as crushers,” Zylstra reveals. Welding Alloys also produces a com- prehensive range of stainless steel and nickel-based alloys and its cobalt-based consumable range is marketed under the STELLOY trade name. Metallurgy of cobalt-based alloys The Stellite alloys are, essentially, cobalt chromium alloys containing 50 to 60% cobalt, hence the term ‘cobalt-based’. Cobalt has a face-centred cubic (FCC) crystal structure, which makes it duc- tile like austenitic stainless steel. “But this structure is somewhat unstable. If exposed to mechanical stress or high temperature, the FCC crystal structure transforms to a hexagonal close packed (HCP) structure, which has less ductility, higher yield stress, a high work harden- ing rate, good fatigue properties and higher toughness,” Zylstra explains. The chromium in the alloy gives it its corrosion resistance. As with stainless steel, the chromium forms a passivation

Micrographs of the STELLOY-clad layer for three different welding positions: left, from the overhead position; centre, from the 3:00 vertical up position; and right, from the downhand position.

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June 2016

AFRICAN FUSION