In this article, Wiehan Zylstra, 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, which was welded out of position using Fronius’ TPS synergic pulsed-GMAW equipment.
“I am going to share a break-through application 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 Alloys South Africa, WASA,” Zylstra begins.
As with all application related-successes, the story starts with the client need. “Sasol have a requirement for a cobalt-based alloy for high-temperature erosion resistance – and erosion involves the combination of corrosion and abrasion, so both wear and corrosion resistance were needed,” Zylstra 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 exploring higher deposition rate welding processes. “The welding was previously applied using SMAW or stick electrode welding with some manual TIG welding using consumable rods. Both these processes have productivity issues, so an additional requirement was specified for a procedure to be developed that reduced welding time,” he says.
Summarising the need, he says that suitable cobalt consumables with good out-of-position weldability were required, along with a process and associated equipment to enable higher productivity.
A short history of cobalt-based alloys
After playing the Beach Boys song, Kokomo, 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, you won’t forget where Haynes came from.”
Haynes contributed significantly to our knowledge of martensitic stainless steels and, in 1905, he designed a motorcar and founded the Haynes 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 registered the trade name ‘Stellite’. His company became Union Carbide in 1920 and Deloro Stellite in 1922. “Haynes’ cobalt-chromium alloys for weld overlay cladding have been known for over 100 years and many different brand names have emerged,” Zylstra informs us, adding 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 Fowlmere, a town near Cambridge in the UK. Originally, WA was 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 business still has production units all over the world. “In South Africa, for example, 52 t/a of our own wires are used to make CrC overlay plate for a range of materials handling applications for mining equipment such as crushers,” Zylstra reveals.
Welding Alloys also produces a comprehensive range of stainless steel and nickel-based alloys and its cobalt-based consumable range is marketed under the STELLOY trade name.
The 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 ductile 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 hardening 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 layer on the alloy’s surface, which forms a barrier to further corrosion. Chromium is also a solid solution strengthener. “One of the fundamentals of metallurgy is that, because of the different atoms in the crystal structure of a solid solution, structures with higher internal stresses are formed, which are more rigid and, macroscopically, stronger,” he explains.
Common grades of the Stellite alloy include: Stellite 1 with 32% Cr, 2.5% carbon (C) and 13% tungsten (W); Stellite 6, which has 27% Cr, 1.0% C and 5.0% W; Stellite 12 with 30% Cr, 1.8% C and 9.0% W; and the very low carbon Stellite 21 with 27% Cr, 0.2% C and 5% Mo, which is often used as a buttering layer. All of these grades (and most others) also contain nickel, iron, silicon and manganese.
“Tungsten and molybdenum are solid solutions strengtheners, while tungsten is also a carbide former. It is the CrC, however, that are the fundamental hardening constituents. They impart the hardness and wear resistance to the Stellite range of alloys,” Zylstra points out.
Showing typical microstructures of deposits, for Stellite 1; Stellite 6 and Stellite 12, Zylstra says: “Note the volume fraction and dispersion of carbides. What you are looking at is the black CrC dispersed around the grain boundaries of the solid solution phase. Clearly, the volume fraction of carbides is highest in the Stellite 1 alloy, which has 2,5% carbon, followed by Stellite 12 with 1.8% and Stellite 6, which only has 1.0% C. This explains why the Stellite 6 alloy is the only machineable cobalt-based alloy in the range,” he says.
In a second series of micrographs, the dispersion of carbides is shown for three different welding processes: GTAW; Oxy-acetylene; and SMAW. “From a dispersion perspective, the TIG process is the best of these three, because the carbides are finer and more evenly distributed around smaller grains.
“This indicates that the welding process choice directly influences the end quality of the clad layer. While the size and distribution of carbides depends on the alloy chemistry of the grade, the cooling rate and heat input associated with welding process also have important roles to play.
The welding solution
Welding Alloys manufactures metal-cored cobalt-based alloy wires for both the GMAW and automated GTAW welding processes. “In term of Stellite 6, we have three or four different variants, with STELLOY 6-G being the wire best-suited to the requirements of Sasol’s applications,” notes Zylstra.
The GMAW process was chosen as it can be welded with heat inputs as low or lower than the GTAW process, but it also offers much better productivity. “WA manufactures both 1.2 mm and 1.6 mm metal cored wires in this grade, which offer excellent weldability and a weld bead that looks almost exactly the same as that from the solid wire GMAW process,” he notes.
Being metal-cored, however, the wires are not as ideal for all-positional welding as a flux-cored wire would be, because flux, particularly fast freezing flux, helps to make out of position welding much easier. Metal-cored wires have less than 4.0 % mineral contents – calcite, rutile, feldspar and fluorspar, for example – which are fluxing agents and slag formers.
Hence the partnership with Fronius and BED.
Fronius’ Transpuls Synergic technology is microprocessor controlled and digitally regulated pulsed, synergic GMAW system. The system is pre-programming with welding parameters for various alloy types, including Stellite alloys, based on years of application experience. “Synergic MIG/MAG welding is a variant of pulsed MIG/MAG, where unit current pulses detach and transfer a single droplet of weld metal of around the same diameter as that of the wire,” Zylstra explains. The deposition rate/wire feed speed can then be varied proportionally, both upwards and downwards, by altering the pulsing frequency.
“Fronius has more than 156 000 synergic programmes for different material grades, wire size and shielding gas combinations,” Zylstra continues, before listing three essential characteristics of synergic welding: pulse parameters are selected automatically; pulse frequency and duration is directly related to wire feed rate; and the electronic control of parameters ensures uniform penetration and weld bead profile.
“Once the consumable grade, diameter and shielding gas has been selected, a single ‘one-knob control’ is used to ‘synergically’ adjust the wire feed rate and pulsed current frequency,” he explains. “This allows the average current and the associated heat input to be reduced below the spray transition current of the wire without having to resort to dip transfer mode. This makes the process ideal for out of position welding at higher deposition rates and with much lower spatter levels,” he adds.
In addition the job-master torch with its integral remote control and weld data display makes the system ideal for in-situ welding – the welder does not have to stop welding and go to the power source to fine-tune the welding parameters.
“Digital arc length control is also applied to maintain the arc length, even with changes in the stick-out length. This further reduced spatter and improves weldability,” Zylstra adds.
Showing a set of etched samples from the procedure qualification records (PQRs), Zylstra highlights the successful results of the collaboration: Overhead, vertical-up and downhand weld samples all show excellent fusion, penetration and surface quality. “A total of 54 samples were sent in for metallurgical evaluation and all passed with flying colours,” he says.
“So it is possible! By choosing a suitable metal-cored wire consumable with good weldability and coupling it to a digitally controlled inverter with advanced synergic technology – along with a skilled welder – out of position in-situ cladding with cobalt-based alloys can be achieved at significantly higher production rates,” he concludes.
