Chapter 8 of 10 · 10 min read
Nickel: sulphur, gases and cleanliness
Understand why nickel and nickel weld metal crack when contaminated with sulphur and similar elements, how dilution brings contaminants in, and how cleanliness protects a nickel repair.
Nickel and its alloys are chosen for corrosion resistance and strength at high temperature, and nickel-based fillers are among the most useful in repair welding, joining cast iron, dissimilar steels and hard-to-weld metals. Nickel welds well when it is clean. When it is not, it cracks, and the contaminants that crack it are found everywhere in a workshop: in cutting oil, paint, marker pens, temperature crayons and even the fuel gas used to heat it.
Kautny understood the central danger in 1915. Nickel, he wrote, has a great affinity for sulphur, with which it forms many compounds. Modern guidance makes the same point more emphatically. This chapter explains why, shows how dilution can carry contaminants from a casting into a nickel weld, and traces a cracked buttering layer on the pump casing to a few crayon marks.
Kautny’s nickel
Kautny gave nickel’s melting point as 1,600 °C; the modern figure from The Engineering ToolBox is about 1,453 °C. He described molten nickel’s capacity to absorb large quantities of gas, particularly oxygen, which remains in the metal as it cools and forms cavities. He rejected the common claim that nickel could not be welded by the flame, while acknowledging the difficulties, and noted that when an alloy of nickel and iron was welded, nickel could separate and appear as small globules in a fractured weld.
His solution, as for copper, was to weld below the melting point where possible. Nickel, he wrote, can be kneaded plastically at a heat below melting, and a joint made at that temperature avoids the harmful absorption of gases. Because nickel conducts heat strongly, the work had to rest on an iron support heated to dark red, 700 to 800 °C, and be hammered with long-handled hammers of about a pound once the nickel reached a bright white heat. Porous places in rolled nickel plate could be repaired by drilling them clean and fusing in a pointed pure nickel wire while the plate rested on the heated anvil. He set three rules: surfaces absolutely free of grease and oil and mechanically clean; work on a heated support; hammering only at the right temperature.
The first of those rules has outlasted all the others.
| Kautny, 1915 | Modern view | |
|---|---|---|
| Melting point | 1,600 °C | about 1,453 °C |
| Affinity for sulphur | great; forms many compounds | removal of sulphur compounds cannot be over-emphasised |
| Heat loss to supports | rest work on anvil at 700 to 800 °C | avoid cold heat sinks; preheat only where the procedure requires |
| Gas absorption | hammer below melting to avoid it | inert gas shielding and clean, dry consumables |
| Cleanliness | free of grease and oil; mechanically clean | also free of paint, markers, crayons and fuel-gas sulphur |
Why nickel fillers are so useful
Nickel’s tolerance of other elements, apart from the embrittling few, is what makes nickel-based fillers so valuable in repair. On cast iron, TWI explains, nickel absorbs the carbon picked up from the casting and stays ductile and machinable where iron-based weld metal would harden. Between dissimilar metals, as Chapter 9 shows, nickel-based fillers accept dilution from both sides without forming brittle structures and can bridge differences in expansion. On hardenable steels, as Chapter 4 noted, nickel or austenitic deposits tolerate hydrogen better than ferritic ones. In each case, the filler’s virtue depends on keeping the embrittling elements out, which is why the same cleanliness rules apply whether the base metal is nickel, cast iron or steel.
Nickel alloys as base metals
When the part itself is a nickel alloy, such as commercially pure nickel, a nickel-copper alloy or one of the nickel-chromium alloys used for heat and corrosion resistance, the same cleanliness applies with added weight, because any contamination now affects the whole heat-affected zone, not just the weld. Fillers are normally chosen to match or overmatch the alloy’s corrosion resistance, following the alloy and filler makers’ data, and many procedures limit heat input and interpass temperature. Where the part will serve at high temperature or in aggressive chemicals, the procedure should be qualified on the actual alloy and, ideally, on material from the same source, because small differences in composition can change how it behaves.
Dilution brings contaminants in
Sulphur and other embrittling elements
TWI’s job knowledge article on welding nickel alloys states that, for defect-free welding of nickel and its alloys, the importance of removing sulphur-containing compounds cannot be over-emphasised. It adds that several other substances can embrittle nickel alloys at high temperature, including lead, phosphorus, boron and bismuth.
The mechanism, in broad terms, is that these elements gather at the boundaries between grains as the metal solidifies or is held hot. Sulphur in particular is associated with low-melting films and weakened grain boundaries in nickel, so that the metal cracks along those boundaries under the shrinkage stresses of welding or when loaded at temperature. Very small amounts are enough. That is why nickel welding is more a matter of cleanliness than of heat.
Where the contaminants come from
TWI lists the common sources, and every one is found in an ordinary workshop.
- Avoid: Oils and greasefrom handling and storage
- Avoid: Cutting fluidsfrom machining the joint preparation
- Avoid: Paintsincluding layout and identification paint
- Avoid: Marker pen inksused to mark weld positions or crack ends
- Avoid: Temperature-indicating crayonsused to check preheat
- Ask: Fuel gas in heatingmay contain sulphur; use electric heating or sulphur-free fuel
The last item surprises many welders. TWI warns that fuel gases frequently contain sulphur, so heat treatment of nickel alloys should use electric furnaces or sulphur-free fuel. A flame playing on a nickel part for a long preheat or post-heat can deliver sulphur directly to the surface.
Dilution brings contaminants in
Nickel weld metal is rarely pure filler. When it is deposited on another metal, some of that metal melts and mixes into the weld; the fraction is called dilution. On cast iron, dilution brings carbon, which nickel tolerates well and which is why nickel fillers are used, but it also brings the casting’s sulphur and phosphorus. TWI notes that nickel deposits high in sulphur and phosphorus picked up by dilution may cause solidification cracking.
This is an illustration. Suppose a nickel filler contains 0.01% sulphur and the casting 0.10%. The weld’s sulphur content is roughly the filler’s share times its sulphur plus the casting’s share times its sulphur.
At 30% dilution the weld carries nearly four times the filler’s sulphur; at 50%, more than five times. That is the quantitative reason TWI’s cast iron procedure butters the groove with small 2.4 or 3 mm electrodes in short runs: low current and small runs melt less cast iron and keep dilution, and the sulphur it brings, down. It is also why the long hot runs in Chapter 5’s failure cracked: high current and wide weaving raised the dilution.
Cleanliness in practice
For nickel and nickel weld metal, preparation means removing contaminants, not just making the joint look clean.
- Mechanical cleaning of the joint and a band either side, by grinding or with clean abrasives and brushes reserved for nickel and stainless work, so that steel or other residues are not transferred.
- Degreasing with a suitable solvent after machining or grinding, since cutting fluids and handling leave films.
- No markers on the joint. Weld positions and crack ends are marked away from the weld area, or with markers known to be free of the harmful elements; any marks near the joint are ground off before welding.
- Temperature measurement by contact thermometer or by crayons suitable for nickel alloys, applied outside the area to be melted.
- Clean heating. Electric heating for preheat and post-heat of nickel alloys, as TWI advises, or care that flames do not play on the joint.
- Clean storage. Nickel consumables kept in closed, labelled containers away from oils, crayons and general workshop dust, so they are as clean when used as when supplied.
Nickel in the pump casing repair
This is an illustration. The pump casing in Chapter 5 is repaired with a nickel buttering layer and a nickel-iron fill. The nickel never meets a nickel alloy part, but it meets everything on the cast iron: oil soaked into the foot from years of service, the marks the inspector made at the crack ends, and the crayons used to check the preheat. Each is a potential source of the elements that crack nickel weld metal, carried in by dilution.
The workshop’s plan therefore added three steps to Chapter 5’s sequence: the casting was degreased and heated in its electric furnace to drive out absorbed oil before the groove was ground; crack-end marks were made with scribed lines rather than marker pen and outside the groove; and preheat was checked with a contact thermometer instead of crayons.
A failure: the cracks that followed the crayon
This is an illustration. During the trial repair on a scrap casting, the welder checked the preheat with a temperature-indicating crayon, marking the groove faces in several places. After buttering, dye penetrant testing showed short cracks running along the centre of some buttering beads, and only some. When the inspector laid a photograph of the crayon marks over the penetrant results, the cracked beads lay over the marks.
The crayon had left residue on the groove faces that the welder had not ground off. Whatever the crayon’s composition, the pattern pointed clearly to contamination as the cause of the solidification cracking, consistent with TWI’s warning about temperature-indicating crayons. The buttering was ground out, the groove re-prepared and degreased, and the trial repeated with a contact thermometer used outside the groove. No cracks appeared. The workshop now stores crayons that are not approved for nickel work separately from nickel consumables, and the procedure specifies how preheat is measured.
The failure also showed the value of the overlay. Cracks that appeared in some beads and not others, with no obvious difference in technique, looked at first like random bad luck or a faulty batch of electrodes. Mapping where they occurred against what had been done to the surface turned a puzzle into a cause in a few minutes. When defects cluster, their positions usually point to their origin.
Silver, gold and Kautny’s pattern
Kautny grouped silver with nickel. Similar effects appear when silver is welded, he wrote, and the hammering process is again advisable. Gold, by contrast, simply melts and joins smoothly, which is why the goldsmiths of his day made ample use of the flame. The contrast is instructive. Gold does not form an oxide that interferes, does not absorb troublesome gases and is not embrittled by common contaminants, so fusion is easy. Nickel and silver absorb gases when molten, so Kautny steered his readers towards joining them hot but solid.
That pattern runs through his whole book: where melting brings trouble, join below the melting point and consolidate by pressure. He applied it to copper alloys, aluminium, nickel and silver, and he closed the book by predicting that this approach would become one of the most important in welding. Modern solid-state processes, the subject of Chapter 10, have proved him right for several of exactly the metals he named. For fusion welding of nickel, the lesson that survives is narrower but just as firm: keep it clean.
Recording cleanliness
Cleanliness is invisible once the weld is made, so it has to be recorded at the time if it is to be shown later. For the casing repair, the workshop’s record noted the degreasing method and solvent, the oil burn-out in the furnace, how the crack ends were marked, how preheat was measured and with what instrument, the brushes and abrasives used, and the batch numbers of the nickel electrodes. None of those entries would be needed if the repair held. If it cracked, they would show whether contamination could be ruled out, and they would let the next repair avoid repeating the cause.
Fume from nickel
Nickel and its compounds in welding fume are a health hazard, and nickel-bearing fillers are used in exactly the repair work where ventilation is often poor: inside casings, under machinery and in confined spaces. Safe Work Australia’s model code of practice on welding processes and the consumable supplier’s safety data sheet set out the controls. A repair plan that chooses a nickel filler for good metallurgical reasons also commits the job to the fume controls that filler needs.
Exercise: a more contaminated casting
The casting from an older machine is found to contain about 0.40% sulphur in a contaminated surface layer, and the nickel filler contains 0.02% sulphur. Calculate the sulphur in a nickel deposit at 30% dilution with this layer, compare it with the 0.037% in the bars above, and say what the workshop should do before welding.
The deposit would contain about 0.7 × 0.02 + 0.3 × 0.40 ≈ 0.134% sulphur, more than three and a half times the 0.037% of the earlier example. The contaminated layer should be removed by grinding back to sound, clean metal before buttering, and the dilution kept low with small electrodes and short runs. A trial weld on the same casting, or on a removable piece of it, checked by dye penetrant, would show whether the remaining metal still cracks nickel weld metal before the real repair is attempted. If the casting is contaminated through its thickness rather than only at the surface, a mechanical repair or a replacement may be the sounder choice, as Chapter 5 discussed.
