Hastelloy is Haynes International's trademarked family of nickel-based corrosion-resistant superalloys, developed for chemical environments aggressive enough to attack standard stainless steels — strong acids, chlorides, and both oxidizing and reducing media used across chemical processing, pollution control, and pharmaceutical equipment. Hastelloy G-3 is the refined, lower-carbon successor to the original Hastelloy G, built on the same nickel-iron-chromium base with a copper addition — roughly 44-47% nickel, 21-23.5% chromium, and 18-21% iron, with 6-8% molybdenum and 1.5-2.5% copper.

The copper is what gives G-3 its specialty: strong resistance to sulfuric acid across a broad concentration and temperature range, along with good performance in phosphoric acid, two of the most demanding process acids in fertilizer and chemical manufacturing. G-3's lower carbon and niobium content compared with the original G alloy give it better resistance to grain-boundary carbide precipitation during welding, meaning it holds up better in the heat-affected zone of fabricated equipment — piping, reactor vessels, and pickling equipment — without needing post-weld heat treatment to restore corrosion resistance the way the original G sometimes did.

Hastelloy G-3 machines much like its G-family sibling — demanding, but slightly more forgiving than the leaner, higher-nickel B and C grades thanks to its substantial iron content. Thermal conductivity is still low compared with steel, so heat from the cut concentrates at the cutting edge rather than dissipating through the chip, driving up tool temperatures even at modest cutting speeds. The alloy work-hardens under light or dwelling cuts, leaving a hardened layer behind the tool that resists the next pass — feed rates need to stay generous enough to consistently cut beneath that layer.

G-3 also tends to gall and adhere to the tool surface rather than shearing away cleanly, producing built-up edge and accelerated flank wear if the tooling isn't well matched to the material. Rigid, low-deflection setups, sharp positive-rake carbide geometry to keep cutting forces down, and coatings selected for hot hardness and resistance to adhesion all pay off in tool life. Cutting speeds should stay low to moderate relative to steel, and a steady, adequate feed — rather than light finishing passes — keeps the cut productive and limits work-hardening between passes.