Alloy 600 is the basic nickel-chromium grade for heat and chloride cracking resistance. Alloy 625 adds molybdenum and niobium for far higher strength and excellent pitting and crevice resistance in seawater and acids. Alloy 718 is precipitation hardenable and reaches the highest strength of the three, which is why turbine and fastener work specifies it.

It work-hardens very rapidly, so any rubbing or dwelling creates a hardened layer the next pass cannot cut, and its low thermal conductivity keeps the heat in the cutting zone rather than carrying it away in the chip. Add abrasive carbides in the structure and tool life is short. Rigid setups, sharp tools, low speeds and positive feeds are essential.

It depends on the grade and on whether strength or bare survival is the requirement. Alloy 718 holds high strength to roughly 700 degrees Celsius before its strengthening precipitates coarsen; alloys 600 and 625 have lower strength but resist oxidation and scaling to considerably higher temperatures, into the region of 1000 degrees and above for 600.

Alloy 625 does, exceptionally well, and it is a standard specification for seawater piping, splash zone components and subsea hardware because its molybdenum content resists pitting and crevice corrosion where stainless steels fail. Alloy 600 is much less resistant in chlorides, so grade selection matters more than the family name.

Routinely, by TIG and MIG with matching or overmatching filler, and it is commonly welded in fabrication. The concerns are hot cracking, careful interpass temperature control and thorough cleanliness, since sulphur and lead contamination from marking pens or lubricants cause cracking. Age-hardenable 718 requires post-weld heat treatment to restore properties.

Round bar, flat bar, sheet, plate, tube, pipe and wire, in the common alloy grades. Because the material is costly and slow to machine, stock is normally selected as close to finished size as possible, and near-net forms such as tube and rings are preferred to hogging a part out of solid bar wherever the design allows.

Generally no - the common grades are austenitic and essentially non-magnetic in the annealed condition, which is occasionally exploited in instrument and sensor housings. Slight magnetic response can appear in some grades after heavy cold work, so it should not be relied on as a material identification test.