How should sheath material be specified for industrial heating elements?

Sheath material for industrial heating elements is normally specified by first setting the maximum operating temperature to narrow the alloy range, then matching the grade to the corrosivity of the atmosphere or bath. Air-circulation and car-bottom furnaces mainly need oxidation resistance, so heat-resistant stainless steels or nickel-base alloys are common; liquid baths such as water, oil or neutral salt need corrosion resistance, and the choice should account for medium type, concentration, flow rate and chloride content, switching to high-nickel alloys or titanium where required. Installation also matters: immersion and radiant heaters differ in hot strength and thermal-shock resistance, and frequent cycling raises thermal-fatigue and scaling concerns. For temperature control, confirm the allowable surface load to avoid local overheating and shortened life. The enquiry should state grade, tube diameter, wall thickness, surface load, heated length and tolerances, and agree on material certificates and visual acceptance criteria; writing only "stainless steel" is too vague for reproducible supply from the supplier.