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Hot components have different duties
A gas baffle, a loaded suspension and a protective temperature-measurement tube may share an industrial furnace while performing different duties. Resistance at the gas-contact surface matters in one component; carrying load at temperature matters in another. Heat-resistant stainless steels therefore include more than one material. Ferritic and austenitic families offer different combinations of properties. Ferritic grades have a useful role in components where low thermal expansion and resistance to particular hot atmospheres are especially important.
What does low expansion contribute?
Metal expands as its temperature rises. In a long furnace component or a broad plate, the dimensional change can be significant. Ferritic stainless steels expand less than common austenitic heat-resistant grades and generally conduct heat more readily. These characteristics influence their behaviour between hotter and cooler zones. The ferritic structure is also magnetic. Attraction to a magnet does not mean the metal is not heat-resistant or stainless; it is a characteristic associated with its structural family.
1.4742 and 1.4762: chromium, aluminium and silicon
Both grades belong to the chromium-aluminium-silicon group. Aluminium and silicon contribute to the surface-protective role of chromium in hot environments. The chromium content of 1.4762 is higher, but that does not establish universal superiority. Furnace elements, burner components, hot-gas dividing surfaces and conveying equipment illustrate applications associated with this family. Exposure to heat is a shared feature, while the load and surrounding gas vary from one component to another.
446 / 1.4749: a separate high-chromium grade
446 is also a high-chromium ferritic heat-resistant steel. It is associated particularly with sulfur-bearing hot gases and heat-recovery equipment in the metals and glass industries. Recuperators and protective tubes are concrete examples. However, 446 is not another name for 1.4762. Its EN number is 1.4749 and its chemical definition differs. Similar application areas do not turn these two materials into one alloy. The high-chromium ferritic category contains several distinct grades.
Where do 309S, 1.4835 and 1.4818 differ?
309S, 1.4835 and 1.4818 are austenitic heat-resistant stainless steels. Their nickel-bearing structures and mechanical properties at temperature offer a different balance from ferritic grades. Silicon, nitrogen and cerium are also present in 1.4835 and 1.4818. Some furnace applications overlap, but behaviour under loading and exposure to gases is different. This explains why heat-resistant steels cannot be described by one maximum temperature alone. Surface resistance, expansion and strength at temperature are separate parts of the overall material behaviour.