Heat-resistant stainless steel

Heat-resistant stainless steel is a special stainless alloy that is resistant to corrosion of the surface in gas environments at temperatures above 550 °C, operating in an unloaded or slightly loaded state. Such steel is often called scale-resistant (scale-resistant stainless steel).

Heat-resistant steel is characterized primarily by significant resistance to oxidation at high temperatures. Such steel is alloyed with chemical elements that change the composition and structure of the scale. As a result, the level of heat resistance increases. As a result of adding the required amount of chromium (Cr) or silicon (Si) to stainless steel, which have a greater affinity for oxygen (O) than iron (Fe), dense oxides based on chromium or silicon are formed on the surface during oxidation. The resulting thin protective film of these oxides significantly complicates the process of further oxidation. To ensure heat resistance up to a temperature of 1100 °C, steel must contain more than 28% chromium. But the best heat resistance results are achieved by simultaneously alloying steel with chromium and silicon.

Heat-resistant stainless steel is a type/kind of steel suitable for use in high-temperature and highly loaded conditions for a long time without noticeable deformation and destruction. The main characteristics of heat-resistant steel are creep and strength. Long-term strength, in turn, is the conditional stress under which steel at a temperature is destroyed after a given long period of time.

The heat resistance levels of stainless steel grades vary depending on the type of grade used. In general, ferritic and martensitic grades have lower heat resistance than austenitic grades. This means that they are better suited for low temperature applications such as welding or brazing. Austenitic grades generally have higher heat resistance levels and can be used where temperatures may exceed 427 °C.

When selecting a metal for use in a particular working environment, it is often necessary to know whether the alloy will be exposed to temperature for a few seconds, a few minutes, or an hour or more. Short, intermittent exposure interrupted by removal from the furnace for cooling is called intermittent exposure, while prolonged immersion in the furnace is called continuous exposure.
The point is that a metal alloy can have different tolerances to high temperatures depending on whether the exposure is continuous or intermittent. Here are some examples of continuous and intermittent temperature limits for stainless steel:
AISI 304
Continuous: 925°C;
Periodic: 870 °C;
AISI 309
Continuous: 1095°C;
Periodic: 980 °C;
AISI 310
Continuous: 1150°C;
Periodic: 1025 °C;
AISI 316
Continuous: 925°C;
Periodic: 870 °C;
AISI 410
Continuous: 705°C;
Periodic: 815 °C;
AISI 420
Continuous: 620°C;
Periodic: 735 °C;
AISI 430
Continuous: 815°C;
Periodic: 870 °C;
You may have noticed an odd and potentially counterintuitive trend in the 300 series stainless steel alloys listed here. Specifically, their recommended maximum continuous use temperature is higher than their intermittent use temperature limit. It is natural to assume that exposing a metal to high temperatures for a shorter period of time will result in less stress on it than long-term exposure.

The intermittent action of temperature on metal creates a stress factor different from a constant temperature factor. This phenomenon is known as "thermal cycling." When a piece of metal rapidly cycles between extreme temperatures, several things can happen.
When metal is heated, it can expand and then contract as it cools. Also, steel alloys in furnace-like conditions can develop scale on their surface - a kind of flaky substance made up of iron and iron oxide. This substance replaces the outer layer of the metal.
When exposed to high and low temperatures repeatedly, scale can begin to crack and split, weakening the metal's shape. This can happen because of the difference in the coefficient of expansion between the stainless steel metal core and its scale surface. In simple terms, the interior of the metal expands or contracts at one rate, while the scale on the surface expands or contracts at another. This difference causes the metal to begin to disintegrate, layer by layer, until it finally fails completely.

Choosing the best stainless alloy for your specific application depends not only on what temperature the alloy can withstand for intermittent and/or continuous use, but also on the cost of that alloy and its performance.
In other cases, you may need to consider the metal's chemical resistance in addition to its operating temperature. This will be able to preserve your structures and parts when exposed to several processes at the same time (temperature and chemical exposure).