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Why is stainless steel rarely seen in high-temperature environments?

After working in the factory for a long time, the experienced workers all have a consensus: stainless steel, which is usually quite "tough", is rare to see once it enters high-temperature areas where the heat waves are rolling. It's like letting a scholar who is used to wearing exquisite suits swing a big hammer in a blacksmith shop, he's really not cut out for it. However, this also depends on how it is said. Some specially made "heat-resistant steels" are specifically designed for this "furnace" area.

1. The "unsuitability to water and soil" of ordinary stainless steel

Ordinary stainless steel, such as the common 304, will expose several fatal weaknesses at high temperatures, making them inadequate in most high-temperature situations.

Weakening of muscles and bones, intense 'diving'

The key to stainless steel is its corrosion resistance at room temperature through the passivation film. But once subjected to high temperatures, this layer of membrane becomes ineffective and its' muscles and bones' will also become weak. ordinary 304 stainless steel, once the temperature exceeds four to five hundred degrees, its strength drops like opening a gate to release water. At this point, carbon steel or heat-resistant alloy steel becomes much more reliable.

Hot expansion and cold contraction, too hot temper

Metal thermal expansion and contraction are natural, but stainless steel has a higher coefficient of expansion than ordinary carbon steel

This means that as soon as the temperature changes, the amplitude of its' stretching arms and kicking legs' becomes particularly large. In areas where the structure is constrained, enormous thermal stress can be suppressed, ranging from mild deformation to severe cracking. A set of hot air ducts, if made entirely of stainless steel and heated up and down several times, will ensure that the flange sealing surface is leak proof, which means that the expansion of various parts is uneven and they compete with each other, causing damage.

Skin peeling, no longer glamorous

Don't think that stainless steel is truly "stainless" forever. At high temperatures, especially above 70-80 degrees Celsius, the protective film on its surface will fail, and the oxide skin will peel off layer by layer, similar to psoriasis. This not only affects the lifespan of the equipment, but if the peeled oxide skin falls on the product, it is a disaster for precision workpieces.

As evidence, room temperature is the main battlefield

This is evidenced by national standards. According to the provisions of GB/T 3098.6-2023 "Mechanical Properties of Fasteners - Stainless Steel Bolts, Screws, and Studs", stainless steel fasteners that meet this standard are tested for mechanical properties at ambient temperatures ranging from 10 ℃ to 35 ℃. Although the standard states that it can be used up to+300 ℃, it also explicitly reminds that when used outside the temperature range of -20 ℃ to+150 ℃, users must consult experts and consider the impact of temperature on performance. This is sufficient to indicate that ordinary stainless steel stages are mainly used at room temperature.

2. "Exception" Appears: The Mastery of Heat Resistant Stainless Steel

As the saying goes, 'there is specialization in every profession.' In this treacherous territory of high temperature, there are naturally 'special forces' who specialize in taming it - heat-resistant stainless steel.

Advantages of heat-resistant stainless steel

These materials, such as 310S (0Cr25Ni20) and 1Cr16Ni35, endow steel with excellent resistance to oxidation and strength maintenance at high temperatures by significantly increasing the content of elements such as chromium (Cr) and nickel (Ni). They can form a dense and strong chromium oxide protective film on the surface, even at temperatures of 1080 ℃ or even higher for a long time, without oxidation or deformation

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High nickel austenitic heat-resistant steels like 1Cr16Ni35 have an oxidation resistance limit of up to 1150 ℃ and a strength retention rate of over 60% at 800 ℃.

The application of heat-resistant stainless steel

Therefore, in the truly high-temperature core parts of the petrochemical industry, such as cracking furnace tubes (operating temperature 950-1100 ℃), heat treatment radiation tubes, and gas turbine combustion chambers, what you see are basically the figures of this type of heat-resistant stainless steel. Their service life is often several times that of traditional materials.

Who is breaking through the "Flame Mountain" of high temperature?        

So, what materials are commonly used to lift beams in the high-temperature "flame mountain" of factories?

Heat resistant steel "special forces":

As mentioned above, they are alloy steels specially designed for high-temperature environments, with complex compositions and naturally higher value than ordinary stainless steel. Refractory material "ascetic monk":

Like refractory bricks and castables, they simply gave up their identity as metals and used ceramic materials such as silicon carbide and alumina, which have extremely high melting points and are born to eat this bowl of high-temperature rice. The furnace lining and burner bricks are none other than them.

Old buddy's perseverance in carbon steel:

In some situations where the temperature is not too high (such as below five or six hundred degrees Celsius) and oxidation is not feared, such as some flues and hot air casings, using carbon steel has low cost and sufficient strength. After a dense layer of skin is oxidized on the surface, it no longer continues to rust inward, becoming a protective shell that is sturdy and durable.

There is no best, only the most suitable

At the end of the day, in a factory that values practicality, selecting materials is always about 'what is suitable is the best'. The standard GB/T 3098.6-2023 reminds us that ordinary stainless steel is not omnipotent. The halo of stainless steel at room temperature - corrosion resistance and high appearance - fades one after another as soon as it enters the high temperature examination room. It's like a meticulous design expert, if you let him do the job of swinging a sledgehammer, that's not his stage.        

And when you see the heat-resistant stainless steel components that are safely in service in the scorching furnace, you should understand that it is not the stainless steel family that cannot withstand high temperatures, but ordinary stainless steel and heat-resistant stainless steel, which are two types of materials with different endowments and each performing its own duties. This' self-awareness' is precisely the thinking principle we should have when selecting equipment.