Misunderstandings Regarding Stainless Steel Flanges

The stainless-steel flange is a pipeline component renowned for its exceptional performance, exhibiting minimal deformation when securely fastened.

Today, our focus lies primarily on dispelling common misunderstandings surrounding stainless steel flanges.

If a magnet adheres to the metal, it indicates that it's not stainless steel.

Stainless steel is categorized into various types: austenitic, ferritic, duplex, martensitic, precipitation hardening, etc. Only austenitic stainless steel is non-magnetic, constituting most of the stainless-steel market (formerly over 70%, currently estimated at around 60%, and projected to decrease to 50% or less). This type is widely utilized. Conversely, all other steel materials exhibit magnetic properties.

For instance, when testing the shiny stainless-steel knives and forks at upscale restaurants with a magnet, they are often magnetic, indicating they are likely made of martensitic stainless steel. Similarly, stainless steel used in car exhaust systems is also magnetic, typically indicating a ferritic composition.

Austenitic stainless steel typically incorporates nickel. However, given the current high global nickel prices, the stainless-steel industry gravitates towards nickel-saving innovations. One promising approach involves the advancement of superior ferritic stainless steel and duplexes. Consequently, it's foreseeable that more magnetic stainless-steel variants will emerge in the future.

Stainless steel is corrosion-resistant and typically does not rust.

It's widely recognized that properly qualified stainless steel (meeting national standards) does not rust under typical atmospheric conditions. This is primarily due to chromium, a vital alloying element in stainless steel (with a minimum chromium content requirement of 13% to ensure excellent corrosion resistance). Chromium enables the formation of a protective passive film on the stainless-steel surface, shielding it from corrosion.

However, despite its corrosion resistance, stainless steel can still experience corrosion due to damage to the surface passivation film and localized factors. Various forms of localized corrosion can occur, such as pitting, crevice, intergranular, stress, and galvanic coupling, collectively known as the five types of localized corrosion in stainless steel.

Firstly, it is essential to avoid exposing stainless steel to prolonged chloride-rich environments. Additionally, maintaining a clean stainless steel environment is crucial to prevent the accumulation of dirt and the formation of contaminants.

Furthermore, it's important to avoid scratching the surface of stainless steel. While a scratched stainless-steel surface can typically regenerate a new passivation film, this new film is generally more vulnerable. Scratches create geometric gaps that are susceptible to crevice corrosion and rust formation.

Which stainless steel is the best?

To my knowledge, there is no universally perfect stainless steel. Each type of stainless steel is tailored for specific applications and designed for particular purposes. When developing and designing materials, it's impractical to address all factors simultaneously, such as cost, corrosion resistance, formability, and weldability, as these considerations often conflict.

When focusing solely on corrosion resistance, it's not feasible to design a product that can withstand all types of corrosion. Instead, it requires selective and targeted enhancements, such as increasing molybdenum content to improve pitting resistance or adding titanium or niobium to enhance resistance to intergranular corrosion. In reality, there are numerous grades of stainless steel, each with its unique composition, amounting to hundreds of varieties. Among these, we commonly encounter the following:

One of the most iconic stainless steels is 304. It boasts a chromium content of 17% to 19% and nickel content ranging from 8% to 11%. Its current market price is approximately 3,000 to 4,000 yuan per ton.

316 stainless steels, renowned for its resistance to seawater corrosion, contain higher levels of molybdenum than 304. Its price typically exceeds 5,000 yuan per ton. On the other hand, 301, 302, and 304 variants have higher carbon content, resulting in slightly lower corrosion resistance.

321 stainless steels, enriched with titanium and niobium to enhance weldability, were previously favored but gradually became less prominent.

Conclusion

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