Stainless steel

What is rust?

Rust is a layer of oxide that forms on the surface of iron or steel as a result of the simultaneous reaction of air and water. The rusting process accelerates if the water contains salt or acids. According to chemical terminology, rust is iron oxide-hydroxide, a chemical compound composed mainly of iron, oxygen, and hydrogen elements.

In technical and scientific contexts, the term "corrosion" is used instead of rust – thus, steel corrodes. Corrosion does not only occur in liquid environments, but also appears after exposure to hot gases, such as the moisture produced by fuel combustion.

How do we protect against corrosion?

We all know how steel structures have been protected against corrosion with varying results by coating the surface with paint or plastic. As long as the protective layer is new and intact, it provides good protection, but even the slightest scratch in a car’s paint allows corrosion to access and progress beneath the surface, eventually leading to the peeling off of large areas of paint.

Better protection can be achieved by covering the steel sheet surface with a thin zinc layer, i.e., galvanizing. The electrochemical reaction between zinc and the water promoting corrosion prevents iron from corroding, but zinc gradually dissolves. As long as the zinc layer remains intact, the iron is protected from corrosion, but as the zinc layer decreases, it becomes vulnerable to corrosion in the same way as untreated iron.

Why is stainless steel rust-resistant?

By selecting the right type of steel, corrosion can be completely avoided in most environments. The special properties of stainless steel are due to the chromium it contains, which must make up at least 12% of the total mass of the steel. This allows the formation of an invisible chromium oxide layer on the surface, which protects against corrosion. Once one oxide layer is destroyed, a new protective layer immediately forms due to the oxygen in the air. In addition to chromium, corrosion resistance can be enhanced using metals like nickel and molybdenum. Generally, it can be said that the corrosion resistance of stainless steel improves with the increasing content of alloying elements.

Applications

The most common stainless steel contains 18% chromium and 9% nickel (this steel type is commonly referred to as type 304), with the rest primarily consisting of steel. This type of steel can resist corrosion in air and fresh water environments, i.e., in the most common situations. We encounter this type of steel regularly in everyday life, as it is used to make many tools, knives, pots, pans, sinks, dishwashers, washing machines, etc. The main reasons why stainless steel is used in such cases are its attractiveness, durability, and ease of cleaning.

The dominant application of stainless steel is in the manufacturing industry, where high-alloy steel is used for equipment operating in various corrosion-prone environments, such as the chemical and fuel chemical industries, the pulp and paper industry, and the food industry. Other special types of stainless steel are known for their high resistance, making them suitable for the construction and transport sectors.

There are also grades of stainless steel that can withstand high temperatures and hot gases produced in metallurgical industry furnaces.

Stainless steel is significantly more expensive than regular unalloyed steel, but it is often more economical to use stainless steel because there is no need for surface coating or constant maintenance.

Some typical uses where stainless steel properties shine are oil platforms, swedish high-speed train and chemical tankers.

Oil platforms

The seabed of the North Sea is rich in oil and natural gas. The drilling of these products takes place on gigantic oil platforms, anchored near the discovery sites.

Due to the high salt content, seawater is highly corrosive to regular steel. To ensure reliable protection against corrosion, suitable metals such as copper alloyed with 10% nickel or special stainless steel containing 6-7% molybdenum are used. In this case, stainless steel should be preferred as it is stronger and can withstand the currents. Stainless steel also allows for the use of much smaller diameter pipes, making the platform structure lighter.

Swedish high-speed train

In the early 1990s, when the Swedish high-speed train was designed, ASEA BROWN BOVERI AB conducted extensive research to find the best material for the train's car body. Regular steel, stainless steel, and aluminum were compared. Ultimately, the decision was made to use stainless steel for the construction, as it was lighter and cheaper than the alternative solutions.

The fact that stainless steel does not require painting or regular maintenance was a sufficient argument to offset its slightly higher cost compared to regular steel. Moreover, the reduction of weight by 2 tons, achieved through the omission of paint, filler materials, and frame sealing, resulted in energy savings. The advantage of a lighter train is energy efficiency.

Chemical tankers

Tankers used for transporting oil are made of regular steel, as oil is not a corrosion-prone substance. However, transporting liquid chemicals, food products, and beverages requires different specifications and needs. For food and especially beverages, it is important that the appearance and taste remain unchanged. Therefore, stainless steel is generally chosen for the containers and piping.

Stainless steel melting

The melting process primarily uses stainless steel waste, i.e., waste produced during sheet metal manufacturing, as well as outdated equipment from old factories. This approach allows for the economical recycling of valuable elements in the steel industry. Incoming steel waste is formed into a batch, whose chemical composition does not differ significantly from the desired outcome.

The waste is fed into an electric arc furnace, where the necessary energy to melt the metal is generated through graphite electrodes and high-voltage current. The furnace can be connected to a ceramic-lined vessel capable of withstanding high temperatures. The waste mixture in the furnace is later supplemented with alloying elements such as nickel, ferrochrome, and ferromolybdenum to achieve the desired steel composition.

Once the melting process is complete, the contents of the arc furnace are emptied into a crane bucket. The bucket is an open container lined with ceramic material. The molten metal is directed into a converter, where impurities such as carbon, silicon, and sulfur are removed through a process involving oxygen and argon.

Samples are taken from the molten steel and analyzed. If necessary, the steel is modified by adding additional alloying elements in the converter or crane bucket.

Steel casting

Once the refining process is complete, the contents of the converter are emptied into a crane bucket, and the steel is cast into a continuous casting unit. After the molten metal’s static pressure is reduced, it is allowed to cool in a quadrangular water-cooled copper mold. The steel solidifies immediately, forming a crust that is then fed through the mold’s base. The resulting quadrangular steel mass is drawn downward and sprayed with powerful jets of water until fully hardened. Guided by rollers on both sides, the steel is bent horizontally and cut to the desired length using an oxygen lance. The produced panels can be rolled into sheets.

Hot rolling of plates

The next stage in the production chain is hot rolling, which involves heating a steel plate in a gas-fired furnace to 1200°C. At this temperature, the steel is relatively flexible and can be easily shaped to the desired thickness in the rolling mill. During the rolling process, the plate’s length and width increase in proportion to the reduction in thickness, depending on how many times the plate passes through the rollers.

Before cutting to the desired format, individually hot-rolled plates (quarter plates rolled in a four-roll mill) undergo heat treatment (annealing) at approximately 1200°C. This treatment ensures uniform internal structure, which guarantees mechanical strength and corrosion resistance. As a result of the heat treatment, a dark oxide layer forms on the plate surface, which is removed using acid (pickling acid). The surface of the steel plate is then washed with water.

Hot rolling can also be performed using a “Steckel” rolling mill, where the steel sheet is rolled back and forth onto coils located on both sides of the mill. The coils are built into a gas furnace so that the material does not cool during the process. The steel sheet being processed can reach lengths of up to 1000 meters. After rolling in the Steckel mill, the hot strip is wound onto a coil.

After processing at high temperatures and with acid (see above), the finished hot strip is cut to the correct size or cold-rolled to achieve a smooth and glossy surface.

Cold-rolled plate and strip

Before cold rolling, the steel strip is heated in a gas-fired furnace. Immediately afterward, it undergoes treatment with acid and other liquid chemicals, as well as electrolytic pickling, to remove the oxide layer. The steel is then rinsed with water, dried with hot air, and rewound onto a coil. This treatment serves the same purpose as hot rolling – to ensure material uniformity.

The actual cold rolling operation involves passing the steel strip between narrow polished rollers arranged systematically with supporting rolls and bearings. During rolling, the material is subjected to strong pressure and tension from the coils on both the front and back sides of the mill. Different rolling mills are used for steel strips of varying thickness. The widest strip is 2 meters, with other common widths of 1.3m, 1m, and 0.8m. The thickness ranges from 0.1mm to 6mm.

During cold rolling, stainless steel gradually becomes harder, and this process is called cold hardening. After rolling, the steel strip is reheated and treated with acid (annealed and pickled) to further process and shape it as needed.

Welded pipe production

Welded pipes are made from cold-rolled steel sheet coils. The steel strip is fed through a rotating mechanism that gradually bends the material into a cylindrical shape.

The two edges of the sheet are pressed together and welded using longitudinal welding techniques to form a pipe, which is then cut to the required length. Larger pipes are bent from the cut plate and welded similarly.

Energy and the environment

Steel production is an energy-intensive process, with electricity being the dominant energy type. Melting alone requires more than 500 kilowatt-hours per ton. Considering the subsequent rolling and processing, the required energy doubles. Additional energy is needed for fuel heating, heat treatment, rolling, and local heating processes. The electric arc furnace's melting energy cannot be replaced by any other type of energy. To ensure sufficient melting power in the furnaces, electricity must be produced at a very high intensity per unit of time, requiring a very high power output of 50-100 megawatts per furnace.

For alternative energy sources such as wood, solar power, wind energy, and heat pumps, it is not reasonable to generate such large power. These are more suitable for heating or energy generation.

Growing concern about environmental pollution has forced steel producers to install extensive equipment to filter air emissions, neutralize used acid baths, and isolate production facilities to reduce noise levels, etc. Today’s steel producers actively monitor legislative and other requirements that protect and preserve the environment for future generations. Compared to other countries, Swedish steel production is currently at the forefront in terms of production technologies and environmental protection advancements.