About Stainless Steel

Stainless steel is an iron alloy.It should contain min 10.5% Cr and max 1.2% C element. The importance of the Cr element used in stainless steel is that it forms an oxide layer (Cr2O3) on the surface of the material and thus protects the material from oxygen.

The Cr and passivation rate in stainless steel determines the stainless quality of the steel.

The oxide layer formed on the surface of the material is too thin to be seen with the eye and is waterproof.

• Ferritic Stainless Steels
• Austenitic Stainless Steels
• Martensitic Stainless Steels
  
• Ferritic-Austenitic (Duplex) Stainless Steels
  

This type of stainless steels contains low carbon and 12-18% Cr. They provide moderate corrosion resistance. They cannot be easily shaped like austenitic steels. They exhibit magnetic properties and their resistance cannot be increased by heat treatment.

Certain areas of usage: Kitchenware, decorative applications, automobile chassis parts, exhaust elements, hot water tanks…

If stainless steels with an austenitic structure at high temperatures and a Cr content of more than 0.1% are quenched, the austenitic structure turns into a martensitic structure. Martensitic structure is a structure with high strength and hardness, but its brittleness is also relatively higher. Since excessive hardness will also cause brittleness, its hardness can be reduced somewhat by annealing during the manufacturing phase and its strength can be brought to the desired level.

They possess medium level of corrosion resistance, heat treatment can be applied to them, they are magnetic and possess low capacity of being welded.

Certain areas of usage: Knives, surgical instruments…

When nickel is added to stainless steel, an austenitic structure is obtained at room temperature. The austenitic structure obtained at room temperature provides the materials with the opportunity to be plastically shaped, high strength and good corrosion resistance. Their mechanical strength cannot be increased by heat treatment, but it can be increased by cold working. These types of steels, which are normally non-magnetic, can also gain partial magnetic properties with cold working.

They possess the capacity to be fine welded, they are hygienic, their impact resistance at low temperature levels is fine.

Certain areas of usage: Machinery and manufacturing industry, architectural applications, food processing equipment…

This type of steels have both ferritic and austenitic structures at the same time. Since they have these two structures together, they provide both high strength, hardness and toughness properties at the same time. Fatigue resistance in corrosive environments is very good. They have good weldability and forming properties.

Certain areas of usage: In marine and salt water environments, in the petrochemical sector, in heat exchanger manufacturing…

Stainless steels are generally considered to be resistant to acids. However, this issue is closely related to the type of acids, their concentration, the type of steel and the ambient temperature. For example, although type 904 stainless steels are resistant to high concentrations of sulfuric acids at room temperature, grade 304 stainless steels cannot withstand even low concentrations.

While 304L and 430 quality stainless steels are resistant against nitric acid, hydraulic acid can damage any type of stainless steel.

300 Series stainless steels can resist bases at high concentrations. However, the same series of stainless steels are not resistant to “Sodium Hydroxide” which is a stronger base type.

The point to be noted here is that stainless steel should never be considered as rust-proof. If each type of stainless steel is used in places that are suitable for its own properties, it can be used for a long time without wear. However; if it is used in places that are not suitable for its own chemical and mechanical resistance, it loses its stainlessness feature.

Stainless steels should be cleaned before use. Contaminated deposits on the surfaces of such steels can cause the stainless steel to rust in the future. Cleaning can be done with a clean cloth and water or special cleaning solutions. However, the point to be considered here is to remove water or solvent residues from the surface after cleaning. Because water droplets on the surface can locally rust the steel.

In addition, during storage, stainless steels should be kept out of contact with the soil and should be kept away from places where carbon steels are stored or processed. Because when any carbon steel shaving gets into contact with stainless steel, stainless steel can start to rust.

For example; if purchased stainless tubes are stored on the ground in a way that they will be exposed to rain, some rainwater will accumulate inside the tubes, and if the wet parts of the tube are not dried in the following days, they will start to rust from the inner surface of the tube or from the contact areas where water remains.

Another example to be given regarding the rusting of stainless materials is as follows;

For instance, stainless steel tubes will be installed in a factory. During the installation phase, these tubes are integrated into a carrier skeleton when they are assemblied. The skeleton part that will carry the tubes is usually made of carbon steel material. Due to the nature of the assembly, stainless steel tubes will come into contact with the carbon steel carrier skeleton and will be used in that way. It is very important to take precautions regarding this issue. Because, if carbon steel is in contact with stainless steel, there will be an interaction between them and therefore rust will start to take place at the contact points. In the later stages, rust can increase and the tubes can reach the point of perforation. In order to prevent this issue, it is inevitable for the company performing the assembly to take certain precautions.

During assembly process, the carbon steel carrier frame and stainless steel tubes should never come into contact. Plastic materials can be placed on the contact points to prevent contact and avoid rust.

A common misconception in use is that this type of steel will never rust. If stainless steels are not stored in an isolated environment, they will rust after a certain period of time.

When choosing a stainless steel type, it is important to make a choice that is suitable for where it is used. Price should also be taken into consideration when making a choice. However; if price is at the forefront of importance, it is likely to encounter problems in terms of use in the future.

If the place where the steel is used and the properties of the material to be purchased are correctly matched, the stainless steel to be purchased can be used for many years without encountering any problems.

If the supplied material is preferred because it is cheap, this may cost the consumer more in the future. For example, if 304 quality stainless steel is used instead of 316 quality stainless steel in a place where savings are required, although it may seem like savings are being made at first, choosing the wrong material can cause bigger problems in the future.

• Properties to pay attention to ;
  • Corrosion resistance of material
  • Mechanical resistance of material
  • Physical properties of material
  • Surface features of material
  • Ambient temperature
  • Type of corrosive agents it will be used against.