Stainless Steel Alloys

The high amount of chromium in the alloy is what makes stainless steel stainless. When in contact with air or water, the high concentration of chromium creates a thin chromium oxide layer that protects the surface, preventing its corrosion.

The maximum tensile strength of these alloys may be as high as 1600 MPa, making them excellent for structural applications.

Most stainless steel alloys are recyclable, which contributes to make them an inexpensive material and makes it environmentally friendly.

Corrosion resistance is one of the most important features to consider in high-temperature applications. Stainless steel can be safely used in applications with temperatures as high as 870°C.

It requires low maintenance and anodizing or coatings are only applied in cases in which the environment is extremely hazardous, making it suitable for both industrial and residential purposes.

Stainless steels alloys are sub-divided into four major categories depending on their crystalline structure: Ferritic, Martensitic, Austenitic, and Duplex.

Ferritic stainless steels contain up to 30% chromium and less than 0,12% carbon. They have the highest corrosion resistance and moderate strength. Their formability is average and their cost is very low. Ferritic stainless steels are not heat-treatable.

Having a chromium concentration of less than 17% and carbon up to 1%, martensitic stainless steels have high hardness, strength, and corrosion resistance making them suitable for high-quality knives.
Adding a considerable amount of nickel generates austenitic stainless steel, which has excellent ductility and formability combined with its characteristic corrosion resistance. Nevertheless, austenitic stainless steel has poor weldability.
To overcome the weldability issues of austenitic stainless steel, a Duplex category is produced. The name duplex derives from the 50-50 distribution of ferrite and austenite, providing better weldability combined with excellent mechanical properties.

Most of these categories are suitable for CNC machining, making it possible to achieve very complex shapes with this material.

A brief description of some alloys and their manufacturability is presented below.

1. 303 Stainless Steel (Annealed)

Slightly denser than other stainless steel alloys, the 303 sheet-metal displays excellent bending properties and its high strength allows it to be manufactured as thin plates.

Its machinability is incredible too. Drilling holes and cutting shapes on them is much easier than other alloys due to their sulfur concentration.

√ Mechanical Properties

√ Manufacturing Processes Performance

2. 304 Stainless Steel (Annealed)

This alloy could be considered the counterpart to 303 stainless steel. Its low sulfur concentration compared to 303 makes it hard to machine, but considerably improves its weldability.

Wear resistance, strength, and hardness are also affected. However, this alloy is malleable and frequently used for sheet-metal fabrication for its high-temperature corrosion resistance.

√ Mechanical Properties

√ Manufacturing Processes Performance

3. 316 Stainless Steel (Annealed)

This nickel-rich alloy offers better corrosion resistance than 303 and 304, making it a good option for the chemical industry.

Its mechanical properties are better than 304. 316 stainless steel has fair machinability and excellent weldability. Its ductility, combined with good mechanical properties and wear resistance makes it a good option for sheet metal fabrication.

√ Mechanical Properties

√ Manufacturing Processes Performance

4. 405 Stainless Steel

Being part of the ferritic category, AISI 405 has a regular corrosion resistance because of its relatively low chromium concentration ranging from 11,5 to 14%. It is used in steam nozzles, annealing boxes, quenching racks, and other applications in which the environment is not extremely corrosive, and the material is required to be used in the “as-welded” condition.

AISI 405 is not the strongest grade, but it has a good impact and wears resistance. Even though it has excellent weldability, oxyacetylene welding is not suitable for this grade.

√ Mechanical Properties

√ Manufacturing Processes Performance

5. 420F Stainless Steel

Having a chromium concentration of 13%, this low carbon stainless steel has average strength and good corrosion resistance. The presence of sulfur in the alloy increases its machinability making it suitable for many applications like gears, shafts, bearings, and tools.

√ Mechanical Properties

√ Manufacturing Processes Performance

6. 430 Stainless Steel

With a composition of 79% iron, 16 to 18% chromium, and small amounts of carbon, manganese, and silicon, the AISI 430 is one of the most popular stainless steel alloys. It can’t be age-hardened and is commonly provided in bars due to its acceptable machinability.

This alloy is strong and has excellent corrosion resistance, and since it can withstand being exposed to nitric and other organic acids, it is widely used in the chemical industry. Other applications include dishwashers, fasteners, pipes, and the automotive and construction industry.

√ Mechanical Properties

√ Manufacturing Processes Performance

7. 440C Stainless Steel

This martensitic stainless steel has a carbon concentration ranging from 0,6 to 0,7%, chromium ranging from 16 to 18%, between 78,4 and 83,4% of iron and is also alloyed using small sulfur and silicon concentrations. It can be heat-treated, making it one of the strongest stainless steels.

440C alloy is used to make surgical knives, cutting tools, bearings and, razor blades due to its impressive strength and wear resistance.

√ Mechanical Properties

√ Manufacturing Processes Performance

8. Carlson 2205 Stainless Steel

This duplex alloy has between 22 and 23% chromium, providing an even higher corrosion resistance than other alloys. The 50/50 concentration of ferrite and austenite gives it an excellent cracking resistance when cold formed.

To improve its erosion resistance, it is alloyed with nickel, molybdenum, and nitrogen. It is used in the transportation industry, piping in the oil and naval industries, chemical processing, and in the power industry for heat-exchangers and coolers.

√ Mechanical Properties

√ Manufacturing Processes Performance

Surface finishes help to further improve stainless steel’s inherent corrosion resistance. Removing free iron from the surface of the material prevents the surface from rusting. The most common surface finishes applied to steel are presented below:

1. Acid Solution Treatment

Acid Solution Treatment also is known as passivation, this process intends to remove any impurity from the product’s surface. These impurities could generate rust in the material which may lead to erosion and a decrease in its corrosion resistance.

2. Sandblasting

This technique consists of bursting small sand particles against the material’s surface. The friction generated removes any stain or rust from the surface. It is also commonly used to remove a previous surface finish or to prepare the surface for a painting process.

3. Polishing

Nothing better than a polished surface to improve the appearance of stainless steel. There are two methods frequently used: Belt-polishing and Electro-polishing. Both processes improve the resistance to corrosion and give a better, mirror-like finish to the surface.

4. Black Oxide

This process is achieved by immersing the oil or wax covered product in a high-temperature solution. The chemical reaction occurring creates a black layer that improves corrosion resistance. This coating is excellent for outdoor applications because it diminishes the stainless steel reflexivity.

Stainless steel is an excellent material, it combines high structural strength with its characteristic corrosion resistance, providing a durable product that can be used in even the most extreme environments. Here are some tips to help you out in your stainless steel project:

1.  Use ferritic or duplex stainless steel for welding applications.

2. When used for in-field applications, applying paint or a black oxide coating reduces its reflectivity, making sure that it is comfortable for the eyes while enhancing its corrosion resistance.

3. Stainless steel has a very good appearance, making it suitable for decorative applications.

4. Its corrosion resistance and mirror-like surface make stainless steel a good option for a pool railing or stairs.

5. For CNC machining applications, it is important to use a stainless steel grade containing at least 1% Sulfur.

6. Its inherent corrosion resistance makes it a very economic option, saving both time and money.