Stainless and acid resistant steel

Controlling the process, knowing the elements

Mastering stainless steel

 
Stainless steel is a large group of alloyed steel types in which the main element is iron (>50 %), contains at least 10,5 % Chromium (Cr) and Carbon content of max 1,2 %, with or without other alloying elements.

Chromium is an essential element as it forms a passive chromium oxide layer (Cr2O3), which prevents the material from corroding. Passivation only occurs if oxygen is present and chromium content is high enough.

Stainless steel

Figure 1: The Principle of stainless steel

Chromium oxide

Figure 2: If the metal is cut or scratched and the passive film is disrupted, more oxide will quickly form and recover the exposed surface, protecting it.

 


Getting the perfect alloy

 
Commonly, stainless steel is divided into groups depending on specific amounts of alloying elements, which controls the microstructure and properties of the alloy. The orientation of atoms determines which crystal structure or phase the metal possess. Most common phases in stainless steels are ferrite, austenite, and martensite. Some elements promote the formation of ferrite and some of the austenite and how the elements are mixed affect the properties of the steel.

 
The most common alloying elements in stainless steel:

Chromium (Cr)

Chromium (Cr): Promotes ferrite and is one of the main alloying elements. Essential for corrosion resistance.

 

Nickel (Ni)

Nickel (Ni): A strong stabilizer of the austenitic microstructure. Enhances corrosion and creep resistance.

 

Molybdenum (Mo)

Molybdenum (Mo): Promotes ferrite and has a strong positive effect on the corrosion resistance.

Carbon (C)

Carbon (C): Promotes austenite, has in some grades a positive effect on mechanical strength. Most modern stainless steel keeps the carbon level as low as possible to reduce the risk of intergranular corrosion.

 

Other elements, such as nitrogen, copper, silicon, aluminum, titanium, and cobalt are also used in small quantities in some stainless steel.

 

Bulten - Periodic Table of Elements

 

The main groups of stainless steel

 
Ferritic: The go-to solution for low-cost applications
Today, ferritic stainless steel is mainly used for low-cost applications as the grades have no or minimal costly nickel additions. There are of course exceptions, and ferritic stainless steel is also utilized due to its excellent magnetic properties and low thermal expansion in applications such as magnetic valves. Special ferritic stainless steel with aluminum addition has excellent high-temperature resistance. It has however limited usage for fasteners as the strength is meager and most grades have moderate corrosion resistance.

Martensitic: Pedal to the metal for high hardness
Martensitic stainless steel is the only stainless steel that can be hardened by forming martensite during hardening. The group is characterized by fairly poor corrosion resistance and high hardness. Typical application areas are products that require high hardness and wear resistance such as knives, needles, bearings, shafts, etc. Martensitic stainless steel fasteners are mainly used in moderate corrosive environments, due to its high strength and reasonably low price.

Austenitic: Featured in 70% of all stainless steel production
Austenitic stainless steel is unquestionably the largest and most diverse stainless steel group, accounting for approximately 70 % of all stainless steel production. 18/8 (18 % Cr and 8 % Ni) is the largest stainless steel type and widely used in many consumer and industrial products. It is also used in nearly all industrial segments, ranging from low demanding products such as cookware to exceptional super austenitic products which are used in severe corrosive or high-temperature environments in oil/gas, offshore and petrochemical industry. A2 and A4 are common austenitic stainless steel fasteners.