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A Comprehensive Exploration of Stainless Steel Families

In our previous blog discussion, we looked at how iron-based alloys with a minimum of 10.5% to 30% chromium make up stainless steel. We know that chromium plays a crucial role in producing the distinctive quality of "stainlessness" or corrosion resistance. No matter how much of the surface is removed, the steel is corrosion resistant due to the oxide layer’s capacity to heal itself. This is not the case when organic coatings like paint or metallic coatings like zinc or cadmium shield carbon or low alloy steel against corrosion.

However, the story of stainless steel doesn’t end with chromium alone, other alloying elements are frequently added to improve their qualities. The classification of stainless steel is unique among metals which depends on their metallurgical structure, which refers to the arrangement of the atoms that make up the grains of the steel. The stainless steel family tree is divided into several branches that can be distinguished in a variety of ways, such as their areas of application, the alloying elements used in their manufacture, or, perhaps most accurately, the metallurgical phases present in their microscopic structures, in this blog we will understand types of stainless steel families and the relationship between them.

The Families Of Stainless Steel

Depending on the particular chemical composition of the steel, the microstructure may be composed of the stable phases austenite or ferrite, a duplex mix of these two, the phase martensite formed when some steels are rapidly quenched from high temperatures, or a combination of these two. 

Ferritic Stainless Steel

Ferritic Stainless Steel (for example, grades 1.4512 and 1.4016) are made up of chromium and iron (usually 12.5% or 17%). Nickel is basically absent from ferritic stainless steels. These materials have very little carbon and are not heat treatable, yet they have superior corrosion resistance to martensitic stainless steels and good oxidation resistance. They are ferromagnetic and, while undergoing an impact transition (i.e. becoming brittle) at low temperatures, have adequate formability. Their thermal expansion and other thermal properties are comparable to those of traditional steel. Ferritic stainless steels are easily welded in thin parts, but when welded in thicker areas, grain development occurs, resulting in a loss of characteristics.

Martensitic Stainless Steel 

Carbon (0.2-1.0%), chromium (10.5-18%), and iron make up martensitic stainless steels (for example, grades 1.4006, 1.4028, and 1.4112). These materials can be heat treated in the same way as ordinary steels to create a variety of mechanical qualities, but they are harder and have different heat treatment temperatures. Their corrosion resistance is moderate (i.e., it is lower than that of other stainless steel with the same chromium and alloy concentration). They are ferromagnetic, have a low-temperature impact transition, and have poor formability. Their thermal expansion and other thermal properties are comparable to those of traditional steels. They can be welded with care, however, cracking can occur if matching filler metals are employed. 

Austenitic Stainless Steel

Austenitic Stainless Steel (for example, grades 1.4301 and 1.4833) are composed of chromium (16-26%), nickel (6-12%), and iron. Other alloying elements (for example, molybdenum) may be added or adjusted to produce derivative grades described in standards (for example, 1.4404). The austenitic group has more grades that are widely used than any other type of stainless steel. Austenitic stainless steels outperform ferritic and martensitic stainless steels in corrosion resistance. Corrosion performance can be tailored to a variety of service situations through careful alloy adjustment, such as changing the carbon or molybdenum percentage. Work-hardening is used to strengthen materials that cannot be hardened by heat treatment.

Austenitic stainless steels, unlike ferritic and martensitic grades, do not have a yield point. They have exceptional formability, and their deformation response may be regulated by chemical composition. They are resistant to cryogenic temperatures and do not undergo an impact transition at low temperatures. They have more thermal expansion and heat capacity than other stainless or traditional steels while having lower thermal conductivity. They are generally easily welded, although, for more heavily alloyed types, attention must be taken in the selection of consumables and techniques. Austenitic stainless steels are frequently classified as non-magnetic, although when machined or treated, they can become mildly magnetic.

Duplex Stainless Steel

Duplex Stainless Steel (for example, grade S31803) are composed of chromium (18-26%), nickel (4-7%), molybdenum (0-4%), copper, and iron. These stainless steels have a microstructure composed of austenite and ferrite, which gives a mix of austenitic stainless steel corrosion resistance and increased strength. Duplex stainless steels can be welded, but the correct proportion of austenite and ferrite must be maintained. They are ferromagnetic and, at low temperatures, undergo an impact transition. Their thermal expansion is intermediate between austenitic and ferritic stainless steels, and their other thermal properties are comparable to plain carbon steels. Formability is acceptable, but larger forces are required than for austenitic stainless steels.

Which applications utilize the various families of stainless steel?

The different families of stainless steel find extensive uses and applications across various industries. Ferritic stainless steel is commonly employed in automotive exhaust systems, architectural structures, and kitchen appliances due to its superior corrosion resistance. Martensitic stainless steel is valued in cutlery, surgical instruments, and turbine blades for its hardness and strength. Austenitic stainless steel, the most widely used family, is found in food processing, pharmaceuticals, chemical plants, and construction, owing to its excellent corrosion resistance and formability. Duplex stainless steel, with its combination of strength and corrosion resistance, is utilized in the oil and gas, marine, and chemical processing industries. Understanding and considering the families and their specific grades is crucial for these industries to ensure material suitability, performance, and longevity in their applications. By selecting the right stainless steel family and grade, industries can achieve optimal results, cost-effectiveness, and durability in their products and processes.

In conclusion, Stainless steel is a superior material renowned for its exceptional properties and wide-ranging applications. The quality and performance of stainless steel are greatly influenced by the specific grades within each family, which are carefully engineered to enhance its characteristics. Metalbook plays a vital role in simplifying the procurement process, connecting buyers with reliable suppliers and providing a seamless sourcing experience. With the combined power of advanced stainless steel grades and efficient procurement solutions, industries can continue to harness the exceptional qualities of stainless steel and drive innovation forward.

Pragati Tiwari