Types of Stainless steel & their properties

Types of Stainless steel

Features of stainless steel

By the addition of chromium (Cr) to iron (Fe), iron becomes immune to corrosion in the atmosphere. When the Cr content rises to 11~12% or more, the corrosion resistance of steel becomes very high.

Hence, the steel with such a high amount of Chromium is called stainless steel, here “stainless” means ‘free from stain of rust’.

Why Chromium is added to stainless steel

The reason why stainless steel has good corrosion resistance is that Cr in it, is get oxidized in the air and forms a protective layer as “passive film” on its exposed surface as shown in figure 1.

Depending on the environmental conditions in which stainless steel is planned to use, Cr content is increased, and Ni and other elements are also added to the steel to get different types of corrosion resistant properties.

However, since its corrosion resistance is provided principally with Cr, Cr is an essential element for stainless steel.  Generally stainless steels contain minimum 10.5% Cr. The AWS Welding Handbook (Vol. 4) defines stainless steels as “alloy steels with a nominal Cr content of at least 11%, with or without other alloy additions.”

Stainless steel is highly heat resistant as well as corrosion resistant and thus its use is versatile, from household products to chemical equipment, oil & gas, aerospace, food processing machines, architectural materials and nuclear power equipment etc.

Why Nickel is added to stainless steel

Nickel makes stainless steels austenitic, and counts for the austenitic grade’s excellent properties, namely weldability, formability and toughness etc. Addition of Nickel is not mandatory in Ferrite and austenitic steels.

Various types of stainless steel

Stainless steel can be divided into

  1. Cr stainless steel and
  2. Cr−Ni stainless steel.

These two grades can further be classified based on their metallographic structure as shown in Fig. 2. Cr stainless steel can be divided into martensitic stainless steel and ferritic stainless steel, and Cr−Ni stainless steel can be divided into austenitic stainless steel, austenite−ferrite stainless steel (duplex stainless steel) and precipitation−hardening stainless steel.

Ferritic stainless steel

1. Ferritic stainless steel- are Chromium based with small amounts of Carbon usually maximum 0.10%. They have a ferritic microstructure like to carbon and low alloy steels. Ferritic stainless steels are limited in use to relatively thin sections due to lack of toughness in welds. However, where welding is not required, they offer a wide range of applications. They cannot be hardened by heat treatment. High Chromium steels with additions of Molybdenum can be used in quite aggressive conditions such as sea water. Ferritic steels are also chosen for their resistance to stress corrosion cracking. They are not as formable as austenitic stainless steels, but they are magnetic. Ferritic stainless steels along with Duplex stainless steel are prone to 885 °F 475 °C Embrittlement. As compared with martensitic stainless steel, its corrosion resistance is better and even resistant to nitric acid (HNO3) because its Cr content is higher.

     Table 1 shows typical grades of ferritic stainless steel.

Austenitic stainless steel

2. Austenitic stainless steel- steels are the most common. Their stable microstructure is obtained from the addition of Nickel, Manganese and Nitrogen. It is the same structure as occurs in ordinary steels at much higher temperatures i.e. above lower transformation temperature. This structure gives these steels their characteristic combination of weldability and formability. Continue reading this article for detailed information on the Austenitic Stainless Steels. The commonest grade of austenitic stainless steel is SS304 or AISI 304 (18%Cr−8%Ni). SS316 or AISI 316 (18%Cr−12%Ni−2%Mo) offers better corrosion resistance, which is also used widely.

As austenitic stainless steel offers good corrosion resistance, workability, mechanical properties and weldability, it is widely used for fabrication of storage tanks, heat exchangers, wastewater treatment facilities, kitchen utensils, bath tubs, sinks, and so on.

Table II shows typical grades of austenitic stainless steel.

Martensitic stainless steel

3. Martensitic stainless steel- These steels are similar to ferritic steels having primary alloying element as Chromium but have high Carbon contents as high as 1% to suit various metallurgical properties. This allows martensitic stainless steel to be hardened and tempered much like carbon and low-alloy steels. They are used when high strength and moderate corrosion resistance is required. They have generally low weldability and formability. They are magnetic in nature and can be differentiated from austenitic stainless steel with this property.

A typical grade of martensitic stainless steel is SS410 (AISI 410) (See Table 3.). It contains 13%Cr and its metallographic structure is martensitic at room temperature that is hard and brittle. Though good mechanical properties can be obtained with this grade of steel by appropriate heat treatment (tempering), it is considered to be inferior to other grades of stainless steel in corrosion resistance because its Cr content is low.

Uses of martensitic stainless steel are for turbine blades, valves and shafts that require high strength, and abrasion and heat resistance.

Duplex Stainless Steel

4. Duplex Stainless Steel– These steels have a microstructure which is approximately 50% ferritic and 50% austenitic, this gives them a higher strength than either ferritic or austenitic steels. They are resistant to stress corrosion cracking. The “lean duplex” steels are formulated to have comparable corrosion resistance to standard austenitic steels but with enhanced strength and resistance to stress corrosion cracking. Super Duplex steels have enhanced strength and resistance to all forms of corrosion compared to standard austenitic steels. They are weldable but need care in selection of welding consumables and heat input. They have moderate formability. They are magnetic but not so much as the ferritic, martensitic and PH grades due to the 50% austenitic phase.

Precipitation hardening (PH) Stainless Steel

5. Precipitation hardening (PH) Stainless Steel– These steels can develop very high strength by adding elements such as Copper, Niobium and Aluminium to the steel. Following a suitable “aging” heat treatment, very fine particles form in the matrix of the steel which imparts strength to the alloy. These steels can be machined to quite intricate shapes requiring good tolerances before the final aging treatment as there is minimal distortion from the final treatment makes them suitable for shaft material for aerospace, oil & gas etc.  Corrosion resistance of PH is comparable to standard austenitic steels like 1.4301 (SS304).

Physical properties of stainless steel

Table 6 shows a comparison of physical properties between stainless steel and carbon steel.

Caution is required in welding of stainless steels as there are big differences in physical properties between stainless steel and carbon steel, which affects weldability directly or indirectly.

For instance, though the thermal expansion coefficient of martensitic and ferritic stainless steel is almost the same as that of carbon steel, that of austenitic stainless steel is 1.5 times as much as that of carbon steel. This indicates that deformation and strain become considerably big in welding of austenitic stainless steel than in welding of carbon steel.

Further, if a weld joint that consists of austenitic stainless steel and carbon steel is subjected to thermal cycles, there arise thermal stresses due to difference of the thermal expansion coefficient between the two materials. So, it is a problem to use a weld joint of dissimilar metals including austenitic stainless steel in an environment where temperature changes cyclically.

Still further, as electric resistance of stainless steel is much higher than that of carbon steel, electrode burn tends to occur with stainless steel covered electrodes in shielded metal arc welding. Therefore, the proper welding currents are lower than those for carbon steel electrodes.

Martensitic and ferritic stainless steels are ferromagnetic while austenitic stainless steel is normally nonmagnetic.

However, there are many cases in which austenitic stainless-steel weld metal is designed to contain some ferritic structure; in such cases, it possesses some extent of magnetism.

Existence or non−existence of magnetism is useful for rough judgment of steel grade in relation to welding procedure. For instance, preheating is not applied to non−magnetic stainless steel, but preheating is effective to magnetic stainless steel in many cases.

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