Stainless Steel, also called Inox, Corrosion Resistant Steel (CRES), or Rustless Steel, is an alloys of iron that are resistant to rust and corrosion.

The composition of stainless steel is as follows:

· Minimum 10.5 % chromium and, in most cases, nickel.

• 0.2 % – 2.11 % carbon.

The corrosion resistance of stainless steel is due to the presence of chromium in stainless steel. Chromium creates a passive film on the surface of stainless steel that protects it from corrosion and allows it to heal on its own when exposed to oxygen.

The properties of the alloy, such as its lustre and corrosion resistance, allow it to be used in a wide variety of applications. Stainless steel is rolled into sheet, plate, bar, wire and tubing.

Stainless steel is used in a variety of applications, including:

Cookware Cutlery Surgical instruments Major Appliances Vehicles Construction material Large buildings Industrial equipment Paper mills Chemical plants Water treatment Storage Tanks and Tankers Chemical and food products

Each type of stainless steel is identified by a three-digit AISI number (ISO 15510). The chemical composition of stainless steels according to ISO 15510 is listed in a helpful interchange table below. ISO 15510 lists the chemical composition of stainless steel according to ISO specifications, ASTM specifications, EN specifications, JIS specifications, and GB specifications.

Properties of Stainless steel

CORROSION

Stainless steel can rust, but only on the outer few strata. Its chromium content protects the deeper strata from oxidation. The addition of nitrogen increases corrosion resistance and mechanical strength. There are many types of stainless steel, each with different levels of chromium (11.7%), nickel (8.5%), and moly (10.5%). The alloy must withstand the environment it is made for, so there are different levels of moly. To increase corrosion resistance, increase chromium to over 11.0%, increase nickel to 8.5%, and increase moly to 10.5%, also increasing corrosion resistance.

DURABILITY

Stainless steel with an annealed tensile yield strength of 30,000 psi is the most common type, 304 stainless steel has a strength of 210,000 psi in an annealed state and increases to 153,000 psi in a full-hard state by cold working.

Stainless steels with a precipitation hardening are 17-4 PH stainless steel and Custom 465 stainless steel. These are precipitation hardening stainless steels that can be heat treated up to a tensile yield of 251,000 psi or 1,730 MPa.

MELTING POINT

As a steel, stainless steel has a melting point close to that of normal steel and much higher than aluminium or copper. Like most alloys, stainless steel’s melting point is expressed in a range of temperatures rather than a single temperature. The temperature of stainless steel varies depending on the consistency of the alloy it is alloyed with.

Stainless steel has a temperature range of 1,400–1,530°F (2,550–2,790°C; 1,670–1,800°–3,010–3,250°R)[10].

CONDUCTIVITY

Stainless steel has a melting point close to that of normal steel and much higher than aluminium or copper. Like most alloys, stainless steel’s melting point is expressed in a range of temperatures rather than a single temperature. The temperature of stainless steel varies depending on the consistency of the alloy it is made from.

Stainless has a melting point of 1,400–1,530 °C. This temperature ranges from 2,550–2,790 °F (1,670–1,800 K–3,010–3,250 °R)

MAGNETISM

Stainless steel is magnetic in Martensite, Duplex and Ferritic stainless steel, but is not magnetic in austenite stainless steel. Ferritic stainless steel owes its magnetic properties to its body centered cubic crystal structure. Ferritic steel has iron atoms arranged in cubes, one at each corner and one at the center. The iron atom at the center is responsible for the magnetic properties of ferritic steel. This arrangement limits the absorbance of carbon in ferritic stainless steel to about 0.025%.

Ferritic stainless steel grades with low coercivity field have been used in electro-valve applications in household appliances, and in injection systems in combustion engines.

Stainless steel that is not magnetic can be slightly magnetic by work hardening it. In some cases, the edge of austenite stainless is magnetized when the crystal structure re-arranges itself.

Some austenite stainless steel grades are magnetically permeable after 2 hours of annealing at a temperature of 1050 °C.[16] EN grade 1.4307, 1.4301, 1.4404, 1.4435, μ 1.056, 1.011, 1.100, 1.000

WEAR

Galling (sometimes called cold welding) is a type of serious adhesive wear that occurs when two moving metal surfaces come into contact and come into contact with each other under high pressure.

Thread galling is most common on stainless steel stainless fasteners. Other alloys that produce a protective oxide film on the surface of the metal, like aluminum and titanium are also susceptible to galling.

When this oxide is deformed, it can be broken, and it can be removed from the component, leaving the exposed reactive metal.

If the exposed surfaces are both of the same type, they can easily fuse together.

Separation of the surfaces can cause surface tearing and may even lead to complete seizure of the metal component or fastener.

Galling can be reduced by using different materials (for example, bronze for stainless steel vs. other stainless steels) or by lubricating threaded joints (for example, martensitic for austenitic vs. nitronic 60).

DENSITY

Depending on the alloy, the density of stainless steel can range from 7,500 kilograms per cubic meter to 8,000 kilograms per cubic meter.

DENSITIES                                                      STAINLESS STEEL

                         STAINLESS STEEL TYPES

There are five main types of stainless steel and each type is distinguished by its crystalline structure:

• austenitic
• ferritic
• martensitic
• duplex
• precipitation hardening

AUSTENITIC

stainless steel is not hardenable through heat treatment due to its face centered cubic crystal structure and the maintenance of its microstructure at all temperatures. However, the development of metastable stainless steel (M-ASS) through cold-forming at cryogenic temperatures is important1. M-ASS offers superior cryogenic toughness, ductility, strength, corrosion resistance, and cost-effectiveness, making it suitable for cryogenic pressure vessel (CPV) production1.

In addition to M-ASS, there are different types of austenitic stainless steel commonly used. The 200 series alloys, such as Type 201 and Type 202, use most of the manganese and nickel while minimizing the need for nickel. These alloys have approximately 50% more yield strength than the 300 series stainless sheets1. Type 304 is the most common type of stainless steel, also known as Type 18/8 or Type 18/10, which refers to its composition of 18% chromium, 8% nickel, and 10% manganese1. Type 316, on the other hand, is the second most common type and is known for its higher corrosion resistance and chloride ion resistance1. There are also highly alloyed stainless steel grades, such as Super Duplex stainless steel and precipitation-hardening stainless steels, which are used in specific applications that require extreme conditions1.

Please note that further research may be needed to provide detailed information about the properties and applications of different types of austenitic stainless steel and highly alloyed stainless steel grades.

FERRITIC

Ferrite stainless steels have a body centered cubic crystal structure like carbon steel. They contain 10.5-27% chromium and very little nickel. The microstructure of ferrite steels is present at any temperature due to the addition of chromium. These steels cannot be hardened by cold work and are magnetic. The addition of Niobium(Nb), Titanium(Ti), and Zirconium(Zr) on type 430 allows good weldability, and due to the near absence of nickel, ferrite steels are less expensive and are used in many products.

Ferrite steels are used in automotive exhaust pipes in North America (type 409) and Europe (type 439/441).

Type 430 ferrite steels contain 17-22% chromium and are used in architectural and structural applications.

Types of ferrite steel used in:

Slate hooks

Rooftop roofing

Chimney ducts

MARTENSITIC

A martensitic stainless steel has a body centered cubic crystal structure and offers a broad range of properties. It is used as a stainless engineering steel, a stainless tool steel, and a creep-resistant steel. It is magnetic and is not as corrosion resistant as ferritic steels or austenitic steels because of its low chrome content.

There are four types of martensitic steels, with some overlap between them. These are:

Fe (Fe-Cr)-C (Fe-C-C grades)

These were the first types of martenitic steels and are still used in many engineering and wear resistant applications.

Fe (Cr)-Ni-C (Nickel-based grades)

Some carbon is substituted for nickel-based grades.

Grade EN 1.4342 (Casting Grade CA6NM, containing 13% Cr + 4% Ni, is used in most Pelton (Pelton, Kaplan) and Francis (Hydroelectric) turbines[57] due to its good casting properties and good weldability as well as its good resistance against cavitation erosion (high strength, good toughness, good corrosion resistance, and good corrosion resistance.

Grade 1.45

Creep-resistant grades are made with small amounts of Niobium, Vanadium, Boron, and Cobalt to increase strength and resistance to creep up to 650 °C.

Martensite stainless steels are heat treated to improve mechanical properties. Heat treatment typically takes place in three steps:

1. Austenitizing: steels are heated to a range of 980-1,050°C (1:800-1,920°F) depending on the grade.
2. Quenching: steels are cooled to around 500°C (930°F) and held at that temperature before air-cooling.
3. Martenite transformation: steels are transformed into martenite, which has a face centered cubic crystal structure and is very hard.
4. Quenched martenite is very brittle and some of the remaining austenite remains.
5. High tempering temperatures reduce yield strength and final tensile strength, but increase elongation resistance and impact resistance.

Nitrogen replaces a small amount of carbon in martensite stainless steels.[when?] The solubility of the nitrogen is limited by the process of PESR (Pressure Electroslag Refining) in which the steels are melted under high nitrogen pressure, resulting in a steel containing up to 0,4% nitrogen. This results in higher hardness and strength, as well as higher corrosion resistance. Due to the high cost of PESR, a lower but significant nitrogen content has been achieved using the Standard AOD process.

DUPLEX

Duplex stainless steel has a 50:50 ratio of austenite to ferrite in its microstructure. Commercial alloys may have a 40:60 ratio. Duplex steels have a higher chromium content than austenitic steels (19–32%), a lower nickel content than austenite steels (up to 5%), and overall a lower alloy content than comparable-performing super-astenitic grades.

Duplex steels have approximately twice the yield strength as austenite stainless steel. Their 50:50 microstructure provides better resistance against chloride stress corrosion cracking than types 304 and 316 of the same material. Duplex grades can be broken down into three sub-groupings based on corrosion resistance: the lean duplex grade, the standard duplex grade, and the super duplex grade. Duplex stainless steel has been extensively used in the pulp and paper industry.

Nowadays, oil and gas is the biggest user and has been pushing for more corrosion-resistant grades, resulting in the creation of super-duplex and hyper-duplex grades.

More recently, the low-cost (and slightly lower-corrosion-resistance) lean-duplex grade has been developed and is mainly used in structural applications in the building and construction industry (concrete reinforcement bars and plates for bridges and coastal works) as well as the water industry.

PRECIPITATION HARDENING

Precipitation hardened stainless steels have the same corrosion resistance as austenitic varieties but can be precipitated hardened to even greater strengths. There are three main types of precipitation hardened stainless steels:

Martensite 17-4 HP (AISI: 630 EN 1,4542) contains approximately 17 % Cr, 4 % Ni, 4 % Cu and 0.3 % Nb. Solution treatment at approximately 1,040 C (1,900 F) and quenching produces a relatively ductile Martensite structure. Following aging treatment at 475 C (887 °F), Nb and Cu rich phases are precipitated, resulting in an outstanding strength level of up to 1000 MPa yield. This excellent strength level is widely used in high tech applications such as aerospace, usually after remelting (removal of non-metallurgical inclusions to increase fatigue life). Another significant benefit of this steel is the aging process, unlike tempering, is at a temperature which can be applied (nearly) to finished parts without distortion or discoloration.

Semi-Austenite 17-7 Ph (AISI: 631 EN:4568) contains 17% Cr; 7% Ni; and 1.3% Al. Typical heat treatment consists of solution treatment followed by quenching. The structure remains austenite at this point. Martensite transformation is achieved either by cryogenic treatment at a temperature of −75 °C ( −103 °F). Severe cold work (more than 70% deformation) is carried out (cold rolling or wire drawing) on nearly finished parts; yield stress levels exceed 1400 MPa.

An austenite A286[65] (ASTM 660 EN: 4980) contains 15% Cr; 25% Ni; 2.1% Ti; 1.2% Mo; 1.3% V; and 0.005%

The structure is non-flammable at all temperatures. Typical heat treatment includes solution treatment, quenching, and aging at 715 C (1,319 F). Aging produces Ni3Ti precipitate, increasing the yield strength to approximately 650 MPa at room temperature (94 ksi).

Mechanical properties and creep resistance are excellent at 700 C (1,300 F).

A286 is classified as a Fe based superalloy, which is used in jet engines and gas turbines, as well as turbo parts.