Difference between Stainless Steel, Carbon Steel, and Alloy Steel

6-Aug-2020

The basic difference between stainless steel, conventional alloy steel and carbon steel is that Stainless Steel contains a very high percentage of chromium (11 – 26 percent) and nickel (3.5 – 22 percent).

Through varying chromium content and by addition, substitution of other alloying elements like nickel, molybdenum, copper, titanium, aluminium, silicon, niobium, sulfur, selenium etc., resistance to corrosion, oxidation, abrasion, hardness and a variety of other distinct properties are either created or enhanced.

On the other hand, steel that contains carbon up to about 1.7 percent as an essential alloying constituent and has properties and structure made up mostly of the element carbon is better known as Carbon Steel. Most of the steel produced in the world is carbon steel. Unprotected carbon steel rusts when exposed to air or moisture but stainless steel is almost immune to rusting and ordinary corrosion.

WHAT IS CARBON STEEL

Carbon steels are the base metals widely used in manufacturing in nearly every industry, including aerospace, aircraft, automotive, chemical, defense, and precision. Carbon steel’s strength is due to its crystalline structure. Groups of iron and carbon atoms are arranged in a lattice, with the carbon atoms preventing the iron atoms from slipping over each other, which imparts the steel more rigidity.

The addition of an alloy such as titanium or manganese strengthens this structure by adding different atomic sizes to the lattice. This reinforces steel's rigidity by further impeding molecular movement when the metal is subjected to stresses. There are four types of carbon steel based on the amount of carbon present in the alloy.

Stainless Steel Pipes, Board, Kitchen

Low Carbon Steel – Composition of 0.05%-0.29% carbon and up to 0.4% manganese. They are the most common form of steel commonly known as mild steel, a relatively low-cost material, easy to shape (malleable). Low carbon steels provide material properties that are acceptable for many applications.

Medium Carbon Steel – Composition of 0.29%-0.54% carbon, with 0.60%-1.65% manganese. Medium carbon steel is ductile, strong and has good wear resistance. Because of the above properties, they find use in forging, heavy industries, and automotive components.

High Carbon Steel – Composition of 0.55%-0.99% carbon, with 0.30%-0.90% manganese. It is very strong and holds shape memory well, making it ideal for springs and high-strength wires.

Ultra High or Very High Carbon Steel - Composition of 1%-2.1% carbon. Its high carbon content makes it an extremely strong material. Due to its brittleness, this grade requires special handling. However, ultra high carbon steels can be tempered to great hardness and are used for specialized products such as knives, axles etc. Tighter carbon content control for more consistent heat treatment. Steels with carbon content above 2% are considered to be cast iron.

WHAT IS ALLOY STEEL

Stainless Steel utilities

Alloy Steel is an iron based mixture containing manganese greater than 1.65%, silicon over 0.5%, copper above 0.6%, or other minimum quantities of different alloying elements such as chromium, nickel, molybdenum, or tungsten are present, each of which imparts different properties to alloy steel. Alloy Steels are made by combining elements during the smelting process when the iron is still molten. Chromium is added in smaller amounts (0.5-2%) to increase hardenability and larger amounts (4-18%) to increase corrosion resistance. Molybdenum is added in amounts of 0.25-0.40% to increase the strength of the steel. Nickel is added in smaller amounts (2-5%) to increase toughness and in larger amounts (12-20%) to increase corrosion resistance. Silicon is added to steel in smaller amounts (0.2-0.7%) to increase strength and in larger amounts (>2%) to improve its magnetic properties while addition of Sulfur or Lead is done to increase the weldability. 

WHAT IS STAINLESS STEEL

Stainless steel is an alloy developed in the early 1900’s after metallurgists discovered that chromium added iron alloys displayed superior corrosion resistance to carbon steel alloys. The first products using stainless steel were produced in 1908 and the first patents were granted in 1912.

Stainless Steel is a highly durable alloy containing the following major ingredients:

• 10 - 30 per cent of chromium by mass giving excellent corrosion and oxidation resistance to it.
• Rest about more than 50 percent iron.

The corrosion resistance that is unique to stainless steel is the result of a transparent passive film of chromium oxide forms on the surface of the steel and protects it from oxidation. Higher chromium levels increase the corrosion resistance of the steel, but it creases the brittleness of the metal, making it hard to work with. There are different categories and types stainless steels. To know more please refer to -

• Categories and Types of Stainless Steel: Compositions and Properties
• Grades and Series of Stainless Steel - Compositions and Uses

Categories and Types of Stainless Steels: Compositions and Properties

04-Aug-2020

The article is about an alloy, a material without which perhaps it won’t be possible to pass a single day in our life. Yes, we are talking about stainless steel and as the title suggests the key points that will be described in short here (this is the 1^st Part of this article):

• What is stainless steel (alloy steel);
• Metallurgical classification of stainless steels into various types and categories;
• Stainless steel composition
• Important properties of the various categories of stainless steel, and their specific uses  

Types and Categories of Stainless steel

Stainless Steel is a highly durable alloy of iron containing about 10 – 30 per cent chromium, 0.3 – 1.0 percent carbon and some other metals as minor constituents, possessing excellent oxidation, corrosion and fatigue resistance properties. Stainless steel has also an aesthetic appeal, excellent lusture, low wear, high strength and durability.

Also Read: Difference between Stainless Steel, Carbon Steel, and Alloy Steel

CLASSIFICATIONS – What are the different grades of stainless steel

Stainless Steels are usually grouped into 5 metallurgical categories (grades):

1. Austenitic Stainless Steels
2. Ferritic Stainless Steels
3. Martensitic Stainless Steels
4. Duplex Stainless Steels
5. Precipitation hardening Stainless Steels

Austenitic Stainless Steels (Compositions, Properties)

These steels have an austenitic crystal structure formed through the use of austenitizing elements like nickel, manganese and nitrogen. These steels have austenitic structure from room temperature to temperature below melting range and hence, can not be hardened by heat treatment.

Austenitic Stainless Steels possess the highest corrosion resistance of all Stainless Steels and also have the greatest strength and resistance at high temperatures. These steels are capable of retaining ductility at temperatures as low as zero degree.

Compositions of Austenitic Stainless Steels:

• Chromium: 16 – 26%
• Nickel: 4 – 22%
• Carbon: 0.03 – 0.25%
• Manganese: 2 – 10%
• Silicon: 1 – 2%

The purpose of adding nickel in austenitic grades is to increase density, co-efficient of thermal expansion and also corrosion resistance. Austenitic Stainless Steels have superior impact strength and toughness as compared to those of Ferritic Stainless Steels. The addition of manganese (Manganese Nickel Austenitic Stainless Steels) helps in achieving even higher strength than only nickel bearing grades over a wide range of temperatures. The yield strength also increases by nearly 40 percent while offering greater resistance to stress corrosion cracking carbide precipitation and pitting.     

Ferritic Stainless Steels (Compositions, Properties)

Ferritic Stainless Steels are basically chromium based alloys and are generally ferritic at all temperatures although some of the grades exhibit austenitic structure at high temperatures and can transform to Martensite.

Compositions of Ferritic Stainless Steels:

• Chromium: 11 – 26%
• Carbon: 0.08 – 20%
• Manganese: 1 – 1.5%
• Silicon: 1 – 2%

In annealed condition, these steels show fully ferritic structure at room temperature. Ferritic Stainless Steels are not strengthened by heat treatment. In annealed condition, Ferritic Stainless Steels develop maximum softness, ductility and corrosion resistance.

Ferritic Stainless Steels are ferro-magnetic, ductile and malleable. But at elevated temperatures mechanical properties of Ferritic Stainless Steels are relatively inferior to the Austenitic Stainless Steels. Ferritic stainless steels are more corrosion resistance than Martensitic Stainless Steels. 

Continued to 2nd Part: Grades and Series of Stainless Steels - Compositions and Uses