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What Is Bainite?

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  • Written By: Ray Hawk
  • Edited By: E. E. Hubbard
  • Last Modified Date: 08 November 2016
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Bainite is a microstructural crystalline pattern that forms in steel during heating. It is named after Edgar C. Bain, a US metallurgist who worked on the alloying and heat treatment of steel in Pittsburgh, Pennsylvania, in the United States. Bainite is formed when austenite is cooled rapidly. Austenite is a allotrope, or form of iron known as gamma iron, that contains carbon and a cubical lattice structure when between 1,670° to 2,552° Fahrenheit (910° to 1,400° Celsius).

Two unique temperature conditions have to exist for the bainite microstructure to form. Austenite must be cooled rapidly enough so that pearlite does not form. Pearlite is a alternating layered structure in steel of ferrite and cementite that forms when the steel is slowly cooled, and falls below a temperature of 1,341° Fahrenheit (727° Celsius). Cooling in the austenite must also be delayed long enough to prevent martensite from forming. Martensite is a very hard, brittle crystalline byproduct of austenite production.

If the processing of austenite is done correctly and bainitic steel is formed, it displays some of the characteristics of both pearlite and martensite. It possesses some of the extreme hardness of martensite, as well as the tough structure of pearlite. The bainitic microstructure consists of ferrite, like in pearlite, and a minute dispersion of cementite also.

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Uses for bainitic steel varieties are included in the power generation industry because of their unique quality of creep resistance. They are less likely to deform under stress than other steel types. This quality is enhanced by alloying the steel with chromium and molybdenum to increase hardness.

Another variation on bainitic steel manufacturing is to infuse it with nonmetallic particles, which produce a more disorganized microstructure. This is called nucleated bainite, or acicular ferrite, and it has a greater ability to deflect cracks than traditional bainite. Uses for this variety include in large structural applications that undergo frequent stress, such as oil rigs and bridges.

Variations on the types of bainitic steel produced are often categorized as upper bainite or lower bainite. The upper range is produced during the cooling process at a temperature of between 1,022° to 752° Fahrenheit (550° to 400° Celsius) and resembles a form of steel known as Widmanstatten ferrite. Lower bainitic steel is produced at a temperature cooling level of 752° to 482° Fahrenheit (400° to 250° Celsius), where it resembles the acicular morphology. Though lower bainite is not specifically nucleated bainite, it is somewhere between upper bainite and martensite structures in composition.

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everetra
Post 4

@hamje32 - It seems that the metallurgy involved in creating the bainite steel involves a delicate balancing act, cooling before pearlite is formed and yet delayed so that martensite is not formed.

In my opinion, it’s the balancing act that gives the steel its unique properties and makes it something that is sought after for swords, kitchen knives and industrial steel.

hamje32
Post 3

@MrMoody - You can buy l6 bainite swords online if you’re willing to spend the money. You would have to be a real sword fanatic in my opinion to go this route, but you’ll find places that will sell these swords.

For swords, I think it’s not just the hardness of the sword that is the issue, but it’s ability to absorb shock. Swords, after all, must give and take without breaking. From what I understand swords can be made of many materials but bainite is considered to be the best material.

MrMoody
Post 2

I understand that some fancy sword collectors (no, I am not a Samurai) have been sporting swords made of bainite steel. The bainite sword is harder and yet more flexible at the same time.

I wonder if bainite is also made in household kitchen blades. I think they might benefit from some of the properties of the steel, like its extreme hardness.

Personally, I am not a big fan of the big chef knives that they advertise on TV which can slice and dice with precision and retain their sharpness forever. To me, a knife is a knife; yet some people take their knives very seriously and I think for them having a knife made of the strongest metal would be a necessity.

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