What is the Difference Between Bainite and Martensite

The key difference between bainite and martensite is that bainite structure appears as thin acicular and irregularly shaped laths below martensite start temperature (Ms), whereas martensite appears in the form of laths with wavy boundaries and ledge-like protrusions.

Bainite and martensite are two microstructures of steel. They can cause different types of properties in steel material.

CONTENTS

1. Overview and Key Difference
2. What is Bainite  
3. What is Martensite
4. Bainite vs Martensite in Tabular Form
5. Summary – Bainite vs Martensite 

What is Bainite?

Bainite is a plate-like microstructure formed in steel under 125 – 550 degrees Celsius temperature. However, this temperature depends on the alloy content of the steel. This structure was first described by E.S. Davenport and Edgar Bain. This microstructure is a product that forms when austenite structure is cooled past a temperature at which it is no longer a thermodynamically stable structure with respect to ferrite, cementite, or ferrite and cementite.

Bainite is a fine, non-lamellar structure that contains cementite and dislocation-rich ferrite. There is a large density of dislocations in ferrite and fine-sized platelets, which makes it harder than its normal structure.

The hardness of bainite lies in between that of pearlite and untampered martensite which is in the same hardness range as steel. It can be produced during both isothermal and continuous cooling, which is a big advantage since it facilitates the production of large components without the excessive addition of allowing elements. Alloys based on bainite often do not require any further heat treatment after the transformation to optimize their strength and toughness.

What is Martensite?

Martensite can be described as a very hard form of steel crystalline structure. This material was first described by German metallurgist Adolf Martens. This term can also refer to any crystal structure that forms by diffusion-less transformation.

The formation of martensite is from carbon steels through the rapid cooling (also named quenching) of the austenite form of iron at a high rate; here, there is no time for carbon atoms to diffuse out of the crystal structure in large quantities in the formation of cementite.

The reaction of martensite starts during cooling when the austenite reaches the Ms (martensite start temperature). At this stage, the parent austenite becomes unstable mechanically. When the temperature is decreased, more of the austenite transforms into martensite. The transformation is finally complete when the martensite finish temperature (Mf) is reached.

In steel with 0 – 0.6% carbon, martensite has the appearance of lath, and it is called lath martensite. But, in steel with a carbon percentage greater than 1%, martensite tends to form a plate-like structure named the plate martensite. At the point between these two percentages, the physical appearance of the grains appears as a mix of the two.

What is the Difference Between Bainite and Martensite?

Bainite and martensite are two microstructures of steel types that can cause different types of properties in steel material. The key difference between bainite and martensite is that bainite is in the form of thin acicular and irregularly shaped laths below martensite start temperature, Ms, whereas martensite is in the form of laths having wavy boundaries and ledge-like protrusions. Moreover, in bainite, the transformation involves a structure change followed by the redistribution of carbon, which precipitates as a carbide, while in martensite, the transformation involves the structure change alone.

The below infographic presents the differences between bainite and martensite in tabular form for side-by-side comparison.

Summary – Bainite vs Martensite

Bainite is a plate-like microstructure that tends to form in steel under 125 – 550 degrees Celsius temperature. Martensite is a very hard form of steel crystalline structure. The key difference between bainite and martensite is that bainite has thin acicular and irregularly shaped laths below Ms, whereas martensite has laths with wavy boundaries and ledge-like protrusions.

Reference:

1. “Bainite.” Wikipedia, Wikimedia Foundation, 27 Jan. 2022.

Image Courtesy:

1. “Bainite steel roll” By H K D H Bhadeshia – Tandf Online (CC BY 4.0) via Commons Wikimedia
2. “Martensite” By Scm83x (CC BY-SA 3.0) via Commons Wikimedia