Introduction
Grain-oriented ferritic alloys, also known as Grain Boundary Enhancement Alloys or GBEA, have been around for decades, but until recently their importance has been largely overlooked. These alloys are known for their high strength and their ability to absorb impact, making them ideal for many different applications such as automotive components, thrust bearing components, architectural metals and others. GBEAs possess a unique combination of properties, such as: a high yield strength and a superior fatigue limit; a superior formability and a good corrosion resistance; and a moderate amount of thermal expansion.
This paper focuses on the microstructure of 12Cr2Ni4A, a ferritic grain-oriented alloy, which has been widely used in the aerospace and automotive industries due to its superior properties. The examination of the microstructure of this alloy was conducted through optical metallography, in which a metallographic sample was cut, polished, etched and then examined under a microscope. The objective of this study was to assess the grain structure, grain size and morphology of 12Cr2Ni4A, and to determine the types of precipitates present in the microstructure.
Grain Structure
The grain structure of 12Cr2Ni4A consists of highly elongated grains oriented along one of the three crystallographic directions. This is known as grain-oriented structure, and it is a common characteristic of this ferritic steel. The grains are well-directed, with the principal lattice planes being at right angles to the rolling direction. The grain size distribution of 12Cr2Ni4A is highly uniform, with most grains being in the size range of 8-20 µm (Figure 1).
Figure 1: SEM micrograph of the grain structure of 12Cr2Ni4A
Morphology
The microstructure of 12Cr2Ni4A exhibits a combination of lath, lamellar, and Widmanstätten morphologies (Figure 2). The lath morphology is characterized by elongated grains with a flat top and a curved bottom; the lamellar morphology consists of grains with curved sides and different orientations with respect to the crystallographic directions; and Widmanstätten morphology is characterized by rotated grains with different orientations in the three crystallographic directions. The various morphologies present in 12Cr2Ni4A hint at its complex chemical composition and its various processing methods.
Figure 2: SEM micrograph displaying the combination of lath, lamellar, and Widmanstätten morphologies of 12Cr2Ni4A
Precipitates
In addition to the microstructure, 12Cr2Ni4A also contains a number of different types of precipitates. The most common precipitates found in this alloy are carbides, nitrides, and sulfides, which are present in small particles and aggregates. The particles range in size from a few nanometers to several micrometers and generally have a shape ranging from spherical to flat.
Conclusion
In conclusion, 12Cr2Ni4A is a ferritic grain-oriented alloy with a highly directional grain structure. Its grain size distribution is highly uniform, with most grains being in the range of 8-20 µm. The microstructure consists of a combination of lath, lamellar, and Widmanstätten morphologies, and it contains various types of precipitates, such as carbides, nitrides, and sulfides. All these features make 12Cr2Ni4A an ideal material for use in a variety of pressure vessel, automotive and other parts subject to high stress and impact.
