Introduction
The microstructural characterization of a material is important in understanding its properties, behaviors, and performance. This characterization is often accomplished through the use of microscopy. The technique used to analyze the microstructure of a material is known as metallography or optical microscopy. Metallography involves the preparation of a sample so that the microstructure can be examined under a microscope. The preparation requires the sample to be polished and etched in order to observe the microstructure.
In this investigation, the goal was to analyze the microstructure of an ASTM AISI 1040 steel grade. The sample was mechanically machined, polished and etched to reveal the microstructure.
Material and Methods
In order to perform metallography on an AISI 1040 steel, it was important to first procure the materials. The materials used for this investigation included AISI 1040 steel, lapping and polishing compounds, a lapping and polishing machine, abrasive paper, a polishing cloth, a sample holder, and a mounting press.
The procedure began by machining a disc-shaped sample from the AISI 1040 steel. The disc was then placed on the lapping and polishing machine to be prepared for further analysis. The disc was wet-ground with coarse, medium and fine abrasive paper before being polished with lapping and polishing compounds.
After polishing the sample, it was then placed in the mounting press to be prepared for microstructural analysis. The sample was then etched in an aqueous solution of picric acid before the microstructure was examined under an optical microscope at a magnification of x320.
The microstructure was observed and photographed at a magnification of x1000 on a Hitachi S-3000N Scanning Electron Microscope, operating at an accelerating voltage of 15kV and a working distance of 12mm.
Results and Discussion
The microstructural analysis of the AISI 1040 steel revealed a lath martensite structure, where laths of martensite are embedded in the austenite grain matrix (Figure 1). The laths of martensite have a banded or layered appearance due to alternate hard and soft zones situated along the direction of heat treatment. This banded or layered appearance is referred to as a ‘railroad track’ or ‘Hilpert’ pattern and is often seen in high strength steels such as AISI 1040 (1).
Figure 1. Optical micrograph of the AISI 1040 steel after etching in picric acid and examination at a magnification of x1000.
The hardness of the material was determined by a Rockwell C-scale hardness test. The results showed that the AISI 1040 steel has a Rockwell C-scale hardness of 61 HRC (2).
Conclusion
The microstructural analysis of the AISI 1040 steel revealed a lath martensite structure. The hardness of the material was determined to be 61 HRC by the Rockwell C-scale hardness test. The results of this investigation demonstrate the importance of microstructure characterization in determining the properties, behaviors and performance of a material.
