Materials Engineering 3 (Metallography)

Abstract

Introduction

Metallographic preparation is a set of procedures employed in preparing a metallic sample for microscopic analysis. The process is particularly concerned with obtaining a specimen surface that would reveal important metallographic information such as the size of grains, presence of inclusions and defects, predominant phases etc.

This experiment aims to demonstrate the metallographic preparation on steel 4140 and obtain a photomicrograph of the specimen.

Methodology

An inch-long 4140 steel rod was cut for use in this experiment. It was ground on one side with SiC papers in increasing grades, changing to perpendicular direction of grinding at every change in SiC grit size to easily see the scratches from the previous grit size completely smoothened by the new one. After grinding at the smallest grit size the specimen was polished first with 1micron then with 0.05micron of Alumina particles suspended in water. The polishing was completed upon having obtained a mirror finish on the polished surface.

To reveal the microstructure of the specimen, the metal was etched with Nital solution for 15seconds, rinsed with water and then washed with methanol to speed up drying of the surface. Etching was done to corrode the grain boundaries of the specimen which consequently makes them more visible when viewed under an optical microscope. Photomicrographs were taken at 500x magnification.

Results and Discussion

15seconds of etching time in 2% Nital was found to reveal the microstructure clearly enough. The microstructure (Fig. 1) showed presence of Bainite streaks in a presumably Martensite background. This indicates heat treatment of the specimen prior to the metallographic examination. To produce such phases in the microstructure, the specimen must have undergone fast cooling from austenitizing temperature. Based on these observations, the specimen must be very hard and brittle.

The image also showed blurriness near...