Preparation and Examination of Metallographic Samples

Abstract

            Using the Vickers Hardness Tester, the microhardness of a material can be gauged quickly and easily. Mounting the sample in Bakelite, be it conducting or non-conduction, allows us to effectively polish and prepare the sample for testing. The Vickers hardness tester can be used for many applications, and due to its accuracy, can be used on such things as coatings and coverings that would otherwise be very difficult to test.

Introduction

            The Vickers Microhardness test stemmed from the original idea of a Macrohardness test. Using weights between 1kg and 100kg and pressing the indenter for 15-20 seconds, an impression can be made into the material that is being tested that is measured, giving a hardness number that can be directly compared to other materials. This is useful for many different applications in the industrial world, ranging from testing coatings on any metallics, to testing any number of high grade alloys.

            Steel is a very versatile metal, consisting mostly of iron, with carbon content between 0.2% and 2.14% depending on what properties and grade you are looking for when producing it. The Carbon content makes a very large difference to the properties of steel, the hardness is majorly affected when the carbon content is changed.  The Carbon acts as a hardening agent, preventing dislocations in the atom crystal lattice from sliding past one another. Changing the Carbon Content controls properties such as ductility, hardness, and tensile strength. Increasing the carbon content increases the strength of the steel, but also the makes it brittle, so care has to be taken to make sure the correct amount is added.

            The higher carbon content generally lowers the melting point of the steel, and also its resistance to temperature change. The carbon content influences the yield strength of the steel due to the carbon atoms fitting into the interstitial crystal lattices of the BCC arrangement of the iron atoms [1]. The interstitial carbon reduces the mobility of dislocations, which in turn has a hardening effect on the iron. To get dislocations to move, a high enough stress level must be applied in order for the dislocations to break away. This is because the interstitial carbon atoms cause some of the iron BCC lattice cells to distort [1]

Experimental

Our mounted sample of steel was ground using fine silicon carbide paper. The grades of paper used were 240, 400, 800 and 1200 grit. This was done using progressively finer paper in order to remove the surface scratches present, making the scratches smaller with each level of paper. Water was used as a lubricant and when changing paper, the sample was washed thoroughly under the tap. Once the 1200 grit paper had been used the sample was rinsed under water then rinsed in alcohol and dried under a heater.

The sample was then polished using a polishing wheel. A six micron wheel was used to start with, and once all of the scratches from the previous paper were removed, rinsed with alcohol and dried under a heater. This process was repeated using a one micron wheel. Inspection using a microscope ensured that the sample was correctly polished.

The sample was then etched using 2% nital. The sample was dipped into an etching basin filled with the etching agent for a few seconds. Care was taken to ensure the sample did not turn black, as this would have been over etching and the sample would have had to be re-polished. Once etched the sample was washed in water, rinsed in alcohol and dried under a heater.

The sample was then inspected under a microscope using 400 x magnification and the microstructure drawn.

            Once the microstructure had been examined under the microscope, it was taken to the Vickers Hardness tester. On a Microhardness scale ranging from 10g to 1000g, three measurements were taken on grains of pearlite and ferrite respectively. The results were recorded below.

Results

Microhardness Results

Macrohardness Results

Discussion

            The samples tested came out at roughly 0.4% carbon content, which is the most common among steel samples. The ferrite is a lot harder than the pearlite due to the amount of carbon within the grain.

I’m sorry this is short, I miscalculated the weeks, the next one will be better I promise, I have to go hand in now.

Apologies again.

References

[1] . Dennis ‘Metallurgy of the Ferrous Metals’ , 1963, Sir Isaac Pitman and Sons Ltd, Woking and London