Sohrab Randjbar DaemiMAT 106 – Experimental Programme 3 - Laboratory Report

The relationship between the composition, microstructure, and mechanical properties of different grades of plain carbon steels

IntroductionIn this report, the results of three different experiments will be discussed, to better clarify the relationship between the composition, microstructure and mechanical properties of different grades of plain carbon steels. In these three experiments different samples will be polished and prepared for microscopical analysis, examined with an optical microscope so that the microstructure can be determined and finally tested with a manual Hounsfield machine to determine their mechanical properties.

Methods Preparation of materials for microscopical analysisBefore observing a sample with an optical microscope, it must be prepared so that its microstructure can be clearly observed. At first the sample is fixed in a cold-set resin formed by a liquid and a powdery part. After the resin has set, the sample will undergo three different stages of refinishing. The first stage is consists of grinding the sample with carbide silicon paper mounted on a grinding wheel. This process is repeated various times, using three different grades of paper, starting from a rougher one and proceeding to finer ones. Each time the grade of the paper is changed, the sample must be scratched at right angles so that the scratches from the previous operation have been removed. Although this repeated grinding process removes most of the scratches and polishes partially the sample, when placing it under a microscope it still displays a scratched surface.The next step is then the polishing of the sample. This is achieved by using two grades of diamond paste also mounted on a rotating wheel. These fine diamond pastes, at 6 and 3 microns, scratch the surface of the metal allowing metal removal and producing a plastically deformed surface region. The sample must be rotated in every angle so that the surface is evenly polished in every direction. The surface of the sample is now shiny and mirror-like but it still is not suitable for microscopical analysis as the scratches are less than the wavelength of light.The last step of the preparation is so applied, etching. Etching is obtained by treating the surface of the sample with a chemical etching reagent, in this case a dilute acid solution. Before applying this solution though, the surface of the sample is cleaned with methylated spirits and acetone. This etching process highlights the grain boundaries as the atoms on the grain boundaries are more chemically active than the rest of the grains and since these grooves reflect light at a different angle than the rest of the grains they become clearly visible when observed through an optical microscope. The sample is now ready for microscopical analysis.

Optical microscope observationFour samples are observed through an optical microscope with four different carbon contents: 0.1%, 0.4%, 0.75% and 1.1%. These samples are polished using

the method described previously and are observed through two different magnifications, 15x12.5 and 15x32. Their microstructure is then sketched down and some estimations on the contents of ferrite and pearlite and on the grain diameter are made.

Mechanical testing of materialsThe mechanical testing of materials is a process composed by two parts: the tensile test and the hardness test. The tensile test is done with a hand driven Hounsfield machine on three different samples with different composition and carbon content. These samples, A, D, P have respectively a content of 0.1%, 0.4% and 0.9% of carbon and with the mechanical properties that can be determined with this test are Young’s Modulus, the upper yield stress, lower yield stress, ultimate tensile strength and the fracture stress. All of these characteristics are determined with a graph plotted directly using the hand driven Hounsfield machine.The hardness test is instead performed with the Rockwell hardness machine. The hardness is tested by pushing a shaped indentor into the material under a given force. The hardness is then calculated from the depth of the impression or the area of the impression.

Results Mechanical testingBelow are the graphs of the stress versus the strain of the three samples and a table containing the main information obtained in the mechanical testing.

In order to calculate the stiffness of the machine, the slope of the straight line traced by the machine during the measure of the stiffness itself is calculated. This is done by selecting to random points on the line and measuring the slope, and then applying the formula lΔ s= l-f/S, where lΔ Δ s is the true extension, l theΔ apparent extension, S the machine stiffness and f the force.

S= (10,7-9,1)/(1,62-1,43)=8,42 KN/mm. The force versus extension graphs had their origins corrected as well, to take in account the errors given by the straightening out and bedding in of the various linkages of the machine.

Microscopical Observation

Above are the sketched of the first examined sample. In the first sketch a fairly big grain size can be noticed. An estimated 0.0206mm grain diameter has been measured. In the second sketch the pearlite content can be observed, lying between the grain boundaries.

As the carbon percentage increases, the Pearlite content increases as well but a reduction in the grain sizes can be observed. An estimated grain diameter of 0.0158mm was measured. Also the impurities increase in this sample.

As the carbon content increases, the ferrite amount decreases but the pearlite amount increases, as seen in the sketch above. Average estimated grain size here is 0.014.

In the last sample, the estimated ferrite content is 10%, and the estimated grain diameter is 0.00113mm.

DiscussionEnough information has been gathered for making several observations on the microstructure and its relation to the mechanical properties of the samples.By observing the relevant part of the iron carbon phase diagram, some observations can be made.

Pearlite is formed when the following reaction, named eutectoid reaction, takes place γ(0.77%) => α(0.022%C) + Fe3 C(6.67%C) at 727 °C. This product is a very fine scale mixture between α (ferrite) and Fe3C in which thin plates of Fe3C are sandwiched between thin plates of ferrite. Through the microscopical analysis we can determine that the grain sizes of the samples with a major carbon content, thus a major pearlite composition, is smaller in respect to the grain sizes of the samples with minor carbon content. Since reducing grain size strengthens considerably materials, the samples with higher carbon content can be subjected to a major stress. This is because grain boundaries disrupt the motion of dislocations through a material and having smaller grain diameters increases the number of grain boundaries per unit of area.This relationship between carbon content and toughness can be confirmed by the tensile tests. An immediate comparison between the ultimate tensile strengths of the three samples can be made from the stress versus strain graphs: Sample P can be subjected to a major stress before it starts necking. The other two samples have respectively lower UTS as their carbon content decreases and the average grain boundaries increase. These results can also be confirmed by the hardness testing with the Rockwell testing machine: Sample P has a higher hardness coefficient.

ConclusionBy carefully preparing a sample for microscopical analysis and observing it with an optical microscope, its microstructure can be determined. By then subjecting them to a mechanical test, the various relationships between the microstructure and the actual mechanical properties of the sample can be determined, with the aid of an elaboration of the obtained results.

Reference W. D Callister, Material Science and Engineering (John Wiley)