The purpose of this report is to perform a complete metallurgical analysis on three parts from a North American automotive engine. The three parts had to include a head bolt, ferrous part, and non-ferrous part. The parts chosen were a head bolt, piston, and exhaust manifold. The objective of the analysis was to determine from what material the parts were made and how the metal parts were processed to bring it to its final form. The metallurgical techniques that were used are image analysis, furnaces, hardness tests, and SEM.
2.0 Head Bolt
The head bolt is used to fasten the cylinder head to the engine. Therefore, it should have high strength, especially in tension, as well as good fatigue life. The bolt had a length of 5”, diameter of 3/8”, and head size of 9/16”. The bolt was gold coloured and after sectioning it was found that the gold colour was only on the surface while the inside was silver.
2.1 Tensile Test
Results of tensile tests show that the bolt has a tensile strength of 150 ksi. This information could be used to identify the steel alloy.
2.2 Image Analysis
In order to identify the microstructure of the bolt, it was examined using the optical microscope. A sample of the bolt was first prepared by sectioning a cross section of the bolt. This section was then mounted in bakelite, polished, and etched. Figure 1 and Figure 2 show the cross section of the bolt at 200X and 500X magnification.
Figure 1 – Bolt Cross Section at 200X
Figure 2 – Bolt Cross Section at 500X
From Figure 1 and Figure 2, the microstructure looks like lath martensite which is common in low to medium carbon steels. Pictures of the different types of martensite are shown in Figure 3 for comparison.
Figure 3 – Types of Martensite
However, since the grain size looks finer and more dispersed, the microstructure could also be tempered martensite. If it is tempered martensite, this would mean that the bolt has gone through a quench and temper heat treatment process. The tempering temperature also affects the microstructure. Higher tempering temperatures would result in more spheroidal cementite precipitates whereas lower temperatures would result in more lath type cementite. This is shown in Figure 4.
Figure 4 – Effects of Tempering Temperature
Figure 4 also shows that a lower tempering temperature results in a higher hardness. Thus, performing hardness tests may give a better idea of the tempering temperature.
Hardness tests were performed to determine the hardness of the bolt at various sections. The sections that were analyzed were the head of the bolt, the shaft of the bolt, and the threads of the bolt. Five Rockwell C hardness measurements were taken at the centre and around the edges of the cross sections of both the head and shaft, and these numbers were averaged for the centre hardness and edge hardness. A Vickers hardness measurement was taken at the threads of the bolt and converted to HRC for comparison. The shaft was then heat treated at 900°C for 1 hour, and then quenched in water. Another Rockwell C hardness measurement was taken on the shaft after heat treating. The results of all the hardness tests are shown in Table 1.
as received heat treat
edge centre centre head 37
Table 1 – HRC Hardness
The higher hardness in the centre as opposed to the edge shows that there may have been some surface modifications. In order to find the thickness of the surface modification, a hardness profile was taken from the edge of the shaft cross section to its centre. It is shown in Figure 5.
Figure 5 – Hardness Profile of Head Bolt
Figure 5 shows that from a distance of about 0.005in from the edge of the cross section up until the centre of the cross section, the hardness is uniform. However, at the very edge of the cross section, the hardness is lower. This shows that the