Engineering Material

Brittle Fracture of Plain Carbon Steel at Low Temperature
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Brittle Fracture of Plain Carbon Steel at Low Temperature
The Final Set of Results LINK Excel.Sheet.12 E:\THESNO\Documents\Book11.xlsx Sheet1!R1C1:R4C9 a f 5 h * MERGEFORMAT
Temp deg C 100 50 26 0 -20 -50 -100 -200
0.1% CS 59.16 56.848 62.084 57.12 36.312 2.448 2.244 0.748
0.4% CS 32.164 27.472 21.624 7.208 5.848 4.42 1.564 0.952
1.0% CS 2.38 2.584 0.884 1.088 0.952 0.952 1.156 0.884

Analysis of the DBTT of the 0.1%, 0.4% and 1.0% Carbon Steel
Maximum+minimum impact energy absorbed20.1% CS
Maximum impact energy
47.0+44.32=45.6545.65×1.36=62.084 JMinimum impact energy
0.6+0.52=0.55 0.55×1.36=0.748J62.084J+0.748J2=31.416 J0.4% CS
32.164+0.9522=16.558 J1.0% CS
2.584+0.8842=1.734 J
From the graph, the temperature which corresponds to 31.416 J is -24 0C, 16.558 J is 18 0C, and 1.734 J is 39 0C.
Relationship Between the DBTT with Hardness
Carbon is known to have the ability to increase the hardness making the materials less brittle in addition to decreasing the martensite transformation temperatures thus making the elements develop high strength. From the results, it can be noted that the Vickers hardness of the specimens declined with the rise in temperature and time. In the experiment, it is evident that there was an insignificant change in the DBTT in relation to the change in the Vickers hardness. However, the DBTT of the plain carbon steels that were heated was lower than that of the rest of the steels.

The reason behind the difference in the DBTT of the specimens is the effect of heat on the specimen structure. In this case, the induced heat causes inhomogeneity resulting in substantial scattering of the specimen particles. The inhomogeneity caused during heating process can be reduced through re-heat treatment (Gensamer, 2017, 173). In this case, the heat treatment procedure greatly affects the level of hardening and embrittlement of the specimens. The variation in the carbon contents in the steel because of heat treatment process has a significant influence on the occurrence of the point defects and defect clusters (Sekban et al., 2016, 42). In this case, the increase in heat decreases the hardenability of the specimen hence causing the DBTT to increase.
Comparison between the experimental findings and the Published Data
In the experiment, it can be noted that the low temperature negatively affects the tensile toughness of plain carbon steel. Tensile toughness, in this case, is the measure of the ductility and brittleness of the materials. Ductile materials tend to absorb high amounts of impact energy if a fracturing force is applied. As a result, the materials end up to a tell-tale deformation. On the other hand, brittle materials tend to shutter whenever they are subjected to an impact force. Therefore, materials that have high ductility and high strength as observed in the experiment have good tensile strength. In the events that the temperatures are changed, the toughness of the materials also fluctuates. As a result, there is a possibility that material can shift from being ductile to a brittle material. In the experiment, when the temperatures go beyond specific values, the materials start to behave as a brittle material. In this case, it is the ductile to brittle temperature. This trend is similar to the published data as the low-carbon steel tends to become brittle when there are subject to low temperatures or are undergoing a high amount of strain. The published cause of the change in the DBTT of the low-carbon includes the tendency of the variation in temperature to alter the composition of the microstructure of carbon steel. Also, the higher the carbon content, the higher the strength of the steel and the higher the DBTT.
Measure used to Suppress the DBTT of Low Carbon Steel
To suppress DBTT, it is essential to consider the factors that affect its existence. In this case, it is necessary to find the crystal structure of the materials, the interstitial atoms, the heat treatment applied and the specimen orientations. Factors that can be altered to suppress the DBTT of material include the specimen thickens and its grain size (Gensamer, 2017, 176). It has been proven that only BCC materials tend to be affected by transition temperatures. In this case, when the temperatures are increased, the elements tend to behave more in plastic deformation. Elements such as the FCC and the HCP, on the other hand, are hardly affected by transition temperatures hence they end up maintains the same energy absorptions when subject to different temperatures. The grain size of the plain carbon steel has a high effect on the transition temperature. In this case, the lower the grain size, the more the DBTT curve shifts to the left. As a result, the materials will have a broader range of service temperatures. To lower the grain size of the plain carbon, the type of heat treatments applied to the materials should be one that makes the grain refines.
Application of heat treatments such as recrystallization’s and air cooling when the materials us under hot working conditions will result to the moving the DBTT to the left hence lowering the transition temperature of the material (Boyd, 2016, 22). The temperature and martensitic structures of the carbon steel form the best structure combining the impact toughness and the strength of the material (Gensamer, 2017, 177). Furthermore, to lower the DBTT of low carbon steel, the specimen orientation should be longitudinal instead of transverse. Longitudinal orientation produces materials with best energy absorptions since the occurrence of cracks tends to take place across the fiber alignment (Pineau, Benzerga, and Pardoen, 2016, 429). On the other hand, transverse orientation makes the materials have the least energy absorption performance since when cracks occur, they tend to propagate parallel to the rolling side of the material. The thickness of the low carbon steel should also be checked, and the size reduced. Materials that have large specimens end up developing more constraints hence becoming more brittle (Sekban et al., 2016, 46). As a result, their absorption temperature rises thus increasing the DBTT of the material. The contents of Sulphur in low carbon steel tends to make the materials brittle when it is subjected to low temperatures, therefore, to lower the DBTT, any Sulphur content in the materials should be reduced to insignificant amounts.
Conclusion
In summary, the experiment provides sufficient data to analyze the behavior of different types of plain carbon steel. In this case, the kind of heat treatments and the characteristics of the material during hot working can be defined, and the resulting product ends up having the best functional features with a wide temperature range it can perform. Furthermore, enhancing the characteristics of the materials will provide the opportunity to develop new products that require strong and less brittle carbon steel material.

References
Boyd, G.M. ed., 2016. Brittle fracture in steel structures. Elsevier, pp. 21-25.
Gensamer, M., 2017. Strength and ductility. Metallography, Microstructure, and Analysis, 6(2), pp.171-185.
Pineau, A., Benzerga, A.A. and Pardoen, T., 2016. Failure of metals I: Brittle and ductile fracture. Acta Materialia, 107, pp.424-483.
Sekban, D.M., Aktarer, S.M., Xue, P., Ma, Z.Y. and Purcek, G., 2016. Impact toughness of friction stirs processed low carbon steel used in shipbuilding. Materials Science and Engineering: A, 672, pp.40-48.