Charpy Impact Testing and Brittle Fracture Of A Range Of Carbon Steel - Report Example

FACULTY OF COMPUTING, ENGINEERING and SCIENCE Engineering Material and science СHАRРY IMРАСT ТЕSTING & BRITTLЕ FRАСTURЕ ОF А RАNGЕ ОF САRBОN STЕЕL Abstract This study investigates three steel samples with varying carbon composition namely 0.1%, 0.4 and 1.0% for ductile to brittle transition temperature (DBTT). The experiments couple Charpy impact and Rockwell Hardness testing in order to reach the conclusions. The DBTT temperature is estimated at 0°C as per the graphs plotted for energy absorbed against temperature while it is established that tensile strength for high carbon steels is higher than that of lower carbon steel. Table of Contents Abstract 2 Table of Contents 3 Introduction 4 Aims and Objectives 6 Laboratory Set-Up and Experimental Procedures 7 Results & Discussion (1/2) 10 Results & Discussion (2/2) 11 Conclusion 12 List of References 14 Introduction Material failure can be described as that characteristic that does not accomplish the purpose intended in the long run or inability to function normally. While causes of failure may be known, prevention of failure may not be guaranteed within an engineering environment. Discoveries made over time have concluded that structural elements fail through various mechanisms such as fracture, elastic deformation and yielding. This study focuses on one of the key concepts in materials science known as fracture; the most common cause of failure in alloys occurring below the yield stress limit. The only possible forms of fracture as established by the material engineering fraternity indicate that materials may be ductile or brittle. This is generally attributed to the amount of plastic deformation that a material may undergo prior to fracture occurrence. While ductile materials are known to resist a great deal of plastic deformation, brittle materials incur little or no plastic deformation before fracturing. Further study indicates that a highly ductile material is likely to neck down at the fracture point as compared to a ductile material which indefinitely fractures as shown in figure 1 below. Figure 1: A pictorial demonstration of the difference between ductile and brittle materials (Bailey, 1997). Ductility of materials has been in existence in as much as the evolution of materials has been in progress. It is however the most preferred method of material failure across the universe as compared to brittle fractures which are seen as more catastrophic. As such incidences such as the sinking of the famous titanic have been purely attributed towards brittle failure. This thus requires mechanical assemblies to be carried out at temperatures beyond the transition temperature in order to avoid such catastrophes. This is also dependent on the composition and type of alloy being deployed in a structure; in the case of steel alloys, they are mostly affected by the presence of carbon impurities or additives. It has been established that the transition temperature for most steel alloys is the 0°C in most places across the globe (School of Materials Science and Engineering, 2014). Figure 2: Ductile to Brittle Transition (School of Materials Science and Engineering, 2014). Increasing the carbon content in steel has been found to decrease the fracture toughness thus there is a major observable change in DBT. In as much as steel alloys are observed to have increased hardness with increased carbon percentages, the strength increases exponentially. Apart from physical factors, the chemical factors that are known to affect the DBT of steel include; the crystal structure, grain size, interstitial atom and the heat treatment. In carrying out studies to establish such material behaviours, metallurgists have come up with various mechanisms used to observe the mechanical behaviour. In order to study the change in steel DBT, temperature change can be plotted against the measured impact energy (Joule) for a specimen prepared specially to observe the changes. One such equipment is the Charpy impact tester that is deployed in this experiment in order to ascertain these findings. This instrument allows for determination of energy that is required to initiate or propagate a crack rather than the energy required for fracture. Aims and Objectives This study aims at identifying the range of ductile to brittle transition temperatures for steels of varying carbon content in ascertaining the alloy’s susceptibility to brittle fracture at low temperatures. With specimens of 0.1%, 0.4% and 1.0% carbon percentages availed; the study’s objectives are to impose the alloys to a range of temperatures from 100°C down to -200°C before impact testing them. The results shall hence be utilised in identifying the temperature ranges in which the changes of impact absorption occurs for each of the availed alloys. The objectives are advanced by correlating the findings with the hardness of the alloy using the Rockwell Hardness Tester. Laboratory Set-Up and Experimental Procedures The specimens of steel having the desired range of carbon content i.e. 0.1%, 0.4% and 1.0% were availed to facilitate the tests; normally classified in accordance to the existing norms as low, medium and high carbon steels. A Charpy balanced impact tester and a notching machine was also availed under strict instructions to be utilised under supervision from the laboratory technician. An electronic temperature measurement device, liquid nitrogen, and a kettle were also provided prior to commencement of experiment. Clear commands were given not to get in direct contact with liquid nitrogen, hot water and samples. Handling of the liquid nitrogen or freezing samples was only carried out or performed under the supervision of the laboratory technician. Notching was carried out on steel specimens of three different types of carbon steels (low – about 0.1% by weight, medium – about 0.4% by weight, and high – about 1.0% by weight, all steels were subjected to the same heat treatment) using the apparatus provided to give a V- notch of standard geometry at the centre of each test piece. The room temperature at the time of experimenting was measured and recorded before kicking off the experimentation session. The notched sample of each of the three steel alloys was placed at room temperature in the Charpy impact tester and carry out the test. The impact energy (Joule) absorbed by the three specimens was recorded accordingly. The samples were pre-cooled from room temperature down to -200C using a mixture of alcohol and liquid nitrogen and hot water baths for tests above room temperature prior to testing of samples. For each type of carbon steel, further samples were placed in boiling water (close to 100C) and liquid nitrogen (about -200C) for 3 minutes for the samples to reach the bath temperature. Using gloves each sample was transferred to the Charpy test machine rapidly and the test carried out in a hastened manner to maximise the temperature. It was also noted that in order to obtain an acceptable resolution for the plotting of the data, there was need for at least 8 data points from the following temperatures (approximate) such as -200C, -100C, -50C, -20C, 0C, at room temperature, 50C and 100C. The hardness of the three steel specimens was measured using the Rockwell Hardness tester and noted for each of the test samples as per their carbon contents. The hardness of the three steel specimens was measured using the Rockwell Hardness Tester and noted for each of the test samples as per their carbon contents. In carrying out this exercise, the provided carbon steel samples i.e. 0.1%, 0.4% and 1.0% were imposed on the hardness testing machine in order to fulfil the objectives of this study. The Rockwell tester presented for this study possessed a 100 kilogram load upon which 0.1%, 0.4% and 1.0% steel balls of 1/16 Inch diameter steel balls to be used as comparative samples were directly applied. On receiving the minor load (10kg), it was placed on the indenter together with the specimen. This created a perfect shape upon which the 100kg load was placed in order to create a basis for comparison with the Rockwell Hardness Number scale B. The hardness test was repetitively carried out in order to get a conclusive average which was considered as the hardness number for each sample presented for study. Results & Discussion (1/2) The results obtained for the DBT experiment were obtained and recorded in with respect to the samples and the impact energy measured in ft/lb-f. The energy measurement was converted from ft/lb-f to Joules by multiplying these figures by 1.36. This was done for all the specimens i.e. 0.1%, 0.4% and 1.0% carbon as shown in table 1 below. Table 1: Values recorded from the Charpy balanced impact test. Temp (C) -200.00 -100.00 -50.00 -20.00 0.00 RT 50.00 100.00 0.1% CS 0.82 3.40 2.04 34.68 54.40 63.92 57.80 58.75   0.68 1.09 2.86 37.94 59.84 60.25 55.90 59.57 0.4%CS 0.82 2.18 2.04 4.90 8.16 25.57 29.92 27.88   1.09 0.95 6.80 6.80 6.26 17.68 25.02 36.45 1.0%CS 0.82 1.09 1.09 1.22 0.95 0.95 1.77 2.04   0.95 1.22 0.82 0.68 1.22 0.82 3.40 2.72 This data was transformed into a graph as directed by the instructions of this experiment. It can be observed from the graph that the energy absorption for the low carbon steel at 0.1% carbon percentage showed an extremely different behaviour in comparison to its counterparts. In as much as the transition temperature was expected to be different for each of the samples tested, it was observed that each of them transitioned at 0C. From the observations, it could be concluded that 1.0% carbon steel is a high strength material as compared to 0.1% and 0.4% carbon steel thereby imparting the desired knowledge about ductility and brittle materials together with their respective transition temperatures. On the other hand, the malleability of low carbon steel materials in real life is high in comparison to high steel carbon materials. This gives low steel carbon an advantage in manufacturing equipment across various industries including the automobile parts. Figure 3: A graph of energy absorbed against temperature plotted to establish the ductile to brittle transition temperatures for the given steel samples. Results & Discussion (2/2) The results obtained for the hardness test carried out using Rockwell hardness equipment were also recorded as per the samples as shown in table 2 below. Table 2: Results Obtained from the Hardness Rockwell B-Scale "HRB" (1/16 inch diameter steel ball indenter, 100 kgf of indentation effort). 0.1%CS 66 57 53 49 56 45 54 46 66 45 51 67 60 55 53 59 64 58 0.4%CS 74 82 69 70 70 71 74 73 74 75 68 80 72 77 71 76 78 72 1.0%CS 89 95 93 93 94 93 88 96 87 99 95 95 97 92 98 93 98 99 From the results achieved in this experiment, it was observed that the hardness of a steel sample whose carbon composition was higher achieved the highest Rockwell hardness. In this case, 1.0% carbon steel achieved the highest hardness of 99 in comparison to 0.1% carbon steel which achieved the lowest hardness at 58. Although all the samples exhibited the same ductility to brittleness transition temperature, from a theoretical perspective high carbon steel would yield earlier than low carbon steel as seen in the case of titanic. It is also evident from the findings that high carbon steels or low alloy steels have low resistance in order to impact at low temperatures. The capability of steel to absorb a great amount of impact energy sharply drops as DBTT is approached (Chandler, 1998). Conclusion This experiment clearly showed how the brittle to ductile transition is related to the temperature dependency of the energy impacted upon it. Apart from this observation, it was also noted that high carbon steels possess higher hardness as compared to those with lower composition of carbon hence higher tensile strength. The sections of the failed surfaces clearly exhibited both characteristics i.e. ductility and brittleness at the transition temperature range. This was observed in a range of temperatures which helps in concluding that there is no definite transition temperature for any given sample. Ductile materials will hence be preferred in structural designs due to their plastic deformation nature and slow fracturing that gives the technical departments time to resolve the issues arising. List of References Bailey, D. (1997) Ductile fracture. Available at: http://www.sv.vt.edu/classes/MSE2094_NoteBook/97ClassProj/exper/bailey/www/bailey.html (Accessed: 17 February 2016). Chandler, H. (1998) Metallurgy for the Non-Metallurgist. New York: ASM International. School of Materials Science and Engineering (2014) 2: Ductile to Brittle Transition. Available at: http://www.materials.unsw.edu.au/tutorials/online-tutorials/2ductile-brittle-transition (Accessed: 15 February 2016). Read More

Aims and Objectives This study aims at identifying the range of ductile to brittle transition temperatures for steels of varying carbon content in ascertaining the alloy’s susceptibility to brittle fracture at low temperatures. With specimens of 0.1%, 0.4% and 1.0% carbon percentages availed; the study’s objectives are to impose the alloys to a range of temperatures from 100°C down to -200°C before impact testing them. The results shall hence be utilised in identifying the temperature ranges in which the changes of impact absorption occurs for each of the availed alloys.

The objectives are advanced by correlating the findings with the hardness of the alloy using the Rockwell Hardness Tester. Laboratory Set-Up and Experimental Procedures The specimens of steel having the desired range of carbon content i.e. 0.1%, 0.4% and 1.0% were availed to facilitate the tests; normally classified in accordance to the existing norms as low, medium and high carbon steels. A Charpy balanced impact tester and a notching machine was also availed under strict instructions to be utilised under supervision from the laboratory technician.

An electronic temperature measurement device, liquid nitrogen, and a kettle were also provided prior to commencement of experiment. Clear commands were given not to get in direct contact with liquid nitrogen, hot water and samples. Handling of the liquid nitrogen or freezing samples was only carried out or performed under the supervision of the laboratory technician. Notching was carried out on steel specimens of three different types of carbon steels (low – about 0.1% by weight, medium – about 0.

4% by weight, and high – about 1.0% by weight, all steels were subjected to the same heat treatment) using the apparatus provided to give a V- notch of standard geometry at the centre of each test piece. The room temperature at the time of experimenting was measured and recorded before kicking off the experimentation session. The notched sample of each of the three steel alloys was placed at room temperature in the Charpy impact tester and carry out the test. The impact energy (Joule) absorbed by the three specimens was recorded accordingly.

The samples were pre-cooled from room temperature down to -200C using a mixture of alcohol and liquid nitrogen and hot water baths for tests above room temperature prior to testing of samples. For each type of carbon steel, further samples were placed in boiling water (close to 100C) and liquid nitrogen (about -200C) for 3 minutes for the samples to reach the bath temperature. Using gloves each sample was transferred to the Charpy test machine rapidly and the test carried out in a hastened manner to maximise the temperature.

It was also noted that in order to obtain an acceptable resolution for the plotting of the data, there was need for at least 8 data points from the following temperatures (approximate) such as -200C, -100C, -50C, -20C, 0C, at room temperature, 50C and 100C. The hardness of the three steel specimens was measured using the Rockwell Hardness tester and noted for each of the test samples as per their carbon contents. The hardness of the three steel specimens was measured using the Rockwell Hardness Tester and noted for each of the test samples as per their carbon contents.

In carrying out this exercise, the provided carbon steel samples i.e. 0.1%, 0.4% and 1.0% were imposed on the hardness testing machine in order to fulfil the objectives of this study. The Rockwell tester presented for this study possessed a 100 kilogram load upon which 0.1%, 0.4% and 1.0% steel balls of 1/16 Inch diameter steel balls to be used as comparative samples were directly applied. On receiving the minor load (10kg), it was placed on the indenter together with the specimen. This created a perfect shape upon which the 100kg load was placed in order to create a basis for comparison with the Rockwell Hardness Number scale B.

The hardness test was repetitively carried out in order to get a conclusive average which was considered as the hardness number for each sample presented for study.

Read More