1.0 Introduction
This report covers lab 5 of the Engineering Materials unit, dedicated to metallography and heat treatment. The lab expands on concepts covered in lectures including heat treatment, crystal structure of metals, and mechanical properties of materials by introducing them in a hands-on environment. This lab was composed of three experiments. One involved the study of crystal structure in metal samples provided, and recording them. Another was testing the hardness of aluminium samples to graph hardness to ageing time for precipitation hardened aluminium alloys. The final experiment was a Jominy Bar test. These three experiments all explored different aspects of the materials content, but all linked back to the same core factors, properties and structure. The following report will explore the theory, describe the procedure, provide the results, and discuss and conclude on each experiment.
2.0 Background/Theory
2.1 Metallography of cast and wrought alloys
Metallography is the study of the structure of metals and the properties they exhibit. These properties and structures are due to multiple aspects of a material, including its purity, what elements make it up, and any treatment it has endured. All of these will change crystal size and shape, and as a result, the properties of a material.
Most factors of a material are affected by its grain size. With decreasing grain size, impact, tensile, yield, and fatigue strength all increase, as does hardness. While decreasing grain size favors hardness, it decreases hardenability. It is also important to note that grain strength has a greater effect during the early stages of deformation, as such grain size will have a greater effect upon yield stress than tensile strength.
The materials given to students include multi-phase, cast, and wrought structures, covering both pure metals and alloys. Across these species a number of crystal structures can be seen, from which a number of properties can be deduced.
Before analysis of results begins, it is important to define terms that will be used. A pure metal is one that contains only one element, while an alloy contains more than one element.
A multi-phase material in this setting is a material in which there exists multiple materials with different properties in the same state. Wrought structures have been worked with tools while hot, while cast structures have been poured into moulds while molten and cooled (Done, 2016).
All of these are factors in the crystal structure.
2.2 Solid Solution Hardening and precipitation Hardening: Aluminium alloys
Solid solution hardening is simply the process of adding one metal to another. It is done during casting, while the metals are in liquid form. The element found in majority is the solvent, while any alloying elements in the minority are solutes.
Plastic deformation arises when dislocations move through crystal lattices in a grain of a material. By adding solutes to the crystal structure, the movement of dislocations becomes restricted, strengthening the material.
There are two different types of solid solutions, substitutional, and interstitial. Substitutional occurs when an atom of differing size takes the place of a solvent atom, while interstitial occurs when smaller atoms are in the solute than the solvent, with the smaller atoms moving between those of the solvent. In both cases the crystal lattice is interrupted, meaning that dislocations require greater energy to move past the alloying element (Gedeon 2010).
In age hardening atoms are distributed uniformly throughout the material. Starting with solution treatment at a high temperature, the material will then undergo rapid cooling, going across the solvus line to exceed the solubility limit. The metastable solution can then cool without lattice defects.
While precipitation hardening increases strength, it decreases toughness and ductility, so a compromise must be struck between the desired values.
It is also important to note however, that age hardening is only effective to a point, after which the precipitates become too spread out to be effective
2.3 Effect of rate of cooling on the hardness of plain carbon and alloy steels.
Basic theory starts at definitions. In this section there are two crucial characteristics to define, hardness and hardenability.
Hardness is the resistance a material exhibits towards plastic deformation. This can be easily measured by how resistant a material is to indentation and scratching. Hardenability is the depth to which hardness can be achieved.
Hardenability is dependent upon martensite content, which itself is dependent upon a number of factors. These include the rate at which a material is cooled, austenite grain size, alloying elements, and carbon content.
When a material is cooled faster, austenite has less opportunity to form ferrite and pearlite, increasing martensite content. Increased austenite grain size will also decrease ferrite and pearlite formation, again increasing martensite content. Adding carbon does the same to ferrite and pearlite formation, as does adding alloying elements, both of which increase the hardenability of a material.
While all of these factors affect hardenability the factor in question is the affect of rate of cooling. As has been briefly covered earlier, faster cooling causes increased hardenability. When a material is cooled quickly from a heated material austenite has less opportunity to decompose into ferrite, pearlite, and bainite. This means that martensite can be formed to a greater depth within the material, thereby increasing hardenability (Khaira, 2013).
Martensite is formed when austenite is cooled. More specifically, a material will cool until it reaches a martensitic temperature, at which point austenite becomes mechanically unstable. Because the amount of austenite in a material directly affects the amount of martensite in a material, it is imperative that the maximum amount of austenite make it to martensitic temperature. This can be done by reducing the amount of austenite forming bainite, ferrite, and pearlite.
3.0 Experimental Procedure
3.1 Metallography of cast and wrought alloys
Students were provided with a number of samples broken down into three categories; cast structures, wrought structures, and multi-phase structures. For cast structures there was slow cooled pure aluminium, and rapid cooled pure aluminium. For wrought structures there was pure copper and 70/30 brass, and for multi-phase structures there was 60/40 brass.
Students would examine these samples and draw out the crystal structures of each sample. Once they were drawn out students would then label features, discuss observed characteristics, how they came to be, and how they will affect the properties exhibited by the metal.
Greater atomic size difference between alloying elements provides greater strength to the alloy
3.2 Solid Solution Hardening and precipitation Hardening: Aluminium alloys
There are a number of treatments to metal available, and it is important that students know and understand a number of them. Samples of aluminium are provided to the student, all of which have undergone solution treatment, but have been aged for different amounts of time. The least aged is 0 minutes, while the most aged is 240 minutes. Students measured and recorded the hardness (in Rockwell A) of each aluminium specimen. The data is as follows.
3.3 Effect of rate of cooling on the hardness of plain carbon and alloy steels
Students used the Jominy bar test to derive the hardenability of steel. For this, a 25x25x100mm bar of annealed steel is brought to 900ºC before being quenched on one end. Once cooled hardness will be tested along the bar from its quenching point. Students were shown the heating and cooling of a Jominy bar, and were also shown that even when a metal looks cool, that it could still burn paper, an important fact to keep in mind. Two samples were provided for this testing, plain carbon steel AISI 1040 and alloy steel AISI 4340.
The independent value of this experiment was distance from the quenched end, while the dependent value was its hardness.
4.0 Results
4.1 Metallography of cast and wrought alloys
4.2 Solid Solution Hardening and Precipitation Hardening: Aluminium Alloys
Ageing Time (minutes) Hardness Values (HRa)
1 2 average
0 32 31 31.5
4 30 32 31
8 33 34 33.5
12 36 34 35
16 34 33 33.5
20 36 36 36
24 35 37 36
28 39 41 40
32 39 40 39.5
36 35 39 37
44 34 36 35
52 36 35 35.5
60 33 35 34
80 36 35 35.5
240 33 31 32
4.3 Effect of Rate of Cooling on the Hardness of Plain Carbon and Alloy Steels
Distance (mm) Hardness (HRa)
AISI 1040 AISI 4340
1.5 58 60
3 59 60
4 46.5 60
5 34.5 60
9 31 59
11 31 58.5
13 26.5
15 30.5
20 17
25 21
30 24.5
35 22.5
40 22
45 19
50 18.5
55 16 58
60 16.5 59
65 14 57
70 14.5 58
5.0 Discussion and Conclusion
5.1 Metallography of cast and wrought alloys
Section 4.1 provides photos of drawings of crystal structures. In this section they will be described. While there are two samples of pure aluminium, they are very distinct from each other due to their cooling. The slow cooled aluminium has significantly larger, elongated crystals, while the rapid cooled aluminium has few crystals showing closer to the centre of its cross section, only on its edge. The larger crystals in the slow cooled aluminium suggest a weaker metal, as was explain in section 2.1, while the smaller crystals in the rapid cool would suggest a stronger material. Both the cast and wrought metals had significantly smaller crystals than either aluminium sample, as although they look similar in the diagrams, they were examined under a magnification of 100. Their tighter packed crystals again suggest better mechanical properties such as impact strength, yield strength, and hardness when compared to the aluminium samples.
5.2 Solid Solution Hardening and Precipitation Hardening: Aluminium Alloys
Background theory/knowledge in section 2.2 suggested that up until a certain point, increased ageing time would cause increased hardness. No quantitative data was supplied in the theory section, so no estimate or information on when the hardness would start to decrease was provided. As such it is hard to say that they went against what was provided in analysis, however the hardness did begin to decrease surprisingly early within ageing times, peaking at 28 minutes. The lowest data point on the graph is at 4 minutes, suggesting that there is high variability in the hardness resulting from ageing.
5.3 Effect of Rate of Cooling on the Hardness of Plain Carbon and Alloy Steel
Within the background theory/knowledge section 2.3 it was referenced that more alloying elements would prevent austenite from forming ferrite, pearlite, and bainite, instead favouring the formation of martensite, and increasing hardenability. Section 4.3 shows the results of experiments upon the hardenability of two Jominy bars, AISI 1040, and AISI 4340. Appendix 7.1 shows the chemical composition of AISI 1040, while appendix 7.2 shows the chemical composition of AISI 4340. As can be seen, AISI 4340 has a greater composition of alloying elements. It can also be seen that in results AISI 4340 maintained greater hardness significantly further into the material. The sustained hardness at depth shows a greater hardenability, as was suggested by the background theory. Even at its peak values, AISI 1040 had lower hardness values than AISI 4340, providing evidence of a lesser martensitic composition.
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