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D. Materials Microstructure Background theory and information The way to ensure that any material will perform in service is to make sure that its mechanical properties and physical properties are up to the task for which it has been chosen. In this context the materials "microstructure" is a key element that can be tailored, via a heat treatment, alloying or other modification process, to make it fit for purpose
Metallographic Observation It is possible to predict the general behavior of materials (or their loading history) by observing their microstructure. Besides the crystallographic nature of a material, imperfections inside a material can also influence its mechanical properties, i.e. tensile, fatigue, creep, fracture toughness, impact properties. Some defects such as missing planes of atoms, called dislocations, are responsible for plastic deformation of crystalline solids. Other artifacts such as grain boundaries, precipitates, twin planes and cracks alter stress distribution in a material and the accompanying motion of dislocations. Most defects, with the exception of dislocations and their effects, can be readily seen using a standard optical microscope. The metallographic examination can be used for both quality control and to predict and/or explain the mechanical properties
Microscopy Having described in some detail the methods by which a metallographic specimen is best prepared and subsequently etched for microscopic examination, it is now appropriate to discuss the principles of the metallurgical microscope. A metallurgical microscope differs from a biological microscope in the manner by which the specimen is illuminated. Because of the inability of visible radiation to propagate through a metal specimen, observations are made using light reflected from the polished surface. A horizontal beam of light is deflected by a plane glass reflector, upward and through a microscope objective onto the surface of the specimen. A certain amount of incident light will be reflected from the specimen surface back through the objective lens system and camera to the microscope eyepiece and computer screen. When examining a metallographic specimen, the objective of lowest magnifying power should first be used. Subsequently, greater detail of particular areas can be obtained by using progressively higher magnifications. The different objectives are mounted on a rotating head, so that their focal planes are very nearly at the same level. Thus, after focusing at the lowest magnification, only small adjustments should be necessary at higher magnifications.
Photomicrographs Materials engineers frequently need a record of the results of the metallographic work. The Plymouth University materials laboratory uses a modern closed circuit camera integrated with the microscope to digitally record the images on an attached computer. THE MICROSTRUCTURES OF PLAIN CARBON STEEL Steel is essentially an alloy of carbon and iron (or more correctly a "solid solution" of carbon in iron) which can contain up to 2.0% carbon by weight. By varying the carbon content present in the resultant alloy steel, and carrying out suitable heat treatments, it is possible to make a very useful engineering material which has a vast range of desirable mechanical properties. By including additional alloying elements such as nickel, chromium and molybdenum the properties of the steel can be further extended. During this part of the task you will be learning about the basic metallurgy and microstructure of a number of common "plain carbon" engineering steels. The notes which follow describe the features of a range of plain carbon steels and will introduce the materials science required to help you understand them better. Embedded within the text are a number of active links which will take you to a suitable site where you can extend your understanding and will assist you with learning the topic. Control click will take you to these. Before you access any of these links it is suggested that you first read these notes fully to gain a general introduction to steel. Phase Equilibrium Diagrams The best way to understand the metallurgy of carbon steel is to study the 'Iron - Carbon Phase Equilibrium Diagram'. Here the important concepts for a phase diagram are: • A phase diagram is a chart (essentially a graph) which is used to show the thermodynamic conditions required for distinct phases to exist at equilibrium in a compound or alloy • The components of phase diagram are the elements or compounds that are initially mixed. • Phase diagrams help us to understand and predict the microstructures that can exist in any alloy, and therefore is used also to predict its likely properties. • A phase system depicted in a phase diagram is at equilibrium if its free energy is at a minimum at a given specified combination of temperature, pressure and composition • A phase is a homogenous, physically distinct and mechanically separable portion of the material with a given chemical composition and structure (a, B.Y etc.)
Iron - Carbon Phase Equilibrium Diagram The iron carbon phase equilibrium diagram is the key to understanding steels. This diagram shows the different phases that are present at various temperatures when carbon is dissolved in iron. Conventionally, on the left side of the iron carbon phase diagram, the composition is 100% Fe (iron). As we move to the right along the axis so 13 the % C (carbon) increases. The important part of the iron-carbon alloy system diagram is that that lies between pure iron and the complete formation (not shown) of interstitial compound iron carbide (Fe3C) at about 6.7% C by weight. Although this diagram is usually referred to as an "equilibrium diagram" it is not a true equilibrium diagram, since the term "equilibrium" implies a condition where no further change of phase is possible with time. In fact, the compound iron carbide (FesC), readily decomposes, albeit very slowly. into iron and carbon (graphite). This decomposition takes a very long time at room temperature. In fact it will take several years to form stable graphite. As a consequence the iron carbide phase is referred to as a "metastable" phase diagram. Even though technically this diagram represents metastable conditions, it can still be considered as representing equilibrium changes that take place under relatively slow heating and cooling cycles.
The standard 'Iron - Carbon Phase Equilibrium Diagram' is shown below. The Iron - Carbon Phase "Equilibrium" Diagram 1600 SC 5 8. Le orc Lu 1400 8 1. Fec 1200 Liquid 11 2014 UN Temperature "C 1000 Darc y. 800 Tere 600 FC 400 0 2 3 5 % Carbon Microstructural Details Very low carbon steel is almost entirely made up of "ferrite", which essentially is just pure iron. As the carbon content of the steel increases so its microstructure will change. Hence the microstructure of low carbon steel is usually a mixture of ferrite and "pearlite. These plain carbon steels are generally used in the "as received" condition following hot forming or cold forming processes. Note that low carbon steels cannot be hardened by heat treatment due to their lack of carbon content. They can however be strengthened by work hardening". Increasing carbon content will strengthen steel, and by changing the microstructure, it also makes it amenable to heat treatment.
However, as the strength of the steel increases so its ductility, and therefore toughness, will decrease. To be fully effective steel should possess both strength and toughness. These properties are normally achieved by heat treatment, usually by quenching and tempering the steel Carbon A very small interstitial atom that tends to fit into clusters of iron atoms. It strengthens steel and gives it the ability to harden by heat treatment. It also causes major problems for welding, particularly if it exceeds 0.25% as it creates a hard microstructure that is susceptible to hydrogen cracking. Carbon forms compounds with other elements present. These are usually carbides e.g. iron carbide, chromium carbide etc. Delta Iron (8) Initially as the molten iron cools down it begins to solidify at 1,535 °C into delta (8). This phase has a body-centered cubic (BCC) crystal structure. With continued cooling so additional phase changes occur. These can be seen on the iron-carbon diagram. Austenite (Y) This phase exists only in carbon steel at high temperature. This phase has a "face centre cubic" (F.C.C) atomic structure which can contain up to 2% carbon in solution. Ferrite (a) This phase has a Body Centre Cubic structure (B.C.C) which can hold very little carbon; typically only about 0.0001% at room temperature. It can exist as either so called "alpha" or "delta" ferrite. Pearlite This is a mixture of alternating strips of ferrite and cementite within in a single grain and so is not a phase in its own right since it is composed of two components. The distance between these "lamellar" plates and their thickness depends on the cooling rate of the steel. A fast cooling creates thin plates that are close together and slow cooling creates a much coarser structure possessing less toughness. The name for this structure (pearlite) is derived from its apparent mother of pearl appearance under a microscope. A fully "pearlitic" structure occurs at 0.8% Carbon. Further increases in carbon will create cementite at the grain boundaries, which will start to weaken the steel. Cementite (FesC) Unlike ferrite and austenite, cementite is a very hard intermetallic compound where approximately 6.7% carbon is present with the remainder being iron. It is actually iron carbide, Fe3C. Cementite is very hard, but when mixed with soft ferrite layers its average hardness is reduced considerably. Slow cooling gives course pearlite which is softer and so easy to machine. However its toughness is poor. A faster cooling rate will result in very fine layers of ferrite and cementite and consequently the steel will be harder and tougher. The microstructure consists of pearlite with inter-granular cementite
Ferrite This phase has a Body Centre Cubic structure (BCC) which can hold very little carbon; typically only about 0.0001% at room temperature. It can exist as either so called "alpha" or "delta" ferrite. Ferrite + Pearlite The phases shown here are a mixture of ferrite and "pearlite"A body centred cubic structure (BCC) can hold very little carbon in solution. So as the carbon content increases so iron carbide (Fe3C) can precipitate from this "solid solution". Pearlite This is a mixture of ferrite and cementite within in a single grain of steel. The distance between these "lamellar" plates, and their thickness, depends on the cooling rate of the steel. The name for this structure is derived from its "mother of pearl" appearance under a microscope. Cementite Cementite (iron carbide, Fe3C) is a very hard intermetallic compound where approximately 6.7% carbon is present with the remainder being iron. The microstructure here consists of pearlite with inter-granular cementite.
Plain Carbon Steel Plain carbon steel is the most widely used kind of steel and the properties of this type of steel depend directly on the amount of carbon present. Most carbon steels have a carbon content of less than 1%. Carbon steel is made into a wide range of products. The range of plain carbon steel can be divided as follows. Low Carbon Steel Low carbon steel is one which can contain up to 0.30%C. This largest category of steel normally comes as flat-rolled products (sheet or strip) and is usually in the cold-rolled and annealed condition. The carbon content for these highly formable steels is very low, less than 0.10%C. with up to 0.4%Mn (manganese). These steels possess the best welding characteristics of any steel and they also have the added benefit of being relatively cheap. Some typical applications of these steels are detailed below. Typical Examples 0.1% - 0.2%C Chains, stampings, rivets, nails, wire, pipe, and other applications where a tough and easily deformed steel is required. 0.2% - 0.30%C A general structural steel which has common uses for machine parts e.g. shafts, levers, RSJ's (rolled steet joist) and various other forgings. Consequently these steels are relatively soft and tough. Medium Carbon Steel Medium carbon steels are similar to low-carbon steels except that the carbon ranges from 0.30% to 0.60% and the manganese from 0.60% to 1.65%. Increasing the carbon content to approximately 0.5% with an accompanying increase in manganese allows medium carbon steels to be fully heat treated (quenched and tempered). Typical Examples 0.3% - 0.4% Used for screws, gears, worms, spindles, shafts, crankshafts, couplings, forgings and many other machine parts. 0.4%-0.5%C Used for higher performance crankshafts and gears, axles, mandrels, tool shanks, and heat-treated machine parts. Rails, railway wheels and rail axles may be included in this category 0.6% - 0.7%C This may be used where a keen edge is not necessary, but where shock strength is wanted e.g. drop hammers dies, set screws, screwdrivers, etc. 0.7% - 0.8%C A tough and hard steel and consequently can be used for anvil faces, band saws, hammers, wrenches, cable wire, etc.
High Carbon Steel High-carbon steels contain from 0.60 to 1.00%C with a manganese content ranging from 0.30 to 0.90%. These steels can be easily heat treated due to their higher carbon content. So these steels are used where hardness or wear resistance is needed. However heat treatment can be problematic. Quench cracking is often a problem where fast cooling rates are experienced. High Carbon Steels' toughness, formability and hardenability are lower than the other steels. Welding is not recommended for these types of steels. Usually joined by brazing with low temperature silver alloy making it possible to repair or fabricate tool-steel parts without affecting their heat treated condition. Typical Examples 0.8% - 0.9%C Used for metal punches, rock drills, shear blades, cold chisels, rivet sets and many hand tools. 0.9% - 1.0%C Used for hardness and high tensile strength, springs, cutting tools, press tools, and striking dies. 1.0% - 1.2%C Used drills, taps, milling cutters, knives, cold cutting dies, wood working tools, files, reamers, knives, tools for cutting brass 1.3% - 1.4%C This is very hard steel and one which is used where a keen cutting edge is required, razors, saws, and where wear resistance is important. Hypo-eutectoid and Hyper-eutectoid Steels A general structural steel is usually a low carbon steel and this steel is produced in the greatest tonnages. As the carbon content increases we have seen that the steel will become stronger and harder (up to a point) and is used in applications where these mechanical properties are required. When a composition of 0.8% C is reached the steel is described as "eutectoid" steel. At this composition, and at 723°C, the eutectoid reaction takes place. So steel containing less than 0.8% C is called "hypo-euctectold steel and when it contains more than 0.8% C it is called a "hyper-euctectoid steel. These ranges are shown below on the lower part of the iron-carbon phase equilibrium diagram
The Iron - Carbon Phase "Equilibrium" Diagram (lower part) 1000 910°C Y Foc 800 OT Temperature °C 723°C 0.02% 0.89 600 Hypoeuctectoid Hypereutectoid Steel Steel a Fe C 400 0 % Carbon When this occurs the high temperature austenite phase transforms to ferrite and cementite according to the solid state phase transformation: Y austenite) 723°C a (ferrite) + FesC (cementite) Note At higher temperatures steel also exhibits the following phase reactions (these are included here for completeness). 1. The "eutectic" reaction at 4.3% C and 1147 °C. 1147°C L (liquid) Y austenite) + FesC (cementite) 2. The "peritectic reaction at 0.18 % C and 1493 °C - 1493°C L (liquid) + 8 (delta) Y austenite)
TASKD (30 marks) To consolidate your introduction to the metallurgy of steels you must now complete the following task. 1. Suggest suitable steels (in terms of carbon % content) for the following applications. Provide an explanation for your answers. i. The head of a hammer or axe. il. A surgeon's scalpel. ili. Office staples [10 marks] 2. A metallurgical microscope is suitable for the close examination of steel microstructures. Examination of metal specimens requires prior polishing and etching of their surface to reveal the grain structures. You must arrange a time to visit the Fractures Lab (SMB001) and use the metallurgical microscopes to look at FOUR different pre-prepared steel microstructure specimens (designated as specimens #1, #2, #3 and #4). Each specimen represents a different composition carbon steel. In the circular spaces (see next page) sketch the microstructure you see for each of those specimens. On your drawings identify each of the phases you see. Using the phase equilibrium diagram information state the type of steel (low, medium, high carbon etc.) each one represents. It is suggested that you use a pencil to draw the microstructures you see. [10 marks] 3. Specimen #4 is a steel designated EN9 (British Standard 970: 1955). Research this steel online to find out: i. What is its composition (put this information in a simple table). [5 marks] ii. What it can be used for. [5 marks]