2 Iron and steel Reference Text Higgins RA & Bolton, Materials for Engineers and Technicians, 5th ed, Butterworth Heinemann Section Ch 11 Additional Readings Section
3 Iron and steel Note: This lecture closely follows text (Higgins Ch11)
4 Iron and steel: Intro (Higgins 11.1) Since the onset of the Industrial Revolution, the material wealth and power of a nation has depended largely upon its ability to make steel. Every new country ramping up into industrialisation begins by focussing on steel production Britain and Europe, then US then USSR then Japan and Korea, and the lastest example China The last few decades of Asian development have been good for Australia s mining industry.
5 Iron and steel: Intro (Higgins 11.1) China now dominates steel production almost half the world s production!
6 Smelting (Higgins 11.2) Blast Furnace turns iron ore to pig iron, which has too much carbon. This is removed in later process such as oxygen process, to make steel. Steel from Start to Finish (Promo. US) Steelmaking (UK) Continuous Casting (More modern system that suits electric arc and recycled steel, but not really suited to blast furnace which is a batch process)
7 Smelting (Higgins 11.2) Despite research on 'direct reduction' of iron ore, the blast furnace still dominates iron production. The thermal efficiency of the blast-furnace is very high, also helped by injection of oil or pulverised low-cost coal to reduce the amount of expensive metallurgical coke consumed. A blast furnace runs non-stop for several years (life of the lining) since it is quite a procedure to stop and start it. However, a typical blastfurnace releases about 6600 tonnes of carbon dioxide every day. Hebei province accounts for a quarter of the China's total steel production capacity
8 Steel-making (Higgins 11.3) Converting pig iron to steel is done by oxidation of impurities, so that they form a slag which floats on the surface of the molten steel or are lost as fume. The Bessemer process 1856 brought steel to the masses. That process is now obsolete. The open-hearth process followed but modern processes are basic oxygen processes (1952) or in the electric-arc furnace. Corus Steel (UK) Description of steel making processes
10 Steel-making (Higgins 11.3) Plain-carbon steels: less than 1.7 % C. Ordinary steels: up to 1.0 % Mn (left over from a deoxidisation process that slightly increases strength and hardness, and reduces sulphur content of the steel. Both sulphur and phosphorus are extremely harmful impurities which give rise to brittleness in steels. Usually specify max 0.05% S and Ph, and high quality steels no more than 0.04%. (or as low as 0.002% in modern steel for pipelines). The majority of steel is mild steel and low-carbon steel for structural work, none of which is heat-treated except for stress relief.
11 Cementite (Higgins ) Ordinarily carbon in steel exists as iron carbide (cementite). Cementite is very hard. So increasing carbon content increases the hardness of the steel. Cementite is actually an intermetallic compound in steel alloys with the chemical formula Fe 3 C. This phase has a specific chemical formula, unlike most phases which have ranges of chemical composition. Cementite is hard and brittle. IMAGE: Journal of Molecular Catalysis A: Chemical Volume 269, Issues 1 2, 18 May 2007, Pages
12 Carbon in Steel
13 The Iron-Carbon equilibrium diagram over a very small range of Carbon (0 to 2% by weight, or 0 to 7% by atoms) This is as much carbon as steel can handle before it turns into cast iron, and then into useless rock. This diagram will meet you again soon (not today). Larger version
14 Figure 11.4 The iron-carbon equilibrium diagram. The small dots in the diagrams depicting structures containing austenite do not represent visible particles of cementite they are meant to indicate the concentration of carbon atoms dissolved in the austenite and in the real microstructures would of course be invisible. The inset shows the 'peritectic part' of the diagram in greater detail.
15 Steel grain structures Equilibrium grain structures Identify: Ferrite Cementite Pearlite Austenite is not visible in any of these why not? watlas.mt.umist.ac.uk/internetmicroscope/micrographs/microstructures.html
18 0.4 % C These then are the main stages in the foregoing process of solidification and cooling of the 0.4 per cent carbon steel: 1 Solidification is complete at Si and the structure consists of uniform austenite. 2 This austenite begins to transform to ferrite at Ui, the upper critical temperature of this steel (about 825 C). 3 At 723 C (the lower critical temperature of all steels), formation of primary ferrite ceases, and, as the austenite is now saturated with carbon, the eutectoid pearlite is produced as alternate layers of ferrite and cementite. 4 Below 723 C, there is no further significant change in the structure.
19 Hyper Eutectic
20 Carbon vs Properties Figure 11.8 A diagram showing the relationship between carbon content, mechanical properties, and uses of plain-carbon steels which have been slowly cooled from above their upper critical temperatures.
21 Normalising (Higgins ) The main purpose in normalising is to obtain a structure which is uniform throughout the work-piece, and which is free of any 'locked-up' stresses. Read Higgins
22 Normalising (Higgins ) Larger version
23 Annealing (Higgins ) Three types of annealing: Type 1: Annealing of castings Same as normalising but slower cooling (controlled in furnace) to prevent cracking.
24 Annealing (Higgins ) Type 2: Spheroidisation annealing An annealing process which is applied to high carbon steels in order to improve their machinability and, in some cases, to help with colddrawing.
25 Annealing (Higgins ) Type 3: Annealing of cold-worked steel Recrystallisation of distorted ferrite grains to restore ductility (e.g. to allow further cold working processes).
26 Annealing (Higgins ) Summary of ranges on the Fe-C diagram.
27 Brittle Fracture in Steels (Higgins 11.7) Ferrite is very susceptible to brittle fracture at low temperatures, especially below the transition temperature. This transition temperature can be depressed to a safe limit by increasing the manganese content to about 1.3%. For use at even lower temperatures, it is better to use a low-nickel steel.
28 Online Resources. Teach yourself phase diagrams Handout Scale of material structure Wikipedia: Steel Production
29 GLOSSARY Smelting Pig Iron Basic Oxygen Process Blast Furnace Electric arc furnace Ferrite Cementite Austenite Pearlite Eutectic Eutectoid UCT LCT Hypo eutectoid Hyper eutectoid Normalising Annealing Spheroidisation annealing Work-hardened annealing Brittle fracture transition temperature
30 QUESTIONS Moodle XML: Some questions in Steel 1. Define all the glossary terms. 2. Give at least 4 reasons why iron is by far the most important metal to man. 3. Explain how carbon atoms join the iron structure in equilibrium conditions of solidification. Give the chemical name and the metallurgical name for this structure. Is this structure substitutional, interstitial or intermetallic? Which is the solute and solvent element? Is this non, complete or partial solubility? 4. Describe the cooling of a hypo-eutectoid iron-carbon mixture under equilibrium conditions. What differences are there with a hyper-eutectoid steel? 5. In the Fe-C thermal equilibrium diagram, identify the α β γ and δ phases. Which phases exist at room temperature. At what temperatures do the others exist? Explain why the δ phase gets very little mention. 6. What is the main difference in the process of normalising of a forging vs annealing of a casting? 7. What is the main difference in the process of annealing rolled sheet vs annealing of a casting? 8. Identify Ferrite, Cementite and Pearlite in photomicrographs.