ENMAT101A Engineering Materials and Processes Associate Degree of Applied Engineering (Renewable Energy Technologies) Lecture 15 Cast Iron

Transcription

1 ENMAT101A Engineering Materials and Processes Associate Degree of Applied Engineering (Renewable Energy Technologies) Lecture 15 Cast Iron

2 Cast Iron Reference Text Higgins RA & Bolton, Materials for Engineers and Technicians, 5th ed, Butterworth Heinemann Section Ch 15 Additional Readings Section

3 Cast Iron Note: This lecture closely follows text (Higgins Ch15)

4 Cast Iron (Higgins 15.1) READ HIGGINS 15.1 Ordinary cast iron is similar in composition to the crude pig iron produced by the blast-furnace. Cast Iron is still important today; It is cheap because it is close to the composition of ordinary pig iron. Mechanical rigidity and strength under compression are good. It machines with ease when a suitable composition is selected. Easy to cast because very fluid High-duty cast irons: e.g. spheroidal-graphite irons are strong, whilst malleable irons are tough.

5 Cast Iron (Higgins 15.1) Cast Iron in the Iron-Carbon equilibrium diagram. se-trans/2001/adi/cast.iron.html

6 Composition of cast irons (Higgins 15.2) Ordinary cast irons contain the following elements, Carbon %, (as graphite: grey CI, or iron carbide: white CI) Silicon %, (breaks down Fe3C, prefers grey CI) Sulphur up to 0.1%, (encourages Fe3C, but embrittlement) Manganese %, (toughens by reversing Sulphur, modify ferrite) Phosphorus up to 1.0%, (Brittle: keep to minimum, raise fluidity)

7 Composition of cast irons (Higgins 15.2) White cast iron Higgins Figure 15.2 The effects of silicon content on the structure of cast iron.

8 Composition of cast irons (Higgins 15.2) White cast iron University of Cambridge ms.ac.uk/miclib/m icrograph_record. php?id=797 Source: am.ac.uk/phasetrans/2001/adi/ca st.iron.html.

9 Composition of cast irons (Higgins 15.2) Fine grey iron Higgins Figure 15.2 The effects of silicon content on the structure of cast iron.

10 Composition of cast irons (Higgins 15.2) Coarse grey iron Higgins Figure 15.2 The effects of silicon content on the structure of cast iron.

11 Composition of cast irons (Higgins 15.2) Coarse grey iron COARSE GREY IRON The aggregate of graphite flakes in gray cast iron, deeply etched, SEM. Mag. bar is 30 μm ce/article/pii/s

12 Composition of cast irons (Higgins 15.2) Coarse grey iron COARSE GREY IRON Flake graphite close to the edge of mounted round specimen, OPTICAL. Mag. bar is 50 μm ce/article/pii/s

13 The influence of cooling rate on the properties of a cast iron (Higgins 15.3) Adding silicon makes cementite unstable, decomposing slowly. If cooled rapidly cementite is trapped and forms white iron. If cooled slowly the cementite can decompose into graphite, forming grey iron. More slowly > coarser gray iron.

14 The influence of cooling rate on the properties of a cast iron (Higgins 15.3) Deliberate chilling of a portion of the casting to produce a hard-wearing surface of white iron. The rest of the casting cools more slowly forming the tougher but softer grey iron structure.

15 'Growth' in cast irons (Higgins 15.4) When cast iron is used above 700 C, cementite decomposes into graphite and iron. This swells the iron, causing warping and cracks. To prevent this 'growth' in cast iron, cementite must be eliminated by using a high silicon content, like 5%. (Cementite decomposes during solidification)

16 Ordinary cast irons (Higgins 15.5) Ordinary cast irons fall into two main groups: engineering irons and fluid irons. READ HIGGINS Engineering irons Higgins made-in-china.com

17 Iron Bridge s.wikimedia.or g/wiki/file:iro nbridge_6.jpg

18 Ordinary cast irons (Higgins 15.5) Fluid irons Fluid irons are for decorative castings with intricate detail. High silicon content Si 2.5 to 3.5% High phosphorus content Pup to 1.5% Very brittle Cast Iron Decorative Panel gumtree.com.au

19 Videos: Ordinary Cast Irons Cast Iron Revival Keeps US Foundry Cooking (3:46 min)

20 High-duty cast irons (Higgins 15.6) Graphite flakes in grey cast are sharp, and so act like fractures (stress raisers). Strength and the toughness can be improved by turning the flakes into balls (spheroidal). READ HIGGINS Spheroidal-graphite (SG) cast iron Also 'nodular iron', or 'ductile iron (USA). Graphite flakes converted to rounded globules by adding magnesium Mg (0.1%). Tensile strength up to 775 MPa. Ductile Iron Ductile iron is useful in applications where strength needs between aluminium and steel.

21 Spheroidal-graphite (SG) cast iron Images: Pearlitic matrix Graphite nodules

22 High-duty cast irons (Higgins 15.6) READ HIGGINS Compacted-graphite (CG) cast iron Between ordinary grey iron (flakes) and SG irons. CG Iron CGI Engine Block

23 Malleable cast irons (Higgins 15.7) These are irons would ordinarily cast into a white iron (cementite structure). But with heat-treatment cementite converted into; Small spherical particles of carbon (the 'black-heart' process) Or Remove the carbon completely from the structure (the 'white-heart' process) READ HIGGINS Blackheart malleable iron Whiteheart malleable iron Pearlitic malleable iron Cast Iron Brake Disc ehow.com

24 Alloy cast irons (Higgins 15.8) Alloying elements improve the mechanical properties of an iron, by: Refining the grain size. Stabilising hard carbides. Producing cast irons with a martensitic or austenitic structure. 1 Nickel: Graphitising effect on cementite (tends to grey iron). Refines grains for heavy sections improving toughness. 2 Chromium: Forms chromium carbide (harder than cementite). Wear resistance. Less susceptible to cementite 'growth. 3 Molybdenum: Increases hardness and toughness of thick sections. 4 Vanadium: Increases strength and hardness. Better heat-resistance by stabilising carbides so that they do not decompose on heating. 5 Copper: Not much solubility but resists rusting.

25 (Higgins)

26 A note about Meehanite. Meehanite is a trademark for an engineering process to make many types of cast iron. When a foundry advertises that it produces "Meehanite Iron" this means the foundry is licensed by Meehanite Metal Corporation. The Meehanite is a series of specific processing steps, which, when followed, will produce, according to Meehanite, cast iron with uniform soundness, consistent physical properties and dependable performance in service. It covers most types of cast irons, and is not a specific alloy or material. Meehanite it really a type of quality accreditation with a name that sounds like a metallic grain.

27 Online Resources. Cast Iron Cast Iron Revival Keeps US Foundry Cooking (3:46 min) Wikipedia: Cast Iron

28 GLOSSARY Cast iron Grey iron Fine grey iron White iron Chills Growth of CI Fluid irons SG iron CG iron Malleable CI Blackheart malleable iron Whiteheart malleable iron Pearlitic malleable iron Alloy cast irons Carbide stabilizers Grain refiners

29 QUESTIONS Moodle XML: Some questions in Steel 1. Define all the glossary terms. 2. Grey vs white cast Irons: Compare strengths and weaknesses of grey and white cast irons. What is the main factor that determines whether white or grey iron is formed? What constituents can be added to help ensure (a) white or (b) grey iron is formed? 3. Describe SG iron with the aid of a sketch and explain why it helps, and how it is generated. Compare it with grey iron. 4. Explain the difference between coarse and fine grey iron in terms of both the casting process and the material properties. 5. A large lathe bed needs to be tough and crack resistant but with a hard wear resistant sliding surface (ways). Investigate casting processes and select a suitable method. Determine an appropriate cast iron type and describe some methods of ensuring a hard metal surface along the ways. Explain why cast iron is still used for machine bases and frames even though this has been the standard method for more than a century. 6. Identify suitable cast iron for: Large water pipes, firebox grating for wood burning fire, suspension arm of a truck, diesel engine block, cam shaft, cooking pot.