Structure and Properties of Steels and Non-ferrous Metals Week 1

Advantages of Metals In general they are ductile Stronger metals exhibit greater stiffness & tensile strength Metals are non-porous They are easily alloyed with other metals Strong bonding is easily achieved using welding, brazing & soldering

Disadvantages of Metals Relatively expensive in terms of energy requirements during production Liable to corrosion Have high density – high self weight

Structure of Metals Major advantage with metals is that the metallic bond is a reasonably flexible type of bond As bonds are non-directional, bonding can take place readily between atoms of similar size Most metallic elements will therefore bond with each other in a process called alloying Alloying can be used to vary the properties of pure metals

Properties Properties tend to be divided into two groups: 1.) Structure intensive 2.) Structure sensitive

Structure Intensive Associated with the properties of the atoms themselves & the forces acting between them They do not depend on microstructure Principle properties: Mass Density Modulus of Elasticity Specific heat Thermal expansion coefficient Some chemical & electrical

Structure Sensitive Properties wholly dependent upon the microstructure of the material – depends upon past history: Hot rolled, cold rolled, heating and cooling and rates involved Final microstructure determined by such processes – can be used to manipulate material properties

Important Structure Sensitive Properties Yield strength Fracture strength Ductility Performance under fatigue

Variation in the Properties of Metals Variation in properties can be very useful in construction applications with tensile strength broadly reducing along with melting point Sheet roofing – requires softness & ductility – copper & zinc Structural applications – iron – higher melting point than copper & zinc

Metallic Crystal Structures Valence electrons in metals act collectively & are not confined to individual atoms Metallic crystal structures can be considered in terms of closely packed spheres Bonding tends to be non-specific & non-directional so far as particular atoms are concerned with one neighbour as good as any other Therefore expect atoms to pack closely surrounded by as many neighbours as possible

Typical Atom Valence electrons

Hexagonal Close-Packing (HCP) Tightest arrangement – involves hexagons Results from any triplet of touching atoms defining an equilateral triangle with each angle being 60° Six equilateral triangles define a complete circle of 360° Hexagonal close-packing can therefore be found in most metal crystals

Diagram Representing HCP

Face-Centred Cubic (FCC) Crystal type exhibited by many metals - a cube with an atom at each corner & one at the centre of each face - planes of atoms are in fact hexagonal but widely separated Shear forces from applied stress more likely to produce ‘slip’ of atoms causing them to slide over each other

Diagram Representing FCC

Body-Centred Cubic (BCC) Some metals adopt a more openly packed structure – a cube with an atom at each corner and another at the body centre Structure less densely packed than the HCP & FCC structures

Diagram Representing BCC

Overview of Packing HCP FCC BCC

Yielding Atoms ‘slip’ over each other During yielding bonds continue to reform with new neighbours Yielding does not represent failure Strength depends upon the ability of metals to resist shear stresses due to tensile or compressive stress

Stress/Strain Relationship Ductile Brittle Strain Stress Slope = Modulus of Elasticity

Theoretical Performance Theoretical Predictions of the mechanical strength of metals over estimate performance due to imperfections in the metallic crystal structure

Grain Boundaries Solidification commences almost simultaneously at many different points within the melt

Grain Boundaries These nuclei form many thousands of crystals which grow until they meet – this completes the solidification process Each crystal orientation is different and regions of disorder occur where they meet – these are known as grain boundaries

Cont’d Grain boundaries are surface defects – the surface of each crystal is affected Faster cooling produces a larger number of smaller crystals Can be a source of weakness in metals on account of imperfect bonding

Dislocations Dislocations are a serious form of defect as they result in yielding of metals at much reduced stress Form due to rapid cooling when metals are subjected to formation processes Atoms do not have sufficient time to pack perfectly

Cont’d All metals contain millions of dislocations Shear stress causes just a single atom per plane to swap neighbours Process ends when the dislocation reaches the end of the crystal The material has yielded but only by one atom each time on this plane - occurs at a much lower stress – accounts for lower yield strength than calculated from theoretical bond strength

Cont’d Dislocations are known as line defects Effect can be controlled by: 1.) Limiting grain size – limits scope for travel 2.) Work hardening – plastic range – dislocations meet obstructions in the metal (grain boundaries) 3.) alloying – crystal planes interrupted by different sized atoms (point defects)

Common Building Metals or MPa

Strength & Stiffness There is a correlation between strength & stiffness – both are related to bond strength Note that density shows no correlation

Tensile Strength of Metals Depends on the cooling regime – more rapid cooling tends to produce a finer grain structure & hence higher strength

Yield Stresses Determine the level of load that can be carried in service – usually substantially less than the ultimate stresses

Yield Stresses UTS YS Failure Strain hardening region Necking region

‘Creep’ Zinc & lead tend to ‘creep’ to failure in the long term – this occurs at much lower stresses than those indicated in the table

Impurities Impurities or alloying generally has a pronounced effect on strength – most impurities increase strength – also tend to increase brittleness

Iron Iron – extracted from natural ores – about 4% carbon content makes it almost useless without further industrial processing Pig iron re-melted with scrap iron or steel – produces cast iron – carbon content reduces to approx 2% - used for pipes & service fittings

White Cast Iron Iron carbide – hard & very brittle – not suitable for structural use – used for high resistance to wear & abrasion – earth movers wearing edges

Grey Cast Iron Most of the carbon is present as graphite – softer than white cast iron – easily machined – relatively weak in tension – good strength in compression – used in older structures for columns etc.

Steel The most important ferrous material in the construction industry – alloy of iron & carbon – wide range of structural uses Up to about 0.25% - mild steel or low carbon steel – structural steels 0.3 to 0.6% – medium carbon steels – carbon steels > 0.6% - high carbon steels

Steel Off-cuts

Tensile Strength of Steel Tensile strength tends to increase linearly with increase in carbon content up to 0.8% carbon content Elongation to fracture decreases from 40% to almost zero

Typical Performance 0.5 1.0 1.5 300 600 900 20 60 40 % elongation (50mm GL) tensile strength N/mm2 %C Tensile strength Elongation

Structural steel Iron carbide (cementite) exists as very fine layers in between layers of iron (ferrite) – Known as pearlite 100% pearlite occurs at 0.8% carbon content

Steel Phase Diagram

Cont’d Processed into required sections – hot rolling – normalised microstructure Cold rolled – very low carbon content – accurate dimensional control – lightweight lintels etc. Moderate strength - good toughness & ductility – easily welded due to low proportion of pearlite

Alloying elements Manganese – increases yield strength & hardness of low carbon steels Niobium – Produces smaller crystals (grains) – increases yield strength Others – molybdenum, nickel, chromium & copper

Composition & properties of some structural steels

Steel for Structural Sections (BS EN 10025) Hot rolled – easily welded – ductile – impact resistant – reasonably high yield strength (should yield if overstressed rather than give a brittle failure) Codes – S 275 J2 H S – structural 275 – min yield strength in N/mm2 J2 – impact resistance H – hollow section

Typical structural Sections

Cont.

Reinforcing Steel (BS 4449; BS 4483) Load transferred via concrete Good bond required – use of hooked ends Poor bond gives localised tensile cracking – water ingress Good bond allows the formation of micro-cracks in the concrete – some cracking is inevitable

Cont’d Safe design – under reinforced – steel yields & work hardens rather than concrete failing in shear or compression Steel must therefore: Bond to the concrete Be ductile Be weldable

Mild Steel Types Smooth round section Can form tight bends Excellent ductility Lower yield strength - 250 N/mm2 Uses – links or where tight bends are required

High Yield Types Yield strength – 460 N/mm2 now 500 N/mm2 Bars are deformed – provides additional bond: Type 1 – cold worked by twisting Type 2 – (grade 460A) - up to 16mm diameter - fabric steel - hot rolled - ribs Type 3 – (grade 460B) - hot rolled - ribs