Steel and Plain Carbon Steels

• Lecture 4 covers steel, specifically plain carbon steels

Steel Overview

• Steel consists mostly of iron
• Carbon content ranges between 0.2% and 2.1% by mass
• Carbon is the most common alloying material
• Other alloying elements include manganese, chromium, vanadium, and molybdenum

Properties of Steel

• Increased carbon leads to higher hardness and strength
• Steel contains no free carbon
• Increasing the carbon content can shift the metal to cast iron
• Steel consists of carbon and iron (Fe and C) combined

Microscopic View of Steel

• The microscopic view reveals the boundaries of steel
• Fe + carbon less strong creates Cementite which is very hard
• Fe + Cementite creates pearlite which is hard and strong

Microscopic View of Pure Iron

• Cementite contains 1 part carbon, 14 parts iron (Fe) ferrite
• Pearlite contains 87% (Fe) ferrite, 13% (Fe+C) cementite

Pearlite Composition

• Maximum 0.9% carbon that pearlite can contain if there are 100 parts of pearlite
• 13 parts are cementite
• 87 parts are ferrite
• 15 parts of cementite contain 1 part carbon, 1 cementite contains 1/15 carbon (13 cementite contains 0.87% = 13/15)

Types of Steel

• Types of steel include steel and alloy steel
• Alloy steel can be stainless steel, heat-resisting steel, or high-speed steel
• Steel can then be broken down to alloy steel or also carbon steel
• Alloy steel can then be broken down to low alloys or high alloys
• Types of Carbon Steels include plain carbon steel
• Plain carbon steels included dead mild steel, mild steel, medium carbon steel, and high carbon steel

Plain Carbon Steel

• Plain carbon steel has their properties mainly due to the properties of carbon
• Plain carbon steels are classified based on manufacturing methods
• Plain carbon steels are also classified depending on chemical composition and quantity of alloying element

Classification by Manufacturing

• Puddled steel
• Bessemer steel
• Open hearth steel
• Electric arc steel
• Crucible steel
• Induction steel
• Electrolytic steel

Contents Other Than Carbon and Iron

• Manganese: up to 1.0%
• Silicon: up to 0.30%
• Sulfur: up to 0.05%
• Phosphorus: up to 0.05%

Dead Mild Steel

• Carbon content: 0.1-0.15%
• High ductility due to low carbon content
• Can be pressed into complicated shapes, even when cold
• Slightly weaker than mild steel
• Is not usually machined because its softness causes tearing and poor finish
• Slightly higher strength than wrought iron
• Low cost and easier to produce than wrought iron
• Easily cold worked and malleable and ductile
• Not easily machined to a good finish
• Used for making wires, thin sheets, and solid drawn tubes

Mild (Low Carbon) Steel

• Carbon content: 0.15-0.3%
• Relatively soft and can be drawn into hot and cold conditions
• Can be easily machined
• Has slightly higher strength than dead mild steel
• Low cost
• Easily hot worked and malleable
• Easily cold worked and ductile
• Easily machined to a good finish
• Used for structural sections (girders, reinforcing rods and mesh)
• Used for making sheets, strips, and boiler tubes
• Used for general workshop purposes
• Used for making rods, welding tubes and tubes, wire goods, nuts, and bolts

Medium Carbon Steel

• Carbon content: 0.3-0.7%
• Harder, tougher, and less ductile than mild steel
• Cannot be bent in the cold condition
• Hot forges well, but temperature control is required
• More costly than mild steel, with a more limited range of forms
• Has higher strength than mild steel
• Respond well to heat treatment to further increase its toughness and hardness
• It is used for higher stressed components
• Can be low carbon range, 0.3 - 0.5%)
• Can be high carbon range (0.5 - 0.7%)
• Low carbon range is used for axels, drop hammer die blocks, laminated springs, high tensile tubes, wire ropes, agricultural tools, screw drivers, wood saws, gold chisels, spanners (US: wrench) and hammer heads
• High carbon range is used for Forged blanks for connecting rods, crankshafts, gears and other stressed components, high tensile tubing, hot rolled bars for general workshop use, springs, wire ropes and hammers

High Carbon Steel

• Carbon content: 0.7-1.9%
• Harder, less ductile, and slightly less tough than medium carbon steel
• Cold forging is not recommended, but it hot forges well
• More costly than medium carbon steel
• Available only in the form of hot rolled bars and forging for a limited range of cold drawn wire (Piano wire)
• Low carbon range: 0.7-0.9% for toughness & hardness, makes chisels, some hard tools, shear blades, coil springs, axe heads, knives
• Medium carbon range 0.9-1.1% for hardness, makes drills, taps, screwing dies and general metal cutting tools
• High carbon range is 1.1-1.9% for very hard, makes ball bearings, files, metal turning tools, wood working and fine age tools and wear-resisting tools

Plain Carbon Steels Comparison

• Dead Mild Steel: 0.1-0.15% carbon, used for wires, rods, sheets etc
• Mild Steel: 0.15-0.3% carbon, used for grinders, plates, nuts and bolts etc
• Medium Steel: 0.3-0.5% and 0.5-0.7% carbon, metal ropes, wires, garden tools and dies etc
• High Carbon Steel: 0.7-0.9%, 0.9-1.1%, and 1.1-1.9% carbon, chisels, hammers, drills, files, lathe tools etc

Case Hardening

• Harden processes only the exterior of the steel part
• Creates a hard, wear-resistant skin (the "case")
• Preserves a tough and ductile interior
• Carbon steels are not very hardenable; therefore, wide pieces cannot be through-hardened
• Alloy steels have better hardenability, so can through-harden and do not require case hardening
• Carbon steels are case hardened which give the surface good wear characteristics and the core tough