mechanical engineering

Applications of plain carbon steels

These are alloys of iron and carbon, chemically combined, with other elements such as manganese, silicon, sulphur, phosphorus, nickel and chromium. Properties are governed by the amount of carbon and the heat treatment used. Plain carbon steels are broadly classified as: low carbon (0.05-0.3%C), with high ductility and ease of forming; medium carbon (0.3-0.6%C), in which heat treatment can double the strength and hardness but retain good ductility; and high carbon (> 0.6%C), which has great hardness and high strength and is used for tools, dies, springs, etc.

Applications of plain carbon steels :

Properties of carbon steels (BS 970) :

*BHN =Brinell hardness number....^To BS 950.

Tempering temperature and clolour for carbon steels :

Carbon steels

• Hard-drawn spring steel
  Low cost; general purpose; low stress; low fatigue life. Temperatures below 120°C. Tensile strength up to 1600 N mm -2.
• Piano (music) wire
  Tougher than harddrawn spring steel; high stress (tensile strength up to 2300 N inm- *); long fatigue life; used for 'small springs'. Temperatures below 120°C.
• Oil-tempered spring steel
  General purpose springs; stress not too high; unsuitable for shock or impact loads. Popular diameter range 3-15 mm.

Alloy steels

• Chrome-vanadium steel
  Best for shock and impact loaL,. Available in oiltempered and annealed condition. Used for internal combustion engine valve spriags. Temperatures up to 220 "C.
• Silicon-manganese steel
  High working stress; used for leaf springs; temperatures up to 220°C. Si1 icon-chromium steel Better than silicon-manganese; temperatures up to 220 "C.
• Stainless steels
  Cold drawn; tensile strength up to 1200Nmm-2. Temperatures from sub-zero to 290 "C, depending on type. Diameters up to 5mm.

Non-ferrous alloys

• Spring brass (70/30)
  Low strength, but cheap and easily formed. Good electrical conductivity.
• Phosphor bronze (5%Sn)
  High strength, resilience, corrosion resistance and fatigue strength. Good electrical conductivity. Tensile strength 770N mm-2. Wire diameters 0.15-7mm. Used for leaf and coil switch springs.
• Beryllium-copper (2 1/4%)
  Formed in soft condition and hardened. High tensile strength. Used for current-carrying brush springs and contacts. Tensile strength 1300Nmm-2.
• Inconel
  Nickel based alloy useful up to 370°C. Exceedingly good corrosion resistance. Diameters up to 7mm. Tensile strength up to 1300Nmm-2.

Moduli of spring materials

Grey iron
Grey iron is so called because of the colour of the fracture face. It contains 1.543% carbon and 0.3-5% silicon plus manganese, sulphur and phosphorus. It is brittle with low tensile strength, but is easy to cast.

Properties of some grey irons (BS 1452)

Spheroidal graphite (SG) iron

This is also called nodular iron because the graphite is in the form of small spheres or nodules. These result in higher ductility which can be improved further by heat treatment. Mechanical properties approach those of steel combined with good castability.

Properties of some SG irons (BS 2789)

Malleable irons

These have excellent machining qualities with strength similar to grey irons but better ductility as a result of closely controlled heat treatment. There are three types: white heart with superior casting properties; black heart with superior machining properties; and pearlitic which is superior to the other two but difficult to produce.

Properties of some maUeabie irons

* BHN : Brinell Hardness Number

Electrolytically refined copper (99.95% Pure) is used for components requiring high conductivity. Less Pure copper is used for chemical plant, domestic plumbing, etc. Copper is available in the form ofwire, sheet, strip, plate, round bar and tube. Copper is used in many alloys, including brasses, bronzes, aluminium bronze, cupronickel, nickel-silver and bryllium-copper.

Composition and mechanical properties of some copper alloys

Classification
Alloy steels differ from carbon steels in that they contain a high proportion of other alloying elements. The following are regarded as the minimum levels:

Alloy steels are classified according to increasing proportion of alloying elements and also phase change during heating and cooling as follows:
low alloy steels
medium alloy steels
high alloy steels
and according to the number of alloying elements as follows:
ternary - one element
quarternary - two elements
complex - more than two elements

General description

Low alloy steels

These generally have less than 1.8% nickel, less than 6% chromium, and less than 0.65% molybdenum. The tensile strength range is from 450-620 N mm-’ up to 85O-1000 N mm-2

Medium alloy steels

These have alloying elements ranging from 5-12%. They do not lend themselves to classification. They include: nickel steels used for structural work, axles, shafts, etc.; nickel-molybdenum steels capable of being case-hardened, which are used for cams, camshafts, rolling bearing races, etc.; and nickelchromemolybdenum steels of high strength which have good fatigue resistance.

High alloy steels

These have more than 12% alloying elements. A chromium content of 13-18% (stainless steel) gives good corrosion resistance; high wear resistance is obtained with austenitic steel containing over 11%  manganese. Some types have good heat resistance and high strength.

Antimony
A brittle lustrous white metal used mainly as an alloying element for casting and bearing alloys and in solders.

Beryllium
A white metal similar in appearance to aluminium. Brittle at room temperature. Has many applications in the nuclear field and for electronic tubes. With copper and nickel it produces alloys with high strength and electrical conductivity. Beryllium iron has good corrosion and heat resistance.

Cadmium
A fairly expensive soft white metal like tin. Used for plating and electrical storage batteries. It has good resistance to water and saline atmospheres and is useful as plating for electrical parts since it takes solder readily.

Chromium
A steel-grey soft but brittle metal. Small traces of carbide give it extreme hardness. It is used extensively in alloys and for electroplating and is also used for electrical resistance wire and in magnet alloys.

Lead
A heavy, soft, ductile metal of low strength but with good corrosion resistance. It is used for chemical equipment, roofing, cable sheathing and radiation shielding. It is also used in alloys for solder and bearings.

Lead-tin alloys
These are used as ‘soft solders’, often with a little antimony for strength. Tinman’s solder Approximately 2 parts of tin to 1 part of lead. Used for electrical jointing and tinplate can sealing. Plumber’s solder Approximately 2 parts of lead to 1 part of tin. Used for wiping lead pipe joints. Type metal Contains about 25% tin, with lead and some antimony. Has negligible shrinkage. Bearing metal Lead based ‘white metal’ contains lead, tin, antimony and copper, etc.

Magnesium
A very light metal, only one-quarter the weight of steel and two-thirds that of aluminium, but not easily cold worked. Usually alloyed with up to 10% aluminium and often small amounts of manganese, zinc and zirconium. Used for aircraft and internal combustion engine parts, nuclear fuel cans and sand and die castings. Magnesium and its alloys corrode less in normal temperatures than does steel.

Manganese
A silvery white hard brittle metal present in most steels. It is used in manganese bronze and high nickel alloys and to improve corrosion resistance in magnesium alloys.

Nickel
Nickel has high corrosion resistance. It is used for chemical plant, coating steel plate and electroplating as a base for chromium. Nickel is used for many steel, iron and non-ferrous alloys. Nickel-base alloys Monel Used for steam turbine blades and chemical plant. Composition: 68%Ni, 3o%cu, 2%0Fe. Inconel Good at elevated temperatures, e.g. for cooker heater sheaths. Composition:  8O%Ni, 14%Cr, 6%Fe.
Nimonic A series of alloys based on 70-80%Ni, with small amounts of Ti, Co, Fe, A1 and C. Has high resistance to creep and is used for gas turbine discs and blades, and combustion chambers. Strong up to 900 “C.

Platinum
A soft ductile white metal with exceptional resistance to corrosion and chemical attack. Platinum and its alloys are widely used for electrical contacts, electrodes and resistance wire.

Silver
A ductile malleable metal with exceptional thermal and electrical conductivity. It resists most chemicals but tarnishes in a sulphurous atmosphere. It is used for electrical contacts, plating, bearing linings and as an alloying element.

Tin
A low-melting-point metal with silvery appearance and high corrosion resistance. It is used for tinplate, bearing alloys and solder.

Titanium
An expensive metal with low density, high strength and excellent corrosion resistance. It is used in the aircraft industry, generally alloyed with up to 10% aluminium with some manganese, vanadium and tin. Titanium is very heat resistant.

Tungsten
A heavy refractory steel-grey metal which can only be produced in shapes by powder metallurgy (m.p.3410 “C). It is used as an alloying element in tool and die steels and in tungsten carbide tool tips. It is also used in permanent magnets.

Zinc
Pure zinc has a melting point of only 400 “C so is good for die casting, usually with 1-2%0Cu and 4%A1 to increase strength. Used for carburettors, fuel pumps, door handles, toys, etc., and also for galvanizing sheet steel, nails and wire, and in bronze.

Aluminium

This acts as a deoxidizer to increase resistance to oxidation and scaling. It aids nitriding, restricts grain growth, and may reduce strength unless in small quantities. The range used is 0-2%.

Chromium

A range of O M % , improves wear, oxidation, scaling resistance, strength and hardenability. It also increases high-temperature strength, but with some loss of ductility. Chromium combines with carbon to form a wear-resistant microstructure. Above 12% the steel is stainless, up to 30% it is used in martensitic and ferritic stainless steel with nickel.

Cobalt

Cobalt provides air hardening and resistance to scaling. It improves the cutting properties of tool steel with 8-10%. With chromium, cobalt gives certain high alloy steels high-temperature scaling resistance.

Copper

The typical range is 0.24.5%. It has limited application for improving corrosion resistance and yield strength of low alloy steels and promotes a tenacious oxide film.

Lead

Up to 0.25% is used. It increases machineability in plain carbon steels rather than in alloy steels.

Manganese

The range used is 0.3-2%. It reduces sulphur brittleness, is pearlitic up to 2%, and a hardening agent up to 1 Yo. From 1-2% it improves strength and toughness and is non-magnetic above 5%.

Molybdenum

The range used is 0.3-5%. It is a carbide forming element which promotes grain refinement and increases high-temperature strength, creep resistance, and hardenability. Molybdenum reduces temper brittleness in nickel-chromium steels.

Nickel

The range used is 0.3-5%. It improves strength, toughness and hardenability, without affecting ductility. A high proportion of it improves corrosion resistance. For parts subject to fatigue 5% is used, and above 27% the steel is non-magnetic. Nickel romotes an austenitic structure.

Silicon

The usual range is 0.2-3%. It has little effect below 3%. At 3% it improves strength and hardenability but reduces ductility. Silicon acts as a deoxidizer.

Sulphur

Up to 0.5% sulphur forms sulphides which improve machineability but reduces ductility and weldability.

Titanium

This is a strong carbide forming element. In proportions of O.2-O.75% it is used in maraging steels to make them age-hardening and to give high strength. It stabilizes austenitic stainless steel.

Tungsten

This forms hard stable carbides and promotes grain refining with great hardness and toughness at high temperatures. It is a main alloying element in high speed tool steels. It is also used for permanent-magnet steels

This is a carbide forming element and deoxidizer used with nickel and/or chromium to increase strength. It improves hardenability and grain refinement and combines with carbon to form wear-resistant microconstituents. As a deoxidizer it is useful for casting
steels, improving strength and hardness and eliminating blowholes, etc. Vanadium is used in high-speed and pearlitic chromium steels.

Vanadium

This is a carbide forming element and deoxidizer used with nickel and/or chromium to increase strength. It improves hardenability and grain refinement and combines with carbon to form wear-resistant microconstituents. As a deoxidizer it is useful for casting
steels, improving strength and hardness and eliminating blowholes, etc. Vanadium is used in high-speed and pearlitic chromium steels.

Typical properties of alloy steels

A = ausformed, MA = martempered, CR =cold rolled, P = poor, F = fair, G = good, PH = preheat required, PR = protection required, HT = at high temperature, HTR = when heat treated, FHTR=final heat treatment required.

Type and composition	Condition	Tensile MN/m2	Product	Use
Pure copper 99.95%Cu	O	220	Sheet, strip wire	High conductivity electrical applications
	H	350
98.85%cu	O	220	All wrought forms	Chemical plant. Deep drawn, spun articles
	H	360
99.25%cu +0.5%As	O	220	All wrought forms	Retains strength at high temperatures. Heat exchangers, steam pipes
	H	360
Brasses 9O%Cu, 10%Zn-gilding metal	O	280	Sheet, strip and wire	Imitation jewellery, decorative work
	H	510
7o%cU, 30%Zn-Cartridge brass	O	325	Sheet, strip	High ductility for deep drawing
	H	700
65%Cu, 35%Zn- standard brass	O	340	Sheet, strip and extrusions	General cold working alloy
	H	700
60%Cu, 40%Zn- Muntz metal	M	375	Hot rolled plate and extrusions	Condenser and heat exchanger plates
59%Cu, 35%Zn, 2%Mn, 2%A1, 2%Fe	M	600	Cast and hot worked forms	Ships screws, rudders
58%Cu, 39%Zn, 3%Pb free cutting	M	440	Extrusions	High speed machine parts
Bronzes 95.5%Cu, 3%Sn, 1.5Zn	O	325	Strip	Coinage
	H	725
5.5%Sn, O.l%Zn, Cu	O	360	Sheet, strip and wire	Springs, steam turbine blades
	H	700
10%Sn, 0.03-0.25P, Cu- phosphor bronze	M	280	Castings	Bushes, bearings and springs
10%Sn, O.S%P, Cu	M	280	Castings	General-purpose castings and bearings
10%Sn, 2%Zn, Cu-Admiralty gunmetal	M	300	Castings	Pressure-tight castings, pump,valve bodies
Aluminium bronze 95?'ocU, 5%AI	O	400	Strip and tubing	Imitation jewellery, condenser tubes
	H	770
10Y0A1, 2.5%Fe, 2-5%Ni, Cu	M	700	Hot worked and cast products	High-strength castings and forgings
Cupronickel 75%cu, 25%Ni	O	360	Strip	British 'silver' coinage
	H	600
70%Cu, 30%Ni	O	375	Sheet and tubing	Condenser tubes, good corrosion resistance
	H	650
29%cu, 68%Ni, 1.25%Fe,1.25%Mn	O	550	All forms	Chemical plant, good corrosion resistance
	H	725
Nickel-silver 55%Cu, 27%Zn, l8%Ni	O	375	Sheet and strip	Decorative use and cutlery
	H	650
Ber y llium-copper 1.75-2.5%Be, 0.5% co, c u	WP	1300	Sheet, strip, wire, forgings	Non-spark tools, springs

0 =annealed, M =as manufactured, H =fully work hardened, WP=solution heat treated and precipitation hardened.

The relevant standard is BS 970 ‘Wrought Steels’. The standard is in six parts:
Part 1 Carbon and carbon manganese steels including free-cutting steels
Part 2 Direct hardening alloy steels
Part 3 Steels for Case Hardening
Part 4 Stainless, heat resisting and spring steels
Part 5 Carbon and alloy spring steels
Part 6 SI metric values (for use with Parts 1 to 5)

Each steel is designated by six symbols:
First three digits
000-199: Carbon and carbon-manganese steels. Digits represent 100 times the percentage of manganese.
200-240: Free cutting steels. Second and third digits represent 100 times the percentage of sulphur.
250: Silicon-manganese steel
300-449 : Heat-resistant, stainless and valve steels
500-999: Alloy steels
Letter
The letters A, M, H and S indicate if the steel is supplied to - chemical analysis, mechanical properties, hardenability requirements, or is stainless, respectively.
Last two digits
These give 100 times the percentage of carbon, except for stainless steels.
Example
070M20: A plain carbon steel with 0.2% carbon and 0.7% manganese. The mechanical properties, i.e. tensile strength, yield strength, elongation and hardness, are given in the standard.