Mechanical Engineers Data Handbook Episode 10 pot

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 The st

Positions of welding

In addition to 'flat' welding, which is the ideal position,

three other positions are used: horizontal, vertical and

overhead If one member is vertical and one horizontal

the position is called horizontal-vertical In the last

case a number of passes must be made to overcome the

tendency for molten metal to run out (See figure.)

5.12.9 Welding terminology, throat size

and allowable stress

216 MECHANICAL ENGINEER’S DATA HANDBOOK Allowable stress for welded structural steels \

Stress (N mm-’) 115 160 195

5.13 Limits and f i t s

It is impossible to make components the exact size and

an allowance or ‘tolerance’ must be made which

depends on the process and the application The

tolerance results in two extremes of size which must be

maintained The tolerances of two fitting parts, e.g a

shaft in a bearing, determines the type of ‘fit’ and

makes interchangeability possible

British Standard BS 4500: Part 1 : 1969, ‘IS0 Limits

and Fits’, gives a comprehensive system relating to holes and shafts; it can, however, be used for other components, e.g a key in a keyway

5.13 I Terminology

Taking the example of holes and shafts, there is a ‘basic

size’ and then maximum and minimum sizes for each,

their differences being the tolerances Their differences

from the basic size are called the ‘maximum and

minimum deviations’

Upper deviation Lower deviation Minimum limit Basic of size size E-+ Tolerance Minimum limit of size

Maximum limit 01 size nca

Maximum limit of size

Basic size

r deviation

r deviation

Maximum Minimum Maximum

mum rance

Clearance fit Transition lit Interference fit

Types of .fit 5.13.2 Selected Fits

The fit describes the manner in which two parts go

together A ‘clearance fit’ means that the shaft will

always be smaller than the hole An ‘interference fit’

means that the shaft will always be larger than the hole

and a fitting force will be necessary A ‘transition fit’

means that there may be either clearance or interfer-

ence

Tolerance

BS 4500 gives 18 ‘tolerance grades’ numbered ITO1,

ITO, IT1, IT2, up to IT16 The actual tolerance

depends on the size of the component (see table

below)

BS 4500 ‘Selected Fits’ Gives a much smaller range of fits, the hole tolerance is denoted by the letter H and the shaft by a lower-case letter (see table) For conventionally manufactured parts, the five fits given are usually sufficient (see table)

selected fits (Bs 4500)

Hole H7 H8 H9 H11 Shaft c l l d10 e9 fl g6 h6 k6 n6 p6 s6

Reduced range of fits for conventionally manufactured prts

Running Sliding Keying Press Push or shrink

5.13.3

drawing

Preliminary design drawing

Example of symbols and sizes on

It is convenient to use symbols, e.g 45 mm shaft and

‘transition’ fit Tolerance is given as: 4 45H7/k6

Production drawing

For a 30mm diameter shaft, fit H9/d10:

Hole maximum limit of size= 30.012 mm

Hole minimum limit of size = 30.00 mm

Therefore tolerance = 0.012 mm

Shaft maximum limit of size = 30.015 mm

Shaft minimum limit of size = 30.002 mm

~ Engineering materials

~~~ ~

6 I I Grey iron

Grey iron is so called because of the colour of the

fracture face It contains 1 5 4 3 % carbon and 0.3-5%0

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)

*BHN = Brinell hardness number

6 I 2 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 im-

proved further by heat treatment Mechanical proper- ties approach those of steel combined with good castability

Properties of some SG irons (BS 2789)

6 I .3 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

The strength, hardness, wear resistance, temperature

resistance, corrosion resistance, machinability and

castability of irons may be improved by the addition of

elements such as nickel, chromium, molybdenum, vanadium, copper and zirconium

6.2 I 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

220 MECHANICAL ENGINEER’S DATA HANDBOOK Applications of plain carbon steels

0.05 Dead mild

0.084 15 Mild

0.10-0.30 Mild Steel plate, sections, structural steel

0.254.40 Medium carbon Bright drawn bar

0.30-0.45 Medium carbon High tensile tube, shafts

0.40-0.50 Medium carbon Shafts, gears, forgings, castings, springs

0.554.65 High carbon Forging dies, springs, railway rails

0.654.75 High carbon Hammers, saws, cylinder liners

0.75-0.85 High carbon Chisels, die blocks for forging

0.854.95 High carbon Punches, shear blades, high tensile wire

0.95-1.10 High carbon

Sheet, strip, car bodies, tinplate, wire, rod, tubes Sheet, strip, wire, rod, nails, screws, reinforcing bars

Knives, axes, screwing taps and dies, milling cutters

Properties of carbon steels (BS 970)

Composition (%) Mechanical properties

Tensile strength Elongation Hardness,

Stronger than En2 Good machinability Weldable Increased carbon improves mechanical properties, but slightly less machinable Tough steel used for forgings, nuts and bolts, levers, spanners, etc

Medium carbon steel, readily machinable

Used for motor shafts, axles, brackets and couplings Used where strength is more important than toughness, e.g machine tool parts Increased manganese content gives enhanced strength and toughness

Properties of carbon steels (E 970) (continued)

Composition (YO) Mechanical properties

Tensile strength Elongation Hardness,

where wear is important, e.g gears and pawls

060A96t 0.99-1.0 0.14.7 0.5-0.7 1300 - 500 High carbon spring steel

*BHN =Brinell hardness number

450-700 Up to dark red Great toughness at expense of hardness

6.3 I 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:

222 MECHANICAL ENGINEER’S DATA HANDBOOK

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

6.3.2 General description

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 scal- ing It improves the cutting properties of tool steel with

8-10% With chromium, cobalt gives certain high

alloy steels high-temperature scaling resistance

Low alloy steels

Copper

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-lOoO 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, cam-

shafts, rolling bearing races, etc.; and nickel-

chromemolybdenum 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 1 1 YO

manganese Some types have good heat resistance and

high strength

6.3.3 Effect o f alloying elements

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%

The typical range is 0.24.5% It has limited applica-

tion 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 brittle-

ness, 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 in- creases high-temperature strength, creep resistance, and hardenability Molybdenum reduces temper brit- tleness in nickel-chromium steels

Nickel

The range used is 0.3-5% It improves strength,

toughness and hardenability, without affecting duc- tility 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 promotes

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 propor-

tions 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

6.3.4

Typical properties of alloy steels

Typical properties of alloy steels

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

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 micro- constituents As a deoxidizer it is useful for casting steels, improving strength and hardness and elimina- ting blowholes, etc Vanadium is used in high-speed and pearlitic chromium steels

Tensile Fatigue strength limit Corrosion Machine- Content Type Specification (Nmm-’) (Nmm-’) Weldability resistance ability Formability Low 1 %Cr, Mo 709M40

17% Cr, Ni, AI

14%Cr, Ni, Cu REX 627

15%Cr, Ni, Mo, V AM 355 Allegheny

18%Ni, Co, Mo 300grade

18%Ni, Co, Mo 250grade

1717 PH Armco

Mo, Nh Firth Vickers

Ludlum maraging INCO

1240 540

1550 700

1550 700

1310 620 (1780)

2010 850 (A2630) (A1880)

1390 -

1850

1160 340

2320 960 (A3090)

P H F H

PH/FHTR PH/FHTR

FHTR PJFHTR PHJFHTR

G

G GJFHTR

F

F

FHTR FHTR GIFHTR GIFHTR

WHTR FJHTR PIHTR

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

224 MECHANICAL ENGINEER’S DATA HANDBOOK 6.3.5 Cast high-alloy steels

Composition (YO) Tensile Yield

6.4 Stainless steels

6.4 I Types of stainless steel

Stainless steels comprise a wide range of iron alloys

containing more than 10% chromium They are

classified as austenitic, ferritic and martensitic

Austinitic stainless steels

A standard composition is l8%Cr, 8%Ni (18/8 steel)

These steels have high resistance to corrosion, good

weldability, high toughness, especially at low tempera-

ture, and excellent ductility They may be hardened by

cold working and are non-magnetic Special proper-

ties are produced by the addition of molybdenum,

cadmium, manganese, tungsten and columbium

Ferritic stainless steels

The chromium content is normally 16-20% with

corrosion resistance better than martensitic but in-

ferior to austenitic steels They are used for presswork

because of their high ductility, but are subject to brittle

failure at low temperature They have moderate

strength and limited weldability and are hardenable by

heat treatment The low carbon content makes them

suitable for forming without cracking They are mag-

netic and have low coefficients of thermal expansion

Martensitic stainless steels

The chromium content is 12-18% and the nickel

content is 1-3% These steels are the least corrosion

resistant of all They are unsuitable for welding or cold

forming They have moderate machineability and are

used where high resistance to tempering at high

temperature is important, e.g for turbine blades They

can be heat treated to improve properties and can be made with a wide range of properties They are used for cutlery

6.4.1 Selection of stainless steels

The applications of the different stainless steels are listed below

Austenitic

Window and aoor frames Roofing and guttering Chemical plant and tanks Domestic hot water piping Spoons, forks, knife handles Kitchen utensils Wash- ing machines Hospital equipment Car hub caps, rim embellishers and bumpers Wheel spokes Welding rods and electrodes Wire ropes Yacht fittings, masts and marine fittings Nuts, bolts, screws, rivets, locking wire, split pins Shafts Coil and leafsprings

Ferritic

Mouldings and trim for cars, furniture, television sets, gas and electric cookers, refrigerators, etc Coinage Spoons and forks Domestic iron soles Vehicle si- lencers Driving mirror frames Fasteners Parts to resist atmospheric corrosion Heat-resistant parts, e.g oil-burner sleeves and parts working up to 800 "C

Martensitic

Structural components Tools High temperature tur- bine parts Flat and coil springs Scales, rulers, knives, spatulas Kitchen tools and appliances where high strength and hardness are required with moderate corrosion resistance Surgical and dental instruments Record player spindles Fasteners

226 MECHANICAL ENGINEER’S DATA HANDBOOK

6.4.3 Properties of typical types

Stainless Ferritic stainless

steel 17 steel more

0.14/0.2 1 (max.) 11.5/13.5 0.1410.2 1 (max.) 11.51133 0.14/0.2 1 (max.) 11.5/13.5

0.1210.2 213 0.1210.2 213 0.1210.2 213

16/18

16/18

-

0.12 8.11 17/19 0.12 8.12 17/19