Higgins R.A. Engineering Metallurgy: Applied Physical Metallurgy

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a coherent precipitate is formed and the slight distortion produced in the

martensite lattice impedes the movement of dislocations. The term 'marag-

ing' is derived from this 'age-hardening' or 'ageing' (17.50) of martensite.

The main function of cobalt seems to be in producing more sites for the

nucleation of (Ti, Al, Mo)Ni

precipitates. In these steels about half of the

strength is due to the original iron-nickel martensite and the other half

to the subsequent precipitation hardening. If the precipitation-treatment

temperature exceeds 500

C, 'over-ageing' results and the strength begins

to fall due to reversion of the structure to austenite.

13.102 These steels combine considerable toughness with high strength

in the heat-treated condition and in this respect are much superior to

conventional high-strength, low-alloy steels. This high strength is main-

tained up to 350

C. The wrought grades are very suitable for hot- or

cold-working by most processes, whilst weldability is excellent presenting

few problems. Heat-treatment involves few difficulties since there can be

no decarburisation and no water quenches are required. An added advan-

tage is that these steels, by virtue of composition, are very suitable for

nitriding (19.40). They are, of course not stainless steels and under

atmospheric conditions will rust at about half the rate of ordinary low-alloy

steels.

Hence some surface protection is required. Nevertheless stainless

maraging steels are available and are based on chromium along with nickel,

cobalt and molybdenum.

Among the more exotic uses of these very strong—but expensive—

steels were aerospace components such as rocket-motor cases and parts for

the suspension system in the Lunar Rover Vehicle. In general engineering,

flexible drive shafts for helicopters, pressure vessels, barrels for rapid-firing

guns,

extrusion rams, cold-forming dies and die-casting dies are all

examples which indicate the wide range of uses possible. Some high-

strength maraging steels are shown in Table 13.9.

Steels Containing Boron

13.110 Most readers will be familiar with the non-metal boron in the

form of the salt, borax. Pure boron, however, is a very hard grey solid

with a high melting point (2300

C) and is added to some steels as ferro-

boron. Though limited use is made of boron as an alloying element in

Britain, boron steels are far more popular in the USA and Sweden.

13.111 Very small amounts—of the order of 0.0005 to 0.005%—are

effective, when added to fully-deoxidised steels, in increasing their harden-

ability by displacing the TTT curves to the right, that is, by reducing the

austenite

—»

ferrite + pearlite transformation rates (Fig. 13.15). The bain-

itic transformation rate may also be reduced. The reduction in transforma-

tion rates is most evident in low-carbon steels (0.2-0.4C) but with steels

in the region of the eutectoid composition the effect is negligible. A very

important feature is that, in respect of hardenability, the amounts of other

alloying elements in low-alloy steels can be reduced by as much as

half,

when small quantities of boron are added, without appreciable increase in

TIME (log.scale) (s)

Fig.

13.15 The effects of extremely small quantities of boron in retarding the rates of

transformation of austenite.

Table 13.10 Steels Containing Boron

Typical Composition (%)

0.2

0.17

0.15

0.18

0.45

0.16

0.25

1.5

0.6

1.3

0.85

1.4

1.2

0.55

0.3

1.0

0.45

0.6

0.5

0.6

0.25

0.2

0.25

0.12

0.25

0.20

0.0005

min.

0.003

0.002

Typical Mechanical Properties

Yield

point

(N/mm

)

690

700

630

1300

Tensile

strength

(N/mm

)

790

860

830

900

1000

930

1550

Elong.

(%)

Uses

High-strength

structural steel

High-tensile bainitic

constructional

steels.

Extra high-strength

constructional steels.

Automobile and

general engineering.

Welded structures—

bridges, submarines,

off-shore structures,

cranes, penstocks,

lorries, trucks, etc.

An abrasion

resistant steel—

conveyors, gears,

dumptrucks,

bulldozers,

excavators, presses,

etc.

unstable austenite

bainite

ferrite + pearlite

stable austenite

unstable austenite

transformation rates. In addition to reduction in cost this generally means

that cold formability and machinability are improved.

Low-carbon manganese steels containing boron are used in Britain for

the manufacture of high-tensile bolts, self-tapping screws and thread-rolled

wood screws. Small additions of nickel, chromium and molybdenum are

also made to some of these boron steels (Table 13.10).

Steels Containing Silicon

13.120 Silicon is commonly present in amounts up to 0.35% in steels for

sand castings, since it improves casting fluidity. However, in most wrought

medium-carbon and high-carbon steels silicon is limited to a maximum of

0.25%

because of its graphitising effect on cementite. It has a high affinity

for oxygen and is used as a deoxidant in many steels.

The increase in high-temperature corrosion-resistance imparted by sili-

con has been mentioned (13.71) and silicon is a constituent of some heat-

resistant valve steels (Table 13.6). Silicon-iron alloys have a high magnetic

permeability but a very low hysteresis (14.38) making them particularly

suitable for transformer cores and the like.

13.121 Silicon dissolves in ferrite thus increasing its strength and hard-

ness.

Since it stabilises ferrite (Fig. 13.16(i)) it raises both Ai and A

temperatures. Low-alloy steels containing silicon as the principal addition

are relatively inexpensive, but because silicon has a graphitising effect

these steels also contain up to 1% manganese as a carbide stabiliser. Both

elements combine in strengthening the ferrite and in increasing harden-

ability, so that silicon-manganese steels respond to oil-quenching. Sub-

sequent tempering provides a good combination of strength and impact

toughness. These steels (Table 13.11) have been widely used for coil and

leaf-type springs, as well as for a variety of tools such as punches and

chisels where shock-resistance is necessary.

Table 13.11 Steels Containing Silicon

Typical Composition (%)

BS 970

Part. 5.: C Si Mn Cr Mo Type of steel

250A53 0.53 1.9 0.85 — —

250A61 0.61 1.9 0.85 — — Spring steels.

925A60 0.60 1.9 0.85 0.3 0.25

— 0.07 4.3 0.08 — — Low-hysteresis iron for transformer cores.

Steels Containing Copper

13.130 Although steels containing copper are little used in Britain they

are fairly popular in the USA. Since copper has a FCC structure it tends

to stabilise austenite when added to steel. It is sparingly soluble in ferrite

(Fig. 13.16(ii)); this solubility decreasing from 1.8% at 835°C to about

0.3%

at 0

C. Copper does not form carbides and in fact has a graphitising

effect. Consequently copper is added to low-carbon steels only and then

in amounts of no more than 1.5%.

In steel copper has the following effects:

(i) it improves corrosion resistance;

(ii) it produces an alloy which can be precipitation hardened enabling

an increase of some 175 N/mm

in tensile strength to be obtained.

The steep slope of the solvus line SV (Fig. 13.16(ii) indicates that precipi-

tation hardening will be possible for a steel containing, say, 1% copper if

it is solution treated at some temperature above X and then quenched. If

the carbon content is very low martensite is not produced even if the steel

is quenched from the y-phase region. Some precipitation hardening steels

available in the USA are shown in Table 13.12.

Fig.

13.16 (i) The iron-silicon equilibrium diagram; (ii) the iron-copper equilibrium dia-

gram.

The phase e is solid solution which is, however, almost pure copper.

Table 13.12 Steels

Containing

Copper

Typical Composition (%)

0.25

0.20

0.06

0.20

1.3

1.0

0.5

0.7

1.0

0.4

1.0

1.2

0.4

1.25

1.9

1.3

0.7

1.5

0.02

0.01

Others

Cr 0.3

Mo 0.25

Typical Mechanical Properties

Yield point

(N/mm

)

340

450

550

350

550

Tensile strength

(N/mm

)

490

620

500

700

Elongation

(%)

18 Precipitation

hardening steels.

SILICON (°/o)

COPPER Wo)

liquid

In those steels which are not precipitation hardened a small increase in

yield strength is obtained over those steels containing no copper, though

little improvement in tensile strength is noticed.

HSLA and Other 'Micro-Alloyed' Steels

13.140 Recent decades have seen a continuous improvement in the

chemical purity of dead-mild steels. This in turn has meant that 'micro'

additions of suitable alloying elements can be effective in producing con-

siderable and reproducible improvements in mechanical properties. Pre-

viously, varying amounts of impurities would have masked and made less

predictable such improvements in properties. Increase in strength can be

achieved whilst retaining inherent ductility and toughness.

Very small amounts of vanadium, niobium, titanium and aluminium are

now used in high-strength low-alloy (HSLA) steels employed widely for

bodywork in the automotive industries as well as in the construction of

buildings, bridges and pipelines. The function of these alloying elements

is to combine with small amounts of carbon and nitrogen present to form

minute carbo-nitride precipitates, typically TiC, TiN and NbCN. These

precipitates severely restrict crystal growth which would otherwise take

place during necessary hot-working and annealing processes. The strength

of HSLA steels tends to increase as grain-size decreases, presumably due

to dispersion hardening (8.62) associated with these precipitates. Niobium

additions of 0.06-0.1% will develop yield strengths of 350 N/mm

in cold-

worked annealed products whilst hot-rolled HSLA steels with yield

strengths of 550 N/mm

are produced. A typical off-shore pipeline compo-

sition might be C 0.09; Si 0.3; Al 0.03; Mn 1.35; Nb 0.035; and V 0.07. Such

a 'steel' can be more correctly described as a 'dispersion strengthened ferrite'

alloy and is very different from an ordinary ferrite/pearlite mild steel.

Within the automotive industry developments in cold-reduced annealed

steels are not confined to high-strength products. For example autobody

pressings of difficult shapes require steel with very low strength and high

ductility not obtainable in normal mild steel. Vacuum degassed steels con-

taining very small amounts of titanium and/or niobium are known as 'inter-

stitial free' steels, since these additions combine with what would otherwise

remain as interstitially-dissolved atoms of carbon and nitrogen and instead

form separate precipitates of TiC, TiN and NbCN. Since no carbon or

nitrogen now remain in solid solution in the ferrite the product has a

very low yield stress combined with a high ductility and hence excellent

formability. These 'steels' are useful for severe pressings and are commonly

supplied as hot-dipped metallic-coated sheet (21.82). Consumer pressure

and (in the USA) legislation now demand that motor car manufacturers

give extended anticorrosion warranties of at least five or seven years.

Exercises

Estimate the eutectoid composition of a steel containing 5% nickel. What

would be the lower-critical temperature of such a steel? Say why such a steel

is not manufactured. (Fig. 13.3; Fig. 13.4 and 13.12)

Estimate the eutectoid composition and temperature of a steel containing 10%

tungsten. What properties would such a steel possess? (Fig. 13.3; Fig. 13.4 and

13.90)

Estimate the tensile strength of a steel containing 0.35% carbon; 0.2% silicon;

0.8%

manganese and 1.0% nickel in the normalised condition. (13.17)

A piece of steel sheet containing 0.05% carbon; 8.0% nickel; 23.0% chromium

and 1.0% titanium was heated to 1100

C and cooled in still air. What phases

are likely to be present in the structure? (13.18)

Discuss the statement 'Carbon is the most important alloying element in steel'.

6. What metallurgical reasoning must be applied when alloy steels are being

specified for machine parts?

(i) What effect does a substitutional element have upon the iron-carbon

thermal equilibrium diagram if (a) it is a y-stabiliser (b) it is an a-stabiliser?

Relate these ideas to the ability of alloy steels to be heat-treated.

(ii) What effects do these substitutional elements have upon the other proper-

ties of steels? (13.11 to 13.18)

8. Discuss the use of nickel and chromium on the heat-treatment properties and

microstructures of hardenable steels. (13.40)

9. What is meant by the term 'temper brittleness'?

Show how the mechanical properties of the steel are affected and indicate a

way in which the effect may be minimised. (13.50)

10.

Name the main groups of stainless steel, indicating those which are capable of

being hardened by heat-treatment.

Discuss difficulties which may arise with welded stainless steels. (13.35; 13.44

and 13.46)

11.

Explain, with the aid of constitutional diagrams (where necessary), the effects

of the following alloying elements when added singly to a plain carbon steel:

(i) nickel; (ii) chromium; (hi) manganese.

Why are nickel and chromium often added in conjunction to many low-alloy

steels? (13.21 to 13.24; 13.31 to

13.33;

13.81 to

13.83;

13.40)

12.

A tool is intended for forging hot steel (forging temperature 1250

C).

What conditions will the tool have to withstand? Suggest a material suitable

for this application and justify your choice. (13.91)

13.

Suggest a typical application for each of the steels shown below and explain

the role of the additions. Where appropriate, describe the heat-treatments

required and the microstructures obtained.

(a) 1.2% carbon.

(b) 0.35% carbon; 1.5% manganese; 0.45% molybdenum.

(d) 0.03% carbon; 18% chromium; 8% nickel. (Table 7.1; Fig. 11.10; 13.50

and Table 13.4; 13.36; 13.44)

14.

Give the approximate compositions of alloy steels which would be suitable for

the following:

(i) a rustless kitchen sink unit;

(ii) an aero-engine connecting rod;

(iii) the lip of a dredger bucket;

(iv) a rustless fruit-knife blade;

(v) an extrusion die for copper alloys;

(vi) a high-tensile steel bolt.

Give reasons in each case for the suitability of the steel and outline the heat-

treatment necessary for each steel you specify. (13.35; Table 13.4;

13.83;

13.36;

Table 13.8; 13.111)

15.

What is the derivation of the term 'maraging' as applied to steels?

Outline the principles involved in the heat-treatment of these steels.

Under what circumstances would these expensive materials be used? (13.101)

Bibliography

Bain, E. C. and Paxton, H. W., Alloying Elements in Steels, American Society for

Metals, 1961.

Cobalt Monograph Series,

Cobalt-containing High-strength

Steels, Centre dTnform-

ation du Cobalt, Brussels, 1974.

Hume-Rothery, W., The Structures of Alloys of Iron, Pergamon, 1966.

LuIa, R. A., Stainless Steel, American Society for Metals, 1986.

Peckner, D., and Bernstein, I. M., Handbook of

Stainless

Steels, McGraw-Hill,

1978.

Pickering, F. B., Physical

Metallurgy

and Design of

Steels,

Applied Science, 1978.

Roberts, G. A. and Carey, R. A., Tool Steels, American Society for Metals, 1980.

Thelning, K-E., Steel and its Heat-treatment: Bofors Handbook, Butterworths,

1984.

United States Steel, Isothermal Transformation Diagrams, 1963.

Wilson, R., Metallurgy and Heat-treatment of Tool Steels, McGraw-Hill, 1975

BS 970:1987 and 1988 Wrought Steels in the Form of Blooms, Billets, Bars and

Forgings (Pt. 2:

Direct-hardening

Alloy Steels; Pt. 4:

Stainless,

Heat-resisting

and

Valve Steels; Pt. 5: Carbon and Alloy Spring Steels).

BS 4490:1969 Methods for the Determination of the Austenitic Grain Size of Steel.

Complex Ferrous Alloys

High-speed Steels

14.10

The

development

high-speed steel

had its

origins

1861 when

Robert Mushet

Sheffield

was

investigating

the

value

manganese

the production

Bessemer steel.

made Henry Bessemer's process

viable

deoxidising

the

'wild' steel with manganese,

and

discovered that

one composition containing rather

large amount

manganese hardened

when

it was

cooled

in air.

Analysis showed approximately

tungsten

be present

addition,

and

this alloy subsequently found

its way on to the

market under

the

name

air-hardening steel.

It was

discovered soon

afterwards that cooling from 1100

in an air

blast gave better results

for

tools.

Some twenty years later Messrs. Maunsel White

and

Fredrick

Taylor, both

of the

Bethlehem Steel Company, replaced

the

manganese

in Mushet's steel

chromium,

and

also increased

the

tungsten content.

These changes

composition enabled

the

steel

to be

forged readily

and

produced

superior tool steel. Production engineers will already

familiar with

the

name

Taylor,

who

devoted

great deal

attention

to time-study

and

machine-shop methods.

Modern high-speed steel

was

first introduced

to the

public

at the

Paris

Exposition

in 1900. The

tools were exhibited cutting

at a

speed

about

0.3

m/s

with their tips heated

redness.

It was

found later that maximum

efficiency

was

obtained with

chromium content

of 4% and

about

18%

tungsten,

the

carbon content having been reduced from 2.0%,

in the

origi-

nal Mushet steel,

between

0.6 and

0.8%.

In 1906

vanadium

was

first

added

high-speed steel. Molybdenum

was

investigated

as a

substitute

for some

of the

tungsten,

but

increases

in the

price

molybdenum

in the

1920s suspended this development, particularly since molybdenum steels

were regarded

being inferior

properties. Only

more recent years

has molybdenum replaced much

of the

tungsten

many grades

high-

speed steel. This

particularly true

in the USA, the

World's leading

producer of molybdenum, where the greatest annual tonnage of high-speed

steel is of the molybdenum type.

14.11 The main features of a high-speed steel are its great hardness in

the heat-treated condition, and its ability to resist softening at relatively

high working temperatures. Thus, high-speed steel tools can be used at

cutting speeds far in excess of those possible with ordinary steel tools, since

high-speed steel resists the tempering effect of the heat generated.

In high-speed steels ordinary cementite, Fe

C, is replaced by single or

double carbides of three different groups based on the general formulae

C; M

and MC, where 'M' represents the total metallic atoms. Thus

C is represented by Fe

WiC and Fe

C, whilst M

is present as

O"23C

and MC as WC and VC. Vanadium Carbide is very abrasion-

resistant and so improves cutting efficiency with abrasive materials.

Since vanadium is also an important grain-refiner, this effect is very

useful in high-speed steels because of the very high heat-treatment tem-

peratures involved. It also increases the tendency towards air hardening

by retarding transformation rates. Vanadium tends to stabilise the 6-ferrite

phase (13.11) at high temperatures and this leads to carbide precipitation

so that high-vanadium steels may be somewhat brittle. Nevertheless vana-

dium is now added in amounts up to 5.0% to modern high-speed steels.

Up to 12.0% cobalt is also added to 'super' high-speed steels. Not being

a carbide former it goes into solid solution and raises the solidus tempera-

ture,

thus allowing higher heat-treatment temperatures to be used with a

consequent increase in the solution of carbides and, hence, hardenability

and wear-resistance. Since cobalt promotes excellent red-hardness these

super-high-speed steels are useful for very heavy work at high speeds.

As mentioned above, high-speed steels containing greater proportions

of molybdenum are now widely used. Generally these alloys require

greater care during heat-treatment, being more susceptible to decarburis-

ation than the tungsten varieties. With modern heat-treatment plant, how-

ever, this is not an unsurmountable difficulty. Molybdenum high-speed

steels are considerably tougher than the corresponding tungsten types and

are widely used for drills, taps and reamers.

All of the metallic carbides mentioned above are harder than ordinary

cementite, Fe

C, but the most important feature of high-speed steel is its

ability to resist softening influences once it has been successfully hardened.

This 'red' hardness, or resistance to softening at temperatures approaching

red-heat, is due to tungsten, molybdenum, cobalt, etc being taken into

solid solution in the austenite along with carbon before the steel is

quenched. Further transformations in the resultant martensite are very

sluggish as a result of the large quantities of alloying elements in solid

solution, so that the steel can be raised to quite high temperatures before

softening sets in due to carbide precipitation.

14.12 In the cast condition the structure of a high-speed steel resembles

that of a cast iron. This cast structure is broken up by forging at tempera-

tures between 900 and 1150

C, followed by reheating to about 750

C to

relieve working stresses. If required in the soft condition high-speed steel

is usually annealed by soaking for about four hours at 850-900

C, followed

by a very slow rate of cooling (10 to 20

C per hour) down to 600

C, when

it can be cooled to room temperature in still air. At this stage the ferrite

present contains relatively little dissolved alloying elements. These are

present as massive carbide particles which have precipitated during

annealing.

14.13 Obviously, when we are dealing with a complex alloy containing

up to six different elements, we cannot represent the system by a simple

binary diagram (9.100). However, we can construct a two-dimensional

diagram showing the relationship between microstructure, temperature

and carbon content for a steel containing a fixed quantity of other elements

(in this case 18% tungsten, 4% chromium and 1% vanadium). Such a

diagram (Fig. 14.1) is generally called a pseudo-binary diagram and, whilst

not being a true equilibrium diagram, serves as a useful constitutional

chart.

As is shown in the pseudo-binary diagram, austenite forms when the

steel is heated to 840

C. Since the eutectoid point has been displaced so

far to the left by the influence of tungsten and other added elements this

austenite contains little more than 0.2% dissolved carbon. If the steel were

quenched from 840

C, the martensite ultimately produced would temper

easily and show little advantage over that in ordinary types of tool steel.

°c

liquid

austenite + liquid

austenite

austenite +

complex carbides

ferrite + complex carbides

Fig.

14.1 A 'pseudo-binary' diagram representing the structure of 18-4-1 high-speed steel.

Here the tungsten-chromium-vanadium content is kept constant but carbon remains

vari-

able.

The sketches of microstructures show the effects of solution temperature upon the

extent of solubility of the complex carbides (light) in the matrix (dark) following quenching.

The higher the quenching temperature the greater the solubility of carbides (indicated by the

slope of ES).

CARBON (o/fe)