Week 11 the Surface Hardening of Steels(NF)

8/13/2019 Week 11 the Surface Hardening of Steels(NF)

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Last Updated:14 February 2014 LMS SEGi College 1

EAT104

MANUFACTURING AND MATERIALS

WEEK 11

The surface hardening of steels


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Introduction

Many metal components require a combination ofmechanical properties.

For example, bearing metals must be both hard

and, at the same time, ductile Many steel components, like cams and gears,

need to be strong and shock-resistant, yet also

hard and wear-resistant.


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The problem can be overcome in twodifferent ways:

1. By employing a tough low-carbon steel, andaltering the composition of its surface, either by

case-hardening or by nitriding.

2. By using a steel of uniform compositionthroughout, but containing at least 0.4% carbon,

and heat-treating the surface differently from the

core, as in flame- and induction-hardening.


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Case-hardening

This process makes use of the fact that carbon will dissolve inappreciable amounts in solid iron.

This is due to the fact that carbon dissolves interstitially in iron, thecarbon atoms are small enough to infiltrate between the larger ironatoms, so solid iron can absorb carbon.

Since only face-centred cubic iron will dissolve carbon in this way, itfollows that steel must be carburised at a temperature above theupper critical temperature. As it is generally low-carbon steel which is

carburised, this involves using a temperature in the region of 900-950C.

Thus, carburising consists of surrounding mild-steel components with

some carbon-rich material, and heating them above their uppercritical temperature for long enough to produce a carbon-rich surfacelayer of sufficient depth


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An impression of the penetration by carbon atoms into the

lattice structure of FCC iron (austenite).


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Carburising in solid media

So-called 'pack-carburising. Components to be treated are packed into steel boxes,

along with the carburising material.

Lids are then fixed on the boxes, which are then slowly

heated to the carburising temperature (900- 950C). They are then maintained at this temperature for

up to six hours (much longer periods are sometimesnecessary when deep cases are to beproduced),according to the depth of case required.


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The relationship between time of treatment, temperature, and depth of

case in a carburising process using solid media (0.15% plain-carbon

steel).


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Carburising in liquid media

Liquid-carburising - or cyanide hardening, as it is usuallycalled is carried out in baths of molten salt which

contain 20 to 50% sodium cyanide, together with as much

as 40% sodium carbonate, and varying quantities of

sodium or barium chloride. The cyanide-rich mixture is heated in iron pots to a

temperature of 870 to 950C, and the work, which is

carried in wire baskets, is immersed for periods of about

five minutes upwards, according to the depth of caserequired.

The process is particularly suitable for producing shallow

cases of 0.1 to 0.25 mm.


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The main advantages of cyanide hardening:

The temperature of a liquid salt bath is uniformthroughout, and can be controlled accurately by

pyrometers.

The basket of work can be quenched directlyfrom the bath.

The surface of the work remains clean.


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Carburising by gaseous media

Gas-carburising is carried out in both continuous andbatch-type furnaces. Whichever is used, the componentsare heated at about 900C for three hours or more in anatmosphere containing gases which will deposit carbonatoms at the surface of the components.

The gases generally used are the hydrocarbons methane('natural gas') and propane (a by-product of petroleumproduction). These should be of high purity, otherwise oilysoot may be deposited on the work-pieces.

The hydrocarbon is usually mixed with a 'carrier' gas(generally a mixture of nitrogen, hydrogen and carbonmonoxide) which allows better gas circulation and hencegreater uniformity of treatment.


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The main advantages of gas carburising:

The surface of the work is clean after thetreatment.

The necessary plant is more compact for a given

output. The carbon content of the surface layers can be

more accurately controlled by this method.


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Case-hardening steels

Plain-carbon and low-alloy steels are used forcase-hardening, but the carbon content should

not be more than 0.2% if a really tough core is to

be obtained.


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Nitriding

Nitriding is carried out at the relatively lowtemperature of 500C.

The work is maintained at 500C for between 40and 100 hours, according to the depth of case

required. The treatment takes place in a gastight chamber

through which ammonia gas is allowed tocirculate.

Some of the ammonia decomposes, releasingsingle atoms of nitrogen, which are at onceabsorbed by the surface of the steel.


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NH3= 3H + N (atom)

The relationship between time of

treatment and depth of case

produced in the nitriding process.


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Advantages of nitriding over case-hardening:

Since no quenching is required after nitriding, cracking ordistortion is unlikely, and components can be machine-

finished before treatment.

An extremely high surface hardness of up to 1150 VPN is

attainable with the aluminium-type Nitralloy steels. Resistance to corrosion is good, if the nitrided surface is

left unpolished.

Hardness is retained up to 500C, whereas a case-

hardened component begins to soften at about 200C. The process is clean and simple to operate.

It is cheap if large numbers of components are treated.


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Carbonitriding

Carbonitriding is a surface-hardening processwhich makes use of a mixture of hydrocarbons

and ammonia.

It is therefore a gas treatment, and is sometimesknown as 'dry-cyaniding' - a reference to the fact

that a mixed carbide-nitride case is produced, as

in ordinary liquid-bath cyanide processes.


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Flame-hardening

In this process, the work-piece is of uniformcomposition throughout and the surface

hardening occurs because the surface layers

receive extra heat treatment as compared with

the core material.

The surface is heated to a temperature above its

upper critical temperature, by means of a

travelling oxyacetylene torch, and is immediatelyquenched by a jet of water issuing from a supply

built into the torch-assembly.


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The principles of flame-hardening


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Induction-hardening

This process is similar in principle to flame-hardening,except that the component is usually held stationary whilst

the whole circumference is heated simultaneously by

means of an induction-coil.

This coil carries a high-frequency current, which induceseddy currents in the surface of the component, thus

raising its temperature.

As soon as the surface of the component has reached the

necessary temperature, the current is switched off and thesurface simultaneously quenched by pressure jets of

water, which pass through holes in the induction-block.


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The principles of induction-hardening.


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Summary of surface hardening processes


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Last Updated:14 February 2014 LMS SEGi College 23

References

R.A. Higgins, 2006, Materials for Engineers andTechnicians, 4thEditions, Butterworth-Heinemann.

Week 11 the Surface Hardening of Steels(NF)

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