Surface Hardening

Need

• Application like gears, camshaft, crankshaft require hard and wear resistant surface with tough core to withstand impact loads.

• Difficult to meet by steel of uniform composition • Such problems can be solved by 1. Increase the carbon content at the surface of low carbon or low carbon low alloy steel and subsequently heat treating the component in specific manner to produce hard and wear resistant surface with tough core

2. Introduce nitrogen in the surface of tough steel so as to produce hard nitride surface with no heat treatment 3. Introduce carbon and nitrogen in the surface of tough steel and then heat treatment in specific manner to produce hard and wear resistant surface 4. Hardening the surface without change of composition of surface

Carburizing

Carburizing

• Increasing carbon content at the surface• Heating of steel to austenitic region in contact with a

carburizing medium , holding at that temperature for sufficient period.

• Since in austenitic range solubility of carbon is more and hence carbon from medium diffuses into austenite

• High carbon content at the surface does not mean high hardness unless the part is heat treated to convert in martensite form.

• It is also known as cementation , case carburizing or case hardening

Types

• Depending upon medium used for carburizingA. Solid ( pack or box) B. GasC. Liquid

Solid Carburizing

Solid Carburizing

• Component to be carburized are packed with carbonaceous material in a steel/cast iron boxes and sealed with clay

• Carbonaceous medium= hard wood charcoal, coke • Accelerator = Barium, Sodium or Calcium carbonate • Typical composition 53-55% charcoal, 30-32% coke

and remaining carbonates of Ba, Ca, Na

Solid Carburizing

• Reaction during carburizing are

2

2

23

22

)(2

2)(

)()(

COironinDissolveCCO

COMediumFromCCO

COBaOBaCO

COMediumFromCboxfromO

Solid Carburizing

• CO is active carburizing agent. It try to dissolve into steel and while doing so it form carbon dioxide and carbon.

• Carbon dissolve in steel and CO2 combine with carbon from medium and form CO

• The process goes on repeating and carburizing continues by the above indirect process

• Direct carburizing occur where the steel is in intimate contact of carburizer

Solid Carburizing

• Direct carburizing is not desired as it results in local variation of carbon content (non uniform hardness)

• Maximum carbon at the surface and case depth depend on temperature of carburizing and time of holding

• Higher the temperature higher the carbon content and higher case depth

• Higher temperature results in excessive grain coarsening

Solid Carburizing

• At a given temperature increase in holding time increases the case depth without changing the maximum carbon at surface

• Solid carburizing = for thick case where extreme uniformity in carbon content is not required

• For thinner and more uniform cases gas and liquid carburizing is used

Roll Carburizing

Roll Carburizing

• Used to obtain very thin case• Heat the steel component above 900 °C rolling it in

carburizing compound reheating it to same temperature holding for some time and quenching in water

Gas Carburizing

Gas Carburizing

• Heat the component to austenitic region in presence of carbonaceous gas such as methane, ethane, propane, butane diluted with carrier gas such as flue gas

• To maintain constant and uniform rate of carbon diffusion, gas composition must be properly controlled along with proper circulation.

• This process produce more uniform cases in shorter time than solid carburizing. Also this process can be performed at somewhat lower temperature.

Liquid Carburizing

Liquid Carburizing

• Immerse the steel in carbonaceous fused salt bath medium at a temperature in austenitic region.

• Bath is composed of sodium cynide(10%) sodium carbonate and sodium choride.

• Alkaline earth salts like barium calcium, or strontium are usually added to bath to encourage the cynide shift

Liquid Carburizing

• Reaction occurring during carburizing are

2

322

32

32

2

2

222

224

23

22

COCCO

NCOCONaONaCN

NCOCONaNaCNNaCNOOr

NCCONaNaCNNaCNO

NaCNOONaCN

Liquid Carburizing

• Atomic nitrogen diffuses into steel and thus results in nitriding along with carburizing.

• In the presence of earth salts access oxygen is reduced and if the operating temperature is kept high formation of cynide (NaCNO) is inhibited (Cynide shift)

CCynamideBariumBaCNCNBa

CONaCNBaNaCNBaCO

)()(

)(2

22

3223

Liquid Carburizing

• Under this shift only carburizing take place or slight nitriding along with carburizing

• In this case there is rapid and uniform heat transfer, low distortion, negligible surface oxidation and rapid absorption of carbon

• Used for rapid production of thin carburized cases

Selective Carburizing

Selective Carburizing

• Sometimes it is desired to carburize some area of the steel.

• Cover the area not desired to be carburized by thin layer of copper or by refractory paste of fire clay mixed with asbestos

Heat treatment after Carburizing

Heat treatment after Carburizing

• To obtain optimum toughness in core and optimum hardness and wear resistance at the surface

• Should be such as to give above properties with least danger of cracking spalling and warping during hardening

Direct Quench

• Directly quench in water or oil from carburizing temperature.

• After that tempered to reduce brittleness of case and minimize the danger of cracking during subsequent finish grinding

• Due to faster cooling martensite is formed at surface while reasonably fine ferrite at center.

• Austenite grain size will remain same as it is prior to quenching

Direct Quench

• If the grain size of austenite after carburizing is fine then above heat treatment process give properties adequate for service requirement

• if it is coarser, coarser martensite will form ( excessive brittle) and ferrite in the core will not be so fine to give possible toughness.

• Also there will be more retained austenite in core• So this type of heat treatment is not good for

coarser grained steels.

Double Quench • This process give best results for any kind of steels and

involve following steps 1. Slow cooling from carburizing temperature to room

temperature. ( reduce residual stresses thus tendency of distortion and cracking during subsequent steps)

2. Reheat to above upper critical temperature of core and quenching (fine ferrite core)

3. Reheating to just above the lower critical temperature of the case and quenching.

4. Tempering to relieve stresses and brittleness of case

Nitriding

Nitriding

• Heating of steel in contact with source of atomic nitrogen at a temperature of about 550 °C

• Atomic nitrogen diffuses into steel and combine with iron and certain alloying elements present in the steel and form respective nitrides which increase the hardness and wear resistance

• Molecular nitrogen does not diffuses into steel and hence completely ineffective as a nitriding medium

Nitriding

• In liquid nitriding, nitriding occurs by formation and decomposition of cyanate by the same reactions as in liquid carborizing

• Since temperature of nitriding is less so carbon cannot diffuse into steel(absence of austenite) and hence only nitrogen diffuses into the steel

Nitriding

• In gas nitriding atomic nitrogen is produced due to dissociation of ammonia which diffuses into steel

• Atomic nitrogen react with iron and form continuous layer of iron nitride. This layer does not get etched with most of the common etching regents and appears white under microscope (white layer)

• This layer is extremely hard and brittle and tend to crack or chip during service

Nitriding

• Plain carbon steel form only white layer and thus not suitable for nitriding

• Due to presence of alloying elements respective nitrides are formed. these nitrides are hard and tough and therefore such a layer of nitrides does not crack or chip

• The layer of alloy nitrides can be etched with common regents and appear dark under microscope ( Dark layer)

Advantage of Nitriding • No subsequent heat treatment is required and thus

minimum distortion• Sharply increases fatigue life, achieve better

corrosion resistance then carburized or hardened components

• Nitriding parts have excellent bearing properties• Nitriding parts have higher hardness than

carburized parts and such parts maintain their hardness upto 600 °C where most of the steel temper readily and become soft

Disadvantage of Nitriding • Suitable for alloy steels only. Increase in cost of

already costly materials• Nitriding cases are relatively thin• White layer can be removed by precision grinding of

lapping which are difficult and expensive or that layer can be converted into dark layer by heating

• No heat treatment is possible after nitriding so core properties should be adjusted before the components are nitrided

CARBONITRIDING• Carbonitride: a surface-hardening process in which both carbon and

nitrogen are absorbed into the surface of the steel. • The term 'carbonitriding' is used to describe a process in which gaseous

media are used. • Treatment takes place at 820-875°C in a carburizing atmosphere to

which has been added 3% to 8% ammonia. • The relative proportions of carbon and nitrogen dissolved may be

controlled by varying both the concentration of ammonia and the temperature.

• Since the steel must be in its austenitic condition to permit rapid solution of carbon this is in turn relatively unfavourable to the solution of nitrogen which dissolves fifty times more quickly in ferrite than it does in austenite.

• Nevertheless, solution of nitrogen in austenite is still considerable provided that the treatment temperature is kept below 900°C since the solubility of nitrogen in austenite falls as the temperature rises for a given concentration of ammonia.

Flame Hardening

Flame Hardening

• Heating of surface of hardenable steel above its upper critical temperature by means of oxyacetylene flames followed by water spray quenching to transform austenite into martensite

• It can be done in different ways such as spot method, progressive method spinning method and combination of progressive and spinning method

Flame Hardening • In spot a spot or local area of component is heated by

one or more flames followed by quenching in water• In progressive heating and quenching devices are

moved over the surface at controlled rate• In spinning flames are held against a rotating work

piece and when heating is complete part is quenched by water spray or complete immersion in water

• In combination work is rotated and the flames are traversed for heating followed by quenching by water spray

INDUCTION HARDENING• Heating of the surface is achieved by surrounding the component

with an inductor coil carrying a high-frequency alternating current (in the range 2-500 kHz).

• When an electric current passes through a coil a magnetic field surrounds the coil and a steel bar introduced into the field will carry a magnetic flux.

• Since the magnetic flux in this case is created by a high frequency alternating current, 'eddy currents' are produced in the surface layers of the steel bar which consequently become heated .

• The surface usually reaches its upper critical temperature in a few seconds.

• As soon as the surface has reached the required temperature the component is quenched either by dropping it into a quenching bath or by lowering it automatically into a water spray.

PRECIPITATION/AGE HARDENING• The strength and hardness of some metal alloys may be

enhanced by the formation of extremely small uniformly dispersed particles of a second phase within the original phase matrix; this must be accomplished by phase transformations that are induced by appropriate heat treatments.

• The process is called precipitation hardening because the small particles of the new phase are termed “precipitates.”

• “Age hardening”is also used to designate this procedure because the strength develops with time, or as the alloy ages.

• Examples of alloys that are hardened by precipitation treatments include aluminum–copper, copper–beryllium, copper–tin,and magnesium–aluminum; some ferrous alloys are also precipitation hardenable.

PRECIPITATION/AGE HARDENING• The object of the precipitation strengthening is to

create in a heat-treated alloy a dense and fine dispersion of precipitated particles in a matrix of deformable metal.

• The particles act as obstacles to dislocation motion. • In order for an alloy system to be able to precipitation-

strengthened for certain alloy compositions; there must be a terminal solid solution which has a decreasing solid solubility as the temperature decreases.

• For example: Au-Cu in which maximum solid solubility of Cu in Al is 5.65% at 548 ْ C that decreases with decreasing temperature.

• The precipitation strengthening process involves the following three basic steps:

- Solutionizing (solution heat treatment), where the alloy is heated to a temperature between solvus and solidus temperatures and kept there till a uniform solid-solution structure is produced.

- Quenching: where the sample is rapidly cooled to a lower temperature (room temperature) and the cooling medium is usually water. Alloy structure in this stage consists of supersaturated solid solution.

- Aging: is the last but critical step. During this heat treatment step finely dispersed precipitate particle will form. Aging the alloy at room temperature is called natural aging, whereas at elevated temperatures is called artificial aging. Most alloys require artificial aging, and aging temperature is usually between 15-25% of temperature difference between room temperature and solution heat treatment temperature.

PRECIPITATION/AGE HARDENING

• Precipitation strengthening and reactions that occur during precipitation can be best illustrated using the Al-4%Cu (duralumin) system.

• The Al-rich end of the Al-Cu phase diagram. It can be observed that the alloy with 4%Cu exists as a single phase α-solid solution at around 550 ْ C, and at room temperature as a mixture of α (with less than 0.5%Cu) and an inter-metallic compound, CuAl2 (θ) with 52%Cu. On slow cooling α rejects excess Cu as precipitate particles of θ.

• These particles relatively coarse in size and can cause only moderate strengthening effect.

PRECIPITATION/AGE HARDENING

PRECIPITATION/AGE HARDENING

PRECIPITATION/AGE HARDENING

• By rapidly cooling the alloy, a supersaturated solution can be obtained at room temperature.

• As a function of time at room temperature, and at higher temperatures up to 200⁰ C, the diffusion of Cu atoms may take place and the precipitate particles can form.

PRECIPITATION/AGE HARDENING