Surface Hardening

Surface Hardening Of Steels

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Hardenability-Why study it? Having studied hardness, hardenability comes as an obvious supplement with it. If we know the hardness number of a material this concept stands as a waste. But places such as shafts, gears etc

Definition

Hardenability can be defined as the capacity of a material under a given set of heat-treatment operations to harden “in depth”. Hardenability as the word itself suggests decides the hardness of the specimen under study. The hardness in such cases is not studied and determined by a single hardness test or a hardness number but by a specially configured S-A-C hardenability test which measures the hardness number at two regions basically(a)surface (b)core or by plotting the vicker’s /rockwell hardness as a function of distance from the quenched end.

Hardenability v/s hardness.

Background Hardenability is with the aim of “penetrating hardness” into the specimen. This is achieved by dissolving some of the elements into the original lattice of the specimen. The choice of the added elements has to be careful as they have to be in synchronization with the original lattice, i.e. they should be such that they fit into the lattice without inducing much dislocations, or compressive and tensile forces which may lead to unexpected failure of the specimen. In case of steel(iron-carbon alloy) this has to be done at the austenizing temperature(temperature at which steel is only austenite).The most suitable elements for surface-hardening of steels are carbon, nitrogen and to a small extent boron.

Surface-hardening processes. • Carburizing in various media. • Nitriding. • Flame hardening. • Induction surface

Hardening by localized hardening heat-treatment

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Carburizing Concept:It makes use of the fact that carbon will diffuse into iron provided that the latter is in the FCC(γ) form which exists above 910 0 C Q. Why only the austenite phase is used for the carburizing process when carbon is already present in the various other allotropic forms of carbon? 5. Austenite provides a large range of carbon compositions stable with it starting right from 0.022wt% C to 0.76wt% C. 6. The other phases already have carbon present in combination with iron as a carbide thus limiting the extent of the carburization process. 7. There are other phases present but they are stable only at higher temperatures which are difficult to maintain and achieve. Classification Depending on the carburizing media there are mainly three types of carburizing • Solid • Liquid • Gas Terminology

Fick’s Law The rate at which carbon toms diffuse beneath the surface of the specimen is determined by Fick’s Law which as follows:-

J=-S.D.δc δx J = amount of carbon passing per unit time across an area S in

a direction normal to the surface S (δc/δx) = Concentration gradient of carbon. D= Diffusion coefficient of carbon in gamma-iron (Temperature dependent) Einstein also gave a much simpler law for diffusion as follows:

x=√ 2Dt ‘x’=case depth

Solid carburizing The process involves packing the work into heat-resisting(25Cr;20Ni) boxes along with the carburizing material so that there is a space of 50mm between the components. The boxes are then heated slowly to the carburizing temperature and maintained at this temperature for according to the depth of casing required. The reactions occurring inside the boxes are as follows: 2C(charcoal)+O 2 →2CO At surface of the work it breaks into carbon dioxide releasing carbon 2CO→CO 2 + C Carbon then dissolves interstitially at the surface of steel. The rate of carburization is increased by the addition of 10-15% BaCO 3 BaCO 3→ BaO + CO 2 CO 2 + C(charcoal)

2CO

Liquid/salt-bath carburizing This type of carburizing is carried out in a mixture of salts having compositions varying from20-50% sodium cyanide. 40% sodium carbonate. Traces of sodium or barium chloride. The mixture is then heated in ‘calorised’ pots to a the carburizing temperature and held at the temperature according to the depth of the casing required. Q. What are ‘calorised’ pots? J. Calorised pots are heat-resistant pots developed by a process called calorising wherein steel is put in a chamber containing aluminium powder and air is purged from it by forcing in argon or nitrogen. The result is a fine layer of zinc on the surface of the work.

Reactions taking place during liquid carburizing:The following reactions are believed to take place 2NaCN + O 2 → 2NaCNO Sodium cyanate formed decomposes at the surface of steel as follows 8NaCNO→4NaCN + 2Na 2 CO 3 + 2CO + 4N Dissolves in 2CO→ COsteel 2 + C

Nitrogen released in its elemental form forms nitrides with carbon in steel and aids in increasing the hardness. Cyanide fumes are dangerous and a suitable extractor or filter should be used. Advantages of liquid over solid carburizing: The temperature control over the process is much more satisfactory in a liquid bath. It produces the specimen of required hardness with

Gas carburizing In recent years it has become the most popular method of mass carburizing particularly when thin cases are required. Q. Why is it the most popular method? A. (a) It is a more cleaner process. (b)The required plant size is also compact for a given output. (c)The carbon content of the case can be most accurately and easily controlled than in any kind of carburizing.

The work is heated at the carburizing temperature in an environment of gases which may yield carbon atoms by decomposition at the work surface. These are generally hydrocarbon alkanes (methane and propane) which are partially burnt in the furnace or are diluted with a carrier gas in order to produce an appropriate carbon concentration. Carrier gases are usually the mixture of hydrogen, nitrogen and carbon monoxide. The active agent is CO and the following reactions are believed to take place. 2CO CH 4 CO + H 2 CO 2 + CH 4

CO 2 + At C work surface 2H 2 + C from work H 2 O +Away C surface 2H 2 + 2CO

Investigational work in ‘plasma carburizing’ is taking place. The process if successfully developed would lead to big saving of natural gas. It is claimed that a case of one millimeter depth would be formed in thirty minutes.

Nitriding Nitriding is exactly same as carburizing with the only obvious difference that the function of the agent here is to release nitrogen after getting adsorbed at it’s surface. It is possible to nitride many types of steel but the high surface hardness is obtained with steels such having alloying elements such as Al,Cr,Mo or V which form nitrides as soon as nitrogen is dissociated at it’s surface. Iron dissolves up to 0.1% nitrogen at 5900C and that above this amount it begins to form hard nitride Fe 4 N. This is where the difference lies between carburizing and nitriding. Nitriding has to be performed in the ferritic state at about 500 0 C while carburizing has to be performed in the austenitic state at about 900 0 C.

Since it is carried out at low temperatures, nitriding is made the final operation in the manufacture of the component. The parts are maintained at 5000C for between 40 and 100 hours in a tight gas chamber through which nitrogen is made to circulate.The following reaction has been proved to occur NH 3

3H + N

NOTE: Plain carbon steel responds moderately to nitriding as the affinity of carbon for nitrogen is not high enough thus the diffusing nitrogen gets dispersed to higher depths reducing the hardness whereas it is high for steels having chromium, vanadium etc as they form hard nitrides as soon as they come into contact with nitrogen. Classification(depending on the state of nitrogen entering the steel.) Ion-/plasma nitriding or ion implantation.

Ion-nitriding. As the name suggests this type of nitriding utilizes nitrogen in the ion form. The work is placed in a chamber uniformly surrounded by nitrogen at near-vacuum conditions and is the cathode. A voltage of 5001000 volts is applied due to which N +++ ions are produced. These move towards the negatively charged work and infiltrate the casing forming hard nitrides. The collision of N +++ ions with the work raise the temperature bringing to the desired temperature(about 590 degree Celsius.)This is evident from a uniform glow of nitrogen ions around the work.

Advantages of ion-nitriding:Ø Since no quenching is required after nitriding, cracking or distortion are unlikely to occur. Ø Resistance to fatigue failure is good. Ø As compared to processes such as salt-bath carburizing where the work has to be rinsed for removal of toxic salts and the water is to be disposed of causing harm to the environment ,this is a clean process Ø The process is economical when large number of components are to be produced.

Disadvantages of ion-nitriding:Ø The process is profitable only when large number of components are to be manufactured. Ø If the work after nitriding is over-heated then all the hardness would be lost and the work will have to be nitrided again.

Carbo-nitriding As the name suggests in this type of nitriding carbon as well as nitrogen diffuse into the work-element. Although salt-bath also achieves the same goal but this process is carried out in gaseous media. The gaseous media used consists of carbon monoxide/hydrocarbon atmosphere with 3-8% ammonia added. Since carbon diffuses into steel only at austenitic temperatures this poses an unfavorable situation for nitrogen(nitrogen dissolves 50 times faster in ferritic state than in austenitic phase.).Therefore the temperature is kept at 9000C since the solubility of nitrogen in austenite falls with increase in temperature.

Localised heat-treatment Outline of the technique: The component as a whole is first-treated by quenching and tempering in order to achieve the necessary core properties. It’s surface is then austenised by heating locally and immediately quenched to a hard martensitic structure. the core structure is tempered martensite and the case is martensite. Core and Case are separated by a ‘cushion layer’ of bainite.

Background

The martensite is formed by rapid cooling (quenching) of austenite which traps carbon atoms that do not have time to diffuse out of the crystal structure. This martensitic reaction begins during cooling when the austenite reaches the martensite start temperature (Ms) and the parent austenite becomes mechanically unstable. At a constant temperature below Ms, a fraction of the parent austenite transforms rapidly, then no further transformation will occur. When the temperature is decreased, more of the austenite transforms to martensite. Finally, when the martensite finish temperature (Mf) is reached, the transformation is complete.

Flame-hardening It is a very simple process wherein a flame derived from gases such as ethyne, propane or natural gas is used to heat-treat the required area. If the area is quite small such as tip of a screw driver manually operated torches are used and as the whole area becomes austenitic the component is water quenched. For larger areas torches with built-in water jet system is used which result in progressive hardening of the specimen. For symmetrical objects such as shafts and gears they are spun around at their two centers in a ring burner. As soon as the area becomes austenitic the entire component is quenched giving a hardened surface.

Induction hardening The process differs only from the previous one that here the heating is carried out via a current carrying coil. The material to be hardened is first cast in the form of a bar and is places within the coil and an alternating current is passed through it giving rise to a magnetic field. When the steel bar is introduced in the coil the various magnetic domains start to align with the increasing magnitude and direction of current. As the direction and current starts to decay the various domains start to align in the opposite direction as indicated by a hysteresis curve. Thus hysteresis losses as well as heat induced due to eddy-currents heat the specimen to the austenizing temperature in a few seconds. The entire array is then immersed into a water-spray system quenching and

Hardenability tests Hardenability is determined by the following one test: Jominy End Quench Test.

Jominy End Quench Test 1st Step:First, a sample specimen cylinder either 100mm in length and 25mm in diameter, or alternatively, 102mm by 25.4mm is obtained. Second, the steel sample is austenitised. This is usually at a temperature of 800 to 900°C. Next, the specimen is rapidly transferred to the test machine, where it is held vertically and sprayed with a controlled flow of water onto one end of the sample. Because the cooling rate decreases as one moves further from the quenched end, you can measure the effects of a wide range of cooling rates that vary from rapid at the quenched end to air cooled at the far end.

2nd Step(To ground flat the specimen along it’s length) The specimen is ground flat to a depth of .38mm to remove decarburised material. The hardness is measured at intervals along its length beginning at the quenched end. For alloyed steels an interval of 1.5mm is commonly used where as with carbon steels an interval of .75mm is typically employed. 3rd Step(To measure and plot Vicker’s and Rockwell hardness) The Vicker’s and/or Rockwell hardness are plotted versus distance from the quenched end.