SURFACE HEAT TREATMENT PROCESSES

Prepared ByMr.P.Senthamaraikannan, M.E., (Ph.D).Assistant professor,Department of Mechanical engineering,Kamaraj college of engineering and technology,Virudhunagar.

Case hardening

Low carbon steels cannot be hardened by heating due to the small amounts of carbon present.

Case hardening seeks to give a hard outer skin over a softer core on the metal.

The addition of carbon to the outer skin is known as carburising.

Advantages of case hardening :

Less distortion compared to through hardening steel

Fatigue properties of a part can be controlled and frequently improved

Relatively inexpensive steel can be given wear-resisting properties which would be normally attained through the use of more highly alloyed and more expensive steels

Hardening of the surface of steels which cannot be normally capable of being hardened to a high degree by altering the surface composition

Combination of case and core properties can be attained that are not possible with conventional hardening treatment

.

Scaling and decarburisation are minimized during surface

hardening, offering advantages in producing machined parts

Can be applied to very large parts, which due to very large mass

or because of danger of cracking would be impractical to harden,

by conventional heating and quenching Selected area can be

hardened on any sized place that are difficult with conventional

heating and quenching

High surface hardness, improve resistance to wear and galling,

improve fatigue life, improve corrosion resistance (stainless steel is

an exception)

Carburising Methods

Pack carburising Salt bath Technology or

Liquid Carburising Gas Carburising Vacuum or Low pressure

carburising

CO2 + C ---> 2 COReaction of Cementite to Carbon Monoxide:2 CO + 3 Fe --->Fe3C + CO2

Pack carburising

The component is packed surrounded by a carbon-rich compound and placed in the furnace at 900 degrees.

Over a period of time carbon will diffuse into the surface of the metal.

The longer left in the furnace, the greater the depth of hard carbon skin. Grain refining is necessary in order to prevent cracking.

Pack carburising This process is the simplest and earliest

carburising process. The process involves placing the components to

be treated in metal containers with the caburising mixture, based on powdered charcoal and 10% barium carbonate, packed around the components.

The containers are then heated to a constant temperature (850 oC to 950 oC)for a time period to ensure an even temperature throughout and sufficient to enable the carbon to diffuse into the surface of the components to sufficient depth.

:

Salt bath carburising. A molten salt bath (sodium cyanide, sodium carbonate and sodium chloride) has the object immersed at 900 degrees for an hour giving a thin carbon case when quenched.

Gas carburising. The object is placed in a sealed furnace with carbon monoxide allowing for fine control of the process.

Nitriding. Nitrides are formed on a metal surface in a furnace with ammonia gas circulating at 500 degrees over a long period of time (100 hours). It is used for finished components.

EFFECT OF CARBURIZING GEAR TOOTH

Limitations of Pack Carburising It is difficult to control case depths of less than

0,6mm The normal case depths produced are 0.25mm to

6mm Poor control of surface carbon content and inability

to produce close tolerances of case depth are disadvantages and is seldom used now a days.

Liquid Carburising This process is mostly used for producing shallow case depths in

thin sections.   The components are heated in a bath containing a suitable mix

of sodium cyanide salts and sodium carbonate. The normal case depths for this process are about 0,25 to 0.5mm

with bath strengths of 20% to 30% NaCN. High bath strengths 40% to 50% NaCN are required for case depths of 0,5mm and above.

Liquid Carburising The case resulting from this process includes carbon

and nitrogen.Which makes this process ideal for low carbon sheet metal pressed and machined components.

This process normally works with bath temperatures of 800oC to 930oC for immersion times from 2 to 7 hours depending on the depth required.

However the disposal problems in eliminating the solid waste & wash water have made this process not environmental friendly and is becoming obsolete.

Gas Carburising Gas carburising is very popular and widely used for case depths

ranging from 0.2 mm to 3 mm. Possible to achieve narrow bands of case depth requirements. Good Repeatability of end results possible Different Furnaces and atmospheres are used to suit the end

requirements of the product and cost impact.

Carbonitriding Carbo nitriding is the variation of Carburising

where nitrogen is additionally introduced into the surface along with carbon.

This achieved by passing Ammonia along with regular gases up to 5% of the Total volume of gases used for carburising

Ammonia breaks and gives necessary source of Nitrogen

Carbonitriding is done at temperatures of 830-890 oC and is used for case depths ranging from 0.1 mm to 0.7 mm.

CARBO NITRIDINGGAS NITRIDING

The nitrogen source - Ammonia (NH3). At the nitriding temperature the ammonia dissociates into Nitrogen and Hydrogen. 2NH3 ---> 2N + 3H2

PRINCIPLE OF INDUCTION HARDENING

GEAR TEETH

Induction hardening

Induced eddy currents heat the surface of the steel very quickly and is quickly followed by jets of water to quench the component.

A hard outer layer is created with a soft core. The slideways on a lathe are induction hardened.

Induction Hardening

Flame hardening

Gas flames raise the temperature of the outer surface above the upper critical temp. The core will heat by conduction.

Water jets quench the component.

Age hardening

Hardening over a period of time Also known as precipitation hardening Occurs in duraluminium which is an aluminium alloy

that contains 4% copper. This makes this alloy very useful as it is light yet reasonably hard and strong, it is used in the space industry.

The metal is heated and soaked (solution treatment) then cooled and left.

Pyrometry

The measurement and control of temperature in a furnace is called pyrometry.

Optical pyrometer

Also known as ‘disappearing filament’.

The light intensity of a lamp, which can be adjusted, is compared to the light from a furnace.

Temperature is measured when the filament seems to disappear in the glow from the furnace.

Thermo-electric pyrometer

A thermocouple uses the principle that a small current flows if two dissimilar metals are joined in a loop with different temperatures at the junctions.

A galvanometer at the cold junction detects a change in current at the hot junction in the furnace

Vacuum hardening

Why vacuum hardening?

Development of vacuum concept for heat treatment

Process details1. evacuation of the furnace is started and vacuum level of better than 1X

10-3 mbar is achieved.2. heating of the furnace starts when the vacuum level of 1.33X10-1 mbar

is reached..3. Heating cycle

Room Temperature |10ºC/min

650ºC -120 mts soaking at furnace temperature |10ºC/min 850ºC-120mts |5ºC/min 1000ºC-60mts |5ºC/min 1100ºC-60mts

| 1170ºC-30mts |5ºC/min 1250ºC-30min

Note:- partial pressure of 4X10-1 mbar is maintained above 800ºC

4. Cooling cycle

Temperature v/s time diagram for vacuum hardening process

Benefits

1. Optimum hardening 2. Distortion and crack free hardening of the workload.3. Absence of oxidation, decarburization or

carburization on the surface of work piece.4. Reduced or no post-hardening and finished costs.5. Prevent surface reaction such as oxidation or

decarburizing on work pieces thus retaining a clean surface intact.

6. Remove surface contaminants such as oxide films and residual traces of lubricants resulting from other operations.

Disadvantages

1. Cost for hardening increases.

2. The components are to be thoroughly cleaned before hardening in vacuum furnace

Limitations and problems

1. Volatilization and dissociation in vacuum furnace

2. Deformation or distortion

Vacuum furnaces

TYPES OF VACUUM FURNACE Vacuum furnaces can be grouped into one of the

three basic designsI. Top loading furnaces.II. Bottom loading furnaces.III. Horizontal loading furnaces.

Vertical loading vacuum furnace

Horizontal loading vacuum furnace

Main parts of vacuum furnace

FURNACE VESSEL

HEATING ELEMENT

INSULATION

PUMPING SYSTEM

Vacuum measuring and control Hot filament ionization gauge Pirani gauge Cooling system: The following media with increasing intensity

oheat transfer are used for the cooling of components.

1. Vacuum. 2. Stagnant gas (Ar, N2).3. Agitated recirculating gas (Ar, N2).4. Agitated recirculating gas at

pressure (Ar, N2, He)

Industrial Usage Of Vacuum Furnaces

HARDENING BRAZING PLASMA NITRIDING PLASMA CARBURIZING LOW PRESSURE CARBURIZING

Plasma Nitriding

1. Introduction

Plasma nitriding, known also as ion nitriding is a form of case hardening process.

It is an extension of conventional nitriding process, utilizing plasma discharge physic to diffuse nitrogen into the surface of a ferrous alloy.

 Plasma nitriding can be further branched out into plasma nitrocarburising.

In this process, carbon together with nitrogen was introduced into the metal surface.

The harden case, which is the nitriding layer is commonly known as

‘diffused case’ or ‘diffusion zone’.

Plasma nitriding is achieved using a D.C glow discharge technology, whereby the nitrogen gas inside the furnace is converted into nitrogen ions and absorbed by the metal.

Molecular nitrogen is first broken into atomic nitrogen through direct plasma dissociation.N2 + e- → N + N + e-

Atomic nitrogen is then further converted into nitrogen ion through plasma ionization N + e- → N+ + 2e-

The nitrogen ion, N+, will then diffuse into the metal surface as finely dispersed nitrides, imparting high hardness to the surface.

Thus, case hardening is achieved.

Plasma Nitriding

In Plasma nitriding process, the job part and the cathode inside the furnace will be emitting a purple glow. This is because voltages had dropped sharply at these regions. This provided a large amount of discharged energy, which causes the cathode and job part to glow. 

Plasma Nitriding Process

 Advantages for utilizing plasma nitriding

Ability to automate the system which gives good reproducibility of results

Shorter cycle time , No environmental hazard ,improve control of case depthAbility to select the compound layer type to suit the required usage

Good friction, wear, and fatigue properties

High hardness of the treated surface

Flexibility to nitride stainless steels, titanium alloys

Possibility to lower nitriding temperature and to limit distortion

Typical plasma nitriding process

Carburising Methods

Pack carburising Salt bath Technology or

Liquid Carburising Gas Carburising Vacuum or Low pressure

carburising

Pack carburising This process is the simplest and earliest

carburising process. The process involves placing the components to

be treated in metal containers with the caburising mixture, based on powdered charcoal and 10% barium carbonate, packed around the components.

The containers are then heated to a constant temperature (850 oC to 950 oC)for a time period to ensure an even temperature throughout and sufficient to enable the carbon to diffuse into the surface of the components to sufficient depth.

:

Limitations of Pack Carburising It is difficult to control case depths of less than

0,6mm The normal case depths produced are 0.25mm to

6mm Poor control of surface carbon content and inability

to produce close tolerances of case depth are disadvantages and is seldom used now a days.

Liquid Carburising

This process is mostly used for producing shallow case depths in thin sections.  

The components are heated in a bath containing a suitable mix of sodium cyanide salts and sodium carbonate.

The normal case depths for this process are about 0,25 to 0.5mm with bath strengths of 20% to 30% NaCN. High bath strengths 40% to 50% NaCN are required for case depths of 0,5mm and above.

Liquid Carburising The case resulting from this process includes carbon

and nitrogen.Which makes this process ideal for low carbon sheet metal pressed and machined components.

This process normally works with bath temperatures of 800oC to 930oC for immersion times from 2 to 7 hours depending on the depth required.

However the disposal problems in eliminating the solid waste & wash water have made this process not environmental friendly and is becoming obsolete.

Gas Carburising Gas carburising is very popular and widely used for case depths

ranging from 0.2 mm to 3 mm. Possible to achieve narrow bands of case depth requirements. Good Repeatability of end results possible Different Furnaces and atmospheres are used to suit the end

requirements of the product and cost impact.

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