salt baths.pptx

7/30/2019 Salt Baths.pptx

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Presentation:

Salt Baths


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IntroductionThe receptacle containing the salts (or

mixtures of salts) which when heated, melt toform a liquid medium for heat treatmentprocesses.

Depending on the nature of the salt used for:

Quenching medium

Heat treatments

Case hardening

Dip Brazing


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Construction of Salt Baths

General Design RequirementsSound construction and without points ofweakness.

Integral joints and corners be rounded off.

Important parts are

Suitable refractory lining

Outer shell

Bath coverCatch pits

Heating system

Labeling


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1) Bath Covers & outershellThe provision of suitable

sliding or hinged and counter-

balance covers.

These covers shall be of non-combustible material andshould be fitted into guides.

The covers should be in placewhenever the bath is not inuse, and in particular duringthe heating up and melting ofsolid salts.

Outer shellof Steel


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2) Catch PitsOf suitable non-combustible material.

Catch pits capable of containing the contents

of the bath in the event of any leakageoccurring. For small sized baths.

If the salt is in direct contact with the heatingsource then it is not necessary.

For large baths it will normally be impractical;for this situation provision should be made tochannel leaking salts to a safe area free ofcombustible material where they can solidify.


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3) Labeling

Every bath shall be clearly labeled with a signindicating whether nitrate or cyanide salts bath.

4) Heating systemDirect or indirect heating of the salt in bath.

Electrical and oil/gas fired heating

The fuel for combustion is provided through

pipes from top or bottom.


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5) Refractories

Fire bricks

For lining salt baths, especially high-temperature baths, high-aluminarefractory MKO-72 (TU 14-8-71) withhigh-alumina VT-1 mortar for the joints.

The use of MKO-72 sharply increasesthe service life of salt baths.


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Quench baths

Located at least 24 inches from any salt bath.

It should be lower in height than the salt

bath.The provision of a suitable screen may be

used instead of above two.

Safe discharge of quench bath water to a

suitable drain.

Cyanide salts contaminated water must bechemical treatment before being discharged.


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Installation Requirements forSalt Baths1) Location and flooring of salt baths Salt baths shall preferably be located atground-floor level.

If on an upper level, a bath should be placedon a suitable strengthened concrete floor.Where practicable, the section containing thesalt baths shall be separated from othersections of the workplace by means of fire-resisting walls.


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Installation Requirements forSalt Baths

2)Provision forAccidental Entry ofWater

Provision shall be madeto ensure that watercannot accidentally entera salt bath.

Fire sprinkler systems

shall not be fitted in heattreatment areascontaining salt baths.


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Installation Requirements forSalt Baths

3) Working Space

In rooms where one or moresalt baths are situated, thebaths shall be so placed as toprovide

adequate working space in the

vicinity of each bath for everyperson working in that part ofthe

premises.


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Storage of SaltsGeneral provisionsStorage rooms -should be dry

and clean.

No smoking area - allowed inthat area. A suitable sign should

be displayed to indicate this.Containers

All salts should be stored inappropriately labeled containerswhich are moisture-proof.


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Storage and Handling ofCyanide Salts

Cyanides should not be stored together withnitrates, and acids or acidic materials MUSTNOT be allowed to come into contact withcyanide salts.

The cyanide store shall be locked and in thecharge of a responsible person extraordinarily.

Cyanide containers should be opened only inthe room in which the salt is to be used.

Dry gloves should be worn whenever cyanidesalts are handled, and a scoop isrecommended for transference.


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Storage and Handling of Nitrate/ Nitrite Salts

Nitrates should be stored away fromoxidisable materials and sources of directheat.

Storage containers should be of

noncombustible material.If nitrate or nitrite salts are delivered in

combustible bags, the salts should betransferred to suitable containers and thebags washed after emptying or burnt undercontrolled conditions.

These salts are not much sensitive totransference of salt with scoop.


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Why do we use salt baths?

The salt baths have following advantages overconventional heat treatment furnaces:

Fast and uniform heating

Controlled cooling conditions duringquenching

Low surface oxidation and decarburization

Good control of case depths

Short treatment times


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Fast and Uniform Heating andCooling

Conduction (combined withconvection) through the liquid media(salt bath).

The heat transfer rate in a liquidmedia is much greater than thegases.

Convection in liquid salt bathproduces uniform heating and coolingconditions.


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Controlled cooling conditions during Quenching

In conventional quenching operation

Either water or oil are used as thequenching media.

Cracks and distortions

Oil removal problem

No control over cooling rate.

Not suitable for hi carbon steels.

Less hardenable steels cant bequenched.


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Controlled cooling conditions during Quenching

Salt bath quenching offersControl over the quenchingrate/severity.

Uniform quenching and no thermalstresses.

Particularly suitable for High Carbonsteels and less hardenable steels.

Interrupted quenching techniqueprevents the work part fromcracking and distortion.


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Salt Bath QuenchingMain Variables in Salt Quenching

Temperature,

Agitation,

Water content,

Residence time

Other Variables

Steel composition

Austenitizing temperature

Section thickness

Configuration of parts


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Low surface oxidation and de -C

Good control over the salt bathatmosphere.

The contact of the hot workpart with the atmosphere isminimized when the part is

treated in the salt bath.So, no iron oxide formationand de C chances.


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Heat Treatment Processes inSalt Baths

Austempring

Martempring

Hardening and annealing

Nitriding and nitrocarburizing

Carbonitriding

Carburizing and carbonitridingSolution treatment of aluminum alloys

Cleaning of metallic parts


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Classification of salt baths

Nitrate salt baths

Mixture of nitrate and nitrite salts

Chloride baths

Cyanide baths

Alkali metal hydroxides

Nit t /Mi t ith it it


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Nitrate/Mixture with nitritesSalt baths

Sodium/potassium salt bathsFor Martempering of steels,

Normal tempring.

For austempring.

For solution treatment ofaluminum alloys.

Nitrate salt baths have also foundapplication in the vulcanization ofrubber.

Safe range of use 160-550C.


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Nitrate Salt baths

Salt bath Composition

Meltingpoint

Worktemperature range

Bath I KNO3 100%

337C 350-500C

Bath II NaNO3 100%

370C 400-600C


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Nitrate-nitrite Mixture salts

Compositions Approx.Melting Point

Worktemperaturerange

KNO3 50-60%

NaNO2 50-40%

135C 160-550C

NaNO3 50-60%

NaNO2 50-40%

145C 150-500C

KNO3 50-60%NaNO3 50-40%

225C 260-600C


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Chloride Salt Baths

For Hardening.

For Tempering


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Bath Type Composition Meltingpoint

Worktemperature

rangeI NaCl 10-

15%KCl 20-

30%BaCl2 40-50%CaCl2 15-20%

400C 500-800C

II NaCO3 45-55%

KCl 55-

45%

450C 550-900C

III BaCl2 50% 540C 570-900C


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Cyanide Salt Baths

Extremely Toxic !!!

Applied for:

Liquid Nitriding

A Molten 27 -34 % of SodiumCyanide Salt bath

Liquid Carbonitriding

A molten salt containing 20-25%

of sodium cyanide


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Cyanide salt baths

Liquid Carburizing

A molten saltcontaining10-25% ofsodium cyanide

Only liquid surface treatments.


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Mixture of Alkali MetalHydroxides

Polymeric contamination on metal partssurfaces may be effectively removed byimmersion of the part into a molten salt.

Polymers decompose and burn at thetemperature of the molten salt.

Mixtures of hydroxides and nitrates assalt baths.

A temperature within 343-510C are usedfor cleaning operation.


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Alkali Metal Hydroxides

Salt bath Composition

Meltingpoint

Worktemperature range

Bath I NaOH 75%KOH 19%H2O 6%

140C 160-280C

Bath II KOH 50-

60%NaOH 50-40%

280C 300-400C


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Compatible Materials for SaltBaths

Steels

Aluminum and aluminum alloys.

Titanium and its alloys at hightemperature.

Following are not suitable:

Magnesium alloys

Zinc and its alloys


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Operation of Salt Bath

Charging of salt/mixture of salt in pre-determined amount.

A continuous supply of fresh salt mustbe provided in the burner tube zonewhile firing to ensure that the burnertubes are completely covered in salt atall times.

As salt around the burner tubes melts,more salt must be added to ensurecomplete coverage of the tubes andelimination of hot spots.


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Operation of Salt BathPut the member to be heat treated

in it for the predetermined timefor required heat treatmentprocess.

To close the lid.

Removal of member from the saltbath.

Transfer to Salt Bath Quenchant.

After residence time (according tothe required treatment), member

is cooled.

Desludging of the salt bath.


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Reaction Control IssuesExplosive generation of steam due to water or moisture

being inadvertently introduced into the molten salt.

Overheating and eventual explosive decomposition ofnitrate salts at temperatures above 550 deg C.

Explosive reactions between molten nitrates andaluminium or magnesium alloys under certainconditions.

Molten nitrates and cyanides may decomposeexplosively if mixed together.

Fires caused through the hot molten salts coming intocontact with combustible materials.


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Precautions for reactioncontrol

Good exhaust around the bath is highly recommended.Water sprinklers should notbe installed in and around

any molten salt system.

Installing audio/visual alarms that go off when bathtemperature exceeds a preset limit.

Avoid mixing of nitrate salts and the cyanide salts.

Salt should be stored in well marked, closed containers,which should be kept in a dry location segregated fromincompatible materials such as cyanide salts.

No vaporizing liquid such as carbon tetrachloride, water,

foam, or aqueousextinguishing agent shall be used for fighting fires near

molten salt baths.


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Temperature control of theSalt bath

Good temperature control for electricaland fuel fired heating salt baths.

Every salt bath shall be equipped with anefficient audible and visual alarm device

capable of giving a clear warning in theevent of the temperature of the saltexceeding a safe predetermined level.

Electrical heating gives better heating

efficiency.


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Gas/Oil Heating

Low maintenance cost Not Better Temperature Uniformity and

Controllability

Can Be Used for Higher Temperature Processes and

with this operating cost increases. Explosion Hazards

Flue Gases to Deal With

Pollution or Emissions of NOx Etc. Hi Initial Cost for Furnace

Relatively complex to Install and Operate


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Electrical Heating

100 % Efficient Better Temperature Uniformity and Controllability

Can Be Used for Higher Temperature Processes

Safe - No Explosion Hazards

No Flue Gases to Deal With

No Pollution or Emissions of NOx Etc. Lower Initial Cost for Furnace

Easy to Install and Operate Can Be Easily Automated

Hi maintenance cost.


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Atmospheric Control of Saltbath

Good atmospheric control isachieved in the salt baths.

Avoiding de-C and oxidation of

the surface of steels.


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Health/Environmental Hazards

Cyanide salts may be proved to be health andenvironmental enemy.

Production of highly toxic hydrogen cyanidegas from reaction of acids, or water (undercertain conditions), with cyanide salts.

Burns resulting from contact with the moltensalt. These are especially hazardous whencaused by molten cyanides since absorption viabody tissues can lead to fatal poisoning.

Accidental ingestion of cyanide salts.

Gassing caused by inhalation of fumes given offin the heat treatment process.


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salt baths.pptx

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