hot forging lubricants - · pdf filelubrication system as hot forging lubricants. graphite...

155

Int. J. Mech. Eng. & Rob. Res. 2014 Ujjwal Kumar et al., 2014

HOT FORGING LUBRICANTS

Ujjwal Kumar1*, Ajay Kumar Mishra1 and Rajkumar Ohdar1

*Corresponding Author: Ujjwal Kumar,[email protected]

Importance of lubrication In a Hot Forging process and die lubrication is the key for quality andproductivity. Lubrication, however, is not an easy job and is very dependent on the right selectionof parameters such as: Lubricant and application types, team supervision and operatorexperience, even when fully automated systems are employed. A lubricant for forging dies is achemical compound that has a formulation that makes possible the forging process to be carriedout within the limits and parameters considered by the forging industry. An attempt here hasbeen made to elaborate and discuss the different aspects of the topic.

Keywords: Die, Forging

INTRODUCTIONIn the hot forging processes, the dies arelubricated and cooled by spraying dilute waterbased lubricants on the heated die surfaces.This spray is produced by atomizing lubricantsinto small droplets using spray devices withpressurized air Figure below.

Figure 1, Lubrication spray process (top)modelled as the spread of a single droplet(bottom). The physical parameters are do, d anddmax (the initial, evolving and maximumdiameters of the drop respectively); Vo the initialdroplet velocity.

LUBRICANTSA lubricant for forging dies is a chemicalcompound that has a formulation that makes

ISSN 2278 – 0149 www.ijmerr.comVol. 3, No. 4, October 2014

© 2014 IJMERR. All Rights Reserved

Int. J. Mech. Eng. & Rob. Res. 2014

1 Department of Forge Technology, National Institute of Foundry and Forge Technology, Ranchi 834003, Jharkhand, India.

Research Paper

Figure 1: Overview of Lubrication

Source: Yang and Shivpuri (2006)

156


possible the forging process to be carried outwithin the limits and parameters consideredby the forging industry.

Importance of lubrication in a Hot Forgingprocess is the key for quality and productivity.Lubrication, however, is not an easy job and isvery dependent on the right selection ofparameters such as: Lubricant and applicationtypes. Team supervision and operatorexperience, even when fully automatedsystems are employed.

CHARACTERISTICS OFIDEAL LUBRICANT• Reduce the sliding friction between dies

and work piece (Load and energyrequirement will be reduced).

• Works as a parting agent between die andwork piece. (Prevents sticking).

• Works as an insulator.

• It should develop a balanced gas pressure(Especially in hammer forging withimpression (this is called Dieseling effect)).

• Should be Non-Abrasive and Non-Corrosive.

• Shouldn’t create smoke (Pollution free).

• Availability with reasonable cost.

The lubricant is often a fine suspension offine graphite particles in water, with surfactants(that lower the surface tension) and bindersadded to aid in the spreading and formationof adherent lubricant films. As the phenomenaof spraying, spreading and film formation arepoorly understood, the lubricant is oftenapplied in large quantities resulting inexcessive cooling of the surface, and excessgraphite in the die cavities and the

atmosphere. Consequently, understanding theideal spraying conditions for lubricant spreadand film formation has become critical to boththe forging and lubrication industrialcommunities.

Based on co-efficient of friction lubricantscan be divided in two categories:

1. With low co-efficient of friction (In practice).

2. With high co-efficient of friction.

WHY DO WE USELUBRICATION• Reduction of forging loads

• Resistance to sticking-release properties

• Cooling

• Improved die life

• Assistance to metal flow

• Protection of dies and tools (coatings)

• Reduced reject

• Reduced friction

• Reduced scale-die and forging

TYPES OF FORGINGLUBRICANTSGraphite (A1-Water Based A2-Oil Based):It is a safe, non toxic, lamellar solid offeringexcellent friction reducing properties. Carefulselection of particle size and type allows useover an extremely wide range oftemperatures.

Non-Graphite (B)

Types of GraphiteIt consists of soluble salts and polymers whichperform under less severe conditions,requiring a lower degree of lubrication.

157


Oils(C): Traditional carrier for solid lubricantsoffering good wetting and flow characteristic,used on their own, oils become subjected tofilm breakdown under condition of temperatureand pressure.

Molybdenum Disulphide (D): It is capableof performing under extreme load andrelatively low 3 temperatures, up to 400 °C.

Boron Nitride (E): It is relatively expensivematerial gaining acceptance and newapplications.

Wax (F): Traditional paraffin/mineral wax usedon light alloys. These along with modernpolymers are more often used as additives toimprove performance of lubricants.

Grease (G): It is a traditional product oftencontaining graphite. Used similar to oils, butconvenient for application in low volume andlower temperature application.

Glass (H): Used with superalloys on billet andpreforms offering protection from furnaceatmosphere and forging lubrication in themolten state, composition choice allowsextension of temperature working range.

Nickel (K): A limited specialized platingprocess for high temperature precision forgingof steel and nickel based alloys where glassis unsuitable.

Some other categorization:

• Synthetic compounds based on organicsalts.

• Saw dust (for deep impression).

• Al-Graphite in mineral oil.

• Mg-alloy-Graphite in mineral oil.

• Cu-Graphite in aq. Solution.

• Ti, Ni, W-Glass in slurry/powder.

Graphite• Graphite is one a grey crystalline allotropic

form of carbon with its origin as natural orartificial.

Graphite an allotrope of carbon consideredthe highest grade of coal; just above anthracitealternatively called meta-anthracite is bestknown for Geometry of the particles, Particlesize, Degree of purity for being used inlubrication system as hot forging lubricants.

Graphite can be obtained naturally orartificially.

Differences which lie between the naturalgraphite and the synthetic or artificial graphiteare:

• Differences are more in the productiveprocess rather than in the structure,Properties are almost identical.

Figure 2: Structure of Graphite

Source: http://www.substech.com/dokuwiki/lib/exe/detail.php?id=graphite&cache=cache&media=graphite_structure.png&

DokuWiki=c9040bae6ad67ef164d3f1e1c5e2a95b

158


• Natural Graphite: mineral extraction.

• Artif icial Graphite: Produced frompetroleum coke.

• Only natural or artificial graphite’s ofhexagonal and crystalline structureproperties are appropriate amorphousgraphite’s do not possess adequatelubricating.

Very small particles of graphite are notalways necessary in a given formulation. Thecombination of particle size and containedcarbon contain is what really makes thedifference to be used as lubricants.

LUBRICANT CARRIERSCarriers are the means of transporting thelubricant agent to the die/tool. The conditionsuch as die temperature, die material, forgingconfiguration, will dictate which carrier is mostsuited. Water and Oil are the main carriesemployed and to a lesser degree solvents.

WaterMost commonly used, must be spray appliedto dies and offers a very safe and cleanworking environment. Water is added toconcentrated lubricant dispersions to makethis type of product very economic in use.

OilMore traditional carrier and agent used onhigher temperature tooling or die areas whichare rather inaccessible. Vegetable oils andsynthetic oils are being used to replace mineraloils which can offer health and safetydrawbacks.

Oil-based products are often used assupplied. Excessive quantities of oil that arenot burnt off prior to forging will cause

explosion/dieseling in die pockets and canresulting die and forging damage.

SolventsQuick during alcohols are used for precoatingsof tooling, billets and preforms requiringlubricating or protective coatings.

VARIABLES OF THEFORGING LUBRICANTS ANDTHEIR IMPACTParameters Related to WorkPiece/Die• Material properties of work piece (Flow

stress).

• Die geometry.

• Surface finish of w/p.

• Scale.

Parameters Related to Lubricants• Composition of lubricants.

• Viscosity of lubricants.

• Amount of lubricants.

Process Parameter• Pressure of deformation.

• Die velocity.

• Heat generated due to deformation.

• Sliding length.

Some of the variables which impact theadherence and other properties between thelubricants drops mainly are:

• Degree of dilution of the concentrate

• Density of the lubricant mist

• Temperature of the die

• Pressure of the application of the lubricantover the die.

159


MAIN INGREDIENTS OF AFORGING LUBRICANTA major ingredient used in forging lubricantsmainly consists of:

• Demineralized water

• Crystalline graphite (natural or synthetic)

• Binding agents

• Stabilizers

• Thickeners

• Bacteriological inhibitors

METHODS OF APPLICATIONSwabs, Mops, Brushes and ClothsNot always uniform with regard to thicknessand distribution and can be uneconomical inuse of lubricants. Can sometimes be anadvantage where lubricants are required on aparticular part of the die which is difficult tolubricate by sprays.

SpraysHand Held Spray Gun: It can be used in allforging applications with a variety of lubricants.

Fixed Spray System: It is associated mainlyto high volume manual and automatic forging.

Reciprocating Spraying Machine:Generally it is used for medium to largeimpressions of complex configuration and forvolume application applied with programmedspray arms.

Pumping/Flooding: It is used for specialextrusion processes or extremely high volumeapplication of water or oil based lubricants.

Pre-Coating: Generally used for protective orglaze coatings as batch processes likeelectrostatic spraying, dipping or plating.

Important factors governing spraydeposition can be categorized into threegroups: those related to the hot die surface,those related to the lubricant properties, andthose related to the spray characteristics. Thefactors related to die surface include the initialsurface temperature, its roughness, itsgeometry (cavity design) and surfacetreatments (such as diffusion treatments orcoatings). Important lubricant propertiesinclude surface tension and viscosity. Lastly,the spray characteristics involve droplet size,droplet velocity, dilution ratio (or solid content)and, spray cone angle and nozzle height.Composition of the lubricant and the selectionof the spray parameters are often determinedby the lubrication and cooling needs of theintended application.

CRITERIA FOR SELECTIONOF LUBRICANTSSelection of the most appropriate lubricant forthe individual forging process is mostimportant. Criteria needed to be consideredare as follows:

• Offer no safety hazards

• Material being forged

• Forging process and type of a productionplant

• Must aid and assist die life

• Operate under a minimum of supervision/easy to manage

Die lubrication and formulations areextremely widespread and particularly thegraphite ones. It is possible to design featuresto suit a specific forging application by the typeof raw material used and method of the

160


lubricants manufacture. It is thereforeappropriate to discuss such detail of selectionwith die lubricant supplier.

ROLE OF FORGINGLUBRICANT TO PROMOTEEXTENDED DIE LIFE• In first place forming a bonded film over the

working die surface

• Secondly imparting to the die surface areduced friction coefficient able to promotea smooth metal flow

• In third place imparting cooling properties

• In fourth place not generating build ups onthe die surface

WORKING TEMPERATURE OFTHE FORGING LUBRICANTTo perform within accepted parameters theforging lubricant has to function attemperatures between 200 and 300 gradescentigrade’s.

SOME IMPORTANT FACTORSAND THEIR EFFECTS ONDIFFERENT ASPECTSAffecting Removal of Heat fromthe Dies• Density of the lubricant.

• Pressure of the lubricant deposition over thedies.

Affect of Friction Coefficient• Adherence of the lubricant film to the dies.

• Type and characteristic of the lubricant used.

Influences the Die Temperature• Mass and contact time of the billet over the

dies.

FACTOR NOT ALWAYSTAKEN CARE OF IN THELUBRICATIONSome of the factors which are not taken careof most of the times in forging correlated tolubricants, which should be taken care of are:

• Quality of the water used in the dilution

• Homogeneity of the diluted lubricant

• Variation control of the solid content priorto

• The application of the lubricant

IMPORTANT TARGET TOACHIEVEThe most important target to achieve is to keepthe total cost of the tooling below the 10% ofthe total manufacturing cost.

LUBRICATION CONTROLSTO MAKEIn the Concentrate Product

Figure 3: Ideal Temperatures forApplication of Lubricants

161


Control of the Manufacturer’sSpecification

• Type of graphite.

• Particle size.

• Ash content.

• Total solid content and graphite content.

• Viscosity and Ph of the concentrate.

In the Diluted Product

• Check that the solid content chosen iscorrect and keep it constant during theproduct usage.

• Check that the diluted product be undercontinuous agitation in the reservoir tank.

In and During its Application

• Time, angle, size of the lubricating drop,output and distance from the lubricatingpoint to the areas to lubricate.

In the Lubricating Unit (Robots)

• Time of positioning

• Time of blow

• Time of retract

• Time of water cooling

• Output of water

• Time of lubrication

• Output of lubricant

DIE AND BILLETINTERACTIONThe Interaction between the die and the billetbecomes extremely complicated as:

• Temperature and their control (due tovariation in both).

• Transference of heat and its removal (billetcooling and heating and cooling cycles ofthe die).

• Time of contact billet/die.

• Lubrication.

• Die composition and treatments.

FUTURE PROSPECTIVE ANDTECHNOLOGICAL BARRIERSTO OVERCOMEWith so many advantages of forgingtechnology in every respect futureprospective, advancement in technology andimprovement in present techniques becomesa wide field to be discovered. Some of themost important field to be explored in relationto use and application of FORGINGLUBRICANTS can be:

• Discovery of new ingredients/formulationsthat improve lubrication and reduce itsincidence in the factors affecting die life.

• Better understanding between therelationship of the lubricating film and thedie surface during the forging process.

CONCLUSIONHere in present work we have seen thedifferent aspects of hot forging lubricants fromchoice of proper lubricant to selection ofproper application method, from affect on dielife to measures taken proper care of for bestresults. Quality and free from defects forgingssuch as surface cracks, internal oxide filling,lack of reduction and use of dies below theirmaximum potential will only be an achievabletarget if the culture prevailing within the forgingindustry recognize the importance of forginglubrication and works towards the

162


implementation of standardized methods forthe evaluation, selection and practical controlof the lubricants once in use and also duringits experimental trial phase. This culture mustbe the result of a joint work of the forgingindustry with the developers and manufacturersof Forging Lubricants.

Finally it does not matter how good thelubricant may be, if not properly applied to thedie surface, in the form, angle, pressure,distance and atomization proper for thespecific die it will not impart the expectedperformance.

REFERENCES1. Gustavo G and Schiuma E (2014), Die

Lubricants for Hot Forging.

2. Hancox A, Hancox D and Haigh G (1994),“Codes of Best Forging Practice”, DieLubrication in Hot Forging, June, British& Midland Forging Ltd., Britain.

3. http://www.substech.com/dokuwiki/lib/exe/detail.php?id=graphite&cache=cache&media=graphite_struc ture.png&DokuWiki=c9040bae6ad67ef164d3f1e1c5e2a95b

4. “Ideal Temperature for Application ofLubricants”, http://www.renite.com/forge-lub.htm

5. Manoj Kumar (2013), Institutes ClassLiterature, National Institute of Foundryand Forge Technology, AssociateProfessor, Department of Forge, Ranchi.

6. Yang L and Shivpuri R (2006), “SpreadingBehavior of Water Based GraphiteLubricants on Hot Die Surfaces”,Industrial, Welding and SystemsEngineering, Vol. 55, No. 1, Annals of theCIRP, The Ohio State University,Columbus, Ohio, USA.

163


APPENDIX 1

Source: Hancox et al. (1994)

APPENDIX 2

Source: Hancox et al. (1994)

hot forging lubricants - · pdf filelubrication system as hot forging lubricants. graphite...

Documents