UDDEHOLM TOOL STEEL FOR DIE CASTING · 2 UDDEHOLM STEEL FOR DIE CASTING This information is based on our present state of knowledge and is intended to provide general notes on our

UDDEHOLM STEEL FOR DIE CASTING 1

UDDEHOLM TOOL STEELFOR DIE CASTING

TOOLING APPLICATION HOT WORK

UDDEHOLM STEEL FOR DIE CASTING2

This information is based on our present state of knowledge and is intended to provide generalnotes on our products and their uses. It should not therefore be construed as a warranty ofspecific properties of the products described or a warranty for fitness for a particular purpose.

Classified according to EU Directive 1999/45/ECFor further information see our “Material Safety Data Sheets”.

Edition: 8, 09.2015The latest revised edition of this brochure is the English version,which is always published on our web site www.uddeholm.com


CONTENTS

Introduction 4

Demands on the die cast product 4

Aspects of die design 5

Die making 6

Die performance 10

Demands on die steel for die casting 12

Die economy 17

Product programme– General description 18– Chemical composition 19– Quality comparison 19

Steel and hardness recommendations 20

Selecting a tool steel supplier is a key decision for all parties, including the tool maker, the tool

user and the end user. Thanks to superior material properties, Uddeholm’s customers get

reliable tools and components. Our products are always state-of-the-art. Consequently, we have

built a reputation as the most innovative tool steel producer in the world.

Uddeholm produce and deliver high quality Swedish tool steel to more than 100,000 customers

in over 100 countries. Some markets are served by ASSAB, our wholly-owned and exclusive

sales channel in Asia and in the Middle East. Together we secure our position as a world

leading supplier of tool steel.

Wherever you are in the manufacturing chain, trust Uddeholm to be your number one partner

and tool steel provider for optimal tooling and production economy.

Quite simply, it pays to go for a better steel.


IntroductionPressure die casting offers an economical way ofproducing large quantities of complex, high-tolerance parts in aluminium, magnesium, zinc andcopper alloys.

The continued growth of the die castingprocess depends, to a large extent, on the greateruse of die castings in the automotive industry,where weight reduction is increasingly important.

Long production runs have focused attentionon the importance of obtaining improved die life.During the last years Uddeholm has occupied aleading role in developing die materials to meetthis demand and that of higher die steel specifi-cations. This has resulted in the grades UddeholmOrvar Supreme, Uddeholm Orvar Superior,Uddeholm Vidar Superior, Uddeholm QRO 90Supreme and Uddeholm Dievar.

Die casters are now experiencing real savingsin production and tooling costs by using thesepremium die steel with closely specified heattreatment procedures. Further improvements

Tool for high pressure aluminium die casting.

have been realized by paying close attention togood product and die design and improved diecasting practices.

Demands onthe die cast productIncreasing demands on die cast products willensure continued development of die castingalloys with higher strength and ductility, improvedmachinability, weldability and corrosion resist-ance. The trends in product design are goingtowards:• larger components• thinner wall thicknesses• more complicated shapes• closer tolerances

These factors favor the use of high pressure diecasting over other casting methods like lowpressure and gravity die casting.

Aluminium pump housing f ixedto the f illing system, i.e. runners,

gates and overf lows.


CAVITY

High-strength steel are extremely notch-sensi-tive. It is therefore important that the cavity isdesigned with smooth changes of sections andfillets of maximum possible radius.

In order to reduce the risk of erosion and heatchecking on the die material near the gate, thecavity wall or any cores or inserts should belocated as far from the gate as possible.

COOLING CHANNELS

The location of the cooling channels should besuch that the entire surface of the die cavity hasas uniform a temperature as possible. Surface

Aspects of die designThe design of a die casting die is primarily deter-mined by the shape of the finished component.But there are a number of aspects involved inthe design and sizing of a die which can have aninfluence and important bearing on die life.

✗

✗

smoothness of the channels is important, bothfrom the view point of cooling and from the viewpoint of strength but also for the resistance tocorrosion.

RUNNERS, GATES AND OVERFLOWS

To get optimum casting conditions the coolingsystem must have a heat balance with “the hotparts” (runners, gates, overflows and cavities).This means that the design of the runner, gateand overflow system is of great importance. Inparts which are difficult to fill in the cavity, anoverflow should be located to help casting metalto flow into this part (alternative vacuum assist-ant casting). In multicavity dies with identicalimpressions, it is important that all runners havethe same path length and cross-sectional areaand that the gates and overflows are identical.

The position of the gates and the thickness andwidth of the land is critical for the injectionspeed of metal. The gates should be designed sothat the injected metal flows smoothly and freelyinto all parts of the cavity. Casting metal that issprayed, instead of flowed into the cavity, causesbad castings. Excessive turbulence of castingmetal can cause erosion of the die.

GUIDELINES FOR SIZING

The following are some guidelines for sizing a diefor aluminium to meet strength requirements:

1.Distance from cavity to outer surface>50 mm (2 inch)

2.Ratio of cavity depth to total thickness <1:3

3.Distance from cavity to cooling channel>25 mm (1 inch)Distance from cavity to cooling channel atcorner >50 mm (2 inch)

4. Fillet radiiZinc >0.5 mm (0.02 inch)Aluminium >1 mm (0.04 inch)Brass >1.5 mm (0.06 inch)

5.Distance from gate to cavity wall>50 mm (2 inch)


Die makingWhen manufacturing a die casting die thefollowing are of vital importance:

• machinability

• Electrical Discharge Machining (EDM)

• heat treatment

• dimensional stability

• surface treatment

• weldability

MACHINABILITY

The machinability of martensitic hot work toolsteel is mainly influenced by the amount of non-metallic inclusions like manganese sulfides andthe hardness of the steel.

As the performance of a die casting die can beimproved by lowering the impurities, i.e. sulphurand oxygen, Uddeholm Dievar, Uddeholm OrvarSupreme, Uddeholm Orvar Superior, UddeholmVidar Superior and Uddeholm QRO 90 Supremeare produced with an extremely low sulphur andoxygen level.

The optimum structure for machining is auniform distribution of well spheroidized car-bides in a soft annealed ferritic structure with aslow hardness as possible. The Microdizingprocess gives Uddeholm Dievar, UddeholmOrvar Supreme, Uddeholm Orvar Superior,Uddeholm Vidar Superior and UddeholmQRO 90 Supreme a homogeneous structure witha hardness of approx. 160 HB for UddeholmDievar and 180 HB for Uddeholm OrvarSupreme, Uddeholm Orvar Superior, UddeholmVidar Superior and Uddeholm QRO 90 Supreme.The steel are characterized by a very uniformmachinability.

General machining data for turning, milling anddrilling of Uddeholm Dievar, Uddeholm OrvarSupreme, Uddeholm Orvar Superior, UddeholmVidar Superior and Uddeholm QRO 90 Supremecan be found in the product information bro-chures.

EDM OF HARDENED AND TEMPERED MATERIAL

A Conventional machining.

B Hardening and tempering.

C Initial EDM, avoiding “arcing” and excessivestock removal rates. Finish with “fine-sparking”, i.e. low current, high frequency.

D (i) Grind or polish EDM surface.

(ii) Temper the tool at 15–25°C (30–50°F) lower than the highest previous tempering temperature.

A Conventional machining.

B Initial EDM, avoiding “arcing” and excessivestock removal rates. Finish with “fine-sparking”, i.e. low current, high frequency.

C Grind or polish EDM surface. This reducesthe risk of crack formation during heatingand quenching. Slow preheating, in stages,to the hardening temperature is recom-mended.

ELECTRICAL DISCHARGE MACHINING

The use of Electrical Discharge Machining (EDM)in the production of die casting dies has beenfirmly established since many years.Development of the process has producedsignificant refinements in operating technique,productivity and accuracy. As an alternative toEDM’ing high speed machining is growing.

The basic principles of EDM (spark erosion)are electrical discharges between a graphite orcopper anode and the steel, the cathode, in adielectric medium. During the process thesurface of the steel is subjected to very hightemperatures, causing the steel to melt orvaporize. A melted and brittle re-solidified layeris formed on the surface and beneath that a re-hardened and tempered layer.

The influence of the EDM operation on thesurface properties of the die steel can in un-favorable circumstances destroy the workingperformance of the die. For this reason thefollowing steps are recommended, as a precau-tionary measure:

EDM OF ANNEALED MATERIAL

More information about electrical dischargemachining can be found in the brochure “EDM ofTool Steel”.


HEAT TREATMENT

Hot work tool steel are normally delivered in thesoft annealed condition. After machining, the diemust be heat treated in order to give optimumhot yield strength, temper resistance, toughnessand ductility.

The properties of the steel are controlled bythe hardening temperature and soaking time, thecooling rate and the tempering temperature.

A high austenitizing temperature for a die has apositive influence on the hot yield strength andthe resistance to softening, which reduce theheat checking tendency. In Uddeholm OrvarSupreme, Uddeholm Orvar Superior and Udde-holm QRO 90 Supreme these properties can beenhanced by austenitizing at 1050°C (1920°F)instead of 1020°C (1870°F). For UddeholmDievar 1030°C (1885°F) instead of 1000°C(1830°F) and for Uddeholm Vidar Superior1000°C (1830°F) instead of 980°C (1800°F).

On the other hand, a high austenitizing tem-perature gives an increased risk of grain growth,which can cause a reduction in toughness andductility. Hence the higher austenitizing tempera-ture should only be used for small dies, cores andcore pins.

Similarly, a higher hardness has a positive effecton heat checking, although a hardness exceeding50 HRC is not recommended for aluminium diecasting and similarly not exceeding 46 HRC forbrass. The risk of cracking and total failure in-creases with higher hardness.

However, by developing the higher toughnessin Uddeholm Dievar, Uddeholm Orvar Supreme,Uddeholm Orvar Superior and Uddeholm VidarSuperior the risk of failure is considerably re-duced.

The quenching rate during hardening has agreat significance for Uddeholm Dievar, Udde-holm Orvar Supreme, Uddeholm Orvar Superior,Uddeholm Vidar Superior and UddeholmQRO 90 Supreme and for all other steel of simi-lar type.

A low quenching rate gives the best possibledimensional stability, but the risk for undesirablechanges in the microstructure of the steelincreases.

A too low cooling rate during hardening willreduce the fracture toughness of the steel.

A high quenching rate gives the best possiblestructure and consequently the best die life.

The right balance must be found between thelower costs (less machining) resulting from a lowquenching rate and the better die life achieved byusing a high cooling rate (high toughness). Inmost cases a high quenching rate is to be pre-ferred where the total economy of the die is themajor consideration.

Decarburization and heavy carburization maycause premature heat checking and shall beavoided at all times.

The die should be tempered after cooling to50–70°C (120–160°F). A second temperingoperation is essential to obtain a satisfactorystructure. The tempering temperature should beselected to obtain the desired hardness of thedie. A third temper is generally recommended fordie casting dies.

Aluminium partfor the automotive industry.


DIMENSIONAL STABILITY

DISTORTION DURINGTHE HARDENING AND TEMPERINGOF DIE CASTING DIES

When a die casting die is hardened and tem-pered, some warpage or distortion normallyoccurs. This distortion is usually greater whenusing higher austenitizing temperatures.

This is well known, and it is normal practice toleave some machining allowance on the die priorto hardening. This makes it possible to adjust thedie to the correct dimensions after hardeningand tempering by high speed machining, EDM’ingand grinding etc.

Distortion takes place because of stresses in thematerial. These stresses can be divided into:

• machining stresses• thermal stresses• transformation stresses

MACHINING STRESSES

This type of stress is generated during machiningoperations such as turning, milling and grinding.

If stresses have built up in a part, they will bereleased during heating. Heating reduces strength,releasing stresses through local distortion. Thiscan lead to overall distortion.

In order to reduce distortion while heatingduring the hardening process, a stress-relievingoperation can be carried out. It is recommendedthat the material be stress-relieved after roughmachining. Any distortion can then be adjustedduring fine machining, prior to the hardeningoperation.

Cavity part for high pressure aluminium die casting.

THERMAL STRESSES

These stresses are created when the die isheated or quenched. They increase if heatingtakes place rapidly or unevenly. The volume ofthe die is increased by heating. Uneven heatingcan result in local variations in volume growth,leading to stresses and distortion.

Preheating in stages is always recommended inorder to equalize the temperature in the com-ponent.

Aluminium die forthe automotive industry.


PREPARATION BEFORE WELDING

Parts to be welded must be free from dirt andgrease to ensure satisfactory penetration andfusion.

WELDING OF SOFT ANNEALED MATERIAL

1 Preheat to 325–375°C (620–710°F).

2 Start welding at this temperature.Never let the temperature of the tool go below325°C (620°F). Max. interpass temperature475°C (885°F). The best way to keep a constanttemperature of the tool during welding, is to usean insulated box with thermostatically controlledelectrical elements inside the walls.

3 After welding cool very slowly 20–40°C/h(35–70°F/h) for the first two hours and thenfreely in air.

4 Soft anneal immediately after welding.

WELDING OF HARDENED ANDTEMPERED MATERIAL

1 Preheat to 325–375°C (620–710°F).

2 Start welding at this temperature.Never let the temperature of the tool go below325°C (620°F). Max. interpass temperature475°C (885°F). The best way to keep a constanttemperature of the tool during welding, is to usean insulated box with thermostatically controlledelectrical elements inside the walls.

3 After welding cool very slowly 20–40°C/h(35–70°F/h) for the first two hours and thenfreely in air.

4 Stress temper 25°C (50°F) below the highestprevious tempering temperature for two hours.

WELDING CONSUMABLES

Uddeholm QRO 90 Weld (SMAW), UddeholmQRO 90 TIG-Weld or Uddeholm Dievar TIG-Weld. More information about welding and con-sumables can be found in the brochure “Weldingof Tool Steel”.

An attempt should always be made to heat slowlyenough so that the temperature remains virtuallyequal throughout the die.

What has been said regarding heating alsoapplies to quenching. Very powerful stresses ariseduring quenching. As a general rule, the coolingrates should be as fast as possible, relative to theacceptable distortion level.

It is important that the quenching medium isapplied as uniformly as possible. This is especiallyvalid when forced air or protective gas atmos-phere (as in vacuum furnaces) is used. Otherwisetemperature differences in the tool can lead tosignificant distortion. Step quenching is recom-mended for larger, more complex dies.

TRANSFORMATION STRESSES

This type of stress arises when the microstruc-ture of the steel is transformed. This is becausethe three microstructures in question—ferrite,austenite and martensite—have different densi-ties, i.e. volumes.

The greatest effect is caused by transformationfrom austenite to martensite. This causes avolume increase.

Excessively rapid and uneven quenching canalso cause local martensite formation, causing avolume increase locally in a die giving rise tostresses in some sections. These stresses can leadto distortion and, in some cases, cracks.

SURFACE TREATMENT

Surface treatments like gas nitriding, salt bath orion nitriding can have a beneficial effect like re-sistance to erosion and soldering on certain partsof a die casting die, such as shot sleeves, nozzles,runners, spreaders, gates, ejector pins and corepins. Different steel possess different nitridingproperties, depending on chemical composition.Other surface treatments have also proved ben-eficial in die casting applications.

WELDABILITY

In many cases, it is important that a die castingdie can be repaired by welding. The repair-weld-ing of tool steel always entails a risk of cracking,but if care is taken and heating instructions arefollowed, good results can be obtained.


Preheating Material temperature

Tin, Lead alloys 100–150°C (210–300°F)

Zinc alloys 150–200°C (300–390°F)

Magnesium,Aluminium alloys 180–300°C (355–570°F)

Copper alloys 300–350°C (570–660°F)

It is important that heating is gradual and even.Thermostatically controlled heating systems arerecommended.

When preheating, coolant should be graduallyapplied in order to obtain a state of equilibrium.Shock cooling should be avoided.

Dies containing inserts must be heated at aslow rate so the inserts and holders can gradu-ally expand together.

CORRECT COOLING

The temperature of the die is controlled viacooling channels by water or oil and by the lubri-cant on the die surface.

In order to reduce the risk of heat checking,the cooling water can be preheated to approxi-mately 50°C (120°F). Thermostatically controlledcooling systems are also common. Cooling watercolder than 20°C (70°F) is not recommended.

Die performanceThe life of a die casting die varies considerablydepending on the size and design of the casting,the type of casting alloy, and the care and main-tenance of the die.

The life of a die can be prolonged by suitabletreatment before and during casting by:

• suitable preheating

• correct cooling


• stress tempering

SUITABLE PREHEATING

The initial contact between a cold die casting dieand the hot casting metal causes a severe shockto the die material. Heat checking can start at thevery first shot and quickly lead to total failure.

Further, it is important to note that the impactstrength, i.e. the materials ability to withstandthermal and mechanical shock, is increased signifi-cantly during the first shots by proper preheatingof the tool.

It is essential therefore, that the temperaturedifference between the die surface and themolten metal is not too great. For this reason,preheating is always recommended.

The most suitable preheating temperature isdependent on the type of casting alloy, but nor-mally lies between 150 and 350°C (300 and660°F).

The curves, in the graphs to the left, show therange within which the material can be preheated.It is important not to preheat to an excessivelyhigh temperature, since the die may become toohot during die casting, causing a tempering backof the die material. Observe that thin ribs get hotvery quickly. The following preheating tempera-tures are recommended:

Preheatingrange

Impact strength

QRO 90 SUPREME

DIEVAR

100 200 300 400 500°C 200 400 600 800 1000°F

Testing temperature

ORVAR SUPREME

VIDAR SUPERIOR

ORVAR SUPERIOR

Preheatingrange

Hot yieldstrength

100 200 300 400 500 600°C 200 400 600 800 1000 1200°F

Testing temperature

QRO 90 SUPREMEDIEVAR

ORVAR SUPREMEand ORVAR SUPERIOR

VIDAR SUPERIOR


During breaks longer than a few minutes, theflow of coolant should be adjusted so that thedie does not cool down too much.

SURFACE TREATMENT

To avoid metal-to-die contact it is important thatthe lubricant (parting compound) adheres well tothe die surface. For example, a new or recentlyrepaired die should not have a glossy metalsurface. It is therefore a good idea to coat thedie surface with a thin oxide film to providegood adhesion for the lubricant in the running-inperiod.

The surface of the die can be oxidized byheating to approx. 500°C (930°F) for one hourfollowed by cooling in air. Heating in a steamatmosphere, 500°C (930°F), for 30 minutes alsoproduces a good oxide film, with suitable thick-ness.

To remove built-up deposits of die lubricantsafter a period of use, shot peening of the cavitysurface is recommended. This treatment alsocloses some of the heat checking cracks. It in-duces compressive stresses in the surface layer,which compensate for some of the tensilestresses which cause heat checking. Parts whichare subjected to abrasion and friction, such asejector pins and shot sleeves, may be nitrided ornitrocarburized for longer life.

Die for brassdie casting.

STRESS TEMPERING

During die casting, the surface of the die issubjected to thermal strains derived from thevariations in temperature; this repeated strainingmay result in residual stresses being generated inthe surface regions of the die. In most cases, suchresidual stresses will be tensile in nature andthereby assist initiation of heat checking cracks.

Stress tempering the die will reduce the levelof residual tensile stress and thereby enhance dielife. We therefore recommend that stresstempering shall be performed after the running-in period and then after 1000–2000 and 5000–10 000 shots. The procedure is then repeated foreach additional 10 000–20 000 shots, so long asthe die exhibits only minor amounts of heatchecking. However, there is little point in stresstempering a heat checked die because the forma-tion of surface cracks in itself reduces the level ofresidual stress.

Stress tempering is best carried out at atemperature about 25°C (50°F) below the high-est tempering temperature which has previouslybeen used during heat treatment of the die.Normally, two hours holding time at temperatureshould be sufficient.


Casting temperature Factors which limit Normal life, number of shotsCasting alloy °F °C die life, Die Die Core

Zinc ~800 ~430 Erosion 0.5–2 million 0.5–2 million

Magnesium ~1200 ~650 Heat checking 100 000 50 000Cracking to toErosion 400 000 200 000Indentation

Aluminum ~1300 ~700 Heat checking 60 000 40 000Cracking to toErosion 200 000 150 000Indentation

Copper/Brass ~1780 ~970 Heat checking 5 000 1 000Indentation to toErosion 50 000 5 000Cracking

Demands ondie steel for die castingDie casting dies are exposed to severe thermaland mechanical cyclic loading, which puts highdemands on the die material. There are thus anumber of phenomena which restrict die life.The most important are:

• thermal fatigue (heat checking)

• corrosion/erosion

• cracking (total failure)

• indentation

The number of shots achievable in a die castingdie is strongly influenced by the working tem-perature, i.e. the casting alloy. The die life for aspecific alloy can also vary considerably due tothe design of the cast product, the surface finish,the production rate, the process control, thedesign of the die, the die material its heat treat-ment and the acceptance level of size and surfacefinish variations.

THERMAL FATIGUE

Thermal fatigue is a gradual cracking due to ther-mal stresses from many temperature cycles andis a micro-scale phenomenon taking place only ina thin surface layer.

In use die casting dies are subjected to alter-nate heating and cooling. This gives rise to severe

strains in the surface layer of the die, graduallyleading to thermal fatigue cracks. Typical thermalfatigue damage is a pattern of surface cracksknown as “heat checking”, well-illustrated in thephoto below.

Much attention has been paid to understandingthe thermal fatigue process and to relate theresistance to heat checking to basic materialproperties. For this purpose Uddeholm has builta special device for simulation of the thermalfatigue damage. The aim of these efforts is toimprove and develop the die material and hasresulted in the premium steel grades UddeholmDievar, Uddeholm Orvar Supreme, UddeholmOrvar Superior, Uddeholm Vidar Superior andUddeholm QRO 90 Supreme.


FACTORS WHICHINFLUENCE THERMAL FATIGUE

Thermal fatigue cracks are caused by a combina-tion of thermal cyclic stress, tensile stress andplastic strain. If any one of these factors is notpresent, a thermal fatigue crack will neither initi-ate nor propagate. The plastic strain starts thecrack and the tensile stress promotes the crackgrowth.

The following factors influence the thermalfatigue:• Die temperature cycle

Preheating temperatureSurface temperature of the dieHolding time at peak temperatureCooling rate

• Basic die material propertiesThermal expansion coefficientThermal conductivityHot yield strengthTemper resistanceCreep strengthDuctility

• Stress raisersFillets, holes and cornersSurface roughness

DIE TEMPERATURE CYCLE

PREHEATING TEMPERATURE

It is essential that the temperature differencebetween the die surface and the molten metal isnot too great. For this reason preheating isalways recommended.

The preheating temperature should be mini-mum 180°C (355°F) for aluminium at whichtemperature the fracture toughness isalmost twice as high as at room temperature.

SURFACE TEMPERATURE OF THE DIE

The temperature of the surface layer of the dieis very important for the occurrence of thermalfatigue. Up to 600°C (1110°F) the thermalexpansion and the stresses are moderate for anormal hot work steel but at higher tempera-tures the risk of heat checking becomes signifi-

cant. The surface temperature of the die is mainlydetermined by the preheating temperature, thecasting temperature of the metal, the design ofthe cast product, the die shape and size and thethermal properties of the die material.

HOLDING TIME AT PEAK TEMPERATURE

Longer holding time implies an increased risk ofover-tempering and creep of the die material.This means a reduction of the mechanicalstrength and accordingly a lower resistance tomechanical and/or thermal loadings.

COOLING RATE

The rate at which the surface layer cools is ofconsiderable importance. More rapid coolinggives rise to greater stresses and leads to cracksat an earlier stage. The choice of coolant isnormally a compromise between desired die lifeand production rate but most die casters haveswitched from oil-based lubricants to water-based ones for environmental reasons.

BASIC DIE MATERIAL PROPERTIES

THERMAL EXPANSION COEFFICIENT

The thermal expansion coefficient ought to below to get low thermal stresses.

THERMAL CONDUCTIVITY

A high thermal conductivity reduces the thermalgradients and thereby the thermal stresses. It is,however, very difficult to predict or to investigateexperimentally to what extent the thermal con-ductivity influences this matter.

HOT YIELD STRENGTH

A high hot yield strength lowers the plastic strainand is beneficial in resisting heat checking.

TEMPER RESISTANCE

If a die material with initially high hot yieldstrength becomes softer during use due to hightemperature exposure it means that the heatchecking damage accelerates. It is therefore im-portant that the die material has a good resist-ance to softening at high temperature exposure.


CREEP STRENGTH

The softening associated with temper resistanceis clearly accelerated by mechanical load. The diematerial is exposed both to high temperatureand mechanical load. It is thus obvious that agood die material will possess resistance to thejoint action of high temperature and mechanicalload as quantified by a high creep strength. In fact,it has been proven by experiment that heatchecking cracks also can be produced by con-stant temperature and cyclic mechanical load.

DUCTILITY

The ductility of the die material quantifies theability to resist plastic strain without cracking.At the initiation stage of the thermal fatiguedamage the ductility governs the number ofcycles before visible cracks appear for a given hotyield strength and temperature cycle. At thecrack growth stage the ductility has a declininginfluence.

The ductility of the material is greatly influ-enced by slag inclusions and segregations, i.e. thepurity and the homogeneity of the steel. Thesteel from Uddeholm for die casting dies aretherefore processed in a special way. The ductilityof the steel has been considerably improved bymeans of a special melting and refining technique,a controlled forging process and a special micro-structure treatment. This improvement is espe-cially pronounced in the centre of thick blocks.

STRESS RAISERS

FILLETS, HOLES AND CORNERS

Geometrical stress concentration and increasedthermal gradients increase the stresses andstrains at fillets, holes and corners. This meansthat heat checking cracks start earlier in theseareas than on plane surfaces. The joint action ofheat checking cracks and fillets increases the riskof total failure of the die.

SURFACE ROUGHNESS

Surface defects such as grinding scratches affectthe starting of cracks for the same reasons asfillets, holes and corners. Within the recom-mended grinding range of 220–600 grit, surfaceroughness should not be a cause of heat

checking. One advantage with a not too highlypolished surface, for example sand blasted oroxidized, is that the parting lubricant adheresbetter and is distributed more evenly on thesurface. Further, less soldering takes place and itgives better release of castings. This is especiallyimportant during the running-in of a new die.

CORROSION/EROSION

CORROSION BY MOLTEN CASTING METAL

During die casting, the molten metal is injectedinto the die. In cases where the cavity surfacelacks a protective layer, the cast metal will diffuseinto the die surface. At the same time, alloyingelements within the die (especially iron), willdiffuse from the die surface into the cast metal.These processes can create both dissolution ofthe steel and intermetallic compounds betweenthe cast metal and the die surface. In cases wheresevere formation of intermetallic compoundsoccurs, the cast metal will solder to the die sur-face.

Uddeholm has investigated the corrosiontendency in different molten die casting metals.

FACTORS WHICH INFLUENCE CORROSION

A number of factors influence die corrosion:

• temperature of the casting metal

• composition of the casting metal

• design of the die


TEMPERATURE OF THE CASTING METAL

The die casting alloys have critical temperaturesabove which corrosion attacks increase. Zincstarts to react with steel at about 480°C (900°F)and aluminium at about 720°C (1330°F). Copper

Degree of corrosion

ALUMINIUMZINC BRASS

Notrecomm.

Notrecomm.

400 500 600 700 800 900 1000°C750 930 1110 1290 1470 1650 1830°F

Temperature

Notrecomm.


Erosion.

Soldering damage on a core pin.

COMPOSITION OF THE CASTING METAL

Pure metals attack tool material at a muchgreater rate than commercial alloys. This is validboth for zinc (Zn) and aluminium (Al). The cor-rosion of the die steel also increases when thealuminium melt contains a low iron content.

DESIGN OF THE DIE

Die design is also of importance for corrosion.If molten metal is injected at too high a velocity,the lubricant on the surface of the cavity can be“washed” away. Too high a velocity is usuallycaused by incorrect gating design.

alloys do not seem to have any really criticaltemperature, but corrosion increases slowly withtemperature.

EROSION BY MOLTEN CASTING METAL

Erosion is a form of hot mechanical wear on thedie surface, resulting mainly from the motion ofthe melt.

Erosion depends upon the velocity of the meltas it is injected into the die as well as its tem-perature and composition. Melt speeds in excessof 55 m/s (180 feet/s ) substantially increaseerosion damage.

A high temperature also affects the situation,as the surface of the die is more easily temperedback. Hard particles such as inclusions and/orprecipitated hard silicon particles, in hypereuteticaluminium melts containing more than 12.7%silicon, further increase the risk of erosiondamage.

Most commonly a combination of corrosionand erosion damages occurs on the die surface.The type of damage that is predominant dependslargely on the velocity of the molten metal intothe die. At high velocities, it is normally theerosion damage which is predominant.

A good tempering back resistance and high hotyield strength of the die material are important.

SURFACE TREATMENT

The surface treatment of the die steel is of greatimportance. If metallic contact between the diesteel and the molten metal can be avoided, therisk of corrosion is much less. An oxide film onthe surface provides good protection. Nitridedor nitrocarburized surfaces as well as othercoating methods also give a certain protection.

ALUMINIUM735°C (1355°F)

Non-oxidized surface

Oxidized surface

1000

800

600

400

200

ORVARSUPREME48 HRC

ZINC500°C (930°F)

Material lossmg/cm2

BRASS950°C (1740°F)


INDENTATION

Indentation on the parting lines or sinking of thedie is normally due to too low hot hardness.

At elevated temperatures, the strength of thesteel and therefore its hardness will diminish.This means that the risk of indentation on a hotwork die will increase with the operating tem-perature of the die. Both the locking pressureon the die halves and the metal injection pres-sure are so high that a certain high-temperaturestrength is required. This is especially importantfor die casting of aluminium (Al), magnesium (Mg)and copper (Cu) alloys.

Fracture toughness at roomtemperature (centre, short-transverse direction).

CRACKING (TOTAL FAILURE)

The toughness of the die material is the ability toaccumulate tensile stresses without cracking atsharp notches or other stress raisers. The tough-ness of a die is dependent on the die materialand its heat treatment. Due to the fact that themechanical and thermal stresses in a die arespread in all directions the toughness in the diehas to be considered in all directions—longitu-dinal, transverse and short transverse.

Uddeholm Dievar, Uddeholm Orvar Supreme,Uddeholm Orvar Superior, Uddeholm Vidar Su-perior and Uddeholm QRO 90 Supreme areproduced by a special processing techniquewhich improves the isotropy of the mechanicalproperties.

Thermal shock is total cracking due to occa-sional thermal overloading. It is a macroscalephenomenon and is one of the most frequentcauses of total damage of the die.

FRACTURE TOUGHNESS OF UDDEHOLMDIEVAR AND UDDEHOLM ORVAR SUPREME

The ability of a material to resist stresseswithout unstable cracking at a sharp notch orcrack is called fracture toughness.

The fracture toughness of Uddeholm Dievarand Uddeholm Orvar Supreme at differenthardnesses are shown in the figure below.

Fracture toughness, KIC

ksi(in)1/2, MPa(m)1/2

100

80

60

40

20

44 45 46 48 44 48 49 HRC

60

50

40

30

20

UDDEHOLMORVAR SUPREME

UDDEHOLMDIEVAR


Die economyThe drive for improved tooling economy hasresulted in the development of “premium quality”die steel.

As the tooling cost is in the order of 10–20 per cent of the total cost of the finished alu-minium die cast product, the validity of paying forpremium die steel quality resulting in increasedtool life is obvious.

The most decisive factors that govern tool lifeare the die material, its heat treatment and thedie casting process control. The material in a diecasting die accounts for 5–15 per cent of the diecost while the heat treatment cost is about5–10 per cent. The picture below—The CostIceberg—shows the steel cost in relation tototal tooling costs.

In order to assure a good steel quality anumber of material specifications for die materialhave been developed during the last 20 years.Most of these contain requirements on chemicalanalysis, microcleanliness, microstructure, band-ing, grain size, hardness, mechanical propertiesand internal soundness (quality level).

One of the most advanced specifications forsteel acceptance criteria and heat treatment at

“The Cost Iceberg”

present is the Special Quality Die Steel & HeatTreatment Acceptance Criteria for Die CastingDies #207–2008 released by the North Ameri-can Die Casting Association (NADCA).

Further improvement of tooling economy mustinvolve specifications on the heat treatment ofthe die. This should be optimized to avoid anyexcessive dimensional changes or distortion butto produce the optimal combination of hardnessand toughness. The most critical factors are thehardening temperature and the cooling rateduring quenching. Precautions like proper pre-heating of the die as well as stress tempering willgive a better die economy.

Surface treatments are methods to protect thedie surface from corrosion/erosion and thermalfatigue.

New welding techniques have opened areas formaintenance and repair welding, both importantways to increase the die life.

Everyone involved in the chain—steel pro-ducer, die manufacturer, heat treater and diecaster—knows that there can be large variationsin quality level at every step of this process.

Optimum results can only be achieved bydemanding and paying for premium quality allalong the line.

PRODUCTION ANDMAINTENANCE COST

TOOLCOST

STEEL COST

welding

scrap

preheating

lost production

heat treatment

delivery delays

repairs

etc., etc...

adjustment

DIE MAKING COST

TOTALPRODUCT

COST


UDDEHOLMTOOL STEEL

DIEVAR

UNIMAX

ORVAR SUPREME/ORVAR SUPERIOR

VIDAR SUPERIOR

QRO 90 SUPREME

QRO 90 HT

IMPAX SUPREME

UDDEHOLMHOLDER STEEL

HOLDAX

A premium Cr-Mo-V-alloyed hot work die steel with good high temperaturestrength and excellent hardenability, toughness and ductility. Suitable formedium to big dies in aluminium die casting. It meets the requirements ofNADCA #207-2011.

A premium Cr-Mo-V alloyed steel with a good toughness and ductility up toa hardness of 58 HRC.

Premium Cr-Mo-V-alloyed hot work die steel (H13) with good resistance tothermal fatigue. The steel are produced by a special melting and refiningtechnique. They meet the requirements of NADCA #207–2011.

A premium Cr-Mo-V alloyed hot work die steel (H11 modified) with goodresistance to cracking. It meets the requirements of NADCA #207–2011.

A premium hot work die steel with high hot yield strength and good temperresistance. Especially suited for die casting of copper, brass and for smallinserts and cores in aluminium die casting.

A prehardened QRO 90 Supreme material, supplied at 37–41 HRCand suitable for core pins.

A prehardened Ni-Cr-Mo-steel supplied at 310 HB suitable for die casting ofzinc, lead and tin. Also used as a holder material and prototype dies.

A prehardened steel with very good machinability supplied at ~310 HB forclamping and holding plates.

Product programme for die castingGENERAL DESCRIPTION


SUPPLIED ANALYSIS % HARDNESS

AISI C Si Mn Cr Mo V Others Brinell

DIEVAR – 0.35 0.2 0.5 5.0 2.3 0.6 – ~160

UNIMAX – 0.50 0.2 0.5 5.0 2.3 0.5 – ~185

ORVAR SUPREME H13 0.39 1.0 0.4 5.2 1.4 0.9 – ~180(1.2344)

ORVAR SUPERIOR H13 0.39 1.0 0.4 5.2 1.4 0.9 – ~180

(1.2344)

VIDAR SUPERIOR H11 modified 0.36 0.3 0.3 5.0 1.3 0.5 – ~180(1.2340)

QRO 90 SUPREME – 0.38 0.3 0.8 2.6 2.3 0.9 Micro-alloyed ~180

IMPAX SUPREME P20 modified 0.37 0.3 1.4 2.0 0.2 – Ni 1.0 ~3101.2738

UDDEHOLMHOLDER STEEL

HOLDAX 4140 modified 0.40 0.4 1.5 1.9 0.2 – S 0.07 ~3101.2312

CHEMICAL COMPOSITION

UDDEHOLMTOOL STEEL

QUALTIY COMPARISON

Qualitative comparison of critical die steel properties.All steel tested at 44–46 HRC except for Uddeholm Unimax where 54–56 HRC is used.

Qualitative comparison of resistance to different die failures (the longer the bar, the better).

UDDEHOLMTOOL STEEL

DIEVAR

UNIMAX

ORVAR SUPREME

ORVAR SUPERIOR

VIDAR SUPERIOR

QRO 90 SUPREME

HEAT GROSSCHECKING CRACKING EROSION INDENTATION

TEMPER HOT YIELD HARDEN-RESISTANCE STRENGTH DUCTILITY TOUGHNESS ABILITY

UDDEHOLMTOOL STEEL

DIEVAR

UNIMAX

ORVAR SUPREME

ORVAR SUPERIOR

VIDAR SUPERIOR

QRO 90 SUPREME


Steel and hardness recommendations

* Surface treatment is recommended** For small Mg die inserts where a good erosion resistance is needed.

DIE PART TIN/LEAD/ZINC ALUMINIUM/MAGNESIUM COPPER, BRASS

CLAMPING PLATES HOLDAX HOLDAX HOLDAXHOLDER PLATES (prehardened) ~310 HB (prehardened) ~310 HB (prehardened) ~310 HB

IMPAX SUPREME IMPAX SUPREME IMPAX SUPREME(prehardened) ~310 HB (prehardened) ~310 HB (prehardened) ~310 HB

DIE INSERTS IMPAX SUPREME DIEVAR QRO 90 SUPREME~310 HB 44–50 HRC 40–46 HRCORVAR SUPREME ORVAR SUPREME ORVAR SUPREMEORVAR SUPERIOR ORVAR SUPERIOR ORVAR SUPERIOR46–52 HRC VIDAR SUPERIOR 40–46 HRCUNIMAX 42–48 HRC52–56 HRC UNIMAX**

FIXED INSERTS ORVAR SUPREME DIEVAR QRO 90 SUPREMECORES ORVAR SUPERIOR 46–50 HRC 40–46 HRC

46–52 HRC ORVAR SUPREMEORVAR SUPERIORVIDAR SUPERIOR44–48 HRCQRO 90 SUPREME42–48 HRC

CORE PINS ORVAR SUPREME QRO 90 SUPREME* QRO 90 SUPREME46–52 HRC 44–48 HRC 42–46 HRC

QRO 90 HT* QRO 90 HT

SPRUE PARTS ORVAR SUPREME ORVAR SUPREME QRO 90 SUPREME48–52 HRC ORVAR SUPERIOR 42–46 HRC

46–48 HRCQRO 90 SUPREME44–46 HRC

NOZZLES STAVAX ESR ORVAR SUPREME QRO 90 SUPREME40–44 HRC ORVAR SUPERIOR 40–44 HRCORVAR SUPREME 42–48 HRC ORVAR SUPREME35–44 HRC QRO 90 SUPREME ORVAR SUPERIOR

42–46 HRC 42–48 HRC

EJECTOR PINS QRO 90 SUPREME QRO 90 SUPREME QRO 90 SUPREMEORVAR SUPREME ORVAR SUPREME ORVAR SUPREME46–50 HRC (nitrided) 46–50 HRC (nitrided) 46–50 HRC (nitrided)

PLUNGER ORVAR SUPREME ORVAR SUPREME QRO 90 SUPREMESHOT SLEEVES 42–46 HRC (nitrided) ORVAR SUPERIOR 42–46 HRC (nitrided)

42–48 HRC (nitrided) ORVAR SUPREMEQRO 90 SUPREME ORVAR SUPERIOR42–48 HRC (nitrided) 42–46 HRC (nitrided)


1 Clamping plates2 Holder plates3 Die inserts4 Fixed inserts5 Cores6 Sprue bushing (nozzles)7 Sprue pin (Spreader)8 Ejector pins

1 2 3 3

48

5 67 2 1


www.uddeholm.com

Network of excellenceUddeholm is present on every continent. This ensures

you high-quality Swedish tool steel and local support wherever

you are. We secure our position as the world’s leading supplier

of tooling materials.

Uddeholm is the world’s leading supplier of tooling materials. This

is a position we have reached by improving our customers’ everyday

business. Long tradition combined with research and product develop-

ment equips Uddeholm to solve any tooling problem that may arise.

It is a challenging process, but the goal is clear – to be your number one

partner and tool steel provider.

Our presence on every continent guarantees you the same high quality

wherever you are. We secure our position as the world’s leading

supplier of tooling materials. We act worldwide. For us it is all a matter

of trust – in long-term partnerships as well as in developing new prod-

ucts.

For more information, please visit www.uddeholm.com

Documents

UDDEHOLM TOOL STEEL FOR DIE CASTING · 2 UDDEHOLM STEEL FOR DIE CASTING This information is based on our present state of knowledge and is intended to provide general notes on our