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CORRELATION BETWEEN HEAT-CHECKINGRESISTANCE AND IMPACT BENDING ENERGY

OF HOT-WORK TOOL STEEL DIN 1.2344.

J. BucksteggeEdelstahl Witten-Krefeld GMBH

Testing Department

P.O.B. 10 06 46

D-47706 Krefeld

Germany

B. GehrickeEdelstahl Witten-Krefeld GMBH

Research and Development, Customer Service

Tool Steel

D 58452 Witten

Germany

U. ReichelEdelstahl Witten-Krefeld GMBH

Testing Department

P.O.B. 10 06 46

D-47706 Krefeld

Germany

Abstract The permanentlyincreasingproduction of die castaluminium and magnesiumparts is directly related to an increasing demand in premium hot-work toolsteels for die casting dies.

861

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862 6TH INTERNATIONAL TOOLING CONFERENCE

One of the major failure modes of the die casting tools is the occurrenceof heat checking, a network of cracks on the dies surface, which eventuallyleads to a repair of the die or in the worst case to premature failure.

In order to extend the die life, so-called premium hot-work tool steelshavebeen developed which have to fulfil standards postulated by the die castingindustry. Standards suchas Chryslers NP 2080or the Acceptance References

of the North American Die Casting Association specify the qualitative designof the hot-work tool steels. Their chemical composition, microstructure,hardness as well as toughness (to be measured in impact bending tests) haveto meet given requirements. As no direct relation between the specifiedcharacteristics and the actual heat checking resistance has been found so far,this work intends to verify a correlation between impact bending energy andheat checking resistance.

This study is based on examinations of hot work tool steel 1.2344 out ofdifferent heats. Impact bending tests were conducted in order to describethe materials toughness. The heat checking resistance was determined on aparticularly designed device. An indexing wheel transfers the samples to aninduction loop heating thesamples to the temperature of liquid aluminiumbe-fore being water quenched. After a specified number of cycles these samples

were microscopically studied in order to determine the number and length ofthe cracks occurred on the surfaces.

Keywords: Hot work tool steel, die casting, toughness, ductility, impact bending test,thermal fatigue, heat checking resistance

INTRODUCTION

The demand for light metal components is still increasing. In order toproduce complicated shapes, the die casting process is the most economicway. The economy of this process is strongly influenced by the number ofcomponents produced out of the according die before failure.

The most common failure mode is a network of cracks on the surface of

the dies caused by the cyclic heating and cooling of the cavity surface duringinjection of the liquid material, the cooling and solidifying, the ejection andthe spray cooling of the surface.

This failure mode, also called heat checking, is besides working conditioninfluenced by the thermal fatigue behaviour of the die materials.

The initiation of thermal shock damage could be explained by the cyclicloading and relief of strain of a material surface undergoing a permanentextension and contraction due to cyclic heating and quenching during theapplication. A simplified interpretation of this behaviour is given by the

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Correlation Between Heat-Checking Resistance and Impact Bending Energy...863

"Kindbom-Theory" as it can be seen in Fig. 1. [1,2] Theoretically an un-

Figure 1. Kindbom theory.

clamped surface element extends elastically during heating-up and constrictselastically to its original state after quenching. The theory closer to realityimplies that the surface elements are clamped by neighbour ones and canonly extend in one direction during heating. The resulting state of stresscauses an elastic and in the end a plastic deformation of the clamped surfaceelement. During quenching, the surface element constricts in all directionswhereby cracks between the surface elements are induced.

The most typical resulting kinds of damage due to thermal shock loadingare described in Fig. 1 too. Deformation is the primary stage of thermalshock damage. Cracks are the second stage and they represent the majorresulting kind of damage. The final stage are shellings which normally occurafter cracks have extended or grown together.

So far formulas describing the thermal fatigue behaviour include proper-ties like:

yield strength

thermal conductivity

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Youngs modulus

coefficient of thermal expansion

The toughness and ductility characteristics of the materials are not con-

sidered.During thelast two decades material specifications for hot work tool steelshave been developed with the aim of reaching a better potential for increasedlive times. The special focus was set on impact bending values obviouslydescribing the toughness behaviour of the material. Due to improved pro-duction methods even the highest requirements with regard to toughness(f.e. North American Die Casting Association, General Motors, CNOMO,French automobile producers) can be met, however there was no link so farbetween those values and heat checking behaviour.

GOAL OF THE INVESTIGATION

To prove a possible correlation between heat checking resistance andtoughness behaviour of hot work tool steel 1.2344 corresponding tests werecarried out on specimenswith constant hardness levels. With reference to thedifference of ductility on one hand and toughness on the other hand notchedCharpy-V specimens as well as unnotched specimens were checked. Theresults were compared with the thermal fatigue behaviour of correspond-ing samples on a fatigue testing stand mainly with temperatures as high as700 C.

EXPERIMENTAL PROCEDURE

In order to check the thermal fatigue behaviour a test stand was used which

allows to expose rectangular specimens to a cyclic heating and cooling. Forthis purpose the specimens are mounted on a wheel so that every specimenwhile rotating runs through a heating and subsequently a cooling device,Fig. 2. With respect to a surface near introduction of the thermal energy theheating takes place by induction with a frequency of 250 kHz. The generatorpower is 15 kW. To be quenched the specimens dive into a water bath whichtemperature is kept constant with cooling pipes.

Figure 3 shows all important features of the complete device schemati-cally. The generator power is led to the induction loop across an impedancematching device and automatically switched on after the specimen rotated

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Figure 2. Thermal fatigue testing, set-up for cyclic heating and cooling.

into the heating position. The maximum surface temperature of the spec-imens is controlled by the programmable switch-on time of the generator.Besides the sequenceof therotation of thespecimens as well as the number ofthermal cycles can be computer controlled. By means of the programmableleading control also the specimen temperature, the temperature of the waterbath and the actual number of cycles can be monitored and stored.

If the test stand is equipped with 4 specimens, the quenching time in the

water bath necessarily equals the time in the heating position. However dueto adjustment of the cooling of the water bath the quenching temperature ofthe specimens can be varied slightly. Additionally it is possible to uncouplethe quenching time from the heating time as long as the test stand is equippedonly with two or one specimen with regard to the sample motion profile.

Due to the different adjustment of the generator power the heating speedcan be varied. With 100 % generator power the heating time from 100 Cto700 Cis approximately 3 seconds (Fig. 4). Reducing the generator powerto 75 % the heating time increases to 5 s. Further power reduction to 50respectively 25 % for heating to 700 Cincreases the heating time to 9 re-

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Figure 3. Thermal fatigue test system.

spectively 23 seconds. The size of the specimen is 50 55 10mm3, see

Figure 4. Temperature-time-profile for different heating power P.

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Fig. 5. A fine ground surface finish was used to avoid crack initiation dueto grinding grooves.

The geometric design of the induction loop causes that the electromag-netic energy is transformed into heat only in a certain area of the specimen.The area with the highest specimen temperature can be recognized according

to the tempering colours on the surface of the specimen after a small numberof thermal cycles (Fig. 6). According to this appearance the highest tem-peratures develop only in a relatively small specimen area extending to thesample corners. The most homogenious area lies in the middle of the speci-men approximately 11 mm below the specimen corner. The metallographicevaluation of the heat checking cracks is carried out in this area.

The number of cracks with a length of more than 20 m as well as thesummarized length of those cracks allow judgment of the thermal fatiguebehaviour. Additional features like variations of the specimen geometry canbe used to characterize the heat checking resistance.

As test material the hot work tool steel according to DIN Standard 1.2344

ESR produced to meet the NADCA specification was used. Besides theimpact bending test specimens, which were cut from the short transversedirection in the core area of bars out of different heats, the specimens for theheat checking test were taken out of the transition area of the same material.

Following raw machining the specimens were hardened and temperedtwice to a hardness-level of 45 1 HRC. After heat treatment they were fineground with a surface roughness of 4 m. All specimens where additionallydemagnetized and thoroughly cleaned.

The examination of the heat checking resistance was performed with 4specimens on the test wheel. The generator power was set to 80 %. Themaximum specimen temperature was adjusted to 700 C. With a constantwater bath temperature of 60 Cand a dip in depth of the specimen of 20 mm

the minimum temperature after quenching reached 105 C.Figure 7 shows the temperature flow in the area of the crack rating over a

complete cycle. The heating to 700 Ctakes 3.5 seconds which correspondsto a temperature changing speed of 150 K/s. During quenching this speedis 170 K/s. One complete temperature cycle takes 18 seconds. This is ex-plained due to the fact, that every specimen runs through two free positionsbetween quenching and next heating. At these positions the surface temper-ature of the sample rises of approx. 50 Cdue to heat penetrating form thecore to the surface.

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Figure 5. Specimen for thermal fatigue testing.

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The duration of the test was set to 4000 cycles. Besides the main testedtemperature of 700 Csome tests were carried out with 650 C.

The necessary time to reach the maximum specimen temperature withgiven generator power was established in pretests. During those tests NiCr-Ni thermocouples where spot welded onto the evaluation area in order to

adjust the temperature in correlation to the induction heating time. Forexample a lowering of the specimen temperature from 700 Cto 650 Cfortest material 2344 decreases the heating time from 3.5 seconds to 3 seconds(Fig. 8 ).

To avoid corrosion on the specimens the closed test vessel was floatedwith argon. Additionally the pH-value of the water bath was kept at 10,5.Besides the basic pH-value helps to improve the moistening of the specimensduring quenching.

RESULTS

The periodic change of tension and compression caused by the tempera-

ture changes leads to the typical network of fatigue cracks. Figure 9 showstheir appearance after 4000 temperature cycles between 700 Cand105 C.The cracks predominantly orientate in the direction of the ground surfacefinish but also develop transverse to that direction. For the quantitative judg-ment of the heat checking resistance the elapsed time to the first occuranceof surface cracks would be a suitable but time consuming feature. Thereforethe number of cracks and the summarized crack length examined metallo-graphically on the not etched specimen cross section from the area of themaximum temperature were used.

Corresponding microscopic photographs of the existing fatigue cracksafter 4000 temperature cycles at a specimen temperature of 700 Cas well

as 650

Cfor comparison are shown in Fig. 10 . While after the test at650 Cno cracks could be determined the increased temperature of 700 Cledto numerous fatigue cracks developing in the direction surface to core. Themaximum length of single cracks reaches up to 1 mm, the average cracklength measures 0,1 mm. Additional examination of the crackcharacteristicsshows that the crack propagation is intergranular as well as transgranular.

The thermal loading of the specimens caused by the temperature cyclesleads besides the development of fatigue cracks also to a tempering effectin the surface area. After 4000 cycles at 700 Ca low-load hardness check(Vickers 1 g) shows that the hardness in the surface drops to 350 HV1 (Fig.

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11). Towards the specimen core, a rapidhardness increase is measurable overa depth of approx. 2 mm. In deeper areas the loss in hardness is not as rapidanymoreandtheoriginalhardnessofthespecimenof490HV1isreachedatadepth of about 4,5 mm. Comparedto themaximum temperature of 700 Cthespecimens heated to 650 Cshow a remarkable lower loss of hardness in the

surface area. The surface hardness of these specimens measures 460 HV 1.Already at a depth of 1 mm the original specimen hardness is reached.

In order to judge the resultsan averageout of each specimen set containingfour specimens was built. With regard to the repeatability of the results acomparison of the values of every single specimen in one set is of interest.

Figure 12 shows a corresponding list for the set containing specimens10 Fto 13 F. While the main results are number of cracks and total length of cracksalso the maximum crack length and the average crack length were measured(Fig. 12 ). Apart from specimen 11 F with lower values the number ofcracks as well as the total crack length for the remaining 3 specimens show asatisfying scatter of 3 respectively 9 %. Including specimen 11 F the scatter

increases to 10 % for the number of cracks and 18 % for the total crack length.Against that the maximum length of the developing cracks is not suitable forquantitative judgments because of the bigger variations of the results.

Finally the results of the heat checking tests after 4000 cycles between700 Cand 105 Cwere compared with the impact bending test results of thesame bars (heats). The specimen material was taken out of standard pro-duction according to NADCA requirements (North American Die CastingAssociation). Therefore the impact bending values fall into a relatively nar-row band between 170 and 300 Joule for the unnotched samples and 10 to22 Joule for the Charpy-V-samples.

With regard to Fig. 13, no correlation could be found between the impactbending values of the notched samples characterizing the crack propagating

speed and the number of fatigue cracks or their total length. Therefore thecoefficients of correlation only measure about 0,06 for the number of cracksand 0,28 for the crack length respectively.

Also the impact bending values of the unnotched samples, which char-acterize the ductility of the material, do not show a correlation to the heatchecking resistance (Fig. 14). The according coefficients of correlationamount to 0,03 respectively 0,08 for the number of cracks respectively thetotal crack length.

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To verify the results some specimens were purposely overheated in heattreatment. The material was austenitizedwith a temperature of 1150 Cfollowedby two time tempering to achieve the typical hardness of 45 HRC. The over-heated coarsened structure leads to a loss of toughness and ductility. Theaccording dots are marked specifically (Figs. 13 and 14). The impact bend-

ing test results of the unnotched samples average on 125 Joule, whereas thenotched samples come to 9 Joule. Even the heat checking test results ofthese specimens dont give an additional hint with regard to a correlationbetween heat checking resistance and toughness respectively ductility.

CONCLUSION

Heat checking resistance respectively thermal fatigue behaviour is oneof the most important features with regard to failures in the field of diecasting. Although there areno standardized tests to check thethermal fatiguebehaviour, toughness and ductility values tested in the impact bending testare a part of most worldwide specifications for hot work tool steel for die

casting dies.The aim of this work was to find out whether there is a correlation between

heat checking test results and toughness characteristics. The set-up for theheat checking test was developed by EWK and permits tests with preciseparameters.

The tested specimens out of steel grade DIN 1.2344 ESR do not showan influence of ductility or toughness on the development of thermal fatiguecracks. The results put an other light on the valuation of impact bendingstrength data as a criterion for the performance of the according hot worktool steel during working operation particularly with regard to heat checkingresistance.

Obviouslythis statementis onlytrue for theexaminedparameters. At leastit opens thefield forfurther investigations of the heat checking characteristicsof hot work tool steels and main influencing factors.

REFERENCES

[1] L. KINDBOM: Warmribildung bei der Temperaturwechselbeanspruchung von War-marbeitswerkzeugen. Arch. Eisenhttenwes. 35 (1964 ) 8, p. 773 - 780

[2] T. MULLER, Temperaturwechselbestndigkeit von Warmarbeitssthlen undbeschichteten Bausthlen. Dr.-Ing. thesis, RWTH Aachen, Germany, 1999

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[3] H. BRIEFS and M. WOLF, Warmarbeitssthle". Verlag Stahleisen Dsseldorf 1975.

[4] P. GUMPEL, Deutsche und internationale Normung von Warmarbeitssthlen".Thyssen Edelst. Techn. Ber. 5 (1979) 2, 88 96.

[5] Chrysler Corp. Manufacturing Standards. Hot-Work Tool Steel (NP-2080), Rev. April1975.

[6] Stahl-Eisen-Prfblatt SEP 1614 Mikroskopische Prfung von Warmarbeitssthlen /Microscopic Inspection of Hot-work Tool Steels". Verlag Stahleisen mbH, Dsseldorf,Germany, 1996.

[7] Premium Quality H-13 Steel Acceptance Criteria for Pressure Die Casting Dies:NADCA nr 207-97. North American Die Casting Association, River Grove, Illinois,USA, 1997.

[8] H. BERNS, E. HABERLING and F. WENDL, Einfluss des Glhgefges auf dieZhigkeit von Warmarbeitssthlen". Thyssen Edelst. Techn. Ber. 11 (1985) 2, 150 157.

[9] P. GUMPEL, Untersuchungen ber Primrcarbide in Warmarbeitssthlen" ThyssenEdelst. Techn. Ber. 9 (1983) 2, 121 123

[10] L.-A. NORSTRM,Ductilityand Toughness in Hot-work Die steels: The Importance

of Proper test Procedures". Transactions of the NADCA 15th International Die CastingCongress and Exhibition, St. Louis, Mo., 1989, Paper No. G-T89-014

[11] H. JESPERSON, M. KLAUCK and P. ROCHE, Is Impact Testing Improving DiePerformance?". Die Casting Engineer, (199), 52-60

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Figure 6. Area of maximum specimen temperature acc. to tempering colours after 20cycles at 700 C.

Figure 7. Time-temperature-curve.

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Figure 8. Temperature in relation to heating time.

Figure 9. Crack network on the specimen surface after 4000 cycles at 700 C.

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Figure 10. Cracks after 4000 cycles.

Figure 11. Hardness profile in the cross section (surface-core) of specimen after differenttemperature loads.

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Figure 12. Typical scatter of results in one test set (4 specimens) after 4000 cycles at700 C.

Figure 13. Number of cracks / total crack length in relation to impact bending energy after4000 thermal cycles between 700 Cand 105 C(Charpy-V- specimen, s-t-direction, core).

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Figure 14. Number of cracks / total crack length in relation to impact bending energy after4000 thermal cycles between 700 Cand 105 C(unnotched specimen, s-t-direction, core).

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