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NADCA DIE CASTING CONGRESS & EXPOSITION, (2018), Transaction No. T18-012*This paper was presented at the North American Die Casting Association Die Casting Congress and ispart of the North American Die Casting Association Congress Transactions. It should not be republishedwithout prior approval from the North American Die Casting Association

Evaluation of Heat Checking of Molds through Flow and Solidification
Simulation
N. Umemori, M. Kawano, T. Masuda (Daido Steel Co., Ltd.)ABSTRACT
Generally, the major failure modes of die casting molds are heat checking, soldering and gross cracking.
Our steel manufacturer conducts researches into the influencing factors of each failure mode by performing
die casting tests and numerical analyses. In addition, the appropriate steels1), 2), heat treatment, surface
treatment and repair materials3) for die casting are developed to suppress the occurrence of initial damage
on molds. This study shows thoughts of influential factors and numerical analyses of heat checking throughflow and solidification simulation. Moreover, versatile simulation of initial crack prediction was derived byevaluating the area and the occurrence of heat checking during die casting tests.
1) Main material characteristics which make an effect on heat checking resistance are proof stress andthermal conductivity of mold steel.
2) Proof stress at maximum temperature can be higher by raising hardness of mold steel at room
temperature.
3) Thermal stress becomes lower by increasing thermal conductivity of mold steel.
4) The lower ratio of thermal stress divided by proof stress can extend the occurrence of heat checking.
INTRODUCTION
With the expanding demand of high fuel economy to meet emission regulation, automakers are pursuinglight weight vehicles. To achieve the weight reduction of vehicles, it is projected increasing number ofaluminum die castings will be applied4). Generally, heat checking is the major failure mode of die casting
molds. When we look at the failure mode of die casting molds as shown in Fig. 1, heat checking counts
almost 78 %5). The heat checking causes water leak or the patterns of heat checking are transferred to thedie casting parts. Therefore, to maintain the surface quality of the parts, molds are repaired by welding orreplacing to a new one. Initial crack prediction is important on the plan of maintenance and order of molds.
However, conventional prediction precision is not high because it does not consider flow and solidification
damage of mold surface. Moreover, it is not necessarily clear that the relation between the material
properties and damages of molds. In these circumstances, we have conducted the die casting tests on
systematically changed material properties by using an actual die casting machine. In this paper, it shows
thoughts of influential factors to heat checking. And it evaluated versatile simulation of initial crack
prediction derived through flow and solidification analysis.

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Fig. 1 Failure modes in die casting molds.

Fig. 2 Shape of test mold.
DIE CASTING TESTSTESTS PROCEDURE
The chemical composition of mold steels used in the tests is shown in Table 1. Steel A is H13. Hightemperature strength of steel B is same as steel A but hardenability and thermal conductivity is higher thansteel A. High temperature strength and thermal conductivity of steel C is much higher than steel A and B.
The test block of these mold steels sized 210 x 205 x 65 mm were quenched from 1030 °C (1886 °F)
by rapid cooling rate and then tempered to 38, 43 and 48 HRC. The steel A of 43 HRC only was quenchedby both rapid cooling rate and slow one. After tempering, the mold of Fig. 2 was made from the block. The
test was carried out by using 135 ton actual die casting machine: TOSHIBA DC-135JT. Test mold weighted
18 kg. It has two projections sized 50 mm width and 7 mm height for easy crack initiation. The aluminum
cast was 122 mm square and 12 mm thick weighted 600 g. The conditions using on die casting test are
shown in Table 2. Molten aluminum alloy, ADC12, was injected to the mold through gate with 54 m/sec
and the cycle time was 28 sec including 3 sec spray time2). The surface temperatures of depth 1 mm ofmolds were measured at 6 places by sheathed thermocouple of diameter 1.6mm.
In this test, heating and cooling to mold were promoted in order to be easy to occur heat checking due tohigher thermal stress6). The temperature of molten aluminum is higher from 30 to 60 °F than generaltemperature to strengthen compressive stress on the surface of mold. Moreover, the area and thickness ofaluminum cast created larger than general one in terms of thermal capacity. On the other hand, the spray© 2021 Daido Steel Co., Ltd.

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time was lengthened 3 second to increase tensile stress on the surface of mold. After casting, heat checking
was observed by dye penetrant testing.
Table 1 Chemical composition of steel A, B, and C (mass%).
Steel A
Steel B
Steel CC0.39
0.360.33

Si
1.0
0.50.1Mn
0.4
0.7
0.6

Cr
5.2
5.5
5.5

Mo
1.2
1.2
3.0V
0.9
0.60.9Comments
H13High quality steel

Table 2 Conditions of die casting test.
Items
Al alloy
Molten alloy temperature
WeightCastSize
Low
Injection velocity
HighPressure
Whole time
Solidification
1 cycleAir blowSpray
Air blowTemperature
Water cooling
Volume flux

Conditions
ADC12973 K (1292 °F)
600 g
122 mm×122 mm×14 mm
200 mm/s
1600 mm/s
65 MPa
28 s
8s
0.5 s
3s
1.5 s
298 K (77 °F)
1.83×10-4 m3/s

MICROSTRUCTURE OF MOLDSMicrostructures of steel A, B and C are shown in Fig. 3. These hardness are 43 HRC. The grain size ofaustenite in steel B was only larger than others because steel B had lower V and also had lower VC
precipitate of stopping grain boundaries during quenching. The microstructure of rapid cooling rate in thequenching of all steels was martensite. However, the microstructure of slow cooling rate of steel A wasmartensite and bainite.

Fig. 3 Microstructure after quenching and tempering of 43 HRC.
HEAT CHECKING
The molds of steel A, B, and C after 10000 shot casting are shown in Fig.4-6. In all of the steels, heat© 2021 Daido Steel Co., Ltd.

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checking was occurred in the vicinity of the gate of the mold. However, in high quality steel C at highhardness 48 HRC, there was less heat checking and no crack near the center of the designing surface.

Table. 3 Material properties (Steel A, 43 HRC).
Rapid cooling

Slowercooling

44

Level ofdamaging ofmold
Impact value(2mm U notch)-2-2

52 J・cm

21 J・cm

Fig. 4 Heat checking test result (Steel A, 43 HRC).

Fig. 5 Heat checking test result (43 HRC).
Table 4 Material properties (Rapid cooling, 43 HRC).
Steel A

Steel B

Steel C

Level of damaging of mold

432Impact value (2mm U notch)

52 J・cm

56 J・cm

51 J・cm

Fracture toughness

96 MPa・√m90 MPa・√m56 MPa・√m

Reduction of area at 500 °C (932 °F)

74 %

73 %

62 %

0.2 % proof stress at 500 °C (932 °F)

826 MPa-2-2827 MPa-1Thermal conductivity

26 W・(m・K)

Thermal expansion rate

12.0×10 ℃© 2021 Daido Steel Co., Ltd.

-6-2-1891 MPa-129 W・(m・K)-6-1

12.0×10 ℃-131 W・(m・K)-6-1

11.9×10 ℃

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5/10Fig. 6 Heat checking test result (Steel A).
Table 5 Material properties (Steel A, Rapid cooling).
38 HRC

43 HRC

48 HRC

Level of damaging of mold

541Impact value (2 mm U notch)

72 J・cm-252 J・cm-227 J・cm

0.2 % proof stress

696 MPa826 MPa991 MPa-2

MATERIAL PROPERTIES EFFECT ON HEAT CHECKING
TOUGHNESS, DUCTILITY
The heat checking resistance and Charpy impact value of steel A with different cooling rate of quenching
are shown in Table 3. According to Fig. 4 and Table 3, even if microstructure and Charpy impact value
were very different, crack appearance was almost same. Furthermore, the depth of cracking was not also
shown a marked difference. Therefore, Charpy impact value was not able to explain an effect on heatchecking7).
The heat checking resistance and material properties at 43 HRC of steel A, B, and C are shown in Table
4. The value in this table explains a damage level of molds. The lower the value is, the lower the damage isto molds. Steel C had the lowest fracture toughness, but it showed superior performance to other steels. Asa result, fracture toughness did not show the relation to heat checking.
Ductility was unable to indicate an effect on heat checking because the reduction of area after fracture oftensile test of steel C which had superior was the lowest in the 3 steels.
THERMAL CONDUCTIVITY, THERMAL EXPANSION RATE
Table 4 showed that the higher the thermal conductivity of molds was, the lower the occurrence of heatchecking was. Because higher thermal conductivity of mold caused to lower thermal stress from contact
with molten aluminum and from a spray cooling7).
In general, the thermal stress σ which works on the surface of an object having a difference temperaturein the cross-section surface is calculated by Equation 1 to 3. The λ increasing lets ⊿T and σ decrease.
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The gap of E, α and ν between each mold steel is narrow. These values are shown in Table 4.
σ=C x E x α x ⊿T …Eq.1⊿T=f(λ) …Eq.2
C=f(ν) …Eq.3
E: Young’s modulus
α: Thermal expansion rate
⊿T: Temperature difference from internal and surface
λ: Thermal conductivity
ν: Poisson’s ratio.
PROOF STRESS AT HIGH TEMPERATURE, MECHANICAL FATIGUE STRENGTH
The heat checking resistance and 0.2 % proof stress at 500 °C(932 °F) of steel A with different
hardness are shown in Table.5. Hardness, which is proof stress at high temperature indicated huge effecton heat checking. The reason is that steel C had superior heat checking resistance. It could be explained byhigh thermal conductivity and high proof stress at high temperature.
The mechanical fatigue strength of steel A and C at room temperature are shown in Fig. 7. These steels
had different heat checking resistance, but the rotating bending fatigue strength was same. Therefore,mechanical fatigue strength at room temperature only could not explain relation to heat checking resistance.

Fig. 7 Mechanical fatigue strength.

Fig. 8 Actual temperature and calculated
temperature.

By these knowledge, main attention properties of mold steels in heat checking during die casting were
thermal conductivity and proof stress at high temperature. The difference between each steel of young’s
modulus, thermal expansion rate and Poisson’s ratio which effect on thermal stress were low. That of
mechanical fatigue strength was also low. And that of impact value, fracture toughness and ductility seemed
directly not to have a correlation to heat checking resistance. Furthermore, softening resistance had little
effect on heat checking resistance because the temperature of molds during die casting was not so muchhigh as to be softened hardness of molds7).

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CASTING FACTOR EFFECT ON HEAT CHECKING
Temperature, thermal stress and flow speed were estimated by flow and solidification software
MAGMASOFT®. The actual temperature and calculated value on the surface of mold of steel A were shownin Fig. 8. Maximum temperature Tmax and T minimum temperature Tmin at 1mm depth from surface wereevaluated. Parts b - g in Fig. 8 were correspond to Fig. 4. Maximum error of calculated temperature toactual one was about 20 °F and the accuracy of estimate temperature was better.
TEMPERATURE DIFFERENCE FROM INTERNAL AND SURFACE
The load and damage level of parts a - f in Fig. 4 are shown in Table 6. It means the lower of damage levelis better. On the damage level 1 of part f, heat checking did not occur. ⊿T did not seem the relation todamage of mold.
⊿T: Temperature difference from maximum and minimum on the surface of mold
THERMAL STRESS
σ3min which was minimum of minimum principal stress indicated the most relation to damage of mold.
σ3min is maximum compressive stress which occurs when it contacts with molten aluminum. This result wascorresponding with equation 48). This denominator is equivalent to compressive stress at injection.
Mold life ∝ (σB / (E x α x ⊿T)) …Eq.4
σB: Tensile strength at high temperature⊿T: Temperature difference of molten aluminum and mold⊿PS is σ1max - σ3min, σ1max is maximum of maximum principal stress during spray cooling. ⊿PS wasconsistent with tendency of the level of damage. In this study, we linked ⊿PS /σy to NC. σy is 0.2 % proofstress at maximum temperature Tmax, and NC is a shot number of initial crack occurrence. The relation of⊿PS /σy and NC is shown in Fig. 9. NC was the results of die casting test, and ⊿PS /σy was calculated byflow and solidification software. Moreover, it evaluated the influence of time integration Ivf of flow speed
when flow speed was changing (v1 > v2 > v3). When ⊿PS against σy was higher, initial crack occurredearlier, and the initial crack occurrence became earlier by increasing Ivf. The change of surface roughnessof casting part when steel A is shown in Fig. 10. It is shown part b in front of pouring gate. At 1 shot ofcasting, the smooth grinding surface of mold was transferred to casting part. At 1000 shot, the surface
roughness of mold became about 1.5 times, but did not show heat checking occurrence. At 2000 shot, theroughness became about 4 times, and heat checking occurred.
According to these results, it is confirmed that flow and solidification effect on heat checking occurrence.
It is also supposed that heat checking is thermal stress phenomenon accompanied with erosion.

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Table 6 Level of damage and load (Steel A, 43 HRC).
abcde

F

Level of damaging of mold

654

3

2

1

⊿T = Tmax - Tmin

132

6

5

4

654

3

2

1

12

3

64

5654

3

1245632

1

σ3Principal stress

σ1minmax⊿Ps = σ1
Flow and solidificationmax- σ3minIvf

Fig. 10 Change of surface roughness of mold.

Fig. 9 Influence to crack occurrence on⊿PS /σy and Ivf.
VALIDATION OF DAMAGE PREDICTION MODEL
By using these results of this study, NC was formulated. λ, α, E and ν also are functions of steel gradeand temperature. The damage prediction was confirmed by calculating NC when steel grade, hardness andmold shape were changed through MAGMASOFT®.
NC = f(σy, ⊿PS, Ivf) …Eq. 5
σy = f(steel grade, hardness(HRC), temperature) …Eq. 6⊿PS = f(λ, α, E, ν) …Eq. 7
The NC distribution of steel A, B and C at 43 HRC are shown in Fig. 11. In this figure, the left half is theresult of die casting test, and the right half is the result of NC prediction by flow and solidification software.
The results were confirmed that the reproducibility of the difference of steel grade about heat checking
resistance. For example, the analysis results which steel C had the lower damage was same tendency as thedie casting test. And it was shown that the lower NC was, the more the damage degree is actually. The NC
distribution of steel A of 38, 43 and 48 HRC are shown in Fig. 12. The analysis results were also reproducible
to the hardness difference of mold about heat checking resistance, and higher hardness of mold had lower
damage. In Fig. 11 and 12, the maximum difference from actual and calculated value of NC at the part b© 2021 Daido Steel Co., Ltd.

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which had more damages was up to nearly 25 %, and the prediction precision was better. Even if steel grade
and hardness change, it can predict how times die life becomes.

Fig. 11. Influence to NC distribution on steel grade.

Fig. 12 Influence to NC distribution on hardness of mold.
SUMMARY
The material properties effect on heat checking resistance are proof stress σy at high temperature andthermal conductivity λ. The higher the σy and λ of mold is, the less heat checking occurrence becomes.
Moreover, the higher ⊿PS / σy is, the lower the shot number NC which occurs heat checking become.
⊿PS = σ1max - σ3min
σ1max : maximum principal stress during spray cooling
σ3min : minimum principal stress during contacting with molten aluminum alloy
NC becomes earlier by increasing time integration Ivf of flow speed. It is supposed that heat checking isthermal stress phenomenon accompanied with erosion. Furthermore, through flow and solidification
software, we could conduct crack occurrence prediction model of NC = f(σy, ⊿Ps, Ivf) . And this prediction
model could predict how times die life becomes when steel grade and hardness are changed.

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1) K. Namiki, M. Kawano, T. Schade: High Thermal Conductivity Steel and its Application to Die CastingTools, NADCA DIE CASTING CONGRESS & EXPOSITION, (2012)
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