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NADCA DIE CASTING CONGRESS & TABLETOP, (2013), Transaction No. T13-052*This article is an abridged version of the original one.
**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

Development and Applications of High Hardenability
Special Quality Die Casting Mold Steels
K. Namiki, N. Yokoi, A. Sekiya (Daido Steel Co., Ltd.)T. Yanagisawa (Daido DM Steels Solutions Co., Ltd.)ABSTRACTToughness is becoming more important for die casting mold steels as well as heat checking resistance.
High toughness is required to prevent gross cracking from water lines, especially from the crossing of thesein large size mold such as transmission and crank cases of automobiles. The cause of decrease in toughness
in large molds is Bainitic transformation during quenching. To prevent this high quenching cooling rate isrequired. But, the tradeoff is dimensional change. The right answer from mold steel design is highly
hardenable mold steels. Based on these back grounds two die casting mold steels have been developed:
DH31-EXTM and DHATM-WORLD. The former is 2367 modified double melted high Mo steel and thelatter is H11 improved type steel. In spite of single melt steel, DHA-WORLD shows much higher toughness
than H13 double melted steel even in large mold. These two steels have been applied to large automobiletransmission cases in these three years, the largest one of which weighs 1700 lbs．These molds run in actual
service without any gross cracking from water lines. This report will describe the features of these steels
and the details of application.
INTRODUCTIONTrend of die casting industries, focusing on automotive industries, are summarized in Figure 1. Aluminum
die casts are increasingly used for various components from engine to transaxle parts to manufacture lightweight vehicles for improved fuel economy. Besides long life molds, to stand high applied stress shorter
cycle time in die casting operation is key issue in automotive industries for cost reduction. For these
purposes, high performance and reliable mold steels are required. NADCA specified steels are these kinds.
Main failure modes of die casting molds are heat checking, gross crack and soldering. As far as heat checking
is concerned, molds are used long to planned life by repeating repair in maintenance time. Gross crack, on
the other hand, results in time consuming repairing, or being scrapped in the worst case. Usually crack
initiates from deep heat checking and water lines, especially their crossing positions. Recent main measures
to shorten cycle time are increased number of water lines and decreased distance between water lines andthe mold surface. Therefore, the toughness inside, especially in large molds, becomes key property to rulemold life.

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The low toughness of the mold core is caused by bainitic transformation in quenching. It has long beenclarified that when molds quenched through bainitic transformation region, toughness drops aftertempering1). Then, two measures are adopted to improve the toughness of molds: fast quenching andapplying high hardenability steels. The transition of die casting mold steels in Japan is shown in Figure 22).
Initially, hot work die steels were standardized according to AISI steels, H13, H11 and H12. Among thesethree steels SKD61, equivalent to H13, is widely used as general purpose steel. Along with the developmentof automotive industries and the growth of die cast components, high performance steels have beendeveloped. Low Si and high Mo type steels are ones of these and already registered as NADCA
recommended steels as category C. Not only alloy designing, but also double melted clean steels havebecome popular as premium type steels and the importance of high quenching rate in low temperature
range has been perceived. In this paper, as an example of measures from steel supplier, high hardenability
die casting mold steels and their application to die casting molds of automotive large parts are introduced.

Figure 1 Trend of die casting industries and requirements forFigure 2 Change in die casting mold

mold steel suppliers in Japan.

steels.

FUNDAMENTAL CHARACTERISTICS OF DEVELOPED STEELSCHEMISTRY AND PRODUCTION PROCESS
Toughness of die casting mold steels are decided by quenching cooling rate especially in low temperature.
Examples of optical micrographs taken from the thin and the thick part of a transmission case mold areshown in Figure 3. Charpy impact values (U notch) are shown together with photographs. Slowly quenched
thick part shows coarse microstructure, acicular bainite structure, which results in lower Charpy values due
to easy crack propagation.
Therefore, to harden through without bainitic transformation in large molds, it was intended to delay
bainitic transformation in the chemistries of new steels. DH31-EX is improved 1.2367 type low Si- high Mo
type steel and DHA-WORLD is improved H11 type steel. In both steels, to improve hardenability Mn and© 2021 Daido Steel Co., Ltd.

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Cr contents are closely controlled. Although DH31-EX is ESR melted steel, DHA-WORLD3) is single melt
steel to reduce production cost. But, by combining production technologies from alloy designing, clean
steel-making to forging process, it is intended to give same properties as those of double melted steels.

37.1 J/cm2

16.1 J/cm2

Figure 3 Optical micrographs taken from the thin part (left) and the thick part (right) of a transmissioncase mold.
HARDENABILITY AND TOUGHNESSCCT curves, continuous cooling transformation curves, of these two grades are shown in Figure 4 and 5.
Compared to H13, bainitic transformation areas are shifted to longer time side. Bainitic transformation
starting points of DHA-WORLD and DH31-EX are 2 hours, much longer than that of H13. The effect ofcooling rate in low temperature range on U notch Charpy impact values is shown in Figure 6. Compared toH13, these new two grades show higher absorbed energy especially in slow quenching. Furthermore, Figure
7 shows the Charpy impact energy values of the samples taken from vacuum quenched large blocks sized
200 mm thick, 600 mm width, and 300 mm long. DH31-EX shows the highest values in wide hardness range.
It is noted, here, that single melt DHA-WORLD shows almost equivalent to or better toughness than ESR
melted H13.

Figure 4 CCT diagram of DHA-WORLD.

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Figure 5 CCT diagram of DH31-EX.

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Figure 6 Effect of quenching cooling rate on
Charpy Impact values (U notch).

Figure 7 Charpy impact values of the specimens
taken from vacuum quenched and tempered large
block.

HEAT CHECKING RESISTANCE
An example of heat checking test results is shown in Figure 85). This test was carried out by 135 ton die
casting machine: TOSHIBA DC-135JT. Test mold weighting 18 kg has two projections sized 50 mm widthand 7 mm height for easy crack initiation. The hardness was 42-43 HRC. The aluminum cast was 122 mm
square x 12 mm thick weighting 600 g. Molten aluminum, ADC12, was ejected through gate with 54 m/sec
and the cycle time was 28 sec including 3 sec. spray time. The molds surfaces as of 10,000 shots were shownin Figure 8. Compared to H13-ESR mold, DH31-EX and DHA-WORLD both showed less heat checking.
Especially on the center surface, between two projections, neither cracks in DHA-WORLD nor DH31-EX.
It is noted here DHA-WORLD, single melted and 1.25 % Mo steel, showed better heat checking resistance,
which was caused by high thermal conductivity due to low Si content6).

DHA-WORLD

DH31-EX

H13-ESR

Figure 8 Heat checking test result: The appearance of test specimens after 10,000 shots.

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APPLICATION TO LARGE SIZE DIE CASTING MOLD
LIFE DETERMINING FACTORS IN LARGE MOLDSExamples of large Al die cast automotive components are cases for transmission, torque converter and soforth. Generally, molds are used by repairing in periodical maintenance time to planned life such as 150,000
shots. Usually, heat checking, shallow chipping as shown in Figure 9 and soldering are removed and repair
welded. But, in case of water leakage due to cracks propagated through the surface from water lines, it takes
much time and cost to repair. Figure 10 shows an example of the gross crack resulted in water leakage andFigure 11 is the fracture surface around crack initiation site. The optical micrograph taken from the crack
initiation site is shown in Figure 12. Needle like bainitic structure was observed, which indicated the low
toughness inside resulting in crack initiation from the water line. Furthermore, the surface of water line wasinternally oxidized along austenite grain boundary, which assisted the crack to initiate.

Heat checking

Chipping

Figure 9 Examples of heat checking and chipping.

Figure 11 Fracture surface from water line

Figure 10 An example of grosscrack.

Figure 12 Optical micrograph taken from thecross section of water line

LARGE BLOCK TEST RESULTS
As the importance of toughness was perceived again by the previous analysis of actual mold, the superiorityof developed steels to conventional ones were tested by using large blocks simulated core molds. Large test
blocks of DHA-WORLD and H13 sized 550 mm width and 500 mm height were quenched from 1030 ℃
in oil and tempered to 42/44 HRC. Both steels showed uniform hardness around 43 HRC from surface to© 2021 Daido Steel Co., Ltd.

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inside. On the other hand, Charpy impact values (U notch) were different each other as shown in Figure
13. In H13 block, the value in the center decreased to 10 J/cm2. DHA-WORLD, whereas, maintained highvalues such as 30 J/cm2 even in the center. Optical micrographs Figure 14 taken from the center parts ofthe specimens were showing clearly the difference: tempered martensite in DHA-WORLD and tempered
bainite in H13.

Figure13 Charpy impact values distribution in large test blocks tempered to 43 HRC.

DHA-WORLDSKD61 (H13)

Figure 14 Optical micrographs taken from the center part of the test blocks.
V NOTCH IMPACT PROPERTIESCorrelation between notch geometries, V and U, is to be stated in appendix.
APPLICATION TO TRANSMISSION CASE MOLDHeat treatment of the mold
DHA-WORLD was at first applied to fixed cavity of transmission case mold. A large block sized 824 x 814
x 404 mm and weighting 2.1t was supplied and machined. Rough machined cavity mold was quenched by© 2021 Daido Steel Co., Ltd.

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11bar vacuum furnace, 36" x 36" x 48", by controlling gas pressure and its flow. The temperature inside wasmeasured by setting thermocouples in a water line located in almost the center of the molds. Actual coolingrates at high and low temperature range were shown in Table 1. The rate in low temperature range was
3.6 ℃/min. When referred to the fundamental data shown in Fig. 6, this cooling rate was enough to obtainhigh enough Charpy values higher than 20 J/cm2. This mold was tempered three times at 580, 610, and570 ℃ to meet the specified hardness 43/45 HRC. Hardness obtained by tempering were plotted in thetempering curve as shown in Figure 15.
Table 1 Quenching cooling rate of thetransmission case mold (ºC/min).
Temperature

Surface

CenterTarget

1030-500

51.7

7.9

10.0

500-200

4.7

3.6

3.0

range (ºC)

Table 2 Charpy impact values of the samples
taken from the center of the mold
DataAve.

12

3Hardness: HRC

43.0

43.2

43.1

43.1

Charpy value:
J/cm2 (U notch)

39.9

25.5

39.9

35.1

Figure15 Hardness obtained by tempering.

The hardness of the mold measured on the surface was in good agreement with fundamental tempering
hardness curve. Furthermore, from the center part of the mold, three small samples were cut by EDM andCharpy test was carried out. As shown in Table 2, all values were higher than 20 J/cm2: this value wasempirically deduced criteria to prevent gross cracking in actual molds.
Optical micrographs of Charpy impact specimen was shown in Figure 16. Microstructure was fine and freefrom bainitic structure.

Figure 16 Optical micrograph taken from the center of the mold.
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Results in die casting
After trial shot and some geometry modification, this mold was nitride and started to run in mass
production in the beginning of 2010. As of 13,000 shots fine heat checking started to appear and heatchecking became severer especially at the position near gate with increasing of shot number. But, without
big cracks which triggered water leakage, this mold had reached to planned life 150,000 shots. The surface
appearances of this mold as of 140,000 shots are shown in Figure 17. Small cracks were observed on thesurface, but shallow one. DH31-EX was also used for ejector side and run through to 150,000 shots.
Compare to DHA-WORLD, almost no heat checking at gate area in DH31-EX mold. An example of thesurface is shown in Figure 17. Beside these molds, DHA-WORLD and DH31-EX have been applied toother molds for transmission cases.

DHA-WORLD: Fixed cavity

DH31-EX: Ejector sideFigure 17 Examples of mold surfaces of the molds.
CONCLUSIONS
Two hot work die steels, DH31-EX and DHA-WORLD, were developed to prevent gross cracking in large
die casting molds by increasing the toughness. The features of these steels are high hardenability which
provide high toughness even in large size molds. These two mold steels have been applied to automotivetransmission cases and running well.
*DH31-EX and DHA are trademarks or registered trademarks of Daido Steel Co., Ltd.
ACKNOWLEDGMENTS
We’d like to acknowledge Mr. Mark Baleja of Century Sun Heat Treating for the discussion in the heat
treatment of molds.
REFERENCES
1) N. Uehara, K. Namiki, K. Takahashi: Denki-Seiko (Electric Furnace Steel), vol.81, No.3, p.165 (1981)
2) K. Namiki: Netsu-shori (Journal of the Japan Society for Heat Treatment), vol.50, No.6, p.556 (2010)3) H. Morikawa: Denki- Seiko (Electric Furnace Steel), vol.81, No.1, p.47 (2010)4) NADCA #207-2011, NADCA Publication #229 (2011)5) N. Yokoi, M. Kawano, K. Inoue: Denki- Seiko (Electric Furnace Steel), Vol.81, No.1, p.25 (2010)6) M. Kawano and K. Inoue: Materia Japan, vol.48, p32 (2009)© 2021 Daido Steel Co., Ltd.

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APPENDIX
In the discussion of die toughness of die casting mold steels, V notch Charpy specimen is popular in US,
but U notch in Japan. For easier discussion the correlation of the Charpy impact energies were tested. Fromeach coupon heat treated together with actual die casting molds by attaching on the surface, V notch andU notch Charpy specimens were taken side by side. Test steels used were H13，DHA-WORLD and DH3EX. Coupons were taken from different size forged blocks, heat treated in different chances with different
sized molds.
Figure Appendix 1 shows the correlation between the absorbed energies by V and U notched specimens.
An approximate V notch value is 0.43 times U notch values.

Figure Appendix-1 Correlation between V and U notch absorbed energy.

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