THOUGHNESS OF DIE CASTING MOULDS. COMPARISON OF

SEVERAL TESTING METHODS

C.TORRIANI – SIDERIA CONSULTING Srl , Torino ,Italy

E. ASPESI – LAPANTA STAMPI Srl , Cassano Maniago ,Italy

G.ZAMBARBIERI – SMT Srl , Pozzo D’Adda ,Italy

A.SILIPIGNI- TAG Srl ,Dolzago,Italy

ABSTRACT

Nowadays ductility is the most useful property to judge the quality of the tool steel and of the heat

treatment of a die casting mould . There are many methods to detect the toughness through impact

test by a heat treated die casting mould .Most of them derives from specifications written up from

die casters ,die caster association , famous laboratory or from conventionally practice.

These methods show many similarities ,but the differences could be in some cases significant.

A mould with critical mass of ca 600 Kg was provided of several impact test rough specimens

according to the most used and known specification. The mould has been heat treated in a high

quenching capacity vacuum furnace and tempering vacuum furnaces to a hardness of 45-46 HRC

by temperature-time cycles commonly used by industry of heat treatment.

The obtained impact value have been compared each other .

This work aim to be an help to understand if impact values obtained according one of this

specification are significant for the ductility level of the mould .

KEYWORDS

Toughness , toughness of die casting mould , impact test ,Charpy V , HP1 , heat treatment , quench

, temper . quenching speed .

INTRODUCTION

Aluminium die casting moulds are highly stressed in working conditions. The more frequently

damages of the working surface are due to thermal stress ,like fatigue cracks and tensional cracks.

There are many factors influencing the time life of a die casting mould : Quality of steel and quality

of heat treatment , design of the mould , maintenance ,handling and working conditions , quality of

cast material . Each factor alone may be potentially responsible for failing of the mould . Normally

if a mould fails, it is due to a negative combination of these factors . In order to preview the tool

life is important to govern and check each factor.

CrMoV hot tool steels are normally utilised for this purpose because of their good tempering

resistance , high yield strength at temperature up to 650°C and sufficient thermal conductivity.

They are also tough enough to be utilised with high strength up to 1700N/mm² (ca 50HRC) at

room temperature .

The mechanical properties depend directly from their chemical composition ,the ductility depend

from their metallurgical quality production . An high metallurgical production quality allowed to

produce these steels with very high level of homogeneity and cleanliness , low contents of gas and

undesirable chemical elements. All this enhance the ductility.

The easier way to understand if a steel is ductile is the impact test at room temperature.

There are many methods to determine the impact energy. The difference between these methods are

concerning the shape , unnotched or notched, and the dimensions of the sample.

One of the most popular method is the Charpy V test .This is also the method we choose for this

work.

In order to value the quality of the hot work tool steels in delivery condition the producers are use to

perform an impact test.

The specimens are taken off from the bar in annealed condition, rough machined , singularly

quenched and tempered to a per-defined hardness range and finish machined for testing.

Also for this operation there are several methods. Differences between these methods are

concerning the position of the samples in connection with cross section of the bar and its hot

forming direction, kind of impact test method and hardness of the specimen.

One of the most popular method is following:

N.3 samples 10x10x55mm according to Charpy V (EN 10045) taken off from the core of the bar.

The length of the sample regarding the hot forming direction has to be transverse, the surface to

notch has to lay parallel to the hot forming direction. This is also the method we choose for this

work.

The rough machined specimens are to quench and temper to a conventional hardness range

of 44-46 HRC . For this heat treatment there are only few indication.

Austenitisation may be done in electrical furnace, with or without controlled atmosphere , or in salt

bad . The holding time is normally 20 min. The quench may be done in oil , water and polymer, or

water.

The tempering may be done in electrical furnace with an holding time of min 2 hours.

After finishing the specimens can be broken through a Charpy hammer.

If, the steel quality is bad because of the presence of weak points for example like coarse grain

size, strong banding , or primary carbides, ,the impact test value will be low also under the best

conditions of heat treatment.

But if the steel quality is good the high impact test value are depending only from the heat

treatment conditions ,which are very favourable for the toughness by this high hardness range.

The shape and size of the rough specimens allowed to attain easily the right solution grade of the

carbides in the austenite. Due to the contact with the quenching liquid , the quenching speed is

extremely high ,allowing to block precipitation of pre eutectoid carbides and to obtain

homogeneous supersaturated martensite ,very tempering resistant . Thanks the small sizes of the

specimens the risk of distortion or cracks is extremely little. All this is extremely difficult to attain

by the heat treatment of a die casting mould. The increased sizes , the irregular shape often with

geometrical criticises and the high quantity of retained heat let be very difficult to reach the right

carbide solution grade of the carbides in the austenite. Further, the risk of distortion or quenching

cracks is enormously increased. A practical problem is how to value the toughness by impact test of

a heat treated mould without breaking it.

Also for this there are several methods. The difference between these methods concern the shape

and placement of parent blocks in, or, on the mould before the heat treatment .From the parent

blocks have to be worked out the impact test specimen.

The parent block prescribed by all these methods let be classified into four classes:

Block worked out directly from the mould ( edge or sleeve fitting)

Positive parent block raising from the mould (part of it)

Parent block joint by welding on one surface of the mould

Parent block inserted into the mould ( through a cooling hole)

Block and parent blocks of all this category went applied on a mould medium size

with weight 600 Kg. After the heat treatment the mould has been broken in order to measure impact

test from the surface and from the section. The impact test results has been confronted each other

and with the impact value of the specimens singularly heat treated.

EXPERIMENTAL PART

Steel

The hot work tool steel we choose for the mould is a high performance die casting mould steel

named HP1.

This steel is produced by the primary German tool steel makers Kind&Co Edelstahlwerk.

The chemical analysis is according to Wst.Nr 1,2340 + Nb

Analysis indicative (Weight %)

C

Si

Mn

Cr

Mo

V

Nb

very low content of:

P,S, gas and other undesirable

elements

0,35

0.20

0,30

5,20

1,40

0,55

+

Tab 1

Bar size: 600 x 200 mm

Production method: ESR ,Forged

From Kind&Co declared impact test : Charpy V 30 J . ( Average of three)

Method: originally position core ,tranverse , single heat treated specimens

The purpose of this grades is to join the high toughness at working hardness , typical of the grade

Wst.Nr 1.2340 , with the high mechanical properties at working temperature ,typical of the high Mo

alloyed grade such Wst Nr 1.2367.

Above some technical details about this grade of steel provided from Kind&Co Edelstahlwerk.

KIND & CO

EDELSTAHLWERKAußendiensttagung 2007

Offshore - Seite 10Werkstoff HP1 - Seite 21

ZTU - Schaubild

TTT-diagram (continuous)

Proof and yield strength at elevate temperatures

°C

Tempering curve

The purpose of this grades is to join the high

toughness at working hardness , typical of

the grade Wst.Nr 1.2340 with the high

mechanical properties at working

temperature ,typical of the high Mo alloyed

grade such Wst Nr 1.2367.

Tab 2

Tooling

The company La Pantastampi S.r.l. –Cassano Magnago (ISO 9001:2000 DNV N.CERT 15100-

2004-AQ-MIL-SINCERT ) , mould maker specialised in die casting moulds ,drafted and built a

mould provided with the most frequently geometrical weak points by a die casting mould such

edges , different cross sections and rough cooling holes. The surface finishing was milled in the

shape they normally do for the heat treatment. Size of the mould: mm 590 x 190 x 840.

Warmfestigkeitsschaubild 30 Ø

0

200

400

600

800

1000

1200

1400

1600

1800

2000

2200

2400

RT 300 400 500 600 700

Prüftemperatur °C

0,2

-Gre

nze

un

d Z

ug

fes

tig

ke

it

N/m

m²

0

20

40

60

80

100

120

140

160

180

200

220

240

Bru

ch

ein

sc

hn

üru

ng

%

Anlassschaubild 60ø, 1020°C Öl

32

34

36

38

40

42

44

46

48

50

52

54

56

400 500 600 700

Anlasstemperatur °C

Hä

rte

HR

c

Fig 1

The mould has been provided with 7 parent blocks. 2 sets of three specimens (PB 8 ,PB 9 ) were

taken off directly from the mould after the heat treatment :

.

Parent Block N Size in mm Kind of joint

1 75x12x90 Welded on the surface

2 65x20/15x52 Insert in the mould

3 15x15x200 Welded on the surface

4 12x12x200 Insert in the mould

5 12x12x120 Insert in the mould

6 12x12x80 Insert in the mould

7 60x60x70 Part of the mould (raising)

8 surface Taken off from the head

9 surface Taken of from the section

between P 5 and P6

0

65x20x52

Singularly hardened Tab 3

The parent blocks have been taken off from the front of the bar. The part of the bar section chosen

for taking off the parent blocks is corresponding with the part of the mould they’ve been applied.

Fig 2

The parent blocks have been positioned on the mould as shown below:

Fig 3

Fig 4

Heat treatment

The heat treatment was performed by TAG S.r.l. –Dolzago (Lc) (ISO EN 9100 – Cert.N. DNV-

15266-2007-AQ-IRA) .

For austenizing and quenching has been utilized an high performance quenching vacuum furnace:

Furnace: High vacuum furnace Manufacturer: STS SysthermsGmbH Installation:01.2002

MaxTemperature:1300°C Qualified temperature range: 850 – 1150 °C

Thermal homogeneity < 1000°C class 2 (+/-6°C)

>1000°C class 3 (+/-8°C)

Useful dimension : 850 x 800 x 1150 mm Loading Capacity :Max 1500 Kg

Heating capacity: 210 KW Heating system: electric resistance Heat zones 3

Thermocouples type:N N.thermocouple : process type 3 , loade type 2 ,Safety type 3

Process gas: Nitrogen/argon Max pressure : 10 Bar Circulation : Forced 0÷3000 rpm

Quenching system: radial Tab 4

For the tempering have been utilised vacuum tempering furnaces STS Systherms GmbH , with load

thermocouple, process gas nitrogen , and adjustable forced gas circulation 0÷2000 rpm.

The mould have been treated according the standard procedures of TAG for the heat treatment in

vacuum furnaces of die casting moulds.

The austenizing temperature was 1020°C and the holding time 60 min.

The quench happened through nitrogen at 6 Bar

The load thermocouple which leaded the holding times was positioned 150mm deep inside the hole

of PB 5

Fig 5

The quenching speed of Tc core between 1020°C and 500 °C has been 22.6 °C/min.

The geometrical distortions of the mould were contained within 2mm absolute.

Fig 5

The mould has been tempered three times and the hardness measured on many points of the surface

was 45,5 – 46 HRC

RISULTS AND DISCUSSION

The impact test has been carried out by SMT Srl – Pozzo d’Adda (SINAL CERTIFICATE N 0718)

a very high specialised laboratory for mechanical testing.

Below the test results:

Parent Block N

SMT test report

SMT test N.

Hardness HRC

Values J

Average J

I II III I II III

0 RP-26528

2414

44

45

45

29

29

30

29,3

1 RP-27056

3078

45

45

46

22

22

21

21,6

2 RP-27056

3079

45

45

46

14

14

16

14,6

3 RP-27056

3080

45

46

45

22

18

24

21,3

4

Rejected – handling damages occurred during transport to TAG

5

RP-27056

3082

45

45

19

17

-

18

6

RP-27056

3083

46

16

7

RP-27056

3084

46

45

46

13

15

16

14.6

8

RP-27056

46

45

45

15

13

15

14.3

9

RP-27056

45

46

45

16

17

18

17

Tab 5

Microscopic evaluation were carried out from the broken specimens by Kind&Co Edelstahlwerk

P 1 P 2

M=500 x

M=500 x

P 3 P 9

M=500 x

M=500 x

P5 P6

M=500 x

M=500 x

P7 P8

M=500x

M=500 x

All the specimens show fine-grained high tempered martensitic structure

Tab 6

All specimens show the same kind martensic structure ,from the surface up to 100 mm depht.

The differences of the impact values are probably due to the presence of extremely fine carbide

precipitation at the previous austenite grain boundaries (1) detectable by SEM analysis, which

presence increase as much the quenching rate decrease.

●Impact test values of singularly heat treated specimens P0 (29J). Has been the highest average

value measured and is not directly significant for the toughness of the mould

●Parent block joint by welding on one surface of the mould :

The impact values of P1 ( 21.6) and P3 (21.3) are in the middle between singularly heat treated

specimens and specimens taken off from the mould.

The reason why is probably due to fact ,that the mass of the parent blocks are separated from the

bigger mass of the mould and have followed their own quenching dynamic with higher cooling rate.

The cooling rate has been lower than by P0 because N2 quenching is not so severe like oil

quenching.

●Block cut out directly from the mould ( edge or sleeve fitting)

P8,(14,3 J) and P9 (17 J) shall indicate the real impact toughness of the working surface of the

mould. As working surface we intend the surface and the first 50mm section under the surface.

●Positive parent block raising from the mould (part of it)

P7 ( 14,6 J) show the same impact toughness level like P8 and P2 .Although 5 faces of 6 were

separated from the mould, it seems that the parent block ,with its mass 60x60 mm, has kept the

same quenching dynamic of the mould .

●Parent block inserted into the mould ( through a cooling hole)

P2 (14.6) , P5(18J) and P6 (16J) show that the parent blocks inserted in the mould have kept the

same quenching dynamic of the mould despite the smaller mass and the loose contact.

Impact test value obtained from parent blocks of these three last category are directly comparable

Two or three parent blocks singularly inserted in holes for a deep 120 mm represent a significant

method for measuring the impact toughness of the mould and maybe the most economic.

REFERENCES

[1] P.Sommer , I.Schruff ,P.Gunderjahn : Simulating calculations of vacuum heat treatment of tool

steels 4th

international tooling conference 1996 Bochum.