THOUGHNESS OF DIE CASTING MOULDS. COMPARISON OF SEVERAL TESTING METHODS

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1 ABSTRACT 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 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 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.

2 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 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.

3 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, Nb Analysis indicative (Weight %) C Si Mn Cr Mo V Nb 0, ,30 5,20 1,40 0,55 + very low content of: P,S, gas and other undesirable elements 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 , with the high mechanical properties at working temperature,typical of the high Mo alloyed grade such Wst Nr Above some technical details about this grade of steel provided from Kind&Co Edelstahlwerk.

4 0,2-Grenze und Zugfestigkeit N/mm² Brucheinschnürung % Härte HRc Anlassschaubild 60ø, 1020 C Öl C Anlasstemperatur C TTT-diagram (continuous) Werkstoff Offshore HP1 - Seite - Seite 10 Tempering curve Warmfestigkeitsschaubild 30 Ø The purpose of this grades is to join the high toughness at working hardness, typical of the grade Wst.Nr with the high mechanical properties at working temperature,typical of the high Mo alloyed grade such Wst Nr RT Prüftemperatur C Proof and yield strength at elevate temperatures Tab 2 Tooling The company La Pantastampi S.r.l. Cassano Magnago (ISO 9001:2000 DNV N.CERT 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.

5 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.

6 Fig 2 The parent blocks have been positioned on the mould as shown below: Fig 3

7 Fig 4 Heat treatment The heat treatment was performed by TAG S.r.l. Dolzago (Lc) (ISO EN 9100 Cert.N. DNV AQ-IRA). For austenizing and quenching has been utilized an high performance quenching vacuum furnace: Furnace: High vacuum furnace Manufacturer: STS SysthermsGmbH MaxTemperature:1300 C Qualified temperature range: C Thermal homogeneity < 1000 C class 2 (+/-6 C) >1000 C class 3 (+/-8 C) Useful dimension : 850 x 800 x 1150 mm Installation: 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 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 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

8 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.

9 Fig 5 The mould has been tempered three times and the hardness measured on many points of the surface was 45,5 46 HRC

10 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 ,3 1 RP , RP , RP ,3 4 Rejected handling damages occurred during transport to TAG 5 RP RP RP RP RP 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

11 P 3 P 9 M=500 x M=500 x P5 P6 M=500 x M=500 x P7 P8 M=500x All the specimens show fine-grained high tempered martensitic structure M=500 x Tab 6

12 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 4 th international tooling conference 1996 Bochum.