Metallurgical quality of steels for aluminum high-pressure casting dies

The cost of the steel used in the high-pressure casting dies usually represents an important part of the total cost of the tooling. Depending on the application and the corresponding operational requirements the use of low metallurgical quality steels can lead to serious problems in the manufacturing process of the die and/or premature failures in service. Therefore, one of the most important decisions relies in the specifications and in the selection of the most suitable steel for the expected life service conditions. The increasing requirements on aluminum castings require a continuous development of the steel alloys used in the dies. The innovations are aimed at obtaining higher strength, greater ductility, improved machinability, fitness for welding and resistance to corrosion. Also, there is a tendency to design larger aluminium components with thinner walls, more complex geometries, and increasingly tight tolerances. All these factors favour the use of high pressure die casting as the most suitable manufacturing technology. Steel die materials must meet very demanding quality requirements, as they will be exposed to significant requests during service. The main failure modes in these tools during the high pressure die casting process are erosion, adhesion, thermal fatigue, and failure breaks. Because of that, hot process tool steel grades are usually used for this kind of applications. The continuous thermal cycles experienced in the surface of the dies result in their progressive deterioration, known under the phenomenon referred as “thermal fatigue”. This phenomenon represents approximately 80% of the main failure mechanisms (fig. 1). Thermal fatigue is a microscopic phenomenon that takes place only in a thin layer of the surface of the dies and it implies a gradual increase of cracks due to thermal stresses created by numerous temperature cycles. Thermal fatigue is the consequence of a combination of cyclic thermal stresses, traction stresses and plastic deformation. Factors such as die casting temperature cycles, base material properties and tool design are decisive.    

Figure 1. Incidence of the most common failure mechanisms in the high-pressure casting dies (Source:  UDDEHOLM)

  In the last 20 years a number of material specifications have been developed to ensure a good quality in the base steel for high pressure casting dies. Currently one of the most advanced specifications is the “Special Quality Die Steel & Heat Treatment Acceptance Criteria for Die Casting Dies” No207-2018 published by the North American Die Casting Association (NADCA). The NADCA specification establishes the necessary controls to ensure that a material is suitable for the manufacture of a die. Likewise, it allows to guarantee that the heat treatment applied to the tool has been satisfactory. The most critical parameters regarding the heat treatment of special casting die steels refer to the austenitizing treatment and the quenching speed. This quenching speed must be fast and controlled to achieve optimal metallurgical properties while minimizing distortion and risk of breakage. The tests are carried out from a material batch; this batch consists on an individual ingot that is forged or laminated by a common procedure and annealed in a same furnace load. The tests are divided into two different groups, one in annealing state and the second one with the manufacturing die quench and tempered. Tests performed to the first group ensure that the raw material meets the quality requirements for manufacturing the die. The tests belonging to the second group allow to certify the heat treatment applied to the die. With this aim, it is essential to obtain two samples located in the center of the steel block (fig. 2). Samples for testing shall be 2 1/2″x 3 1/2″x 1/2″. The 2 1/2″ dimension must be parallel to the thickness dimension of the material block. The 3 1/2″ dimension must be parallel to the width of the block and dimension 1/2″ represents the longitudinal direction.    

Figure 2. Location of test specimens in the centre of the base material block. (Source: UDDEHOLM)

 

 Quality requirements of the annealed material

• Chemical composition: C-Mn-P-S-Si-Cr-Mo-V. Special quality die steel grades must meet the chemical requirements defined by the standard.
• Hardness in annealing state, Brinell method (ASTM E10)
• Micro-inclusion Content (ASTM E45, Method A)
• Ultrasounds (ASTM A681 S1.1). This test is performed on the steel block. All blocks must be free from internal defects such as oxides, porosity, cracking, strong segregation, etc. Ultrasonic examination of the original steel material must be carried out in accordance with the recommended practices by ASTM A388 and E114 and the acceptance criteria are those agreed between supplier and customer.
• Potential impact resistance capacity of annealed steel ASTM E23. A set of 5 impact specimens per material batch (fig. 3). The specimens are tempered in oil and are annealed to 44-46 HRC before machining to the final dimensions, at least 2 annealing cycles are applied. The notch is located longitudinally according the base material (fig.5, left).  Heat treatment conditions depend on the steel quality and are properly indicated in the NADCA specification. The specimens must be machined in compliance with the ASTM A370 norm (fig. 5, right). The highest and lowest value are dismissed and the average value obtained from the other 3 specimens is calculated.

Figure 3. Location of the impact and metallography specimens obtained from the base material block (left). A set of 5 impact test specimens is machined and a metallographic sample (right) is obtained (Source: NADCA#207)

   

Figure 4. Location of the notch in the impact test specimens regarding the lamination direction of the material L (left). Dimensions of impact test specimens (right). (Source: NADCA#207)

 

• Grain size. It is determined on a tempered and annealed specimen at at least 590oC (ASTM E112). (Micrograph 1).
• Annealing structure. It is determined in admission state. For this purpose, it is evaluated by means of optical microscope at 500x after being attacked with nital 5%. It is evaluated in accordance with the NADCA Annealing Microstructures Chart #207:2018. (Micrograph 2).
• Segregation Bands. The annealing structure must be free of banding. It is determined at 50x by means of optical microscope. It is evaluated in accordance with the NADCA Banding Chart #207:2018. (Micrograph 3).

  Figure 5. Tool steel micrographs (AZTERLAN): Micrograph 1. Grain size (200x). Micrograph 2. Annealing microstructure (500x). Micrograph 3. Banding (50x). Micrograph 4. Tempering and annealing microstructure (500x 3).

Heat treatment quality requirements

 

• Hardness. The recommended hardness level is within the range of 42-52 HRC. The hardness range must be specified with three indentation points (e.g., 42-44 HRC). The hardness test is performed according to the latest ASTM E18, ASTM E10, ASTM E384 or ASTM A956 review. A minimum of 5 hardness points (4 footprints in the corners and 1 in the center) are performed. It will be agreed with the customer.
• Furnace chart data. The heat treatment provider must have a copy of the heat  treatment applied: it must include preheating, austenitizing, tempering (from the tempering temperature up to 150ºC). The minimum cooling speed between the austenization temperature and the 540ºC is 28ºC/min.
• Tempering structure. A specimen representative of the hardening treatment should be examined. The sample should be taken preferably from an attached coupon of the die that has gone throughout the same hardening process. If this is not possible, a specimen would be obtained from the corner of the die. The sample is etched with Nital 5% and by means of optical microscopy the microstructure is assessed according to the Reference Chart of Thermal Treatment Microstructures. (Micrograph 4).
• Impact resistance. A coupon is attached to the die away from edges and corners (fig. 6). The coupon will accompany the die during preheating, austenitizing, tempering and first annealing. The coupon must be annealed to 44-46 HRC. If the specified final hardness is different from 44-46 HRC, the coupon will be removed from the die after the first annealing and will be independently annealed to 44-46 HRC. From this coupon, 5 Charpy V impact specimens (ASTM A370) will be obtained. The V notch must be parallel to the longitudinal direction of the base material. The largest and lowest values will be despised and the average value obtained from the other 3 specimens is calculated.

 

Figure 6.Specimen attached to the die by welding points. (source: UDDEHOLM)

  Given the strict requirements that casting die steels must meet, at the end of this article we would like to highlight the great importance of having an appropriate characterization of the annealed material, as well as its potential response to the tempering and annealing heat treatments before die is machined. In addition, the heat treatment applied to a machined die must be verified by means of an attached specimen to the tooling, as indicated in the NADCA specification. In order to ensure an adequate service life and a correct traceability of properties of the steel and the die, a proper assessment of the material used and of the heat treatment applied will be obtained if these guidelines are followed. Failures during the manufacturing process, as well as premature failures during the use of dies in the aluminum die casting process are very recurrent and widely studied incidents by the Service Failure Diagnostics area of AZTERLAN Metallurgy Research Centre. Usually more than one root cause such as tooling design, machining conditions, heat treatment applied, service conditions and of course, the suitability of the raw material, converge at the origin of such failures. AZTERLAN develops as well an important research activity in the metallurgical improvement of tooling used by the metal-mechanical processing industry, ranging from the development of new materials to the development of treatments and other processes, aimed at improving their resistance and their correct performance during its service life, as well as at ensuring the efficiency of the production process.