Problem Description
This case study focuses on the analysis of a common and representative quality issue within the high-pressure die casting mold industry: mold cracking defects. Such issues are relatively prevalent in actual production processes and can impact both the service life of the mold and the quality of the cast parts to varying degrees.
The products discussed below are sample parts previously manufactured for the client by other suppliers. These samples were provided by the client for technical analysis purposes; furthermore, the client has granted authorization to utilize the analysis process and results for this case study and for external presentation.
Premature heat checking (surface cracking) phenomena were observed in two product series: the Chain Cover Series (GZF**3,GZFS**7,GZ***8) and the Oil Pan Series (GZ***7,DK***9). From the perspectives of both manufacturing and operational usage, the premature cracking observed in the high-pressure die casting molds for these series shares common underlying characteristics. The following analysis begins by focusing specifically on the GZFS**7 high-pressure die casting mold.
Premature Cracking Status of Two GZFS**7 Molds (both molds are shown at a stage representing approximately 20,000 casting cycles): Mold “E” exhibits widespread heat checking and casting marks across its entire surface. In contrast, Mold “G” shows heat checking and casting marks only in the fillet (rounded corner) areas; the flat surfaces exhibit minimal cracking or marking, indicating a significant overall improvement in surface quality.
Our analysis focuses primarily on the following eight key aspects:
Material chemical composition; material microstructure in the annealed state; material purity; hardness after heat treatment; microstructure after heat treatment; stress distribution in fillet areas; the presence of “white layer” residues from Electrical Discharge Machining (EDM); and the temperature differential (△T) during the high-pressure die casting process.
Material Chemical Composition:
8418 steel with a compliant chemical composition possesses superior high-temperature strength, high resistance to tempering softening, high toughness, high plasticity, and high thermal conductivity.
2.1. Material Microstructure in the Annealed State:
If the material’s microstructure in the annealed state is non-compliant, it will be impossible to achieve a satisfactory martensitic structure following subsequent heat treatment.
Reference Standard: The North American NADCA_207-2003 standard for annealed microstructure. The metallographic structure of the GZFS**7E high-pressure die-casting mold steel in its annealed state is highly satisfactory; both the low-magnification (100X) view of segregation and the high-magnification (500X) view of the spheroidized structure are ideal, essentially meeting the requirements of the North American NADCA_207-2003 standard. Similarly, the metallographic structure of the GZFS**7G mold steel in its annealed state is also highly satisfactory; both the low-magnification (50X) view of segregation and the high-magnification (500X) view of the spheroidized structure are ideal, essentially meeting the requirements of the North American NADCA_207-2003 standard.
In contrast, the metallographic structure of a non-compliant annealed material (specifically, a domestically produced H13 steel) is of poor quality; both the low-magnification (100X) view of segregation and the high-magnification (500X) view of the spheroidized structure are substandard. Consequently, this material essentially fails to meet the North American NADCA_207-2003 standard, and the occurrence of heat checking or catastrophic cracking in high-pressure die-casting molds may be attributed to this underlying cause.
2.2. Material Purity
① The Impact of Impurities on High-Pressure Die-Casting Molds:
◆ Impurities serve as the initiation points for the formation of heat checks (thermal cracks);
◆ Impurities lack inherent strength and are unable to withstand the stresses associated with thermal fatigue cracking;
◆ Impurities compromise the ductility of the material.
② Purity Standards for High-Pressure Die-Casting Mold Steels: Based on the applicable standards, both Series 1 and Series 2 materials are deemed compliant.
The purity inspection images for GZFS**7E and GZFS**7G reveal essentially no impurities exceeding the permissible limits, thereby confirming that the purity of these materials meets the required standards.
2.3. Heat Treatment Hardness
Regarding the relationship between material hardness and strength—based on an analysis of relevant data—the two properties are essentially directly proportional: the higher the hardness, the greater the strength, and the stronger the resistance to cracking. However, excessively high hardness levels increase the susceptibility to “high-temperature tempering embrittlement” (a tendency toward catastrophic cracking). Based on these two principles and practical field experience, the material exhibits its maximum resistance to heat checking when its hardness falls within the range of HRC 46–48. The heat-treated hardness of the GZFS**7E high-pressure die-casting mold was originally measured using an older-model hardness tester.
Furthermore, the timing of the measurement was inappropriate (it should have been performed prior to final machining); consequently, the actual hardness may be approximately 1–2 HRC points lower than the measured values. This suggests that the actual values likely fall within the range of HRC 41–45 (moving die) and HRC 42–46 (fixed die)—values that are significantly too low and highly non-uniform. The severe cracking observed in the early stages of the high-pressure die-casting mold’s service life is directly attributable to this low and non-uniform hardness.
Similarly, the heat-treated hardness of the GZFS**7G high-pressure die-casting mold was measured using an older-model hardness tester. The actual hardness may be approximately 1 HRC point lower than the measured value, suggesting an actual hardness of around HRC 45–46—a result falling toward the lower end of the acceptable tolerance range. The cracking observed in the early stages of this mold’s service life is directly linked to this lower-than-optimal hardness.
2.4. Heat Treatment Microstructure
The data presented here represents the impact test results for two material samples of identical composition and hardness, but subjected to different cooling methods during heat treatment. The material subjected to rapid quenching and cooling achieved an excellent martensitic microstructure, yielding an impact strength of 290 joules. In contrast, the material subjected to slower quenching and cooling developed a microstructure of inferior quality, resulting in an impact strength of only 60 joules. Therefore, a superior heat-treated microstructure correlates directly with higher impact strength.
The reference images depict the microstructure standards for the quenched and tempered state as defined by the North American NADCA_207-2003 specification; the images on the left represent acceptable microstructure grades, while those on the right represent unacceptable grades.
Regarding the quenched and tempered microstructure of the GZFS**7E mold: neither the low-magnification (100X) view—examining segregation—nor the high-magnification (500X) view—examining spheroidization—revealed the presence of any defective microstructures. Consequently, the microstructure substantially complies with the North American NADCA standards. Regarding the quenched and tempered microstructure of the GZFS**7G high-pressure die-casting mold: both the low-magnification (50X) view—examining segregation—and the high-magnification (500X) view—examining spheroidization—present a highly ideal microstructure. As such, it substantially complies with the North American NADCA standards.
The metallographic structure of the domestically produced H13 material in its quenched and tempered state was found to be substandard; both the low-magnification (100X) view—revealing segregation—and the high-magnification (500X) view of the microstructure were of poor quality. Fundamentally, this material fails to meet the North American NADCA standards. The occurrence of early-stage heat checking or catastrophic cracking in the high-pressure die-casting molds may be attributed to this specific issue.
Analysis Summary: Based on the metallographic analysis of the quenched and tempered materials, the two GZFS**7 molds exhibit a microstructure that complies with North American NADCA standards, whereas the domestically produced H13 material does not. Consequently, the early-stage heat checking observed in the two GZFS**7 molds will not, for the time being, be attributed primarily to issues with the quenched and tempered metallographic structure.
2.5. Stress at Fillet Radii
During the injection of high-temperature molten aluminum, the surfaces of the mold’s fillet radii are subjected to compressive stress; conversely, during the ejection and mold-release spraying stages, these surfaces are subjected to tensile stress. If the polishing lines (surface finish texture) are oriented perpendicular to the direction of these stresses, the alternating cycles of compressive and tensile forces can easily induce early-stage heat checking at the mold’s fillet radii.
2.6. EDM Residual “White Layer”
Following Electrical Discharge Machining (EDM), the material’s surface develops a layered structure progressing from the outermost layer inward: a melted and re-solidified layer, followed by a re-quenched layer, a re-tempered layer, and finally the base metal (substrate). The melted and re-solidified layer is highly resistant to chemical etching; consequently, when viewed under a metallographic microscope, it appears white—hence it is commonly referred to as the “white layer.” The presence of the white layer and the adjacent re-quenched layer results in an increase in surface hardness, while the re-tempered layer immediately beneath them causes a reduction in hardness. This specific structural configuration—resembling an eggshell atop the egg white—renders the surface particularly susceptible to fracture under external mechanical loads, thereby serving as a primary source for the initiation of early-stage microcracks.
Analysis Summary: Based on the evidence regarding the residual white layer, it is highly probable that this phenomenon exists within the molds in question. Therefore, this factor is identified as a primary cause contributing to the early-stage heat checking observed in the high-pressure die-casting molds.
2.7.Mold Temperature Differential (△T):
Regarding the relationship between mold temperature and thermal stress: when the mold temperature is elevated (specifically during the molten aluminum injection phase), the magnitude of compressive stress is significant; conversely, when the mold temperature is lower (following the application of the mold-release agent), the magnitude of tensile stress becomes significant. A disparity between high and low mold temperatures creates a temperature differential (△T) across the mold surface; this △T is a critical factor contributing to early-stage mold cracking.
Regarding the relationship between mold temperature and fracture toughness: lower mold temperatures correspond to lower toughness; therefore, trial runs should be avoided while the mold is in a cooled state.
The operating temperature of the fixed mold typically falls between 200°C and 250°C, with a temperature differential (△T) of approximately 50°C; both the operating temperature and the temperature differential for the fixed mold are considered relatively reasonable.
In contrast, the operating temperature of the moving mold varies widely—ranging from 120°C to 340°C—indicating a significant imbalance between high and low extremes. Furthermore, the temperature differential (△T) lacks a consistent, reasonable value; consequently, both the operating temperature and the temperature differential of the moving mold are deemed unreasonable.
Analytical Summary: For the GZFS**7G mold, the operating temperature and temperature differential (△T) of the fixed mold are relatively reasonable compared to those of the moving mold. As a result, the severity of cracking observed on the fixed mold is less pronounced than that on the moving mold. This factor has been identified as the primary cause of the mold’s early-stage cracking.
Summary of Early-Stage Mold Cracking
Based on the analysis of the GZFS**7 mold, the following preliminary conclusions can be drawn:
① The material composition, metallographic structure, and purity of the high-pressure die-casting mold meet the required standards; therefore, these factors are unrelated to the early-stage cracking observed. ② The early-stage cracking is directly linked to the following factors: insufficient hardness resulting from heat treatment, low operating temperatures during molding, and a significant temperature differential (△T) occurring before and after the mold-spraying process.
③ While the effects of fillet stress and the EDM “white layer” on the high-pressure die-casting mold cannot currently be quantitatively measured, theoretical analysis suggests that these factors may potentially contribute to early-stage cracking in aluminum die-casting molds.
Subsequently, our company acquired imported hardness testers (replacing domestic models, which suffered from data instability and a tendency to yield readings higher than actual values) to conduct comprehensive hardness inspections on all heat-treated components. This measure ensures that component hardness meets the specified requirements—thereby preventing early-stage cracking caused by insufficient hardness (and consequently low strength), as well as preventing catastrophic failure (bursting) caused by excessive hardness (and consequently high brittleness).
Currently, the overall surface quality of the GZ***G mold has shown significant improvement, demonstrating the effectiveness of these corrective measures. Based on the data collected to date, molds GZF**3, GZ***8, GZ***7, and DK***9 exhibit the following issues: low hardness (below HRC45) and severe early-stage cracking in the high-pressure aluminum die-casting molds; conversely, molds with appropriate hardness (HRC46–48) do not experience severe early-stage cracking.
Summary
Based on the aforementioned analysis and practical experience, Raidymold is able to effectively identify and mitigate potential risks throughout the entire process of die-casting mold development and manufacturing. By implementing proactive controls and continuous optimization regarding critical quality factors, we ensure that our products meet anticipated requirements in terms of performance, service life, and stability, thereby achieving consistent and reliable high-quality delivery.
If you would like to learn more about cracking defects, please click on “Analysis of Causes and Improvement Strategies for Cracking in HPDC Molds.”
