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Thermal Fatigue Resistance: The Technical Analysis of Thermodur® E40K Superclean
In the die casting sector, die failure is an operational cost and a critical process interruption. The main cause is well-known: 80% of failures are due to thermal fatigue. To meet this challenge, the hot work tool steel Thermodur E40K Superclean was developed. Bonomi Acciai, through our technicians E. Prati and A. Magistrelli, managed the publication of the technical analysis of this material in the prestigious journal “La Metallurgia Italiana,” the official publication of the Italian Association for Metallurgy (AIM). This in-depth article analyzes the data from that paper, demonstrating why this steel represents a fundamental evolution for die lifespan.
The technical article curated by our engineers, published in the May 2020 issue of “La Metallurgia Italiana”, the official journal of the Italian Association for Metallurgy (AIM).
The Problem: Failure due to Thermal Shock
Die casting dies undergo extreme cyclic stress: the cavity surface is rapidly heated by the injection of the molten alloy and then cooled. This continuous expansion and contraction generate a network of micro-cracks, known as heat checking or thermal fatigue, which inevitably grow until they cause failure. While other factors like stress cracking (9%) or corrosion (7%) have an impact, thermal fatigue resistance remains the critical parameter for a die’s service life.
The Solution: Thermodur E40K Superclean
Thermodur E40K Superclean was developed by Deutsche Edelstahlwerke (part of Swiss Steel Group) specifically to maximize thermal fatigue resistance, combined with high toughness and wear resistance at high temperatures. It is an Electro-Slag Remelted (ESR) steel, featuring a homogeneous microstructure and a low Silicon content to improve toughness. It also complies with the rigorous standards of the NADCA #207 specification.
To validate its performance, the material was compared with standard industry steels: H11 and H13.
Data Analysis: Proof of Technical Superiority
The laboratory tests, presented in the article, focused not only on hardness but on the properties that truly determine a die’s life.
1. The Critical Test: The Thermal Fatigue Test
To simulate operating conditions, samples of the different steels (quenched and tempered to 45±1 HRC) were subjected to thousands of thermal cycles: induction heated to 650°C and immediately quenched in 60°C water. The results are unequivocal:
- Visually: After just 4,000 cycles, micrographs show that Thermodur E40K Superclean is much less susceptible to crack formation compared to H11 and H13.
- Quantitatively: Analyzing the maximum crack depth and total crack length, Thermodur E40K Superclean shows the lowest values at every step (2,000, 4,000, 6,000, and 8,000 cycles). Its advantage becomes even more pronounced after 6,000 cycles, demonstrating superior resilience over time.
2. The "Why" Behind the Success: The Instrumented Impact Test
The Charpy-V test alone is not enough to explain this behavior. Therefore, the instrumented impact test was used, which allows for distinguishing the energy used to initiate the crack from the energy needed to propagate it. This is where the real difference emerged:
- Thermodur E40K Superclean showed the highest absorbed impact energy (152 J).
- It was followed by H11 (136 J) and, much further down, H13 (92 J). This does not mean H13 is brittle, but that Thermodur E40K and H11 possess a significantly higher resistance to crack propagation. Once the micro-crack has formed (an inevitable event), E40K is the steel that best resists its advancement.
3. High-Temperature Performance: Toughness
Further confirmation comes from toughness tests (Charpy-V) at high temperatures. While many steels lose toughness, Thermodur E40K Superclean shows exceptional behavior: its toughness increases with rising temperature, reaching a peak of approximately 82 J at 500°C. It therefore maintains its optimal performance precisely at the operating temperatures of die casting.
The analysis published in “La Metallurgia Italiana” is clear: the superior thermal fatigue resistance of Thermodur E40K Superclean is not a simple claim, but a proven technical fact. It is not just about hardness.
Its superiority stems from an optimized combination of high-temperature stability and toughness, achieved through careful chemical design (low Si) and metallurgical processing (ESR process). This superior toughness is what slows crack propagation, leading to a measurable increase in die service life and a reduction in machine downtime.
As Bonomi Acciai, the Italian partner of Swiss Steel Group, we are proud to have managed the dissemination of this fundamental research for the Italian market, confirming our role as a technical partner for foundries and die makers seeking excellence.
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