| 1. |
- Yuan, Miwen, 1990, et al.
(författare)
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A Case Study for a Worn Tool Steel in the Hot Stamping Process
- 2023
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Ingår i: Journal of Materials Research and Technology. - : Elsevier BV. - 2238-7854 .- 2214-0697. ; 22, s. 1065-1075
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Tidskriftsartikel (refereegranskat)abstract
- A good understanding of failure mechanisms can help us improve the lifetime of the dies. This paper presents a case study investigating the wear behavior of a QRO90 die insert utilized for stamping uncoated boron-alloyed high-strength steel sheets. Topography and microstructure were characterized by means of scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), hardness measurement and X-ray photoelectron spectroscopy (XPS). Severe galling due to accumulated layers transferred from the boron-alloyed steel workpieces occurred on the die surface. Material softening was detected in the sublayer of the tool steel (up to ∼200 μm). In addition, white layers with a in a thickness of 1-2 μm were frequently observed on the surface of the round corner of the tool. The main wear mechanisms are discussed. Galling caused by surface softening and the spallation of white layers are considered to be the primary wear mechanisms for the tool.
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| 2. |
- Yuan, Miwen, 1990, et al.
(författare)
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Comparison of Softening Behavior and Abrasive Wear Resistance between Conventionally and Additively Manufactured Tool Steels
- 2024
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Ingår i: Steel Research International. - 1869-344X .- 1611-3683. ; 95:6
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Tidskriftsartikel (refereegranskat)abstract
- Directed energy deposition (DED) is a powerful method for hard-facing the existing tool components. Herein, three tool steel grades including a newly developed maraging steel (NMS), a cold work tool steel (V4E), and a high boron steel (HBS) are cladded on a hot work tool steel substrate. After tempering, the cladded tool steels are exposed to high temperatures at 600, 700, and 800 °C for 3 h to evaluate their softening resistance. The results show that all three tool steel grades have a significant hardness drop when the softening temperature reaches 700 °C. The investigated NMS has the lowest initial hardness in the tempered state but the best softening resistance. In contrast, HBS has the highest initial hardness but the worst softening resistance among the three steel grades. V4E tool steel has a moderate softening resistance compared to the other two steel grades. The factors contributing to the softening resistance have been discussed. Corresponding conventional tool steels are also adapted as references for comparison. Except for the NMS, the additive manufacturing tool steel samples have a better softening resistance than the conventional counterparts owing to more alloying elements trapped in the matrix. The abrasive wear resistance of the tempered and softened tool steels manufactured by DED and conventional methods is also evaluated and the wear mechanism is discussed. Besides the hardness, the free spacing λ between coarse hard particles is one of the major factors influencing the abrasive wear resistance.
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| 3. |
- Yuan, Miwen, 1990, et al.
(författare)
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Softening Behavior of a Cold Work Tool Steel and High-Boron Tool Steel Fabricated by Directed Energy Deposition
- 2024
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Ingår i: Steel Research International. - 1869-344X .- 1611-3683. ; 95:3
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Tidskriftsartikel (refereegranskat)abstract
- Prolonging the life of hot stamping dies in the automotive industry is challenging. Surface modification is an effective way to improve the durability of the dies. In this study, two tool steel grades, one cold work tool steel (V4E) and one high-boron tool steel (HBS), are deposited on a tool steel substrate using directed energy deposition, followed by tempering. Softening behavior at high temperatures of 550 and 600 °C is investigated. In the key findings, it is revealed that both steels exhibit remarkable hardness, surpassing the substrate even after extended exposure to high temperatures. HBS shows excellent softening resistance in terms of hardness at 550 °C but experiences a significant drop at 600 °C. V4E demonstrates an overall superior softening resistance due to its thermal stable MC (M represents metal) carbides and the relatively stable dislocation density. Microstructural analysis highlights some unique features, such as borides in HBS and dendritic structures in V4E. In this study, the correlation between microstructure characteristics and hardness evolution is revealed, providing some insights into how these materials resist softening to enhance the longevity and performance of hot stamping dies.
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| 4. |
- Yuan, Miwen, 1990, et al.
(författare)
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Study of defects in directed energy deposited Vanadis 4 Extra tool steel
- 2022
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Ingår i: Journal of Manufacturing Processes. - : Elsevier BV. - 1526-6125. ; 76, s. 419-427
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Tidskriftsartikel (refereegranskat)abstract
- A cold work tool steel with varied number of layers was deposited on a substrate of hot work tool steel for hard-facing by directed energy deposition technique. This study deals with the defects and microstructure in the as-cladded tool steels. Defects, including pores and cracks, were found in the deposited zone, the number of which increased with the building height or number of layers deposited. Large irregular pores were mainly located at the lower regions of the deposited layers. The formation of this type of pores was attributed to the segregation of alloy elements on the pore surface and insufficient heat input. Non-equilibrium eutectic microstructure was a characteristic feature in the regions adjacent to the pores. Cracking, on the other hand, tended to occur in the upper part of the deposited layers. Two important contributing factors were identified for cracking. The first one was the microstructural gradient, which was modified from fine cellular dendrite to columnar dendrite when moving from the bottom to the top deposited layer. Second, the deposited cold work tool steel exhibited a relatively large temperature range of solidification, as simulated by Thermocalc software, leading to high sensitivity to hot cracking.
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| 5. |
- Yuan, Miwen, 1990, et al.
(författare)
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Time and temperature dependent softening of a novel maraging steel fabricated by laser metal deposition
- 2022
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Ingår i: Materials and Design. - : Elsevier BV. - 1873-4197 .- 0264-1275. ; 224
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Tidskriftsartikel (refereegranskat)abstract
- Durability is a critical factor for hot stamping dies from an economic point of view. Refurbishing the dies by depositing new material instead of replacement is a promising method to reduce the cost. For this reason, a newly developed maraging steel (NMS) was cladded on a hot work tool steel by means of directed energy deposition. After an optimized tempering, exposures at high temperatures were carried out on the cladded NMS in order to examine the softening resistance. The microstructural evolution of the material was systematically characterized using a combination of optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), Auger electron spectroscopy (AES) and transmission electron microscopy (TEM). The precipitate in the cladded steel was identified as Laves phase. The coarsening of this phase is considered as the main reason for the thermal softening of the steel at high temperatures. The coarsening behavior was also simulated by using the revised Langer-Schwartz-Wagner (LSW) model, which was in good agreement with experimental observations. Moreover, a mathematical model of precipitate strengthening was successfully applied to evaluate the softening behavior of the steel. This model can be used to predict the hardness/strength evolution of the investigated tool steel during its high-temperature service.
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