Microstructure and Properties of Additively Manufactured Tool Steels for Hot Stamping
Doctoral thesis, 2023
Hot work tool steels are commonly used to produce dies for hot stamping, where the steels are exposed to cyclic thermal and mechanical loads. There is a constant demand to improve the lifetime of the dies. Additive manufacturing (AM) provides new solutions for tool design. For example, laser beam powder bed fusion (L-PBF) can print die with complex cooling channels, which can improve cooling efficiency and extend mould life. Directed energy deposition (DED) can easily do a hard facing for the tool surface and refurbish a worn die. This thesis evaluated the microstructure and properties of hot stamping tool steel fabricated by both L-PBF and DED techniques. The softening resistance was also assessed at elevated temperatures.
Before addressing the properties of AM tool steels, a case study was performed on the worn surface of a hot stamping insert die. Galling was observed, which was a result of accumulated layers transferred from the steel workpieces to the die. Material softening of the die was detected in the sublayer of ~ 200 μm. It is the softening of the die material that promotes galling. Galling together with the spalling of the white layer are supposed to be the primary wear mechanisms for the tool.
A modified H13 (M-H13) hot work tool steel was fabricated by L-PBF. The effect of two types of post-processing, direct tempering from as-built condition (DT) and conventional quenching followed by tempering (QT), on the microstructure and mechanical properties was evaluated. The softening resistance at elevated temperatures was investigated. Its correlation with the microstructure was also focused on. The evolution of carbides was discussed based on the microanalysis results and the JMatPro simulation.
Three different types of tool steels, Vanadias 4 Extra (V4E), a high-boron steel (HBS) and a newly developed maraging steel (NMS), were cladded on a hot work tool steel by means of DED for hard-facing purpose. For all tool steels, a near-dense cladded zone was obtained except V4E. Defects, including pores and cracks, were found in the deposited zone of V4E, the number of which increased with the building height or number of layers deposited. The factors that contribute to the formation of pores and cracks were identified.
After being tempered, the cladded tool steels were exposed at high temperatures to assess the softening resistance in terms of hardness. The abrasive wear resistance of the tempered and softened tool steels manufactured by DED was also evaluated at room temperature. A comparison with conventional counterparts on softening resistance and wear resistance was made. The microstructural evolution as a function of temperature and time was characterized and the precipitates were identified. Numerical simulations were applied to NMS to analyze the coarsening behavior of the precipitate and its influence on the mechanical property. The wear mechanism was discussed, and the governing factors were proposed.
Before addressing the properties of AM tool steels, a case study was performed on the worn surface of a hot stamping insert die. Galling was observed, which was a result of accumulated layers transferred from the steel workpieces to the die. Material softening of the die was detected in the sublayer of ~ 200 μm. It is the softening of the die material that promotes galling. Galling together with the spalling of the white layer are supposed to be the primary wear mechanisms for the tool.
A modified H13 (M-H13) hot work tool steel was fabricated by L-PBF. The effect of two types of post-processing, direct tempering from as-built condition (DT) and conventional quenching followed by tempering (QT), on the microstructure and mechanical properties was evaluated. The softening resistance at elevated temperatures was investigated. Its correlation with the microstructure was also focused on. The evolution of carbides was discussed based on the microanalysis results and the JMatPro simulation.
Three different types of tool steels, Vanadias 4 Extra (V4E), a high-boron steel (HBS) and a newly developed maraging steel (NMS), were cladded on a hot work tool steel by means of DED for hard-facing purpose. For all tool steels, a near-dense cladded zone was obtained except V4E. Defects, including pores and cracks, were found in the deposited zone of V4E, the number of which increased with the building height or number of layers deposited. The factors that contribute to the formation of pores and cracks were identified.
After being tempered, the cladded tool steels were exposed at high temperatures to assess the softening resistance in terms of hardness. The abrasive wear resistance of the tempered and softened tool steels manufactured by DED was also evaluated at room temperature. A comparison with conventional counterparts on softening resistance and wear resistance was made. The microstructural evolution as a function of temperature and time was characterized and the precipitates were identified. Numerical simulations were applied to NMS to analyze the coarsening behavior of the precipitate and its influence on the mechanical property. The wear mechanism was discussed, and the governing factors were proposed.
Tool Steels
Wear
Hot stamping
Additive Manufacturing
Softening Resistance
Virtual Development Lab (VDL)
Opponent: Massimo Pellizzari, University of Trento, Italy.
Author
Miwen Yuan
Chalmers, Industrial and Materials Science, Materials and manufacture
A Case Study for a Worn Tool Steel in the Hot Stamping Process
Journal of Materials Research and Technology,; Vol. 22(2023)p. 1065-1075
Journal article
Characteristics of a modified H13 hot-work tool steel fabricated by means of laser beam powder bed fusion
Materials Science & Engineering A: Structural Materials: Properties, Microstructure and Processing,; Vol. 831(2022)
Journal article
Study of defects in directed energy deposited Vanadis 4 Extra tool steel
Journal of Manufacturing Processes,; Vol. 76(2022)p. 419-427
Journal article
Time and temperature dependent softening of a novel maraging steel fabricated by laser metal deposition
Materials and Design,; Vol. 224(2022)
Journal article
M. Yuan, L. Nyborg, C. Oikonomou, S. Karamchedu, Y. Fan, L. Liu, Y. Cao, Softening behavior of a cold work steel and high-boron tool steel fabricated by directed energy deposition
M. Yuan, L. Nyborg, C. Oikonomou, Y. Fan, L. Liu, J. Ye, Y. Cao, Comparison of Softening Behavior and Abrasive Wear Resistance between Conventionally and Additively Manufactured Tool Steels
Additive Manufacturing (AM) is a newly developed technique that builds components by adding new material to existing parts, rather than subtracting material. From the sustainability point of view, a large fraction of raw materials can be saved. In addition, it opens the doors to the free design of the products. It has the capability to produce complicated hollow structures, such as turbine blades and rocket nozzles. Meanwhile, AM technique can reduce the steps of assembly, which shortens the leading time of the product.
In the automobile industry, AM also has the advantage to manufacture hot stamping dies with internal cooling channels, which can largely increase the cooling efficiency and extend the tool life. On the other hand, the dies can also be protected by pre-cladding hard materials on their surface with the directed energy deposition technique (DED, one of the AM techniques). If the die wears after usage over a long period of time, it can be repaired with DED as well. The purpose of these actions is to extend the tool’s life.
This thesis focuses on the microstructure and properties of a few selected tool steels produced by AM methods. Due to the unsatisfied properties of the as-printed parts, post-heat treatments are usually needed. The responses to these treatments have been investigated and discussed. The suitable post-heat treatments for the AM parts are identified. The tool steels in this study are supposed to be used at high temperatures. Hence, the material behaviors in a long term are examined at high temperatures. Their response at higher temperatures for a short term is also inspected. We investigate the evolution of the microstructure, hardness, wear resistance, and their correlations. Additionally, the comparison between conventionally and additively produced materials is performed as well. The insights in this thesis can improve the understanding of the field.
In the automobile industry, AM also has the advantage to manufacture hot stamping dies with internal cooling channels, which can largely increase the cooling efficiency and extend the tool life. On the other hand, the dies can also be protected by pre-cladding hard materials on their surface with the directed energy deposition technique (DED, one of the AM techniques). If the die wears after usage over a long period of time, it can be repaired with DED as well. The purpose of these actions is to extend the tool’s life.
This thesis focuses on the microstructure and properties of a few selected tool steels produced by AM methods. Due to the unsatisfied properties of the as-printed parts, post-heat treatments are usually needed. The responses to these treatments have been investigated and discussed. The suitable post-heat treatments for the AM parts are identified. The tool steels in this study are supposed to be used at high temperatures. Hence, the material behaviors in a long term are examined at high temperatures. Their response at higher temperatures for a short term is also inspected. We investigate the evolution of the microstructure, hardness, wear resistance, and their correlations. Additionally, the comparison between conventionally and additively produced materials is performed as well. The insights in this thesis can improve the understanding of the field.
Subject Categories
Materials Engineering
Infrastructure
Chalmers Materials Analysis Laboratory
Areas of Advance
Materials Science
ISBN
978-91-7905-786-2
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5252
Publisher
Chalmers