Estudo de processamento e pós-processamento do aço ferramenta AISI H13 por fusão em leito de pó

Author(s)

Eduardo Bertoni da Fonseca

• Document type: Doctoral Thesis
• Press: Campinas, SP.
• Institution: Universidade Estadual de Campinas (UNICAMP). Faculdade de Engenharia Mecânica
• Defense date: 2023-07-07

Examining board members

Éder Sócrates Najar Lopes; Claudemiro Bolfarini; Kahl Dick Zilnyk; João Batista Fogagnolo; Milton Sergio Fernandes de Lima

• Advisor: Éder Sócrates Najar Lopes

Abstract

Additive manufacturing technologies produce parts with complex geometries typically in a layer-by-layer fashion. Thus, cost reduction and the desired quality of parts can be achieved in a shorter production chain. Among the applications of additive manufacturing is the fabrication of tools, such as molds and dies for injection molding, die casting, and forging. Typically, AISI H13 tool steel is the most employed grade in such applications due to its high mechanical resistance and toughness at high temperatures. However, the processability of H13 steel by laser powder bed fusion and subsequent heat treatment routes need further development. This work initially addresses the processability of H13 steel, exploring the consolidation, defects, microstructure, and hardness as a function of laser power and scanning speed using the powder bed fusion technique. The use of dimensionless processing parameters allows the evaluation of processability in different machines and urges researchers to report more data on processing conditions. The microstructural characterization of the as-built samples reveals a cellular/dendritic solidification structure due to partitioning of alloying elements, besides a microstructural heterogeneity related to the intrinsic thermal cycle during processing, which is elucidated by the analysis of the processed layers. Afterwards, the phase transformations associated to direct tempering heat treatment of the as-built microstructure are assessed. Microsegregation stabilizes a large fraction of retained austenite at the cell walls, which undergoes decomposition during tempering following two pathways. In the para-equilibrium pathway, retained austenite transforms into fresh martensite upon cooling, promoting hardness increase and retaining the solidification structure. Carbon diffusion is associated with retained austenite destabilization, while diffusion of other alloying elements remains negligible. In the equilibrium pathway, retained austenite decomposes isothermally into ferrite and carbides, while the cellular structure degenerates. Finally, an evaluation of the mechanical performance of parts subjected to different heat treatment routes is carried out to determine mechanical properties relevant to typical applications of tool steels, such as mechanical strength, fracture toughness and wear resistance. Among the evaluated heat treatment routes, a direct tempering route and a traditional austenitizing, quenching and tempering route stand out. In the direct tempering route, superior mechanical strength is observed due to the refined microstructure obtained after processing. The choice of temperature for the second tempering cycle allows for adjusting the mechanical properties between a material with high mechanical and wear resistance due to the secondary precipitation of carbides (at 550 °C), or a material with high fracture toughness due to martensite desaturation and coarsening (at 650 °C). The austenitizing heat treatment dissolves the microsegregation and forms new grains, resulting in a microstructure similar to the wrought material produced by conventional routes. As a result, greater fracture toughness is achieved, but with lower mechanical and wear resistance. The results provide background for the processing and heat treatment of H13 tool steel processed by additive manufacturing, correlating microstructural features with mechanical properties (AU)

• FAPESP's process: 19/06276-4 - Study of processability and post-processing of AISI H13 tool steel by powder bed fusion: micro/nanostructural and mechanical characterization
• Grantee: Eduardo Bertoni da Fonseca
• Support Opportunities: Scholarships in Brazil - Doctorate