Tempering of an additively manufactured microsegregated hot-work tool steel: A high-temperature synchrotron X-ray diffraction study

Author(s)

Fonseca, Eduardo B. ; Escobar, Julian D. ; Gabriel, Andre H. G. ; Ribamar, Giovani G. ; Boll, Torben ; Lopes, Eder S. N.

• Document type: Journal article
• Source: ADDITIVE MANUFACTURING; v. 55, p. 13-pg., 2022-04-20.

Abstract

Rapidly solidified microstructures, produced by concentrated heat sources, have gained increasing attention due to the widespread expansion of additive manufacturing techniques. In laser powder bed fusion of tool steels, the as-built microstructure consists of a microsegregated network of cell-like walls decorated with retained austenite (RA). This inhomogeneous elemental distribution leads to phase transformation sequences dictated by local - instead of global - equilibrium. This work uses synchrotron X-ray diffraction to study the kinetics of retained austenite decomposition during tempering of a microsegregated as-built AISI H13 tool steel. The RA decomposition products are also characterized by hardness tests, scanning electron microscopy, and atom probe tomography. Present results evidence the formation of high temperature para-equilibrium (T < 650 degrees C) or equilibrium (T >= 650 degrees C) microstructures. The para-equilibrium microstructural path evidences isothermally stable RA, which transforms into fresh martensite upon cooling due to carbon depletion. However, the cell-like microsegregation arrangement is retained due to insufficient homogenization of substitutional elements. The equilibrium microstructural path results in isothermal transformation of RA into ferrite + carbides, as well as dissolution of the cell-like microsegregation structure. These results provide insights towards the understanding and development of optimized heat treatment cycles appropriate for such increasingly common inhomogeneous microstructures. The results are relevant not only to laser powder bed fusion, but also to other localized melting/remelting processes with associated high cooling rates. (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
• FAPESP's process: 18/21251-5 - Characterization of the temperature-activated secondary hardening effect of fire-resistant steels during fire simulations
• Grantee: Julian David Escobar Atehortua
• Support Opportunities: Scholarships in Brazil - Post-Doctoral