Abstract
The solidified microstructure and carbide precipitation behavior in an S390 high-speed steel processed by electron beam melting (EBM) have been fully characterized. The as-EBM microstructure consists of discontinuous network of very fine primary carbides dispersed in auto-tempered martensite matrix together with a limited amount of retained austenite. The carbide network consists of M2C/M6C and MC carbides. Both the columnar and near-equiaxed grain structures were found in as-EBM microstructure and the presence of inter-dendritic eutectic carbides assisted in revealing the dendritic solidification nature. The top-layer microstructure observation confirmed that the columnar dendritic structured grains were located adjacent to the micro-melt pool boundary, indicating an epitaxial growth with the average growth direction parallel to the maximum thermal gradient. At the center of the micro-melt pool, the near-equiaxed grains were developed by dendritic growth parallel to the beam traveling direction. The carbide decomposition was revealed by scanning transmission electron microscopy and confirmed by transmission Kikuchi diffraction. The MC carbides (rich in V followed by W) nucleated at the interface between M2C (W, Fe, Mo, and Co in the order of significance) and the matrix and then grew from the outside inward, but their nucleation might occur from the M2C carbide itself. The thermal effect induced by the adjacent scan lines seems to trigger a solid-state phase transformation of MC → M2C + γ-Fe. The elemental migration was theoretically calculated and compared with the experimental results. The high hardness of ~ 65 HRC and good transverse rupture strength of ~ 2500 MPa in as-EBM S390 means that EBM processing can be used to fabricate highly alloyed tool steels. With the help of the post-processing heat treatment, the best Rockwell hardness of 73.1±0.2 HRC and transverse rupture strength of 3012±34 MPa can be obtained.
Highlights
COMPARED to conventional ingot casting and powder metallurgy, electron beam melting (EBM) has successfully demonstrated its capability to fabricate near-net-shape parts with the very fine microstructure and minimum segregation due to its intrinsic rapid solidification
In contrast to selective laser melting (SLM) that operates at lower temperatures, EBM can work at high temperatures of up to 1200 °C
scanning electron microscopy (SEM) micrograph in Figure 3 shows the presence of spherical-shaped gas pores with a typical size of 45.1 ± 10.6 lm in sample 1, which is the
Summary
COMPARED to conventional ingot casting and powder metallurgy, electron beam melting (EBM) has successfully demonstrated its capability to fabricate near-net-shape parts with the very fine microstructure and minimum segregation due to its intrinsic rapid solidification. METALLURGICAL AND MATERIALS TRANSACTIONS A manufacturing (AM).[1] In contrast to selective laser melting (SLM) that operates at lower temperatures, EBM can work at high temperatures of up to 1200 °C. This in situ heating characteristic makes EBM attractive for processing high-performance alloys that are susceptible to thermal stress-induced cracking or solidification/hot cracking. EBM was applied to produce precipitation-strengthened nickelbase superalloys,[2,3,4,5,6] cobalt-base alloys,[7,8] as well as titanium aluminide intermetallic alloys.[9,10,11] little effort has been made regarding EBM high-speed steels that are susceptible to cracking. Note the EBM work[12] on H13 steel that has a carbon content of 0.37 wt pct and limited degree of alloying does not represent those highly alloyed high-speed steels
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