Hot work tool steels with medium C contents are known to be difficult to process by laser powder bed fusion (L-PBF). Cold and, to a lesser extent, hot cracking occurs in these alloys. Cold cracks are attributed to the low ductility and large residual stresses due to the complex thermal profiles. These can be avoided by platform preheating, which may introduce additional costs and side-effects on microstructure and properties. Therefore, the market trend is to develop new steel grades with improved 3D-printabilty. In this work, a prototype alloy with a leaner C content is proposed. To compensate for the negative effect of reduced C, computational thermodynamics was used to define chemistries with an optimized balance of carbide forming elements, and Si. The prototype tool steel shows enhanced L-PBF processability, and properties meeting and/or exceeding those of wrought AISI H13 in terms of hot strength, tempering and thermal fatigue resistance.
Abstract Mold repair is a viable strategy for saving energy and reducing CO2 emissions. Papers in the literature show that repairing a limited damaged area of the mold instead of producing a new one is becoming increasingly attractive, especially considering the latest European and international regulations introduced with the green deal. In this paper, the authors are pleased to present some preliminary results related to the repair of AISI H13 tool steel molds by Laser-Directed Energy Deposition. Steel blocks (20 x 55 x 100 mm3), previously tempered at 435±10 HV, were machined to reproduce the material removal of the damaged part of the mold. Subsequently, the region was repaired by L-DED using commercial H13 powder. The process parameters were optimized to obtain a defect-free welded area. Since the microstructure of the deposited tool steel consists of hard (730±10 HV) and brittle (7 J Charpy impact toughness) martensite, a series of post-process heat treatments were performed at different temperatures to restore a hardness compatible with that of the base steel. However, this goal was only partially achieved due to the different tempering behavior of L-DED-deposited and bulk H13 steel. In particular, the tempering temperature had to be limited to avoid softening of the base steel. In the best case, double tempering at 620 °C resulted in a toughness recovery of up to 42 J. Thermal fatigue tests showed better resistance to crack propagation after tempering, as evidenced by the shallower penetration depth compared to the as-built material.
Near-full density and crack-free AISI H13 hot-work tool steel was fabricated using laser-directed energy deposition (L-DED). Two different heat-treatment scenarios, i.e., direct tempering (ABT) from the as-built (AB) condition and systematization and quenching prior to tempering (QT), were investigated, and their effect on the microstructure, hardness, fracture toughness (Kapp), and tempering resistance of the L-DED H13 is reported. For this purpose, the optimal austenitization schedule was identified, and tempering curves were produced. At a similar hardness level (500 HV1), QT parts showed higher Kapp (89 MPa√m) than ABT (70 MPa√m) levels. However, the fracture toughness values obtained for both parts were comparable to those of wrought H13. The slightly larger Kapp in the QT counterpart was discussed considering the microstructural homogenization and recrystallization taking place during high-temperature austenitization. The tempering resistance of the ABT material at 600 °C was slightly improved compared with that of the QT material, but for longer holding times (up to 40 h) and higher temperatures (650 °C), ABT showed superior resistance to thermal softening due to a finer martensite substructure (i.e., block size), a finer secondary carbide size, and a larger volume fraction of secondary V(C,N) carbides.
To address the challenges in processing medium-carbon hot work tool steels by laser-based additive manufacturing (AM), a recently developed hot work tool steel with improved processability was processed by both laser powder bed fusion (L-PBF) and laser-directed energy deposition (L-DED). Microstructure and phases in as-built (AB) and quenched (Q) states were compared for both processing routes. Hardness, Charpy V notch impact toughness, tempering- and thermomechanical softening- resistance, after direct double tempering from AB condition (DT) and quenching and tempering (QT) were measured and assessed. Properties were then compared with those of AM-, as well as wrought- AISI H13 hot work tool steel. The results suggest that the new steel exhibits comparable mechanical and thermomechanical properties to steel H13. Finally, practical case studies of repairing tools made from H13, using the new tool steel (L-DED), and producing relatively large molds with complex geometries (L-PBF) were demonstrated.
Abstract In laser additive manufacturing (AM) of hot work tool steels, direct tempering (DT) of the tool from as-built (AB) condition without prior conventional austenitization and quenching results in enhanced tempering resistance. To date, intercellular retained austenite (RA) decomposition, leading to a shift in secondary hardening peak temperature, and finer martensite substructure are reported to be responsible for such a behavior. In this work, authors aimed at studying the strengthening contributions by performing isothermal tempering tests for long times (up to 40 hours) at elevated temperatures (up to 650 °C) on DT and quenched and tempered (QT) specimens. The thermal softening kinetics and the microstructural evolution were evaluated with the support of computational thermodynamics. The results suggest that the main contributor to enhanced temper resistance in DT condition is the larger fraction of thermally stable and extremely fine (~ 20 nm) secondary (tempering) V(C,N) compared with QT. This could be explained by the reduction of available V and C in austenitized and quenched martensite for a later secondary V(C,N) precipitation during tempering, because of equilibrium precipitation of relatively large (up to 500 nm) vanadium-rich carbonitrides during the austenitization process. A complementary effect of the substructure refinement ( i.e. , martensite block width) in rapidly solidified highly supersaturated martensite was also quantified in terms of Hall–Petch strengthening mechanism. The significant effect of secondary V(C,N) was successfully validated by assessing a laser AM processed vanadium-free hot work tool steel in QT and DT condition, where no significant differences in strength and temper resistance between the two conditions were evident. Graphical Abstract
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