Showing papers on "Tempering published in 2019"

Heat treatment and properties of a hot work tool steel fabricated by additive manufacturing

Abstract: Additive manufacturing (AM) is increasingly used for the manufacturing of tools and dies; in this respect, apart from the optimization of processing parameters, it is important to establish the most proper heat treatment conditions for the fabricated parts. In this paper, the microstructure, and some properties of H13 hot work tool steel fabricated by selective laser melting (SLM) have been evaluated after direct tempering and in quenched and tempered condition. The as-built microstructure consists of a partially tempered martensite and a much higher amount (up to 19%vol) of retained austenite (RA) compared to the quenched steel (RA

Influence of post treatment on microstructure, porosity and mechanical properties of additive manufactured H13 tool steel

Abstract: Additive manufacturing (AM) is an attractive manufacturing technology in tooling applications. It provides unique opportunities to manufacture tools with complex shapes, containing inner channels for conformal cooling. In this investigation, H13, a widely used tool steel, was manufactured using a laser powder bed fusion method. Microstructure, tensile mechanical properties, hardness, and porosity of the AM H13 after stress relieve (SR), standard hardening and tempering (SR + HT), and hot isostatic pressing (SR + HIP + HT) were investigated. It was found that the microstructure of directly solidified colonies of prior austenite, which is typical for AM, disappeared after austenitizing at the hardening heat treatment. In specimens SR + HT and SR + HIP + HT, a microstructure similar to the conventional but finer was observed. Electron microscopy showed that SR and SR + HT specimens contained lack of fusion, and spherical gas porosity, which resulted in remarkable scatter in the observed elongation to break values. Application of HIP resulted in the highest strength values, higher than those observed for conventional H13 heat treated in the same way. The conclusion is that HIP promotes reduction of porosity and lack of fusion defects and can be efficiently used to improve the mechanical properties of AM H13 tool steel.

Austenite reversion kinetics and stability during tempering of an additively manufactured maraging 300 steel

Abstract: Reverted austenite is a metastable phase that can be used in maraging steels to increase ductility via transformation-induced plasticity or TRIP effect. In the present study, 18Ni maraging steel samples were built by selective laser melting, homogenized at 820 °C and then subjected to different isothermal tempering cycles aiming for martensite-to-austenite reversion. Thermodynamic simulations were used to estimate the inter-critical austenite + ferrite field and to interpret the results obtained after tempering. In-situ synchrotron X-ray diffraction was performed during the heating, soaking and cooling of the samples to characterize the martensite-to-austenite reversion kinetics and the reverted austenite stability upon cooling to room temperature. The reverted austenite size and distribution were measured by Electron Backscattered Diffraction. Results showed that the selected soaking temperatures of 610 °C and 650 °C promoted significant and gradual martensite-to-austenite reversion with high thermal stability. Tempering at 690 °C caused massive and complete austenitization, resulting in low austenite stability upon cooling due to compositional homogenization.

Improving strength and ductility of H13 die steel by pre-tempering treatment and its mechanism

Abstract: Retained austenite plays an important role in alloy steel to obtain high strength and excellent ductility. In this work, the pre-tempering process was developed to refine the microstructure of H13 steel. The mechanical properties were effectively improved with the tensile strength ∼1921 MPa, yield strength ∼1534 MPa, impact toughness ∼13.8 J‧cm-2, tensile elongation ∼11.8% and hardness ∼53.2 HRC. What's more, the tensile strength and hardness of the samples maintained above 83% after holding in 550 °C for 50 h, which were higher than the tensile strength (1569 MPa) and hardness (48.3 HRC) of samples treated by traditional 600 °C tempering treatment. It implied that the strength was improved by tempered martensite with high density dislocation and dispersed carbides in H13 steel, while the ductility was improved by the stable lath retained austenite. It is expected that the pre-tempering process can be applied widely in other die steel for superior performance.

High-Temperature Properties and Microstructural Stability of the AISI H13 Hot-Work Tool Steel Processed by Selective Laser Melting

Abstract: The microstructural stabilities, softening resistance, and high-temperature tensile properties of the H13 hot-work tool steel by selective laser melting (SLM) were systematically studied. A series of tempering procedures were performed on the as-SLMed specimens. Afterwards, the mechanism of softening resistance behavior was discussed based on the XRD, SEM, EBSD observations, hardness measurements, and high-temperature tensile tests. It was found that the as-SLMed H13 consisted of α-iron and γ-iron. The carbide-stabilizing elements aggregated as the cell-like substructures for the rapid solidification of the SLM process. After the softening resistance treatment, the retained austenite transformed to ferrite and carbide mixtures. The cell-like substructures dissolved slowly into the matrix when the temperature was below 550 °C. These factors increased the hardness and retarded the softening of the material. When the temperature was 600 °C, the microstructural constituents transformed to soft ferrite and globular carbides, which lead to a considerable decrease of the hardness. Due to the grain refinement, solid solution strengthening, and residual stress, the as-SLMed H13 exhibited better mechanical properties than that of the wrought counterparts.

Comparative Corrosion Behavior of Five Microstructures (Pearlite, Bainite, Spheroidized, Martensite, and Tempered Martensite) Made from a High Carbon Steel

Abstract: The present work discusses the comparative corrosion behavior of five microstructures of steels, namely, pearlite, bainite, spheroidized, martensite, and tempered martensite, which have been processed, respectively, by air cooling, isothermal transformation, spheroidizing, quenching, and tempering of a steel with composition 0.70C, 0.24Si, 1.12Mn, 0.026P, 0.021S, 0.013Nb, 0.0725Ta, and 97.7Fe (all are in wt pct). Dynamic polarization and alternating current (AC) impedance spectroscopic tests in freely aerated 3.5 pct NaCl solution show that the corrosion resistance of the steel specimens consisting of the preceding five microstructures decreases in the following sequence: pearlitic – bainitic – spheroidized – martensitic – tempered martensitic steels. The variation in the corrosion rate has been attributed to the shape, size, and distribution of the ferrite and cementite.

Ultra-high strength martensitic 420 stainless steel with high ductility

Abstract: Martensitic 420 stainless steel was successfully fabricated by Selective laser melting (SLM) with >99% relative density and high mechanical strength of 1670 MPa, yield strength of 600 MPa and elongation of 3.5%. X-ray diffraction (XRD) and scanning electron microscopy disclosed that the microstructure of SLM 420 consisted of colonies of 0.5–1 μm sized cells and submicron martensitic needles with 11 wt. % austenite. Tempering of as-built SLM 420 stainless steel at 400 °C resulted in ultra-high strength material with high ductility. Ultimate tensile strength of 1800 MPa and yield strength of 1400 MPa were recorded with an elongation of 25%. Phase transformation analysis was carried out using Rietveld refinement of XRD data and electron backscattered diffraction (EBSD), which showed the transformation of martensite to austenite, and resulted in austenite content of 36 wt. % in tempered SLM 420 stainless steel. Transformation induced plasticity (TRIP), austenite formation and fine cellular substructure along with sub-micron martensite needles resulted in stainless steel with high tensile strength and ductility. The advanced mechanical properties were compared with conventionally made ultra-high-strength steels, and the microstructure-properties relationships were disclosed.

Exploring the relationship between the microstructure and strength of fresh and tempered martensite in a maraging stainless steel Fe–15Cr–5Ni

Abstract: Hierarchical microstructure engineering is an efficient design path for ultra-high strength steels. An excellent example of this is maraging stainless steel, which achieves its high-performance by combining the hierarchic martensitic microstructure and nano-sized precipitates. Relating this complex microstructure with mechanical properties, e.g. strength, is not trivial. In the present work, we therefore explore the relationship between the hierarchic microstructure, evolving with the tempering of a Cu-containing maraging stainless steel 15–5 PH, and its strength. Comprehensive microstructure characterization, including the quantification of dislocation density, effective grain size, precipitates and retained austenite fraction is performed after quenching and tempering at 500 °C. The microstructure data is subsequently used as input for assessing the evolution of individual strength contributions and thus the increase in strength of tempered martensite contributed by Cu precipitation strengthening is evaluated. It is found that the Cu precipitation and dislocation annihilation are two major factors controlling the evolution of the yield strength of the tempered martensite. The Cu precipitation strengthening is also modelled using our previous Langer-Schwartz-Kampmann-Wagner model based predictions of the Cu precipitation, and modelled precipitation strengthening is compared with the evaluated Cu precipitation strengthening from the experiments. The work exemplifies the promising approach of combining physically based precipitation modelling and precipitation-strengthening modelling for alloy design and optimization. However, more work is needed to develop a generic predictive framework.

Effect of quenching and tempering temperature on microstructure and tensile properties of microalloyed ultra-high strength suspension spring steel

Abstract: Effects of quenching-tempering heat treatment on microstructural evolution and fracture behavior of microalloyed high strength suspension spring 55SiCrVNb were investigated. The results showed that an optimal combination of mechanical properties was obtained at the heat-treatment of oil quenching from 900 °C and tempering at 400 °C. Specifically, the ultimate tensile strength, yield strength, elongation and area reduction reached 2021 MPa, 1826 MPa, 10.3% and 42.7%, respectively. With the increasing austenitizing temperature, the grain size increased monotonically with more carbides dissolved in the matrix, and the strength first increased significantly and then decreased slowly. Furthermore, the fracture behavior transformed from ductile fracture to brittle fracture. In addition, the strengths were weakened dramatically as the tempering temperature increased but the elongation and area reduction were enhanced. Three kind of carbides are identified at different tempering temperatures, namely the coherent M2.5C and MC carbides gradually mature into large-sized non-coherent M3C and M7C3 carbides. Moreover, the dislocation reduction, lath boundary and twin martensite decomposition and carbide coarsening were exacerbated at higher tempering temperature while the fracture behavior changed from brittle fracture to ductile fracture.

Effect of tempering on laser remelted AISI H13 tool steel

Abstract: The effect of tempering temperature on surface hardened AISI H13 tool steel by laser remelting process using an Yb-fiber laser has been investigated. Single remelting tracks were produced in argon environment at different laser power and scan speed in the range of 400–600 W and 200–1600 mm/min respectively, maintaining the laser spot diameter fixed at 3 mm. Their effects on geometrical aspects, microstructure and microhardness were analyzed considering the thermal history of molten pool recorded using an IR pyrometer. Microstructure was found to be associated with cooling rate and melt pool lifetime, estimated from the temperature signal. Thereafter, remelted surface with 30% overlapping tracks was generated and subjected to 1 h tempering cycle at different temperatures in 500–900 °C range followed by hardness and wear tests. Hardness was retained fully up to 500 °C and significant softening occurred at 600 °C and 700 °C. At 800 °C and higher temperatures, effects of laser remelting were impaired completely, but substrate got hardened through martensite formation in a conventional manner. Wear resistance followed the trend of hardness. Changes in microstructure and formation of various phases were found to be the reasons behind the modifications in hardness.

Strong and ductile steel via high dislocation density and heterogeneous nano/ultrafine grains

Abstract: High strength and high ductility are critical to metallic materials and have always been pursued. A novel strategy to produce a medium Mn stainless steel with high dislocation density and heterogeneous nano/ultrafine-grains was designed and outstanding mechanical properties were achieved. The special heterogeneous microstructure was fabricated by cold rolling followed by rapid reverse annealing at intercritical temperature and a final step of low-temperature tempering. The results of this study demonstrated that the heterogeneous microstructure consisted of 74% reversed austenite, 16% deformed austenite and 10% tempered martensite. The dislocation density in austenite of the final microstructure had a high value of 1.12 × 1015 m−2 and the average grain size was 0.63 μm. The high yield strength (1324 MPa) and tensile strength (1401 MPa) of the steel were achieved and combined with high uniform elongation of 34.7%. The combination of high dislocation density (dislocation strengthening) and heterogeneous nano/ultrafine grains (fine-grain strengthening) resulted in such high yield strength. Simultaneously, twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP) effect occurred within the heterogeneous nano/ultrafine grains leading to high strain hardening rate and good uniform elongation in the steel.

Improvement of the mechanical properties of Al–Mg–Si alloys with nano-scale precipitates after repetitive continuous extrusion forming and T8 tempering

Abstract: Repetitive continuous extrusion forming (R-Conform) followed by T8 tempering treatment was found to be a viable approach for obtaining superior strength with high ductility in an Al–Mg–Si alloy. The evolution of mechanical properties and microstructure of the Al–Mg–Si alloy, and its mutual relationship during this novel approach were investigated. The results showed that a homogeneous and refined microstructure was formed during the R-Conform process, which improved the mechanical properties of the alloy. Concurrently, the R-Conform process caused effective dynamic strain redissolution of β″ precipitates, and the precipitation kinetics were affected by the subsequent T8 tempering process. The refined (sub)grains and induced high densities of the dislocations accelerated the formation of numerous dispersive nano-scale β″ precipitates during an additional T8 tempering treatment at a low artificial aging temperature (120 °C) and a short aging time (3 h). Therefore, remarkable improvements in both the tensile strength and ductility were achieved by 7 R-Conform passes and T8 tempering.

Role of Bead Sequence in Underwater Welding.

Abstract: This paper presents examinations of the role of the bead sequence in underwater welding. Two specimens of wet welded layers made by covered electrodes with the use of normalized S355G10+N steel were welded by a reasonable bead sequence. For each specimen, metallographic macro- and micro-scopic tests were done. Then, Vickers HV10 hardness measurements were conducted for each pad weld in the welded layer. The results show that welding in the water environment carries many problems in the stability of the welding arc, which influences the properties of the welds. The effects of refining and tempering the structure in heat-affected zones of earlier laid beads was observed, which provides a reduction of hardness. The possibility of applying two techniques while welding the layer by the wet method is described. It is stated that a reasonable bead sequence can decrease the hardness in heat-affected zones up to 40 HV10. Tempering by heat from next beads can also change the microstructure in this area by tempering martensite and can decrease susceptibility to cold cracking.

Process development and impact of intrinsic heat treatment on the mechanical performance of selective laser melted AISI 4140

Abstract: The low alloy steel AISI 4140 (German grade 42CrMo4) is one of the most frequently used Quench & Tempering (Q&T) steels with a wide range of applicability. Until now, commercially available iron powders for additive manufacturing can be summed up by their low amount of carbon. Fusion welding of Q&T steels often leads to cracks due to brittle martensitic transformation and the associated volume change. Therefore, the selection of appropriate process parameters in laser powder bed fusion (LPBF) plays a key role for the final material properties and is achieved through utilization of a new process development strategy and evaluation of microstructural features of test cubes. In this work tensile specimens were successfully produced with optimal process parameters and mechanical tests of additively built samples indicate mechanical performance comparable with a 450 °C tempered state of conventionally cast material. By correlating the measured mechanical properties of LPBF samples to those of a conventional Q&T state, an estimation of the intrinsic heat treatment during LPBF was carried out using an inverse transient Hollomon–Jaffe approach. This analysis indicates that a rapid reheating rate of 103 − 105 °C/s to ≈700–800 °C of the previous built layers is the determinant for the low hardness found in the LPBF process of AISI 4140. This is also in accordance with the finely dispersed carbide precipitates in the as built condition. Furthermore, the effect of bed pre-heating on the final material tempering state was found to be negligible. This shows the importance of a balanced match between LPBF process parameters and subsequent application demands as well as necessary postprocessing steps.

Aging phenomenon in low lattice-misfit cobalt-free maraging steel: Microstructural evolution and strengthening behavior

Abstract: We elucidate here the impact of aging temperature on microstructural evolution and strengthening behavior on low lattice misfit cobalt-free maraging steel. The best combination of high-strength (1850 MPa) and high-toughness (125.4 MJ m−3) was obtained at the optimal aging temperature of 520 °C, without sacrificing ductility. Electron back scattered diffraction studies suggested that preferred orientations of {101}, fraction of high-angle grain boundary (HAGB) and total length of grain boundary per unit area (μm/μm2) were increased with increase of aging temperature, which was beneficial to both strengthening and toughening of maraging steel. The strengthening contribution from the precipitates was transformed from shearing mechanism to bypass mechanism when the aging temperature is greater than 520 °C. The aging tempered steel of 520 °C provided maximum strengthening increment of 1463 MPa through shearing mechanism, while granular reverted austenite at this temperature contributed to high toughness.

Microstructure, mechanical, and tribological properties of M3:2 high-speed steel processed by selective laser melting, hot-isostatic pressing, and casting

Abstract: In this work, the influence of different manufacturing techniques of M3:2 high-speed steel on the resulting microstructure and the associated material properties was investigated. Therefore, microstructure as well as the mechanical and tribological properties of cast steel (with subsequent hot-forming) and steel powder processed by two techniques: hot-isostatic pressing (HIP) and selective laser melting (SLM) were compared. A detailed SLM parameter analysis revealed that the porosity of SLM specimens can be decreased towards a smaller point distance and a longer exposure time (high energy input). A rise in preheating temperature is associated with a reduction in the crack density or the complete avoidance of cracks. In this context, the high-speed steel showed outstanding densification behavior by SLM, even though this steel is considered to be hardly processable by SLM due to its high content of carbon and hard phase-forming elements. In addition, the reusability of steel powder for SLM processing was investigated. The results indicated that multiple reuse is possible, but only in combination with powder processing (mechanical sieving) after each SLM cycle. The microstructure of SLM-densified high-speed steel consists of a cellular, fine dendritic subgrain segregation structure (submicro level) that is not significantly affected by preheating the base plate. The mechanical and tribological properties were examined in relation to the manufacturing technique and the subsequent heat treatment. Our investigations revealed promising behavior with respect to hardness tempering (position of the secondary hardness peak) and tribology of the M3:2 steel processed by SLM compared to the HIP and cast conditions.