Impact of in Situ Heat Treatment Effects during Laser-Based Powder Bed Fusion of 1.3343 High-Speed Steel with Preheating Temperatures up to 700 °C

For laser-based powder bed fusion (PBF-LB) of high carbon steels, preheated build platforms can reduce thermal stresses and crack formation inside the generated material. Furthermore, the heat distribution during PBF-LB is affected by laser energy input and heat transfer into the surrounding area. Depending on the preheating temperature and the thermal conditions during PBF-LB, thermal gradients and different thermal exposure times of the manufactured layers can lead to in situ heat treatment effects. As a result, gradients in microstructures and properties are observed in the manufactured material. The effects are investigated on AISI M2 high-speed steel (1.3343). Specimens are manufactured at platform preheating temperatures between 200 and 700 °C. Base plate and surface temperatures in the building layer are monitored by thermocouples and pyrometry. Local variations in the material microstructure and properties are determined and the effects of temperature distribution on microstructure and hardness are discussed.     

Additive Manufacturing of Tool Steels

The public funded project “AddSteel” aims to develop functionally adapted steel materials for additive manufacturing (AM). Based on the AM process laser powder bed fusion (LPBF), the holistic process chain, including alloy design, powder atomization, AM, and postheat treatment, is considered to achieve this objective. Tool steels are usually characterized by higher carbon content and limited weldability, leading to limited processability for LPBF. To extend these limitations, different approaches for tool steels are investigated: for high-carbon tool steels, the effects of lower martensite start temperature are investigated using the alloy 1.2842 as an example. A low martensite start temperature seems to be advantageous for crack-free processing with LPBF. In order to avoid a high hardness level after rapid cooling, the use of a hot work steel with a carbon content of 0.2 wt% is investigated. Due to the chemical composition of the material, a moderate preheating temperature <300 °C is required. In addition, very high scanning speeds are possible with an improved shielding gas flow. Finally, the experience along the process chain with the standard steels is used for a modification of the alloy 1.2344. The effects of this modification on AM and heat treatment are investigated.     

Microstructure and Properties of a Novel Carbon-Martensitic Hot Work Tool Steel Processed by Laser Additive Manufacturing without Preheating

Laser additive manufacturing (LAM) techniques, such as laser-powder bed fusion (L-PBF) or laser-directed energy deposition (L-DED), allow for the production of complex-shaped parts by either the local melting of a metallic powder bed by a laser beam (L-PBF) or a local application and laser beam melting of powder material by a nozzle (L-DED). In the case of carbon-martensitic tool steels, their cold crack susceptibility limits their LAM processability and is usually counteracted by substrate preheating. As preheating can increase the oxygen take-up of the powder and alter the part microstructure, it can be disadvantageous for part quality and powder reusability. In this study, it is investigated a carbon-martensitic steel designed for the production of parts with low crack density by LAM without preheating, focusing on the microstructure and hardness of the L-PBF- and L-DED-manufactured steel. The steel can be LAM-processed without preheating, resulting in specimens with low crack densities and martensitic microstructure with retained austenite. The hardness of the as-built material (L-PBF: 542HV30 and L-DED: 623HV30) is increased by quenching and tempering up to 693HV30. Direct tempering of the as-built specimen without previous quenching leads to a shift of the secondary hardness maximum from 500 to 530 °C.     

The mechanical stability of austenite and cryogenic toughness of ferritic Fe-Mn-AI Alloys

In an attempt to understand the role of retained austenite on the cryogenic toughness of a ferritic Fe-Mn-AI steel, the mechanical stability of austenite during cold rolling at room temperature and tensile deformation at ambient and liquid nitrogen temperature was investigated, and the microstructure of strain-induced transformation products was observed by transmission electron microscopy (TEM). The volume fraction of austenite increased with increasing tempering time and reached 54 pct after 650 °C, 1-hour tempering and 36 pct after 550 °C, 16-hour tempering. Saturation Charpy impact values at liquid nitrogen temperature were increased with decreasing tempering temperature, from 105 J after 650 °C tempering to 220 J after 550 °C tempering. The room-temperature stability of austenite varied significantly according to the(α + γ) region tempering temperature;i.e., in 650 °C tempered specimens, 80 to 90 pct of austenite were transformed to lath martensite, while in 550 °C tempered specimens, austenite remained untransformed after 50 pct cold reductions. After tensile fracture (35 pct tensile strain) at -196 °C, no retained austenite was observed in 650 °C tempered specimens, while 16 pct of austenite and 6 pct of e-martensite were observed in 550 °C tempered specimens. Considering the high volume fractions and high mechanical stability of austenite, the crack blunting model seems highly applicable for improved cryogenic toughness in 550 °C tempered steel. Other possible toughening mechanisms were also discussed. Formerly Graduate Student, Seoul National University.     

Towards the Optimization of Post-Laser Powder Bed Fusion Stress-Relieve Treatments of Stainless Steel 316L

The use of post-processing heat treatments is often considered a necessary approach to relax high-magnitude residual stresses (RS) formed during the layerwise additive manufacturing laser powder bed fusion (LPBF). In this work, three heat treatment strategies using temperatures of 450 °C, 800 °C, and 900 °C are applied to austenitic stainless steel 316L samples manufactured by LPBF. These temperatures encompass the suggested lower and upper bounds of heat treatment temperatures of conventionally processed 316L. The relaxation of the RS is characterized by neutron diffraction (ND), and the associated changes of the microstructure are analyzed using electron backscattered diffraction (EBSD) and scanning electron microscopy (SEM). The lower bound heat treatment variant of 450 °C for 4 hours exhibited high tensile and compressive RS. When applying subsequent heat treatments, we show that stress gradients are still observed after applying 800 °C for 1 hour but almost completely vanish when applying 900 °C for 1 hour. The observed near complete relaxation of the RS appears to be closely related to the evolution of the characteristic subgrain solidification cellular microstructure.

     

The effect of grain size and retained austenite on the ductile-brittle transition of a titanium-gettered iron alloy

The effect of microstructural changes on the ductile-brittle transition temperature (DBTT) was studied in a titanium-getter ed Fe-8Ni-2 Mn-0.15 Ti alloy. A fairly strong grain size dependence of the transition temperature, 8°C/mm^−1/2, was found. Grain size refinement from 38 μm (ASTM #6.5) to 1.5 μm (ASTM #15.5) through a four-step thermal treatment lowered the transition temperature by 162°C. A small amount of retained austenite was introduced into this grain-refined microstructure, and the transition temperature was reduced by an additional 120 ~ 150°C. The reduction of the DBTT due to retained austenite was smaller when the austenite was in a large-grained structure (64°C). The distribution and stability of retained austenite were also studied.     

Influence of Austempering Temperature on Microstructure and Mechanical Properties of High-Silicon Carbide-Free Bainitic Steel

The microstructural evolution of a novel high-silicon carbide-free bainitic steel at different austempering temperatures is investigated. The microstructure is evaluated by means of optical and electron microscopy, X-ray diffraction, microhardness, and nanohardness. Results show a variation in the amount of stabilized retained austenite changing the temperature of the isothermal treatment. In particular, it is observed an increase in the retained austenite volume fraction increasing the temperature up to 350 °C, while further increase leads to a reduction. Moreover, increasing the isothermal holding temperature from 250 °C, through 300, 350, and 370 °C, a progressive bainite coarsening and an increase in the amount of stabilized carbon-enriched retained austenite are observed. Tensile tests reveal an excellent combination of mechanical properties: mechanical strength in the range 1276–1988 MPa and total elongation 0.18–0.44.     

Thermally Stable Ni-rich Austenite Formed Utilizing Multistep Intercritical Heat Treatment in a Low-Carbon 10 Wt Pct Ni Martensitic Steel

Austenite reversion and its thermal stability attained during the transformation is key to enhanced toughness and blast resistance in transformation-induced-plasticity martensitic steels. We demonstrate that the thermal stability of Ni-stabilized austenite and kinetics of the transformation can be controlled by forming Ni-rich regions in proximity of pre-existing (retained) austenite. Atom probe tomography (APT) in conjunction with thermodynamic and kinetic modeling elucidates the role of Ni-rich regions in enhancing growth kinetics of thermally stable austenite, formed utilizing a multistep intercritical (Quench-Lamellarization-Tempering (QLT)-type) heat treatment for a low-carbon 10 wt pct Ni steel. Direct evidence of austenite formation is provided by dilatometry, and the volume fraction is quantified by synchrotron X-ray diffraction. The results indicate the growth of nm-thick austenite layers during the second intercritical tempering treatment (T-step) at 863 K (590 °C), with austenite retained from first intercritical treatment (L-step) at 923 K (650 °C) acting as a nucleation template. For the first time, the thermal stability of austenite is quantified with respect to its compositional evolution during the multistep intercritical treatment of these steels. Austenite compositions measured by APT are used in combination with the thermodynamic and kinetic approach formulated by Ghosh and Olson to assess thermal stability and predict the martensite-start temperature. This approach is particularly useful as empirical relations cannot be extrapolated for the highly Ni-enriched austenite investigated in the present study.     

Effect of thermomechanical processing on the retained austenite content in a Si-Mn transformation-induced-plasticity steel

The influence of hot deformation on the microstructure of a hot-rolled Si-Mn transformation-induced-plasticity (TRIP) steel was evaluated in an effort to better control retained austenite content. In this study, axial compressive strains varying in amounts from 0 to 60 pct were imposed in the austenite phase field, and effects on the formation of polygonal ferrite, bainite, and retained austenite were determined. In addition, modifications in simulated coiling temperature from 420 °C to 480 °C and cooling rates from the rolling temperature, between 10 °C/s and 35 °C/s, were assessed. Fast cooling rates, low coiling temperatures, and low degrees of hot deformation were generally found to decrease the amount of polygonal ferrite and increase retained austenite fraction. Unexpectedly, a sharp increase in polygonal ferrite content and decrease in retained austenite content occurred when the fastest cooling rate, 35 °C/s, was coupled with extensive hot deformation and high coiling temperatures. This effect is believed to be due to insufficient time for full recovery and recrystallization of the deformed austenite, even in the absence of intentional microalloying additions to control recrystallization kinetics. The resultant decrease in hardenability allowed the ferrite transformation to continue into the holding time at high (simulated) coiling temperatures.     

Microstructure Evolution and Mechanical Behavior of a CMnSiAl TRIP Steel Subjected to Partial Austenitization Along with Quenching and Partitioning Treatment

The present study investigated the microstructure evolution and mechanical behavior in a low carbon CMnSiAl transformation-induced plasticity (TRIP) steel, which was subjected to a partial austenitization at 1183 K (910 °C) followed by one-step quenching and partitioning (Q&P) treatment at different isothermal holding temperatures of [533 K to 593 K (260 °C to 320 °C)]. This thermal treatment led to the formation of a multi-phase microstructure consisting of ferrite, tempered martensite, bainitic ferrite, fresh martensite, and retained austenite, offering a superior work-hardening behavior compared with the dual-phase microstructure (i.e., ferrite and martensite) formed after partial austenitization followed by water quenching. The carbon enrichment in retained austenite was related to not only the carbon partitioning during the isothermal holding process, but also the carbon enrichment during the partial austenitization and rapid cooling processes, which has broadened our knowledge of carbon partitioning mechanism in conventional Q&P process.     

Effects of Pre-tempering on Intercritical Annealing in Fe-2Mn-0.3C Alloy

As-quenched martensite was pre-tempered at 623 K and 923 K (350 °C and 650 °C), and then it reverted to austenite by intercritical annealing at 998 K (725 °C) in a Fe-2Mn-0.3C alloy. Pre-tempering at 623 K (350 °C) accelerates austenite formation, while pre-tempering at 923 K (650 °C) significantly retards it. It is proposed that austenite nucleation is accelerated by increasing the number density and particle size of cementite during tempering, whereas austenite growth is retarded by Mn enrichment in cementite during tempering at high temperature, leading to opposite effects of pre-tempering on reversion kinetics.     

Formation of Reversed Austenite in High Temperature Tempering Process and its Effect on Mechanical Properties of Cr15 Super Martensitic Stainless Steel Alloyed with Copper

Formation mechanism of the reversed austenite of Cr15 super martensitic stainless steel (SMSS) alloyed with copper after high temperature tempering was investigated by means of thermo‐calc software, transmission electron microscope (TEM), and X‐ray diffraction (XRD). The mechanical properties of the SMSS were also tested. The experimental results show that the reversed austenite with low dislocation density is formed at high temperature tempering processing. The transformation of the martensite to reversed austenite is a diffused phase transformation, and the growth of the reversed austenite is closely related to the diffusion process of Ni. The bulk reversed austenite with large amount of stacking faults is formed with the increase of the tempering temperature. The volume fraction of reversed austenite increases at first and then decreases with increasing tempering temperature, and the maximum amount of the reversed austenite is obtained at 650°C. The reversed austenite is unstable at the tempering temperature above 650°C and the martensitic phase transformation will occur at the following cooling process. The mechanical properties of Cr15 super martensitic stainless steel are significantly influenced by the volume fraction of reversed austenite.     

Spark plasma sintering of Al-Si-Cu-Fe quasi-crystalline powder

This article presents the results of a study on the microstructure and mechanical properties of Al-Si-Cu-Fe specimens produced by the spark plasma sintering (SPS) technique. The microstructure of the starting powder and bulk specimens was analyzed by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The formation of the icosahedral and decagonal quasi-crystalline phases in the as-gas-atomized powders is described for the first time. It is then shown that these metastable phases transformed into the 1/1 cubic-approximant phase upon heating at about 600 °C. Second, the effects of SPS process parameters such as the temperature and time have been investigated. Owing to the generation of a spark discharge between neighboring powder particles, dense cylindrical samples were obtained after a short sintering time of 30 minutes at the temperature of 650 °C. The highest values of the Vickers microhardness, about 8.9 GPa, were obtained when the powders were sintered in the temperature range of 600 °C to 650 °C for a holding time of 30 minutes, while the fracture toughness was found to be inversely proportional to the sintering temperature. However, at the sintering temperature of 650 °C, the fracture toughness increased from about 1.40 to 1.52 MPa √m as the holding time increased from 10 to 60 minutes. As compared to cast specimens, the enhanced mechanical properties are explained by the refined microstructure resulting from the low temperature and short sintering time applied during SPS processing.     

Spark plasma sintering of Al-Si-Cu-Fe quasi-crystalline powder

This article presents the results of a study on the microstructure and mechanical properties of Al−Si−Cu−Fe specimens produced by the spark plasma sintering (SPS) technique. The microstructure of the starting powder and bulk specimens was analyzed by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The formation of the icosahedral and decagonal quasi-crystalline phases in the as-gas-atomized powders is described for the first time. It is then shown that these metastable phases transformed into the 1/1 cubicapproximant phase upon heating at about 600°C. Second, the effects of SPS process parameters such as the temperature and time have been investigated. Owing to the generation of a spark discharge neighboring powder particles, dense cylindrical samples were obtained after a short sintering time of 30 minutes at the temperature of 650°C. The highest values of the Vickers microhardness, about 8.9 GPa, were obtained when the powders were sintered in the temperature range of 600°C to 650°C for a holding time of 30 minutes, while the fracture toughness was found to be inversely proportional to the sintering temperature. However, at the sintering temperature of 650°C, the fracture toughness increased from about 1.40 to 1.52 MPa √m as the holding time increased from 10 to 60 minutes. As compared to cast specimens, the enhanced mechanical properties are explained by the refined microstructure resulting from the low temperature and short sintering time applied during SPS processing     

The effect of austenitizing temperature on the microstructure and mechanical properties of as-quenched 4340 steel

The effect of austenitizing temperature on both the plane strain fracture toughness,K _IC , and the microstructure of AISI 4340 was studied. Austenitizing temperatures of 870 and 1200°C were employed. All specimens austenitized at 1200°C were furnace cooled from the higher austenitizing temperature and then oil quenched from 870°C. Transmission electron microscopy revealed an apparent large increase in the amount of retained austen-ite present in the specimens austenitized at the higher temperature. Austenitizing at 870°C resulted in virtually no retained austenite; only minor amounts were found sparsely scat-tered in those areas examined. A considerably altered microstructure was observed in specimens austenitized at 1200°C. Fairly continuous 100 to 200Å thick films of retained austenite were observed between the martensite laths throughout most of the area exam-ined. Additionally, specimens austenitized at 870°C contained twinned martensite plates while those austenitized at 1200°C showed no twinning. Plane strain fracture toughness measurements exhibited an approximate 80 pct increase in toughness for specimens austen-itized at 1200°C compared to those austenitized at 870°C. The yield strength was unaffected by austenitizing temperature. The possible role of retained austenite and the elimination of twinned martensite in the enhancement of the fracture toughness of those specimens austen-itized at the higher temperature will be discussed.     

Evolution of the Microstructure of Laser Powder Bed Fusion Ti-6Al-4V During Post-Build Heat Treatment

The microstructure of additively manufactured Ti-6Al-4V (Ti64) produced by a laser powder bed fusion process was studied during post-build heat treatments between 1043 K (770 °C) and just above the β transus temperature 1241 K (1008 °C) in situ using high-energy X-ray diffraction. Parallel studies on traditionally manufactured wrought and annealed Ti64 were completed as a baseline comparison. The initial and final grain structures were characterized using electron backscatter diffraction. Likewise, the initial texture, dislocation density, and final texture were determined with X-ray diffraction. The evolution of the microstructure, including the phase evolution, internal stress, qualitative dislocation density, and vanadium distribution between the constituent phases were monitored with in situ X-ray diffraction. The as-built powder bed fusion material was single-phase hexagonal close packed (to the measurement resolution) with a fine acicular grain structure and exhibited a high dislocation density and intergranular residual stress. Recovery of the high dislocation density and annealing of the internal stress were observed to initiate concurrently at a relatively low temperature of 770 K (497 °C). Transformation to the β phase initiated at roughly 913 K (640 °C), after recovery had occurred. These results are meant to be used to design post-build heat treatments resulting in specified microstructures and properties.

     

Metallurgy of continuously annealed high strength TRIP steel sheet

The effects of heat‐treatment conditions on mechanical properties are comprehensively investigated to optimise the industrial process of the 590 MPa grade TRIP steel sheet with the metallurgical understanding. The substantial effect of the thermal conditions are first clarified by laboratory investigation, which includes the effects of annealing conditions, cooling conditions from intercritical temperature to austempering temperature and austempering conditions. The results indicate that the optimum annealing temperature is between 800 and 850 °C and the mechanical properties are hardly influenced by the annealing time between 30 and 120 s at an annealing temperature of 825 °C. It is also suggested that the optimum quenching rate is 45 °C/s to obtain the stable properties of the products and the optimum austempering conditions are 425 °C with over 300 s in case of a constant temperature austempering. Based on the laboratory investigation, mill trial is performed using the NKK No.4‐CAL in Fukuyama works. The heat treatment conditions are intentionally varied to examine minutely the stability of the production. The mechanical properties are sensitive to the austempering start temperature, when the austempering temperature is gradually decreased during austempering in the industrial conditions for the stable operation without meanders. Excellent mechanical properties can be obtained by controlling the austempering start temperature between 445 and 460 °C. On the contrary, the properties deteriorate in case of the austempering start temperature over 470 °C although the amount of retained austenite is the same or slightly larger than the material which exhibits excellent properties. This is because the retained austenite is less stable in the high‐temperature austempered material caused by less bainite transformation.     

<Emphasis Type="Italic">In-situ</Emphasis> reactive processing of nickel aluminides by laser-engineered net shaping

Nickel aluminide intermetallics (e.g., Ni₃Al and NiAl) are considered to be attractive materials for high-temperature structural applications. Laser-engineered net shaping (LENS) is a rapid prototyping process, which involves laser processing fine metal powders into three-dimensional shapes directly from a computer-aided design (CAD) model. In this work, an attempt has been made to fabricate aluminide intermetallic compounds via reactive in-situ alloying from elemental powders using the LENS process. In-situ reactive alloying was achieved by delivering elemental Ni and Al powders from two different powder feeders, eliminating segregation observed in the samples deposited by using the premixed elemental powders. Nickel aluminides of various compositions were obtained easily by regulating the ratio of their feed rates. The aluminide deposits exhibited a high solidification and subsolidus cracking susceptibility and porosity formation. The observed porosity resulted from a water-atomized Ni powder and can be minimized or eliminated by the use of a N₂-gas-atomized Ni powder of improved quality. Cracking was due to the combined effect of the high thermal stresses generated from the LENS processing and the brittleness of the intermetallics. Crack-free deposits were fabricated by preheating the substrate to a temperature of 450 °C to 500 °C during LENS processing. Compositionally graded Ni-Al deposits with a gradient microstructure were also produced by the in-situ reactive processing.     

The effect of grain size and retained austenite on the ductile-brittle transition of a titanium-gettered iron alloy

The effect of microstructural changes on the ductile-brittle transition temperature (DBTT) was studied in a titanium-getter ed Fe-8Ni-2 Mn-0.15 Ti alloy. A fairly strong grain size dependence of the transition temperature, 8°C/mm^?1/2, was found. Grain size refinement from 38 μm (ASTM #6.5) to 1.5 μm (ASTM #15.5) through a four-step thermal treatment lowered the transition temperature by 162°C. A small amount of retained austenite was introduced into this grain-refined microstructure, and the transition temperature was reduced by an additional 120 ~ 150°C. The reduction of the DBTT due to retained austenite was smaller when the austenite was in a large-grained structure (64°C). The distribution and stability of retained austenite were also studied.     

Third Generation 0.3C-4.0Mn Advanced High Strength Steels Through a Dual Stabilization Heat Treatment: Austenite Stabilization Through Paraequilibrium Carbon Partitioning

In excess of 30 vol. pct austenite can be retained in 0.3C-4.0Mn steels subjected to a dual stabilization heat treatment (DSHT) schedule—a five stage precisely controlled cooling schedule that is a variant of the quench and partition process. The temperature of the second quench (stage III) in the DSHT process plays an essential role in the retained austenite contents produced at carbon-partitioning temperatures of 723 K or 748 K (450° C or 475 °C) (stage IV). A thermodynamic model successfully predicted the retained austenite contents in heat-treated steels, particularly for a completely austenitized material. The microstructure and mechanical behavior of two heat-treated steels with similar levels of retained austenite (~30 vol. pct) were studied. Optimum properties—tensile strengths up to 1650 MPa and ~20 pct total elongation—were observed in a steel containing 0.3C-4.0Mn-2.1Si, 1.5 Al, and 0.5 Cr.