Precipitation hardened (PH) martensitic grade stainless steels are commonly used in additive manufacturing (AM) processes, but the heat treatment response can vary depending upon the powder feedstock ...composition. As-built AM 17–4 PH grade stainless steel fabricated using argon and nitrogen atomized feedstocks in separate powder bed fusion systems responded differently to standard heat treatment cycles. Materials fabricated from argon atomized feedstocks, containing low levels of nitrogen (0.01 wt%) and retained austenite (< 1%), responded as expected to standard solutionizing and aging heat treatment cycles. In contrast, materials fabricated from nitrogen atomized feedstocks, containing between 0.06 and 0.12 wt% nitrogen and up to 81% retained austenite, did not display peak aging with standard heat treatments and deviated from the expected overaging response with increasing aging temperatures. At the highest nitrogen and retained austenite levels, peak aging is found to occur at temperatures in excess of 680 °C, even though the material still contains retained austenite levels in the 20% range. These unexpected changes in the heat treat response are closely linked to differences in the nitrogen composition of the powder feedstock. Changes in the Ni and Cr equivalent values determined by other primary alloying elements also impact the heat treat response, even though the alloy compositions still fall within standard alloy element composition ranges.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
In this study, a new precipitation hardening stainless steel (PHSS), C-X stainless steel, was manufactured using selective laser melting (SLM) technology. Following SLM fabrication, a series of heat ...treatments were applied to improve the mechanical properties of the as-built samples. The microstructure precipitates distribution and evolution, and mechanical properties of SLM C-X stainless steels in the as-built and heat-treated conditions were systematically studied using scanning electron microscope (SEM), X-ray diffraction (XRD), and transmission electron microscope (TEM). The XRD spectrum revealed that solution treatment resulted in the formation of a complete martensite phase, and a reverted austenite (γ’) phase formed after aging treatment. The TEM analysis indicated that numerous dislocations and nanoprecipitates were dispersed within the martensite matrix for both the as-built and aged samples. The rod-like NiAl precipitates with a size range of 3–25 nm for the as-built samples and 7–30 nm for the solution-aged samples were determined through high-resolution TEM (HRTEM), selected area electron diffraction (SAED), and energy-dispersive X-ray spectroscopy (EDS). Furthermore, the microhardness of the SLM C-X stainless steel parts was found to significantly improve from 350 HV0.2 in the as-built state to 510 HV0.2 in the solution-aged state. The ultimate tensile strength (UTS) of the SLM C-X stainless steel parts also increased from 1043 MPa in the as-built state to 1601 MPa after solution-aging heat treatment.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Today, a large number of different steels are being processed by Additive Manufacturing (AM) methods. The different matrix microstructure components and phases (austenite, ferrite, martensite) and ...the various precipitation phases (intermetallic precipitates, carbides) lend a huge variability in microstructure and properties to this class of alloys. This is true for AM-produced steels just as it is for conventionally-produced steels. However, steels are subjected during AM processing to time-temperature profiles which are very different from the ones encountered in conventional process routes, and hence the resulting microstructures differ strongly as well. This includes a very fine and highly morphologically and crystallographically textured microstructure as a result of high solidification rates as well as non-equilibrium phases in the as-processed state. Such a microstructure, in turn, necessitates additional or adapted post-AM heat treatments and alloy design adjustments. In this review, we give an overview over the different kinds of steels in use in fusion-based AM processes and present their microstructures, their mechanical and corrosion properties, their heat treatments and their intended applications. This includes austenitic, duplex, martensitic and precipitation-hardening stainless steels, TRIP/TWIP steels, maraging and carbon-bearing tool steels and ODS steels. We identify areas with missing information in the literature and assess which properties of AM steels exceed those of conventionally-produced ones, or, conversely, which properties fall behind. We close our review with a short summary of iron-base alloys with functional properties and their application perspectives in Additive Manufacturing.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Precipitation hardening maraging steels possess of great importance with attractive properties in various industrial applications. However, a major confusion about the fine precipitate evolutions and ...the relationships between the strengthened particles and mechanical properties of PH13–8Mo steels under various heat treatment conditions remains to be further investigated. To shed light on the direct relationships between the fine nano-phase and mechanical properties, a combination of thermodynamics predictions and experimental measurements were carried out to model the issues. The investigation results reveal that the coherent NiAl nano-precipitates distributed on the lath-like martensite matrix firstly presented a steady growth and then a significant coarsening of approximately 9 nm at 593 °C for 5 h. The acicular or block-like diffusion-controlled reverted austenite enriched in Ni element with a tendency of growth was clarified, which maintains the K–S orientation relationship with martensite matrix after diverse aging treatments. The current studied PH13–8Mo steel displays comparable mechanical properties despite over-aging conditions, and the sharp drops in hardness and the steady increment in impact energy were systematically examined. The excellent work hardening behaviors of the present steel were modeled, which indicates that the modified Ludwik model displays significantly improved agreement with the experimental data than the widely accepted Hollomon model. The five strengthening mechanism models are discussed and the C–O-M model exhibits high consistency with the experimental data with a value of 652 MPa. The growth of nano-precipitates maybe prohibits the dislocations from cutting the particles, which promotes the hardening behaviors. This work offers a valuable reference for further quantitively experimental illustrates.
•The present investigated PH13–8Mo steel displays promising balanced mechanical properties despite over-aging conditions.•The modified Ludwik model is more suitable to predict the work hardening behavior.•The fine NiAl precipitate evolutions under various heat treatment are systematically investigated.•The current estimation results of the C–O-M model exhibit strong applicability to the tensile yield strength after the five strengthening mechanisms' discussion.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Laser additive manufacturing is attractive for the production of complex, three-dimensional parts from metallic powder using a computer-aided design model
. The approach enables the digital control ...of the processing parameters and thus the resulting alloy's microstructure, for example, by using high cooling rates and cyclic re-heating
. We recently showed that this cyclic re-heating, the so-called intrinsic heat treatment, can trigger nickel-aluminium precipitation in an iron-nickel-aluminium alloy in situ during laser additive manufacturing
. Here we report a Fe19Ni5Ti (weight per cent) steel tailor-designed for laser additive manufacturing. This steel is hardened in situ by nickel-titanium nanoprecipitation, and martensite is also formed in situ, starting at a readily accessible temperature of 200 degrees Celsius. Local control of both the nanoprecipitation and the martensitic transformation during the fabrication leads to complex microstructure hierarchies across multiple length scales, from approximately 100-micrometre-thick layers down to nanoscale precipitates. Inspired by ancient Damascus steels
-which have hard and soft layers, originally introduced via the folding and forging techniques of skilled blacksmiths-we produced a material consisting of alternating soft and hard layers. Our material has a tensile strength of 1,300 megapascals and 10 per cent elongation, showing superior mechanical properties to those of ancient Damascus steel
. The principles of in situ precipitation strengthening and local microstructure control used here can be applied to a wide range of precipitation-hardened alloys and different additive manufacturing processes.
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FZAB, GEOZS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Whereas conventional precipitation hardening is well-known to feature a single hardness peak, recently, double-peak precipitation hardening was observed, where the first peak hardness is higher than ...the second conventional one, thus offering a new approach to strengthen materials. Yet, classical precipitation strengthening models fail to predict such high strengthening in the early aging stage. In this work, molecular dynamics simulations were firstly performed to obtain a realistic dislocation-precipitate interaction law at the nano-scale, which was introduced into the discrete dislocation dynamics (DDD) method so as to investigate the precipitation hardening effects at the micro-scale. The DDD simulations correctly predict the double-peak hardening, namely, the critical resolved shear stress (CRSS) for a dislocation passing through a precipitate field first decreases, then increases, and finally decreases, as the precipitate radius rp increases. Then, a precipitate shearing model was developed, which agrees well with the DDD simulations and experimental observations. Based on the DDD simulations and theoretical analysis, the three CRSS regimes were found to be controlled by coherency strengthening (CRSS∝rp−1/2), chemical strengthening (CRSS∝−rp−1) and Orowan mechanism (CRSS∝rp−1), respectively. Finally, a universal law for the inverse relation between the CRSS and precipitate size at the second, conventional peak was unveiled, while the first peak was found to occur favorably for rapid precipitation in the early aging stage. This work provides new insights into precipitation hardening in general and double-peak hardening in particular, which are of great importance for alloy design.
•A dislocation-precipitate interaction law was obtained by atomic simulations, and fed into discrete dislocation dynamics (DDD) method.•DDD simulations correctly predict the double-peak hardening.•A precipitate shearing model was developed to describe both the DDD simulations and experimental observations.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Precipitation hardening stainless steel 17-4PH coatings were successfully deposited on the AISI 304 substrate via friction surfacing (FS) for enhancing the surface hardness. Through FS, the ...incoherent precipitates (Cu and CrN, 25–50 nm) originally existed in the as-received (AR) 17-4PH consumable rod (345 HV1) were re-dissolved into the martensitic matrix of the FSed 17-4PH coatings (400 HV1). The strain incompatibility caused by severe plastic deformation (SPD) in FS induced the formation of Cr-rich (Ni and Cu-depleted) δ-ferrite (1.8–2.7%) along the plastic flow direction with stress concentration. The shear deformation and dynamic recrystallization (DRX), as evidenced by the−J shearing and RTc’ recrystallization components in the FSed coatings, refined the grain size of the prior austenite. Increase in hardness of the FSed coatings was mainly attributed to the high dislocation density introduced by SPD. There was 4.8% acicular reversed austenite formed displacively, which possessed Kurdjumov-Sachs (K-S) relationship with surrounding martensite in the AR 17-4PH rod after solutionized treatment (ST) and aging (H900). The hardness of the AR 17-4PH after H900 was increased (439 HV1) because of the increment of dislocation density and micro-strain in the martensitic matrix introduced by the early-stage precipitation of coherent Cu. Owing to existence of δ-ferrite in the FSed coatings, some globular reversed austenite formed diffusively besides acicular ones after H900. There was more austenite (9.2%) in the FSed coating after H900 (456 HV1). The globular reversed austenite, which nucleated on the shear deformed martensitic grains in the FSed coatings, possessed {110}fcc//{120}bcc with adjacent δ-ferrite. SPD-induced dislocations in the FSed coating were annihilated during H900 leading to decrease in hardness but compensated by the precipitation hardening of coherent Cu. After FS, the micro-scale MnS and NbP2S8 inclusions in the AR 17-4PH were broken into nano-sized fragments which were dissolved into martensitic matrix during H900.
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•Nano-sized incoherent precipitates in 17-4PH rod were re-dissolved in the martensitic matrix of 17-4PH coatings.•Strain incompatibility in FS induced formation of little δ-ferrite along plastic flow direction in the coatings.•After aging, more reversed globular austenite formed diffusively in the coatings due to existence of δ-ferrite.•FS fragmented the micro-scale sulfides inclusions in 17-4PH rod into nano-sized.•The nano-sized inclusions in the coatings dissolved in martensitic matrix during aging.•Hardness of 17-4PH was improved by FS, and further improved by subsequent aging.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
In present research article, microstructure evolution in P91 steel and their weldments are reviewed in as-virgin and heat treatment and creep exposure condition. The thermal stability of P91 steel is ...derived from solid solution strengthening, sub-grain hardening and precipitation hardening. The initial microstructure plays an important role in deciding the mechanical properties of P91 steel and their weldment in long-term ageing and creep exposure condition. Effects of various alloying elements present in P91 steel and their related phase have also been discussed in details. The role of grain coarsening, Cr/Fe ratio, lath widening and dislocation density on creep rupture life of base metal and weldments are discussed. The combined effects of lath martensitic microstructure, residual stress and diffusible hydrogen content on performance of P91 steel material are also discussed.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Selective laser melting (SLM) is an additive manufacturing (AM) technique that uses powders to fabricate 3Dparts directly. The objective of this paper is to perform an experimental investigation of ...selective laser melted 17-4PH stainless steel. The investigation involved the influence of separate processing parameters on the density, defect, microhardness and the influence of heat-treatment on the mechanical properties. The outcomes of this study show that scan velocity and slice thickness have significant effects on the density and the characteristics of pores of the SLMed parts. The effect of hatch spacing depends on scan velocity. The processing parameters, such as scan velocity, hatch spacing and slice thickness, have effect on microhardness. Compared to the samples with no heat-treatment, the yield strength of the heat-treated sample increases significantly and the elongation decreases due to the transformation of microstructure and the changes in the precipitation strengthening phases. By a combination of changes in composition and precipitation strengthening, microhardness improved.
•The separate processing parameters' influence of SLM is investigated.•The RN and Dm distribution was used to characterize the defect.•The Cu precipitation phase plays an important role in changing the microhardness.•The impact of changes in composition and precipitation is proposed.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
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