Medium Mn steels possess a composite like microstructure containing multiple phase constituents like metastable austenite, ferrite, δ-ferrite and α′-martensite with a wide range of fractions for each ...constituent. The high mechanical contrast among them and the deformation-driven evolution of the microstructure lead to complex fracture mechanisms. Here we investigate tensile fracture mechanisms of medium Mn steels with two typical types of microstructures. One group consists of ferrite (α) plus austenite (γ) and the other one of a layered structure with an austenite-ferrite constituent and δ-ferrite. Samples with the first type of microstructure show a dimple-type fracture due to void formation primarily at the ferrite/strain-induced α′-martensite (α′) interfaces. In contrast, the fracture surface of δ-ferrite containing steels shows a combination of cleavage in δ-ferrite and dimple/quasi-cleavage zones in the γ-α (or γ/α′-α) constituent. The embrittlement of δ-ferrite is due to the formation of B2 ordered phase. Failure of these samples is govern by crack initiation related to δ-ferrite and crack-arresting ability of the γ-α layers. Austenite stability is critical for the alloys' fracture resistance, in terms of influencing void growth and coalescence for the first type of microstructure and crack initiation and termination for the microstructure containing δ-ferrite. This effect is here utilized to increase ductility and toughness. By tailoring austenite stability towards higher fracture resistance, the total elongation of δ-ferrite containing steels increases from ∼13% to ∼33%. This approach opens a new pathway towards an austenite-stability-controlled microstructural design for substantially enhanced damage tolerance in steels containing metastable austenite and δ-ferrite.
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•δ-Ferrite in welded seam and fusion line was successfully suppressed via inserting Ni interlayer.•Full martensite was obtained in welded seam and fusion line of Al-Si coated PHS with ...Ni interlayer.•Al content had a similar content in welded seam and fusion line without segregation.•Tensile strength of welded joint with Ni interlayer was similar to that of removing Al-Si coating.
Mechanical properties of Al-Si coated press-hardened steel laser welded joint can not meet requirement due to the presence of δ-ferrite. This study investigated the suppression mechanism of δ-ferrite using a nickel interlayer during fiber laser welding. The fraction of primary solidification δ-ferrite was reduced to 57% with a 5.85% Ni in welding pool, and completely transformed into austenite during peritectic reaction (L + δ → γ) and solid phase transformation (δ → γ). Full martensite microstructure was obtained in welded seam and fusion line without Al-segregation. Tensile strength of welded joint had a ∼17.1% improvement compared to as-received welded, similar to that of removing Al-Si coating.
Medium Mn steels are promising candidates for the third-generation advanced high strength steels. However, it shows a distinct limit of mechanical property under conventional hot-rolling and ...intercritical annealing procedures. Aiming at improving the combination of strength, ductility and toughness, the present study proposed a multi-alloyed design principle together with a direct intercritical rolling process. The results show that the microstructure of the hot rolled steel consisted of wide delta ferrite, lath martensite, twinned-martensite and coarse retained austenite with a low stability, corresponding to a product of strength and elongation (PSE) of 24.0 GPa% and an impact toughness of 13.8 J/cm2 at −40 °C. In contrast, the intercritical rolling produced a laminated microstructure containing thin delta ferrite, submicron recrystallized ferrite and retained austenite. The PSE and impact toughness at −40 °C simultaneously increased to 46.1 GPa% and 46.7 J/cm2, respectively, which was also much higher than the hot rolled steel subjected to low temperature and high temperature annealing. The significant enhancement of mechanical property was mainly attributed to the highest content and optimal stability of retained austenite caused by the submicron grain size, high density of dislocations, granular morphology, and elemental enrichment. The optimized austenite stability provided persistent transformation-induced plasticity (TRIP) effect for work hardening and damage resistance. Moreover, the nano-sized VC precipitates in the intercritical rolled steel played a significant role in reaching an ultrahigh yield strength of ∼1 GPa. This study not only provides a very simple pathway to improve the mechanical property of medium Mn steel, but also sheds light on the design and development of high-performance materials.
The mechanical and corrosion properties of gas metal arc additive manufacturing (GMA-AM) 316L could be optimized by modifying the volume fractions of sigma (σ) and delta-ferrite (δ) phases through ...heat treatment. Results show that the heat treatment at 1000°C to 1200°C for one hour will not obvious influence the morphology of grains in steel but largely influence the contents of σ and δ phases. The heat treatment at 1000°C effectively increases the amount of σ phase in steel, causing both increase of UTS and YS but decrease of El and RA. The heat treatment at 1100°C to 1200°C completely eliminates σ phase, leading to the decrease of UTS and YS but increase of El and RA. The σ phase has better strengthening effect than δ phase, but which may degrade ductility and increase the possibility for cracks generation in steel. Meanwhile, limiting the number of both σ and δ phases through heat treatment can improve the corrosion resistance of steel. And σ phase appears more detrimental impact on degradation the corrosion resistance of steel than δ phase.
•Strain accommodation between delta-ferrite and martensite improves toughness.•Three-fold increase in ductility is noticed while retaining the original strength.•Multiple strengthening mechanisms ...provide innovative method to increase toughness.
The presence of the delta-ferrite phase in steel is often considered detrimental, as it reduces both ductility and toughness. However, this study reports that in a B2-NiAl precipitation-reinforced modified PH13-8Mo steel, delta-ferrite can impart a toughening effect similar to that of retained austenite, exhibiting a three-fold increase in ductility without sacrificing material strength. Using several analysis techniques, the toughening effect was attributed to strain accommodation between delta-ferrite and martensite. This was owing to multiple strengthening mechanisms in delta-ferrite, including precipitation, orientation, solid solution, fine grain, and the elimination of grain boundary brittleness. These findings provide an innovative method for improving the toughness of maraging steels.
In this work, 316L cubes were produced by Directed Energy Deposition (DED) process. To evaluate the effect of deposition patterns on the microstructure, mechanical performance and residual stress of ...316L samples, two different deposition strategies are selected (67° and 90°). The general microstructure is revealed, and then the effect of deposition pattern on the microstructure of 316L alloy is evaluated through the Primary Cellular Arm Spacing (PCAS) analysis. The cooling rate in each sample is estimated according to the PCAS values. Interestingly, it is found that by increasing the rotation angle per layer, the PCAS value decreases as a consequence of increment in the cooling rate. On the other hand, in both cases, by increasing the distance from the substrate, due to the changes in cooling mechanisms, the cooling rate at first decreases and then at the last layers increases again. The phase composition analysis of 316L samples confirms the predictions that suggested the presence of residual δ-ferrite in the final microstructure. In fact, the final microstructure of samples is characterized by austenitic dendrites together with some residual δ-ferrite in the interdendritic regions. Moreover, the microstructural evaluations exhibit that during the DED process, some metallic inclusions are formed within the 316L samples that consequently deteriorates their mechanical properties. Tensile results show that the samples with 90° rotation per layer have a better mechanical performance such as slightly higher ultimate tensile strength and almost 35% higher elongation to fracture, mainly owing to their finer microstructure and slightly less oxide content. However, in both cases, the elongation of the 316L samples is lower than the typical elongation of this material produced via DED. This discrepancy is found to be as a result of higher inclusions contents in the samples produced in this work with respect to those of literature. Lastly, it is found that the residual stresses on the top surfaces are similar for both deposition patterns, although higher stress values are observed on the lateral surfaces of the cubes produce using the 90° rotation per layer.
Metal additive manufacturing (AM) offers exceptional design freedom, but its high thermal gradients often generate non-equilibrium microstructures with chemical and interfacial instabilities. Steels ...that solidify as δ-ferrite often experience a further solid-state phase transformation to austenite during AM. The detailed nature of this phase transformation during AM is yet to be fully understood. Duplex stainless steel, which is known for its unique combination of high corrosion resistance and mechanical properties, is a suitable alloy to further study this phase transformation.
The current study aims to gain novel insights into solid-state phase transformations and mechanical properties of duplex stainless steels during laser powder-bed fusion (LPBF). As-printed microstructures exhibit significant deviations when compared to conventionally manufactured counterparts in terms of phase balance and morphology, elemental partitioning, and interface character distribution. During LPBF, only a small fraction of austenite forms, mostly at the ferrite-ferrite grain boundaries, via a phase transformation accompanied by diffusion of interstitials. Austenite/ferrite boundaries are shown to terminate on {100}F//{111}A planes. This is due to the character of parent ferrite-ferrite boundaries which is dictated by the sharp texture and geometry of austenite grains induced by directional solidification and epitaxial growth of ferrite. Benchmarking mechanical properties against a wrought counterpart demonstrates that AM offers high strength but relatively low ductility and impact toughness. A short heat treatment reverts the microstructure back to its equilibrium state resulting in balanced tensile and toughness properties, comparable to or even better than those of wrought counterparts.
Directed energy deposition (DED) has been employed to produce AISI 316L samples. Microstructure and primary cellular arm spacing (PCAS) are studied analysing the relationship with the cooling rate at ...the different heights of DED processed 316L stainless steel sample. It is found that, by increasing the deposition distance from the substrate, the PCAS of the sample increases from 2.9 to 4.5 μm, as a consequence of the decreased cooling rate and thermal gradient. On the other hand, in the last deposited layers, the PCAS of the sample decreases from 4.5 to 3.3 μm, because of the changes in cooling mechanisms. The phase composition of samples after deposition is revealed and compared with the predictions based on the Schaeffler and Pseudo-binary diagrams. It is revealed that the final microstructure is characterized by austenitic dendrites together with some residual delta ferrite located at dendritic arms location. Lastly, the effect of using fresh or recycled powders, on the microstructure and mechanical properties of DED 316L stainless steel parts is investigated. It is found that the samples fabricated using recycled powders have rather similar tensile strength levels, but much lower elongation than those produced using fresh powder due to a lower inclusions content and of their average lower size. The nature of these inclusions is discussed as well as the reason for their increase both in numbers and size.
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