Showing papers on "Austenite published in 2019"

Effect of heat treatment on microstructure and mechanical properties of 316L steel synthesized by selective laser melting

Abstract: The influence of annealing at different temperatures (573, 873, 1273, 1373 and 1673 K) on the stability of phases, composition and microstructure of 316L stainless steel fabricated by SLM has been investigated and the changes induced by the heat treatment have been used to understand the corresponding variations of the mechanical properties of the specimens under tensile loading. Annealing has no effect on phase formation: a single-phase austenite is observed in all specimens investigated here. In addition, annealing does not change the random crystallographic orientation observed in the as-synthesized material. The complex cellular microstructure with fine subgrain structures characteristic of the as-SLM specimens is stable up to 873 K. The cell size increases with increasing annealing temperature until the cellular microstructure can no longer be observed at high temperatures (T ≥ 1273 K). The strength of the specimens decreases with increasing annealing temperature as a result of the microstructural coarsening. The excellent combination of strength and ductility exhibited by the as-synthesized material can be ascribed to the complex cellular microstructure and subgrains along with the misorientation between grains, cells, cell walls and subgrains.

Wire and arc additive manufacturing of HSLA steel: Effect of thermal cycles on microstructure and mechanical properties

Abstract: Wire and arc additive manufacturing (WAAM) is a viable technique for the manufacture of large and complex dedicated parts used in structural applications. High-strength low-alloy (HSLA) steels are well-known for their applications in the tool and die industries and as power-plant components. The microstructure and mechanical properties of the as-built parts are investigated, and are correlated with the thermal cycles involved in the process. The heat input is found to affect the cooling rates, interlayer temperatures, and residence times in the 800–500 °C interval when measured using an infrared camera. The microstructural characterization performed by scanning electron microscopy reveals that the microstructural constituents of the sample remain unchanged. i.e., the same microstructural constituents—ferrite, bainite, martensite, and retained austenite are present for all heat inputs. Electron backscattered diffraction analysis shows that no preferential texture has been developed in the samples. Because of the homogeneity in the microstructural features of the as-built parts, the mechanical properties of the as-built parts are found to be nearly isotropic. Mechanical testing of samples shows excellent ductility and high mechanical strength. This is the first study elucidating on the effect of thermal cycles on the microstructure and mechanical properties during WAAM of HSLA steel.

Tailoring the microstructure and mechanical properties of AISI 316L austenitic stainless steel via cold rolling and reversion annealing

Abstract: Tensile properties of cold rolled AISI 316L stainless steel after full reversion of martensite to austenite, recrystallization of retained austenite, and grain growth were studied at 850, 950, and 1050 °C. At higher temperatures, it was found that the kinetics of the reversion and recrystallization processes enhance but coarser grain sizes will be obtained at the end of recrystallization. At 1050 °C, appreciable grain growth was observed after the completion of the recrystallization process, which was not the case for a low temperature of 850 °C. At the stage of full recrystallization, by decreasing the annealing temperature, the yield stress (YS) and the ultimate tensile strength (UTS) values increased and total elongation decreased, which was related to the grain size strengthening by the Hall-Petch law. However, the Hall-Petch slope for the UTS was found to be much smaller than that of YS, which reveals that YS has greater grain size dependency. The latter was ascribed to the improved work-hardening behavior and enhanced transformation-induced plasticity (TRIP) effect by coarsening of grain size. To obtain high-strength and ductile steel with tensile toughness higher than 300 MJ/m3 and yield ratio of ∼0.5, the average grain size of ∼3 μm was found to be desirable.

Ultrahigh-performance TiNi shape memory alloy by 4D printing

Abstract: For additively manufactured components, it's widely accepted to have high enough energy input to facilitate nearly full density and low enough energy input to avoid cracking tendency. In this work, ultrahigh-performance Ti 50.6 Ni 49.4 (at.%) shape memory alloy (SMA) was manufactured by selective laser melting (SLM) under high enough energy inputs (155–292 J/mm 3 ). The microstructure, phase transformation behaviors, mechanical and shape memory properties of the SLM-manufactured SMA were investigated by various characterization methods of X-ray diffraction, scanning and transmission electron microscopies, differential scanning calorimetry, room temperature and stress-controlled cyclic tensile tests, etc. Results show that the martensite content and the austenite and martensitic transformation temperatures decrease with the decrease of laser energy input (the increase of laser scanning speed). Interestingly, the SLM-manufactured SMA exhibits ultrahigh tensile strength of 776 MPa and elongation of 7.2% under room-temperature tensile condition. In addition, stress-controlled cyclic tensile tests under 400 MPa indicate that the SLM-manufactured SMA has ultrahigh shape memory effect of 98.7% recovery ratio and 4.99% recoverable strain after ten times loading-unloading cycle. The ultrahigh mechanical and shape memory properties are associated to the combined effects of dispersedly distributed nano-sized Ti 2 Ni precipitates, ultrafine grains and profuse dislocations in the SLM-manufactured SMA. This work substantiates, for the first time, high enough energy input in SLM can be applied to manufacture ultrahigh-performance TiNi SMAs.

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.

Effect of heat treatment on the microstructure and mechanical properties of maraging steel by selective laser melting

Abstract: This work investigates the solution phenomenon, aging behavior and room-temperature mechanical properties of maraging steel manufactured by selective laser melting (SLM). Different heat treatment experiments, including solution treatment (ST), direct aging treatment (DAT) and solution + aging treatment (SAT) are designed. Microstructure analysis indicates that ST and SAT will eliminate the cellular and lath structures, but DAT has little effect on these. The content of austenite increases with the addition of DAT temperature and holding time. While austenite is almost undetectable in ST and SAT samples. Meanwhile, both the elongation and toughness of the samples with DAT gain a slight improvement with the temperature increasing. Importantly, DAT yields similar microhardness, tensile strength and impact toughness to SAT, although the resultant microstructures are completely different. The results demonstrate that DAT can achieve the similar mechanical properties to SAT samples. Samples with high mechanical properties (microhardness of 653.93 HV and ultimate strength of 2126.30 MPa) have been obtained by DAT at 520 °C for 6 h as well as solution treatment at 900 °C for 1 h and aging treatment at 520 °C for 6 h. This investigation reveals the evolution regularity of microstructure, microhardness, tensile performance and impact toughness of maraging steel manufactured by SLM after different heat treatments.

Process-Structure-Property Relationships of AISI H13 Tool Steel Processed with Selective Laser Melting.

Abstract: Due to a good combination of high hardness, wear resistance, toughness, resistance to high operating temperatures, and fairly low material cost, AISI H13 tool steel is commonly used in the manufacture of injection molds. Additive manufacturing (AM) such as selective laser melting (SLM), due to the layer-wise nature of the process, offers substantial geometric design freedom in comparison with conventional subtractive manufacturing methods, thereby enabling a construction of complex near-net shape parts with internal cavities like conformal cooling channels. The quality of SLM-manufactured parts mainly depends on the part geometry, build orientation and scanning strategy, and processing parameters. In this study, samples of H13 tool steel with a size of 10 × 10 × 15 mm3 were SLM-manufactured using a laser power of 100, 200, and 300 W; scanning speed of 200, 400, 600, 800, 1000, and 1200 mm/s; and hatch spacing of 80 and 120 µm. A constant layer thickness of 40 µm, 67° scanning rotation between subsequent layers, and a stripe scanning strategy were maintained during the process. The samples were built considering a preheating of 200 °C. The relative density, surface roughness, crack formation, microstructure, and hardness were evaluated. The relative density is shown to increase with increasing the volumetric energy density up to a value of about 60 J/mm3 and then no significant increase can be pointed out; the maximum relative density of 99.7% was obtained. A preheating of 200 °C generally aids to increase the relative density and eliminate the crack formation. The microstructure of built samples shows fine equiaxed cellular-dendritic structure with martensite and some retained austenite. The microhardness of the as-built samples was found to vary from 650 to 689 HV 0.2, which is comparable to a conventionally produced H13 tool steel.

High strength and ductility combination in nano-/ultrafine-grained medium-Mn steel by tuning the stability of reverted austenite involving intercritical annealing

Abstract: The structure–property relationship in 0.06C–5.5Mn steel subjected to different annealing temperatures and time was studied. Mn played a stronger effect on stabilizing austenite in comparison with Ni, and low-C medium-Mn steel possessed excellent hardenability. The reverse transformation of martensite to austenite occurred during intercritical annealing, and the volume fraction was first increased and then decreased on increasing annealing temperature or prolonging annealing time, indicative of change in thermal stability by element partitioning and coarsening of grain size. Correspondingly, the elongation was first increased and then decreased, consistent with the variation in the stability of reverted austenite. The yield strength was gradually decreased because of several factors, including recrystallization of α′ martensite, decreased stability of reverted austenite, and coarse grain size. The maximum product of strength and ductility was obtained on annealing at 650 °C for 10 min, which was attributed to the optimal stability of reverted austenite rather than the highest volume fraction, and tensile strength and elongation were 1120 MPa and 23.3%. The strain partitioning behavior of two phases was elucidated by analyzing Luders straining and continuous work hardening after yield point elongation, and the deformation mechanism was strongly related to the stability of reverted austenite.

Formation of strain-induced martensite in selective laser melting austenitic stainless steel

Abstract: The austenitic stability of selective laser melted austenitic stainless steel was investigated by the tensile tests in the 80–300 K temperature range. The selective laser melting process greatly improved the yield strength, but severely suppressed the stain-induced α′ martensitic transformation compared with the conventional austenitic stainless steel. The high density of low-angle grain boundaries and fine cellular microstructures were observed in the selective laser melted austenitic stainless steel, which suppressed dislocation slip and deformation twinning during deformation, resulting in the reduction in the nucleation sites of α′ martensite and the enhancement of the austenitic stability.

The study of casting defects in steel 35HGSL

Abstract: In this research work the classification of defects of castings obtained by electric arc smelting is considered. Of particular interest to researchers is the rock-like and naphthalene fractures. A stone-like fracture is characterized by a clearly defined uniform surface over which the fracture occurs. Grain boundaries are partially soluble in the austenite phase, consisting of fine individual particles or films formed from molten eutectics. It is also worth noting that most often the stone-like fracture is observed at the grain boundaries.

Influence of different heat-affected zone microstructures on the stress corrosion behavior and mechanism of high-strength low-alloy steel in a sulfurated marine atmosphere

Abstract: The stress corrosion cracking (SCC) behavior and mechanism of the simulated heat-affected zone (HAZ) of high-strength low-alloy (HSLA) steel in a sulfurated marine atmosphere were surveyed in detail using electrochemical measurements and slow strain rate tensile (SSRT) tests combined with microstructure analysis. The SCC of the simulated HAZs is controlled by both anodic dissolution (AD) and hydrogen embrittlement (HE), which are attributed to the synergistic effect of Cl− and SO42−, as Cl−-induced localized dissolution causes microcrack initiation, and SO42--catalyzed acid regeneration facilitates microcrack propagation. The intercritical HAZ and fine-grained HAZ present high crack numbers because of the high amount of prior austenite grain boundaries (PAGBs), lath bainite boundaries (LBBs), and martensite/austenite (M/A) constituents, which act as preferential sites for hydrogen trapping and crack initiation. However, coarse-grained HAZ exhibits the highest SCC susceptibility because of the coarse PAGBs, wide lath bainites (LBs), and high local dislocation density, which promote crack propagation.

Laser Powder Bed Fusion of Stainless Steel Grades: A Review

Abstract: In this paper, the capability of laser powder bed fusion (L-PBF) systems to process stainless steel alloys is reviewed. Several classes of stainless steels are analyzed (i.e., austenitic, martensitic, precipitation hardening and duplex), showing the possibility of satisfactorily processing this class of materials and suggesting an enlargement of the list of alloys that can be manufactured, targeting different applications. In particular, it is reported that stainless steel alloys can be satisfactorily processed, and their mechanical performances allow them to be put into service. Porosities inside manufactured components are extremely low, and are comparable to conventionally processed materials. Mechanical performances are even higher than standard requirements. Micro surface roughness typical of the as-built material can act as a crack initiator, reducing the strength in both quasi-static and dynamic conditions.

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.

Laser metal deposition as repair technology for 316L stainless steel: Influence of feeding powder compositions on microstructure and mechanical properties

Abstract: Laser metal deposition was used to repair grooves on 20 mm thickness 316L stainless steel plates using two different 316L stainless steel commercial powders – Fe-0.15C-11.8Cr-0.15Mn-0.2Ni-0.031P-0.56Si-0.05S (wt.%) powder and Fe-0.09C-17.05Cr-1.2Mn-11.28Ni-0.019P-0.46Si-0.09S (wt.%) powder. Good comprehensive performance of fine metallurgical bonding with were successfully achieved under optimized processing parameters. The microstructure and mechanical properties (micro-hardness, ultimate tensile strength, bending strength, low-temperature impact toughness) of the repaired specimens were investigated. Results indicated that chemical composition (different element contents) of the powders played an important role in determining the microstructure, phases and properties of the specimens. It was found that the microstructure of the specimens repaired with Fe-0.15C-11.8Cr-0.15Mn-0.2Ni-0.031P-0.56Si-0.05S (wt.%) powder was homogeneous and consisted of Cr-rich martensite while a number of cellular dendrite were presented in microstructure of specimens repaired with Fe-0.09C-17.05Cr-1.2Mn-11.28Ni-0.019P-0.46Si-0.09S (wt.%) powder and the repaired specimens consisted of ferrite and austenite. Due to solid solution strengthening, the average hardness of the specimens repaired with former powder was higher than that of the specimens with later powder. However, in the aspect of mechanical properties, LMD with Fe-0.09C-17.05Cr-1.2Mn-11.28Ni-0.019P-0.46Si-0.09S (wt.%) powder had better performance than the specimens repaired with another kind of powder. The relationship between the microstructure characteristics and mechanical performances of the repaired specimens was also discussed.

On the influence of deformation mechanism during cold and warm rolling on annealing behavior of a 304 stainless steel

Abstract: In the present study, a 304 stainless steel was deformed at different temperatures of room temperature (cold rolling) and 200 °C (warm rolling) for elucidating the distinct deformation mechanisms, and then, the influence of the deformed microstructures on the subsequent microstructural evolution and texture development during annealing treatments was studied. The results indicated that deformation-induced martensite transformation was the dominant deformation mechanism during cold rolling, and a mixed microstructure consisting of ~60% volume fraction of α'-martensite and deformed austenite was obtained. Deformation twinning of austenite dominated the deformation during warm rolling. The underlying reason was the increasing SFE with deformation temperature, which inhibited the martensitic transformation but promoted the formation of deformation twins. On annealing, the recrystallization of retained austenite in cold-rolled samples was retarded because of martensitic reversion compared to the warm-rolled samples. Completely recrystallized structure with average grain size of ~0.7 µm was obtained in both cold and warm-rolled samples after annealing at 800 °C. Texture evolution indicated that martensitic reversion enhanced the intensity of {110} texture in cold-rolled and annealed samples. However, the final recrystallization texture components were consisted of {110} and {113} regardless of rolling process. Furthermore, structure-property relationship was established, and combination of high strength and excellent plasticity can be obtained in both cold and warm-rolled sheets by tuning the subsequent annealing parameters.