Lath Boundaries Research Articles

Advantageous Implications of Reversed Austenite for the Tensile Properties of Super 13Cr Martensitic Stainless Steel.

The advantageous implications of the microstructure and volume fraction of reversed austenite for the tensile properties of super 13Cr martensitic stainless steel (13Cr SMSS) in an experiment with quenching and double-step tempering treatment in the temperature range of 550-750 °C were investigated. The results show that, with increases in one-step tempering temperature, the content of reversed austenite was enhanced considerably from 0.9% to 13.3%. The reversed austenite distributed in the martensitic lath boundary conformed to the (11¯1)γ//(011)α' and [011]γ//[1¯1¯1]α' Kurdjumov-Sachs orientation relationship with the matrix. When tempered at 675 °C for 3 h for the first stage and 600 °C for 2 h for the second stage, the maximum volume fraction of reversed austenite was approximately 13.3%, achieving uniform elongation of 10.4% and total elongation of 27.2%. Moreover, the product of strength and elongation (PSE) was 23.5 GPa·% higher than other samples. The outstanding combination of high strength and commendable plasticity was due to the phase transformation of the reversed austenite into secondary martensite during tensile straining. The reversed austenite consumed the plastic energy at the tip of the microcrack and made the crack tip blunt, which hindered the further propagation of the crack, consequently increasing the total elongation and improving toughness.

Effect of aging temperature on microstructure evolution and strengthening behavior of L-PBF 18Ni(300) maraging steel

This work investigated the microstructure evolution with aging temperature of laser-powder bed fusion ( L -PBF) 18Ni(300) maraging steel at different scales. Important issues of cell structure changes, precipitation sequence, and austenite reversion were evaluated. The dominant strengthening factor in each state was elucidated based on yield strength modeling. The as built fully martensitic matrix consists of dislocation cell structures, and its yield strength comes from the martensite lath and solid solution strengthening with comparable contributions. Aged at 440 °C, the cell walls kept stable and massive ω particles prominently strengthened matrix by shearing mechanism. At 490 °C, the cell walls were partially destroyed and austenite reversion just began along the lath boundary. Densely distributed η-Ni 3 Ti particles produced the peak strength dominantly by the Orowan mechanism, but the shearing process may also exist meanwhile. At 540 °C, cell walls further degraded and reverted austenite formed along the lath boundaries and around some retained cell walls. Its strength decreased mildly mainly due to the wider inter-particle spacing, but was still maintained quite high by Laves-Fe 2 Mo and η-Ni 3 Ti particles via the Orowan process. The reverted austenite with limited fraction had a minor role in its strength decrease, but improved ductility significantly. • Cell structures change, precipitation sequence and autenite reversion were discussed. • Precipitate types were rigorious identified based on crystallographic information. • Dominant strengthening factors in as built and aged states were elucidated.

Effect of microstructure on slip-induced crack initiation and early propagation of martensitic steel during high cycle fatigue

Martensitic steel presents high fatigue crack resistance due to its complex and hierarchical microstructure, but its mechanisms are unclear. This work experimentally investigated effect of the lath and the prior austenitic grain (PAGs) on crack initiation and the early propagation behavior in high cycle fatigue. It is found that micro cracks initiated from the slip bands in the interior of thin monocrystalline laths, in which the dislocation intensity zones deflected the crack path. Subsequently, the micro cracks coalesced through the lath boundaries, and resulted in an intermittent and staggered appearance of crack path in the PAGs. Finally, the crack propagation across the boundaries of the primary PAG are extremely slow and presents significant zigzag manners due to the misorientation of the laths in neighboring PAGs. The above observations demonstrate that the crack initiation and early propagation are significantly hindered by the multi-scaled microstructure of the martensitic steel.

Effects of Cu-rich phases on microstructure evolution and creep deformation behavior of a novel martensitic heat-resistant steel G115

In this work, effects of Cu-rich phases on the microstructure evolution and creep deformation behavior of G115 steels with 1 (1Cu steel) and 2 wt% Cu (2Cu steel) were systematically investigated under 160–220 MPa at 650 °C. The results showed that dispersedly distributed Cu-rich phases played a vital role in retarding the microstructural degeneration and improving the creep strength. The Cu-rich phases inside grains could induce dislocation tangle and prohibit dislocation annihilation during creep, but they could be dissolved into the matrix due to the cutting effect of mobile dislocation lines. The Cu-rich phases at grain boundaries could hinder the migration of martensitic lath boundaries and thus delayed the creep failure behavior. Moreover, the independent Cu-rich phases at grain boundaries could provide heterogeneous nucleation sites for Laves phases, which strengthening pinning effect. As a result, compared with the 1Cu steel, the creep threshold stress of the 2Cu steel was enhanced from 103.36 to 117.93 MPa, and the creep lifetime was also prolonged markedly.

Si segregation and relevant effect on the corrosion propagation in F/M steel exposed to liquid lead-bismuth eutectic

ABSTRACT Clarification of the relationship between oxygen diffusion and microstructural evolution in liquid lead-bismuth eutectic (LBE) is of great importance for understanding and consequently tailoring the oxidation process. In the present study, a Si-modified ferritic martensitic steel was exposed to oxygen-saturated LBE at 500°C at various times and the interface between the internal oxidation zone and matrix was investigated. Microstructural characterisation reveals that grain boundaries and martensitic lath boundaries provide a fast pathway for oxygen diffusion, resulting in the high amount of oxidation products, as well as cracks. The internal stress caused by the formation of oxidation products along grain boundary results in the proliferation of dislocations, which subsequently play as the new diffusion channel for oxygen. The chemical analysis demonstrates that Si preferably concentrates along the dislocations in a form of Si-rich oxide, suggesting the oxidation of Si is prior to the Cr-rich oxide.

The role of delta ferrite in hydrogen embrittlement fracture of 17-4 PH stainless steel

The role of δ-Fe in hydrogen embrittlement (HE) of 17-4 PH steel is studied in this work. Scanning Kelvin probe force microscopy result indicates that δ-Fe is a hydrogen trapping site. Accordingly, δ-Fe can reduce the hydrogen concentration of surrounding martensite and prior austenite grain boundaries (PAGBs) and imped the brittle fracture along lath boundaries and PAGBs, which can be beneficial to the HE resistance improvement. However, a cleavage fracture of δ-Fe can occur under the synergetic action of hydrogen-enhanced localized plasticity (HELP) and hydrogen enhancement of the strain-induced generation of vacancies (HESIV). These findings indicate a new path to improving HE resistance of high strength martensitic steels.

Characterization of High-Temperature Tensile Properties and Thermal Stability in Gas Tungsten Arc Welds of Ti-added Reduced Activation Ferritic/Martensitic Steel

In this study, the effects of Ti addition on the microstructure, the tensile properties at 550 ℃, and the thermal stability in gas tungsten arc (GTA) welds of reduced activation ferritic/martensitic (RAFM) steel were studied. Ti was added to promote precipitation of MX in order to enhance high-temperature properties. For reference, Ti-free RAFM (reference RAFM steel with the composition of 9Cr-1W-0.2V-0.1Ta-0.1C) was compared with 0.013 wt% Ti added RAFM steel (Ti-added RAFM). The addition of Ti contributed to the increase in the area fraction of MX precipitates (2.0 → 4.6 %) and a decrease in the average size of M23C6 (149 → 119 nm) in the base metal. After the cross-weld tensile test at 550 ℃, the tensile properties of Ti-added RAFM steel were superior to that of Ti-free RAFM steel. Both steels were fractured at inter-critical heat-affected zone (ICHAZ) showing the lowest hardness due to over-tempering. However, the higher area fraction of MX precipitates in the Ti-added RAFM produced more significant strengthening, compared to the Ti-free RAFM steel. After heat exposure at 550 ℃ for 500 h, Ti-added RAFM steel was highly resistant to degradation; hardness distribution and tensile properties were almost similar before and after thermal exposure. ICHAZ exhibited the substantial retention of fine laths and high density of dislocation with marginal recovery even after thermal exposure. It is conceivable that excellent thermal stability of Ti-added RAFM steel can be attributed to the high fraction of MX particles by suppressing lath boundary migration.

Ultrastrong and stress corrosion cracking-resistant martensitic steels

This study aims to reveal the atomic-scale effects of tempering on the complex substructures and stress corrosion cracking (SCC) resistance of high-strength martensitic steels. The SCC resistance and strength of boron-doped Fe-0.3C-0.3Si-1.0Mn-1.0Ni-0.5Cr (wt%) martensitic steel increase concurrently without low-temperature tempering. Notably, the degradation of SCC resistance caused by tempering is in contrast with the known effect. To explore this unexpected result, subboundaries inside the martensitic microstructure are investigated via atomic-nano-micro-scale analyses. The strongly segregated carbon at the lath boundaries during tempering is a precursor to the harmful cementite, which acts as severe SCC initiation sites. Eventually, intensive crack grew along the lath boundaries, deteriorating the SCC resistance of the material.

In-depth understanding of the relationship between dislocation substructure and tensile properties in a low-carbon microalloyed steel

At present, the microstructure-tensile property relationship of low carbon microalloyed steel has been widely investigated, mainly with the effects of microstructure type and its simple morphology including size and shape etc. The correlation between dislocation substructure and tensile properties, however, seems more essential and needs in-depth analysis. In this study, four typical Mn containing (from 1.08 to 1.77 wt%) low-carbon microalloyed steels were prepared by Thermo Mechanical Control Process, and the correlations among Mn content, dislocation substructure and tensile properties were investigated. A mixed microstructure consisting of polygonal ferrite, degenerated pearlite, granular bainitic ferrite and martensite/austenite (M/A) constituents formed in each steel, with highly complex dislocation configuration and distribution. For the 1.08% Mn steel, the dislocation substructures appeared mainly in dislocation wall distributing in the ferrite matrix of degenerated pearlite, and dislocation tangle in the granular bainitic ferrite around the M/A constituents and the bainitic ferrite lath boundary. As the Mn content increased from 1.45% to 1.77%, the dislocation cells appeared and increased. Simultaneously with the rising of Mn content, the mean equivalent diameter (MED) of all the ferrite matrix decreased. The ferrite grains whose MED was defined by the misorientation tolerance angles (MTAs) from 2 to 8°, featured a variety of dislocation substructures including dislocation cell boundaries, dislocation walls, bainitic ferrite lath boundaries, and dislocation tangles. They effectively governed the yield strength due to their determined close Hall–Petch relationship. Moreover, the significant increase in the amount of M/A constituents enhanced the strain hardening capacity, leading to an elevated tensile strength. Dislocation tangles exhibiting around the M/A caused a microstrain concentration, promoted additional microvoids and deteriorated the elongation.

Microstructural evolution and the effect on strength of G115 steel after long-term creep at 923 K

The microstructural evolution and the effect on the strength of G115 steel after creep under 923 K and 130 MPa for up to 7288 h were systematically investigated using OM, SEM and TEM. The results show that the variation tendency of total calculated strengthening contribution is in a good agreement with the measured hardness of G115 steel after creep at 923 K. A new nucleation mechanism of Laves phase nucleated adjacent to Cu-rich phase in the grain interiors is found in G115 steel during creep at 923 K. Compared with Laves phase at grain boundaries and martensitic lath boundaries, the growth velocity of Laves phase nucleated adjacent to Cu-rich phase is lower. Laves phase and Cu-rich phase play a vital role in the strength of G115 steel during creep at 923 K in addition to M23C6 carbides. Not only Laves phase and Cu-rich phase in the grain interiors help to increase the strength by enhancing the dislocation strengthening and precipitation strengthening, but also Laves phase and Cu-rich phase affect the strength variation by influencing the precipitation strengthening obviously. In contrast, Laves phase at martensitic lath boundaries is easy to coarsen, which may be unfavorable to the strength of G115 steel due to the weakened martensitic lath strengthening. In order to increase the beneficial effect of Laves phase and Cu-rich phase, it is necessary to increase the volume fraction of Cu-rich phase and Laves phase in the grain interiors, and control the growth of Laves phase. Increasing the amount of Cu-rich phase can promote the precipitation of Laves phase with low growth rate by providing more nucleation sites for Laves phase in the grain interiors.

Microstructure and sulfide stress corrosion cracking of the Inconel 625/X80 weld overlay fabricated by cold metal transfer process

The use of cold metal transfer process (CMT) is an excellent alternative to depositing nickel-based alloys on oil and gas pipelines due to the relatively small dilution. However, sulfide stress corrosion cracking (SSCC) of dissimilar weld joints has proven to be an issue. Therefore, this study investigates the microstructure and SSCC behavior of an Inconel 625/X80 weld overlay fabricated CMT process focusing on the tempering effect. Four-point bending tests show a high SSCC susceptibility of the non-overlap zone due to the formation of coarse structure in the heat affected zone and narrow martensite layer along the fusion boundary. However, microstructural characterization indicates a tempering effect in the overlap zone, producing homogenized microstructure and less residual strain in the heat affected zone. Therefore, the SSCC resistance is greatly improved, especially for the fusion boundary, because reversed austenite in the martensite layer could decrease the accumulation of hydrogen at lath boundaries.

Temperature-dependent crack induced by microstructure evolution in 20Mn2SiMoCuV bainite wheel steel

Several severe cracks were found in novel 20Mn2SiMoCuV bainite wheel tread after 100,000 km test running, which will significantly affect bainite wheel’s service safety. The reason for cracks generation was investigated through microstructure observation and wheel/shoe contact analysis. Results showed that cracks were caused by microstructure evolutions resulting from temperature variations during quickly heating and cooling repeatedly in braking at contact interface of wheel tread and brake shoe. Temperature increased to above 500 °C at contact area will reduce the bainite wheel microstructure stability, leading to retained austenite decomposition to brittle martensite and cementite precipitation along grain boundary and lath boundary. Both above brittle martensite and cementite provided location for crack initiation and propagation. Therefore, some measures can be suggested to prevent initiation and propagation of cracks such as optimizing heat treatment process parameters to improve microstructure stability and adjusting matching of wheel tread/brake shoe to coordinate thermal conductivity.

Effect of Long-Term Thermal Aging on Microstructure Evolution and Creep Deformation Behavior of a Novel 11Cr-3W-3Co Martensite Ferritic Steel.

This paper focused on the microstructure evolution under different thermal aging times at 650 °C and its effect on creep behavior in 11Cr-3W-3Co heat-resistant steel. After short-term thermal aging at 650 °C (>750 h), a Laves phase was found in the regions adjacent to the PAG boundaries, martensitic lath boundaries, and M23C6 carbides, and gradually swallowed adjacent M23C6 carbides with the aging time increased. Higher contents of Si and P are good promoters of the nucleation of the Laves phase during long-term aging. In addition, the coarsening behavior of the Laves phase, M23C6, and MX were investigated. As the aging time increases, the coarsening behavior among precipitated phases in the above-mentioned example exhibits remarkable variability, which is discussed in detail in this paper, and the evolution of the subgrain size was also analyzed in detail. The increasing rate of subgrain size is, in general, consistent with that of the M23C6 carbide size. The evolution of dislocation density in different aging times shows an obvious difference, and the decreasing rate of dislocation density is significantly affected by the precipitated phase after long-term aging time. The creep performance of the material decreases significantly as the aging time increases, which is closely related to the coarsening of the precipitates such as M23C6 carbides and subgrain during long-term aging.

In situ tempering of martensite during laser powder bed fusion of Fe-0.45C steel

During laser powder bed fusion (L-PBF), materials experience cyclic re-heating as new layers are deposited, inducing an in situ tempering effect. In this study, the effect of this phenomenon on the tempering of martensite during L-PBF was examined for Fe-0.45C steel. Detailed scanning electron microscopy, transmission electron microscopy, atom probe tomography, and hardness measurements indicated that martensite was initially in a quenched-like state after layer solidification, with carbon atoms segregating to dislocations and to martensite lath boundaries. Subsequent tempering of this quenched-like martensite was the result of two in situ phenomena: (i) micro-tempering within the heat affected zone and (ii) macro-tempering due to heat conduction and subsequent heat accumulation. Hardness measurements showed that although both influenced martensite tempering, micro-tempering had the most significant effect, as it reduced martensite hardness by up to ∼380 HV. This reduction was due to the precipitation of nano-sized Fe3C carbides at the previously carbon-enriched boundaries. Lastly, the magnitude of in situ tempering was found to be related to the energy input, where increasing the volumetric energy density from 60 to 190 J/mm3 reduced martensite hardness by ∼100 HV. These findings outline the stages of martensite tempering during L-PBF and indicate that the level of tempering can be adjusted by tailoring the processing parameters.

Lath martensite substructure evolution in low-carbon microalloyed steels

Lath martensite substructures in as-quenched plain carbon steels exhibit dislocation-like contrast in the transmission electron microscope. More recent observations reported internal twins and nanoscale auto-tempered intra-lath carbides as additional lath substructures in ultra-low-C binary Fe–C steels. Modern microalloyed steels often have similar ultra-low C contents besides microalloying elements like Ti, Nb or V and, more recently, Mo, to achieve high strength, toughness and weldability. Nonetheless, little is known about the lath substructure evolution in the as-quenched state of microalloyed steels. This study investigates the hierarchical martensite substructure evolution post-quenching of microalloyed Nb and NbMo steels with 0.1 wt% C. Hierarchical microstructure characterization was done using scanning and transmission electron microscopy, and electron backscatter diffraction methods including parent grain reconstructions with MTEX. Thermokinetic simulations using MatCalc to determine the carbide evolution during auto-tempering were corroborated with site-specific transmission electron microscopy. Mo addition led to lowering of the martensite start temperature, yet the Nb steel showed a finer hierarchical microstructure. Finer laths with in-lath dislocations, short and long twins, and lath boundary decoration of carbides were found in the Nb steel. Conversely, laths in the NbMo were wider, with frequent intra-lath auto-tempered precipitates in the vicinity of dislocations, without twins.

On the nucleation site and formation sequence of secondary phases during isothermal high-temperature aging of wire arc additively manufactured PH13–8Mo stainless steel

For the first time, the nucleation site and the formation sequence of secondary phases during isothermal high-temperature (600 °C) aging process of a wire arc additively manufactured PH13–8Mo martensitic stainless steel were investigated herein. The microstructural characterizations revealed that the aging process starts with the nucleation of homogenously distributed spherical nano-sized β-NiAl phase within the martensite laths, followed by the formation of ellipsoidal M23C6 carbides, which promoted the formation of reverted austenite within the martensite laths and also along the lath boundaries at intermediate aging times. Lastly, new M23C6 carbides were formed within the reverted austenite at the final stage of the aging process.

The effect of reversed austenite on mechanical properties of 13Cr4NiMo steel: a CPFEM study

In order to investigate the effect of reversed austenite on the mechanical properties of 13Cr4NiMo steel, a meso scale crystal plasticity finite element model (CPFEM) was established to simulate the deformation induced martensitic transformation (DIMT) and micromechanical behavior during tensile deformation. The results demonstrate that the mechanical stability of the film-like austenite embedded in the martensite lath boundary is greater than blocky austenite within the martensite lath. The film-like austenite will preferentially undergo DIMT during initial plastic deformation prior to blocky austenite. After deformation, the tempered martensite yields and deforms plastically, while adjacent new martensite is elastically deformed. The incompatible deformation leads to the introduction of dislocations in the tempered martensite and increases the work hardening rate. The new martensite, that is transformed from film-like austenite, significantly increases the true strain of the adjacent martensite matrix, which is beneficial to the plasticity and toughness, but has little contribution to the increase of matrix stress. New martensite, transformed from blocky austenite, increases the von Mises stress of the adjacent martensite matrix, which improves the tensile strength and strain hardening capacity, but makes little contribution to the increase of matrix strain.

Effect of Zr on the microstructure and mechanical properties of 12Cr ferritic/martensitic steels

The effect of Zr in the range of 0–∼1.36 wt.% on the microstructure and mechanical properties of 12Cr ferritic/martensitic steels was investigated. The steels were prepared by melting and hot rolling, then normalized at 1050 °C for 30 min and tempered at 760 °C for 90 min. The steels with 0–∼0.11 wt.% Zr had a mixture of martensite and ferrite, while the steels with ∼1.36 wt.% Zr showed fully ferrite. The area fraction of the ferrite phase in steels increases as an increase in the Zr content, which indicates that Zr is a ferrite forming alloy element in steels. The precipitates in steels with 0–∼0.11 wt.% Zr were mainly distributed along martensitic lath boundary and ferritic/martensitic interface. The coarser precipitates distributed at the ferrite/martensite interface were M23C6 carbides and obey Kurdjumov-Sachs orientation relationship, namely (110)α-Fe//(111)M23C6 and [-111]α-Fe//[011]M23C6. There were two kinds of precipitates in the steels containing ∼1.36 wt.% Zr. The coarser precipitates, which distributed randomly, were Zr-rich (Fe, Zr) compounds. The Laves phases found in smaller precipitates were distributed at the grain boundary. Tensile tests showed that the strength decreased and the ductility increased as an increase in the Zr content. The steels with 0–∼0.11 wt.% Zr demonstrated a superior strength balance, of which the tensile strength and elongation at room temperature were 750–830 MPa and 19%–22%, respectively. At the elevated temperature (600 °C), the tensile strength of the steels decreased to 390–450 MPa and the elongation of steels increased to 27%–34%.

Influence of 650 °C Thermal Aging on Microstructure and Creep–Fatigue Behaviors of P92 Steel

Abstract In this paper, P92 steel was subjected to thermal aging treatment at 650 °C for 800 h, and then basic mechanical and creep–fatigue test was performed. The creep–fatigue cycle response trend is consistent before and after aging. Subsequently, microscopic observation shows that P92 steel after aging still has typical lamellar martensite and prior austenite grains. The thermal aging of 650 °C resulted in more precipitates of martensite lath, obvious lath boundary, coarsening of martensite lath, and decreased dislocation density. Furthermore, thermal aging results in the increase of precipitates (Laves phase) and martensite width of P92 steel. The fine Laves phase located on the grain boundary can effectively nail the grain boundary and play the role of precipitation strengthening. Besides, the Laves phase located on the dislocation has the effect of diffusion strengthening, which prevents dislocation slip and improves the creep–fatigue resistance of P92 steel. Finally, four creep–fatigue life model parameters of aging P92 steel were obtained according to the test, including strain range partitioning (SRP), strain energy partitioning (SEP), frequency separation life model (FSL), and strain energy density exhaustion model (SEDE). The prediction results of the four models fall within the double tolerance zone. The SRP and SEP are found to be conservative, while the FSL and SEDE are recommended herein due to their suitability of predicting creep–fatigue life of aging P92 steel.

Solute segregation and precipitation across damage rates in dual-ion–irradiated T91 steel

Dual-ion irradiations using 5.0 MeV defocused Fe2+ ions and co-injected energy degraded 2.00 to 2.85 MeV He2+ ions were conducted on a Fe9CrMo ferritic-martensitic steel T91 to 17 dpa at a damage rate range of 5 × 10−5 dpa/s to 3 × 10−3 dpa/s at 445°C, followed by characterization of the microstructure using conventional and scanning transmission electron microscopy. Radiation induced Ni/Si clusters and radiation induced segregation were quantified using energy dispersive X-ray spectroscopy at each condition and were compared with the same material irradiated in the BOR-60 reactor and in the as-received condition. No significant Cr segregation was found at lath boundaries after dual-ion irradiation, while Ni and Si enrichments both decreased with increasing damage rate leading to a sharp decrease in the density of Ni/Si clusters with damage rate. Increased point defect recombination at higher ion damage rates likely reduced the Ni/Si cluster density compared with BOR-60. Although the overall vacancy concentration and diffusion are enhanced by the irradiation damage rate, the lack of time for thermal diffusion and ballistic displacements of solutes are significant limiting factors for Ni/Si cluster formation. This work demonstrates the effect of irradiation damage rate on elemental segregation and clustering when using ion irradiation to simulate reactor irradiation.