Martensite Lath Boundaries Research Articles

Deformation twinning in irradiated ferritic/martensitic steels

Two different ferritic/martensitic steels were tensile tested to gain insight into the mechanisms of embrittlement induced by the combined effects of displacement damage and helium after proton/neutron irradiation in SINQ, the Swiss spallation neutron source. The irradiation conditions were in the range: 15.8–19.8 dpa (displacement per atom) with 1370–1750 appm He at 245–300 °C. All the samples fractured in brittle mode with intergranular or cleavage fracture surfaces when tested at room temperature (RT) or 300 °C. After tensile test, transmission electron microscopy (TEM) was employed to investigate the deformation microstructures. TEM-lamella samples were extracted directly below the intergranular fracture surfaces or cleavage surfaces by using the focused ion beam technique. Deformation twinning was observed in irradiated specimens at high irradiation dose. Only twins with {112} plane were observed in all of the samples. The average thickness of twins is about 40 nm. Twins initiated at the fracture surface, became gradually thinner with distance away from the fracture surface and finally stopped in the matrix. Novel features such as twin-precipitate interactions, twin-grain boundary and/or twin-lath boundary interactions were observed. Twinning bands were seen to be arrested by grain boundaries or large precipitates, but could penetrate martensitic lath boundaries. Unlike the case of defect free channels, small defect-clusters, dislocation loops and dense small helium bubbles were observed inside twins.

Fine–scale structural features of intercritically aged HSLA-100 plate steel and their influence on yield strength and low-temperature impact toughness

Fine-scale structural features of an HSLA-100 steel intercritically aged at 950K (677°C) for 4.2ks (70min.) were investigated via transmission electron microscopy. The microstructure consisted of approximately 90vol.pct. of low‑carbon lath martensite. Average lath width was about 0.25μm, unchanged from the as-quenched condition, owing to a high number density of epsilon‑copper precipitates at these boundaries. About 10vol.pct. of medium‑carbon austenite existed as coarsened, broken-up islands between laths; martensite lath boundary coverage by austenite was notably reduced compared to the as-quenched condition. Greater continuity between adjacent low‑carbon martensite laths was accompanied by coarse copper precipitates within laths and an order of magnitude lower dislocation density than the as-quenched martensite. Collectively, these features produced a sub-grain coarsening effect. Austenite islands larger than about 0.5μm were unstable with respect to the formation of fresh, medium‑carbon martensite regions. Copper precipitation within austenite islands (owing to carbon and copper enrichment) might also have contributed to austenite transformation upon quenching. Intercritical aging was associated with a strong-and-undesirable deviation from strength-toughness behavior at sub-critical aging temperatures. Two key factors were identified as being responsible for this negative outcome: (i) a sub-grain coarsening effect associated with “delamellarization” of low‑carbon martensite and austenite and (ii) the formation of brittle martensite upon quenching. Quantitative estimates of the changes in strength and impact toughness are provided for microstructural differences between intercritically aged and subcritically aged material.

Hydrogen induced mechanical degradation in tungsten alloyed steels

The present work considers the effect of hydrogen on the mechanical properties of generic Fe-C-W alloys. Thermal treatment aimed at a martensitic microstructure and two conditions are compared; i.e. an as-quenched state and a quenched and tempered state with lower dislocation density and with W based carbides formed during tempering. The hydrogen induced mechanical degradation is evaluated by in-situ tensile tests, while thermal desorption spectroscopy, hot/melt extraction and permeation experiments are performed to understand the observed hydrogen embrittlement degree. The hydrogen induced ductility loss increases with increasing carbon content due to the higher amount of hydrogen trapped by the denser martensitic lath boundaries. Furthermore, the quenched and tempered condition shows a lower susceptibility to hydrogen. This is correlated to the reduced hydrogen content when tempered due to a decreased dislocation density and the fact that the tempered induced W2C particles did not trap hydrogen. Moreover, corresponding observations to this interpretation are perceived by permeation experiments since, on the one hand, the diffusion coefficient decreases with carbon content due to the increasing trapping ability of alloy A➔B➔C, and, on the other hand, the diffusivity increases when tempering is applied, which is also linked to the decrease in dislocation density and the disability of the carbides to hinder hydrogen diffusion by efficient trapping.

Microstructure evolution and toughness degeneration of 9Cr martensitic steel after aging at 550 °C for 20000 h

The China Low Activation Martensitic (CLAM) steel is one of 9Cr martensitic steels, and it has been selected as the primary candidate structural material for future fusion reactors in China. It is essential to understand the thermal stability of CLAM steel to ensure the operation safety during exposures at operating temperatures. In this paper, the thermal aging behavior of CLAM steel at 550 °C for 20000 h was studied. Microstructure evolution and mechanical property change were investigated. The results showed that Laves phase was mainly precipitated along the martensitic lath boundaries and grain boundaries in CLAM samples after aging for more than 4000 h, and its size increased faster than that of M23C6. The size of TaC had no obvious changed but its number increased after aging. The microstructure evolution resulted to degradation of the mechanical property, especially the ductile-to-brittle transition temperature (DBTT), which was increased from − 92 to − 43 °C after aging for 20000 h. The possible mechanism of DBTT increasing had been analyzed based on the interaction between precipitations and grain boundaries.

Interaction of Pb -16Li melt with EUROFER97 under higher temperature and neutron irradiation

The behaviour of ferritic-martensitic steel EUROFER97 was evaluated after its exposure in a liquid Pb-16Li environment under neutron flux simulating the conditions of operating parameters of the HCLL test blanket module using a test train inserted into the LVR 15 research reactor in CV Řež. The interaction of the Pb-16Li alloy with neutron flux results in radioisotope formation due to transmutation reactions. The formation of tritium and polonium was measured and compared with calculated values. The Pb-16Li corrosion effect leads to dissolution of steel matric. Two reaction layers were observed on the exposed EUROFER97 surface. The first continuous surface layer is characterized by uneven thickness, the second subsurface layer confirms that dissolution of the metal matrix takes place along martensitic lath boundaries. The performed examinations proved that the mechanism of corrosion in the liquid melt was volume dissolution of steel.

Influence of Laves phase on creep strength of modified 9Cr-1Mo steel

The influence of Laves phase in the microstructure on the creep strength of a modified 9Cr-1Mo tempered ferritic steel, F91, is investigated in this research. Through additional aging heat treatment up to 5000h, a pronounced increase of Mo-rich Laves phase is found around Cr-rich carbide M23C6, which is pinned on the prior-austenite grain boundaries and martensitic lath boundaries. This secondary precipitation hardening provided by the Laves phase is rapidly offset by its coarsening into large clusters during long-term thermal exposure and under creep conditions, leading to earlier creep rupture than the pristine material. Uniaxial constant-load creep tests are conducted on both aged and non-aged coupons under the same creep conditions. The creep rupture behavior of both materials is rationalized with the assistance of a deformation-mechanism-based true-stress creep model that was developed in the previous work. The effects of Laves phase on the creep strength of F91 are related to its influence on the well-recognized existing deformation mechanisms: dislocation glide, dislocation climb, and grain boundary sliding.

Mechanical properties and microstructure of 3D-printed high Co–Ni secondary hardening steel fabricated by laser melting deposition

The mechanical properties and microstructure of the 3D-printed high Co–Ni secondary hardening steel fabricated by the laser melting deposition technique was investigated using a material testing machine and electron microscopy. A microstructure investigation revealed that the samples consist of martensite laths, fine dispersed precipitates, and reverted austenite films at the martensite lath boundaries. The precipitates are enriched with Co and Mo. Because the sample tempered at 486°C has smaller precipitates and a higher number of precipitates per unit area, it exhibits better mechanical properties than the sample tempered at 498°C. Although the 3D-printed samples have the same phase constituents as AerMet 100 steel, the mechanical properties are slightly worse than those of the commercial wrought AerMet 100 steel because of the presence of voids.

Brittle Fracture Behaviors of Large Die Holders Used in Hot Die Forging

Brittle fracture of large forging equipment usually leads to catastrophic consequences. To avoid this kind of accident, the brittle fracture behaviors of a large die holder were studied by simulating the practical application. The die holder is used on the large die forging press, and it is made of 55NiCrMoV7 hot-work tool steel. Detailed investigations including mechanical properties analysis, metallographic observation, fractography, transmission electron microscope (TEM) analysis and selected area electron diffraction (SAED) were conducted. The results reveal that the material generated a large quantity of large size polyhedral M23C6 (M: Fe and Cr mainly) and elongated M3C (M: Fe mainly) carbides along the martensitic lath boundaries when the die holder was recurrently tempered and water-cooled at 250 °C during the service. The large size carbides lead to the material embrittlement and impact toughness degradation, and further resulted in the brittle fracture of the die holder. Therefore, the operation specification must be emphasized to avoid the die holder being cooled by using water, which is aimed at accelerating the cooling.

Influence of Tool Rotational Speed and Post-Weld Heat Treatments on Friction Stir Welded Reduced Activation Ferritic-Martensitic Steel

The effects of tool rotational speed (200 and 700 rpm) on evolving microstructure during friction stir welding (FSW) of a reduced activation ferritic-martensitic steel (RAFMS) in the stir zone (SZ), thermo-mechanically affected zone (TMAZ), and heat-affected zone (HAZ) have been explored in detail. The influence of post-weld direct tempering (PWDT: 1033 K (760 °C)/ 90 minutes + air cooling) and post-weld normalizing and tempering (PWNT: 1253 K (980 °C)/30 minutes + air cooling + tempering 1033 K (760 °C)/90 minutes + air cooling) treatments on microstructure and mechanical properties has also been assessed. The base metal (BM) microstructure was tempered martensite comprising Cr-rich M23C6 on prior austenite grain and lath boundaries with intra-lath precipitation of V- and Ta-rich MC precipitates. The tool rotational speed exerted profound influence on evolving microstructure in SZ, TMAZ, and HAZ in the as-welded and post-weld heat-treated states. Very high proportion of prior austenitic grains and martensite lath boundaries in SZ and TMAZ in the as-welded state showed lack of strengthening precipitates, though very high hardness was recorded in SZ irrespective of the tool speed. Very fine-needle-like Fe3C precipitates were found at both the rotational speeds in SZ. The Fe3C was dissolved and fresh precipitation of strengthening precipitates occurred on both prior austenite grain and sub-grain boundaries in SZ during PWNT and PWDT. The post-weld direct tempering caused coarsening and coalescence of strengthening precipitates, in both matrix and grain boundary regions of TMAZ and HAZ, which led to inhomogeneous distribution of hardness across the weld joint. The PWNT heat treatment has shown fresh precipitation of M23C6 on lath and grain boundaries and very fine V-rich MC precipitates in the intragranular regions, which is very much similar to that prevailed in BM prior to FSW. Both the PWDT and PWNT treatments caused considerable reduction in the hardness of SZ. In the as-welded state, the 200 rpm joints have shown room temperature impact toughness close to that of BM, whereas 700 rpm joints exhibited very poor impact toughness. The best combination of microstructure and mechanical properties could be obtained by employing low rotational speed of 200 rpm followed by PWNT cycle. The type and size of various precipitates, grain size, and evolving dislocation substructure have been presented and comprehensively discussed.

Role of evolving microstructure on the mechanical properties of electron beam welded ferritic-martensitic steel in the as-welded and post weld heat-treated states

The microstructure and mechanical properties of electron beam welded joints of reduced activation ferritic-martensitic steel in the as-welded and post-weld heat treatment (PWHT) states have been explored. The as-received base metal (BM) was in normalised and tempered condition. The PWHTs employed include post-weld direct tempering (PWDT) at 760°C/90min/air cooling and (ii) re-austenitizing at 980°C/30min/air cooling+ tempering at 760°C/90min/air cooling (PWNT). The BM microstructure was composed of fully tempered lath martensite with prior austenite grain and martensite lath boundaries decorated with M23C6 type carbides whereas intra-lath regions majorly displayed MX type carbides. In the as-welded state, the fusion zone (FZ) contained martensite in coarse grains and small amount of δ-ferrite with no evidence for precipitation of M23C6 and MX either in intra- or inter-granular regions. The heat affected zone (HAZ) was made up of martensite in fine grains without any δ-ferrite and with subtle variations in microstructure across the HAZ. The as-welded joints exhibited high hardness in the FZ and HAZ due to the occurrence of martensite during the weld thermal cycle. The impact toughness of the as-welded joint was inferior compared to that of the BM due to the combined influence of the martensite, coarse grains and presence of δ-ferrite in the weld zone. Tensile strength of as-welded joint was higher than that of BM. PWHTs were beneficial in decreasing the hardness in the FZ and HAZ. PWDT could not fully eliminate the pronounced variation of hardness observed in the transverse section of welded joint. Though the impact toughness of the weld joint was improved marginally compared to as-welded state after PWDT, it was much lower than that recorded in the case of BM. PWNT treatment minimised the variation in hardness across the transverse section of weld joint and the impact toughness surpassed than that achieved in BM. The tensile properties of BM, welded joints in as-welded and in PWHT conditions were determined at room temperature and correlated with the prevailing microstructures.

Carbon Redistribution and Carbide Precipitation in a High-Strength Low-Carbon HSLA-115 Steel Studied on a Nanoscale by Atom Probe Tomography

HSLA-115 is a newly developed Cu-bearing high-strength low-carbon martensitic steel for use in Naval structural applications. This research provides, for the first time, a comprehensive compositional analysis of carbon redistribution and associated complex phase transformations in an isothermal aging study of HSLA-115 at 823 K (550 °C). Specifically, we characterize carbon segregation at lath boundaries, grain-refining niobium carbonitrides, cementite, and secondary hardening M2C carbides, in addition to copper precipitation, by 3D atom probe tomography (APT). Segregation of carbon (3 to 6 at. pct C) is observed at martensitic lath boundaries in the as-quenched and 0.12-hour aged microstructures. On further aging, carbon redistributes itself forming cementite and M2C carbides. Niobium carbonitride precipitates do not dissolve during the austenitizing treatment and are inherited in the as-quenched and aged microstructures; these are characterized along with cementite by synchrotron X-ray diffraction and APT. Sub-nanometer-sized M2C carbide precipitates are observed after the formation of Cu precipitates, co-located with the latter, indicating heterogeneous nucleation of M2C. The temporal evolution of the composition and morphology of M2C carbides at 823 K (550 °C) is described using APT; their precipitation kinetics is intertwined with Cu precipitates, affecting the bulk mechanical properties of HSLA-115. Phase compositions determined by APT are compared with computed compositions at thermodynamic equilibrium using ThermoCalc.

Effect of Tempering on Microstructure and Mechanical Properties of Laser Welded and Post-Weld Treated AHSS Specimens

An advanced high strength steel (0.08 %C, 1.79 %Mn, 0.23 %Si) was subjected to different post-weld heat treatments by quenching & tempering treatments (Q&T) after laser welding to reduce the risk of martensite formation in a few seconds based on an idea of quench and partitioning (Q&P), mechanism. The thermal stability of retained austenite, microstructure development and mechanical properties have been studied at 2 tempering temperatures of 440°C (Ms) and 636°C (Bs), both for 15 minutes, by means of electron microscopy, dilatometry, hardness profile and tensile tests. Dilatometer study unveiled that redistribution of carbon atoms and precipitation of transition carbides occur around 150°C and austenite decomposition occur at 600°C. Tempering at 636°C resulted in notable effect on the mechanical properties, while no significant difference was detected at 440°C, except a slight hardness drop. The strength increased up to 12% for the different specimens without significant loss in ductility for all specimens tempered at 636°C, which may be caused by precipitation hardening and recrystallization of martensite lath boundaries during tempering around 600°C.

The relation between aging temperature, microstructure evolution and hardening of Custom 465® stainless steel

The microstructure and hardness of Custom 465® precipitation hardened stainless steel were characterized following 4 hours aging at temperatures ranging from 482°C to 648°C. Dilatometry measurements and thermodynamic calculations were used to understand the mechanism of martensite-to-austenite reversion. Three major stages of aging were observed. The first stage is the peak hardening at aging temperatures of 480–510°C, where both η-Ni3Ti precipitation and austenite reversion begin independently of each other. The second stage is the initial stage of overaging observed above 538°C as reduction in hardness is due to a slight increase in both the precipitate dimensions and the reverted austenite volume fraction. The third, progressive overaging stage occurs above 593°C in which a dramatic hardness decrease is caused by a significant increase in both precipitate dimensions and reverted austenite volume fraction. Transmission Kikuchi diffraction orientation mapping revealed that globular austenite randomly nucleates at martensite grain boundaries, while acicular austenite nucleates along martensite lath boundaries. Thermodynamic calculations indicated that the dissolution of η-Ni3Ti precipitates does not govern the austenite reversion, although both phases are in competition for Ni. While the thermodynamic calculation predicts increase in the fraction of austenite with increase in aging temperature, the fraction of austenite found at room temperature in aged samples does not vary when the aging temperature, in which austenite reversion occurs, is increased above 593°C. This is explained by the change in composition of the austenite as a function of aging temperature.

Study of Mechanical Degradation of 9Cr-1Mo-V-Nb Steel Under Creep-Fatigue Interaction Conditions

Creep-fatigue tests of 9Cr-1Mo-V-Nb steel have been conducted under load control with further study of the steel mechanical degradation via microscopic observation, static-mechanical analysis and ultrasonic evaluation. The tempered specimens exhibited a tempered martensitic structure with a high dislocation density in the lath interior and fine precipitates on the previous austenite grain and martensite lath boundaries. However, the major microstructure changes to the 9Cr-1Mo-V-Nb steel caused by creep-fatigue were the coarsening of Cr23C6 precipitates, the recovery of dislocations due to rearrangement and annihilation, an increase in the martensite lath width, and the formation of cavities. The ultrasonic velocity was observed to increase rapidly within the initial fatigue life fraction (stage I), but the attenuation decreased during this stage. During stage II, there was a slight increase in the ultrasonic velocity and the attenuation subsequently decreased. The ultrasonic velocity decreased and the attenuation increased during the final period (stage III) of fatigue life.

Phase structures and morphologies of tempered CA6NM stainless steel welded by hybrid laser-arc process

The post-weld tempered microstructure of hybrid laser-arc welded CA6NM, a cast low carbon martensitic stainless steel, was investigated. The microstructural evolutions from the fusion zone to the base metal were characterized in detail using optical microscopy, scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), X-ray diffraction (XRD) and microhardness techniques. The fusion zone, in its post-weld tempered condition, consisted of tempered lath martensite, residual delta-ferrite with various morphologies, reversed austenite and chromium carbides. The reversed austenite, which can be detected through both EBSD and XRD techniques, was found to be finely dispersed along the martensite lath boundaries, particularly at triple junctions. Based on the EBSD analysis, the orientation relationship between the reversed austenite and the adjacent martensite laths seemed to follow the Kurdjumov-Sachs (K-S) model. The results also revealed the presence of the reversed austenite in the different regions of the heat affected zone after post-weld tempering. The microindentation hardness distribution was measured, and correlated to the evolution of the corresponding microstructure across the welds.

Influence of the Initial Microstructure on the Reverse Transformation Kinetics and Microstructural Evolution in Transformation-Induced Plasticity–Assisted Steel

The reverse transformation behavior upon heating to intercritical temperature was studied in Fe-0.21C-2.2Mn-1.5Si (wt pct) alloy with three initial microstructures. One is the cold-rolled (CR) structure and two others are martensite having different fractions of retained austenite. The CR structure exhibits slower reverse transformation kinetics than martensite due to the lesser population of potent nucleation sites and coarse cementite particles. The film type of retained austenite at the martensite lath boundary contributes to the earlier start of the reverse transformation, because it can proceed as the growth of pre-existing retained austenite, which makes the nucleation process less critical. Besides, the growth of interlath austenite plays an essential role in the evolution of fine lath-type reverse-transformed microstructure, which was difficult to obtain from similar initial microstructures of martensite having negligible fraction of interlath austenite.

Influence of carbides on the high-temperature tempered martensite embrittlement of martensitic heat-resistant steels

The high-temperature temper embrittlement of martensitic heat-resistant 10Cr12Ni steel was studied. The results demonstrate that there is some irreversible temper embrittlement when the steel is tempered at 625°C. The irreversible temper embrittlement can be overcome by re-tempering at higher temperature. The tempered martensite embrittlement at 625°C is attributed to the precipitation of M23C6-type carbides along the martensite lath and prior austenite grain boundaries. The tempering process can be divided into three stages: In the first stage, the martensite laths recover and M7C3-type carbides precipitate inside the martensite laths, leading to the improvement of the toughness. In the second stage, M7C3-type carbides dissolve, and M23C6-type carbides precipitate along the martensite laths and prior austenite grain boundaries. The M23C6-type carbides play a nucleating role in the development of cracks, leading to tempered martensite embrittlement. In the third stage, the toughness gradually recovers with the further recovery of the martensite laths.

A methodology for the measurement of the interfacial excess of solute at a grain boundary

Segregation of solutes on grain boundaries is usually quantified by the peak value of the concentration profile measured across the grain boundary. This procedure is inadequate both from the theoretical and from the practical point of view. Instead, we put forward the concept of interfacial excess of solute, as having a thermodynamic background and being of operative efficiency. Our analyses reveal carbon and boron co-segregation at a martensite lath boundary of a Dual Phase steel.

Microstructure Evolution and Precipitation Behavior of 0Cr16Ni5Mo Martensitic Stainless Steel during Tempering Process

The microstructure, morphology of precipitates and retained austenite and the volume fraction of retained austenite in 0Cr16Ni5Mo stainless steel during the tempering process were analyzed using optical microscope (OM), transmission electron microscope (TEM), X-ray diffraction (XRD) and scanning transmission electron microscope (STEM). The results show that the microstructure of the tempered steel is mainly composed of tempered martensite, retained austenite, and delta ferrite. In the case of samples tempered from 500 to 700 °C, the precipitates are mainly M23 C6, which precipitate along the lath martensite boundaries. The precipitate content increases with the tempering temperature. During the tempering process, the content of retained austenite initially increases and then decreases, the maximum content of retained austenite being 29 vol. % upon tempering at 600 °C. TEM analysis of the tested steel reveals two morphology types of retained austenite. One is thin film-like retained austenite that exists along the martensite lath boundary. The other is blocky austenite located on packet at the boundary and the original austenite grain boundary. To further understand the stability of reversed austenite, the Ni content in reversed austenite was measured using STEM. Results show a significant difference in nickel concentrations between reversed austenite and martensite.

Influence of Heat Treatments on the Microstructural Evolution and Resultant Mechanical Properties in a Low Carbon Medium Mn Heavy Steel Plate

In this study, the microstructural evolution and resultant mechanical properties in a low carbon medium Mn heavy steel plate were investigated in detail. The results show that the introduction of medium manganese alloy design in the heavy steel plate has been shown to achieve the outstanding combination of strength, ductility, low-temperature impact toughness, and strain hardening capacity. It has been found that the austenite phase mainly displays at martensitic lath boundaries and shows lath shape for the heat treating at 873 K (600 °C) for 1 to 10 hours or 893 K (620 °C) for 2 hours, and not all the austenite phase obeys the K–S or N–W orientation relationship with respect to abutting martensitic lath. Although the microstructure in the steel after heat treating at 873 K (600 °C) for 1 to 10 hours is similar to each other, the resultant mechanical properties are very different because the volume fraction and stability of retained austenite vary with the heat treatments. The best low-temperature impact toughness is achieved after heat treating at 873 K (600 °C) for 2 hours due to the formation of a considerable volume fraction of retained austenite with relatively high stability, but the strain hardening capacity and ductility are disappointing because of insufficient TRIP effect. Based on enhancing TRIP effect, the two methods have been suggested. One is to increase the isothermal holding temperature to 893 K (620 °C), and the other one is to prolong the isothermal holding time to 10 hours at 873 K (600 °C). The two methods can significantly increase strain hardening capacity and ductility nearly without harming low-temperature impact toughness. In addition, the stability of retained austenite has been discussed by the quantitative analysis and it has been demonstrated that the stability of retained austenite is related to the chemical composition, size, and morphology. Moreover, the isothermal holding temperature has a great effect on the stability of retained austenite, while the effect of the isothermal holding time is relatively poor.