Martensite Packet Research Articles

Influence of refined hierarchical martensitic microstructures on yield strength and impact toughness of ultra-high strength stainless steel

The hierarchical martensitic features in ultra-high strength stainless steel (UHSSS), including the prior austenite grains, martensite packets, blocks and laths with the descending size, were refined to various extents by employing different thermomechanical processes and then carefully characterized. Their relation to yield strength and impact toughness was analyzed. We conclude that the refinement of martensitic structures could lead to the significant increase of yield strength, which follows the Hall-Petch relation with the effect grain size defined by high angle boundaries (HABs). Impact toughness of UHSSS depends on the frequency and capability for retained austenite (RA) grains at both HABs and martensite lath boundaries to trap the propagating cracks via strain-induced transformation, in which the film-like RA grains at lath boundaries appear to make the greater contribution.

The effect of TiN inclusions on the fracture mechanism of 20CrMnTi steel with lath martensite

This study aims to investigate the effect of TiN inclusions on the fracture mechanism of 20CrMnTi steel with martensite. Size of martensite packets, blocks and TiN inclusions were characterized, and the room-temperature tensile properties, impact toughness and fracture toughness were tested of 20CrMnTi steel quenched at different temperatures. The effects of TiN inclusions on the impact toughness and fracture toughness were investigated according to the Hall-Petch relationship. The results show that, TiN inclusions are high temperature stable phases which insoluble to the matrix, mostly squared in shape and dispersed. The impact toughness and fracture toughness of 20CrMnTi steel decrease with increasing sizes of the initial austenite grains, martensite packets and blocks as the quenching temperature increases. Interestingly, the TiN inclusions strongly affect the toughness of the 20CrMnTi steel in the fracture and the fine-grained sample has a better toughness. Under high-stress concentrations, TiN inclusion particles can initiate cleavage cracking.

Measurement of martensitic packet size by EBSD in the high strength steel X38CrMoV5-1 and estimation of its influence on cleavage fracture

Measurement of martensitic packet size by EBSD in the high strength steel X38CrMoV5-1 and estimation of its influence on cleavage fracture

Simulation of Austenite Formation During Continuous Heating from Low Carbon Martensite with Poly-dispersed Cementite

The nucleation and growth of austenite during continuous heating in plain carbon martensite is simulated using classical nucleation and diffusion growth theories assuming that austenite is nucleated on cementite particles at prior austenite grain boundaries and martensite packet, block, and interlath boundaries. A critical nucleus model on the spherical substrate was modified to take into account the influence of the boundary energy on which cementite particles formed. Simulations were carried out using the particle size distribution of cementite measured in an Fe-0.2 mass pct C alloy heated to near eutectoid temperature (Ae1). Austenite nucleation stopped in a very short time regardless of boundary site or particle size of cementite due to the fast decrease in carbon supersaturation and the depletion of nucleation sites. The fraction of austenite nucleated on cementite at prior austenite boundaries and martensite packet boundaries etc was much greater than that nucleated on cementite at interlath boundaries. While cementite particles dissolved quickly after austenite was nucleated, a large proportion of cementite particles at lath boundaries remained undissolved until they disappeared at 30 °C to 40 °C above Ae1. The evolution of austenite grain size was also simulated after austenitization was completed, and compared with experiment.

TEM Study of Martensite Microstructure in the Junctions of Laths in Low-Carbon Chromium Steel after LCF III

TEM study of junctions between martensite packets (laths) and their microstructure in low-carbon martensitic steel were studied. It was revealed that formation of slip bands commences at junctions between laths. Heavy changes in microstructure occur at junctions of packets and in the near-boundary laths because of plastic deformation of martensite. A number of typical junctions between the laths after LCF are considered. Deformation process within the individual packet occurs inhomogeneously, some of the laths deform more heavily than the others. The coarser slip bands group within large laths and run along their whole lengths. It was shown that the above microstructure changes strongly affect the cyclic fracture of the steel. Some concrete sites of microcrack initiation are indicated.

TEM Study of Junctions between Martensite Laths and Changes in Microstructure of Low-Carbon Steel before LCF II

The joining points between martensite packets (laths) and their microstructure in low-carbon martensitic steel were TEM studied. In order to determine the real microstructure of the packet, martensite examinations were conducted before low-cycle fatigue (LCF) tests, considering the structure of the packets and types of their joining. The changes in microstructure occurred in the above places after austenite-martensite transformation were also analyzed. It was shown that after jointing some packets initiate arch-like contours in the laths, exhibiting a presence of local stresses. Several types of joints are considered, including the penetration of laths of one packet into that of neighboring one. It was revealed that the microstructure changes are exhibited in joining points without any external deformation, and result in the localized plastic deformation at LCF. It is assumed that microcrack initiation and commencement of fatigue failure of the material should be expected to happen just in these areas. All the above is explained from point of view of the peculiarities of martensitic transformation.

Microstructure and mechanical properties of a low-carbon HDQ&P steel

ABSTRACTThe microstructure and mechanical properties of Fe-0.19C-1.46Mn-1.50Si, a low-carbon hot-rolling direct quenching and partitioning steel, were investigated, and particular emphasis was placed on the effects of the rolling temperature and the subsequent cooling method. Once rolling occurred above the recrystallization temperature, the grain size of prior austenite could be reduced from ∼88.2 to ∼11.8 μm. The corresponding packet and block sizes of martensite could be reduced from ∼17.6 to ∼4.3 μm and from ∼9.0 to ∼1.8 μm, respectively, after a typical two-step quenching and partitioning (Q&P) treatment following rolling immediately. Once rolling occurred below the recrystallization temperature, deformed prior austenite with a grain length and width of ∼150 and ∼60 μm, respectively, were obtained. Consequently, only the block size would be reduced to ∼1.9 μm. The volume fraction of retained austenite (RA), which was ∼11% in the specimens, was not significantly affected by the rolling temperature. Further, if the typical two-step Q&P treatment was replaced by air cooling after the rolled specimen was quenched to the martensitic start temperature, the volume fraction of RA in the specimen was reduced to ∼7.4%, and the corresponding carbon depletion degree in the martensitic matrix was decreased as well. Tension and impact tests indicated that the cooling method was more critical than the rolling temperature for determining the mechanical properties. The presence of sufficient amounts of RA and carbon depletion in the refined martensitic matrix contribute to the strong strain hardening and energy absorption capabilities of the steel together. As a result, rolling at a temperature that was slightly higher than the recrystallization temperature combined with the typical two-step Q&P treatment could be an optimized process to obtain excellent comprehensive mechanical properties. In particular, the ductile-to-brittle transition temperature (DBTT) could be reduced from −10°C to −30°C.

Structure and properties of layer, surfaced on HARDOX 450 steel by boron containing wire

Analysis of structure phase states and properties of the layers formed on HARDOX 450 low alloy steel by welded-on wires with boron content of 4.5 and 6.5 % wt. was made by the methods of modern physical material science. In the initial state HARDOX 450 steel has the structure of tempered martensite, in the volume and along the boundaries of crystals of it the cementite particles are located. The particles located in the volume have acicular shape and those along boundaries are mainly round. The presence of extinction bend contours has been revealed, indicative of the curvature torsion of crystal lattice of the material’s portion. They originate and finish on the interfaces of martensite crystals. Scalar density of chaotically located dislocations and forming the netlike substructure is 6.2·1010 cm–2. The layer welded on HARDOX 450 steel has microhardness increasing by more than two-fold that of the base. Analysis of state diagrams of Fe – C, Fe – B, B – C systems and polythermal cross–sections in Fe – C – B system has shown that the rapid cooling of Fe23C6 – Fe23B6 alloys from liquid state would facilitate the formation of multiphase structural states. It is stated by the methods of transmission electron diffraction microscopy that the reasons for the high microhardness level of the surface layers are the following: formation of iron borides and crystals of ultafine-dispersion (up to 100 nm) packet martensite with high level (~1011 cm–2 ) of scalar density of dislocations; presence of nanodimentional particles of iron and boron carbides in the volume and on the boundaries of martensite crystals; high level of curvature torsion of crystal lattice of iron borides and α-phase grains, caused by the internal stress fields along interphase (interface of iron boride crystals and α-phase grains) and intraphase boundaries (interface of iron borides and martensite crystals packet). Increase in boron concentration from 4.5 to 6.5 % is accompanied by the sufficient increase (by 1.2 – 1.5 times) in hardness of welded layer. It is caused by the increase of dimensions and relative content of iron boride regions by 1.5 – 2.0 times.

Effect of microstructure on the Charpy impact properties of directed energy deposition 300M steel

Direct manufacturing techniques, such as directed energy deposition (DED), are able to produce complex components efficiently. In this study, microstructure evolution and impact toughness of DED 300M ultra-high strength steel are investigated. The results show that the microstructure of the as-deposited DED 300M ultra-high strength steel is mainly composed of martensite and some blocky bainite. The micro-segregation of elements is observed within the interdendritic area. After heat treatment, the microstructure becomes uniform and consists of martensite and lower bainite. The impact toughness of the as-deposited DED 300M ultra-high strength steel is 9 J/cm2, while it is significantly increased to 25 J/cm2 after heat treatment. Furthermore, it is observed that the fracture mode of the as-deposited sample is quasi-cleavage fracture. During the process of propagation, the main cracks would go across the martensite packet and deflect in the another one, and secondary cracks also deflected in the high-angle grain boundaries. By contrast, the fracture mode of heat-treated DED 300 M steel is ductile fracture.

Microstructure and property of a selective laser melting process induced oxide dispersion strengthened 17-4 PH stainless steel

Selective laser melting (SLM) process of 17-4 PH stainless steel has been a subject of research interest due to its complex microstructure-property correlation associated with SLM process. In this work, an oxide dispersion strengthened 17-4 PH stainless steel was successfully fabricated directly by SLM process with an oxygen content of 500 ppm in the chamber. It contained a matrix of ultrafine packet of martensite and reverted austenite with a dispersion of nano Mn-Si oxide particles that could provide strengthening by generation of geometrically necessary dislocations, making 17-4 PH an oxide dispersion strengthened material after the SLM process. Furthermore, post heat treatments could increase fractions of martensite and further induce either formation of ultrafine reverted austenite to promote significant transformation-induced plasticity during work hardening process or Cu-rich phase precipitation strengthening. This article not only describes a clear microstructure-property correlation for a SLM processed 17-4 PH, but also it shows the potential advantage of the SLM process in microstructure engineering.

Investigation of constitutive relationship and dynamic recrystallization behavior of 22MnB5 during hot deformation

In order to analyze the softening behavior of 22MnB5 steel and further predict the constitutive relationship during hot sheet metal forming, a series of isothermal hot compression tests were conducted at the temperature range of 800–950 °C and strain rate range of 0.01–0.8 s−1 on BAEHR 805 A/D thermo-mechanical simulator system. Based on the friction corrected flow curves, the characteristic strain and stress of dynamic recrystallization (DRX) were derived from the Kocks-Mecking plots and expressed as a function of Zener-Hollomon parameter. Moreover, a physical constitutive model considering work hardening (WH), dynamic recovery (DRV) and DRX as well as corresponding JMAK-type DRX kinetics were developed. The results showed that the established physical equations can accurately predict the flow behavior with a correlation coefficient of 0.997 and average absolute relative error of 3.89%. Optical observation of the microstructure after hot compression revealed that the established DRX kinetics accurately reflects the reality, and then a Zener-Hollomon parameter dependent dynamic recrystallized grain size model was developed. Furthermore, EBSD analysis was carried out to study the effect of deformation conditions on martensite morphology and the results show that a lower temperature and higher strain rate lead to a finer martensite packet while the martensite block width becomes larger under the higher strain rate.

Microstructural Stability During Creep Exposure of 9Cr-1Mo Steel Treated at Different Normalization Temperatures

A modified 9Cr-1Mo steel has been exposed to three separate austenitization temperatures, i.e., at 950, 1025, and 1100 °C for normalization. After subsequent tempering at 750 °C, the normalized and tempered samples were creep-tested at temperatures of 550, 600, and 650 °C. The creep strength of the investigated samples was evaluated in terms of minimum creep strain rate and time to rupture. The effects of microstructure, precipitate and boundary misorientation on the creep behavior of the samples have been studied with TEM and EBSD analyses. Further, the evolution of crystallographic texture after creep tests has also been studied. The presence of an intermediate size of martensitic microstructural units (i.e., prior-austenite grain, martensitic packets, etc.) and combination of fine coherent and incoherent Nb(C,N) precipitates has provided superior creep strength for the samples normalized at 1025 °C, when these are subsequently subjected to low-temperature (i.e., 550 °C) and high-temperature (i.e., 650 °C) creep tests, as compared to other conditions of normalizing heat treatment.

Investigations of fatigue damage in tempered martensitic steel in the HCF regime

With increasing requirements for high service life and mechanical strength of components undergoing cyclic deformation, identification of quantitative relationships between material properties and its microstructure is of substantial significance. To understand the interaction mechanisms between microstructure, the fatigue crack initiation and the early stage fatigue crack growth fundamental fatigue experiments at 95 Hz and a stress ratio of R = −1, applied to a 0.5C-1.25Cr-Mo tempered martensitic steel were carried out. The present study is focused on both the observation of the crack initiation and, as main objective, the subsequent short crack growth by in-situ observations of a well-selected surface area. With the help of the in-situ crack observations, the correlation with the local microstructure and FIB cross-sections, it was shown that early crack propagation is driven by different damage mechanisms, which take place within martensite blocks as well as on block boundaries. A transgranular crack growth mechanism on the hierarchical martensite block level was correlated to single slip system activities within small blocks or alternating slip system activities within coarse blocks. Interestingly, also intergranular crack propagation on martensite block boundaries was observed within single martensite packets with nearly perpendicular spatial orientation of the crack path to the loading direction. This indicates not only the crack propagation by shear mechanisms, but also brittle crack propagation below the stress intensity threshold value and a high interaction with the underlying microstructure.

Effect of Interaction Field-related Parameters on Creep Rupture Process in Lath Martensite Packet Structures based on FTMP

Effect of Interaction Field-related Parameters on Creep Rupture Process in Lath Martensite Packet Structures based on FTMP

Кристаллографические особенности структуры alpha-фазы гафния и сплавов гафний--титан

AbstractThe structure of a hafnium crystal undergoing β → α (bcc → hcp) polymorphic transformation upon gradual cooling and the structure of Hf_55Ti_45 and Hf_30Ti_70 alloys formed under various kinetic conditions of polymorphic transformation are studied. The structure of the α phase in cast hafnium is shown to consist of lath crystals grouped into packets. The misorientations between separate laths in a packet are less than 1°. The Hf–Ti alloys in the cast state exhibit a mixed structure consisting of α-phase crystals of several morphological types. A structure of packet martensite is observed in the Hf–Ti alloys after quenching. Each packet includes laths of several crystallographic orientations. There is no regular alternation of differently orientated laths in the packet. The same set of α-phase orientations within an initial β-phase grain is observed independently of the cooling rate of the Hf–Ti alloys upon β → α polymorphic transformation. The misorientation of substructural elements within an α-phase crystal in the Hf–Ti alloys is ~5° for the cast state and ~2.2° after quenching.

Modelling of constitutive relationship, dynamic recrystallization and grain size of 40Cr steel during hot deformation process

Abstract The hot deformation and dynamic recrystallization (DRX) behaviors of 40Cr steel were studied by hot compression tests over a wide range of deformation temperature (1023 K–1323 K) and strain rate (0.01 s−1–10 s−1). The constitutive relationship, DRX kinetics and DRXed grain size were modelled, and numerical simulation was performed to verify their predicting accuracy. Moreover, the effects of deformation parameters on martensite structure after hot compression and quenching were analyzed. The results show that the strain compensated Arrhenius can accurately predict the flow stress with the average absolute relative error of 3.89% and correlation coefficient of 0.992. The fraction and grain size of DRX are strongly related to deformation conditions, and low strain rate and high temperature are beneficial for the occurrence of DRX. Moreover, the equation of DRX fraction was developed based on Avrami model, and the relationship between DRXed grain size and Zener-Holloman parameter was also established. The simulated results of true strain, DRX fraction and grain size agree well with experimental findings. Finally, it was found that the size of martensite packet became larger under high strain rate, while the aspect ratio of martensite lath increases at higher deformation temperature.

Crystallographic Features of the α-Phase Structure in Hafnium and Hafnium–Titanium Alloys

The structure of a hafnium crystal undergoing β → α (bcc → hcp) polymorphic transformation upon gradual cooling and the structure of Hf55Ti45 and Hf30Ti70 alloys formed under various kinetic conditions of polymorphic transformation are studied. The structure of the α phase in cast hafnium is shown to consist of lath crystals grouped into packets. The misorientations between separate laths in a packet are less than 1°. The Hf–Ti alloys in the cast state exhibit a mixed structure consisting of α-phase crystals of several morphological types. A structure of packet martensite is observed in the Hf–Ti alloys after quenching. Each packet includes laths of several crystallographic orientations. There is no regular alternation of differently orientated laths in the packet. The same set of α-phase orientations within an initial β-phase grain is observed independently of the cooling rate of the Hf–Ti alloys upon β → α polymorphic transformation. The misorientation of substructural elements within an α-phase crystal in the Hf–Ti alloys is ~5° for the cast state and ~2.2° after quenching.

Strengthening mechanisms in an ultrafine grained powder metallurgical hot work tool steel produced by high energy mechanical milling and spark plasma sintering

The strengthening mechanisms in an ultrafine grained (UFG) powder metallurgical (PM) tool steel produced by high energy mechanical milling (MM) and spark plasma sintering (SPS) have been investigated. In order to gain a proper insight to the Hall-Petch relation, bimodal grain size microstructures were produced by mixing different fractions of the nanocrystalline (NC) MM and the coarse grain (CG) gas-atomized powders to achieve different equivalent grain sizes. Dislocation strengthening (~ 435 MPa) and Hall-Petch strengthening (~ 388 MPa) are the major contributing mechanisms in strengthening the as-sintered steel. The presence of 34 vol% retained austenite (RA) had a negative influence on the strength (~ 555 MPa) and highlights the need of heat treatments (i.e., quenching and tempering) to achieve a fully tempered martensitic microstructure with a dispersion of fine secondary carbides. Recovery of dislocations and defects accompanying the precipitation of carbides reduces the effect of grain size (i.e., martensite packet size) on the strength after tempering. However, dislocations acting as preferential sites for secondary carbides nucleation during tempering, provide a finer and more homogenous particles dispersion compared to the CG counterpart. In this condition, the strength is mainly governed by the carbides size and distribution (~ 210 MPa) confirming that the indirect beneficial effects of MM can still be observed after heat treatment.

Tensile and impact behaviour of thermo mechanically treated and micro-alloyed medium carbon steel bar

In the present study, a comparison between thermo mechanically treated (TMT) and direct air cooled micro-alloyed hot rolled (MAHR) steel bar has been made in terms of microstructure, texture and mechanical properties. Tensile and Charpy impact tests were performed as a part of the mechanical investigation. Tensile strength and impact toughness values of TMT bar were higher than those of MAHR bar. It was also noticed that TMT bar exhibited better resistance to both ductile and brittle fracture at high and low temperatures compared to MAHR steel. This can be corroborated owing to the presence of fine tempered martensite at the periphery, a mixed microstructure of granular bainite, ferrite, and degenerated pearlite as an intermediate layer and a mixture of ferrite and pearlite at the core of the TMT bar. On the other hand, a coarse ferrite-pearlite phase mixture was present throughout the cross section for MAHR steel and sounded responsible for lower strength and impact toughness properties. Additionally, fractions of higher angle grain boundaries (HAGBs) in the core and smaller martensite packet around the rim section were also contributed beneficially to resist the cleavage crack propagation in TMT rebar. The higher intensity of gamma fiber of MAHR rebar has contributed lowest ductile-brittle transition temperature. Numerous cleavage cracks which were initiated from inclusions or carbide particles were also found responsible for crack propagation during both ductile and brittle types of fracture.

Effect of Micro‐Alloying Elements on Microstructure and Mechanical Properties in C–Mn–Si Quenching and Partitioning (Q&P) Steels

The authors design two kinds of Mo–Nb and Ni–Mo–Ti microalloyed steels based on C–Mn–Si steel using quenching and partitioning (Q&P) processing. The results indicate that the addition of microalloying elements increases retained austenite fraction, however, the average carbon concentration in retained austenite is reduced. Nano‐sized NbC and TiC precipitates are separately observed in the Mo–Nb and Ni–Mo–Ti microalloyed steel. Both precipitates can refine prior austenite grains as well as martensite packets and laths. The yield strength of Mo–Nb microalloyed steel treated by Q&P processes is about 87–155 MPa higher than that of the typical C–Mn–Si steel due to precipitation strengthening of NbC and refinement of martensitic structure. However, in the Ni–Mo–Ti microalloyed steel, the increment of yield strength based on precipitation strengthening of TiC is offset by the decrease of solution strengthening due to carbon dilution of the martensitic matrix, and finally the yield strength is not improved. Additionally, although the addition of alloying elements is not associated with strengthening, the product of tensile strength and total elongation (PSE) for both Mo–Nb and Ni–Mo–Ti microalloyed steels approach or exceed 20 GPa%, which indicates a superior comprehensive performance.