Martensite Packet Research Articles

Revealing hydrogen-induced delayed fracture in ferrite-containing quenching and partitioning steels

Hydrogen-induced delayed fracture (HIDF) of two ferrite-containing quenching and partitioning (QP) steels, QP980 and QP1180, has been studied by deep-drawn cup tests and slow strain rate tests (SSRT) in the present work. It is found that QP980 has better delayed fracture resistance than QP1180 due to both volume fraction and distribution of ferrite. Other factors, such as residual stress, plastic strain, retained austenite and strain-induced fresh martensite, are also carefully considered, but cannot account for the difference. In the QP steels, hydrogen-induced cracks nucleate in sensitive regions such as retained austenite or strain-induced fresh martensite, and prefer to propagate along prior austenite grain boundaries (PAGBs) and martensite packet boundaries. We also found that ferrite in QP steels can be used to blunt hydrogen-induced cracks, and interrupt the continuity of PAGBs and packet boundaries. QP980 steel has a higher volume fraction of interconnected ferrite, and discontinuous PAGBs and martensite packet boundaries, which are the main mechanisms responsible for the better HIDF resistance of QP980. Therefore, it is proposed that an appropriate volume fraction and distribution of ferrite phase should be present in QP steels for better HIDF resistance.

Effect of Martensite Structure and Carbide Precipitates on Mechanical Properties of Cr-Mo Alloy Steel with Different Cooling Rate

Abstract The effect of cooling rate on martensitic structure and carbides precipitation behavior was investigated based on Cr-Mo alloy steel with different quenching media of oil, water and 10% NaCl-water, respectively. The influence mechanism of martensite structure and carbide precipitates on mechanical properties was also studied. The results showed that martensite packets and blocks were refined with water quenching, however, they were coarse with oil or 10% NaCl-water quenching. Martensite laths were refined and dislocation density increased with increasing cooling rate. The carbides in tempered steel were coarse obviously with 10% NaCl-water quenching. The impact toughness deteriorated significantly with 10% NaCl-water quenching due to coarsening of martensite structure and carbides.

Effects of Austenitizing and Quenching Parameters on Microstructures and Mechanical Properties of Press Hardened WHT1500HF Steel

Abstract The effect of austenitizing temperature and the cooling rate during quenching on the microstructure and mechanical properties of press hardening steel were investigated. The as-quenched microstructure and prior austenite grain size were characterized by optical microscopy and transmission electron microscopy. The mechanical properties were evaluated by uniaxial tensile tests. The results showed that the ultimate tensile strength and yield strength values were the highest and the formability index value was excellent for the water-quenched samples compared to the oil-quenched samples or the die-quenched samples. After quenching at different cooling rates, the microstructure was fully martensite, in which the martensite exhibited lath morphology. As the cooling rate increased, the martensite laths became finer and the martensite packets size decreased. The martensite laths for the specimens with the highest cooling rate were finer than for those of specimens with lower cooling rates, and there was a larger number of fine needle-like ϵ-Fe2C in the matrix of martensite laths. For the WHT1500HF steel, the optimum austenitizing temperature is 900°C, which resulted in a mean austenitic grain size of 9.4 μm. Therefore, the properties of the hot stamping steel can be improved through using an optimal austenitizing temperature and increasing the quenching rates by changing the physical die design.

SEM Study of the Influence of Microstructure on Low Cycle Fatigue Crack Growth in Martensitic Steel I

Distribution of fatigue cracks in chromium martensitic steel after low cycle fatigue (LCF) tests at room temperature has been studied using SEM, and the experimental evidences of localized plastic flow (LPF) are presented. The influence of the location of LPF and the microstructure elements on the trajectory and growth of microcracks is also considered. The dimensions of plastic zones ahead of macrocrack tip as well as at its edges were measured in the process of crack propagation inside of the sample. The processes occurring in plastic zone, particularly ahead of macrocrack tip, were analyzed. Distribution, orientation and the reasons of slip bands’ formation as well as the microstructure elements at which they were nucleated have been studied. The impact of the slip bands’ orientation on the process of macrocrack growth was also analyzed. In addition the interactions of a crack with the boundaries of former austenite grains, martensitic packets, martensitic laths, slip bands and precipitates have been discussed.

Ductile and intergranular brittle fracture in a two-step quenching and partitioning steel

A two-step quenching and partitioning steel exhibits ductile fracture under uniaxial tension and unexpectedly intergranular brittle fracture in the case of pre-cracked configuration. A gradual retained austenite-to-martensite transformation because of low stress triaxiality under uniaxial tension, leads to ductile fracture with voids formation between fresh and tempered martensite resulting from the large strain partitioning and localization. When the retained austenite fast transforms to martensite in the near crack tip region because of high stress triaxiality, intergranular fracture is promoted by the high maximum principal stress level and by the formation of brittle martensite necklace along tempered martensitic packet boundaries.

″Complete twin″ and ″the Transgranular Twin Line″ of Common Steels

The microstructures of 20, 30, 40, 45, T9 and T11 commercial carbon steels were observed by optical microscopy, scanning electron microscopy and transmission election microscopy. It was found that the internal twins in martensite of common steels is basically a complete twin, and internal twin plane of mutually parallel martensite units in a martensite packet have a same crystallographic orientation, namely, forming ″the transgranular twin line″ or ″interpenetration twinline″. The underlying mechanism of ″complete twin″ and″ the transgranular twin line″ were explored. Both all are to form in the martensite transformation process. In order to promote the growth of martensitic nuclei in initial stage, the associated nucleus with twinning relationship had been formed at a semi-coherent end plane of nuclei which possesses high energy, and forming transformation twins through ″double change″. Therefore, transformation twins are usually all ″complete twin″. In the martensite packet, all the martensite plates of mutual parallelism are to form the associated nucleus with twin interface plane at a side of original ″ twin block″ of the martensite and grow up, so a martensite packet all have the identical ″trans-granular twin line″. In the formation process of ″complete twinning″ and″ trans-granular twin line″, don’t involve the mechanics properties of material, and mainly decide from nucleation work and nucleus growth work. It is thus clear that, having the complete twins in the martensite doesn’t mean that the critical resolved shear stress (CRSS) of slipping exceeded the CRSS of twinning.

Exploring on Stereo Morphology and Habit Plane of Martensite in Ferrous Alloy

Themicrostructures of 20, 30, 40, 45, T9, T10, T11, 20Cr, 40Cr, 45CrNi, 65Mn, 9CrSi, CrWMn commercial carbon and alloy steels were observed by optical microscopy, scanning electron microscopy and transmission election microscopy, meanwhile the habit plane, the substructure and stereo shapeof martensit have been proposed. It was verified that the packet martensite should be divided into packet thin sheet martensite and packet fine martensite, the latter principally can be divided into {557}A packet fine plate martensite, {1 1 1}A packet fine plate martensite and {2 2 5}A packet fine plate martensite according to the carbon content. Space morphology of low carbon packet martensite units is sheet-like but not lath-like; that of martensite medium and high carbon packet martensite units is plate-like. There is no noticeable difference between the substructures of lath and plate martensite. Martensite without internal twin is not certainly lath martensite, while all martensite with numerous internal twins is certainly plate martensite.

Optimization of hot workability and microstructure control in a 12Cr-10Ni precipitation hardenable stainless steel: An approach using processing maps

Optimization of hot working parameters and corresponding microstructure control were explored in a 12Cr-10Ni precipitation hardenable martensitic stainless steel. From hot compression tests, 1073–1373 K (800°–1100 °C) and five (0.001–10 s−1) different strain rates, processing maps were developed, and various domains of microstructure evolution (in the austenite phase) were identified. The stable domain of the processing maps appeared to indicate dynamic recrystallization (DRx) in the austenite phase: as evident by the appearance of inflection point in the strain hardening rate versus flow stress plots. The suspected DRx domain was further confirmed by detailed microstructural investigations of the respective water-quenched specimens. The DRx domain showed a refinement of the martensite packet size and of the prior transformation austenite grain size. The later was shown with optical microstructure and reconstructed (modelled) microtexture data of the prior austenite grains. Additionally, the DRx domain showed a significant difference in activation energy as estimated from appropriate constitutive modelling.

Characterization of lower bainite formed below M S *

Abstract Lower bainite can indeed grow below martensite start temperature (M S ) in a commercial alloy. Ultrahigh-strength low-alloyed 42CrMo4 steels are austempered below M S . The preliminary martensite and lower bainite packets formed during austempering partition the prior austenite grains effectively. The obtained ultrafine-grained nanostructured microstructure enhances the mechanical properties of the alloys significantly. Morphology and overall microstructure are examined by reflection optical microscope and scanning electron microscopy. Substructures of lower bainite and virgin/tempered martensite are revealed by transmission electron microscopy. With increasing duration of austempering, a clear transition from cleavage to ductile rupture was observed by fractography. Effects of grain partitioning and the risk of embrittlement have been discussed concretely. The better understanding of the microstructure austempered below M S will feedback to the materials development activities both in conventional and new materials areas.

Effect of the Thermomechanical Treatment on Structural and Phase Transformations in Cu–14Al–3Ni Shape Memory Alloy Subjected to High-Pressure Torsion

The possibilities of controlling the structure and properties of a Cu–Al–Ni shape memory alloy due to the use of different schemes of the thermomechanical treatment, including forging, homogenizing in the austenitic state and subsequent quenching, and high-pressure torsion have been found. For the first time, an ultrafine-grain structure has been produced in this alloy via severe plastic deformation using high-pressure torsion. It has been detected that high-pressure torsion using ten revolutions of the anvils leads to the formation of a nanocrystalline structure with a grain size of less than 100 nm. The subsequent short-term heating of the alloy to 800°С (10 s) in the temperature region of the existence of the homogeneous β phase made it possible to form an ultrafine-grain structure with predominant sizes of recrystallized grains of 1 and 8 μm. The quenching after heating prevented the decomposition of the solid solution. The refinement of the grain structure changed the deformation behavior of the alloy, having provided the possibility of the significant plastic deformation upon mechanical tensile tests. The coarse-grained hot-forged quenched alloy was brittle, and fracture occurred along the boundaries of former austenite grains and martensite packets. The highstrength ultrafine-grained alloy also experienced mainly the intercrystalline fracture along the high-angle boundaries of elements of the structure, the grain size of which was less by two orders than that in the initial alloy. This determined an increase in its relative elongation upon mechanical tests.

Effect of Nb on microstructure and yield strength of a high temperature tempered martensitic steel

Martensitic steels based on a composition of 25CrMo47NbVTi with different concentrations of Nb (0.003%–0.060%) were quenched (Q) at 900 °C and tempered (T) at 700 °C to obtain oil country tubular goods (OCTG) with higher yield strength. The precipitation and microstructures were characterized and quantified by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and x-ray diffraction (XRD). The results show that the increased Nb content led to an enhanced overall precipitation, the rising solution-precipitation temperature, the increased mass or volume fraction of the Nb-containing precipitates, and the decreased average diameter of Nb-containing particles. With the enhanced precipitation of small sized Nb-containing particles, the austenite grain and corresponding martensitic packet and block were evidently refined. In addition, the dislocation density increased slightly with increasing Nb addition. The yield strength was experimentally measured and quantitatively estimated. The findings based on theoretical calculations indicated that as a consequence of intensified strengthening from grain boundaries, precipitates and dislocations, the yield strength was enhanced significantly by Nb addition.

The relationships of microstructure-mechanical properties in quenching and partitioning (Q&P) steel accompanied with microalloyed carbide precipitation

A low carbon steel microalloyed with Nb-V-Ti were heat treated by means of two-step quenching and partitioning (Q&P) process after partial austenitization. Combined use of SEM equipped with EBSD, XRD and TEM showed that the microstructure was composed of intercritical polygonal ferrite, tempered lath martensite, secondary twin martensite and carbon-enriched retained austenite, as well as microalloyed carbides (mainly VC) precipitated inside ferrite and martensite. When increasing partitioning time from 0 s to 3600 s, the volume fraction of retained austenite increased up to 6.5% with mean carbon content in austenite keeping a constant level. The retained austenite kept K-S orientation relationship with adjacent tempered lath martensite and neighboring secondary twin martensite in different martensite packet. As partitioning time increased up to 3600 s, the initial lath martensite tempered gradually to ferrite without any cementite formation.

In situ postweld heat treatment of transformation induced plasticity steel resistance spot welds

ABSTRACTTransformation-induced plasticity (TRIP) steel resistance spot welds are delicate to low-energy interfacial failure via crack propagation through martensitic fusion zone during cross-tension (CT) loading. This paper addresses the effect of three different types of in situ postweld heat treatment (PWHT) on the mechanical properties of TRIP steel resistance spot welds. Depending on the post weld second pulse current level, three different strengthening mechanisms were found including (i) martensite tempering with reduced hardness, (ii) refining of martensite packets with improved toughness and (iii) nugget re-melting/enlargement combined with possible reduction of grain boundary impurity segregation. All designed in situ PWHTs were enabled to promote pullout failure mode with improved load-bearing capacity and energy absorption capability during CT loading.

Impact of Martensite Spatial Distribution on Quasi-Static and Dynamic Deformation Behavior of Dual-Phase Steel

The effects of microstructure parameters of dual-phase steels on tensile high strain dynamic deformation characteristic were examined in this study. Cold-rolled steel sheets were annealed using three different annealing process parameters to obtain three different dual-phase microstructures of varied ferrite and martensite phase fraction. The volume fraction of martensite obtained in two of the steels was near identical (~ 19 pct) with a subtle difference in its spatial distribution. In the first microstructure variant, martensite was mostly found to be situated at ferrite grain boundaries and in the second variant, in addition to at grain boundaries, in-grain martensite was also observed. The third microstructure was very different from the above two with respect to martensite volume fraction (~ 67 pct) and its morphology. In this case, martensite packets were surrounded by a three-dimensional ferrite network giving an appearance of core and shell type microstructure. All the three steels were tensile deformed at strain rates ranging from 2.7 × 10−4 (quasi-static) to 650 s−1 (dynamic range). Field-emission scanning electron microscope was used to characterize the starting as well as post-tensile deformed microstructures. Dual-phase steel consisting of small martensite volume fraction (~ 19 pct), irrespective of its spatial distribution, demonstrated high strain rate sensitivity and on the other hand, steel with large martensite volume fraction (~ 67 pct) displayed a very little strain rate sensitivity. Interestingly, total elongation was found to increase with increasing strain rate in the dynamic regime for steel with core–shell type of microstructure containing large martensite volume fraction. The observed enhancement in plasticity in dynamic regime was attributed to adiabatic heating of specimen. To understand the evolving damage mechanism, the fracture surface and the vicinity of fracture ends were studied in all the three dual-phase steels.

Effect of Austenitizing Temperature on Mechanical Properties of the Mixed Bainite - Martensite Microstructure in CrMoV Steel

The effect of austenitizing temperature on mechanical properties of the mixed bainite - martensite microstructure in CrMoV steel was studied in the present work. The result showed that at low austenitizing temperature (910°C - 1000°C), the mixed microstructures containing 12-28% volume fraction of lower bainite showed higher yield and tensile strength than fully martensitic microstructure. The partitioning of the prior austenite grain by lower bainite was found to cause a refinement of the martensite packet size. In addition the strength of the lower bainite in the mixed microstructure is enhanced by plastic constraint induced by the surrounding stronger martensite. By increasing the austenitizing temperature from 1000°C to 1200°C (40min), the YS, UTS, %EL, %RA and CVN impact energy decreased for all samples. This is attributed mainly to the increase in austenite grain size and width of bainite sheaves.

Features of the microstructure of Ti–Nb alloy obtained via selective laser melting

Ti–Nb alloy with 40 wt % of Nb is obtained from a composite Ti–Nb powder by means of selective laser melting. The Ti–Nb alloy has a two-phase microstructure. The main β-phase of the solid titanium–niobium solution forms grains ranging in size from ~2 to 20 μm. A nonequilibrium α″-phase is found in the forms of lamellar, globular, and packet martensite inside the grains of the β-phase and along their boundaries.

Origin of low temperature toughness in a 12Cr-10Ni martensitic precipitation hardenable stainless steel

Though ductile to brittle transition (DBT) is a typical feature of body centered cubic materials, the present 12Cr-10Ni precipitation hardened martensitic stainless steel exhibited excellent low temperature impact toughness. This was, however, dependant on the aging temperature. 250°C aging led to higher toughness both at room temperature and at −196°C (77K). Specimens aged at 400 and 500°C, on the other hand, displayed significantly lower sub-zero impact properties. Though martensite packet size, and size distribution, were identical between the two ageing treatments; there was clear evidence of second phase coarsening: from very fine precipitates of less than ~ 10nm to relatively coarser second phase of ~ 5–25nm range. It is suggested that precipitate coarsening and associated loss of coherency are the limiting factors to the DBT performance of this important class of material.

Effect of Ti on microstructure and strengthening behavior in press hardening steels

Effect of Ti addition on the microstructure and strengthening behavior in press hardening steels (PHS) was analyzed by optical metallography (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The results show that the microstructure of PHS is martensite, and two sizes of particles disperse in the martensite matrix during the forming and quenching process. The size of the bigger particles is between 100 and 200 nm, and the small particles are nanometer-sized. The quantity of the particles has a positive relation with the Ti content. More importantly, the microstructure and strengthening mechanism are affected by the precipitating behavior of the particles. Besides the prior austenite grain, martensite packet, block and lath are refined by Ti addition. The steels are strengthened by the fine grains, martensite substructure and precipitates. The uniformly distributed dislocation in the martensite lath, the density of which is between 3.0×1014 cm–2 and 5.0×1014 cm–2, strengthens the steels through associating with fine carbide particles.

SEM Study of Fatigue Crack Propagation in Chromium Martensitic Steel after LCF

Crack propagation after low-cycle fatigue (LCF) deformation has been studied in the chromium martensitic structural steel. Although the study of a fundamental mechanism of fatigue crack growth has received much attention over the last decade, it still remains a sufficiently complex problem and needs full understanding. Moreover, the recent studies show that the cracks propagate discontinuously even on the millisecond timescale, and their growth rate significantly depends on a microstructure of the material. In the present work the boundaries of the former austenitic grains were revealed on the polished surfaces of the thermally treated samples, which subsequently were undergone low-cycle fatigue tests. The experimental studies show that fatigue macrocracks mainly grow along the boundaries of the former austenitic grains, and changetheir propagation direction when crossing the grain boundary, however, remain within 45 ̊ interval with regard the cycling axis. In particular cases, when the boundaries of a martensite packets and those of the former austenite grains lay along the length of a packet, the macrocrack is better formed and with regular borders. After a macrocrack reaches a definite length ~30-50μ, a microcrack is nucleated ahead of the macrocrack tip, and is oriented along the substructure element of the steel. Further deformation tests provide an increase in the length of the main crack via aggregation of microcracks initiated ahead of it during the LCF. In the cases when the macrocrack is deviated, slip bands are formed in martensitic structures along the boundaries of martensite packets (laths). A correlation is revealed between the microcrack components and the substructure elements of the steel as well. The same results were obtained by fractography of the tested and fractured samples. However, in the latter case correlation was established between the cleavage facets and the dimensions of packets.

Hydrogen Environment Assisted Cracking of Modern Ultra-High Strength Martensitic Steels

Martensitic steels (Aermet®100, Ferrium®M54™, Ferrium®S53®, and experimental CrNiMoWV at ultra-high yield strength of 1550 to 1725 MPa) similarly resist hydrogen environment assisted cracking (HEAC) in aqueous NaCl. Cracking is transgranular, ascribed to increased steel purity and rare earth addition compared to intergranular HEAC in highly susceptible 300M. Nano-scale precipitates ((Mo,Cr)2C and (W,V)C) reduce H diffusivity and the K-independent Stage II growth rate by 2 to 3 orders of magnitude compared to 300M. However, threshold K TH is similarly low (8 to 15 MPa√m) for each steel at highly cathodic and open circuit potentials. Transgranular HEAC likely occurs along martensite packet and {110}α′-block interfaces, speculatively governed by localized plasticity and H decohesion. Martensitic transformation produces coincident site lattice interfaces; however, a connected random boundary network persists in 3D to negate interface engineering. The modern steels are near-immune to HEAC when mildly cathodically polarized, attributed to minimal crack tip H production and uptake. Neither reduced Co and Ni in M54 and CrNiMoWV nor increased Cr in S53 broadly degrade HEAC resistance compared to baseline AM100. The latter suggests that crack passivity dominates acidification to widen the polarization window for HEAC resistance. Decohesion models predict the applied potential dependencies of K TH and da/dt II with a single-adjustable parameter, affirming the importance of steel purity and trap sensitive H diffusivity.