Effects of martensite morphology and tempering on dynamic deformation behavior of dual-phase steels

The effects of martensite morphology and tempering on the quasistatic and dynamic deformation behavior of dual-phase steels were investigated in this study. Dynamic torsional tests were conducted on six steel specimens, which had different martensite morphologies and tempering conditions, using a torsional Kolsky bar, and then the test data were compared via microstructures, tensile properties, and fracture mode. Bulky martensites were mixed with ferrites in the step-quenched (SQ) specimens, but small martensites were well distributed in the ferrite matrix in the intermediate-annealed (IA) specimens. Under a dynamic loading condition, the fracture mode of the SQ specimens was changed from cleavage to ductile fracture as the tempering temperature increased, whereas the IA specimens showed a ductile fracture mode, irrespective of tempering. These phenomena were analyzed in terms of a rule of mixtures applied to composites, microstructural variation, martensite softening and carbon diffusion due to tempering, and adiabatic shear-band formation.     

Influence of martensite content and morphology on the toughness and fatigue behavior of high-martensite dual-phase steels

A series of high-martensite dual-phase (HMDP) steels exhibiting a 0.3 to 0.8 volume fraction of martensite (V _m ), produced by intermediate quenching (IQ) of a vanadium and boron-containing microalloyed steel, have been studied for toughness and fatigue behavior to supplement the contents of a recent report by the present authors on the unusual tensile behavior of these steels. The studies included assessment of the quasi-static and dynamic fracture toughness and fatigue-crack growth (FCG) behavior of the developed steels. The experimental results show that the quasi-static fracturetoughness (K _ICV ) increases with increasing V _m in the range between V _m =0.3 and 0.6 and then decreases, whereas the dynamic fracture-toughness parameters (K _ID , K _D , and J _ID ) exhibit a significant increase in their magnitudes for steels containing 0.45 to 0.60 V _m before achieving a saturation plateau. Both the quasi-static and dynamic fracture-toughness values exhibit the best range of toughnesses for specimens containing approximately equal amounts of precipitate-free ferrite and martensite in a refined microstructural state. The magnitudes of the fatigue threshold in HMDP steels, for V _m between 0.55 and 0.60, appear to be superior to those of structural steels of a similar strength level. The Paris-law exponents (m) for the developed HMDP steels increase with increasing V _m , with an attendant decrease in the pre-exponential factor (C).     

Effects of microstructural morphology on quasi-static and dynamic deformation behavior of Ti-6Al-4V alloy

The effects of microstructural morphology on quasi-static and dynamic deformation behavior of a Ti-6Al-4V alloy were investigated in this study. Quasi-static and dynamic torsional tests were conducted using a torsional Kolsky bar for Widmanstätten, equiaxed, and bimodal microstructures, which were processed by different heat treatments, and then, the test data were analyzed in relation to microstructures, tensile properties, and fracture mode. Quasi-static torsional properties showed a tendency similar to tensile properties and ductile fracture occurred in all three microstructures. Under dynamic torsional loading, maximum shear stress of the three microstructures was higher and fracture shear strain was lower than those under quasi-static loading, but the overall tendency was similar. In the Widmanstätten and equiaxed microstructures, adiabatic shear bands were found in the deformed region of the fractured specimens. The possibility of the adiabatic shear band formation under dynamic loading was quantitatively analyzed, depending on how plastic deformation energy was distributed to either void initiation or adiabatic shear banding. It was found to be most likely in the equiaxed microstructure, whereas it was least likely in the bimodal microstructure.     

Influence of martensite content and morphology on tensile and impact properties of high-martensite dual-phase steels

A series of dual-phase (DP) steels containing finely dispersed martensite with different volume fractions of martensite (V _m) were produced by intermediate quenching of a boron- and vanadium-containing microalloyed steel. The volume fraction of martensite was varied from 0.3 to 0.8 by changing the intercritical annealing temperature. The tensile and impact properties of these steels were studied and compared to those of step-quenched steels, which showed banded microstructures. The experimental results show that DP steels with finely dispersed microstructures have excellent mechanical properties, including high impact toughness values, with an optimum in properties obtained at ∼0.55 V _m. A further increase in V _m was found to decrease the yield and tensile strengths as well as the impact properties. It was shown that models developed on the basis of a rule of mixtures are inadequate in capturing the tensile properties of DP steels with V _m>0.55. Jaoul-Crussard analyses of the work-hardening behavior of the high-martensite volume fraction DP steels show three distinct stages of plastic deformation.     

The influence of martensite shape,concentration, and phase transformation strain on the deformation behavior of stable dual-phase steels

A continuum model is developed to examine the influence of martensite shape, volume fraction, phase transformation strain, and thermal mismatch on the initial plastic state of the ferrite matrix following phase transformation and on the subsequent stress-strain behavior of the dual-phase steels upon loading. The theory is developed based on a relaxed constraint in the ductile matrix and an energy criterion to define its effective stress. In addition, it also assumes the martensite islands to possess a spheroidal shape and to be randomly oriented and homogenously dispersed in the ferrite matrix. It is found that for a typical water-quenched process from an intercritical temperature of 760 °C, the critical martensite volume fraction needed to induce plastic deformation in the ferrite matrix is very low, typically below 1 pct, regardless of the martensite shape. Thus, when the two-phase system is subjected to an external load, plastic deformation commences immediately, resulting in the widely observed “continuous yielding” behavior in dual-phase steels. The subsequent deformation of the dual-phase system is shown to be rather sensitive to the martensite shape, with the disc-shaped morphology giving rise to a superior overall response (over the spherical type). The stress-strain relations are also dependent upon the magnitude of the prior phase transformation strain. The strength coefficienth and the work-hardening exponentn of the smooth, parabolic-type stress-strain curves of the dual-phase system also increase with increasing martensite content for each selected inclusion shape. Comparison with an exact solution and with one set of experimental data indicates that the theory is generally within a reasonable range of accuracy. Formerly Visiting Professor, Department of Mechanical and Aerospace Engineering, Rutgers University     

Effects of volume fraction of tempered martensite on dynamic deformation properties of a Ti-6Al-4V alloy having a bimodal microstructure

The effects of the volume fraction of tempered martensite on the tensile and dynamic deformation properties of a Ti-6Al-4V alloy having a bimodal microstructure were investigated in this study. Five microstructures having various tempered-martensite volume fractions were obtained by varying heat-treatment conditions. Dynamic torsional tests were conducted on them using a torsional Kolsky bar. The test data were analyzed in relation to microstructures, tensile properties, and adiabatic shear-band formation. Under a dynamic loading condition, the maximum shear stress increased with increasing tempered-martensite volume fraction, whereas the fracture shear strain decreased. Observation of the deformed area after the dynamic torsional test indicated that a number of voids initiated mainly at α-phase/tempered-martensite interfaces, and that the number of voids increased with increasing martensite volume fraction. Adiabatic shear bands of 6 to 10 μm in width were formed in the specimens having lower martensite volume fractions, while they were not formed in those having higher martensite volume fractions. The possibility of adiabatic shear-band formation was explained by concepts of absorbed deformation energy and void initiation. jointly appointed with the Materials Science and Engineering Department, Pohang University of Science and Technology     

Effects of volume fraction of tempered martensite on dynamic deformation properties of a Ti-6Al-4V alloy having a bimodal microstructure

The effects of the volume fraction of tempered martensite on the tensile and dynamic deformation properties of a Ti-6Al-4V alloy having a bimodal microstructure were investigated in this study. Five microstructures having various tempered-martensite volume fractions were obtained by varying heat-treatment conditions. Dynamic torsional tests were conducted on them using a torsional Kolsky bar. The test data were analyzed in relation to microstructures, tensile properties, and adiabatic shear-band formation. Under a dynamic loading condition, the maximum shear stress increased with increasing tempered-martensite volume fraction, whereas the fracture shear strain decreased. Observation of the deformed area after the dynamic torsional test indicated that a number of voids initiated mainly at α-phase/tempered-martensite interfaces, and that the number of voids increased with increasing martensite volume fraction. Adiabatic shear bands of 6 to 10 μm in width were formed in the specimens having lower martensite volume fractions, while they were not formed in those having higher martensite volume fractions. The possibility of adiabatic shear-band formation was explained by concepts of absorbed deformation energy and void initiation.     

Acoustic emission during deformation of dual-phase steels

The study of acoustic emission (AE) during deformation of dual-phase steels consisting of ferrite (F) and pearlite (P)/martensite (M) indicates that the AE peak in the yielding region always appears at the beginning of the macroplastic deformation. After macroyielding starts, the AE decreases because the dislocation velocity, ν_d, decreases. The total AE energy, ∑E_lp, emitted during Lüders band propagation is related to the Lüders strain, ε_l and ∑E_lp/ ε_l, decreases as ε_l increases because of the increase in ε, resulting from the decrease in dislocation velocity, ν_d. After quenching from the two-phase region at cooling rates larger than a certain critical value, a second AE peak, which is produced by the cracking mainly at the interfaces between M and F and secondly in martensitic particles, appears in a certain plastic strain range in addition to the one in the yielding region. As the cooling rate becomes too fast, the AE peak in the yielding region disappears, and the second AE peak cannot be completed due to the brittle fracture.     

The morphology and substructure of martensite in maraging steels

Thin foil transmission electron microscopy, X-ray diffraction and dilatometric techniques have been used to study the martensitic γ → α transformation in three steels with nominal contents of 8 pct nickel and 0.2 pct beryllium and chromium contents of 12, 14 and 16 pct. In each case the martensite formed as laths with a habit plane close to {225}_γ. With increasing chromium content and increasing cooling rate greater numbers of the laths were observed to be internally twinned. Detailed analysis of the martensitic transformation suggested that the internally twinned laths are formed by a sequence of γ→ ε or faulted γ→ ά. The orientation relationships between the three phases γ, ε and α, determined from selected area diffraction analysis, corresponded to Kurdjumov-Sachs.     

合金元素对双相钢形变及退火织构的影响

分析了双相钢冷变形过程中形变织构的演变规律以及合金元素对双相钢形变织构的影响规律。分析结果表明,虽然存在有少量珠光体,但双相钢冷变形织构与体心立方金属相近;非碳化物形成元素Si的加入对形变织构没有明显的影响;碳化物形成元素Cr和Nb导致形变织构在α取向线的{112}<110>取向位置上的取向密度增强。测试分析了双相退火过程中钢的织构演变规律。结果表明,退火时晶粒的取向将沿着α取向线向γ取向线位置转变,Si或Cr对晶粒取向改变的阻碍作用不明显,微合金元素Nb明显阻碍晶粒取向改变,退火织构仍保持很高的α取向线织构组分。对上述的实验结果进行了理论分析。     

Fatigue crack propagation in dual-phase steels: Effects of ferritic-martensitic microstructures on crack path morphology

microstructures with maximum resistance to fatigue crack extension while maintaining high strength levels. A wide range of crack growth rates has been examined, from ~10^-8 to 10^-3 mm per cycle, in a series of duplex microstructures of comparable yield strength and prior austenite grain size where intercritical heat treatments were used to vary the proportion, morphology, and distribution of the ferrite and martensite phases. Results of fatigue crack propagation tests, conducted on “long cracks” in room temperature moist air environments, revealed a very large influence of microstructure over the entire spectrum of growth rates at low load ratios. Similar trends were observed at high load ratio, although the extent of the microstructural effects on crack growth behavior was significantly less marked. Specifically, microstructures containing fine globular or coarse martensite in a coarse-grained ferritic matrix demonstrated exceptionally high resistance to crack growth without loss in strength properties. To our knowledge, these microstructures yielded the highest ambient temperature fatigue threshold stress intensity range ΔK₀ values reported to date, and certainly the highest combination of strength and ΔK₀ for steels (i.e., ΔK₀ values above 19 MPa√m with yield strengths in excess of 600 MPa). Such unusually high crack growth resistance is attributed primarily to a tortuous morphology of crack path which results in a reduction in the crack driving force from crack deflection and roughness-induced crack closure mechanisms. Quantitative metallography and experimental crack closure measurements, applied to currently available analytical models for the deflection and closure processes, are presented to substantiate such interpretations. Formerly Lecturer and Research Engineer in the Department of Materials Science and Mineral Engineering, University of California     

Effects of martensite morphology on the aging behaviour of virgin martensite

The low temperature aging behaviour of virgin martensite with two different morphologies, thin plate and lenticular martensites, has been studied in the present work. It is revealed by means of internal friction and electrical resistivity measurements that the aging behaviour between these two morphological types of martensite is quite different at temperatures below about 200 K. However, when temperature is above about 250 K, electrical resistivity results show a similar tendency during aging between these two types of martensite, which is in line with the internal friction results obtained. It has also been found that the electrical resistivity started to change even at temperatures as low as 22 K.     

Microstructure and tensile behavior of nitrogen-alloyed,dual-phase stainless steels

Two alloys of high-nitrogen stainless steel have been heat treated to produce dual-phase microstruc-tures. The first alloy, N10CrNiMol7 1, a Ni-containing stainless steel, was processed conventionally. The second alloy, N20CrMol7, a Ni-free stainless steel, was processed to obtain a higher nitrogen content by pressurized electroslag remelting. The martensite in N10CrNiMol7 1 was homogeneously distributed in the ferrite and obtained a near-constant volume fraction as a function of intercritical annealing temperature. Microprobe analysis and microhardness measurements of the martensite con-stituent suggested that up to 0.4 pct N was dissolved in the austenite before quenching. Austenite formation, martensite transformation, undissolved nitrides, and retained austenite were evaluated by transmission electron microscopy (TEM). The Ni-containing alloy exhibited classic dual-phase tensile behavior in that continuous yielding was observed together with good combinations of ultimate tensile strength and total elongation. The martensite constituent in alloy N20CrMol7 was concen-trated within bands. Comparison of tensile properties of the two alloys at similar volume fractions and hardness levels of martensite and ferrite showed that the microstructure containing banded mar-tensite had inferior combinations of strength and ductility. The degradation of tensile ductility was accompanied by a fracture mode transition from microvoid coalescence to transgranular cleavage. The deformation and fracture behavior of both alloys were related to the microstructure.     

The influence of Mo2C morphology and distribution on the fatigue crack initiation and propagation behavior of Fe-C-Mo dual-phase steels

Dual-phase microstructures consisting of ferrite with carbides (Mo₂C) surrounding equiaxed martensite packets have been developed in two alloys, Fe-O. 2C-4Mo and Fe-O. 2C-2Mo. These alloys were chosen because of the presence of two distinct carbide morphologies: (1) a needle-shaped interphase carbide structure, and (2) a fibrous carbide structure. Isothermal transformations were used to control the carbide morphology and distribution in the ferritic regions of the dual-phase microstructures. In the present research the effects of changes in carbide structure on low cycle fatigue (LCF) and fatigue crack growth (FCG) behavior were studied. Crack initiation was observed at prior austenite grain boundaries in the fibrous microstructure, and along intrusion/extrusion defects in the interphase needle microstructures for LCF tests. TEM studies revealed a carbide free region at prior austenite grain boundaries where initiation occurs for the fibrous case. The cyclic stress/strain response of the ferritic portions of the microstructure is determined by the ability of the carbides to homogenize the strain found there. This affects the stress/strain distribution in the composite ferrite-martensite microstructure by changing the hardness ratio of the two phases and subsequently alters the fatigue crack growth behavior and the macroscopic cyclic stress/strain response. Strain localization was also found to affect the roughness induced closure found for fatigue crack growth tests for low R tests (R = 0.1).     

The effects of deformation induced martensite on the sensitization of austenitic stainless steels

This paper compares the effects of deformation which induces martensite in austenitic stainless steel with deformation which does not on the sensitization and corrosion susceptibility of these alloys. We show that deformation which induces martensite causes rapid sensitization at temperatures below 600 °C, leads to extensive transgranular corrosion, and can produce rapid healing. The martensite is also an area of extensive carbide precipitation. Deformation alone noticeably increases the kinetics of sensitization only at temperatures where undeformed samples are readily sensitized. Without the presence of martensite, intergranular corrosion is always the predominant corrosion path, rapid healing is not observed, and most carbides precipitate along the grain boundaries.     

Cyclic deformation behavior of a transformation-induced plasticity-aided dual-phase steel

Cyclic hardening-softening behavior of a TRIP-aided dual-phase (TDP) steel composed of a ferrite matrix and retained austenite plus bainite second phase was examined at temperatures ranging from 20 °C to 200 °C. An increment of the cyclic hardening was related to (1) a long-range internal stress due to the second phase and (2) the strain-induced transformation (SIT) behavior of the retained austenite, as follows. Large cyclic hardening, similar to a conventional ferrite-martensite dual-phase steel, appeared in the TDP steel deformed at 20 °C, where the SIT of the retained austenite occurred at an early stage. This was mainly caused by a large increase in strain-induced martensite content or strain-induced martensite hardening, with a small contribution of the internal stress. In this case, shear and expansion strains on the SIT considerably decreased the internal stress in the matrix. With increasing deformation temperature or retained austenite stability, the amount of cyclic hardening decreased with a significant decrease in plastic strain amplitude. This interesting cyclic behavior was principally ascribed to the internal stress, which was enhanced by stable and strain-hardened retained austenite particles.     

Effect of martensite content on friction and oxidative wear behavior of 0.42 Pct carbon dual-phase steel

In order to explore the tribological potential of the dual-phase (DP) steel as a wear-resistant material, the friction and wear characteristics have been investigated for this steel with varying amounts of martensite from 42 to 72 vol pct, developed by varying holding time at the intercritical annealing temperature of 740 °C. Dry sliding wear tests have been conducted on DP steels containing 0.42 wt pct carbon using a pin-on-disk machine under different normal loads of 14.7, 19.6, 24.5, 29.4, and 34.3 N and at a constant sliding speed of 1.15 m/s. Weight loss has been measured at different time intervals on the same specimen. The variation of cumulative volume loss with sliding distance has been represented by two linear segments signifying the run-in and the steady state of wear. The mechanism of wear is primarily oxidative in nature, which has been confirmed by X-ray diffraction patterns of the wear debris generated during sliding. The wear rate varies linearly with load in both the run-in and the steady state. The wear rate decreases linearly with increasing volume fraction of martensite in DP steels reflecting the effect of hardness imparted by the increasing amount of martensite, which is a hard and load bearing phase. The average coefficient of friction also decreases linearly with increasing load as well as with increasing martensite volume fraction. In the run-in stage, the wear coefficient does not change significantly between DP1 and DP2 steels containing 42 and 51 vol pct martensite, respectively, but decreases sharply as one moves from DP2 to DP4 containing, respectively, 51 and 72 vol pct martensite. But in the steady state, the wear coefficient decreases almost linearly with increasing volume fraction of the martensite. The decrease in wear coefficient may be attributed to the decreasing wear rate dominating over the decrease in real area of contact due to increasing hardness.     

Influence of austenite and martensite strength on martensite morphology

The martensite morphology and austenite flow strength have been determined in a variety of ferrous alloys chosen so that the austenites were paramagnetic, ferromagnetic, substitutional strengthened, and interstitial strengthened. It is demonstrated that two of the most important variables in determining the habit plane (and thus morphology) of martensite in a given alloy are the resistances to dislocation motion in austenite and in ferrite (i. e., martensite). In the wide variety of alloys where martensite with a {259}_γ habit plane was observed, the austenite flow strength atM _s is greater than 30,000 psi. At lower austenite strengths, either {225}_γ or {111}_γ habit planes are found depending on the resistance to dislocation motion in ferrite. Thus, {225} martensites are not always found as part of the spectrum between {111} and {259} martensites but only in the cases (e. g., interstitial strengthening) where ferrite is preferentially strengthened relative to austenite. All of the observations are consistent with the idea that the habit plane observed in a given alloy is the one involving the minimum plastic work for the lattice invariant shear.