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.     

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.     

Effects of transformed ferrite growth on the tensile fracture characteristics of a dual-phase steel

The effects of transformed ferrite growth on the tensile fracture characteristics of a dual-phase steel were investigated by observing crack initiation, propagation, and fracture behaviors. Crack initiation occurred by decohesion between martensite and ferrite. However, cracks propagated along the ferrite-martensite interface in a high temperature quenched specimen, whereas in specimens quenched from lower temperature cracks propagated into the martensite particle. Tensile fracture behaviors were not strongly influenced by the cooling rate. At both cooling rates of 5.6 and 0.1 °C/sec, specimens quenched from high temperature fractured by partially brittle fracture mode, but fracture mode changed to ductile mode as the quenching temperature decreased. The effect of transformed ferrite on the fracture mode was not substantially different from that of retained ferrite. However, the crack initiation and propagation was influenced by the variation in martensite distribution caused by different growth behavior of transformed ferrite.     

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.     

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     

冲击载荷下低屈服比高强钢板的拉伸性能

通过室温下的仪器化冲击试验和静态拉伸试验,研究一种低屈服比高强度钢板在冲击载荷下的力学性能和断裂机理.结果表明:试验钢的组织由细小岛状马氏体与针状铁素体为主构成,马氏体体积分数为27.6%.与静态拉伸性能相比,在名义应变速率为100 s-1的冲击载荷作用下,试验用钢屈服强度提高31.6%,延伸率不降低.在静态和动态载荷下,该钢均以显微空洞长大聚集的方式发生韧性断裂,但显微空洞的形核和长大方式不同.在静态载荷下,显微空洞形核于颈缩区的铁素体晶粒内部或铁素体-马氏体两相界面处,空洞主要通过两相界面的脱开而形成长大;在动态载荷下,显微空洞主要形核于颈缩区的两相界面处,空洞主要通过马氏体粒子的开裂而形成长大.     

Influence of martensite composition and content on the properties of dual phase steels

A study has been made of the mechanical properties of dual phase (martensite plus ferrite) structures produced when Fe-Mn-C alloys are quenched from the austenite plus ferrite phase field, so as to give a series of alloys with constant ferrite and martensite compositions but varying percent martensites. It is found that the strength of a dual phase structure is dependent on the ferrite grain size and the volume fraction of martensite, and is independent of the composition and strength of the martensite. In agreement with previous work the ductility of these steels is superior to that for standard HSLA steels at the same tensile strength. As shown in a previous paper the strength and ductility as a function of percent martensite are in agreement with Mileiko’s theory of composites of two ductile phases. This theory and the results indicate that the superior ductility of dual phase steels is largely a consequence of the high strength (fine grained), highly ductile (low interstitial content) ferrite matrix.     

Effect of hydrogen on fracture behavior of a quenched and tempered medium-carbon steel

This work examined the effects of hydrogen on fracture of quenched and tempered 1045 steel. Tests were made at room temperature on tensile, Charpy impact, and 4-point notched bend specimens. This steel exhibits tempered martensite embrittlement (TME) for tempering temperatures between 300 and 375 °C. Thus hydrogen in most cases affected fracture by increasing the amount of intergranular fracture. In bend specimens, hydrogen also induced quasicleavage (QC) fracture at points of maximum normal stress below the notch root, points which appeared to be the locations of crack initiation. Tear ridges on theseQC surfaces were at martensite lath packet boundaries. Crack orientations were largely mode I in uncharged specimens, with mode II appearing at the notch root in most hydrogen-charged specimens. These observations are in general agreement with earlier work on martensitic steel. Formerly graduate student, Carnegie-Mellon University     

Static and dynamic fracture toughness of a medium carbon microalloyed steel of three different microstructures

A multiphase ferrite-bainite-martensite (F-B-M) microstructure was developed in an automotive grade V-bearing medium carbon microalloyed steel, 38MnSiVS5. It was characterized using optical, scanning, and transmission electron microscopy. The tensile, Charpy impact, and static and dynamic fracture toughness behaviors were evaluated. The results are compared with those of ferrite-pearlite (F-P) and tempered martensite (T-M) microstructures of the same steel. Although the tensile properties of the multiphase microstructures were superior, the Charpy impact and static and dynamic fracture toughness properties were inferior compared with those of the other two microstructures. The F-P condition displayed the highest plane strain fracture toughness value (K_IC), while the T-M condition was characterized by the highest dynamic fracture toughness (conditional) value (K_IDQ). The Charpy impact energy of the T-M condition was greater than that for the other two conditions. An examination of the surfaces of fractured samples revealed predominant ductile crack growth in the F-P microstructure and a mixed mode (ductile and brittle) crack growth in the T-M and the F-B-M microstructures. Although the Charpy impact energy, plane fracture toughness (K_IC), and conditional dynamic fracture toughness (K_IDQ) of the multiphase microstructure were inferior to those of the T-M and the F-P microstructures, the toughness properties were comparable to those of medium carbon low alloy steels having bainite-martensite (AISI 4340) or tempered martensite microstructures.     

Effect of Heat Treatment on Microstructure, Mechanical Properties and Fracture Behaviour of Ship and Dual Phase Steels

 Grade A (GA) and high strength steel DH36 ship steels possessing different chemical compositions were used, and strength properties of GA steel and DH36 steel were compared. Additionally, 4 types of dual phase (DP) steels with different martensite volume fractions (MVFs) were produced from GA steel by means of heat treatment and they were compared with other steels through conducting microstructure, microhardness, tensile and impact tests. The fracture surfaces of specimens (DH36, GA and DP steels) exposed to tensile and Charpy impact tests were investigated by scanning electron microscope. Furthermore, it was found that the specimens quenched from 800 and 900 ℃ had better strength than DH36 steel. The tensile test results indicated that the tensile strength of DP steel water quenched from 900 ℃ was 3 times that of GA steel and twice that of DH36 steel.     

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.     

Effect of size and shape of tungsten particles on dynamic torsional properties in tungsten heavy alloys

The effect of the size and shape of tungsten particles on dynamic torsional properties in tungsten heavy alloys was investigated. Dynamic torsional tests were conducted on seven tungsten alloy specimens, four of which were fabricated by repeated sintering, using a torsional Kolsky bar, and then the test results were compared via microstructure, mechanical properties, adiabatic shear banding, and deformation and fracture mode. The size of tungsten particles and their hardness were increased as sintering temperature and time were increased, thereby deteriorating fracture toughness. The dynamic torsional test results indicated that in the specimens whose tungsten particles were coarse and irregularly shaped, cleavage fracture occurred predominantly with little shear deformation, whereas shear deformation was concentrated into the center of the gage section in the conventionally fabricated specimens. The deformation and fracture behavior of the specimens having coarse tungsten particles correlated well with the observation of the in situ fracture test results, i.e., cleavage crack initiation and propagation. These findings suggested that there would be an appropriate tungsten particle size because the cleavage fracture mode would be beneficial for the “self-sharpening” of the tungsten heavy alloys.     

铍铝合金室温拉伸及原位疲劳失效分析

通过室温拉伸试验和SEM原位疲劳测试,研究了金属型复合材料铍铝合金的拉伸性能及拉伸和原位疲劳的断裂机制.结果表明,合金在室温下具有较好的拉伸性能,且对应变速率有明显的敏感性.拉伸断裂是由脆性铍相的解理断裂和韧性铝相的延性断裂构成的混合型断裂.在拉拉疲劳载荷下,疲劳裂纹萌生于铍相颗粒,断裂模式仍由铍相的解理断裂和铝相的韧性断裂组成.     

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.     

Tempered martensite embrittlement in SAE 4340 steel

The toughness of SAE 4340 steel with low (0.003 wt pct) and high (0.03 wt pct) phosphorus has been evaluated by Charpy V notch (CVN) impact and compact tension plane strain fracture toughness (K _1c) tests of specimens quenched and tempered up to 673 K (400°C). Both the high and low P steel showed the characteristic tempered martensite embrittlement (TME) plateau or trough in room temperature CVN impact toughness after tempering at temperatures between 473 K (200°C) and 673 K (400°C). The CVN energy absorbed by low P specimens after tempering at any temperature was always about 10 J higher than that of the high P specimens given the same heat treatment. Interlath carbide initiated cleavage across the martensite laths was identified as the mechanism of TME in the low P 4340 steel, while intergranular fracture, apparently due to a combination of P segregation and carbide formation at prior austenite grain boundaries, was associated with TME in the high P steel.K _IC values reflected TME in the high P steels but did not show TME in the low P steel, a result explained by the formation of a narrow zone of ductile fracture adjacent to the fatigue precrack during fracture toughness testing. The ductile fracture zone was attributed to the low rate of work hardening characteristic of martensitic steels tempered above 473 K (200°C).     

Influence of Isothermal Deformation Conditions on The Mechanical Properties of 22MnB5 HPF Steel

In the present study, the aluminized 22MnB5 Hot Press Forming (HPF) steel was deformed isothermally in the temperature range of 600–800 °C. The specimens were deformed with 0%, 10% and 30% engineering strain at a strain rate of 0.5/s. Mechanical stabilization of austenite during phase transformation was observed. The influence of strain rate was also studied. It was found that the martensite transformation was affected by the combined effect of mechanical stabilization and dynamic recovery. The influence of high temperature deformation on the final microstructure, microhardness and mechanical properties were studied. Deformation applied below 750 °C resulted in a significant decrease of the strength and the hardness. The ferrite fraction and morphology had a clear effect on the room temperature tensile fracture morphology.     

Through-thickness fracture of a Ti-V-N plate steel

The effects of microstructure on through-thickness fracture properties of a Ti-V-N plate steel have been determined directly by through-thickness tensile tests and indirectly by studying delamination fractures in longitudinal tensile and Charpy tests. The initiation of ductile fracture is primarily controlled by inclusions, but overall ductility is influenced by microstructure such that the tensile fracture strain is higher for ferrite-pearlite microstructures than for ferrite-bainite or ferrite-martensite. The cleavage fracture stress is lower for steels which have been rolled belowAr ₃ and contain deformed ferrite than for steels finish rolled aboveAr ₃. Measurements of true stress and true strain for fracture initiation qualitatively fit a model which assumes cleavage fracture occurs at a critical stress, ductile rupture at a critical strain, and a transition fracture mode comprising ductile initiation followed by cleavage. Formerly Visiting Research Fellow, Metals Technology Laboratories, CANMET, Ottawa, ON, Canada Formerly Research Scientist, Metals Technology Laboratories, CANMET     

Through-thickness fracture of a Ti-V-N plate steel

The effects of microstructure on through-thickness fracture properties of a Ti-V-N plate steel have been determined directly by through-thickness tensile tests and indirectly by studying delamination fractures in longitudinal tensile and Charpy tests. The initiation of ductile fracture is primarily controlled by inclusions, but overall ductility is influenced by microstructure such that the tensile fracture strain is higher for ferrite-pearlite microstructures than for ferrite-bainite or ferrite-martensite. The cleavage fracture stress is lower for steels which have been rolled belowAr ₃ and contain deformed ferrite than for steels finish rolled aboveAr ₃. Measurements of true stress and true strain for fracture initiation qualitatively fit a model which assumes cleavage fracture occurs at a critical stress, ductile rupture at a critical strain, and a transition fracture mode comprising ductile initiation followed by cleavage. Formerly Visiting Research Fellow, Metals Technology Laboratories, CANMET, Ottawa, ON, Canada Formerly Research Scientist, Metals Technology Laboratories, CANMET     

深冷对逆相变处理中锰钢组织性能的影响

为了研究原始组织状态对逆相变退火中锰钢微观组织和力学性能的影响,对淬火处理的中锰钢再进行-74℃深冷处理,采用SEM、EBSD、XRD等手段评价了逆相变退火处理后的微观组织,用单轴拉伸和冲击试验评价力学性能.研究结果表明:淬火态中锰钢的组织由马氏体和体积分数约为33％的残余奥氏体组成,深冷处理后得到残余奥氏体体积分数小...