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     

Microstructural effects on fatigue crack growth in a low carbon steel

A study of the influence of microstructure on fatigue crack growth in an AISI 1018 steel has been carried out. Two distinctly different duplex microstructures were investigated. In one microstructure ferrite encapsulated islands of martensite; in the other martensite encapsulated islands of ferrite. The latter structure resulted in a significant increase in threshold level (18 MPa√mvs 8 MPa√m) together with an increase in yield strength. Fractographic analysis was used to investigate the influence of microstructure on the mode of fatigue crack growth. Formerly at the University of Connecticut     

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).     

Near threshold fatigue crack growth behavior of a titanium alloy: Ti-6A1-4V

Near threshold fatigue crack growth behavior of Ti-6A1-4V alloy was investigated as function of Widmanstatten microstructures. In particular, the effect of colony size on ΔK_th, ΔK_eff,th and K_cl,th has been studied. It has been found that crack growth rates are strongly affected by the size of microstructural features such as colonies and α laths. However, the microstructural units controlling crack growth are colonies in fast cooled microstructures consisting of fine Widmanstatten colonies while they are α laths in relatively slow cooled ones with coarse colonies. As a result, the projection in literature of increased fatigue crack growth resistance at large colony sizes in titanium alloys can not be generalized. This distinction appears to be brought about by the thick continuous interplatelet β phase present in slow cooled structures. In fast cooled structures, thin discontinuous β phase is seen to be ineffective in arresting slip or crack. However, in slow cooled ones thick β phase appears to effectively retard slip/crack in fatigue. The thickness, composition, intrinsic properties such as modulus, ductility of β phase and an added environmental effect have been suggested to be important in this respect. The crack growth rates and the magnitudes of ΔK_th and ΔK_eff,th can be uniquely ordered when compared in terms of the controlling microstructural units in respective microstructures. However, crack closure levels at threshold appear to be dependent on colony size. In addition, an increase in the intrinsic crack growth resistance ΔK_eff,th appears to exist when the cyclic plastic zone size approaches the thickness of α laths in the microstructure.     

The effect of microstructure and environment on fatigue crack closure of 7475 aluminum alloy

The effects of slip character and grain size on the intrinsic material and extrinsic closure contributions to fatigue crack growth resistance have been studied for a 7475 aluminum alloy. The alloy was tested in the underaged and overaged conditions with grain sizes of 18 μm and 80 μm. The fracture surface exhibited increased irregularity and planar facet formation with increased grain size, underaging, and tests in vacuum. These changes were accompanied by an increased resistance to fatigue crack growth. In air the 18 μm grain size overaged material exhibited relatively poor resistance to fatigue crack growth compared with other microstructural variants, and this was associated with a lower stress intensity for closure. All materials exhibited a marked improvement in fatigue crack growth resistance when tested in vacuum, with the most significant difference being ˜1000× at a ΔK of 10 MPa m^1/2 for the 80 μm grain size underaged alloy. This improvement could not be accounted for by either an increase in closure or increased crack deflection and is most likely due to increased slip reversibility in the vacuum environment. The intrinsic resistance of the alloy to fatigue crack growth was microstructurally dependent in vacuum, with large grains and planar slip providing the better fatigue performance.     

Strength and fatigue properties in medium carbon duplex steels

The effect of microstructure on strength and fatigue properties has been investigated in two medium carbon alloy steels (BS 817M40 and BS 835M30) by developing dual-phase, ferritic-martensitic microstructures. Hardness-strength relationships and fatigue resistance at comparatively high strength levels were investigated by producing various microstructures. Conventional quenching and tempering, intercritical annealing and step quenching were used to vary the proportion, morphology and distribution of the ferrite and martensite phases. The results of the present study show that both hardness and strength increase with increasing proportion of martensite and/or hardness of the second phase. The relationship between hardness or strength and martensite percent is not in good agreement with a simple “law of mixtures” but is compatible with a more rapid strength increase at high martensite contents. The dual phase microstructures from the present study show superior near threshold ΔK_TH values than normal tempered martensite. The results also show a high degree of correlation between Paris equation m values and fracture toughness K_IC, showing that for high m values K_IC is low and vice versa. The present experiments show that although crack initiation resistance in dual-phase steels is excellent crack propagation rates are higher than in quenched and tempered microstructures for a given ΔK.     

The effect of microstructure on fatigue crack propagation in iron-carbon alloys

The dependence of fatigue crack growth rate on the cyclic stress intensity factor was determined for six iron-carbon alloys ranging in carbon content from 0.23 to 1.08 wt pct carbon. Both ferrite/pearlite and ferrite/free iron carbide microstructures were studied. Scanning electron microscope fractography studies correlated the fatigue mechanism with microstructure. It was found that when the predominant mode of crack growth was ductile, the crack growth rateda/dN could be related to the cyclic stress intensity factor ΔK by an equation of the formda/dN = (ΔK)_m where andm are constants. The constantm was approximately equal to four when the crack growth mechanism presumably was the blunting and resharpening of the crack tip by slip processes. The constantm was greater than four when the crack growth mechanism was void coalescence in the interlamella ferrite of pearlite colonies. The preferred fatigue crack path through the pearlitic alloys was through the free ferrite phase. formerly Research Assistant at Materials Science and Engineering Department and Materials Research Center, Northwestern University.     

The effect of tempering temperature on near- threshold fatigue crack behavior in quenched and tempered 4140 steel

Fatigue crack growth in compact tension samples of high purity 4140 steel quenched and tempered to various strength levels was investigated. Tempering temperatures of 200, 400, 550, and 700 °C produced yield strengths from 1600 to 875 MPa, respectively. Crack propagation and crack closure were monitored inK-decreasing tests performed underR = 0.05 loading conditions in laboratory air. Results indicated that as the yield strength increased the crack growth rate increased at a given ΔK and ΔK_th decreased. Threshold values varied from 2.8 MPa m^1/2 (200 °C temper) to 9.5 MPa m_1/2 (700 °C temper). Cracks in the 200 °C tempered samples grew by an intergranular mechanism following prior austenite grain boundaries probably caused by hydrogen embrittlement or tempered martensite embrittlement. Tempering above 200 °C produced transgranular fatigue crack growth. The level of crack closure increased with tempering temperature and with crack propagation in a given tempered condition. Crack closure was caused by a combination of plasticity-induced and oxide-induced mechanisms. The use of an effective stress intensity range based on crack closure consolidated the fatigue crack growth curves and the threshold values for all tempering temperatures except 200 °C. Formerly Graduate Research Assistant, Department of Materials Science and Engineering, Stanford University, Stanford, CA. Formerly Professor, Department of Materials Science and Engineering, Stanford University, Stanford, CA.     

Fatigue crack propagation of metastable beta titanium-vanadium alloys

The fatigue crack propagation behavior of three titanium-vanadium alloys (24, 28, and 32 wt pct V) which have tensile deformation modes ranging from coarse twinning to wavy and planar slip has been measured in laboratory air and correlated with their low cycle fatigue properties and microstructure. The fatigue crack growth rate of alloys with similar microstructures but different deformation modes, and of alloys with similar deformation modes but different microstructures have been compared. Increasing the deformation barrier mean free path and improving low cycle fatigue properties has been observed to reduce the fatigue crack growth rate at low and inter mediate ΔK levels. The fatigue crack growth data have been compared with that calculated from equations which use microstructure and low cycle fatigue parameters. The predictive capability of these equations which contain only measurable parameters has been found to be quite adequate.     

连续退火与连续热镀锌590MPa级双相钢的疲劳性能

采用拉-拉疲劳实验研究了连续退火与连续热镀锌590MPa级冷轧双相钢的疲劳性能,绘制了它们的S-N曲线,并采用扫描电镜对其疲劳断裂特征进行了分析。结果表明,连续退火与连续热镀锌冷轧双相钢在应力比R=0.1,加载频率为15Hz的条件下的疲劳极限分别为217和216MPa,通过对疲劳试样表面的观察发现,表面铁素体/马氏体界面开裂是疲劳失效的主要原因。     

Cu-bearing high-strength low-alloy steels: The influence of microstructure on the initiation and growth of small fatigue cracks

The fatigue characteristics of a Cu-bearing high-strength low-alloy (HSLA) steel were investigated in air, relative humidity ≈50 pct, as a function of microstructure, which was altered by heat treatments and welding. Small fatigue cracks (≈30-Μm long) were naturally initiated from smooth specimens and grown past the transition length (≈200 Μm), where they exhibited the characteristics of large fatigue cracks. The number of cycles to crack initiation depended on stress magnitude but not on microstructure, although the site of initiation was microstructurally dependent. Small cracks in all microstructures grew at δK values below the large crack threshold. The as-received (polygonal ferrite) microstructure and one of the lath microstructures that resulted from heat treatment exhibited the same growth rate correlation as large cracks in the linear (Paris) region, and could be considered as an extension of the large crack growth region down to the point of initiation. Small cracks grew at rates faster than expected through one of the heat-treated and the weld microstructures; therefore, the number of cycles required for growth from initiation to the transition to large crack growth decreased about threefold, which is a potentially important factor in predicting lifetimes of structures made from this steel.     

A Novel Ni-Containing Powder Metallurgy Steel with Ultrahigh Impact,Fatigue, and Tensile Properties

The impact toughness of powder metallurgy (PM) steel is typically inferior, and it is further impaired when the microstructure is strengthened. To formulate a versatile PM steel with superior impact, fatigue, and tensile properties, the influences of various microstructures, including ferrite, pearlite, bainite, and Ni-rich areas, were identified. The correlations between impact toughness with other mechanical properties were also studied. The results demonstrated that ferrite provides more resistance to impact loading than Ni-rich martensite, followed by bainite and pearlite. However, Ni-rich martensite presents the highest transverse rupture strength (TRS), fatigue strength, tensile strength, and hardness, followed by bainite, pearlite, and ferrite. With 74 pct Ni-rich martensite and 14 pct bainite, Fe-3Cr-0.5Mo-4Ni-0.5C steel achieves the optimal combination of impact energy (39 J), TRS (2170 MPa), bending fatigue strength at 2 × 10⁶ cycles (770 MPa), tensile strength (1323 MPa), and apparent hardness (38 HRC). The impact energy of Fe-3Cr-0.5Mo-4Ni-0.5C steel is twice as high as those of the ordinary high-strength PM steels. These findings demonstrate that a high-strength PM steel with high-toughness can be produced by optimized alloy design and microstructure.     

Short fatigue crack growth behavior in a ferritic-bainitic steel

Short fatigue crack growth behavior was studied in a ferrite-bainite microstructure in C-Mn steel with respect to microstructural variations. Specimens were subjected to cyclic loading at three different stress levels: 559, 626, and 687 MPa. The crack propagation rates varied from 10^-4 to 10^-2 μm/cycle. Crack lengths were measured using a replication technique. The growth rates were systematically decreased at microstructural heterogeneities up to a length of 3 to 4 grain diameters. A two-stage short fatigue crack growth model previously developed by Hussain et al. was modified to predict the crack growth behavior. The calculated values were within 10 pct error of the experimentally determined results. The model was then used to present the effect of grain boundaries on cracks propagating at constant rates. It was shown that the mode of presenting of the fatigue data can help in understanding different practical problems in stage I. These include situations such as block loading and short-duration stress spikes in nonpropagating crack regimes.     

Mechanisms of ambient temperature fatigue crack growth in Ti-46.5Al-3Nb-2Cr-0.2W

Fatigue crack growth studies have been conducted on a two-phase alloy with a nominal composition of Ti-46.5Al-3Nb-2Cr-0.2W (at. pct), heat treated to produce duplex and lamellar microstructures. Fatigue crack growth tests were conducted at 23 °C using computer-controlled servohydraulic loading at a cyclic frequency of 20 Hz. Several test methods were used to obtain fatigue crack growth rate data, including decreasing-load-range-threshold, constant-load-range, and constant-K _max increasing-load-ratio crack growth control. The lamellar microstructure showed substantial improvement in crack growth resistance and an increase in the threshold stress intensity factor range, ΔK _th , when compared with the behavior of the duplex microstructure. The stress ratio had a significant influence on crack growth behavior in both microstructures, which appeared to be a result of roughness-induced crack closure mechanisms. Fractographic characterization of fatigue crack propagation modes indicated a highly tortuous crack path in the fully lamellar microstructure, compared to the duplex microstructure. In addition, limited shear ligament bridging and secondary cracking parallel to the lamellar interfaces were observed in the fully lamellar microstructure during fatigue crack propagation. These observations were incorporated into a model that analyzes the contribution of intrinsic vs extrinsic mechanisms, such as shear ligament bridging and roughness-induced crack closure, to the increased fatigue crack growth resistance observed for the fully lamellar microstructure.     

The Effect of the Initial Microstructure on Recrystallization and Austenite Formation in a DP600 Steel

The effects of initial microstructure and thermal cycle on recrystallization, austenite formation, and their interaction were studied for intercritical annealing of a low-carbon steel that is suitable for industrial production of DP600 grade. The initial microstructures included 50 pct cold-rolled ferrite–pearlite, ferrite–bainite–pearlite and martensite. The latter two materials recrystallized at similar rates, while slower recrystallization was observed for ferrite–pearlite. If heating to an intercritical temperature was sufficiently slow, then recrystallization was completed before austenite formation, otherwise austenite formed in a partially recrystallized microstructure. The same trends as for recrystallization were found for the effect of initial microstructure on kinetics of austenite formation. The recrystallization–austenite formation interaction accelerated austenization in all the three starting microstructures by providing additional nucleation sites and enhancing growth rates, and drastically altered morphology and distribution of austenite. In particular, for ferrite–bainite–pearlite and martensite, the recrystallization–austenite formation interaction resulted in substantial microstructural refinement. Recrystallization and austenite formation from a fully recrystallized state were successfully modeled using the Johnson–Mehl–Avrami–Kolmogorov approach.     

Mechanisms of ambient temperature fatigue crack growth in Ti−46.5Al−3Nb−2Cr−0.2W

Fatigue crack growth studies have been conducted on a two-phase alloy with a nominal composition of Ti−46.5Al−3Nb−2Cr−0.2W (at. pct), heat treated to produce duplex and lamellar microstructures. Fatigue crack growth tests were conducted at 23°C using computer-controlled servohydraulic loading at a cyclic frequency of 20 Hz. Several test methods were used to obtain fatigue crack growth rate data, including decreasing-load-range-threshold, constant-load-range, and constant-K _max increasing-load-ratio crack growth control. The lamellar microstructure showed substantial improvement in crack growth resistance and an increase in the threshold stress intensity factor range, ΔK _th, when compared with the behavior of the duplex microstructure. The stress ratio had a significant influence on crack growth behavior in both microstructures, which appeared to be a result of roughness-induced crack closure mechanisms. Fractographic characterization of fatigue crack propagation modes indicated a highly tortuous crack path in the fully lamellar microstructure, compared to the duplex microstructure. In addition, limited shear ligament bridging and secondary cracking parallel to the lamellar interfaces were observed in the fully lamellar microstructure during fatigue crack propagation. These observations were incorporated into a model that analyzes the contribution of intrinsic vs extrinsic mechanisms, such as shear ligament bridging and roughness-induced crack closure, to the increased fatigue crack growth resistance observed for the fully lamellar microstructure. S.J. BALSONE, formerly with the United States Air Force, Wright Laboratory, Materials Directorate     

Influence of heat treatment on the fatigue crack growth rates of a secondary hardening steel

The relationships between microstructure and fatigue crack propagation behavior were studied in a 5Mo-0.3C steel. Microstructural differences were achieved by varying the tempering treatment. The amounts, distribution, and types of carbides present were influenced by the tempering temperature. Optical metallography and transmission electron microscopy were used to characterize the microstructures. Fatigue fracture surfaces were studied by scanning electron microscopy. For each heat treatment the fatigue crack growth properties were measured under plane strain conditions using a compact tension fracture toughness specimen. The properties were reported using the empirical relation of Paris [da/dN = C_oΔK^m]. It was found that secondary hardening did influence the fatigue crack growth rates. In particular, intergranular modes of fracture during fatigue led to exaggerated fatigue crack growth rates for the tempering treatment producing peak hardness. Limited testing in a dry argon atmosphere showed that the sensitivity of fatigue crack growth rates to environment changed with heat treatment.     

预处理组织对低碳钢IQ&P工艺下组织及性能影响

采用γ单相区和γ+α双相区轧制并淬火工艺以及双相区再加热-淬火-碳配分(IQ&P)工艺,研究预处理组织对低碳钢室温状态多相组织特征及力学性能的影响规律.实验用低碳钢经两种工艺轧制并淬火处理,获得马氏体和马氏体+铁素体的预处理组织,再经双相区IQ&P工艺处理后均获得多相组织.马氏体预处理钢的室温组织由板条状亚温铁素体、块状回火马氏体以及一定比例的针状未回火马氏体和8.2%的针状残余奥氏体组成;马氏体+铁素体预处理钢由板条状亚温铁素体、块状和针状未回火马氏体以及14.3%的短针状或块状残余奥氏体组成.在相同的双相区IQ&P工艺参数下,预处理组织为马氏体的钢抗拉强度为770 MPa,伸长率为28%,其强塑积为21560 MPa·%;而预处理组织为马氏体+铁素体的钢抗拉强度为834 MPa,伸长率增大到36.2%,强塑积达到30190 MPa·%,获得强度与塑性的优良结合.      

Effect of Ultra Fast Cooling on the Alloy Cost Reduction of the X80 Pipeline Steel

The microstructures and mechanical properties of X80 pipeline steels produced by both novel ultra fast cooling and conventional‐accelerated continuous cooling modes are investigated. Results showed that different levels of Mo addition had a remarkable effect on the microstructures and mechanical properties of the investigated pipeline steels. The proeutectoid ferrite and pearlite formation is inhibited in the high‐Mo steel and acicular ferrite is obtained over a wide range of cooling rates, whereas the dominant acicular ferrite microstructure can only be obtained when the cooling rates reach up to 5 C s^?1. Very similar microstructures and mechanical properties are obtained in the low‐Mo steel produced with ultra fast cooling and in the high‐Mo steel produced by the conventional‐accelerated continuous cooling. It was proved by simulation and industrial trials that high‐strength low‐alloy steels such as pipeline steels, can be produced using the novel ultra fast cooling which also reduce alloy cost.