Fatigue crack growth behaviour of a Si–Mn steel with carbide-free lathy bainite

An investigation has been performed to examine the fatigue crack propagation (FCP) rate and fatigue threshold of an Si–Mn steel containing carbide-free lathy bainite. Compact tension specimens prepared from this steel were given four different heat treatments to produce four different austenite contents. The fatigue test was carried out at stress ratio of –1 in a room temperature ambient atmosphere. The results show that the FCP threshold of the steel increases with an increase in the volume fraction of carbon-saturated austenite. The crack growth behaviours show that the deformation strengthening ability of the austenite has a significant effect on the FCP in the threshold region. The effect of austenite on the FCP threshold is seven times that of the bainite.     

Fatigue crack growth in TRIP steel under positive R-ratios

Load-controlled fatigue tests are conducted for four positive R values on a low-alloy TRIP steel for two different heat treatments: an optimal treatment leading to a multiphase microstructure containing retained austenite, ferrite, bainite and martensite, and a non-optimal treatment leading to a ferritic–martensitic dual-phase microstructure. A significantly increased resistance to fatigue crack growth is found for the optimal case with respect to the non-optimal case. The amount of crack closure is found to be larger in case of the non-optimally treated (ferritic–martensitic) steel. Close to the crack tip, an increased hardness suggests martensite formation. An EBSD technique is used to quantify the volume of retained austenite ahead of the crack tip, within the plastic zone. It is found that martensite formation only occurs within the monotonic plastic zone during fatigue. By evaluation of the retained austenite fraction during straining in static tensile tests, the plastic strain levels within the plastic zone are assessed. Additionally, the effect of martensite formation on fracture toughness is estimated.     

Plastic deformation — its role in fatigue crack propagation

Recognizing the fact that the effective driving force (ΔK _eff) determines the fatigue crack propagation (FCP) rate and that the shear strain, which is considered to develop due to an occurrence of crack closure, primarily contributes to the plastic deformation, an effort is made here to elucidate the role of plastic deformation in FCP by developing a correlation between the ΔK _eff and the applied driving force (ΔK) with shear strain as variable. The effect of the degree of plastic deformation (i.e. shear strain level) on the FCP rates at higher values of ΔK, where ΔK _eff approaches ΔK, approaching the Paris regime, appears minimal. On the other hand, the disparity between ΔK _eff and ΔK, which apparently increases with shear strain level, persists at lower values of ΔK. This suggests a strong influence of the degree of localized deformation on the FCP rates in the near threshold level. Hence, an improvement of FCP rates in the near threshold level should follow an effort that promotes the plastic deformation near the crack tip to a greater degree. This approach could, therefore, form the basis to explain the effect of the grain size, microstructure, environment,R-ratio and crack size on the near-threshold FCP rates.     

THE EFFECT OF MICROSTRUCTURE AND FRACTURE SURFACE ROUGHNESS ON FATIGUE CRACK PROPAGATION IN A Ti-6A1-4V ALLOY

Characteristics of fatigue crack propagation (FCP) have been studied on materials with three different microstructures of a Ti-6A1-4V alloy, prepared with different heat treatments. The effect of microstructure on the FCP behaviour was attributed to the development of crack tip shielding, primarily resulting from the role of crack path morphology in inducing crack closure and crack deflection. Roughness-induced crack closure played an important role on the near-threshold FCP behaviour at a stress ratio of 0.05, but the FCP data plotted in terms of the effective stress intensity factor range, δK_eff (allowing for crack closure), still exhibited the effect of microstructure. Fractographic examinations were performed, using a scanning electron microscope (SEM) with the aid of image processing, which enabled a three-dimensional reconstruction of the fracture surface using a stereo pair of SEM micrographs. Fracture surface roughness was evaluated quantitatively by the ratio of the real area of the reconstructed fracture surface to its projected area. As fracture surface roughness was taken into account in evaluating the FCP data in addition to crack closure, the effect of microstructure disappeared, indicating that the intrinsic FCP resistance was the same in all the materials. Thus, it was concluded that fracture surface roughness was a dominating parameter in controlling the FCP of the Ti-6A1-4V alloy.     

In-situ SEM study of short fatigue crack propagation behavior in a dissimilar metal welded joint of nuclear power plant

In-situ Scanning Electron Microscopy (SEM) fatigue experiments were carried out to study short fatigue crack propagation (FCP) behavior of various regions (weld zone, interface region and heat affected zone (HAZ)) in a domestic dissimilar metal welded joint of nuclear power plant. The local microstructural effect on short fatigue crack initiation and propagation behavior was investigated with its influence on both material fatigue and structure fatigue analyzed. Considering material fatigue, in the weld region, crack grows along δ ferrites when propagating parallel to the dendrite, and deflects or branches along δ ferrite, γ austenite dendrite, δ/γ interface and grain boundaries when propagating perpendicular to the dendrite; in safe ends, the crack grows along slip lines and coalesces with secondary cracks; in A508 HAZ, the crack propagates or branches along martensite transgranularly. In terms of structural fatigue, the crack tends to deflect when propagating across the weld/A508 interface or weld/316 L interface with the influence of local microstructure, and the weld/A508 interface region has a resistance to FCP due to its high strength. The fatigue crack propagation rate of each region was compared and analyzed. The fatigue fractography was also characterized under SEM to analyze the crack propagation process.     

Fatigue resistance of impact-modified thermoplastic copolyesters

The effect of impact-modifier concentration and particle size on the fatigue resistance of an amorphous thermoplastic copolyester of dimethyl terephthalate with ethylene glycol and 1,4-cyclohexanedimethanol (KODAR®^* PETG) was determined. Fatigue crack propagation (FCP) rate at a given value of stress intensity factor range, K, decreased by a factor of four with the addition of an impact modifier, if the inherent viscosity of the PETG remained constant. The FCP rate did not vary with modifier concentration or particle size. However, the value of K at instability did increase with increasing modifier content and particle size appeared to alter the FCP mechanism. in fatigue tests on unnotched specimens, neat PETG was more fatigue resistant than the impact-modified blends, suggesting that the modifier particles reduced the number of cycles required to initiate a crack.     

Threshold and growth mechanism of fatigue cracks under mode II and III loadings

ABSTRACT The fatigue crack growth behaviour of 0.47% carbon steel was studied under mode II and III loadings. Mode II fatigue crack growth tests were carried out using specially designed double cantilever (DC) type specimens in order to measure the mode II threshold stress intensity factor range, ΔK_IIth. The relationship ΔK_IIth > ΔK_Ith caused crack branching from mode II to I after a crack reached the mode II threshold. Torsion fatigue tests on circumferentially cracked specimens were carried out to study the mechanisms of both mode III crack growth and of the formation of the factory‐roof crack surface morphology. A change in microstructure occurred at a crack tip during crack growth in both mode II and mode III shear cracks. It is presumed that the crack growth mechanisms in mode II and in mode III are essentially the same. Detailed fractographic investigation showed that factory‐roofs were formed by crack branching into mode I. Crack branching started from small semi‐elliptical cracks nucleated by shear at the tip of the original circumferential crack.     

Multiphase microstructure formation and its effect on fracture behavior of medium carbon high silicon high strength steel

This work investigated the evolution of multiphase microstructure and impact fracture behavior of medium carbon high silicon high strength steel subjected to the austempering treatment at 240,360,and 400 ℃.The results show that martensite,bainite,and retained austenite (RA) are the main microstructural phases.The austempering treatments at 360 and 400 ℃ caused the formation of carbon-poor ferrite in the matrix,and the transformation of ultrafine bainite into coarse lath bainite and granular bainite,respectively.Thick filmy RA was distributed between bainite laths.The polygonal martensiteaustenite islands and blocky RA formed along the grain boundaries.The average carbon concentration in the matrix decreased with the temperature increase,while the impact toughness initially increased and then dropped with temperature.The quasi-cleavage brittle fracture dominated the impact fracture mechanism of the sample austempered at 240 ℃ by forming tearing surfaces and tearing steps.The microcracks disappeared in the RA on the prior austenite grain boundaries.On the other side,the fracture surface of the sample austempered at 360 ℃ exhibited ductile fracture with deep dimples and brittle fracture with cleavage river patterns.The polygonal martensite-austenite islands or blocky RA constrained the microcracks.After austempered at 400 ℃,the brittle fracture was dominant,showing river patterns,and the microcracks propagated through the granular bainite without any resistance.     

Microstructure and mechanical properties of C–Si–Mn(–Nb) TRIP steels after simulated thermomechanical processing

Abstract

Continuous and discontinuous cooling tests were performed using a quench deformation dilatometer to develop a comprehensive understanding of the structural and kinetic aspects of the bainite transformation in low carbon TRIP (transformation induced plasticity) steels as a function of thermomechanical processing and composition. Deformation in the unrecrystallised austenite region refined the ferrite grain size and increased the ferrite and bainite transformation temperatures for cooling rates from 10 to 90 K s^-1. The influence of niobium on the transformation kinetics was also investigated. Niobium increases the ferrite start transformation temperature, refines the ferrite microstructure, and stimulates the formation of acicular ferrite. The effect of the bainite isothermal transformation temperature on the final microstructure of steels with and without a small addition of niobium was studied. Niobium promotes the formation of stable retained austenite, which influences the mechanical properties of TRIP steels. The optimum mechanical properties were obtained after isothermal holding at 400°C in the niobium steel containing the maximum volume fraction of retained austenite with acicular ferrite as the predominant second phase.     

Effect of bainitic transformation on mechanical properties of 0.6C-Si-Mn steel

The mechanical properties of 0.6C-Si-Mn steel transformed isothermally in the bainitic temperature region (593 and 648 K) were investigated. The mechanical properties of the steels were improved with increasing bainite and retained austenite and the corresponding decrease in martensite. Marked benefits of the mechanical properties were obtained for the steels containing the maximum content of retained austenite in the bainite matrix, independent of transformation temperature. For isothermal transformation at 593 K, the 0.2% yield stress, _y, ultimate tensile stress, _u, and notch tensile stress (NTS) were improved significantly, while the advantage of the per cent elongation and Charpy 2 mm V-notch (CVN) impact energy was relatively small. As a result of isothermal transformation at 648 K, the per cent elongation and CVN impact energy were dramatically improved, while the superiority of _y, _u and NTS was not much greater than isothermal transformation at 593 K. Compared to 0.6C steels transformed isothermally at the same temperatures, in which little appreciable retained austenite was found, the isothermally transformed steels having a microstructure consisting of bainite and retained austenite improved the mechanical properties remarkably. These results are described and discussed.     

高强度结构钢及其接头热影响区的疲劳及断裂性能分析

采用焊条电弧焊对48 mm厚高强度结构钢进行焊接,对焊接接头热影响区疲劳裂纹扩展门槛值ΔK_th、裂纹扩展速率da/dN和断裂韧度K_IC进行研究并与基体进行对比。结果表明,在室温下,焊接接头热影响区具有更好的疲劳和断裂性能;随着与熔合线距离的增大,热影响区的组织依次为粗大板条状贝氏体+奥氏体薄膜、细粒状贝氏体、回火索氏体+细粒状贝氏体,硬度逐渐下降;在室温下,焊接接头热影响区和基体冲击韧性均位于上平台。热影响区的残余奥氏体薄膜和硬度较高的贝氏体是影响其疲劳和断裂性能的重要因素。      

回火温度对两相区退火海工钢组织和性能的影响

对690 MPa级海工钢进行“淬火+两相区退火+回火”三步热处理,研究了回火温度对其组织和性能的影响、分析了力学性能变化与组织演变和残余奥氏体体积分数之间的关系。结果表明：回火后实验钢的显微组织为回火贝氏体/马氏体、临界铁素体和残余奥氏体的混合组织。随着回火温度的提高贝氏体/马氏体和临界铁素体逐渐分解成小尺寸晶粒,而残余奥氏体的体积分数逐渐增加;屈服强度由787 MPa降低到716 MPa,塑性和低温韧性明显增强,断后伸长率由20.30%增至29.24%,-40℃下的冲击功由77 J提升至150 J。残余奥氏体体积分数的增加引起裂纹扩展功增大,是低温韧性提高的主要原因。贝氏体/马氏体的分解和残余奥氏体的生成,引起组织细化、晶粒内低KAM值位错的比例逐渐提高和小角度晶界峰值的频率增大,使材料的塑性和韧性显著提高。     

Fatigue crack propagation of bainitic nodular cast iron

The effects of austempering temperature and isothermal transformation time on fatigue crack growth rate in a ductile iron with a bainitic structure have been studied. Crack growth rates in austempered samples were compared with those in materials with a ‘bullseye’ casting structure. Using scanning electron microscopy, the mechanism of the fatigue crack growth can be understood by observing the fracture surface of a fatigue specimen. X-ray diffractometry was used to determine the volume fraction of retained austenite. It can be concluded that the volume fraction of retained austenite, the fracture mode and the matrix microstructure are closely related to the fatigue crack propagation rate and the fracture mode.     

Effect of continuous-cooling transformation structure on mechanical properties of 0.4C-Cr-Mo-Ni steel

Commercially available 0.4C-Cr-Mo-Ni steel was studied to determine the effects on its mechanical properties of various microstructures produced by continuous-cooling transformation after austenitization. A good combination of strength and notch toughness was obtained independently of test temperatures (293 and 193 K) when the steel was austenitized at 1173 K and then continuously cooled at an average rate of 3.1 K s^–1 (expressed as the average cooling rate from 823 to 573 K) before final rapid cooling. The microstructure of the steel consisted of a mixed structure of martensite and 10–15 vol% lower bainite, which appeared in acicular form in association with the martensite. Slower cooling had a detrimental effect on the mechanical properties of the steel; the microstructure of this steel consisted of a mixed structure of martensite and upper bainite, which appeared as masses in the matrix. As the average cooling rate increased, the lath size and internal stringer-carbide size in the upper bainite were larger, and retained a somewhat increased austenite content.     

低合金高强度钢的微观组织和疲劳裂纹扩展行为

用透射电镜观察了３０ＣｒＭｎＳｉＮｉ２Ａ钢等温的微观组织，疲劳裂纹扩展行为、裂纹尖端塑性区和位错结构，结果表明，等温状态组织由马氏体和贝氏体组成。在一个奥氏体晶粒内一般存在四个板条领域、裂纹尖端的塑性区内存在主位错带，疲劳断裂的基本组织单元为板条晶或板条束。裂纹遇到板条束界时方向发生较大偏斜。     

Fatigue crack propagation behaviour of unidirectionally solidified γ/γ′-δ eutectic alloys

Fatigue crack propagation (FCP) studies were conducted on a series of/- (Ni-Nb-Al) alloys by subjecting them to cyclic four-point bending loads at room temperature. The aluminium contents of the alloys investigated ranged from 1.5% to 2.5% by weight, with the niobium contents adjusted to maintain controlled eutectic microstructures. In addition to studies of as-grown alloys, heat treatments were performed on several of the alloys to determine the effect of resultant changes in microstructure on FCP behaviour. Crack growth rates from approximately 2×10^–6 to 10^–3 mm/cycle were recorded as a function of the crack tip stress intensity factor range. The growth rates for the heat-treated alloys differed little from the as-grown FCP behaviour. Comparison with other published results indicated that the addition of aluminium was beneficial to FCP resistance, although the level of aluminium addition (within the investigated range of 1.5 to 2.5% by weight) did not influence the crack growth rates. Based on a comparison with previously reported results, chromium additions were seen to have a detrimental effect on FCP behaviour. Fractographic studies revealed the superior fatigue behaviour of the/- eutectic composite to be a result of repeated grain boundary delamination as the crack progressed through the microstructure.     

Effects of simulated on line accelerated cooling processing on transformation temperatures and microstructure in microalloyed steels Part 2—Plate processing

The effects of three on line accelerated cooling parameters (accelerated cooling start temperature T_A, cooling rate ?, and cooling interrupt temperature T₁) on transformation temperatures and microstructure in a low carbon microalloyed plate steel were studied by laboratory simulations in a quench deformation dilatometer. Varying the on line accelerated cooling parameters changes the austenite condition and transformation path. In general, the transformation path shifts from polygonal ferrite towards bainite with increasing T_A, increasing ?, and decreasing T₁. There is also a corresponding refinement in the microstructure and increase in hardness. In comparison with the laboratory thermomechanical processing treatments, the multipass industrial rolling schedule produces a much more heavily deformed austenite structure than laboratory thermomechanical processing treatments, which would favour high transformation temperatures, fine polygonal ferrite microstructure, and lower hardness.

MST/3425     

PASSIVATION EFFECTS ON CRACK CLOSURE IN THE FATIGUE CRACK PROPAGATION OF A PEAK-AGED Al-Li-Zr ALLOY IN NaCl AND Na2SO4 SOLUTIONS

Abstract Fatigue cracking of a peak-aged Al-Li-Zr alloy was investigated by measuring crack closure as a function of applied anodic potential in 0.6 M NaCl and 0.5 M Na₂SO₄ solutions with an unloading elastic compliance technique, and by comparison with crack closure in dry air. The present work involves complementary anodic behaviour of the Al-Li-Zr alloy in both solutions by potentiodynamic polarization and potentiostatic current transient experiments. From the repassivation rates in the passivation potential range in both solutions, it is indicated that a more stable passive film is formed at lower applied anodic potential than at higher applied anodic potential. The intrinsic fatigue crack propagation (FCP) rates under unstable passivation potential in both solutions were significantly larger than those obtained in dry air. Under stable passivation potential in both solutions, however, the intrinsic FCP rates in the low ΔK_eff range were slightly lower than those obtained in dry air. The crack closure in the low ΔK_eff range increased under stable passivation potential, in dry air and under unstable passivation potential. The high crack closures appearing in the low ΔK_eff range were characterized by a tortuous fracture surface in dry air, and the occurrence of various crack paths such as rolling plane delamination under unstable passivation potential. The difference between environmental crack closures under stable and unstable passivation conditions is discussed in terms of environment-assisted crack-tip damage processes.     

Study on carbide-bearing and carbide-free bainitic steels and their wear resistance

Carbide-free and carbide-bearing bainitic steels have been obtained. The relationship between the bainitic microstructure and wear resistance has been studied. Results show that carbide-free upper and lower bainitic microstructures obtained in the steel with Si?+?Al mainly consist of bainitic ferrite and retained austenite. Carbide-bearing upper and lower bainitic microstructures obtained in the steel without Si?+?Al consist of bainitic ferrite, carbide and trace amounts of retained austenite. The carbide-free bainite exhibits higher strength and toughness than carbide-bearing bainite, especially the toughness. Under lower wear loading, carbide-bearing lower bainite (LB) exhibits higher wear resistance. Under higher wear loading, carbide-free LB exhibits higher wear resistance, which results from the improved surface hardness due to strain-induced martensitic transformation from the retained austenite.     

Alloy optimisation of bainitic steel for large plastic mould

Based on the phase transformation theories, especially the T₀ concept of bainite transformation, alloy optimisation of bainitic steel with carbides has been carried out aiming at the produce of plastic mould with large cross-section. The effect of manganese and silicon on proeutectoid ferrite and bainite transformation is explored by dilatometric analysis, XRD and different microscopy techniques. The results show that after the alloy optimisation, the transformation of proeutectoid ferrite is suppressed and when the cooling rate is lower than 0·1°C?s^??1, the new lower bainite transformation appears by decreasing carbon capacity of austenite and promoting carbide precipitation. Industrial production proves that the optimised alloy SDP1 can meet the demand for the plastic mould with the thickness of 1050?mm.