Low cycle fatigue behavior of a quenched and tempered niobium bearing HSLA steel

The low cycle fatigue behavior of a quenched and tempered niobium (columbium) bearing high strength low alloy steel heat treated to give tempered martensitic microstructures presumably with and without fine niobium carbides was studied by transmission electron microscopy, stress relaxation, X-ray diffraction line broadening and strain-controlled fatigue testing. The steel without the niobium carbides cyclically softened rapidly at all strain amplitudes studied. This softening was attributed to the rearrangement of the dislocation substructure into a cell structure and to the accompanying decrease in internal stress. The steel presumably containing the fine niobium carbides cyclically softened to a lesser extent. This correlated with the observation that dislocations in this steel did not rearrange themselves into a cell structure and, hence, there was less change in the internal stress during cycling. The steel without the niobium carbides exhibited somewhat better strain-life behavior at large strain amplitudes. This was attributed to the cell structure being able to accommodate a greater amount of plastic strain in that steel. Formerly a Graduate Student, Marquette University     

Fatigue behavior of a precipitation hardening Ni−Al−Cu medium carbon steel

The fatigue behavior of an Fe-0.3 wt pet C-4 wt pet Ni-1 wt pet Al-1 wt pet Cu precipitation hardening steel was investigated in three different heat treated conditions which give similar tensile strengths but different microstructures. One heat treatment produced a lightly tempered lath martensite having fine carbides and a high dislocation density. The other two heat treatments produced highly tempered martensite with coarse carbides, fine intermetallic precipitates and a relatively low dislocation density. The steel in the lightly tempered condition showed marked softening on strain cycling while the highly tempered conditions resulted in both hardening and softening. The lightly tempered structure had better low cycle fatigue resistance but the two highly tempered structures had better high cycle resistance. The dislocation substructure in the lightly tempered steel rearranges itself and accommodates plastic strain during cyclic deformation while the substructure in the highly tempered structures containing fine precipitates resists rearrangement. This difference is suggested as the reason for the differences in behavior. The three conditions show little variation in their resistance to fatigue crack propagation. However, the highly tempered, precipitate containing structures were much more resistant to fatigue crack initiation in notched specimens.     

Fatigue behavior of a precipitation hardening Ni-Al-Cu medium carbon steel

The fatigue behavior of an Fe-0.3 wt pct C-4 wt pct Ni-1 wt pct Al-1 wt pct Cu precipitation hardening steel was investigated in three different heat treated conditions which give similar tensile strengths but different microstructures. One heat treatment produced a lightly tempered lath martensite having fine carbides and a high dislocation density. The other two heat treatments produced highly tempered martensite with coarse carbides, fine intermetallic precipitates and a relatively low dislocation density. The steel in the lightly tempered condition showed marked softening on strain cycling while the highly tempered conditions resulted in both hardening and softening. The lightly tempered structure had better low cycle fatigue resistance but the two highly tempered structures had better high cycle resistance. The dislocation substructure in the lightly tempered steel rearranges itself and accommodates plastic strain during cyclic deformation while the substructure in the highly tempered structures containing fine precipitates resists rearrangement. This difference is suggested as the reason for the differences in behavior. The three conditions show little variation in their resistance to fatigue crack propagation. However, the highly tempered, precipitate containing structures were much more resistant to fatigue crack initiation in notched specimens. Former Postdoctoral Research Associate, Department of Materials Science and Engineering, and Walter P. Murphy Professor of Materials Science and Engineering     

Ultrasonic Fatigue Damage Behavior of 304L Austenitic Stainless Steel Based on Micro-plasticity and Heat Dissipation

Very high cycle fatigue behavior(107-109 cycles)of 304 Laustenitic stainless steel was studied with ultrasonic fatigue testing system(20kHz).The characteristics of fatigue crack initiation and propagation were discussed based on the observation of surface plastic deformation and heat dissipation.It was found that micro-plasticity(slip markings)could be observed on the specimen surface even at very low stress amplitudes.The persistent slip markings increased clearly along with a remarkable process of heat dissipation just before the fatigue failure.By detailed investigation using a scanning electron microscope and an infrared camera,slip markings appeared at the large grains where the fatigue crack initiation site was located.The surface temperature around the fatigue crack tip and the slip markings close to the fracture surface increased prominently with the propagation of fatigue crack.Finally,the coupling relationship among the fatigue crack propagation,appearance of surface slip markings and heat dissipation was analyzed for a better understanding of ultrasonic fatigue damage behavior.     

Fatigue behavior of a nial precipitation hardening medium carbon steel

The fatigue behavior of an Fe-0.30C-4.48Ni-l.32Al steel tempered to give three different microstructures of the same ultimate tensile strength has been investigated by light and electron microscopy, low and high cycle fatigue tests, X-ray line broadening and stress relaxation measurements. The three different heat treatments produced the following structures: I) a conventional quenched and tempered microstructure with a high density of dislocations and elongated carbides, II) a microstructure of high dislocation density, coarse carbides and fine coherent NiAl precipitates and III) a highly tempered micro-structure with a recovered dislocation substructure, coarse carbides and fine coherent NiAl precipitates. In low cycle, strain controlled fatigue cyclic softening in Treatment I was accompanied by a rearrangement of the dislocation substructure and a reduction in both the internal stress and lattice microstrain. Treatment II, which remained cyclically stable during the initial portion of the fatigue life, showed little change in the internal stress and dislocation density and showed a slight increase in lattice microstrain. Treat-ment III, which initially cyclically hardened, exhibited a rise in internal stress, lattice microstrain and dislocation density. The behavior of Treatments II and III is attributed in part to the presence of the fine NiAl precipitates which appear to reduce the tendency of the transformation induced dislocation substructure to rearrange itself into a cell structure during fatigue. In high cycle, stress controlled fatigue Treatment II showed the best fatigue resistance and Treatment I the worst. Improvement in life was attributed to improved resistance to crack initiation. Formerly Graduate Student, Marquette University,     

Fracture toughness and fatigue behavior of matrix II and M-2 high speed steels

The effects of heat treatment and of the presence of primary carbides on the fracture toughness,K _Ic and the fatigue crack growth rates,da/dN, have been studied in M-2 and Matrix II high speed steels. The Matrix II steel, which is the matrix of M-42 high speed steel, contained many fewer primary carbides than M-2, but both steels were heat treated to produce similar hardness values at the secondary hardening peaks. The variation of yield stress with tempering temperature in both steels was similar, but the fracture toughness was slightly higher for M-2 than for Matrix II at the secondary hardening peaks. The presence of primary carbides did not have an important influence on the values ofK _Ic of these hard steels. Fatigue crack growth rates as a function of alternating stress intensity, ΔK, showed typical sigmoidal behavior and followed the power law in the middle-growth rate region. The crack growth rates in the near threshold region were sensitive to the yield strength and the grain sizes of the steels, but insensitive to the sizes and distribution of undissolved carbides. The crack growth rates in the power law regime were shifted to lower values for the steels with higher fracture toughness. SEM observations of the fracture and fatigue crack surfaces suggest that fracture initiates by cleavage in the vicinity of a carbide, but propagates by more ductile modes through the matrix and around the carbides. The sizes and distribution of primary carbides may thus be important in the initiation of fracture, but the fracture toughness and the fatigue crack propagation rates appear to depend on the strength and ductility of the martensite-austenite matrix.     

Use of the nanoindentation technique for studying microstructure/crack interactions in the fatigue of 4340 steel

The objectives of this research are to study the influence of microstructure on the fatigue crack growth behavior in 4340 steel and to explore the application of the nanoindentation technique for determining the plastic deformation zone at a fatigue crack tip. Two heat treatment conditions were chosen for the steel: annealed and quenched plus tempered. The annealed steel consists of coarse pearlite and proeutectoid ferrite, while the quenched and tempered steel consists of fine tempered martensite. Fatigue crack propagation tests were conducted on disklike compact (DCT) specimens. Subsequently, the nanoindentation technique was applied to quantitatively determine the plastic deformation zone at fatigue crack tips. The plastic deformation zone size determined by the nanoindentation test seems larger than the cyclic deformation zone calculated using the fracture mechanics equation, which involves many assumptions. The fatigue crack growth test results show that the annealed steel has a higher resistance to crack growth than the quenched and tempered steel. The fatigue crack in the annealed steel tends to grow along pearlite domain boundaries, or the cementite/ferrite interfaces within a pearlite domain. In contrast, the fatigue crack in the quenched and tempered steel tends to traverse the fine martensite laths. Consequently, the actual crack path in the annealed steel is rougher than in the quenched and tempered steel and more secondary cracks are observed in the annealed steel.     

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.     

高温渗碳轴承钢旋弯疲劳裂纹萌生与扩展行为

 为了提高航空轴承的服役寿命,借助QBWP-10000X型旋转弯曲疲劳试验机,研究了高温渗碳轴承钢的旋转弯曲疲劳性能和裂纹萌生扩展行为。结果表明,钢的中值疲劳强度达到913.3 MPa。有效渗层中大量M₂₃C₆和少量M₆C碳化物显著提高了试验钢的表面硬度,渗层不同碳浓度导致马氏体先后发生相变而形成408 MPa表面压应力,进而提高了钢的疲劳性能。疲劳裂纹主要萌生在表面缺陷和次表面碳化物,分别占比71.4%和 28.6%。萌生裂纹缺陷特征尺寸及承载应力对应力强度因子和循环次数影响显著,深犁沟形状由于涉及应力集中而直接影响疲劳循环次数,承受相同加载应力碳化物特征尺寸越大,循环次数越低。裂纹萌生后沿渗碳层碳化物边界快速扩展同时向芯部缓慢扩展,最后在试样疲劳源对侧近边缘区域发生准解理和韧性混合断裂。     

Fatigue crack growth in a type 316 stainless steel and a 20 pet Cr/25 pet Ni/Nb stainless steel at elevated temperature

Fatigue failure in metals and alloys occurs by the nucleation and controlled propagation of a surface crack. At ambient temperature propagation is transgranular and is controlled, to a large extent, by continuum plasticity effects at the crack tip. At elevated temperatures this simple process might be affected by oxidation and the tendency towards intergranular propagation. The elevated temperature fatigue crack propagation behavior of a 20/25/Nb stainless steel and a type 316 stainless steel is studied by optical measurement of the crack growth rate of artificially-induced notches under conditions where gross plastic straining is present in the bulk of the material. Tests conducted at ambient temperature are included for comparison with elevated temperature behavior. By reference to fatigue life data for smooth unnotched specimens, tested under identical conditions to the crack growth tests, an attempt is made to rationalize the roles of crack initiation and propagation in the fatigue process and indicate the relevance of crack growth data in predicting fatigue life data.     

Microstructural examination of

Fatigue tests were performed to examine how microstructural conditioning influences crack initiation and propagation in SA508 class 3 low-carbon steel. A 3-mm-long crack was introduced in compact tension (CT) fatigue test specimens under four different loads in order to obtain crack tip plastic zones at different stress intensity factor ranges, ΔK = 18, 36, 54, and 72 MPa√m. The microstructure of the plastic zones around the crack tip were examined by trans- mission electron microscopy (TEM) and selected area electron diffraction (SAD). Micro- orientation of the dislocation cells in the plastic zones of all of the CT samples increased to 4 deg from the level of an as-received sample. Four-point bending fatigue tests were performed for plate shape samples with a large cyclic strain range. The SAD value of the bending samples was also 4 deg in the damaged area where cracks already initiated at an early stage of the fatigue process. These test results indicate that the microstructural conditioning is a prerequisite for the fatigue crack initiation and propagation in SA508. These observations may lead to better under- standing of how fatigue initiation processes transit to cracks.     

Fatigue crack propagation through weld heat affected zones

Fatigue tests were performed on specimens containing weld heat affected zones at two orientations to the stress axis. Two heat affected zones were studied, one in Ducol W30 (a low alloy steel) and the other in mild steel. Under conditions of constant alternating and maximum stress intensity a fatigue crack only propagated at a uniform rate when it was remote from the heat affected zone. A heat affected zone which was harder than either the parent plate or weld metal was found to reduce crack propagation rates by a factor of up to 2 by restricting the plastic zone size around the crack tip. The changes in crack propagation rate could not be related uniquely to the conditions of the material immediately adjacent to the crack tip. Furthermore, the shape of the plastic zone was found to influence the direction of the propagation of a fatigue crack which always deviated toward regions of lower flow stress. A crack was never found to follow the interface between the weld metal and the parent metal heat affected zone because the flow stresses were not the same on either side of the interface. There was no difference in crack propagation mechanism between the parent plate and its heat affected zone for the stress conditions imposed. Formerly with Central Electricity Research Laboratories, Materials Division, Leatherhead, Surrey, England     

The Small Fatigue Crack Growth Behavior of an AM60 Magnesium Alloy

The effects of thermomechanical processing and subsequent heat treatment on the small fatigue crack growth (FCG) behavior of an AM60 (Mg-6.29Al-0.28Mn wt pct) alloy were evaluated. The effects of mechanical loading parameters, such as maximum stress and load-ratio, on the small FCG behavior were also determined. Maximum stress did not appear to affect the crack propagation rate of small cracks in the stress and crack size ranges considered. Materials with different microstructures and yield stresses, introduced by different processing conditions, showed similar crack growth rates at equivalent stress intensity factor ranges. The effect of load ratio on small crack growth rates was recorded. Fracture surface characterization suggested that the fatigue crack propagation mechanism was a mixture of transgranular and intergranular cracking. Porosity and other material defects played respective important roles in determining the fatigue crack initiation and propagation behavior.     

The Significance of Crack Initiation Stage in Very High Cycle Fatigue of Steels

Different stages of the Very High Cycle Fatigue (VHCF) crack evolution in tool steels have been explored using a 20 kHz ultrasonic fatigue testing equipment. Extensive experimental data is presented describing VHCF behaviour, strength and crack initiating defects in an AISI H11 tool steel. Striation measurements are used to estimate fatigue crack growth rate, between 10^?8 and 10^?6 m/cycle, and the number of load cycles required for a crack to grow to critical dimensions. The growth of small fatigue cracks within the “fish‐eye” is shown to be distinctively different from the crack propagation behaviour of larger cracks. More importantly, the crack initiation stage is shown to determine the total fatigue life, which emphasizes the inherent difficulty to detect VHCF cracks prior to failure. Several mechanisms for initiation and early crack growth are possible. Some of them are discussed here: crack development by local accumulation of fatigue damage at the inclusion – matrix interface, hydrogen assisted crack growth and crack initiation by decohesion of carbides from the matrix.     

Fatigue Initiation Study of TMT Eutectoid Steel

Eutectoid steel and 1060 steel samples were given a variety of thermomechanical treatments (TMT), described in Table I, which varied the pearlite interlamellar spacing, the cementite and inclusion orientation, the degree of cold-work in the ferrite matrix, and (for 1060 steel only) the proeutectoid grain boundary ferrite network. These samples were then evaluated as to resistance to fatigue crack initiation. The TMT designated, G, involves a subcritical anneal and results in a partially recrystallized condition which shows not only excellent resistance to fatigue crack initiation in the near threshold region but is second only to a fine pearlite microstructure, B, in fatigue crack propagation threshold value. It is believed that the excellent properties of the TMT, G, are related to the formation of subgrains in the interlamellar ferrite. On a scale normalized to tensile strength,(ΔK/√ρ)/σ_u, fine oriented pearlite in a soft ferrite matrix (rapid up-quench TMT such as E, F) shows the best resistance to fatigue crack initiation. A proeutectoid ferrite grain boundary network is poor at resisting fatigue crack initiation but good at resisting fatigue crack propagation. It should be emphasized that the combined high resistance of the subcritical TMT (G) toboth fatigue crack initiation and propagation, coupled with its much easier implementation relative to similar microstructures produced by difficult rapid up-quench TMT's (E, F) make it a very promising approach to improving overall fatigue resistance.     

Strain controlled fatigue deformation and fracture of nitrogen alloyed 316LN austenitic stainless steel

In this paper, Low cycle fatigue (LCF) behavior of 316LN austenitic stainless steel alloyed with 0.078 and 0.22 wt% nitrogen, designated as N078 and N022 steels respectively, is compared in the temperature range 300–873 K by strain controlled fatigue tests at ± 0.6% strain amplitude. Interestingly, N022 steel showed continuous decrease in fatigue life with temperature in contrast to N078 steel which showed maximum in fatigue life at 573 K. Drastic reduction in fatigue life is observed in both the steels in the temperature range 673–873 K and has been attributed to the occurrence of dynamic strain aging. Both steels exhibited manifestations (for ex.: decrease in plastic strain and anomalous stress response with increase in temperature) corresponding to the occurrence of Dynamic Strain Ageing (DSA) in the above temperature range. Under all testing conditions, fracture surfaces revealed transgranular crack initiation and transgranular crack propagation.     

Influence of microstructure on fatigue behavior and surface fatigue crack growth of fully pearlitic steels

This work examined the influence of microstructure on the surface fatigue crack propagation behavior of pearlitic steels. In addition to endurance limit or S(stress amplitude)-N(life) tests, measurements of crack initiation and growth rates of surface cracks were conducted on hourglass specimens at 10 Hz and with aR ratio of 0.1. The microstructures of the two steels used in this work were characterized as to prior austenite grain size and pearlite spacing. The endurance tests showed that the fatigue strength was inversely proportional to yield strength. In crack growth, cracks favorably oriented to the load axis were nucleated (stage I) with a crack length of about one grain diameter. Those cracks grew at low ΔK values, with a relatively high propagation rate which decreased as the crack became longer. After passing a minimum, the crack growth rate increased again as cracks entered stage II. Many of the cracks stopped growing in the transition stage between stages I and II. Microstructure influenced crack propagation rate; the rate was faster for microstructures with coarse lamellar spacing than for microstructures with fine lamellar spacing, although changing the prior austenite grain size from 30 to 130 jμm had no significant influence on crack growth rate. The best combination of resistance to crack initiation and growth of short cracks was exhibited by microstructures with both a fine prior austenite grain size and a fine lamellar spacing. Formerly with Carnegie Mellon University     

Creep-fatigue interaction in austenitic stainless steels

Low cycle fatigue failures occur by the initiation and controlled growth of a surface crack. The development of crack propagation models, based on continuum mechanics, have enabled successful predictions of fatigue life at both room and elevated temperatures. This paper attempts to extend such models to cover the situations in which creep damage, introduced during periods of stress relaxation, influences the rate of growth of the surface fatigue crack. Equations predicting fatigue life as a function of hold period are in good agreement with experimental data, for Type 304 stainless steel, Type 316 stainless steel and Incoloy-800.     

Effect of silicon particles on the fatigue crack growth characteristics of Al-12 Wt Pct Si-0.35 Wt Pct Mg-(0 to 0.02) Wt Pct Sr casting alloys

Fatigue crack growth (FCG) characteristics and mechanisms in Al-Si-Mg eutectic casting alloys containing 0.35 wt pct Mg and 0 to 0.02 wt pct Sr were investigated as a function of stress ratio,R, stress-intensity-factor range, ΔK, and silicon (Si) particle size. The fatigue crack propagation behavior was compared with that observed in commercial casting alloy A356. At the same applied ΔK level, the crack growth rate was found to increase with increasing stress ratio and Si particle size. Modified (fine Si morphology) and A356 alloys showed better FCG resistance than the unmodified (coarse Si morphology) ones, for a constant applied ΔK, due to increased closure. The effects of roughness-induced and plasticity-induced crack closures, crack branching, and crack meandering on the fatigue crack propagation observed in these alloys have been discussed. The fatigue crack propagation path is found to be dependent on the Si particle characteristics. The mechanisms of silicon particle decohesion and cracking are also discussed. Formerly Research Associate, Département des Sciences Appliquées, Université du Québec à Chicoutimi