Fatigue crack tip deformation

The shape of a fatigue crack tip as influenced by an air or a vacuum environment has been investigated in two stainless steels and an aluminum alloy. Under plane strain conditions and at crack growth rates in the Paris region, the crack tip opening displacement (CTOD) is much larger in vacuum than in air, a circumstance attributed to strain localization in air due to the presence of moisture and the absence of strain localization in vacuum. In type 304 stainless steel, a strain-induced transformation from austenite to martensite occurs at the crack tip, and the extent of this strain-induced transformation in type 304 stainless steel is consistent with the degree of blunting taking place at the crack tip as influenced by the environment. In air, the extent of transformation is a function of the ΔK level, and as a result, the crack opening level is found to differ in a ΔK decreasing test as compared to aAK increasing test. Fatigue striations are observed in air but are absent in vacuum. It is proposed that the greater extent of blunting in vacuum is responsible for the absence of striations in vacuum.     

Deformation structure and subsurface fatigue crack generation in austenitic steels at low temperature

In order to progress in the understanding of fatigue crack generation for high-strength alloys, the subsurface fatigue crack initiation sites were characterized and the deformation structure was investigated for the solution-treated 24Cr-15Ni-4Mn-0.3N and 32Mn-7Cr-0.1N austenitic steels. High-cycle fatigue tests of those steels were carried out at 4, 77, and 293 K. Subsurface crack initiation was detected in the lower-peak stress and/or in the longer-life range at the three temperatures. The subsurface crack initiation sites were intergranularly formed. The localized deformation and/or strain concentration by dislocation arrays of the (111)–〈110〉 system assisted intergranular cracking due to incompatibility at grain boundaries. Dislocation movements were restricted to their slip planes. Even at the lower stress level, dislocations had generated in more than one slip system and piled up to a grain boundary. The peak cyclic stress was lowered with the increasing size of the subsurface crack initiation site. The dependence of the subsurface crack size on the peak cyclic stress was discussed.     

Interaction between recrystallization and precipitation during the high temperature deformation of HSLA steels

A new mechanical method is described for following the progress of precipitation in niobium-modified steels. The technique is based on the determination of the strain to the peak stress in high temperature, constant strain rate compression tests. The peak strain is sensitive to holding or aging time prior to testing, thus permitting the kinetics ofstatic precipitation to be determined in either the re crystallized or the predeformed condition. A modification of this technique permits the determination of the kinetics ofdynamic pre-cipitation. The rates of static and dynamic precipitation measured in this way are generally ‘faster’ than the kinetics determined by other methods. The results indicate that the addition of niobium to austenite retards recrystallization in two distinct ways. There is a significant delay introduced by what appears to be a solute effect. In addition, under conditions where precipitation is more rapid thansolute- retarded recrystallization, the operation of the recrystallization process is prevented or retarded until precipitation is complete or nearly complete. This paper is based on a presentation made at a symposium on “Recovery Recrystallization and Grain Growth in Materials” held at the Chicago meeting of The Metallurgical Society of AIME, October 1977, under the sponsorship of the Physical Metallurgy Committee.     

铸造奥氏体不锈钢的疲劳裂纹扩展

用紧凑拉伸试样研究了载荷比、单峰过载和两步高-低幅加载对Z3CN20-09M铸造奥氏体不锈钢疲劳裂纹扩展速率的影响.当应力强度因子范围相同时,疲劳裂纹扩展速率随载荷比的增大而增大.单峰过载使裂纹扩展速率先有短暂的增加后长距离的减速扩展,出现裂纹扩展迟滞现象.两步高-低幅加载时,若两步的最大载荷不同,第二步裂纹扩展也会出现迟滞现象.用两参数模型和Wheeler模型能够预测恒幅载荷和变幅载荷下的疲劳裂纹扩展行为.     

Thermal effects during uniaxial straining of steels

When metals are deformed, most of the strain energy absorbed is converted to heat resulting in a temperature increase. Such temperature increases could affect mechanical properties during forming operations and were studied during rapid uniaxial tensile straining at strain rates of 3 x 10³ and 10–² _s–¹ in a dual phase steel, a high strength low alloy (HSLA) steel, and a plain carbon steel, using an infrared thermometer. The maximum temperatures observed were directly related to the strain energy absorbed, as measured by the area under the stress-strain curve. The dual phase steel absorbed the largest amount of strain energy and therefore registered the largest temperature increase. However, the observed temperature increases were lower than those predicted by calculations assuming adiabatic heating, indicating that such heating did not occur at the strain rates studied.     

Fatigue crack propagation in martensitic and austenitic steels

Fatigue crack growth rates were measured in an annealed and in an aged maraging steel and in three different austenitic steels. Microhardness measurements were used to determine the plane strain plastic zone sizes as a function of ΔK and to evaluate the cyclic flow stress of the material near the crack tip. The presence of a reversed cyclic plastic zone within the monotonic plastic zone was confirmed. The two maraging steels work soften near the tip of the crack while the three austenitic steels work harden. The fatigue crack growth rates of the maraging steels are independent of the monotonic yield stress and are typical of the growth rates of steels with a bcc crystal structure. The crack growth rates in the stainless steels are an order of magnitude lower than for maraging steels for ΔK< 30 ksi √in. The excellent fatigue crack growth resistance of austenitic stainless steels is related to the de-formation induced phase transformations taking place in the plastic zone and to the low stacking fault energy of the alloys.     

Internal hydrogen-induced subcritical crack growth in austenitic stainless steels

The effects of small amounts of dissolved hydrogen on crack propagation were determined for two austenitic stainless steel alloys, AISI 301 and 310S. In order to have a uniform distribution of hydrogen in the alloys, they were cathodically charged at high temperature in a molten salt electrolyte. Sustained load tests were performed on fatigue precracked specimens in air at 0 ‡C, 25 ‡C, and 50 ‡C with hydrogen contents up to 41 wt ppm. The electrical potential drop method with optical calibration was used to continuously monitor the crack position. Log crack velocityvs stress intensity curves had definite thresholds for subcritical crack growth (SCG), but stage II was not always clearly delineated. In the unstable austenitic steel, AISI 301, the threshold stress intensity decreased with increasing hydrogen content or increasing temperature, but beyond about 10 wt ppm, it became insensitive to hydrogen concentration. At higher concentrations, stage II became less distinct. In the stable stainless steel, subcritical crack growth was observed only for a specimen containing 41 wt ppm hydrogen. Fractographic features were correlated with stress intensity, hydrogen content, and temperature. The fracture mode changed with temperature and hydrogen content. For unstable austenitic steel, low temperature and high hydrogen content favored intergranular fracture while microvoid coalescence dominated at a low hydrogen content. The interpretation of these phenomena is based on the tendency for stress-induced phase transformation, the different hydrogen diffusivity and solubility in ferrite and austenite, and outgassing from the crack tip. After comparing the embrittlement due to internal hydrogen with that in external hydrogen, it is concluded that the critical hydrogen distribution for the onset of subcritical crack growth is reached at a location that is very near the crack tip. Formerly Research Assistant, Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign.     

Effects of test temperature on internal fatigue crack generation associated with nonmetallic particles in austenitic steels

Internal crack generation associated with nonmetallic inclusions or precipitates has been investigated on high-cycle fatigue at 4 K, 77 K, and 293 k of 25Mn-5Cr high-manganese austenitic steel and nitrogen-strengthened 25Cr-13Ni austenitic stainless steel. In both steels, the internal crack initiation typically occurred at 4 K or in long-life range over 10⁶ cycles at 77 K. Particles such as inclusions and precipitates were responsible for the internal crack-generation behavior, and the origins were identified as mainly Al₂O₃ inclusions in 25Mn-5Cr steel and AIN precipitates in 25Cr-13Ni steel, respectively. We discuss the crack-generation stage I mechanism and the relationship between stress range and size of crack-initiation site. The generation of fatigue cracks associated with the nonmetallic particles in the specimen interior involved a stage I crack. A threshold condition assumption was proposed, that the crack propagation occurred at any stress level when the local stress intensity factor range reached over a constant at or around the initiation crack associated with defects.     

Effects of deformation on hydrogen permeation in austenitic stainless steels

Transient and steady state fluxes of hydrogen were measured for annealed and deformed AISI 301, 304 and 310 austenitic and annealed AL 29-4-2 ferritic stainless steel membranes using a gas phase permeation technique at T = 100–350°C. Permeability and effective diffusivity and solubility constants were calculated from these data. Up to 80% deformation of the stable AISI 310 alloy made only a relatively small change in the transport parameters. Deformation of AISI 301 and 304 resulted in various amounts of stress-induced α′ martensite, which greatly enhanced the effective hydrogen diffusivity and permeability. The relationship between phase changes and hydrogen transport parameters was modeled using various assumptions about the microstructure. Effective solubility and diffusivity values are discussed in terms of dislocation trapping and transport.     

The noncontinuum crack tip deformation behavior of surface microcracks

The crack tip opening displacement (CTOD) of small surface fatigue cracks (lengths of the grain size) in Al 2219-T851 depends upon the location of a crack relative to the grain boundaries. Both CTOD and crack tip closure stress are greatest when the crack tip is a large distance from the next grain boundary in the direction of crack propagation. Contrary to behavioral trends predicted by continuum fracture mechanics, crack length has no detectable effect on the contribution of plastic deformation to CTOD. It is apparent from these observations that the region of significant plastic deformation is confined by the grain boundaries, resulting in a plastic zone size that is insensitive to crack length and to external load.     

Formation of a submicrocrystalline structure in metastable austenitic steels during severe plastic deformation and subsequent heating

The structure and the mechanical properties of metastable austenitic steels after severe plastic deformation by four or six passes of equal-channel angular pressing (ECAP) at a temperature of 400°C are studied. It is shown that ECAP results in strain hardening mainly due to the formation of a submicrocrystalline structure, which is retained after subsequent heating to 500°C.     

Phase instabilities during high temperature exposure of 316 austenitic stainless steel

Although Type 316 austenitic stainless steel is widely used in steam generating plants and nuclear reactors the knowledge about aging reactions, nature of precipitates, and precipitation kinetics during high temperature exposure is limited. Time-temperature-precipitation (TTP) diagrams were determined between 400° and 900°C for up to 3000 hr as a function of carbon content, solution treatment temperature, and cold work. The nucleation and growth phenomena, morphology, and composition of the various carbide (M₂₃C₆, M₆C) and intermetallic phases (σ, χ, η were determined. The complex sequence of phase instabilities can be explained on the basis of the carbon content, effect of molybdenum and chromium on the carbon solubility, thermodynamic stability of the phases, and the kinetics of the various precipitation reactions. B. WEISS, formerly Senior Research Engineer, Westinghouse Research Laboratory, Pittsburgh, Pa.     

The effect of high temperature low cycle fatigue on the corrosion resistance of austenitic stainless steels

The effect of high temperature (650 °C) low cycle fatigue on the corrosion behavior of five austenitic stainless steels (Types 304, 316L, 321, and Incoloy Alloys 800 and 800H) has been investigated. For comparison, corrosion tests were also performed on samples of as-received material as well as material which had been solutionized and material which was sensitized at 650 °C. It was observed that cyclic loading at high temperature reduces the corrosion resistance to a much greater extent than does just the exposure of unstressed material to elevated temperatures. Formation of chrome carbides during cycling and depletion of chromium from the matrix is responsible for the decrease in corrosion resistance. Of the alloys tested, Type 304 exhibited the lowest corrosion resistance. Superior corrosion resistance of the other alloys was due to the following: (a) a lower carbon content, (b) a higher chromium content, and (c) the presence of a strong carbide forming element (stabilized material).     

Effect of weld heat input and creep strain on the elevated temperature and crack growth properties of austenitic steels

A potential material class for use at 600°C and more, e.g. for steam turbines with improved thermal efficiency, are austenitic steels. Using these steels with welded joints, it is to be considered that, by superposition of weld residual stresses and service stresses, extensive creep strains – and in the worst case crack formation – can occur locally. To assess the influence of these effects on service behaviour, different material states of CrNi-steels and Incoloy 800 were investigated with respect to strength, ductility and, especially, to crack and creep crack growth in the temperature range around 600°C. It is shown that creep embrittlement, not microstructural changes as effected by weld heat input, causes heat affected zone (HAZ)-reheat cracking. Creep embrittlement can be avoided by special design and fabrication rules.     

The role of crack tip strain rate in the stress corrosion cracking of high strength steels in water

Creep tests have been performed on fracture mechanics specimens of as-quenched 4340 and 3.5NiCrMoV rotor steel to confirm the importance of crack tip strain rate in causing stress corrosion cracking. By allowing creep in a noncracking environment, dry air for the high strength steels tested, cracking did not occur when water, the corrosive solution, was later added to the system. Thus, it is possible to inhibit stress corrosion in spite of conditions otherwise conducive to crack growth. Conditions necessary to restart cracking were also tested. The importance of this result in terms of the mechanism of stress corrosion and difficulties in measuring K_ISCC is discussed. I. O. Smith, formerly Associate Professoraf with the Department of Mining and Metallurgical Engineering, University of Queensland     

On the crack susceptibility of high alloyed tool steels during continuous casting and in the temperature region of hot working

Hot tensile tests after applied prior melting down of the specimens were carried out on mainly ledeburitic tool steels and their strength and ductility were determined in the temperature range between liquidus and 900°C. The test parameters and specimen microstructures were adapted to the conditions and structures prevailing in continuous casting and primary hot working of blooms. The metallurgical processes leading to the embrittlement of the material were examined by means of metallographic investigations. The temperature ranges of internal crack susceptibility and low ductility were evaluated.     

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

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

Dislocation-particle interactions during high temperature deformation of two-phase aluminium alloys

Aluminium-silicon, and aluminium-nickel alloys containing various dispersions of secondphase particles have been deformed in compression at elevated temperatures. It is found that there is a sharp transition in the mechanical behaviour, microstructure, and microtexture, at a critical temperature or strain rate, which is dependent on particle size. The results are compared with various models of stress relaxation at particles, and it is shown that both interface and bulk diffusion controlled mechanisms are operative. The relevance of the transition to the hot working of two-phase alloys is discussed.     

Recovery and recrystallization during high temperature deformation of α-iron

The high temperature deformation of vacuum-melted iron and zone-refined iron has been studied in the temperature range 500° to 800°C over a wide range of strain rates in torsion. Changes in stress-strain behavior and metallographic observations show a transition in the dynamic restoration process from recovery at high stresses to recrystallization at low stresses. The results are discussed in terms of a model for dynamic recrystallization. It is shown that the dependence of the critical strain for the onset of recrystallization on experimental conditions is the most important factor in determining the deformation characteristics. This paper is based upon a thesis submitted by G. GLOVER in partial fulfillment of the requirements of the degree of Doctor of Philosophy at the University of Sheffield.