A THERMAL SHOCK FATIGUE STUDY OF TYPE 304 AND 316 STAINLESS STEELS

–Thermal fatigue crack initiation and propagation promoted by thermal quenches have been studied in AISI 316 and 304 stainless steels and correlated with isothermal strain cycling fatigue. Axially unconstrained specimens of rectangular section were held at bulk temperatures of 250°C to 500°C and symmetrically water-quenched on the narrow faces to give equivalent surface strain ranges from 2.8 ×10^?3 to 5.4 ×10^?3. Crack initiation in smooth samples showed an apparent threshold at a surface strain range of 2.8×10^?3 equivalent to a thermal amplitude of 150°C with no cracking being produced in 500,000 cycles. The crack growth in prenotched samples was evaluated by direct observation and by subsequent fractography and showed two modes of growth. The crack growth was strain controlled during the early stages of propagation where the crack tip was within the surface zone under conditions of fully plastic cyclic yield. At greater depths the propagation rates in the remaining elastically cycled material were found to correlate with calculated stress intensity values. In the chosen symmetrical quenched axially unconstrained configuration the crack growth rates decreased towards the centre of the specimen, indicating a crack arrest condition as expected from analysis. The results indicated a good correlation with the fracture behaviour observed from isothermal strain cycling fatigue behaviour in an air environment.     

EFFECTS OF NEUTRON IRRADIATION ON LOW-CYCLE FATIGUE AND TENSILE PROPERTIES OF AISI TYPE 304 STAINLESS STEEL AT 298 K

Abstract— Room temperature studies have been made of the effect of neutron damage on the mechanisms concerned with the low-cycle fatigue and tensile test behaviour of stainless steel AISI Type 304. Samples were irradiated in the HFR at Petten to a fast fluence of 5·10²⁴ n m⁻² ( E > 0·1 MeV) at 333 K followed by mechanical testing at room temperature. The low temperature irradiation caused irradiation hardening: the 0·2 yield stress increased from 230 MN m⁻² for the unirradiated material to a lower yield point value of 540 MN m⁻². Irradiation had no significant effect on fatigue life. The loop type damage was removed by glide dislocations resulting in cyclic softening. Dislocation substructures were observed after fatigue testing: cell structures were more pronounced after fatigue testing to failure the higher the applied strain ranges.
The formation of fatigue cracks at the surface of the specimens was observed in a series of specimens exposed to an increasing number of fatigue cycles.     

LOW-CYCLE FATIGUE UNDER NON-PROPORTIONAL LOADING

A series of strain-controlled, low-cycle fatigue experiments have been conducted on 42CrMo steel under various loading paths including circular, square, cruciform, and rectangular paths. Present experiments have shown that there is additional hardening under non-proportional cyclic loading. Non-proportional cyclic additional hardening also results in a shorter life for multiaxial low cycle fatigue. A non-proportionality measure of strain path based on both a physical basis and macromechanical phenomena is proposed. The loading path effect on additional hardening is also described well. Low-cycle fatigue damage accumulation and the evolution process under non-proportional loading is analysed via the Continuum Damage Mechanics Model of Chaboche. A non-proportinality measure is introduced in the damage evolution equation and a modified Coffin-Manson type formula is derived. A novel fatigue life prediction approach based on the critical-plane concept of Brown and Miller is proposed.     

FATIGUE MICROCRACKS IN TYPE 304 STAINLESS STEEL AT ELEVATED TEMPERATURE

Microcracking of type 304 stainless steel at 538°C has been studied, in particular, the initiation, growth and coalescence, of fatigue microcracks on smooth specimens via surface replicas and photomicrographs. Quantitative information, such as, initiation period, growth and coalescence behaviour, statistical distributions of crack length, density of cracks, distribution patterns and crack growth properties, were obtained. Knowledge of these parameters is critical for the application of fracture mechanics to fatigue life assessment and the damage evaluation of structures at elevated temperature.     

THERMAL-MECHANICAL FATIGUE OF MAR-M 509 SUPERALLOY. COMPARISON WITH LOW-CYCLE FATIGUE BEHAVIOUR

Abstract— The thermal-mechanical fatigue behaviour of a cast cobalt based superalloy, MAR-M 509 was investigated. Hollow smooth specimens were submitted to temperature cycling between 600 and 1050°C. The mechanical strain vs temperature cycle shows a diamond shape with peak strains at intermediate temperatures. The stress-strain behaviour as well as fatigue life curves are reported. A significant part of total life is spent in crack initiation. Comparison with isothermal low cycle fatigue was made at the temperature where peak strains occurred. Isothermal low cycle fatigue life to crack initiation was shown to give a conservative estimate of thermal mechanical fatigue life provided extra strains due to differences in thermal expansion coefficients of the oxide scale and base alloy were taken into account.     

CREEP-FATIGUE CRACK GROWTH AND FRACTOGRAPHY OF A TYPE 304 STAINLESS STEEL AT ELEVATED TEMPERATURE

Abstract— Creep-fatigue crack growth behaviour of a Type 304 stainless steel under four types of reversed loading patterns (P-P, P-C, C-P and C-C) was investigated and the results are discussed in the light of fracture mechanics and fractography. The crack growth rate for all of the four types of loading was successfully correlated in terms of the cyclic integral range λ J. It was unnecessary, for practical purpose, to divide Ay into a fatigue component, λ J _f, and a creep component, λ J _c, as has been done elsewhere. The transition of the correlating fracture mechanics parameter from fatigue to creep was not necessarily associated with the fracture morphology. This was related to the longer transition hold time in morphology in C-C type loading compared to C-P type loading, and was attributed to recovery of grain boundary sliding during the compression hold in the C-C type loading.     

IN-REACTOR LOW-CYCLE FATIGUE OF TYPE 0Kh16N15M3B STAINLESS STEEL

An investigation has been carried out on austenitic stainless steel 0Kh16N15M3B under normal conditions and also to neutron irradiation of 6.8 × 10¹⁶nm^-2s^-1 ( E > 0.1 MeV) intensity. Thin-walled torsion cylindrical specimens were tested in strain-controlled fully reversed loading mode at 923 K. Various ranges of strain, pre-loading fluences and half-cycle hold times (1, 5 and 30min) were applied. Neutron irradiation was found to result in hardening of the steel, stimulating cyclic stress relaxation and a reduction in cyclic life. When acting together, neutron irradiation and static loads cause a more significant reduction in the number of cycles to failure than if summed up as independent factors. Application of a kinetic failure criterion based on a damage parameter enables an estimation to be made of the limiting state of the steel under high-temperature cyclic loading with hold periods.     

CREEP, FATIGUE AND CREEP-FATIGUE CRACK GROWTH RATES IN PARENT AND SIMULATED HAZ TYPE 321 STAINLESS STEEL

Abstract— Crack growth rate data are presented from a range of fully reversed displacement-controlled fatigue and creep-Fatigue tests and from static load-controlled creep crack growth tests on aged 321 stainless steel (parent and simulated HAZ) at 650 ° C. In the creep fatigue tests, constant displacement tensile hold periods of 12–192 h were used. Crack growth rates comprised both cyclic and dwell period contributions. Cyclic growth contributions are described by a Paris-type law and give faster crack growth rates than those associated with pure fatigue tests. Dwell period contributions are described by the C* parameter. The total cyclic crack growth rates are given by summing the cyclic and dwell period contributions. Estimates of C* using a reference stress approach together with the appropriate stress relaxation creep data are shown to correlate well with experimentally measured C* values. Crack growth rates during static load-controlled tests correlate well with C* . Good agreement is obtained between crack growth rates during the static tests and those produced during the hold period of the creep-fatigue tests.     

FATIGUE DAMAGE IN AUSTENITIC-FERRITIC DUPLEX STAINLESS STEELS

Fatigue damage in two austenitic-ferritic duplex stainless steels, with the structure of a natural composite and different levels of nitrogen content, was studied in low-cycle fatigue. Both steels show initial cyclic hardening followed by softening and a long stabilisation period. The cyclic stress-strain curve increases with the nitrogen content while Manson-Coffin curves of both steels intersect at medium fatigue lives. The study of the surface relief reveals intensive slip markings both in ferrite and in austenite. Their density is influenced by the nitrogen content. Both the intensity and density of the persistent slip band (PSB) markings are higher in the ferrite. Crack initiation was found to appear predominantly in PSBs in the ferritic grains at the low strain amplitudes, and in the ferritic and austenitic grains at the highest strain amplitudes. The level of the cyclic stress-strain response and the fatigue lives are discussed in terms of the cyclic strain localisation and of the effect of texture and nitrogen content on the strength and fatigue damage. The increased strength of the austenitic phase, due to high nitrogen alloying, results in cyclic slip localisation in the ferrite, and the decrease of fatigue life, compared with the steel with the lower nitrogen content.     

STRESS RESPONSE UNDER THERMAL-MECHANICAL STRAIN CYCLING FOR A 1 CrMoV FERRITIC STEEL AND TWO 316 STAINLESS STEELS

Abstract— Isothermal and thermal-mechanical strain fatigue tests were conducted in air on representative service alloys; a 1 CrMoV steel, and two batches of 316 stainless steel. Data was obtained for thermal-mechanical in-phase and out-of-phase cycles, and also for isothermal tests at the maximum, minimum, and mid-temperature of the thermal-mechanical cycle. Dwell periods were also incorporated in the cycle to assess their effects.
A comparative evaluation has been made on the basis of the materials' cyclic stress response. In general, the results have shown that the thermal-mechanical strain cycling tests cause a large increase in stress range over those tested under isothermal conditions at maximum temperature. In addition, mean stress and strain offsets were developed in continuous cycle thermal-mechanical tests, whereas negligible offsets occurred in isothermal tests.
It appears that the response of the materials could not always simply be explained by reference to the temperature change itself.     

LIFE PREDICTION OF 316 STAINLESS STEEL UNDER CREEP-FATIGUE LOADING

Life prediction for creep-fatigue loading conditions should be related to creep damage mechanisms. In order to examine the effect of the creep damage mode on rupture life under creep-fatigue loading, a “combined creep-fatigue loading test” was carried out on 316 stainless steel. In this method, creep loading and fatigue loading are repeated alternately. The fracture criteria under combined loading closely depend on the creep fracture modes of the static creep test. A new life prediction method which uses this new fracture criterion is proposed. The criteria are changed when the creep damage mode varies. In order to verify the adequacy of this method, fatigue tests with a tensile strain-hold wave form were carried out. It is clear that rupture life in such fatigue tests is dependent on the chosen fracture criteria.     

CREEP-FATIGUE BEHAVIOUR OF FOUR CASTS OF TYPE 316 STAINLESS STEEL

Low-cycle fatigue tests including hold periods up to 24 h at maximum tensile strain have been conducted on four casts of Type 316 stainless steel in the temperature range 570 to 625°C. These resulted in failure times up to 4000 h. In general, fatigue life is reduced as the hold period increases, but the extent of life reduction varies from material to material and with test temperature. For two casts material this initial life reduction is followed by an increase in life as hold periods are extended. The results are rationalised in terms of prevailing fracture processes, and the implication for the extrapolation of short-term data is discussed.     

MICROSTRUCTURAL AND ENVIRONMENTAL EFFECTS ON FATIGUE CRACK PROPAGATION IN DUPLEX STAINLESS STEELS

Abstract— Fatigue crack initiation and propagation in duplex stainless steels are strongly affected by microstructure in both inert and aggressive environments. Fatigue crack growth rates in wrought Zeron 100 duplex stainless steel in air were found to vary with orientation depending on the frequency of crack tip retardation at ferrite/austenite grain boundaries. Fatigue crack propagation rates in 3.5% NaCl solution and high purity water are increased by hydrogen assisted transgranular cyclic cleavage of the ferrite. The corrosion fatigue results are interpreted using a model for the cyclic cleavage mechanism.     

INFLUENCE OF CREEP-FATIGUE INTERACTION ON HIGH TEMPERATURE FATIGUE CRACK GROWTH BEHAVIOR of Ti-1100

Abstract— A series of crack growth experiments has been preformed on the near alpha titanium alloy, Ti-1100, to determine the mechanism of the creep-fatigue interaction. Based on pure creep crack growth results, the increase in the creep-fatigue crack growth rate is not amenable to separate contributions of creep crack growth and fatigue crack growth.
A mechanism has been proposed to account for the increase in creep-fatigue crack growth rate that is based on the planar slip of titanium alloys which results in the formation of dislocation pileups at the prior beta grain boundaries and leads to intergranular fracture. This mechanism has been validated through crack growth experiments preformed on a Ti-1100 that has been microstructurally modified through the precipitation of internal slip barriers. These show that the intergranular fracture and increase in crack growth rate are absent.     

A SIMULATION OF THE BEHAVIOUR OF MULTI-SURFACE FATIGUE CRACKS IN TYPE 304 STAINLESS STEEL PLATE

Abstract— Fatigue tests were carried out to study the growth and coalescence behaviour of multi-surface cracks which were initiated at semi-circular surface notches, and an existing crack growth simulation program was developed to predict and compare with the experimental results. Additional comparisons with ASME and BSI conditions were also carried out to enhance the reliance and integrity of structures and machine elements. The results presented in this paper show that the simulation procedure has utility for fatigue life prediction.     

NOTCH EFFECT ON TWO-LEVEL CUMULATIVE LOW-CYCLE FATIGUE LIFE UNDER DIFFERENT BIAXIAL LOADING MODE SEQUENCES

Abstract Two-level cumulative low-cycle fatigue lives of AISI 316 stainless steel notched specimens with different biaxial loading mode sequences were experimentally analyzed in this paper. Forty-eight cases were conducted in the experimental program by considering the loading level sequence effect, the biaxiality of two levels and the cycle ratios. Results show that interlock effect caused by the characteristic fracture surfaces of the different biaxial states is beneficial to the cumulative fatigue lives. On the other hand, the tensile loading of the second level will accelerate the opening of cracks and decrease fatigue strength. Miner's rule predicts most fatigue lives within 30% error bands, and the loading level sequence effect was not found in this research involving complex fracture modes. Fractography of specimens in the cumulative fatigue tests is reported and discussed in this paper.     

FRACTURE MECHANISMS AND LIFE ASSESSMENT UNDER HIGH-STRAIN BIAXIAL CYCLIC LOADING OF TYPE 304 STAINLESS STEEL

Abstract— In order to investigate the elevated-temperature low-cycle fatigue characteristics of Type 304 stainless steel under biaxial loading, strain-controlled push-pull and torsional fatigue tests were conducted at 550°C under proportional and nonproportional loading conditions with phase differences of 0, α/6, α/4, α/3 and α/2 between the applied sinusoidal axial and torsional strains. Based on the experimental results, this study presents a discussion on fracture mechanisms and failure criteria. In the case of proportional loading, the fracture mode was found, from fractographic observations, to be classified into either Mode I or Mode II, while the fractures were of mixed mode under nonproportional loading. However, failure lives under both the loading conditions could be correlated well in this study with the equivalent shear strain range, Δy, defined by extending the β -plane theory of Brown and Miller. The failure life under nonproportional loading could also be predicted by applying a strain-energy-based approach.     

HIGH-TEMPERATURE LOW-CYCLE FATIGUE RESISTANCE UNDER SUPERIMPOSED STRESSES AT TWO FREQUENCIES

Abstract— Damage accumulation in both strain and stress control type fatigue tests are considered under duplex frequency loading conditions. The damage accumulation can be considered as a summation of damage due to static loading plus damage at each of the frequencies studied. An equation is derived that illustrates the distribution of total damage to failure under a variety of loading conditions.     

FATIGUE DESIGN OF SPOT-WELDED AUSTENITIC AND DUPLEX STAINLESS SHEET STEELS

The fatigue strength of spot-welded stainless sheet steels has been investigated. The main part of the fatigue tests was performed on a cold rolled austenitic stainless sheet steel (AISI304) in air at ambient temperature. For comparison, a duplex stainless steel (SAF2304) of similar yield strength as AISI304 was also incorporated into the test programme. Since the fatigue strength of spot-welded joints depends on the mode of loading, both shear-loaded and peel-loaded joints were tested. The fatigue strength of the spot-welded stainless steels was found to decrease with decreasing sheet thickness. Furthermore, the fatigue strength for peel-loaded joints is lower than that of shear-loaded joint for sheets of equal thickness.
The local loading conditions at the weld edge have been analysed in terms of finite element calculations and fracture mechanics. A design parameter derived from a fracture mechanics analysis was defined for spot-welded stainless sheet steels. It was shown to predict the fatigue life of the present steels and joint configurations in a satisfactory way.     

THE INFLUENCE OF NITROGEN ON HIGH TEMPERATURE LOW CYCLE FATIGUE BEHAVIOUR OF AUSTENITIC STAINLESS STEELS

Abstract— High strain fatigue properties of AISI 316 , AISI 316N and Sandvik 253MA have been investigated at 600°C. The two latter alloys, which contain significant amounts of N, exhibit a higher resistance to fatigue than 316. This effect is accompanied by a planar dislocation slip mode in 316N and 253MA as opposed to a wavy slip mode in 316. The results provide strong evidence that N improves fatigue strength in austenitic stainless steels, by inhibiting cross-slip of screw dislocations.