Study on creep-fatigue failure prediction methods for type 304 stainless steel

Creep-fatigue failure is one of the principal failure modes to be avoided in elevated-temperature components of liquid metal fast breeder reactor (LMFBR) plants. To prevent this failure during the plant life with sufficient confidence, accurate and reliable methods should be employed for evaluating creep-fatigue endurance. A number of creep-fatigue tests have been conduced to establish a reliable creep-fatigue design methodology applicable to LMFBR plants in the last two decades but the conditions of these tests are generally far from those expected in actual plants. For the purpose of studying the characteristics of various creep-fatigue life prediction methods in conditions closer to actual plant conditions, the authors initiated creep and creep-fatigue tests for type 304 austenitic stainless steel with a special emphasis on tests with longer durations than past tests. Interim results are summarized in this paper. Two representative life prediction methods, linear damage fraction rule and ductility exhaustion method, were then applied to these test conditions. It was found that both methods can predict the failure lives with reasonable accuracy. Some comparisons were made regarding the characteristics of these two methods.     

Time dependence of J-integral

Tests performed within the framework of earlier RWTÜV projects together with results obtained elsewhere with regard to the time dependence of fracture mechanics data show that time effects reduced the toughness of materials, according to the nature of the test (extremely slow load rate or hold times with sustained load).

Reduction in toughness has an effect on the following:

• - decrease in critical material data (J₀, δ_i)

• - levelling off of the crack resistance curve J = J(Δa) and in consequence a decrease of tearing modulus.

This tendency is confirmed quantitatively by recent test results. These tests were performed with the material 15 Mn Ni 63 at room temperature with hold times under sustained load and according to the appropriate standards (without hold times). The tests show that hold times cause additional stable crack growth. The resulting J − Δa curve is lower and less sloping than the curve obtained in a standardized test. The time effect should be taken into account in a safety analysis.     

Pipe fracture safety analysis using limit load and J-integral techniques

J-integral fracture toughness tests were performed on full scale pipe specimens to assess the fracture safety performance of two reactor piping alloys. The two alloys investigated were ASTM A106 Grade B carbon steel and circumferentially welded Type 304 stainless steel.The full scale pipe fracture tests were performed on 1.2 m long, circumferentially cracked pipes loaded in four-point bending on a variably compliant test bed. Results of the experiments were analyzed using the limit load approach currently being considered for inclusion in Section XI of the ASME Code. The results were also evaluated using two tearing instability approaches. One approach assumed elastic-perfectly plastic material behavior and the other accounted for material hardening by requiring actual load and displacement data.The limit load analysis provided a good prediction of the maximum load carrying capacity of the pipe specimens in most cases. The results were especially good for the ASTM A106 steel pipes when the materials property data was used to calculate the flow stress. The J-integral tearing instability analysis was shown to accurately describe the ductile tearing instability behavior of the ASTM A106 steel pipe providing material hardening was taken into account.     

Heat-to-heat variations in the fatigue and creep-fatigue behavior of AISI type 304 stainless steel at 593°C

Results from experimental tests designed to characterize differences in the fatigue or creep-fatigue properties of heats of Type 304 stainless steel are reported. These tests included strain controlled fatigue with and without tensile hold times as well as cyclic stress—strain curve generation at 593°C. Although differences in cyclic hardening characteristics were found in four heats of stainless steel tested, little or no difference in cyclic lifetime occurred, with or without tensile hold times, for three of the heats tested. The fourth heat (Ht. No. 8043813) showed an increased resistance to creep-fatigue damage which was tentatively attributed to subtle differences in residual element chemistry. Bilinear constants were also calculated for the four heats and comparisons are made between bilinear and cyclic stress-strain curves.     

Ductile fracture behavior of circumferentially cracked type 304 stainless steel piping under bending load

A ductile pipe fracture test program has been conducted in Japan Atomic Energy Research Institute (JAERI) to investigate the ductile fracture behavior of circumferentially cracked pipes and to demonstrate the validity of the leak before break concept in LWR pipings.In the paper are described the scope of the pipe test program and current test results for 6-inch diameter type 304 stainless steel pipes. Test pipes with a through-wall or a part-through crack in the circumferential direction were bent under low or high compliance condition, and stable or unstable pipe fracture behavior was investigated. J based tearing instability criterion and the net section collapse criterion are compared with the pipe test results, and the validity of these fracture criteria is discussed. Furthermore, geometries of acceptable flaws in pipes are evaluated considering the leak before break condition.     

Radiation-induced segregation in desensitized type 304 austenitic stainless steel

Radiation-induced segregation (RIS) in desensitized type 304 stainless steel (SS) was investigated using a combination of electrochemical potentiokinetic reactivation (EPR) test and atomic force microscopy (AFM). Desensitized type 304 SS was irradiated to 0.43 dpa (displacement per atom) using 4.8 MeV protons at 300 °C. The maximum attack in the EPR test for the irradiated desensitized SS was measured at a depth of 70 μm from the surface. Grain boundaries and twin boundaries got attacked and pit-like features within the grains were observed after the EPR test at the depth of 70 μm. The depth of attack, as measured by AFM, was higher at grain boundaries and pit-like features as compared to twin boundaries. It has been shown that the chromium depletion due to RIS takes place at the carbide-matrix as well as at the carbide-carbide interfaces at grain boundaries. The width of attack at grain boundaries after the EPR test of the irradiated desensitized specimen appeared larger due to the dislodgement of carbides at grain boundaries.     

Ferrite formation in neutron-irradiated type 304L stainless steel

Austenitic (γ) to ferrite (α) transformation was observed using transmission electron microscopy in type 304L stainless steel that had been irradiated at ~500°C to fast-neutron (E > 0.1 MeV) fluences greater than ~ 3 × 10²²n/cm². Previous studies on similar unirradiated stainless steels found no such transformation, indicating that the γ → αtransformation was irradiation-induced. The α phase appeared to nucleate on stacking faults, indicating that the presence of large Frank loops was the critical step in the transformation. After an entire grain of austenite had transformed, the only remaining γ phase existed as shells around voids. Coincidence of rapid swelling behavior with γ → α transformation indicated that the two were related, perhaps by reaction of both phenomena to the effects of irradiation and temperature on microchemical segregation. A volume expansion of about 2.5% was found to be associated with the transformation. Inferences are drawn relating this behavior in type 304L steel to the effects of radiation on other reactor structural materials such as type 316 stainless steel, which is also a metastable austenitic composition.     

A comparison of the use of the modified J-integral and the deformation J-integral in procedures for estimating the critical crack length for CANDU pressure tubes

The paper presents the results of a theoretical investigation whose objective has been to see whether there are advantages to be gained from using the modified J-integral in procedures for estimating the critical crack length for CANDU pressure tubes. For typical operation conditions, and with irradiated tubes having critical crack lengths over a wide range, it is shown that the slope of the modified J-integral J_M-Δa crack growth resistance curve for a pressure tube crack is only marginally greater than the slope of the corresponding deformation J-integral J_D-Δa curve; the results are expressed in terms of the parameter Z_*, which is dJ_M/da − dJ_D/da and the parameter Q, which is the fractional difference between dJ_M/da and dJ_D/da. In the light of these findings, there would appear to be little advantage to be gained in using J_M, rather than J_D, as a characterizing parameter for crack growth in a CANDU pressure tube.     

Application of the J-integral concept to thermal shock loadings

In the frame of our analytical work the applicability of ductile fracture mechanical J-integral concept on mechanical and thermal shock loaded structures with flaws is investigated. By that the behaviour of possible flaws in components of power plants during accidents can be described (e.g. reactor pressure vessel and piping during emergency cooling).The analyses presented in this paper have been performed with a version of the finite element code ADINA [1] extended by fracture mechanical options. The postanalyses of the first series of pressurized thermal shock experiments (PTSE-1A, B, C) performed at ORNL show stress intensity factors (K_I) calculated from J-integrals which are about 10% lower than values of OCA programs [2] based on the linear elastic K-concept usually used for brittle materials. The discrepancy may be referred to different treatment of the influence of plasticity. The results assessed in the frame of the cleavage fracture concept coincide well with the measured times respectively crack tip temperatures at crack initiation and arrest.In the first thermal shock experiment (NKS-1) performed at the MPA-Stuttgart a circumferentially deep cracked test cylinder with overall upper shelf material conditions has been investigated. The postcalculations based on the J-integral with J_R-controlled crack growth show good coincidence between analytical determined and measured structure and fracture mechanical quantities but they are accompanied with numerical problems due to unloading and large plasticity effects.     

In-reactor deformation of solution annealed type 304L stainless steel

Over the past six years at EBR-II, a great deal of information has been obtained on the in-reactor behaviour of solution annealed-Type 304L stainless steel. This information consists of the following: (1) Irradiation induced swelling results in the form of immersion density and transmission electron microscope (TEM) measurements on unstressed material that extends over a temperature range of 395° to 530°C and a neutron fluence range of 1.8 to 9.3 × 10²² n/cm² (E > 0.1 MeV). (2) Irradiation induced creep results from helium pressurized capsules irradiated at a temperature of 415°C. The hoop stress range covered in the experiment was 0 to 27.3 ksi, and the peak neutron fluence obtained to date is 7 × 10²² n/cm² (E > 0.1 MeV). (3) Residual stress measurements (slit tube technique) with complementary TEM gradient studies on stressed and unstressed capsules. (4) Comparative swelling studies of stressed cladding material and unstressed capsule material from encapsulated EBR-II driver fuel experiments over wide ranges of temperature and neutron fluences. The deformation information derived from the four above studies represent an extensive data base from which to obtain an understanding of the in-reactor deformation of austenitic stainless steel. It is the purpose of this paper to review our information on the in-reactor deformation of solution annealed Type 304L stainless steel.     

A parametric study on J-integral under pressurized thermal shock by using statically indeterminate fracture mechanics

The method of statically indeterminate fracture mechanics (SIFM) is application of elastic-plastic fracture mechanics to statically indeterminate problems. Application of SIFM has been developed for axially cracked cylinder problems under axisymmetric pressurized thermal shock, PTS loading. This method allows us to evaluate the J-integral in an explicit form and is efficient in clarifying the mechanical characteristics of the PTS event. This paper describes a parametric study of the J-integral under PTS loading by using SIFM.     

Residual stress measurements in irradiated solution-annealed type 304 stainless steel tubing

Immersion density and residual stress measurements were made on solution-annealed type 304 stainless steel capsule tubing irradiated up to fluence levels of 9 × 10²² n/cm² (E > 0.1 MeV). The measured residual stress is dependent on the competition between differential swelling which builds up differential stresses, and irradiation creep which relaxes these stresses. The measurements were analyzed using a bilinear swelling equation formulated with swelling data from the same heat of material. The temperatures and fluence levels of the swelling and slit tube data were each calculated with the same computer code. At high fluence, when swelling was in the steady-state region, the effective irradiation creep rate increased by a factor of about three. Further analysis was made assuming that stress-enhanced swelling and swelling-enhanced irradiation creep were the enhanced relaxation mechanisms.     

Stable and unstable crack growth in type 304 stainless steel plate

Experimental and theoretical results on stable as well as unstable fractures for Type 304 stainless steel plates with a central crack subjected to tension force are given.In the experiment using a testing machine with a special spring for high compliance, the transition points from the stable to the unstable crack growth are observed and comparisons are made between the test results and the finite element solutions.A round robin calculation for the elastic-plastic stable crack growth using one of the specimens mentioned above is also given.     

Thermal fatigue strength of type 304 stainless steel in simulated BWR environment

Thin-walled cylindrical carbon steel specimens were thermally fatigued in a pressurized autoclave. Since high and low temperature pure water were alternately supplied into the autoclave, the specimens were subjected to homogeneous thermal stress through the wall thickness. The thermal fatigue life was defined as the number of cycles to crack penetration to the inside of the cylindrical specimen. The thermal fatigue strength was compared with the mechanical fatigue strength performed in air and in high temperature water. Even if taking account of the Higuchi-Iida formula, which considers the effects of strain rate, dissolved oxygen concentration and water temperature on fatigue life, the thermal fatigue lives of carbon steel were found to be slightly shorter than the mechanical fatigue lives.     

The fracture toughness measured on sensitized 304 stainless steel in simulated reactor water

The environmental conditions chemically equivalent to BWR primary water, e.g. 288°C, 0.2 ppm O₂ and/or 98°C, air-saturated, were found to influence considerably the in-water fracture toughness values of furnace-sensitized Type 304 stainless steel.Notched compact tension and three point bend specimens sampled from two heats of standard materials (0.06% C) showed significant reduction in dJ/da values reflecting consistently the effects of loading rate, temperature, dissolved oxygen concentration and degree of sensitization. In particular the crack enhancement with lowering the loading rate was significant. The effect became apparent with dJ dt at and below 1× 10−1 kg·mm/mm²/min (1.6 × 10 J/m²/s) in the typical BWR environment.Based on the results, it is suggested that a critical consideration is needed on the significance of such an environmental effect in the LWR structural safety evaluation, in particular that the probability of instable fracture at the “rings” of sensitized material near welded joints is subject to reviewing.     

Effect of nonproportional loading on creep-fatigue properties of 304 stainless steel at low strain ranges near the elastic region

The effect of nonproportional strain path on fatigue/creep-fatigue properties was investigated with 304 stainless steel at 550°C under strain controlled biaxial conditions. The fatigue/creep-fatigue life reduction due to nonproportional strain path occurred even at the lowest strain range investigated, that is, 0.2% for fatigue loading and 0.3% for creep-fatigue loading. The Mises-type path-dependent equivalent strain range was employed in order to evaluate the fatigue/creep-fatigue strength under nonproportional loading conditions. Stress relaxation behavior under nonproportional loading was examined. It was shown that stress relaxes proportionally toward the origin of stress plane even under nonproportional loading. Fatigue damage and creep damage were calculated based on the linear damage summation rule. Life prediction was shown to be possible within an accuracy of a factor of about 2 for nonproportional loading along with other waveforms including pure axial loading, pure torsional loading and combined proportional loading.     

J integral estimation fracture analysis for circumferentially cracked carbon steel piping

The three-segment fitting method is presented to describe the material stress-strain curves with yield plateaus. A J integral estimation approach for carbon steel piping with circumferential through-wall cracks was developed. Failure assessment curves obtained using three options in the CEGB R6 approach were proposed for GB20 carbon steel piping under bending. The initiation and maximum moments predicted by the J estimation approach presented in this paper are quite close to the experimental values.     

Effect of cyclotron-injected helium on void-formation characteristics of type 304 stainless steel

Samples of Type 304 stainless steel were injected with helium by cyclotron bombardment to concentrations ranging between $\text{1.1 × 10}{}^{\mathrm{?7}}$ and $\text{1 × 10}{}^{\mathrm{?4}}$ ppma. Following cyclotron injection, the samples were given a variety of heat treatments prior to insertion in EBR-II for irradiation at 450 °C to a total dose of $\text{1 × 10}{}^{21}\text{n/cm}{}^2$. Samples that were not heat treated or that were annealed at 650 °C following cyclotron injection formed few voids and dislocation loops after EBR-II irradiation. This behavior is apparently due to the precipitate clusters that were formed during the helium injection. These precipitates were analyzed by electron microscopic techniques and found to have spherically symmetric strain fields that were of interstitial character. Samples that were annealed at 760 °C following cyclotron injection formed a larger number density of both voids and dislocation loops than did the control sample after EBR-II irradiation. The void volume also exceeded that of the control. Clustering of the dislocation loop population near grain boundaries and precipitate particles was observed in the control and low helium concentration samples.     

Investigation of the ductile fracture properties of type 304 stainless steel plate, welds, and 4-inch pipe

J-integral fracture toughness tests were performed on welded 304 stainless steel 2-inch plate and 4-inch diameter pipe. The 2-inch plate was welded using a hot-wire automatic gas tungsten arc process. This weldment was machined into 1T and 2T compact specimens for single specimen unloading compliance J-integral tests. The specimens were cut to measure the fracure toughness of the base metal, weld metal and the heat affected zone (HAZ). The tests were performed at 550°F, 300°F and room temperature. The results of the J-integral tests indicate that the J_Ic of the base plate ranged from 4400 to 6100 in lbs/in² at 550°F. The J_Ic values for the tests performed at 300°F and room temperature were beyond the measurement capacity of the specimens and appear to indicate that J_Ic was greater than 8000 in lb/in². The J-integral tests performed on the weld metal specimens indicate that the J_Ic values ranged from 930 to 2150 in lbs/in² at 550°F. The J_Ic values of the weld metal specimens tested at 300°F and room temperature were 2300 and 3000 in lbs/in² respectively. One HAZ specimen was tested at 550°F and found to have a J_Ic value of 2980 in lbs/in² which indicates that the HAZ is an average of the base metal and weld metal thoughness. These test results indicate that there is a significant reduction in the initiation fracture toughness as a result of welding.The second phase of this task dealt with the fracture toughness testing of 4-inch diameter 304 stainless steel pipes containing a gas tungsten arc weld. The pipes were tested at 550°F in four point bending. Three tests were performed, two with a through wall flaw growing circumferentially and the third pipe had a part through radial flaw in combination with the circumferential flaw. These tests were performed using unloading compliance and d.c. potential drop crack length estimate methods. The results of these test indicate that the presence of a complex crack (radial and circumferential) reduces in the initiation toughness and the tearing modulus of the pipe material compared to a pipe with only a circumferentially growing crack.     

Oxidation of 304 stainless steel in high-temperature steam

An experiment on oxidation of 304 stainless steel was performed in steam between 900°C and 1350°C, using the spare cladding of the reactor of the nuclear-powered ship Mutsu. The temperature range was appropriate for a postulated loss of coolant accident (LOCA) analysis of a LWR. The oxidation kinetics were found to obey the parabolic law during the first period of 8 min. After the first period, the parabolic reaction rate constant decreased in the case of heating temperatures between 1100°C and 1250°C. At 1250°C, especially, a marked decrease was observed in the oxide scale-forming kinetics when the surface treated initially by mechanical polishing and given a residual stress. This enhanced oxidation resistance was attributed to the presence of a chromium-enriched layer which was detected by use of an X-ray microanalyzer. The oxidation kinetics equation obtained for the first 8 min is applicable to the model calculation of a hypothetical LOCA in a LWR, employing 304 stainless steel cladding.