Effect of ratcheting deformation on fatigue and creep-fatigue life of 316FR stainless steel

Components of fast breeder reactor (FBR) plants will be subjected to large thermal load, and progressive deformation with loading cycles (ratcheting) and creep-fatigue damage should be considered in their design. To clarify the effect of ratcheting on fatigue and creep-fatigue life, a series of fatigue and creep-fatigue tests coupled with strain progress were carried out for 316FR stainless steel. It was found that tensile ratcheting decreases the failure life to a large extent at small strain range, while compressive ratcheting does not decrease the failure life. Measurement of striation intervals on fracture surface showed small influence of strain increment on the crack propagation rate, suggesting that the main cause of the life reduction is the decrease in the crack initiation life. It was also found that failure life in various conditions is correlated well with a product of strain range and tensile peak stress.     

Study on creep-fatigue life prediction methods for low-carbon nitrogen-controlled 316 stainless steel (316FR)

Low-carbon, nitrogen-controlled 316 stainless steel called 316FR was developed and is regarded as a principal candidate for a main structural material of liquid metal-cooled fast breeder reactor plants in Japan. To develop a creep-fatigue evaluation method suitable for this steel, a number of uniaxial creep-fatigue tests have been conducted for three products of this steel. Long-term data up to about 35,000 h were obtained and applicability of failure life prediction methods was studied based upon their results. Cruciform shaped specimens were also tested under biaxial loading conditions to examine the effect of stress multiaxiality on failure life under creep-fatigue condition.     

Clarification of strain limits considering the ratcheting fatigue strength of 316FR steel

The effect of ratcheting on fatigue strength was investigated in order to rationalize the strain limit as a design criterion of commercialized fast reactor systems. Ratcheting fatigue tests were conducted at 550 °C. Duration of the ratchet straining was set for a certain number of strain cycles taking the loading condition of fast reactors into account, and the number of cycles for strain accumulation was defined as the ratchet-expired cycle. Fatigue lives decrease as the accumulated strain by ratcheting increases. Mean stress increased during the ratcheting cycle and its maximum value depended on the accumulated strain and the ratchet-expired cycle. Fatigue life reduction was negligible when the maximum mean stress was less than 25 MPa, corresponding to an accumulated strain of 2.2%. Accumulated strain is limited to 2% in the present design guidelines and this strain limit is considered effective to avoid reducing fatigue life by ratcheting. Microcrack growth behaviors were also investigated in these tests in order to discuss the life reduction mechanisms in ratcheting conditions.     

The effect of a temperature change on void-swelling in electron-irradiated stainless steel type 316

The void-swelling behaviour of stainless steel type 316 has been investigated in electron-irradiation experiments to doses of 40 dpa, involving a temperature change between 475°C and 575°C, or vice versa, after 20 dpa. In the high/low temperature cycle the swelling was commutative. In the low/high temperature cycle the swelling rate, even at 575°C, was characteristic of 475°C. The dominant point-defect sinks are the dislocations and the effects can be understood in terms of changes in the dislocation density. A low/high temperature cycle is beneficial as long as the dislocations dominate the voids as point-defect sinks and thermal vacancies are unimportant. Implications for the operation of fast breeder reactors are discussed.     

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.     

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.     

Dynamic strain aging in stress controlled creep-fatigue tests of 316L stainless steel under different loading conditions

Stress controlled fatigue-creep tests were carried out for 316L stainless steel under different loading conditions, i.e. different loading levels at the fixed temperature (loading condition 1, LC1) and different temperatures at the fixed loading level (loading condition 2, LC2). Cyclic deformation behaviors were investigated with respect to the evolutions of strain amplitude and mean strain. Abrupt mean strain jumps were found during cyclic deformation, which was in response to the dynamic strain aging effect. Moreover, as to LC1, when the minimum stress is negative at 550 °C, abrupt mean strain jumps occur at the early stage of cyclic deformation and there are many jumps during the whole process. While the minimum stress is positive, mean strain only jumps once at the end of deformation. Similar results were also found in LC2, when the loading level is fixed at −100 to 385 MPa, at higher temperatures (560, 575 °C), abrupt mean strain jumps occur at the early stage of cyclic deformation and there are many jumps during the whole process. While at lower temperature (540 °C), mean strain only jumps once at the end of deformation.     

Low-cycle fatigue strength of 10Kh18N9 stainless steel at 773 K in sodium containing a hydroxide impurity

Translated from Atomnaya Énergiya, Vol. 66, No. 5, pp. 333–335, May, 1989.     

Behavior of annealed type 316 stainless steel under monotonic and cyclic biaxial loading at room temperature

This paper addresses the elastic-plastic behavior of type 316 stainless steel, one of the major structural alloys used in liquid-metal fast breeder reactor components. The study was part of a continuing program to develop a structural design technology applicable to advanced reactor systems. Here, the behavior of solution annealed material was examined through biaxial stress experiments conducted at room temperature under radial loadings (√3τ = σ) in tension-torsion stress space. The effects of both stress limited monotonic loading and strain limited cyclic loading were determined on the size, shape, and position of yield loci corresponding to a small offset strain (10 microstrain) definition of yield.In the present work, the aim was to determine the extent to which the constitutive laws previously recommended for type 304 stainless steel are applicable to type 316 stainless steel. It was concluded that for the conditions investigated, the inelastic behavior of the two materials are qualitatively similar. Specifically, the von Mises yield criterion provides a reasonable approximation of initial yield behavior and the subsequent hardening behavior, at least under small offset definitions of yield, is to the first order kinematic in nature.     

Mechanism of dynamic strain aging and characterization of its effect on the low-cycle fatigue behavior in type 316L stainless steel

Low-cycle fatigue tests were carried out in air in a wide temperature range from 20 to 650 °C with strain rates of 3.2 × 10⁻⁵–1 × 10⁻² s⁻¹ for type 316L stainless steel to investigate dynamic strain aging (DSA) effect on the fatigue resistance. The regime of DSA was evaluated using the anomalies associated with DSA and was in the temperature range of 250–550 °C at a strain rate of 1 × 10⁻⁴ s⁻¹, in 250–600 °C at 1 × 10⁻³ s⁻¹, and in 250–650 °C at 1 × 10⁻² s⁻¹. The activation energies for each type of serration were about 0.57–0.74 times those for lattice diffusion indicating that a mechanism other than lattice diffusion is involved. It seems to be reasonable to infer that DSA is caused by the pipe diffusion of solute atoms through the dislocation core. Dynamic strain aging reduced the crack initiation and propagation life by way of multiple crack initiation, which comes from the DSA-induced inhomogeneity of deformation, and rapid crack propagation due to the DSA-induced hardening, respectively.     

Swelling of type-316 stainless steel at high fluences in EBR-II

A test to measure swelling induced by fast neutron irradiation in unstressed specimens of type-316 stainless steel has completed irradiation in the EBR-II reactor. Results are reported and discussed which describe the swelling as a function of neutron fluence, temperature of irradiation and extent of cold work in the alloy. Density determinations showed swellings of up to 15% $\text{ΔV}\text{V}{}_{\mathrm{f}}$ for 20% cold worked type-316 stainless steel at a neutron fluence level of 1.4 × 10²³n/cm², E > 0.1 MeV (70 dpa). The peak swelling temperature range was 550°C–600°C regardless of the extent of cold working. Increasing the cold work level reduced the swelling and tended to broaden the swelling temperature peak. Transmission electron microscopy (TEM) investigations showed that cold working had reduced the average void sizes compared to those observed in the solution annealed material.     

Long-term creep rupture behavior of smoothed and notched bar specimens of low-carbon nitrogen-controlled 316 stainless steel (316FR) and their evaluation

Low-carbon, nitrogen-controlled 316 stainless steel is regarded as a principal candidate for a main structural material of future fast breeder reactor plants in Japan. To grasp creep deformation and rupture behavior of this steel whose modeling is indispensable in the design of high-temperature components, a number of uniaxial tensile creep tests have been conducted for four products of this steel at 550 °C and higher temperatures. Long-term creep rupture data up to about 94,000 h were obtained and used to examine the applicability of rupture and deformation estimation methods developed earlier. In addition, two tests were conducted using round-bar specimens with circumferential notches to make investigation of the effect of stress multiaxiality on creep damage.     

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.     

Effects of stress on swelling in 316 stainless steel

Stress was found to increase the magnitude of irradiation-induced swelling in 316 stainless steel. Measurement of the densities of pressurized tube specimens, irradiated at temperatures of ~ 430–475°C to peak fluences of $\text{~ 9 × 10}{}^{22}\text{n/cm}{}^2$ ($\text{E > 0.1 MeV}$) in EBR-II, has indicated increased swelling in both the annealed and 20% cold worked conditions of this alloy. Swelling in the annealed specimens was observed to increase linearly with hoop stress up to ~ 20 ksi (130 MPa), whereupon further increases in stress resulted in reduced swelling. Swelling in the cold worked material was linear with stress up to levels of ~ 28 ksi (193 MPa).     

Low temperature fatigue crack propagation in neutron-irradiated type 316 steel and weld metal

The fast cycling fatigue crack propagation characteristics of type 316 steel and weld metal have been investigated at 380°C after irradiation to 1.72?1.92 × 10²⁰ n/cm²($\text{E > 1}$ MeV) and 2.03 × 10²¹ n/cm² ($\text{E > 1}$ MeV)at the same temperature. With mill-annealed type 316 steel, modest decreases in the rates of crack propagation were observed for both dose levels considered, whereas for cold-worked type 316 steel irradiation to 2.03 × 10²¹ n/cm² ($\text{E > 1}$ MeV) caused increases in the rate of crack propagation. For type 316 weld metal, increases in the rate of crack propagation were observed for both dose levels considered.The diverse influences of irradiation upon fatigue crack propagation in these materials are explained by considering a simple continuum mechanics model of crack propagation, together with the results of control tensile experiments made on similarly irradiated materials.     

Creep rupture strength of activated-TIG welded 316L(N) stainless steel

316L(N) stainless steel plates were joined using activated-tungsten inert gas (A-TIG) welding and conventional TIG welding process. Creep rupture behavior of 316L(N) base metal, and weld joints made by A-TIG and conventional TIG welding process were investigated at 923 K over a stress range of 160-280 MPa. Creep test results showed that the enhancement in creep rupture strength of weld joint fabricated by A-TIG welding process over conventional TIG welding process. Both the weld joints fractured in the weld metal. Microstructural observation showed lower δ-ferrite content, alignment of columnar grain with δ-ferrite along applied stress direction and less strength disparity between columnar and equiaxed grains of weld metal in A-TIG joint than in MP-TIG joint. These had been attributed to initiate less creep cavitation in weld metal of A-TIG joint leading to improvement in creep rupture strength.     

Low temperature fatigue crack propagation in neutron-irradiated type 316 steel and weld metal

The fast cycling fatigue crack propagation characteristics of type 316 steel and weld metal have been investigated at 380°C after irradiation to 1.72–1.92 × 10²⁰n/cm² (E>1 MeV) and 2.03×10²¹n/cm² (E>1 MeV) at the same temperature. With mill-annealed type 316 steel, modest decreases in the rates of crack propagation were observed for both dose levels considered, whereas for cold-worked type 316 steel irradiation to 2.03 ×10²¹ n/cm² (E>1 MeV) caused increases in the rate of crack propagation. For type 316 weld metal, increases in the rate of crack propagation were observed for both dose levels considered.The diverse influences of irradiation upon fatigue crack propagation in these materials are explained by considering a simple continuum mechanics model of crack propagation, together with the results of control tensile experiments made on similarly irradiated materials.