Small punch and uniaxial creep fracture behaviours of modified SUS304H steel at various temperatures

Commercial austenitic stainless steel SUS304H with small amount of vanadium addition was used in this study. Small punch (SP) creep and uniaxial tensile creep tests were conducted at 650, 700, and 750 °C to measure creep lives and the minimum displacement rates or the minimum creep strain rates. The measured parameters were compared between the two test methods, seeking empirical relationships among the parameters using Larson-Miller Parameter and Monkman-Grant relation. Magnitude of the applied stress (MPa) in the uniaxial tensile creep test was approximately equal to the applied load value (N) in the SP creep test at all test temperatures. It was shown that during the creep deformation of the SP creep specimen, crack initiation and accompanying crack growth occur simultaneously. Competing failure mechanisms of creep deformation and crack growth may affect the SP creep life and consequently determine the proportionality function, α, in the relation between the SP load and the tensile creep rupture stress in creep tests.     

Assessment of power law creep constants of Gr91 steel using small punch creep tests

A method for determining the power law creep constants using the small punch (SP) creep test is studied. We performed elastic-plastic-secondary creep finite-element (FE) analysis of Gr91 (ASTM A387 GR91 CL2) steel using the properties at 565 °C to investigate the evolution of stress and strain rate at the weakest location of the SP creep specimen, i.e. at the annular region located at about 0.7 mm from the centre of the specimen. Empirical relations that correlate the applied load to the equivalent stress and the punch displacement rate to the equivalent creep strain rate are suggested on the basis of the finite-element stress analysis results. These simple relations enable us to achieve the constitutive relation of equivalent stress and equivalent creep strain rate under small punch creep test condition. To validate this approach, SP creep tests were conducted and creep constants were evaluated by using the proposed relations. These evaluated creep constants were then compared with those measured from standard uniaxial creep test. It is shown that creep constants evaluated from the SP creep test and the proposed method are in a good agreement with those from the uniaxial creep test.     

FATIGUE CRACK GROWTH BEHAVIOUR OF METALLIC BIOMATERIALS IN A PHYSIOLOGICAL ENVIRONMENT

Fatigue crack propagation (FCP) in Ringer's solution has been studied on four metallic biomaterials, i.e. pure titanium (TP35H), stainless steel (SUS304), dual-phase stainless steel (SUS329J4L), and titanium alloy (Ti–6Al–4V). For TP35H and SUS329J4L, the crack growth characteristics in Ringer's solution were almost the same as those in room air in the entire ΔK region examined, and thus the effect of Ringer's solution was not observed. The crack growth rates of SUS304 were enhanced in Ringer's solution in the region of ΔK ≥ 25  MPa m as compared to room air, while below this ΔK level they were similar in both environments. Also in Ti–6Al–4V, similar crack growth behaviour in Ringer's solution was found. From the comparison among the da/dN–ΔK relationships of the four materials in Ringer's solution, TP35H exhibited the fastest growth rates and then Ti–6Al–4V, SUS304, SUS329J4L in the order of decreasing growth rate. On the contrary, if the data were plotted in terms of ΔK_eff /E, Ti–6Al–4V and TP35H showed superior crack growth resistance to the stainless steels.     

Experimental evaluation of the interaction effect between plastic and creep deformation

An experimental study of plasticity-creep interaction effects is reported. The combined stress tests are performed on thin wall tubular specimens of SUS 304 stainless steel at room temperature and high temperature (600° C). The plastic behaviors subsequent to creep pre-strain and creep behaviors subsequent to plastic pre-strain are obtained for loading along straight stress paths with a corner. The inelastic behaviors including both plastic and creep deformations are experimentally investigated. The interaction effects between plastic and creep deformations are quantitatively estimated with the equi-plastic strain surface.     

Structural instability of cold worked alloy 304 in 650 °C service

A heavily worked 304 stainless steel wire basket recrystallized and distorted while in service at 650 °C (1200 °F). This case study demonstrates that heavily cold worked austenitic stainless steel components can experience large losses in creep strength, and potentially structural collapse, under elevated temperature service, even at temperatures more than 300 °C (540 °F) below the normal solution annealing temperature. The creep strength of the recrystallized 304/304L steel was more than 1000 times less than that achievable with solution annealed 304H. These observations are consistent with limitations (2000 Addendum to ASME Boiler and Pressure Vessel Code) on the use of cold worked austenitic stainless steels for elevated temperature service.     

Estimation of long-term creep strength in austenitic power plant steels

This paper presents the characteristics of the performance of austenitic steel Super 304H and HR3C used for construction of boilers with supercritical and ultra-supercritical steam parameters. Characteristic microstructure images of the examined steels in the as received condition observed with the scanning electron microscope are shown. The method for strength assessment based on abridged creep tests carried out at a temperature higher than the design one is presented on the example of examined steels. It has been demonstrated that the obtained results do not deviate from the values of creep strength determined in long-term creep tests. The maximum difference is ±?20%, which is in compliance with the acceptable scatter band. The presented methodology can be used for verification of creep strength.     

The small punch creep test: some results from a numerical model

Obtaining accurate estimates of remanent creep life is of great importance to the power generating industry. The small punch creep test promises to be a useful way forward in this respect. However, a major concern with the test revolves around the ability to convert small punch test data into the required uniaxial equivalents. Experimental results within the literature have given contradictory results partly due to the large experimental scatter inherent within the test and so this article reports some results from a recently developed stochastic finite element model of the small creep punch test that provides guidance on this matter. The uniqueness of the model is based on its realistic creep deformations laws, including strain hardening, thermal softening and damage accumulation that enables it to produce life predictions for virgin material as well as for material with pre existing damage. It is shown that the model produces excellent life predictions for virgin 0.5Cr–0.5Mo–0.25V steel and for damaged 1.25Cr–1Mo steel over a wide range of test conditions. The model also predicts that the dependency of the time to failure on minimum displacement rates is such that small punch test data can be converted into uniaxial data using relatively simple analytical expressions.     

Determination of creep behaviour of Ti60 alloy by small punch creep test

The small punch (SP) creep test has distinct advantages in the creep property assessment of materials at elevated temperatures. However, there are few creep properties of Ti alloys obtained by the SP creep test in the current literature. In this paper, the SP creep behaviour of Ti60 alloy has been evaluated under various loads in the range 550–800 N over a temperature range 873–973 K. The SP creep curves obviously indicated the primary, secondary and tertiary stages of creep. The test results have been compared with those of conventional creep tests. The European Code of Practice (CoP) for Small Punch Testing, Dorn equation and Monkman–Grant relationship have also been used to analyse the results of the SP creep tests. The ratio of load of the SP creep tests to equivalent stress of conventional creep tests, the load exponent value of steady deflection rate and activation energy for creep deformation were estimated from the SP creep tests. In conclusion, it was found that dislocation creep may be the main mechanism that dominates the SP creep deformation of Ti60 alloy in the range of load and temperature.     

Experimental study of remaining creep life of SA-304L stainless steel using small punch creep test

In this paper, the remaining creep life of SA-304L stainless steel at elevated temperatures is studied. The small punch creep tests (SPCT) were performed on SA-304L virgin and aged materials at constant loads. Times to fracture and the minimum deflection rate in SPCTs were recorded, and the time-temperature parametric analysis was performed, based on experimental results. The constants of the Larson–Miller parameter and the Monkman–Grant relationship were obtained for different consumed creep life ratios, and eventually the equations were established to explain the variation of constants of the Larson–Miller parameter and the Monkman-Grant relationship with respect to the consumed creep life ratio.     

Small punch testing and its numerical simulations under constant deflection force conditions

A comparison of results of small punch tests on miniaturized discs under a constant force with their simulation by means of FEM is presented. A heat-resistant steel of type CSN 41 5313 (EN 10CrMo9-10) was selected for our investigations. The small punch tests as well as the necessary conventional creep tests on massive specimens were performed at 873 K. For simulations, the Norton power-law and the exponential relationships were applied in the FEM model of the SPT arrangement. Parameters of both relationships were derived from stress dependences of minimum creep rate obtained from the conventional creep tests. While at higher loads the Norton power-law yields results more comparable with those obtained from experiments, at lower loads the exponential relationship gives better results. The investigation also confirms the simple relation between stress in conventional tests and force in small punch tests resulting in identical time to fracture of both types of tests. __________ Translated from Problemy Prochnosti, No. 1, pp. 32–35, January–February, 2008.     

Behavior of AISI 316L Steel Subjected to Uniaxial State of Stress at Elevated Temperatures

This paper presents an experimental investigation on an AISI 316L stainless steel regarding mechanical properties and short uniaxial creep tests at elevated temperatures. The short time creep tests were carried out under different but constant stresses. The obtained data of ultimate tensile strength, yield strength, creep curves and effects of elevated temperatures on mechanical properties were presented. For a selected rheological model,material parameters were obtained. As a justification, such rheological model is implemented in the finite element procedure for an uniaxially stressed specimen in selected environmental conditions.     

Two different small size testing technique for the creep residual life evaluation of an equiaxed Ni-based superalloy for GT blade and vane application

Two miniaturised creep testing techniques have been applied to a polycrystalline Ni-based superalloy employed for gas turbine blades and vanes. The test campaign was conducted on as-delivered material to assess both testing techniques and evaluate their effectiveness in determining creep residual life and creep residual properties of operated material. The first technique involved small size uniaxial creep testing, conducted on samples with a diameter lower than 3 mm. The diameter effect was deeply studied and a calibration was done to correlate results from standard creep test pieces with those from small diameter samples. The second technique involved the small punch creep test, and the assessment on virgin material enabled determination of the correlation factor, k_sp, to compare small punch test results with uniaxial creep tests. All the collected results were related to the coarse grain macrostructure that characterises this class of alloys and that increases the scatter of the collected experimental results.     

Effect of Nb on long-term creep life of JIS SUS304HTB and JIS SUS347HTB steels – heat-to-heat variation and life assessment of stainless steels

Heat-to-heat variation in creep life has been investigated for the 9 heats of JIS SUS 304HTB (18Cr–8Ni steel) and also for the 9 heats of JIS SUS 347HTB (18Cr–12Ni–Nb steel) in the NIMS Creep Data Sheets, mainly taking the effect of Nb into account. The heat-to-heat variation in creep life of 304HTB is mainly caused by the variation in precipitation hardening due to fine NbC carbides at short times, while it is mainly caused by the variation in available nitrogen concentration, defined as the concentration of nitrogen free from AlN and TiN, at long times. The heat-to-heat variation in creep life of 347HTB is mainly explained by the variation of boron concentration, 3–27 ppm, but not by the variation of solution temperature, Nb/C atomic ratio and phosphorus concentration. Boron reduces the coarsening rate of fine M₂₃C₆ carbides along grain boundaries, which enhances the grain boundary precipitation hardening.     

Numerical modelling of small disc creep test

Abstract

The small disc creep test is described. The disc creep curves observed need to be related to the creep properties of the material obtained by conventional testing. This relationship should include a means of assessing the creep damage which is initially present in the material and therefore to an estimate of remanent life. There are difficulties with such correlations and these are shown to be related to an imperfect understanding of the complex nature of the small punch test. This understanding can be improved by suitable physical modelling of the deformation occurring. The present paper describes a suitable finite element model and verifies the model against experimental observations of disc creep tests. The model uses a realistic creep deformation law which includes strain hardening, thermal softening and damage accumulation for 0·5Cr0·5Mo0·25V steel. It is shown that the results obtained from the test are sensitive not only to the initial condition of the specimen but also to the conditions of the test (e.g. the values of frictional forces at the punch/specimen interface). The paper makes suggestions of how the model may be used to optimise the disc creep test.     

Use of small specimen creep data in component life management: a review

Small specimen creep testing techniques are novel mechanical test techniques that have been developed over the past 25 years. They mainly include the sub-size uniaxial test, the small punch creep test, the impression creep test, the small ring creep test and the two-bar creep test. This paper outlines the current methods in practice for data interpretation as well as the state-of-the-art procedures for conducting the tests. Case studies for the use of impression creep testing and material strength ranking of creep resistant steels are reviewed along with the requirement for the standardisation of the impression creep test method. A database of small specimen creep testing is required to prove the validity of the tests.     

Multi-step Load Impression Creep Tests for a 1/2Cr1/2Mo1/4V Steel at 565°C

Abstract: Impression creep tests under multi-step load conditions were performed for a service-aged 1/2Cr1/2Mo1/4V steel at 565°C, corresponding to uniaxial stresses in the range of 100–150 MPa. Results of the tests have shown that the minimum creep strain rate data of the material, produced from the creep curves obtained under different load histories, are in good agreement. The results obtained indicate that the results from a multi-step load test of an impression creep specimen, which is particularly useful when the test material is very limited, can be used to obtain the secondary creep properties for a material from a very small test sample.     

Experimental Study of the Creep Behaviour of the Cold‐Drawn 304L Stainless Steel

Abstract: In this article, the material and physical parameters for the creep constitutive equations of cold‐drawn 304L stainless steel have been determined using experimental data. Austenitic stainless steel 304L is used mostly in power generation and petrochemical industries because of its high‐temperature creep resistance even at above yield stresses. Test samples have been obtained from cold‐drawn bars, and the material conforms to ASTM A276‐05a specifications. The creep behaviour and properties have been examined for this material by conducting uni‐axial creep tests. Constant temperature and constant load uni‐axial creep tests have been carried out at three temperatures of 680,700 and 720 °C, subjected to constant loads which produce below and above yield initial stresses of 200, 250, 320, 340 and 360 MPa. The experimental data have been used to obtain the creep constitutive parameters using numerical optimisation techniques. In addition, the temperature and stress dependency of the creep properties for this alloy have been investigated using Larson–Miller and Monkman–Grant parameters.     

Property evolution on annealing deformed 304 austenitic stainless steel

Property changes and microstructural evolution of deformed and subsequently annealed austenitic stainless steel are investigated. SUS304 samples were deformed by high pressure torsion to obtain 100 % α′ volume fraction. When plastically deformed SUS304 is annealed in the temperature range of 200–600 °C for 1 h, peak maxima in hardness, electrical resistivity, and saturation magnetization appears at 400 °C. In this study, SUS304 containing 100 % α′ was investigated on the basis of changes in microstructure and mechanical properties after annealing at the temperature of 400 °C for up to 500 h. The observed property changes are attributed to solute segregation, strain relaxation, and G phase precipitation.     

Investigation of bonding strength and sealing behavior of aluminum/stainless steel bonded at room temperature

This article reports the direct bonding of aluminum (Al) [99.999% (5N), 99% (2N)] and stainless steel SUS (304, 316) without heating for sealing in the ultra high vacuum (UHV) components. For bonding, the smooth surfaces of the Al and SUS specimens were activated using argon fast atom beam (Ar-FAB) for 1-60 and 60 min, respectively, in a background pressure of 6.0 × 10⁻⁵ Pa followed by close contact under an external pressure of 960 N. High bonding strength resulted in the bonded mates of Al and SUS304 activated for 30 and 60 min, respectively, due to the adhesion forces of the surface atoms. Tensile pulling tests showed bulk fractures in Al with impurity dependent bonding strength. The bonding strengths for the Al5N/SUS304 and Al2N/SUS304 specimens were higher than 60 and 100 MPa, respectively. For the sealing test, the smooth surface of the SUS316 flange containing a hole was bonded with Al after surface activation 60 and 30 min, respectively. Leak rates for Al5N/SUS316 and Al2N/SUS316 specimens were 1.5 × 10⁻¹¹ and 2.0 × 10⁻¹¹ Pa m³/s, respectively. These results satisfy the permissible leakage of a large-sized UHV chamber. Time dependence of the leak test behavior for both specimens shows a stable leak rate. Therefore, the sealing of Al/SUS316 may be utilized for the fabrication of corrosion free joints for fluid flow in the cooling of electron guns of small size equipment such as portable scanning electron microscopes in UHV pressure.