Impression Creep Tests Research Articles

Creep Deformation and Rupture Behavior of Single- and Dual-Pass 316LN Stainless-Steel-Activated TIG Weld Joints

Creep deformation and rupture behavior of single-pass and dual-pass 316LN stainless steel (SS) weld joints fabricated by an autogenous activated tungsten inert gas welding process have been assessed by performing metallography, hardness, and conventional and impression creep tests. The fusion zone of the single-pass joint consisted of columnar zones adjacent to base metals with a central equiaxed zone, which have been modified extensively by the thermal cycle of the second pass in the dual-pass joint. The equiaxed zone in the single-pass joint, as well as in the second pass of the dual-pass joint, displayed the lowest hardness in the joints. In the dual-pass joint, the equiaxed zone of the first pass had hardness comparable to the columnar zone. The hardness variations in the joints influenced the creep deformation. The equiaxed and columnar zone in the first pass of the dual-pass joint was more creep resistant than that of the second pass. Both joints possessed lower creep rupture life than the base metal. However, the creep rupture life of the dual-pass joint was about twofolds more than that of the single-pass joint. Creep failure in the single-pass joint occurred in the central equiaxed fusion zone, whereas creep cavitation that originated in the second pass was blocked at the weld pass interface. The additional interface and strength variation between two passes in the dual-pass joint provides more restraint to creep deformation and crack propagation in the fusion zone, resulting in an increase in the creep rupture life of the dual-pass joint over the single-pass joint. Furthermore, the differences in content, morphology, and distribution of delta ferrite in the fusion zone of the joints favors more creep cavitation resistance in the dual-pass joint over the single-pass joint with the enhancement of creep rupture life.

Low temperature friction stir welding of P91 steel

Abstract Bead-on-plate friction stir welds were made on P91 alloy with low and high rotational speeds (100 and 1000 RPM) to study their effects on weld microstructural changes and impression creep behavior. Temperatures experienced by the stir zone were recorded at the weld tool tip. Different zones of welds were characterized for their microstructural changes, hardness and creep behavior (by impression creep tests). The results were compared with submerged arc fusion weld. Studies revealed that the stir zone temperature with 100 RPM was well below Ac1 temperature of P91 steel while it was above Ac3 with 1000 RPM. The results suggest that the microstructural degradation in P91 welds can be controlled by low temperature friction stir welding technique.

The microstructure and impression creep behavior of cast Mg–4Sn–4Ca alloy

Because of low creep properties of magnesium–aluminum alloys, magnesium–tin alloys have received much attention in applications where high mechanical properties in high temperatures required. In this study creep properties of Mg–4Sn–4Ca alloy were investigated by the aim of impression creep test, scanning electron microscopy, energy dispersion spectrometry and X-ray diffraction analysis. The impression creep tests were carried out under different shear modulus normalized stress at high temperatures. According to the measured stress exponent values and activation energies the climb-controlled dislocation creep was determined as the dominant mechanism. The creep resistance of this alloy was related to the presence of Ca–Mg–Sn and Mg2Ca phases which are distributed uniformly in the matrix and exhibit high thermal stability.

High Temperature Mechanical Properties of an Extruded Mg–TiO2 Nano‐Composite

The microstructure, high‐temperature shear strength, hot hardness, and creep resistance of pure Mg and its composite, reinforced with 1 vol% TiO2 nano‐particles, were investigated. The shear yield stress (SYS) and ultimate shear strength (USS), measured by the shear punch test (SPT), together with hot hardness were evaluated in the temperature range 25–225 °C. Impression creep tests were performed at 150 °C and under punching stress of 225 MPa. Depending on the test temperature, the shear yield stress (SYS) increased by 4–10 MPa, and ultimate shear strength (USS) by 4–12 MPa in the composite material. Similarly, the hardness increased by 4–8, and impression rate decreased from 10−4 to 0.3 × 10−4 mm s−1. This was attributed to the dispersive distribution of the nano‐sized TiO2 particles in the Mg matrix.

Impression creep deformation behaviour of 316LN stainless steel

The impression creep deformation behaviour of 316LN SS was investigated from microstructure, substructure, microhardness and profilometry studies of the creep deformed region. Impression creep tests were conducted on 316LN SS in the temperature range of 923–973 K, at different punching stresses in the range of 472–760 MPa. The impression creep deformation was characterised by a hemispherically shaped plastic zone which developed around the indentation. The study revealed the distinct regions under the punch undergoing deformation to different extents. The deformation was found to occur predominently on (111) planes. The dislocations in the highly deformed region were well dispersed in the matrix. The size of the plastic zone was estimated to be ∼1·5 times the diameter of the indenter based on the microhardness and profilometry studies. The critical spacing to be maintained between the adjacent indentations was estimated to be >5 times the diameter of indenter.

Effect of tin on the microstructure and impression creep behavior of MRI153 magnesium alloy

The effect of tin on the microstructure and creep behavior of as cast magnesium MRI153 alloy was investigated in the present study. Creep behavior of the alloys were studied using impression creep test under shear modulus normalized stress ranging from 0.025 to 0.04 at temperatures of 152–217°C. Results showed that adding tin to MRI153 alloy reduces the amount of Al2Ca via forming CaMgSn phase, whereas the continuity and value of Mg17Al12 phase are increased. Solubility of calcium in Mg17Al12 phase is also reduced in the presence of tin. It was shown that adding tin to MRI153 alloy decreases the alloy creep strength, which can be attributed to the reduced value of Al2Ca phase and formation of CaMgSn phase instead as well as reduced thermal stability of Mg17Al12 phase and its increased value and continuity. By calculating creep activation energy and stress exponent, it was shown that although climb-controlled dislocation creep is the dominant mechanism in the creep of these alloys, the creep activation energy is decreased with increasing the amount of tin.

Effect of solution treatment on the microstructural evolution and mechanical properties of an aged Mg–4Zn–0.3Ca alloy

The effect of solution treatment on the microstructure, mechanical properties, and creep resistance of cast Mg–4Zn–0.3Ca (wt%) alloy was investigated. Creep behavior was assessed by the impression creep testing at 175 and 225°C, while shear yield stress (SYS) and ultimate shear strength (USS) values were measured by the shear punch testing (SPT) at room temperature and 175°C. A semi-continuous network of Ca2Mg6Zn3 phase was found to form at grain boundaries after a single-step (SS) solution treatment (500°C/4h). This phase was, however, completely dissolved into the matrix after a double-step (DS) solution treatment (350°C/20h+510°C/3h). The SS treated samples exhibited better mechanical properties and creep resistances, due to the presence of the thermally stable Ca2Mg6Zn3 phase, mainly at grain boundaries. This phase, however, made the SS treated samples more susceptible to brittle fracture by acting as preferred sites for initiation of micro-cracks that could easily propagate through the intergranular phase network. On the other hand, DS treated samples had inferior mechanical properties and creep resistances, as compared to the SS treated condition, due to the elimination of the intergranular phases and activation of twinning during deformation.

Impression Creep Behavior of Sn-3.5Ag-0.7Cu/Cu Brazed

Brazing, as one of the major connection technology, has been widely used in different areas such as aviation, aerospace, electronics and chemical industries etc. Sometimes creep fracture can be found in the brazed joints when they are used at high temperature. The conventional characterized method for creep properties cannot be used to study the brazed joints due to their small size. The impression technology has the potential to be used to study the creep properties of brazed joints, since no special requirements are needed for the size and shape of to-be-measured materials. In this investigation, Sn-3.5Ag-0.7Cu/Cu brazed joint is created and its creep properties is measured by impression creep testing. The effect of punching stress and temperature is studied on the impressing depth, which change from 70 to 100MPa and 80-130 oC, respectively. The microstructure of Sn-3.5Ag-0.7Cu/Cu is examined by Optical Microscope (OM), Scanning Electron Microscope (SEM) and Energy Dispersive Spectra (EDS). The results show that the impressing creep depth increases with the rise of temperature and punching stress. Creep resistance of the diffusion region has obvious effect on the creep resistance of the weld. The experimental results can provide the basis for the structural integrity analysis of brazed joints.

Effect of aging treatment on the microstructure, creep resistance and high-temperature mechanical properties of Mg–6Zn–3Cu alloy with La- and Ce-rich rare earth additions

Abstract Effect of aging treatment on the microstructure, creep resistance, and high temperature mechanical properties of the Mg–6Zn–3Cu alloy (ZC63) with La- and Ce-rich rare earth additions was investigated by impression creep and shear punch tests (SPT). Impression creep tests were performed in the temperature range 423–498 K and under punching stress in the range 150–700 MPa for dwell times up to 3600 s. The ultimate shear strength (USS) was measured by the SPT in the temperature range 298–498 K. The results showed that aging treatment increased the shear strength of the cast alloys, whereas it had detrimental effects on the creep strength of materials. These observations were discussed on the basis of the microstructural features and phase constituents of the investigated alloys in the as-cast and aged conditions.

Impression creep behavior of the extruded Mg–4Zn–0.5Ca and Mg–4Zn–0.5Ca–2RE alloys

Abstract The creep characteristics of the extruded Mg–4Zn–0.5Ca and Mg–4Zn–0.5Ca–2RE (wt%) alloys were investigated using impression creep tests in the temperature range of 423–498 K. The creep behavior of both alloys was well described by the Garofalo hyperbolic sine relationship modified for the threshold stress concept. The stress exponent was 5 and the activation energies for creep were found to be close to that of the lattice diffusion in the α-Mg, i.e., 135 kJ mol –1 . Lattice diffusion-controlled climb of dislocations was thus determined as the dominant rate-controlling mechanism. At lower temperatures and higher stresses, the power–law breakdown was observed. The appearance of the threshold stress was attributed to the finely dispersed second phase particles that are present in the microstructure. Threshold stresses decreased linearly with increasing temperature. The RE-containing alloy showed higher threshold stresses and better creep resistance, due to the effect of load transfer resulting from high volume fraction of the second phases and formation of the thermally stable (MgZn) 12 RE particles.

Evaluation of creep deformation behaviour of different microstuctural zones of 316LN SS weld joint using impression creep testing technique

Impression creep technique, which uses flat ended cylindrical indenter, is an attractive material non-invasive technique for evaluating creep properties as it enables probing small volume of material and requires short time for testing. The present investigation discusses the application of impression creep technique for characterising the creep behaviour of different microstructural regions in a high nitrogen grade of 316LN SS weld joint. Impression creep tests were carried out at 923 K on the weld metal, the base metal and the heat-affected zone in 316LN SS weld joint. The impression creep curves exhibited the first two characteristic stages of the creep processes, namely, a primary creep stage and a secondary creep stage. The rate at which the cylindrical flat punch penetrated into the test specimen was controlled by the time dependence of the movement of the material underneath the punch. The weld metal, the base metal and the heat affected zone exhibited distinct creep behaviour. The difference in creep behaviour of each zone was correlated with the difference in microstructure and morphology of the weld joint. A comparison was made between the data obtained from impression creep tests and from independent uniaxial creep tests on the weld metal and the base metal. A good correlation was established between impression creep and uniaxial creep results.

Indentation creep of a cast Mg–6Al–1Zn–0.7Si alloy

Creep behavior of an Mg–6Al–1Zn–0.7Si cast alloy was investigated by indentation creep in order to evaluate the correspondence of indentation creep results with those obtained by other creep test methods. All creep tests were carried out in the temperature range 423–523K and under constant loads of 5 and 20N. Based on the steady-state power-law creep relationship, the average stress exponents of about 5.8 and 11.3, and activation energies of 82.3 and 127.8kJ/mol were obtained in the low- and high-stress regimes, respectively. These results are in good agreement with those of the impression and conventional compressive creep tests reported in the literature. It is suggested that the operative creep mechanism is pipe-diffusion-controlled dislocation viscous glide in the low-stress regime, and dislocation climb with some sort of back stress, similar to those noted in dispersion strengthening alloys, in the high-stress regime.

Effect of Phosphorous Inoculation on Creep Behavior of a Hypereutectic Al-Si Alloy

Creep behavior of Al-Si hypereutectic alloys inoculated with phosphorus was investigated using the impression creep testing. The results showed that at stress regimes of up to 400-450 MPa and temperatures up to 300 °C, no significant creep deformation occurred in both uninoculated and inoculated specimens; however, at temperatures above 300 °C, the inoculated alloys presented better creep properties. Creep data were used to calculate the stress exponent of steady-state creep rate, n, and creep activation energy, Q, for different additive conditions where n was found varied between 5 and 8. Owing to the fact that most alloys have lower values for n (4, 5), threshold stress was estimated for studied conditions. The creep governing mechanisms for different conditions are discussed here, with a particular attention to the effect of phosphorous addition on the microstructural features, including number of primary silicon particles, mean primary silicon spacing, and morphology and distribution of eutectic silicon.

The microstructure and creep characteristics of cast Mg–4Zn–0.5Ca and Mg–4Zn–0.5Ca–2RE alloys

The effect of 2wt% Ce-rich rare earth (RE) addition on the microstructure and creep behavior of the Mg–4Zn–0.5Ca (wt%) alloy was investigated by impression creep tests in the temperature range of 423–498K and under stresses in the range of 250–800MPa for dwell times up to 3600s. Addition of RE was shown to significantly improve the creep resistance of the base alloy. Formation of the thermally stable (MgZn)12RE and Mg17RE2 particles was responsible for such an improvement. The creep behavior can be divided into two stress regimes, with a change from the low-stress regime to the high-stress regime occurring, depending on the test temperature, around 0.024<(σ/G)<0.038 and 0.031<(σ/G)<0.045 for the base- and RE-containing alloys, respectively. In the low-stress regime, stress exponents were in the range 5.0–5.2 and 5.1–6.2, and activation energies were found to be 65±4 and 57±7kJmol−1 for the base and RE containing alloys, respectively. The activation energies were stress dependent and this implied that the stress assisted dislocation climb controlled by pipe diffusion is the rate-controlling mechanism. In the high-stress regime, stress exponents were in the range 10.8–11.7 and 12.1–14.0, and activation energies were 105±20 and 85±18kJmol−1 for the base and RE containing alloys, respectively. Power-law creep was invalid in this region and activation energies were stress dependent, suggesting that the stress-assisted dislocation climb controlled by lattice diffusion, accelerated due to high concentration of vacancies resulting from applying high stresses, is the rate-controlling mechanism.

Study on creep deformation mechanism of 316LN stainless steel from impression creep tests

Impression creep tests were carried out on 316LN stainless steel (SS) at various temperatures in the range 898–973 K. The stress dependence of the steady state impression velocity followed the power-law with stress exponent n = 6. The temperature dependence of the steady state impression velocity obeyed Arrhenius type rate equation. The apparent activation energy for creep deformation (Qc) was estimated to be 500 kJ mol−1. Based on the n and Qc values, it is concluded that the rate controlling mechanism is dislocation creep.

Shear punch creep behavior of cast lead-free solders

Creep behavior of the tin-based lead-free Sn–2Bi, Sn–5Sb, and Sn–9Zn, binary alloys was studied by the newly-developed shear punch creep testing (SPCT) technique in the temperature range 298–375K. Assuming a power law relationship between the creep rate and shear stress, stress exponents of 8.4–11.4, 8.5–11.5, and 6.4–9.3 and average activation energies of 63.4, 61.4 and 40.2kJmol−1 were obtained for Sn–2Bi, Sn–5Sb, and Sn–9Zn, respectively. These creep parameters are in good agreement with those determined by tensile, impression, and indentation creep testing of the same materials reported in the literature. Among all tested materials, as indicated by their minimum creep rates, the solution hardened Sn–2Bi showed the highest creep resistance followed by Sn–5Sb with weak solution and particle hardening effects, and Sn–9Zn that benefits merely from particle hardening. Different creep behaviors of the materials were discussed in terms of their microstructural features.

A Basis for Selecting the Most Appropriate Small Specimen Creep Test Type

Many components in conventional and nuclear power plant, aero-engines, chemical plant etc., operate at temperatures which are high enough for creep to occur. These include plain pipes, pipe bends, branched pipes etc., the manufacture of such components may also require welds to be inserted in them. In most cases, only nominal operating conditions (i.e., pressure, temperatures, system load, etc.) are known and hence precise life predictions are not possible. Also, the proportion of life consumed will vary from position to position within a component and the plant. Hence, nondestructive techniques are adopted to assist in making decisions on whether to repair, continue operating or scrap certain components. One such approach is to use scoop samples removed from the components to make small creep test specimens, i.e., sub-size uniaxial creep test specimens, impression creep test specimens, small punch creep test specimens, and small ring (circular or elliptical) creep test specimens. Each specimen type has its own unique advantages and disadvantages and it may not be obvious which one is the most appropriate test method to use. This paper gives a brief description of each specimen and associated test type and describes their practical limitations. The suitability of each of the methods for determining “bulk” material properties is described and it is shown that an appropriate test type can be chosen.

Creep damage modelling of P92 pipe weld at 675°C

Creep data from ferritic steel P92 parent material, weld metal, cross-weld and heat affected zone specimens tested at 675°C are used to establish material behaviour models necessary for describing the creep behaviour of a P92 pipe with a circumferential weld. The method for determining the material properties for the heat affected zone of a P92 weld, in a single state variable creep damage constitutive model, using the data from cross-weld and impression creep tests, is described. The material properties for the parent material and weld metal regions have been already been published.Creep continuum damage finite element analyses are subsequently performed for a P92 pipe containing a circumferential weld, subjected to internal pressure and end loading, in order to assess the high temperature integrity and failure behaviour of a typical power plant component, from which the lifetime and potential failure modes can be predicted.

Indentation Creep of Lead-Free Sn-5Sb Solder Alloy with 1.5 wt% Ag and Bi Additions

Creep behavior of the ternary Sn-5Sb-1.5Ag and Sn-5Sb-1.5Bi alloys was studied by indentation testing at 298 and 370 K, and compared to that of the binary Sn-5Sb base alloy. Among all tested materials, as indicated by their minimum creep rates, Sn-5Sb-1.5Bi showed the highest creep resistance followed by Sn-5Sb-1.5Ag and Sn-5Sb. The superiority of the Bi-containing alloy is mainly due to the microstructural refinement and solid solution hardening effects of Bi in the Sn matrix, while the improvement in the creep resistance of the Ag-containing alloy is caused by the formation of spherical and rod-shaped Ag3Sn particles. The stress exponent values in the range 11.0–12.2 are close to those determined by tensile and impression creep testing of the same alloys reported in the literature. These high stress exponents together with the activation energies of 58–70 kJ/mol may suggest that the dominant creep mechanism is dislocation creep.

Compressive and impression creep behavior of a cast Mg–Al–Zn–Si alloy

Creep behavior of an Mg–6Al–1Zn–0.7Si cast alloy was investigated by compression and impression creep test methods in order to evaluate the correspondence of impression creep results and creep mechanisms with conventional compression test. All creep tests were carried out in the temperature range 423–523K and under normal stresses in the range 50–300MPa for the compression creep and 150–650MPa for impression creep tests. The microstructure of the AZ61–0.7Si alloy consists of β-Mg17Al12 and Mg2Si intermetallic phases in the α-Mg matrix. The softening of the former at high temperatures is compensated by the strengthening effect of the latter, which acts as a barrier opposing recovery processes. The impression results were in good agreement with those of the conventional compressive creep tests. The creep behavior can be divided into two stress regimes, with a change from the low-stress regime to the high-stress regime occurring, depending on the test temperature, around 0.009<(σ/G)<0.015 and 0.021<(σimp/G)<0.033 for the compressive and impression creep tests, respectively. Based on the steady-state power-law creep relationship, the stress exponents of about 4–5 and 10–12 were obtained at low and high stresses, respectively. The low-stress regime activation energies of about 90kJmol−1, which are close to that for dislocation pipe diffusion in the Mg, and stress exponents in the range of 4–5 suggest that the operative creep mechanism is pipe-diffusion-controlled dislocation viscous glide. This behavior is in contrast to the high-stress regime, in which the stress exponents of 10–12 and activation energies of about 141kJmol−1 are indicative of a dislocation climb mechanism similar to those noted in dispersion strengthening mechanisms.