Small Punch Creep Research Articles

Investigation of creep behavior of Ni3Al-base superalloy by small punch creep

This study aimed to investigate the creep behavior of Ni3Al-base superalloy through small punch creep test. For this purpose, Rene 80 specimens in both casting and heat treatment conditions, after welding by Nd:YAD pulsed-laser, were subjected to a small punch creep test at 700 °C temperature and forces 350 N, 450 N. The results showed that the lowest and highest creep life in the cast specimen is related to heat-affected zone and weld metal, respectively. Coarsening, liquation, and grain boundary cracking in heat-affected zone were the most important factors in reducing creep life. Cr-Mo and Ni-Zr phases were the most important factors that were eliminated in heat-treatment specimen due to dissolution of these factors.

The effect of heat treatment on creep behavior of GTD-111 superalloy welded by pulsed Nd:YAG laser using small punch test

This work is about the evaluating creep characterizations of GTD-111 superalloy, and its weldment, which has high demand in use in turbine blades of aero-engines and land-based gas turbine industries. For this purpose, plates with a thickness of 3 mm have subjected to Nd:YAG pulsed-laser welding. Test specimens were machined from different parts of weldments and prepared in the shape of disks with a thickness of 0.5 mm, including weld metal, heat-affected zone, and base metal. Small punch creep tests were carried out at 700 °C with loads of 350 N and 450 N to find the weakest location in creep consideration of GTD-111 weldment. Analysis of creep diagrams based on time-displacement showed that the weld metal has the highest creep life due fine γ′ particles and MC carbide. The HAZ of the heat-treated specimen showed a longer creep life of about 17 h than the cast specimen. This was due to the reduction of harmful phases for welding, including γ′, γ-γ′, MC, and Cr-M, which were the leading cause of the defect and reduced creep life in HAZ.

A proposal of a conversion method between small punch creep properties and uniaxial ones of Mod.9Cr-1Mo steel at the operating temperature of fast reactors

A proposal of a conversion method between small punch creep properties and uniaxial ones of Mod.9Cr-1Mo steel at the operating temperature of fast reactors

Evaluation of Creep Strength in Small Turbine Wheels by using Very Small Punch Creep Specimen, and Relationship between Creep Strength and Microstructure

Evaluation of Creep Strength in Small Turbine Wheels by using Very Small Punch Creep Specimen, and Relationship between Creep Strength and Microstructure

An Experimental Approach to the Application of Digital Image Correlation to Small Punch Creep Testing

Deterioration assessment of materials is essential to the continued effective operation of critical components operating, at high temperatures and stresses, in thermal power plants. Within progressive inspection strategies, small punch creep testing (SPCT) presents an effective method for establishing the degree of creep exhaustion of in-service power engineering alloys on account of the small volume of material required (Ø8 mm × 0.5 mm discs). These tests make the application of complex strain fields, analogous to the strain field experienced by the service components, possible. In addition, this work explores the application of a noncontact digital image correlation (DIC) method to the full-field measurement of strains which is not possible through traditional extensometry. The accelerated SPCT tests were performed on 12 % Cr ex-service steels subjected to 600 °C. Experimental methodologies and challenges associated with ex situ DIC-SPCT are discussed including equipment and parameter selection, speckle pattern application, preforming requirements and loading method. It is shown that the developed DIC technique using a stereo microscope, airbrushed speckle patterns, sombrero-preformed discs and interrupted tests can be used to compare surface strain development in various aged forms of the ex-service steel. Such an approach is valuable to understanding strain development in small punch tests and to assist in the comparative damage assessment of ex-service power plant materials.

Anisotropy of small punch creep performance of selective laser melted GH4169 at 650 °C

The large temperature gradient and the epitaxial growth of grains during selective laser melting process usually lead to the mechanical anisotropy of metal parts. To evaluate creep anisotropy of selective laser melting-fabricated GH4169 alloy, in this paper creep performance of GH4169 was investigated by means of small punch creep test method. The effect of the small punch loading-induced prestrain on creep properties of the specimens was examined. The results indicate that under the same heat-treatment and the loading conditions, the small punch creep specimens loaded along the building direction have the longer creep life and the larger creep plasticity than the ones loaded along the scanning direction. The finding reveals that loading deformation-induced dynamic recrystallization has a significant effect on improving creep properties by weakening anisotropy properly. The small punch creep properties of the specimens loaded along the building direction superior to that of the specimens loaded along the scanning direction are mainly attributed to the higher deformability of heterogeneous microstructure, the lower recrystallization activation energy and the deformation activation energy.

Fully plastic J-integral and C* equations for small punch test specimen with a surface crack

Small punch (SP) creep test specimen with an elliptical surface crack was suggested for use with creep crack growth testing. Estimation equations for fracture parameters are needed. K-equation was derived previously for the crack in the SP specimen.In this study, estimation equations for J and C* were derived. 3D elastic-plastic finite element analyses were carried out to determine J value along the front of the surface cracks of the same geometries as were employed for the K-estimation. The dimensionless function, h1 was determined for various crack locations according to the EPRI scheme. The plastic limit load solution was also determined. The C*-equation of the cracked SP specimen was derived based on the Jp solution using the analogy between the secondary creep constitutive law and the Ramberg-Osgood plasticity law. The C*-equation may be able to be used to characterize the creep crack growth behavior under the extensive creep condition of the cracked SP specimen.

Small punch creep performance of heterogeneous microstructure dominated Inconel 718 fabricated by selective laser melting

Creep performance of additively-manufactured components is strongly related to additive manufacturing process-induced microstructures and loading direction. Here, the creep life and the cracking behavior of Inconel 718 fabricated by selective laser melting were investigated systematically at 650°C and 400 N by means of the small punch creep testing method. The results show that the periodic and spatially heterogeneous microstructures composed of V-shaped grains and columnar grains are produced, and play an important role in the creep performance. Under the same heat-treatment condition, the creep resistance of the specimens loaded along the scanning direction is much higher than that of the ones loaded along the building direction. While under the same loading direction, homogenization/aging-treated specimens have the longest creep life and the solution/aging-treated specimens are of the shortest creep life. The creep cracking mechanism of the alloy associated with the spatially heterogeneous microstructures was elucidated. The theoretical calculation suggests that the higher energy input or the larger overlap ratio of the melting pools may decrease the area fraction of the V-shaped grains to enhance the creep life. The finding provides a potential strategy to design microstructures for the high creep performance of selective laser melting-fabricated Inconel 718.

Anisotropic creep behavior of stainless steel produced by selective laser melting

In this study, cubic blocks of 316L austenitic stainless steel (SS316L) were fabricated from gas-atomized powder with certain process parameters for selective laser melting (SLM). The microstructural characteristics and tensile properties were investigated for the blocks made in two different directions of the material by using the SLM method. Series of small punch (SP) creep tests were conducted at 650 °C under various applied load levels in the vertically oriented specimen and horizontally oriented specimen. The results suggested that the creep life and the creep resistance of the vertical specimen were significantly higher than those of the horizontal specimen. The creep voids and micro-cracks nucleated and grew along the grain boundaries and the intra-granular cellular networks, where the accumulated dislocations has caused creep cavity nucleation and generated physical damages at both the boundaries. An image analysis method was used to measure the area fraction of the internal creep damage. Furthermore, the results for the horizontal and the vertical specimens were compared after the SP creep tests. The difference in creep damage mechanism between the two directions was explained by the melt pool boundary (MPB) characteristics. Two types of MPB such as “layer-layer” and “track-track” MPBs, were generated by overlapping of multiple melt pools between the layers and between the tracks during the SLM process. The bonding force of the “layer–layer” MPBs seemed to be stronger than that of the “track–track” MPBs. The optimal direction with a high creep resistance was determined for vertical specimens. The creep test results obtained by SLM showed slightly weaker creep resistance compared with the reported data of the conventionally manufactured 316L stainless steel.

Investigation of creep behavior of 316L stainless steel produced by selective laser melting with various processing parameters

Creep behavior of 316L stainless steel was experimentally investigated using small punch (SP) specimens produced by the selective laser melting (SLM) with various processing parameters. Five rectangular blocks were fabricated from gas atomized powder using the SLM. Various combinations of the processing parameters were used in the SLM process, such as the scanning speed and the laser power, when the energy density was kept as a constant. The scanning speed was increased from 336 to 784 mm/s, while the laser power was simultaneously increased from 165 to 385 W. The microstructural characteristics of each block were examined to verify the manufacturing conditions. Internal defects such as pores and un-melted powder were observed in the specimens fabricated when the low laser power was used. Small punch (SP) creep tests were carried out at 650 °C in the applied load range of 320–550 N. Creep-rupture life and Norton’s power law creep deformation constants were measured. The results showed that even though the energy density condition was same for all of the creep specimens, those with higher laser power showed better creep resistance. Since it was known that a higher scanning speed could deteriorate the creep resistance, the higher creep resistance under the high scanning speed condition was indebted to the higher laser power used. The observed creep behavior of the SLM blocks was explained in relation to the microstructures.

Effects of scanning speed on creep behaviour of 316L stainless steel produced using selective laser melting

AbstractIncreasing numbers of critical components for the aerospace and power industries are fabricated using additive manufacturing (AM). To increase the productivity of high‐temperature components by AM, the scanning speed needs to be maximized without sacrificing the required creep properties. In this study, five rectangular blocks were manufactured using selective laser melting while varying the scanning speed from 420 to 980 mm/s. Small punch creep tests were conducted at 650°C using 10 × 10 × 0.5 mm specimens machined from each block. Creep deformation and rupture life were measured. Power law creep constants were also determined. Difference of creep behaviours were explained based on the microstructures showing pore defects and the measured metal density values. A 3D response surface plot was employed to predict the creep life as a function of the scanning speed and the energy density. As a consequence, a scanning speed range of 416 to 572 mm/s is recommended to maximize productivity and to increase creep resistance.

Small Punch Creep Test: An Overview

Small punch creep (SPC) test has an important role in the remaining life calculation of in-service components functioning at high temperature and harsh ambience. Since the extraction of conventional full scale specimens from the components in service impairs its integrity, SPC test can be used as an alternative technique. This test method requires very thin disc similar to those needed for transmission electron microscopy (~ 3 mm diameter and 0.5 mm thick). The pre-requisite to employ this test technique is to establish correlations between uniaxial creep and SPC tests in priori. The purpose and scope of this work is to present briefly the principle of SPC testing method in comparison with uniaxial creep test. The differences in terms of specimen geometry and deformation process have been clearly brought out. The correlations developed by several researchers for determining the creep properties from SPC test along with uniaxial creep results for comparison is presented. The contributions by several authors in the direction of numerical simulation and fractography of SPC test is also briefed.

Design and high temperature behavior of novel heat resistant steels strengthened by high density of stable nanoprecipitates

International efforts have been focused on the development of new 9Cr Ferritic/Martensitic steels capable of operating at temperatures above 700 °C to improve thermal efficiency of new power plants. The distribution and size of the MX nanoprecipitates present in these steels have been reported to be the key factor for improving the thermal stability of the microstructure during creep. Bearing in mind these findings, three different heat resistant steels were designed to have a higher number density of MX nanoprecipitates than commercial 9Cr Ferritic/Martensitic steels. The manufacturing and subsequent microstructural characterization carried out showed that this goal was achieved. Preliminary high temperature strength results performed by means of Small Punch Creep Tests (SPCT) demonstrated that these new steels improve significantly the high temperature strength without an important loss of ductility as compared to a current commercial 9Cr Ferritic/Martensitic steel.

Small punch testing of Fe-Al based alloys with Ti and Nb additions

Three Fe-Al-based alloys, a binary with 22 at. % of Al, a ternary with 22 at. % of Al and 7 at. % of Ti and a quaternary with 22 at. % of Al, 4 at. % of Ti and 4 at. % of Nb prepared by arc melting to small button type ingots were studied by small punch test and small punch creep test in order to obtain the high temperature tensile and creep properties. Evaluation of the results shows a significantly improved strength at high temperatures and creep resistance of the ternary and quaternary alloys compared to the binary alloy. The observation of the punched discs fracture surfaces related to the initial microstructure also helps to better understanding of the deformation and fracture behavior of these alloys at high temperatures.

Inconel 617 alloy creep life augmentation by graphene transfer coating

Presently, countries are trying to increase their energy efficiency. Therefore, the operating temperatures of very high temperature reactor (VHTR) and high temperature gas-cooled reactor (HTGR) are increasing in order to increase overall efficiency. A metallic material’s intrinsic creep property is a limitation in this scenario. Inconel 617 alloy is a candidate material used in the intermediate heat exchangers of the VHTR and HTGR. Therefore, in this work, graphene, which is a promising ultra-thin material, is utilized as an efficient coating to augment the creep property of the Inconel 617 alloy substrate. The Inconel 617 alloy substrate is coated with graphene via a transfer methodology. The small punch creep test methodology is applied in order to measure the creep lifetime of the specimens. From the experimental results it can clearly be seen that the creep lifetime of the Inconel 617 alloy substrate specimen increases by a significant 5~20 % with the transferred graphene coating compared to the as-received Inconel 617 alloy specimen.

Degradation and reduction of small punch creep life of service-exposed Super304H steel

To ensure the safety and structural integrity of a power boiler in thermal power plants, residual life management of superheater tubes at elevated temperature is needed. Over the decades, small punch (SP) creep testing has been widely used as an effective method for measuring creep life and creep properties of the boiler tube materials. In this study, a series of SP creep tests were performed at 650 °C with virgin and service-exposed Super304H stainless steels. The service period was 54750 h and 68550 h, respectively. The residual creep rupture life of the 68550 h serviced Super304H material decreased significantly when it was compared with the virgin and 54750 h serviced materials. Coarsening of the M23C6 precipitates along the grain boundaries made the adjacent region Cr-depleted, which could accelerate the formation of creep cavities at the grain boundaries. These microstructural degradations reduced the creep rupture life of the service-exposed materials. The Larson–Miller curve and the Monkman–Grant relationship were applied to predict the creep rupture life of service-exposed Super304H steels from the measured short creep rupture data.

Determination of creep damage properties from small punch creep tests considering pre-straining effect using an inverse approach

Although the small punch creep test (SPCT) has been commonly accepted as an effective small specimen material characterisation technique (e.g. the European Code of Practice (CoP) for small punch test was produced in 2006 to provide a practical guideline for the experimental work and data interpretation of small punch test), the conversion from the experimental results obtained from the SPCT, i.e. displacement-time data, to the corresponding strain-time data obtained from conventional uniaxial creep tests has not been thoroughly understood due to the complex nature of the deformation and material failure mechanisms involved in the SPCT, e.g. contact mechanism, material nonlinearity, large deformation and initial plasticity damage.The current paper presents an inverse method to determine the creep properties directly from the SPCT experimental results, without converting them to the uniaxial creep test data. A finite element (FE) model taking account of the conventionally overlooked pre-straining effect has been employed for an inverse optimisation method for the first time. The inverse method has been proven to be capable of effectively predicting the creep behaviours of a P91 steel at 600 °C with a good agreement to the experimental results.

Investigating creep rupture and damage behavior of 41Fe-25.5Cr-23.5Ni alloy small punch creep specimens using a novel microstructure meshing approach

The Voronoi tessellation technique was used to model an idealized polycrystalline microstructure. This novel meshing approach was also implemented to study the crack initiation and growth behaviors of Sanicro25 alloy through small punch creep tests using a modified creep damage constitutive model. Furthermore, the influence of specimen thickness on crack fracture mechanics was examined. The suitability of this novel method was validated on the basis of the accuracy of simulation results. Transgranular fracture patterns were found at the primary stage, which were transformed into intergranular cracks as the creep time progressed. Cracks in small punch specimens were predominantly initiated on the outer surface, where high stress triaxiality occurred. Simultaneously, the high stress and stress triaxiality deviation controlled crack growth behaviors were determined by considering the various stress states.

A direct assessment of creep life based on small punch creep test

The uniaxial creep test (UCT) has generally been used to identify creep properties of materials. However, it consumes an enormous amount of testing time and requires a substantial specimen volume. The small punch creep test (SPCT) is introduced as an alternative test method to overcome the shortcomings of the UCT. It uses a fingernail-sized sheet as a specimen (10 × 10 × 0.5 mm3) and a spherical punch ball to apply load to the specimen. Despite these positive aspects, SPCT has rarely received much attention in industrial fields due to a complex interpretation process that requires additional processes such as UCT, FEM, and CEN code. In this paper, a direct assessment method for SPCT is proposed to evaluate creep life directly from the SPCT. In this method, a stress formulation was derived to convert force into one equivalent stress. Moreover, a non-dimensional correction factor was defined to consider the friction effect between the punch ball and specimen. To demonstrate this method, STS316L stainless steel was used as a test specimen for both the UCT and SPCT at 650 °C. A comparison of Larson-Miller models derived from each test result showed good reliability of the proposed direct assessment method.

Determination of creep properties of an advanced Fe-Cr-Ni alloy using small punch creep test with a modified creep strain model

In this study, creep properties and fracture behaviors ranging from 400 to 450 N in small punch creep (SPC) tests at 700 °C and 725 °C were evaluated by experiment and simulation for Sanicro25 alloy. The similar creep performances were obtained experimentally in both SPC tests and uniaxial creep (UC) tests, and the correlation between loads in SPC tests and stresses in UC tests at different temperatures was determined. In addition, different crack growth patterns which gradually changed from intergranular fracture (IF) to combined model of IF and transgranular fracture (TF), and finally TF was found. Based on specimen geometrical deformation, a modified cone model was proposed, which considered the effect of various thicknesses in different deformation regions. Then, a reasonable and feasible SPC creep strain model was developed to describe the strain as a function of time. According to the proposed model, Monkman-Grant (MG) relationship and modified MG relationship in SPC tests were evaluated. Compared with UC tests, the rupture creep strain played an important role in analyzing of the creep properties in SPC tests.