Damage and Fracture Behaviours in Aged Austenitic Materials during High-Temperature Slow Strain Rate Testing

Theoretical and Applied Mechanics Letters

2014

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In this study, slow strain rate tensile testing at elevated temperature is used to evaluate the influence of temperature and strain rate on deformation behaviour in two different austenitic alloys. One austenitic stainless steel (AISI 316L) and one nickel-base alloy (Alloy 617) have been investigated. Scanning electron microscopy related techniques as electron channelling contrast imaging and electron backscattering diffraction have been used to study the damage and fracture micromechanisms. For both alloys the dominante damage micromechanisms are slip bands and planar slip interacting with grain bounderies or precipitates causing strain concentrations. The dominante fracture micromechanism when using a slow strain rate at elevated temperature, is microcracks at grain bounderies due to grain boundery embrittlement caused by precipitates. The decrease in strain rate seems to have a small influence on dynamic strain ageing at 650 • C.Energy production by power plants must be more efficient to meet the increasing energy consumption. A higher efficiency can be established by increasing temperature and pressure in the boiler sections. This gives higher demands on the materials that should operate within these power plants. Not only is great high-temperature corrosion resistance of importance but also the mechanical behaviour during slow deformation because these alloys can undertake low deformation rates from 10 −5 s −1 (low cycle fatigue) to 10 −7 s −1 (creep) during service. 1,2 Current study was focused on damage and fracture micromechanisms related to low strain rate and high-temperature in two austenitic materials. Using uniaxial slow strain rate tensile testing (SSRT) at high-temperatures, the influence of low strain rates on these mechanisms could be investigated. Also precipitation due to high-temperature and deformation could be coupled to the damage and fracture behaviours.The austenitic stainless steel, AISI 316L and the nickel-base alloy, Alloy 617 were used in this study. Both types were solution treated before testing, AISI 316L at 1 050 • C for 10 min and Alloy 617 at 1 175 • C for 20 min. Table 1 shows the chemical composition of the materials.For the SSRT a Roell-Korthaus and an Instron 5982 tensile test machines were employed. The tensile test machines were equipped with a MTS 653 furnace and Magtec PMA-12/2/VV7-1 a) Corresponding author.

mentioning

confidence: 93%

mentioning

confidence: 94%

Deformation behaviour in advanced heat resistant materials during slow strain rate testing at elevated temperature

Theoretical and Applied Mechanics Letters

2014

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“…This means that for example during service performed at RT components risk to be damaged or fractured which in turn means that the safety of the whole power plant can be jeopardized. However, at an elevated temperature as 700 °C the σ-phase seems not to be brittle during slow deformation [9].…”

Section: Resultsmentioning

confidence: 99%

Long Term High-Temperature Environmental Effect on Impact Toughness in Austenitic Alloys

2014

KEM

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Structural integrity is crucial for the safety of power plants with higher efficiency to meet the increasing global energy consumption. High-temperature environment will demand not only improved high-temperature corrosion resistance but also a maintained sufficient toughness. This study investigates how long term high-temperature environment influence the impact toughness of two austenitic stainless steels (AISI 304 and Sandvik SanicroTM 28) and one nickel-bas alloy (Alloy 617). Alloy 617 has shown increasing impact toughness with both increasing temperature and time, up to 700°C and 3 000 hours, while the two austenitic stainless steels have shown the opposite for the same conditions. At 10 000 hours the impact toughness of Alloy 617 has decreased but the alloy still possess great toughness. Both austenitic stainless steels show embrittlement due to brittle σ-phase and Alloy 617 seems to gain good impact toughness performance from small evenly distributed precipitates.

“…However, with a low strain rate the elongation to failure increases compared to a higher strain rate (2x10 -3 /s) for all tested temperatures. This could be due to a lower deformation hardening at RT when using a low strain rate [14]. At elevated temperature and slow strain rate softening due to DRV and DRX could be the reason for the increase in elongation to failure.…”

Section: Thermec 2013 Supplementmentioning

confidence: 99%

Influence of Deformation Rate on Mechanical Response of an AISI 316L Austenitic Stainless Steel

2014

AMR

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Austenitic stainless steels are often used for components in demanding environment. These materials can withstand elevated temperatures and corrosive atmosphere like in energy producing power plants. They can be plastically deformed at slow strain rates and high alternating or constant tensile loads such as fatigue and creep at elevated temperatures. This study investigates how deformation rates influence mechanical properties of an austenitic stainless steel. The investigation includes tensile testing using strain rates of 2*10-3/ and 10-6/s at elevated temperatures up to 700°C. The material used in this study is AISI 316L. When the temperature is increasing the strength decreases. At a slow strain rate and elevated temperature the stress level decreases gradually with increasing plastic deformation probably due to dynamic recovery and dynamic recrystallization. However, with increasing strain rate elongation to failure is decreasing. AISI 316L show larger elongation to failure when using a strain rate of 10-6/s compared with 2*10-3/s at each temperature. Electron channelling contrast imaging is used to characterize the microstructure and discuss features in the microstructure related to changes in mechanical properties. Dynamic recrystallization has been observed and is related to damage and cavity initiation and propagation.