Determination of Long-Term Creep Properties for 316H Steel Using Short-Term Tests on Pre-strained Material

Determination of long-term creep rupture properties for 316H steel is both costly and time-consuming and given the level of scatter in the data would need substantial number of tests to be performed. The primary objective of this study is to estimate the long-term creep properties of cross-weld (XW) and as-received (AR) 316H stainless steel by performing accelerated tests on pre-compressed (PC) material. In this work, uniaxial creep rupture tests have been performed on XW specimens and the results have been used to establish a correlation with accelerated test results on the PC material. Moreover, tensile tests have been performed on XW specimens at room temperature and 550 °C to examine the pre-conditioning effects on the mechanical response of the material. Similar power-law creep properties have been found for the creep strain rate and rupture time behaviour of the XW and PC specimens. It also has been found that the creep ductility data points obtained from XW and PC specimens fall upon the estimated trend for the AR material at 550 °C when the data are correlated with the applied stress normalised by 0.2% proof stress. The results show that the long-term creep properties of the XW and AR material can be estimated in much shorter time scales simply by performing tests on the PC material state.

Effect of Hydrogen on Creep Properties of SUS304 Austenitic Stainless Steel

The objective of this study is to derive mechanistic insight into the degradation of metals in high-temperature hydrogen in order to enable the safety of evolving hydrogen technologies that operate at elevated temperature. Creep testing was carried out in argon and hydrogen gases under absolute pressure of 0.12 MPa at 873 K. The material was JIS SUS304 austenitic stainless steel. Results revealed that the creep life (time-to-failure) and creep ductility (strain-to-failure) of the SUS304 in hydrogen gas and in argon displayed opposite trends. While the creep life (time-to-failure) of the SUS304 in hydrogen gas was significantly shorter than that in argon, creep ductility (strain-to-failure) was higher in hydrogen. Associated with the relatively higher creep ductility, evidence of transgranular microvoid coalescence was more prevalent on fracture surfaces produced in hydrogen compared to those produced in argon. In addition, analysis of the steady state creep relationships in hydrogen and argon indicated that the same creep mechanism operated in the two environments, which was deduced as dislocation creep. Regarding the mechanisms governing reduced creep life in hydrogen, the effects of decarburization, carbide formation and the hydrogen-enhanced localized plasticity (HELP) mechanism were investigated. It was confirmed that these effects were not responsible for the reduced creep life in hydrogen, at least within the creep life range of this study. Alternately, the plausible role of hydrogen was to enhance the vacancy density, which led to magnified lattice diffusion (self-diffusion) and associated dislocation climb. As a consequence, hydrogen accelerated the creep strain rate and shortened the creep life.

Metallurgical Evaluation of an Additively Manufactured Nickel-Base Superalloy for Gas Turbine Guide Vanes

In this paper, AM produced test samples of a IN939 derivative nickel-based alloy were tested for tensile, fatigue and creep properties at temperatures up to 871°C and compared to the traditional cast material. Initial results showed improved tensile and fatigue strength, but a reduction in both long-term creep rupture strength and creep ductility in the AM produced material compared to the cast baseline. Microstructural observations in the AM produced material showed a significant difference in the overall metallurgical characteristics beyond grain size compared to the castings. In addition to the laboratory studies and to provide a direct comparison between AM and traditional castings, both AM and cast components were tested in live engine trials exceeding 4,000 hours. Detailed scanning electron microscopy techniques were used to evaluate the evolution of grain size, gamma-prime, MC carbide and secondary M23C6 carbide size and distribution throughout a 5-step heat treatment process. Post-test evaluations for creep rupture specimens of the AM material showed creep cavitation near grain boundaries. The results from the AM produced material are discussed in comparison to expected properties and characteristics from traditional casting methods. Results have shown that material production and short-term metallurgical properties are sufficient to produce quality high temperature stationary guide vanes, but additional research and development is needed to optimize the AM process to achieve high-temperature creep behavior comparable to castings.

Fabrication of Grade 91 Materials Experience in Oil and Gas Applications

A modified 9-Cr alloy was developed by Oak Ridge National Laboratory, in early 1980s, to increase high temperature capabilities of ferritic steels for superheater tubing. The material improved high temperature creep properties by controlling alloying elements and microstructure. The material was added to ASME BPVC in 1983 (thru Code Case 1943) as Grade 91. Higher yield and tensile strength, in comparison to other low-Cr alloy steels (like Grade 22), allowed for fabrication of thinner component wall thickness. This in-turn reduced susceptibility to through-wall thermal stresses during transient events. Directionally this also reduced material costs. Consequently, petrochemical industries have utilized Grade 91 in applications at Heat Recovery Steam Generation units (HRSG), Steam Methane Reformers (SMR), CO2 Boilers and as convection coils in Ethane conversion units. Grade 91 material has complex microstructure and requires careful control of welding parameters to assure crack free welds that provide adequate creep ductility and retain creep strength at high temperatures. The current guidelines documented in API 582 and Technical Report 938 provide limited insights on success factors for weldability. Grade 91 material use has been growing in the recent past Petrochemicals Complex and in offshore applications, at once-throw steam generators (OTSG). The aim of this paper is to share experience on welding parameters. Guidance needs be adjusted to specific projects and repair activities.

High Pressure Heat Treatment of AM Parts—Combining HIP and Heat Treatment

Hot Isostatic Pressing (HIP) has been used for several decades within different industries for a wide variety of applications [1]. During the recent years HIP has become an important post process for metal additive manufacturing (AM) to secure material performance and quality. The HIP process uses a high isostatic pressure and elevated temperature to densify additively manufactured material by eliminating internal defects. The elimination of defects results in improved material properties such as fatigue, creep, ductility and fracture toughness [2-8]. HIP have historically been used only for densification and defect elimination and any modification and optimization of a material’s microstructure is usually performed after the HIP process in a separate heat treatment step in separate equipment e.g. a vacuum furnace. The main reason that these processes have been performed separately is that the achievable cooling rates in HIP systems have traditionally been relatively low, lower than what many materials require for heat treatment to for example create martensite or a super saturated condition.

Exploratory what-if analysis of some debated canister failure modes in the review of a licence application for the construction and operation of a spent nuclear fuel repository in Sweden

Regulatory review of the licence application for construction and operation of a spent fuel repository at the Forsmark site in Sweden involves detailed assessment of both expected and hypothetical failure modes of the copper canister. The copper canister, which is supported by the bentonite buffer and the surrounding crystalline rock in the KBS-3 concept, is expected to provide complete containment of radioactive elements for very long timescales. Detailed assessment shows that there is a small probability on such timescales of canister failure due to corrosion following loss of buffer as well as mechanical failure due to large earthquakes. During the regulatory review process, it was proposed that canisters might also fail due to: (i) corrosion in anoxic oxygen gas free water, (ii) pitting corrosion, (iii) stress corrosion cracking, (iv) creep brittle failure, (v) hydrogen embrittlement. We here provisionally accept a number of alternative assumptions related to these processes as a basis for what-if analysis of their implications. The focus is not to determine the merit or to estimate probability of these cases, but rather to explore their potential significance in the context of the available knowledge about the repository environment. Simplified estimates are made of the consequences in terms of number and timing of canister failures as well as radiological impact. It is judged that poor creep ductility of copper would have larger potential consequences compared to localised corrosion phenomena. Potential corrosion failures are expected to be associated with the small fraction of deposition holes that are most extensively exposed to corrodants.

Longitudinal Seam Welded Piping Assessment in Refinery Reformer Units

Creep is progressive deformation of material over an extended period when exposed to elevated temperature and stresses below the yield strength. Poor Creep ductility and cracking can be a problem above 900 °F (482°C) in the HAZ of low alloy (Cr-Mo) steel. High stress areas, including supports, hangers and fittings are more vulnerable to cracking. Creep cracking has occurred in longitudinal pipe welds with excessive peaking or welds with poor quality. Numerous incidents of cracking in low alloy (Cr-Mo) steel have been reported in the power industry and in refineries with major concern in longitudinal seam welds as well as highly stressed welds in reactors-heaters interconnecting piping. This paper presents the results of an assessment performed on reactors-heaters interconnecting piping in a catalytic reformer unit with a maximum operating temperature of about 950 °F (510 °C) at 250 psig (1.7 MPa) (> 40 years in-service). Comprehensive inspection including visual, dye penetrant testing, thickness measurements and peaking measurements have been performed. Phased Array Ultrasonic Testing (PAUT) was utilized to detect crack-like defects and flaws. Detailed pipe stress analysis and finite element analyses (FEA) were also performed.

Development of a New Austenitic Stainless Steel (Low-C-18Cr-11Ni-3Cu-Mo-Nb-B-N) With High Sensitization Resistance and High Temperature Strength

Austenitic materials with high sensitization resistance and high temperature strength are required for furnace and reaction tower of desulfurizing plants in the petroleum refinery industry. For these requirements, a new steel (LowC-18Cr-11Ni-3Cu-Mo-Nb-B-N) has been developed. The steel shows no intergranular stress corrosion cracking in polythionic acid environment after aging in the temperature range from 565 to 700 °C for up to 10,000 hours. This excellent PTA-SCC resistance is attributed to the prevention of M23C6 carbide precipitation along grain boundary due to extra low carbon content with high ratio of niobium to carbon. The maximum allowable tensile stress of this steel is estimated to be more than 30% higher than that of ASME SA213 Type347H. This excellent strength is based on the precipitation strengthening effect due to fine precipitates of a copper rich phase which are coherent with the austenite matrix in addition to Z-phase (NbCrN). Moreover, boron addition improves creep strength and creep ductility of the steel. From these results, it is concluded that the newly developed steel is a promising material not only for refinery processes but also for other elevated temperature usages.

Creep-Ductility of High Temperature Steels: A Review

A number of measures of the creep-ductility of high temperature steels are reviewed with an ultimate focus on intrinsic creep-ductility. It is assumed that there will be a future requirement for the determination of long duration creep ductility values for design and product standards in the same way as there is currently for creep strength values. The determination of such information will require specialist modelling techniques to be applied to the complex nature of multi-temperature, multi-heat (multi-cast) data collations, and possible solutions are considered. In service, the exhaustion of creep-ductility is most likely to occur at stress concentrations, and for this, a knowledge of the multiaxial creep-ductility is required, and its relationship to uniaxial creep-ductility. Some practical applications requiring a knowledge of creep-ductility are reviewed.