Herein we investigate how the oxygen content in hot isostatically pressed (HIP'd) 316L stainless steel affects the mechanical properties and tensile fracture behavior. This work follows on from previous studies, which aimed to understand the effect of oxygen content on the Charpy impact toughness of HIP'd steel. We expand on the work by performing room-temperature tensile testing on different heats of 316L stainless steel, which contain different levels of interstitial elements (carbon and nitrogen) as well as oxygen in the bulk material. Throughout the work we repeat the experiments on conventionally forged 316L steel as a reference material. The analysis of the work indicates that oxygen does not contribute to a measureable solution strengthening mechanism, as is the case with carbon and nitrogen in austenitic stainless steels (Werner in Mater Sci Eng A 101:93-98, 1988). Neither does oxygen, in the form of oxide inclusions, contribute to precipitation hardening due to the size and spacing of particles. However, the oxide particles do influence fracture behavior; fractography of the failed tension test specimens indicates that the average ductile dimple size is related to the oxygen content in the bulk material, the results of which support an on-going hypothesis relating oxygen content in HIP'd steels to their fracture mechanisms by providing additional sites for the initiation of ductile damage in the form of voids.

Herein, we have performed J-Resistance multi-specimen fracture toughness testing of hot isostatically pressed (HIP'd) and forged 304L austenitic stainless steel, tested at elevated (300°C) and cryogenic (À 140°C) temperatures. The work highlights that although both materials fail in a pure ductile fashion, stainless steel manufactured by HIP displays a marked reduction in fracture toughness, defined using J 0.2BL , when compared to equivalently graded forged 304L, which is relatively constant across the tested temperature range.https://doi.org/10.1007/s11661-018-4466-x Ó The Author(s) 2018. This article is an open access publication Hot isostatic pressing (HIP) is a component manufacturing technique, which employs the use of high temperature, and isostatically controlled pressure to consolidate metal alloy power of desired chemistry into bulk metal under an inert (usually argon) atmosphere. [1] The advantages of HIP are well documented, [1][2][3][4] the most significant being within HIP's ability to produce near-net shape components; components with exceedingly complex geometries thus eliminating the need for subsequent machining/welding procedures on the manufactured component. This may not only reduced the costs associated with the overall manufacture process, but through the elimination of welded joints, produces components of homogenous metallurgy; omitting common issues associated with welding of components; hot cracking, different metallurgical zones, induced residual stresses, etc. This is clearly an advantage for components which will be subjected to high stress conditions throughout their lifetime. The degree of metallurgical homogeneity which HIP produces results in no grain directionality, like that commonly seen in forgings, due to the isostatically controlled pressure and temperature and therefore HIP materials display isotropic mechanical properties. Finally, HIP produces material with a comparatively smaller grain size than that of forgings and castings, which not only improves the yield strength and ultimate tensile strength, also lends itself to easier inspection view non-destructive examination techniques.Because of HIP's ability to increase design freedom, there have been increased efforts to demonstrate that components produced by HIP have equivalent or better material properties than those of equivalently graded forged materials. However, the authors have recently shown that the fracture behavior is subtly different between equivalently graded HIP and forged austenitic stainless steel, with HIP 304L and 316L exhibiting a reduction in impact toughness [5,6] as well as HIP 304L exhibiting a reduction in J-integral fracture toughness at ambient temperature. [7,8] This difference in fracture behavior was attributed to the presence of a comparatively large volume fraction of non-metallic oxide inclusions in the HIP microstructure, which lower the energy required to cause fracture via an unzipping effect, whereby ductile void growth is unable to occur on the same scale as in forged stainless steel...

Hardness mapping permits quantification of the properties of materials over microstructurally significant lengths. A technique has been developed whereby hardness maps can be generated to account for specific weld geometry and further refined using an adaptive approach. Once a preliminary map is produced, subsequent indents are placed in high hardness gradient locations to decrease interpolation distances between indentation sites. The method is demonstrated with three test cases: an Al-7010 friction stir weld, an Alloy 600/82 (NeT TG6) weld and an SA508-4N/Alloy 82/316LN dissimilar metal weld. The results show that the method has identified and resolved high regions with elevated hardness gradients. This provides the ability to resolve weld regions rapidly across large areas minimising indent counts.

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Transition welds represent a challenge for the assessment of structural integrity of nuclear plant due to the complexity of the microstructure, properties and local stress state. This paper presents the initial findings of a study aimed at characterising the local microstructure and properties of a transition weld between SA508-4N ferritic steel and SS316LN austenitic stainless steel using a nickel-base filler of Alloy 82. The local microstructures and local composition of the material interfaces are characterised using backscattered electron imaging and Energy-dispersive X-ray spectroscopy. The ferritic steel shows significant grain refinement in the heat affected zone compared to the base metal. This refinement is also observed in the heat affected zone of the austenitic stainless steel although not as significant. Micro-hardness testing has also been incorporated to provide an indication of the influence of local microstructure on flow properties across the weld region. The results indicate a hardness range of between 180–340HV across the weld with the highest value in the heat affected zone of the ferritic steel and the lowest in the austenitic stainless steel. Yield and flow properties derived from flat transweld tensile tests incorporating digital image correlation are related to the micro-hardness results and microstructural characterisation, and an initial assessment of the fracture mechanism performed using fractography.

Herein we assess the differences in Charpy impact behavior between Hot Isostatically Pressed and forged Inconel 690 alloy over the temperature range of 300°C to À 196°C. The impact toughness of forged 690 exhibited a relatively small temperature dependence, with a maximum difference of ca. 40 J measured between 300°C and À 196°C, whereas the HIP'd alloy exhibited a difference of approximately double that of the forged alloy over the same temperature range. We have conducted Charpy impact testing, tensile testing, and metallographic analyses on the as-received materials as well as fractography of the failed Charpy specimens in order to understand the mechanisms that cause the observed differences in material fracture properties. The work supports a recent series of studies which assess differences in fundamental fracture behavior between Hot Isostatically Pressed and forged austenitic stainless steel materials of equivalent grades, and the results obtained in this study are compared to those of the previous stainless steel investigations to paint a more general picture of the comparisons between HIP vs forged material fracture behavior. Inconel 690 was selected in this study since previous studies were unable to completely omit the effects of strain-induced martensitic transformation at the tip of the Chary V-notch from the fracture mechanism; Inconel 690 is unable to undergo strain-induced martensitic transformation due to the alloy's high nickel content, thereby providing a sister study with the omission of any martensitic transformation effects on ductile fracture behavior.

Dissimilar metal welds represent a challenge for structural integrity assessments of nuclear plants due to the complex microstructure and variation in properties over short length scales. This paper presents the findings of a study aimed at investigating the fracture behaviour of a dissimilar metal weld of SA508-4N steel joined to 316LN stainless steel via a filler weld of Alloy 82. Fracture toughness observations have shown the crack growth direction extended from the SA508-4N heat affected zone into the Alloy 82 weld metal. Finite element modelling has been used to simulate the fracture toughness testing with the crack-tip positioned in different material regions. The results have shown that the location of the crack-tip in relation to the fusion boundary and material properties being sampled by the plastic zone has a significant effect on the fracture behaviour of the dissimilar metal weld. Finally X-ray tomography of tested material has provided significant insights into the level of initial porosity within the dissimilar metal weld and development of ductile damage in the materials surrounding the crack-tip. This has been used to infer parameters for a continuum ductile damage model that is used to simulate crack growth.

Where a crack has grown such that it crosses a material interface, such as in a clad pressure vessel, there is an additional assessment requirement, above the surface and depth points to evaluate the material interface region. This is typically in terms of fatigue crack growth behaviour and crack extension by ductile tearing. Generally, elastic stress intensity factor (KI) solutions, such as those in R6, only consider the surface and deepest point locations, and therefore do not provide a methodology to account for any interactions at material interfaces. Therefore, for a material interface, an assessment may look to describe the region by the maximum of the surface or deepest point values, or it may be argued to be represented by the value from either the surface or deepest point, depending on which is geometrically closer. However, this has the potential to lead to overly conservative interpretations in terms of KI and the elastic-plastic equivalent, KJ, at the material interfaces, depending on the fracture toughness and growth laws used. The work presented here investigates the influence of the clad to plate interface on stress intensity factor under elastic (KI) and elastic-plastic (KJ) conditions for a number of defect geometries, loading conditions and material properties. The results presented in this paper seek to provide guidance on the suitability of KI and KJ for the surface and deepest points, extracted from models containing only plate material properties, to describe the behavior of a crack which crosses a material interface region in a clad pressure vessel wall.