Study of Hot Cracking Behaviour of Nitrogen-Enhanced Austenitic Stainless Steels using Varestraint and Hot Ductility Tests
TL;DR: In this paper, the role of high nitrogen content on hot cracking susceptibility of this class of steels using Varestraint and hot ductility tests was discussed and compared with other steels.
Abstract: Hot cracking is a major problem in the welding of austenitic stainless steels (SS), particularly the fully austenitic grades. A group of alloys of high nitrogen stainless steel is being developed for structural components of the Indian Fast Reactor programme. Studying the hot cracking behaviour of this nitrogen-enhanced austenitic stainless steel is an important consideration during welding, as this material solidifies without any residual delta-ferrite in the primary austenitic mode. Nitrogen has potent effects on the solidification microstructure; hence, it is expected to have a strong influence on the hot cracking behaviour. Both Varestraint and hot ductility tests were used to evaluate its solidification and liquation cracking susceptibility. Different heats of this material were investigated, which included fully austenitic (high nitrogen stainless steels) containing 0.07–0.22 wt. (%) nitrogen. Varestraint tests were carried out on these alloys using specimens of 3 mm thickness at four strain levels between 0.5 and 4.0%. The Brittleness Temperature Range (BTR) was also evaluated from these tests. The Varestraint test results showed that the solidification cracking susceptibility is higher for 0.22 wt. (%) steel and the liquation (HAZ) crack significantly increases with increasing nitrogen content. Hot ductility tests were conducted on these alloys using a thermomechanical simulator and the Nil Strength Temperature (NST), Nil Ductility Temperature (NDT) and Ductility Recovery Temperature (DRT) were determined. The hot ductility test results showed that the nil ductility range (NDR), the difference between NST and DRT of the nitrogen-enhanced steel containing 0.22 % N, is higher (50 °C) than that of the alloys containing 0.07 % N (40 °C) and 0.14 % N (30 °C), indicating high susceptibility of the 0.22 % N alloy to liquation cracking. This paper presents and discusses the role of high nitrogen content on hot cracking susceptibility of this class of steels using Varestraint and hot ductility tests.
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TL;DR: In this article, Nitrogen content in the 316 LN steel has been increased for optimizing the tensile, creep, fatigue and creep-fatigue interaction strength, whereas the fatigue and fatigue interaction strength have optimum values at around 0.12-0.13% for optimum combination of strengths.
21 citations
TL;DR: In this paper, a nitrogen-enhanced 316LN austenitic stainless steel (SS) with improved high-temperature properties is developed, where the effect of nitrogen on its tensile, creep and low cycle fatigue behavior has been investigated.
Abstract: For the future sodium-cooled fast reactors (SFRs), which are envisaged with a design life of 60 years, nitrogen-enhanced 316LN austenitic stainless steel (SS) with improved high-temperature properties is being developed. To optimize the enhanced nitrogen content in 316LN SS, the effect of nitrogen on its tensile, creep and low cycle fatigue behavior has been investigated. For different heats of 316LN SS containing 0.07-0.22 wt% nitrogen, the tensile and creep properties increased with increase in nitrogen content, while low cycle fatigue properties peaked at 0.14 wt% nitrogen. Finally, based on the evaluation of the hot cracking susceptibility of the different heats of 316LN SS with varying nitrogen content, using the Varestraint and Gleeble hot-ductility tests, the nitrogen content for the nitrogen-enhanced 316LN SS has been optimized at a level of 0.14 wt%. The 0.14 wt% nitrogen content in this optimised composition shifts the solidification mode of the weld metal to fully austenitic region, including that due to dilution of nitrogen from the base metal, thereby increasing its hot cracking susceptibility. This necessitated development and qualification of welding electrodes for obtaining weld metal with 0.14 wt% nitrogen by optimising the weld metal chemistry so as to obtain the requisite delta ferrite content, tensile properties, and very importantly impact toughness both in the as-welded and aged conditions. Studies on localised corrosion behaviour of nitrogen-enhanced 316LN SS indicated the beneficial effect of nitrogen addition to sensitization, pitting, intergranular corrosion, stress corrosion cracking and corrosion fatigue.
11 citations
TL;DR: In this article, a new method for conducting Trans-Varestraint tests for assessing hot cracking susceptibility is proposed, and experiments were carried out, to validate the new method, with an industrial scale rig using tungsten inert gas welding.
Abstract: A new method for conducting Trans-Varestraint tests for assessing hot cracking susceptibility is proposed. Experiments were carried out, to validate the new method, with an industrial scale rig using tungsten inert gas welding. The hot cracking susceptibility of API-5L X65 and EN3B steel was compared. The results indicated that, by using the new method, the strain applied to the welding bead and consequently to the solidification front was controlled in a repeatable and reliable way. The results also indicated that EN3B has a maximum crack length (a parameter in the test) higher than X65 and it is reached at lower augmented strain thus demonstrating it is more susceptible to hot cracking, while also indicating that there is a capability of predicting the initiation position of hot cracks during welding. By using the method proposed, the capability of setting standardized test procedures for Trans-Varestraint tests is improved. It is recommended that future tests for assessing hot cracking susceptibility should employ the proposed method in order for the results to be comparable and to also study the effect of strain rate in hot cracking of materials.
10 citations
TL;DR: In this paper, a nitrogen-enhanced 316LNSS with 0.12-0.14 wt-% N has been developed for its welding, which has better combination of mechanical properties.
8 citations
Dissertation•
01 Jan 2013
TL;DR: In this paper, a literature review was conducted concerning the behavior of Inconel Ni-base alloys and low-alloy steels in DMWs for nuclear applications, which was centered on the metallurgical changes occurring with post-weld heat treatment (PWHT) at the interface of ferritic/austenitic DMMs, on the weldability of Alloy 52 mock-up manufactured in the SINI project and on the narrow-gap welding (NGW) technique emerging in the NPP design.
Abstract: Dissimilar metal welds (DMWs) between low-alloy steels (LAS), stainless steels (SS) and nickel-base alloys are very important in the design of conventional and nuclear power plants (NPPs). They help to reach better performances for high temperature environment but they can promote premature failure of components. Failure is often related to cracking in the heat affected zone of base materials. In this study, a literature review was conducted concerning the behavior of Inconel Ni-base alloys and LAS in DMWs for nuclear applications. It was centered on the metallurgical changes occurring with post-weld heat treatment (PWHT) at the interface of ferritic/austenitic DMWs, on the weldability of Inconel filler metals and on the narrow-gap welding (NGW) technique emerging in the NPP design. The aim was to characterize a NGW present in modern pressurized water reactor (PWR) design, which uses an Inconel filler metal to join the reactor pressure vessel nozzle to its safe-end. In addition, the behavior of Alloy 690 was studied. Eight samples were characterized. A narrow-gap Alloy 52 mock-up manufactured in the SINI project was studied in the as-welded condition and after PWHT. It showed that PWHT resulted in increased carbon depletion in the LAS side and in an extensive chromium carbide precipitation in the weld metal. It was responsible for a sharp hardness peak in the weld metal. Samples from EPRI (Electric Power Research Institute) were characterized for ENVIS project, showing different weld
6 citations
Cites background from "Study of Hot Cracking Behaviour of ..."
...(Srinivasan et al. 2010) Trace elements often cause GB embrittlement, but additions of beneficial elements can counteract it....
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References
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Book•
01 Oct 1988
TL;DR: In this article, the importance of the Constitution diagram for the understanding of Welding Phenomena is discussed, and a detailed description of the Welding and post-weld surface treatment of Fabrications and Welded Components made from Austenitic Stainless Steels is given.
Abstract: Contents: Significance of Constitution Diagrams for the Understanding of Welding Phenomena * Metallurgical Processes During Solidification and Cooling in Stainless Steel Weld Metal * Metallurgical Phenomena in Secondary Crystallization of Stainless Steels and Weld Metals * Precipitation Phenomena in Stainless Steel and Weld Metals * Hot Cracking Resistance During the Welding of Austenitic Stainless Steels * Welding Metallurgy of Ferritic Stainless Chromium Steels with Carbon Contents Below 0.15 per cent * Welding Metallurgy of Low Carbon Chromium-Nickel Martensitic Stainless Steels (Soft Martensitic Steels) * Welding Metallurgy of Duplex Austenitic-Ferritic Stainless Steels * Welding Metallurgy of Austenitic Stainless Steels * General Instructions for the Welding and Post-Weld Surface Treatments of Fabrications and Welded Components Made from Austenitic Stainless Steel * Welding Metallurgy of Heat Resisting Steels * Welding Metallurgy of Austenitic-Ferritic Dissimilar Joints * Appendix: Abbreviations and Short Designations * References * Author Index * Subject Index.
394 citations
"Study of Hot Cracking Behaviour of ..." refers background in this paper
...In fully austenitic stainless steels, in the absence of a small amount of residual delta ferrite, hot cracking is a serious concern during welding, especially in the enrichment of impurity elements like sulphur, phosphorous and silicon [3]....
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TL;DR: The microstructures of austenitic stainless steel welds typically contain a variety of complex austenite-ferrite structures and this behavior is related to solidification cracking susceptibility as discussed by the authors.
Abstract: The microstructures of austenitic stainless steel welds typically contain a variety of complex austenite-ferrite structures. These structures are a result of both the solidification behaviour and subsequent solid state transformations which are controlled by both composition and weld cooling rates. The solidification cracking susceptibility is directly related, in a complex fashion, to the solidifying weld structure. The solidification and solid state transformations which occur during welding are reviewed in detail and this behaviour is related to solidification cracking susceptibility. Improved understanding of these phenomena offers an opportunity to improve commercial applications of austenitic stainless steels.
275 citations
TL;DR: In this paper, the WRC-92 diagram is used as a general guide to maintain a desirable solidification mode during welding, which is a significant problem during the welding of austenitic stainless steels.
Abstract: Solidification cracking is a significant problem during the welding of austenitic stainless steels, particularly in fully austenitic and stabilized compositions. Hot cracking in stainless steel welds is caused by low-melting eutectics containing impurities such as S, P and alloy elements such as Ti, Nb. The WRC-92 diagram can be used as a general guide to maintain a desirable solidification mode during welding. Nitrogen has complex effects on weld-metal microstructure and cracking. In stabilized stainless steels, Ti and Nb react with S, N and C to form low-melting eutectics. Nitrogen picked up during welding significantly enhances cracking, which is reduced by minimizing the ratio of Ti or Nb to that of C and N present. The metallurgical propensity to solidification cracking is determined by elemental segregation, which manifests itself as a brittleness temperature range or BTR, that can be determined using the varestraint test. Total crack length (TCL), used extensively in hot cracking assessment, exhibits greater variability due to extraneous factors as compared to BTR. In austenitic stainless steels, segregation plays an overwhelming role in determining cracking susceptibility.
202 citations
Journal Article•
TL;DR: In this paper, the rationale behind the selection of materials for the different components of the 500 MWe sodium cooled Prototype Fast Breeder Reactor (PFBR) was discussed, and the major factors considered in the selection process were operating conditions, availability of design data in nuclear codes, ease of fabrication, international experience and cost.
Abstract: This paper discusses the rationale behind the selection of materials for the different components of the 500 MWe sodium cooled Prototype Fast Breeder Reactor (PFBR). The major factors considered in the selection of materials include operating conditions, availability of design data in nuclear codes, ease of fabrication, international experience and cost. Attempt has been made to minimise the number of materials and welding consumables in order to avoid mix up of materials during fabrication, and to reduce the cost of Research and Development on materials development and characterisation. From consideration of radiation damage, 20% cold worked Alloy D9 (15Cr-15Ni-Mo- Ti-Si) has been chosen for the initial core of the PFBR. Type 316L(N) stainless steel (SS) has been chosen for structural components of reactor assembly, other than the core components, operating at temperatures above 700 K while 304L(N) SS is the choice for components operating at lower temperatures. Modified 9Cr-1Mo steel is the choice for steam generator while carbon steel has been chosen for top shield components of the reactor assembly. Stringent specifications for chemical compositions and other mechanical properties have been drawn for PFBR materials with the view to improve reliability of components.
163 citations
TL;DR: The influence of nitrogen on tensile properties of 316L stainless steels has been studied for nitrogen levels of 0·07, 0·11,0·14 and 0·22 wt-%.
Abstract: The influence of nitrogen on tensile properties of 316L stainless steels has been studied for nitrogen levels of 0·07, 0·11, 0·14 and 0·22 wt-%. Tensile tests have been carried out at several temperatures in the range 300–1123 K. Nitrogen was found to be beneficial for tensile strength at all the test temperatures. Yield strength and ultimate tensile strength were found to increase linearly with increase in nitrogen content at all the test temperatures. Tensile ductility showed a non-monotonic variation with nitrogen content and test temperature. Equations have been developed to predict yield strength and ultimate tensile strength of 316L stainless steel as a function of nitrogen content and tensile test temperature.
74 citations