Welding in Energy-Related Projects (eBook)

Sam Stuart (Autor)

eBook Download: EPUB

2013 | 1. Auflage
502 Seiten
Elsevier Science (Verlag)
978-1-4831-4513-6 (ISBN)

Infos

Welding in Energy-Related Projects contains the proceedings of the Welding Institute of Canada's Second International Conference held in Toronto, 20-21 September 1983, on the theme ''Welding in Energy-Related Projects.'' The contributions to the conference offer a unique overview of many areas of technology from research and development studies to construction and operation, and as such provide a comprehensive reference source. This volume contains 44 papers organized into eight sections. Section I contains studies on materials and weldability of steels for energy structures. Section II covers welding techniques such as flux-cored arc welding, root pass welding, and automatic welding. Section III on welding control systems includes studies on such as integrated robotic welding and microprocessor technology in automatic integrated welding systems. Sections IV and V presents studies on welding of high-alloy systems and welding procedure optimization, respectively. Section VI covers quality assurance and inspection of piping systems. Section VII takes up the properties of welds. Section VIII presents stress and strain analyses of welds.

Dr. Sam Stuart is a physiotherapist and a research Fellow within the Balance Disorders Laboratory, OHSU. His work focuses on vision, cognition and gait in neurological disorders, examining how technology-based interventions influence these factors. He has published extensively in world leading clinical and engineering journals focusing on a broad range of activities such as real-world data analytics, algorithm development for wearable technology and provided expert opinion on technology for concussion assessment for robust player management. He is currently a guest editor for special issues (sports medicine and transcranial direct current stimulation for motor rehabilitation) within Physiological Measurement and Journal of NeuroEngineering and Rehabilitation, respectively.

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Welding in Energy-Related Projects contains the proceedings of the Welding Institute of Canada's Second International Conference held in Toronto, 20-21 September 1983, on the theme "e;"e;Welding in Energy-Related Projects."e;"e; The contributions to the conference offer a unique overview of many areas of technology from research and development studies to construction and operation, and as such provide a comprehensive reference source. This volume contains 44 papers organized into eight sections. Section I contains studies on materials and weldability of steels for energy structures. Section II covers welding techniques such as flux-cored arc welding, root pass welding, and automatic welding. Section III on welding control systems includes studies on such as integrated robotic welding and microprocessor technology in automatic integrated welding systems. Sections IV and V presents studies on welding of high-alloy systems and welding procedure optimization, respectively. Section VI covers quality assurance and inspection of piping systems. Section VII takes up the properties of welds. Section VIII presents stress and strain analyses of welds.

Textauszug

PROPERTIES OF WELDS IN THICK SECTION Nb-CONTAINING STEELS

R.B. Lazor* and A.G. Glover*,     *Welding Institute of Canada

R.D. McDonald**,     **CANMET, EMR

ABSTRACT

Three C/Mn steels microalloyed with Nb or Nb/Ti were subjected to thermal simulations using a Gleeble 1500 to produce structures representative of coarse-grained and fine-grained regions of weld heat-affected zones. Peak temperatures of 1350°C and 1000°C were used as were cooling times between 800°C and 500°C of 11, 16, 43, and 200 s. Microstructures were examined and Charpy specimens were tested at −40°C.The microstructures of samples heated to 1350°C were predominantly mixtures of bainite and martensite, while some polygonal ferrite and proeutectoid ferrite were observed at slower cooling rates, Martensite content was directly related to the carbon content. The Ti alloyed steel developed the finest overall microstructural appearance in terms of prior austenite grain size, bainitic lath width, and ferrite grain size. For 1000°C peak temperature, the microstructures were similar to those of the original baseplates and there was negligible grain growth.The impact values showed marked differences amongst the simulations. Poor toughness of the Nb steel suggests that dissolution of Nb precipitates can occur during welding and the grain size increases rapidly. The better properties obtained with the Nb/Ti steel, in which a relatively fine grain size was maintained, shows that TiN precipitates do not dissolve upon reheating and are available for ferrite nucleation during cooling. Overall, the Nb/Ti steel exhibited better toughness than the steel microalloyed with niobium.

KEYWORDS

Welding

heat-affected zones

microalloyed steels

Gleeble simulations

toughness

precipitates

INTRODUCTION

The balance of strength and toughness of C/Mn microalloyed steels has led to their use in shipbuilding, offshore structures, and transportation. These applications quite often involve high heat input welding which has, on occasion, led to poor toughness of the weld zones. The embrittlement has been commonly attributed to the additions of Nb and V and their effects on microstructural transformations and precipitation.

The precipitation of niobium and/or vanadium carbonitrides during controlled rolling produces a fine grain size and good toughness. During the heating cycle of welding, some or all of the carbonitrides may dissolve. This results in austenitic grain growth adjacent to the fusion boundary and possible excessive re-precipitation in the ferrite phase during cooling. It has been shown that the coarsegrained zones of Nb/V steels are similar to those containing no grain refining elements at all (1). Toughness can be maintained by restricting grain growth at high temperatures and reducing the possibility of precipitation in the ferrite phase. The literature indicates that Nb microalloyed steels provide better toughness than V steels in the coarse-grained HAZ. The problem of excessive HAZ grain growth can be controlled also through Ti additions. The use of Ti also reduces the amount of free nitrogen which is also beneficial to toughness.

In view of the uncertainty with respect to HAZ behaviour of C/Mn microalloyed steels and the improved toughness reported with Ti additions, a limited study was undertaken using Gleeble simulations of heavy section plates. The results and discussion are concerned only with the properties of the heat-affected zone of steels alloyed with Nb and Nb/Ti combined.

MATERIAL

The materials chosen for the tests were intended for application to offshore structures and marine applications. They include a steel supplied to Lloyd’s grade EH36, British Standard BS4360 grade 50D, and a commercially produced low carbon steel, 272−2. The analyses of the plates are given in Table 1. These plates are supplied in the normalized condition and exhibit yield strengths above 340 MPa (50 ksi) and tensile strengths between 490 and 620 MPa (70 to 90 ksi).

TABLE 1

Chemical Composition of Steels

WELD SIMULATION

Different regions of the weld heat-affected zone were produced on a Gleeble 1500 thermal simulator using peak temperatures of 1350°C and 1000°C, and several cooling rates. The cooling rates were programmed as outlined in Table 2 to cover a range of conditions which could be expected in practice. The two conditions for the 50D steel, t8−5 = 11 s and t8−5 = 16 s, correspond to heat inputs of 2 kJ/mm and 3 kJ/mm, respectively. The other two steels were programmed for t8−5 = 43 s and t8−5 = 200 s. The 43 second tests correspond to the cooling of welds made using 3.4 kJ/mm and a preheat of 200°C. The 200 second cooling time would be obtained through very high heat inputs (∼ 8 kJ/mm) such as for electroslag welding.

TABLE 2

Parameters and Results of Gleeble Testing

RESULTS AND DISCUSSION

The microstructures of the Gleeble samples are shown in Figures 1, 2 and 3 for the three steels. In Fig. 1, steel EH36 heated to 1350°C and cooled between 800°C and 500°C for 43 s transformed to a mixture of bainite and martensite with delineated grain boundaries. At the slower cooling rate (t8−5 = 200 s, the bainitic lath width increased and the prior austenite grain boundaries were less discernible. This sample also exhibited regions of polygonal ferrite and grain boundary ferrite which replaced most of the martensite.

Fig. 1 Microstructures of Gleeble simulations of steel EH36 (500x).

Fig. 2 Microstructures of Gleeble simulations of steel 272-2 (500x).

Fig. 3 Microstructures of Gleeble simulations of steel 50D (500x).

The samples heated to 1000°C both exhibit microstructures of polygonal ferrite and pearlite, with a marked increase in grain size at the slower cooling rate.

The microstructures of steel 272−2 (Fig. 2) were similar to those observed for the EH36 for a 1350°C peak temperature, except that steel 272−2 developed a finer overall microstructural appearance in terms of prior austenite grain size, bainitic lath width, and ferrite grain size. For t8-5 = 200 s, the structure is a mixture of ferrite and bainite with some polygonal ferrite. The lath dilineation and austenite grain boundaries at both cooling rates was less distinctive than for EH36.

As a comparison, Fig. 3 shows the microstructures of steel 50D heated to a 1350°C peak temperature and two cooling rates. The faster cooled test (t8-5 = 11 s) is principally low carbon martensite (75%) with the remainder being bainite (25%). Prior austenite grain boundaries were not readily apparent. The sample cooled at t8-5 = 16 s was almost the same mixture of low carbon martensite (70%) and bainite (29%), but there was some development of proeutectoid ferrite (1%). Carbide precapitation was also observed between bainitic laths for this sample as was some grain growth.

The samples for steel EH36 heated to 1000°C for both cooling times exhibit microstructures of polygonal ferrite and pearlite, with a marked increase in grain size at the slower cooling rate. For steel 272−2 at 1000°C, the final structures were composed entirely of polygonal ferrite at both cooling rates. The structures of EH36 and 272−2 compared for a cooling time of t8-5 = 200 s and 1000°C peak temperature reflect the differences in the base metal microstructures.

Charpy specimens were prepared from steels EH36 and 272−2 with through-thickness notches and tested at −40°C. The energies reported in Table 2 reflect changes with peak temperatures and composition.

The peak temperatures of the Gleeble simulations produced marked differences in the microstructures which can be explained with reference to the chemical composition and the impact values.

For steel EH36 (no Ti), the niobium precipitates dissolve completely when heated to 1350°C and the austenite grain growth is essentially unrestricted. Impact values of 5 J at −40°C for both cooling rates show that although the transformation structures are more refined at the faster cooling rate, the cleavage resistance is very poor. The austenite grain size of steel 272−2 for a 1350°C peak temperature is slightly less than EH36. This, combined with a finer structure, results in improved toughness, although only slightly. The impact energy was better for the faster cooling rate which could be partly a result of the apparent mixture of structures and more numerous ferrite grains. The ferrite transformations suggest that TiN precipitates do not dissolve upon reheating and are therefore available for nucleation. TiN precipitate clusters were observed at both cooling rates for a peak temperature of 1350°C.

At 1000°C, the impact energy for EH36 shows an improvement but the slower cooling rate has a higher value...

Produkt-Details

• Erscheint lt. Verlag: 22.10.2013
• Sprache: englisch
• Themenwelt: Naturwissenschaften ► Physik / Astronomie
• Themenwelt: Technik ► Elektrotechnik / Energietechnik
• Themenwelt: Wirtschaft
• ISBN-10: 1-4831-4513-1 / 1483145131
• ISBN-13: 978-1-4831-4513-6 / 9781483145136