Abstract. 1 Introduction

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1 Transactions on the Built nvironment vol 11, 1995 WIT Press, ISSN The research on usability of 14HNMBCu and high-strength low-alloy steels for marine applications J. Cwiek, J. Labanowski Mechanical ngineering Faculty, Technical University ofgdansk, Abstract A paper presents results of investigation carried out on quenched and tempered plates, 12 mm in thickness, made of high-strength low-alloy steel grades (with minimum yield strength 690 MPa) and (with minimum yield strength 620 MPa). The obtained results show that examined plates and their welded joints meet et Norske Veritas' (NV) requirements for marine constructions. Susceptibility to cold cracking of tested steels was evaluated eventually. 1 Introduction Advanced marine constructions, like navy ships, tankers and offshore platforms require high-strength low-alloy (HSLA) weldable steels for weight reduction. There are three kinds of HSLA weldable steels i.e. quenched and tempered, thermo-mechanical controlled processed (TMCP), and precipitation hardened with copper addition. In Poland only quenched and tempered HSLA steel grades are currently produced. They are of high strength but are susceptible to cold cracking during welding. Weldable steel should possess: sufficient strength and toughness in both welded joint and parent metal.

2 Transactions on the Built nvironment vol 11, 1995 WIT Press, ISSN Marine Technology and Transportation welded joint without any cracks. Thus we checked if tested steels and their joints meet requirements of et Norske Veritas [1,2] for marine constructions. Than susceptibility to cold cracking was evaluated with the use of controlled thermal severity (CTS) and Tekken tests. Minimum preheat temperatures for avoiding cold cracking were found. The paper presents results of investigation carried out at order of Naval Academy in Gdynia, Poland. 2 xperimental procedure and results Quenched and tempered plates, 12 mm in thickness, made of steel grades and, produced by Czgstochowa Mill in Poland were examined with the use of two melts of each steel grade. Chemical composition of tested steels is presented in Table 1. Table 1. Chemical composition of tested steels Chemical composition, % C Mn Si P S Ni Cr Mo Cu B Mechanical properties of steel plates were evaluated according to et Norske Veritas requirements [1,2]. Table 2 shows transverse mechanical properties of tested plates. In order to evaluate mechanical properties of welded joints the following three most common welding methods were used: shielded metal arc welding

3 Transactions on the Built nvironment vol 11, 1995 WIT Press, ISSN Marine Technology and Transportation 305 (), submerged arc welding (), and metal active gas welding (). Two test butt welded joints 300x500 mm for each steel melt were produced under following conditions: - low-hydrogen coated electrode B 160 (ISO 515 B20) 0 4 mm, arc energy = kj/cm,. - SAB OK Autrod wire 0 5 mm with SAB OK Flux 10.71, arc energy = kJ/cm, - SpGlSN2pr wire mm, shielding gas Ar + 20 % 2, arc energy = kj/cm. Table 2. Transverse mechanical properties of tested plates T.S. MPa Tensile properties Y.P. MPa l. % RA. % Charpy-V Impact Temp. C KV T.S. - tensile strength, Y.P. - yield point, l. - elongation, R.A. - reduction of area, KV - absorbed energy J NV grade Streng Tough -th -ness Table 3 shows results of tensile and bending tests on welded joints. Table 4 presents results of Charpy-V impact test on weld metal (W.M.), fusion line (F.L.) and heat affected zone (with notch 2 and 5 mm distant from fusion line).

4 Transactions on the Built nvironment vol 11, 1995 WIT Press, ISSN Marine Technology and Transportation Table 3. Results of tensile and bending tests on welded joints Welding method T.S. MPa Bending angle * o bending radius = 3t, t- plate thickness, T.S - tensile strength NV strength group Table 4. Results of Charpy-V impact test on welded joints Welding Impact energy in J/ at temperature in C NV method WM F.L. HAZ2 HAZ5 grade 32/-40 34/-40 27/-60 28/-20 29/-60 27/-60 30/-40 39/-40 33/-40 35/-40 33/-40 36/-40 27/-40 39/-60 38/-60 34/-60 35/-40 29/-40 30/-20 38/-20 36/ /-20 71/-20 30/-20 45/-20 38/-20 38/-20 30/-20 29/-60 27/-60 48/-40 31/-60 50/-60 30/-60 29/-40 27/-40 34/-60 29/-60 41/-40 33/-40

5 Transactions on the Built nvironment vol 11, 1995 WIT Press, ISSN Marine Technology and Transportation 307 Susceptibility to cold cracking was evaluated in CTS and Tekken tests with the use of process. For each steel melt 9-12 CTS and 9-12 Tekken samples were prepared according to Polish Standard PN-90/M The Tekken samples were preheated within the temperature range of C in a chamber furnace. Initial temperature of the CTS samples ranged from -20 C to +20 C. Below an ambient temperature CTS samples were cooled and welded in the special fridge welding chamber. Table 5 presents minimum preheat temperatures for avoiding cold cracking in CTS and Tekken tests. Table 5. Minimum preheat temperatures for avoiding cold cracking in CTS and Tekken tests 14HNMBCu Minimum preheat temperature, C CTS Tekken Summary Table 2 shows that tested plates meet et Norske Veritas' requirements for strength groups and. Welded joints, made with the use of, and methods, maintain strength for particular groups (c.f. Table 3). It should be noted, however, that results of bending test are rather negative. Table 6 presents NV toughness grades for both parent metal and welded joints. It can be seen that only shielded metal arc welding of one grade melt decreased the toughness grade of the plate.

6 Transactions on the Built nvironment vol 11, 1995 WIT Press, ISSN Marine Technology and Transportation Table 6. NV toughness grades for plates and welded joints Parent Welding method metal 4 Conclusions The obtained results show that examined plates and welded joints meet et Norske Veritas' requirements for marine constructions. eveloping safe joints without cold cracks requires special welding conditions, i.e. the use of low-hydrogen process and the control of heat input and preheat temperature. High-strength low-alloy plates of and steels may be used for ships and offshore constructions. References 1. Rules for Ships. et Norske Veritas, January Rules for classification of mobile offshore units. et Norske Veritas, January 1985.