Development of High Strength Steel Weld Metals – Potential of novel high-Ni compositions

Neural network modelling has been applied to search for novel high strength steel weld metal compositions, potentially offering improved tolerance to variations in the weld thermal cycle. An experimental 7 wt% Ni manual metal arc electrode, formulated on these predictions, was used to produce all-weld metals and welds in high strength steel. Mechanical properties were correlated to weld metal microstructure and compared with those of a conventional 3 wt% Ni weld metal for different welding conditions. Light optical and transmission electron microscopy proved the 7 wt% Ni weld metal microstructure to be predominantly bainitic with some martensite. Also the 3 wt% Ni weld metals were bainitic/martensitic but with relative fractions being strongly cooling rate dependent. Similar strength and toughness were measured for both types of weld metals at intermediate cooling rates (∆t8/5≈10 s). However, the properties of the 7 wt% Ni weld metal varied significantly less with welding conditions and this composition clearly offers an advantage in terms of tolerance to variations in the weld thermal cycle. Introduction Steels with minimum yield strength in excess of 690 MPa have been welded on a limited scale, although with many precautions, after matching strength weld metals became available in the 1960s. In recent years, a greater demand has arisen for strong steels in applications such as offshore structures, cranes and pipelines. Such applications require a wider range of welding processes, offering flexibility and higher productivity, to be competitive. There is therefore a growing need for high strength steel welding consumables whilst at the same time maintaining toughness and ease of use. Reasonable insensitivity of the weld metal properties to variations in welding procedure parameters, such as heat input and interpass temperature, is therefore desirable, especially during manual welding. Current high strength steel welding consumables typically have compositions in the range 0.04-0.08C, 12Mn, 0.2-0.5Si, 1-3Ni wt% along with some additions of Cr, Mo and sometimes Cu [1-3]. As alloying content and strength increase, bainite and martensite gradually become the dominant microstructural components rather than the softer phases associated with strength levels less than 690 MPa. Although well-balanced mixed martensitic/bainitic/ferritic microstructures can offer attractive combinations of properties, the microstructure, and hence the properties, tend to become sensitive to the cooling rate [3, 4]. Greater tolerance to variations in the weld thermal cycle may require higher alloy content [3] and perhaps a radical departure from established alloying practices. Neural network modelling was therefore applied to search for promising compositional domains to investigate [5, 6]. This paper describes how the potential of a novel high strength steel weld metal with 7 wt% Ni has been explored and a comparison is made with a conventional 3 wt% Ni high strength weld metal. Microstructures were characterised and correlated to the mechanical properties of all-weld metals and welds in high strength steel for a range of welding parameters. Eurojoin 5, Vienna, 13-14 May, 2004 L. Karlsson et al. Page 2 of (8) Neural Network Modelling A neural network model was created with the task of exploring new compositions which might be suitable for high strength steel weld metals as described in [5, 6]. Figure 1 shows toughness simulations at -40°C as a function of the Mnand Ni-concentrations at 0.03 wt% C. Based on these predictions and preliminary tests, it was decided to investigate in more detail a weld metal composition of 0.06C, 7Ni, 0.5Mn wt%. Figure 1 Contour plot showing the predicted behaviour of weld impact toughness at -40°C as a function of Mnand Niconcentrations at 0.03 wt% C. Experimental Manual metal arc (MMA) welding electrodes of nominal compositions 0.06C, 2Mn, 3Ni wt% and 0.06C, 0.5Mn, 7Ni wt% were used to deposit all-weld metals in 20 mm mild steel plates according to ISO 2560. Joint faces were buttered to avoid dilution with parent material and a range of heat inputs and interpass temperatures were used to vary the cooling time between 800 ̊C and 500 ̊C (∆t8/5) (Table 1). Table 1 Interpass temperatures, nominal heat inputs and calculated cooling times (∆t8/5 calculated according to EN 1102, Annex D) 3Ni 7Ni Weld No. 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 Interpass temperature (oC) 125 150 200 200 200 125 200 250 150 100 150 200 200 200 150 Nominal heat input (kJ/mm) 1.4 1.3 1.1 1.3 1.3 1.8 1.7 1.7 0.6 1.5 1.4 1.3 1.3 1.4 2.7 ∆t8/5 (s) 7.7 7.8 8.0 9.4 9.5 10 12 19 3.6 7.6 8.4 9.4 9.5 10 25 For comparison purposes and to study effects of dilution with plate material it was also decided to produce half-V butt welds in 20 mm high strength steel with nominal heat inputs of 1.4 and 2.8 kJ/mm. Weldox 700 plate material with actual yield strength of 827 MPa, actual tensile strength of 862 MPa and a chemical composition of 0.13C, 0.31Si, 0.99Mn, 0.24Cr and 0.17Mo wt% was selected as representative of steels with a minimum specified yield strength of 700 MPa. Eurojoin 5, Vienna, 13-14 May, 2004 L. Karlsson et al. Page 3 of (8) Samples of weld metal were chemically analysed using optical emission spectrometry and Leco combustion equipment. Standard 10 by 10 mm Charpy-V impact toughness testing and tensile testing of longitudinal 10 mm diameter specimens were performed. Specimens from the weld metal cross section, perpendicular to the welding direction were mounted in bakelite for analysis with light optical microscopy (LOM). For transmission electron microscopy (TEM) studies, 3 mm disc shape specimens perpendicular to the welding direction were prepared from the last bead and from central regions reheated by subsequent weld passes. Results All-weld metals Compositional ranges of the tested all-weld metals are presented in Table 2. The chemical composition varied within a very narrow range for the 7 wt% Ni weld metals whereas larger variations were found for the 3 wt% Ni variant. These welds are henceforth referred to as 7Ni and 3Ni respectively. The larger compositional variations for the leaner weld metals are most likely caused by the consumables being from several experimental batches whereas the 7 wt% Ni electrodes were from two different batches. Table 2 Weld metal chemical compositions (wt%). Element C Si Mn Cr Ni Mo O (ppm) N (ppm) Welds 3Ni: 1-8 0.047-0.079* 0.19-0.30 2.07-2.16 0.27-0.44 2.56-3.16 0.60-0.66 280-410 90-160 Welds 7Ni: 1-7 0.059-0.061 0.32-0.34 0.54-0.56 0.14-0.15 6.60-6.84 0.35-0.39 310-350 80-160 *3Ni-4: 0.079, 3Ni-5: 0.047 The yield strength was typically around 850 MPa for both types of weld metals at ∆t8/5 of about 10 s (Table 3). However, the span between the highest and the lowest value was almost 250 MPa for the 3Ni weld metals compared to 100 MPa for the 7Ni type. Tensile strength was somewhat higher for the 3Ni than for the 7Ni weld metals at comparable cooling rates and varied somewhat more with cooling rate. Room temperature Charpy-V impact toughness was similar for both Ni-levels but varied less with cooling rate and test temperature for the more highly alloyed consumable. Table 3 Mechanical properties of experimental 3Ni and 7Ni weld metals. 3Ni 7Ni Weld No. 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 Tensile properties Rp0.2 (MPa) 978 885 922 885 844 939 858 734 882 889 871 865 848 858 789 Rm (MPa) 994 942 953 929 938 972 931 924 947 939 926 904 911 895 893 A5 (%) 18 20 19 22 20 18 19 18 16 14 18 18 18 18 22 Charpy-V impact toughness +20oC (J) 86 101 94 105 104 84 87 78 86 87 100 99 93 109 91 -40oC (J) 74 87 77 64 78 77 74 50 68 72 86 92 87 90 78 Eurojoin 5, Vienna, 13-14 May, 2004 L. Karlsson et al. Page 4 of (8) Butt welds in 20 mm plates Weld metal chemical composition and mechanical properties of butt weld in 20 mm Weldox 700 plate material are presented in Table 4. Dilution with fused parent material was most clearly seen in the higher Cand lower Ni-contents compared to all-weld metal analyses in Table 2. Strength and toughness of butt welds were on similar but slightly lower levels compared to all-weld metals for comparable cooling rates. Table 4 Effect of heat input on mechanical properties of a weld in a high strength steel produced with 7Ni experimental MMA electrodes.