Instrumentiranim Šarpijevim klatno je ispitana udarna žilavost metala šava dva valjana mikrolegirana čelika, zavarena elektrolučno u zaštiti mešavine gasova. Prethodno određena optimalna mešavina gasova (Ar+5%CO2+0.9%O2) je korišćena sa različitim vrednostima unete energije da bi se odredio njen uticaj na žilavost metala šava na različitim temperaturama. Optimalne vrednosti unete energije su određene za oba čelika, koje su davale najveću energiju rasta prsline u prisustvu acikularnog ferita, kao dominantne mikrostrukture.Impact toughness of weld metals of two hot rolled microalloyed steels, welded by gas shielded metal arc process, has been evaluated by using the instrumented Charpy pendulum. Previously determined optimum gas mixture (Ar+5%CO2+0.9%O2) was used with different energy inputs to determine its effect on weld metal toughness at different testing temperatures. For both steels the optimum energy input has been determined, providing maximum crack propagation energy due to presence of acicular ferrite, as a dominant microstructure

Milosavljević, Anđelka

Prokić-Cvetković, Radica

Sedmak, Aleksandar

FME Transaction

English

Machinery - Repository of the Faculty of Mechanical Engineering, University of Belgrade

    Radica Proki}-Cvetkovi} Assistant Professor Andjelka Milosavljevi}   Professor Aleksandar Sedmak  Professor   University of Belgrade Faculty of Mechanical Engineering    The Effect of Input Energy on Toughness of Weld Metal Made by Gas Metal Arc Welding of Microalloyed Steel    Impact toughness of weld metals of two hot rolled microalloyed steels, welded by gas shielded metal arc process, has been evaluated by using the instrumented Charpy pendulum. Previously determined optimum gas mixture (Ar+5%CO2+0.9%O2) was used with different energy inputs to determine its effect on weld metal toughness at different testing temperatures. For both steels the optimum energy input has been determined, providing maximum crack propagation energy due to presence of acicular ferrite, as a dominant microstructure.  Keywords: microalloyed steels, energy input, total impact energy, crack initiation energy, crack propagation energy    1. INTRODUCTION Welding by metal arc process with mixture of shielded gases has been increasingly popular in recent years. For welding of microalloyed steels mixtures of argon (Ar), carbon-dioxide (CO2), and/or oxygen (O2) are often used. A compound of gas mixture significantly affects weldment properties, especially weld metal toughness. Previously determined optimum gas mixture (Ar + 5% CO2 + 0.9% O2) [1] is used with different energy inputs, to determine its effect on weld metal toughness at diffe-rent testing temperatures. The energy input is generally very important parameter which affects significantly mechanical properties of a weldment, and specifically for microalloyed steels which are well known for their sensitivity to energy input level. The property most affected by energy input is the toughness and thus the investigation will be focussed on it.  2. EXPERIMENTAL PROCEDURE Two microalloyed steels (hot rolled plates) were used for welding, one alloyed with Nb (denoted as N steel) of thickness 11 mm, and the other one alloyed with Nb, V and Ti (denoted by T steel) of thickness 7.2 mm. The composition and properties for both steels and filler material are given in tab. 1 and 2, respectively. The filler material was commercially available wire VAC 60 Ni (produced by ′Jesenice′, Slovenia), ∅1.2 . 1 and 2, respectively. Preheating has not been applied, since the C equivalent was CE = 0.2 (steel N), and CE = 0.34 (steel T), [1]. In the previously performed investi-gation the optimum gas mixture (Ar + 5% CO2 + 0.9% O2) was determined  [1]  and used for welding by the gas metal arc process. The energy input is calculated by:     Q = 60U Iw η⋅10-3  ,                        (1)  where Q is the energy input [kJ/cm], U arc voltage [V], I current intensity [A], w welding speed [cm/min], and η effective coefficient, taken as 0,70 for MIG/MAG  [2]. The input energy was 5, 7 and 12 kJ/cm for steel N, and 4, 7 and 10 kJ/cm for steel T. For both steels the coupon plates with V grooves were welded and used for testing, Fig. 1. The specimens for weld metal toughness testing were cut out, with dimen-sions in accordance with standard ASTM E23-89, which allows reduced thickness B: 55×10×9 mm for steel N, 55×10×6 mm for steel T, Fig. 2. The standard 2 mm deep V notch was machined along 10 mm dimension, Fig. 2. Since the instrumented Charpy testing has not been stan-dardized yet, experimentally verified recommendations, prescribed by ESIS [3], have be used instead. Testing has been done at the room temperature, -40°C and -55°C.  Figure 1. ″V″ groove.  mm, with composition and properties shown in TabReceived: May 2002, revised: September 2003, accepted: October 2003. Correspondence to: Radica Prokić-Cvetković,  Faculty of Mechanical Engineering, University of Belgrade,27. Marta 80, 11120 Belgrade, SCG E-mail: rprokic@mas.bg.ac.yu © Faculty of Mechanical Engineering, Belgrade. All rights reserved. FME Transactions (2003) 31, 87-90        87    88  ▪  Vol. 31, No 2, 2003        FME Transactions       Figure 2. Charpy specimens with "V" notch in weld metal.  During the instrumented Charpy testing the following diagrams can be obtained: force (F) - time (τ); energy (E) - time (τ); force (F) - displacement (Df). These diagrams provide additional informations about material behaviour and failure mechanism. Typical diagram force (F) - displacement (Df) obtained by the instrumented Charpy pendulum is show in Fig. 3, while the schematic procedure of energy separation is shown in Fig 4.   Figure 3. Typical diagram force-displacement obtained on instrumented Charpy pendulum [3].   Figure 4. Schematic separation of total impact energy into crack initation and propagation energies  The impact toughness testing has been significantly im-proved after introduction of instrumented Charpy pen-dulum, i.e. after an application of oscilloscope, what enabled separation of total impact energy into crack ini-tiation and crack propagation energy:      Eu = Einc + Elom ,                                         (2) where Eu stands for the total impact energy, Einc for the crack initiation energy, and Elom for  the crack propaga-tion energy. Even if two materials exhibit the same toughness value, i.e. the total impact energy, their beha-vior can be different from point of view of crack initia-tion and propagation. For example, although the total impact energy overcomes the critical value, the initia-tion energy may be dominant, leaving only small contri-bution for propagation energy. In that case, the value of total energy itself is not enough to guarantee avoidance of catastrophic fracture  [3]. Therefore, if crack already exists, what can not be precluded when considering wel-ded joints  [4], the critical toughness value should take into account only crack propagation energy. One should notice different literature data on the critical toughness value, from 28 J  [5-7], to 35 J  [2] and 40 J  [8], specifi-cally for microalloyed steels.  3. RESULTS AND DISCUSSION  Results of toughness testing at different temperatures are shown in Fig. 5-7. The energies obtained are scaled to the standard thickness of 10 mm.   Figure 5. Impact energy vs. temperature.  Figure 6. Impact energy vs. temperature.  FME Transactions     Vol. 31, No 2, 2003 ▪  89     Figure 7. Impact energy vs. temperature.  From these results one can see that the total impact energy, Eu, as well as the crack initiation, Einc, and crack propagation energies, Elom, are higher for steel T in all cases. With reduced temperature both Eu and Elom significantly reduce, while the effect on Einc is much smaller. The reduction of Elom is more expressed for steel N than for steel T. At room temperature, Eu is the highest for weld metal made with energy input 7 kJ/cm (Fig. 5), being 174 J for steel N and 208 J for steel T. Both increase and decrease of energy input reduces the impact energy, Fig. 6 and 7. The initiation energy Einc for both steel is almost the same and does not show any significant influence of the energy input, while the propogation energy Elom depends on the energy input in the way similar to total energy, Eu. The propogation energy is higher than the initiation energy in all cases, except for steel N welded by energy input 12 kJ/cm, when it is slightly smaller. The total impact energy Eu, at –40 °C, has also the high-est values for welde metals made with the enrgy input 7 kJ/cm, being 97 J for steel N and 125 J for steel T. To some extent the initiation energy follows the behaviour of total energy, whereas the propagation energy is com-pletely similar. Contrary to room temperature testing the initiation energy is higher than the propagation energy for all three energy input levels in the case of steel N, whereas the oposite holds for steel T. With further temperature reduction all enbergy values reduce. So, at –55 °C, the total energy is 70 J for steel N, and 93 J for steel T. In all cases Einc is higher than Elom for both steels. In order to explain the obained results a microstructural investigation has been performed, as shown in more de-tails in  [1]. Here only one characteristic micrograph is show, Fig. 8, indicating clearly the acicular ferrite as a dominant microstructure in the weld metals obtained by the optimum energy input. The beneficial effect of aci-cular ferrite has been known for some time,  [4-8], and the basic aim of  [1] was to investigate this effect and to establish the optimum gas mixture and energy input for typical microalloyed steels. The investigation presented here has proved the beneficial role of acicular ferrite.   Figure 8. Typical weld metal microstructure with dominant acicular ferrite 4. CONCLUSIONS Based on the analysis of experimental results, the fol-lowing conclusions can be derived: − The impact energy for both steels depends signifi-cantly on the energy input. One can recommend 7 kJ/cm as the optimal energy input for both steels. Both smaller and higher energy inputs are detrimen-tal for toughness. The effect is more pronounced with higher enrgy input, e.g. crack propagation ener-gy for steel N, welded with 12 kJ/cm, is smaller than crack initiation energy already at room temperature. − Steel N has sufficient toughness at -40 °C only with the optimum energy input and in any case is not re-commended at –55 °C. Steel T has significantly bet-ter toughness, since its toughness is sufficient at –40 °C in all cases and even at –55 °C it can be used with the optimum energy input − Microstructural examinations have shown the the amount of acicular ferrrite in weld metal is propor-tional with its toughness.  REFERENCES  [1] Prokić-Cvetković R., PhD Thesis, Faculty of Mechanical Engineering, , Belgrade (2000). [2] Hrivnak I., Theory Welding Metal and Alloys, Published by Slovak Academy of Science, Bratislava (1989). [3] Proposed Standard for the Instrumented Charpy V Impact Test on Metallic Materials, ESIS (1994). [4] Thaulow C., at al., IIW Doc.X-119-86 (1986) 1-50. [5] Evans G. M., The Effect of Nickel on the Micro-structure and Properties of C-Mn All-Weld Metal Deposits, IIW Doc. II-A-791-89. [6] Evans G.M., The Effect Chromium on the Micro-structure and Properties of C-Mn All-Weld Metal Deposits, IIW Doc. II-A-738-88. [7] Evans G.M., The Effect of Molybdenum on the Microstructure and Properties of C-Mn All-Weld Metal Deposits, IIW Doc. II-A-666-86.  90  ▪  Vol. 31, No 2, 2003        FME Transactions    [8] Onsoen M.I., Liu S. and Olson D.L., Welding Journal (1996) 216. [9] Gerić K., Sedmak S., Welding and Welded Structure, Vol. 42, (1997) 31.    UTICAJ UNETE ENERGIJE NA @ILAVOST METALA [AVA DOBIJENOG ELEKTROLU^NIM ZAVARIVANJEM MIKROLEGIRANIH ^ELIKA U ZA[TITI ME[AVINE GASOVA  R. Proki}-Cvetkovi}, A. Milosavqevi},  A. Sedmak  Instrumentiranim arpijevim klatno je ispitana udarna жilavost metala шava dva vaљana mikrolegirana чelika, zavarena elektroluчno u zaшtiti meшavine gasova. Prethodno odreђena optimalna meшavina gasova (Ar+5%CO2+0.9%O2) je koriшћena sa razliчitim vrednostima unete energije da bi se odredio њen uticaj na жilavost metala шava na razliчitim temperaturama. Optimalne vrednosti unete energije su odreђene za oba чelika, koje su davale najveћu energiju rasta prsline u prisustvu acikularnog ferita, kao dominantne mikrostrukture.    Table 1. Composition of the base and filler metals. elements, mass % Steel C Si Mn P S Cu Al Nb Ti Cr Ni V N 0.07 0.15 0.66 0.016 0.010 0.13 0.092 0.077 - 0.042 0.036 - T 0.056 0.32 1.28 0.012 0.005 0.031 0.049 0.045 0.02 - - 0.054 VAC 60 Ni 0.08-0.1 - 1.4-1.6 P+S<0.025 - - - - - 1-1.2 -  Table 2. Mechanical properties of the base and filler metals. Re, [N/mm2] Rm, [N/mm2] A5, [%] KV(-20°C), [J] Steel N 448-456 543-551 33-34 129-156 Steel T 510-537 571-595 37-42 152-197 VAC 60 Ni 440-510 560-630 22-30 80-125; 30-35 (-400C) Re, Rm, A5 – measured along the rolling direction,  KV – measured normal to the rolling direction      

The effect of input energy on toughness of weld metal made by gas metal arc welding of microalloyed steel

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The effect of input energy on toughness of weld metal made by gas metal arc welding of microalloyed steel

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