Weldability of a 2205 duplex stainless steel using plasma arc welding
Introduction
Use of duplex stainless steels is one the increase thanks to their combination of excellent anti-corrosion properties with good mechanical behaviour, especially in temperature-sensitive components such as heat exchangers and chemical reactors used in the chemical and petrochemical industries [1], [2], [3]. Good mechanical properties (high strength combined with high toughness) are associated with the presence of a duplex structure with a good balance in the proportion of austenite/ferrite, which is usually 50/50.
Given this increased use of duplex stainless steels, we need to gain a better understanding of those metallurgical factors that influence weldability. Conventional fusion welding processes required for construction assembly have a considerable impact on duplex structure, both in the fusion zone (FZ) and in the heat affected zone (HAZ). It is well known that impact toughness of the welds in duplex stainless steels decreases with the increase of δ-ferrite in the HAZ [4], since the local duplex structure is severely ferritized by the high peak temperature and by the fast cooling rate of the thermal cycle. Another problem associated with fusion welding of these materials is their susceptibility to solidification cracking, which is greater than that of the 304 L austenitic stainless steels [5]. The precipitation of undesirable phases such as intermetallic compounds, carbides and nitrides can cause a drastic deterioration in toughness and corrosion resistance, for instance in the case of σ phase, which has very fast formation kinetics [6].
Therefore it is necessary to assure the continuity of duplex structure properties across the weld by controlling the phase balance both in the FZ and in the HAZ. For practical application of this kind of welded joints, an adequate proportion of ferrite in the FZ would be in the range of 30–70%. This ferrite content depends on the chemical composition of the FZ and the cooling rates of the weld, which are related to the input energy applied during welding [7], [8]. For this reason, the present research is intended to determine the optimal welding conditions for autogenous welding (without filler) of duplex stainless steels while controlling the input energy.
The welding technique used in this research is plasma arc welding (PAW), in which the electric arc generated between a non-consumable tungsten electrode and the working piece is constrained using a copper nozzle with a small opening at the tip. By forcing the plasma gas and arc through a constricted orifice, the torch delivers a high concentration of energy to a small area, giving higher welding speeds and producing welds with high penetration/width ratios, thus limiting the HAZ dimensions. For these reasons PAW is a very useful technique for welding austenitic steels and can also be applied to duplex stainless steels [9], [10], [11].
In addition to the melt-in welding mode, which is usually adopted in conventional welding processes (such as gas tungsten arc welding-GTAW), the keyholing mode can also be used in PAW in certain ranges of metal thickness (e.g. 2.5–6 mm). With a proper combination of orifice gas flow, travel speed and welding current, keyhole forming is possible, allowing higher welding speeds than GTAW with full penetrations. The present work addresses the use of both the melt-in or conduction mode and the keyhole mode of welding, for each of which it defines the minimum net energy input needed to achieve proper operation and metallurgical weldability in a 3 mm thick 2205 duplex stainless steel sheet.
Section snippets
Experimental procedure
Parent material used in this research was a 3 mm thick rolled sheet of 2005 commercial duplex stainless steel. Its chemical composition in weight percentages is given in Table 1. Prior to welding, sheets were solution annealed at 1000 °C for 30 min and quenched to achieve a homogenized microstructure (as indicated by ASTM A923-A) and eliminate any intermetallic phases [12].
Autogenous butt joints were made by transferred plasma arc welding (TPAW). Standard coupons were obtained by welding 25 mm wide ×
Influence of the net input energy on operative weldability
The influence of the net input energy (Hnet), defined as the proportion of heat input per unit of length reaching the workpiece, on the penetration, shape and size of the welds was evaluated as indicated in Eq. (1). To that end the values of Hnet were calculated considering the plasma welding conditions – I, E and υ together with the values of energy transfer efficiencies (η) – for both groups of welds (melt-in and keyhole). The values of ηmelt-in and ηkeyhole used were typical of plasma arc
Conclusions
- (1)
Good operative weldability by PAW in 3 mm thick sheet of a 2205 duplex stainless steel is achieved by welding with a net input energy in the range 2500–3200 J/cm, if the keyhole mode is used.
- (2)
Welds produced by keyhole PAW have higher penetration/width ratios than welds produced in the melt-in mode.
- (3)
Welds produced in duplex stainless steels under high energy conditions in conduction mode are characterized by an increase of ferrite contents inside the fusion pools to over 45% more than in the parent
Acknowledgements
The authors wish to thank the Spanish Ministerio de Educación y Ciencia for the financial support given to the present research (MAT 2001-1123-C03-03). Also are grateful with Mr. Gilberto del Rosario from Centro de Apoyo Tecnológico (CAT-URJC) for his contribution to the quantitative X-ray diffraction study of parent sheets.
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