The numerically computed values of the peak

The numerically computed values of the peak temperature and the analytically estimated values of the maximum shear stress, τmax, can be used further to provide an assessment of the longevity of the FSW tools with polygonal pin shapes. A tool durability index is therefore presumed here as the ratio of the shear yield strength of the tool pin material at the computed peak temperature and τmax experienced by the corresponding pin. Thus a tool pin with a higher value of durability index is expected to be lesser susceptible to premature failure during actual FSW operation. For example, the peak temperature and τmax experienced by the triangular pin are estimated as 662 K and 581.76 MPa, respectively, for the welding conditions considered here. The corresponding durability index can therefore be estimated as (615.0/581.76) where 615 MPa is the shear yield strength of tool pin material at 662 K [25]. The estimated values of τmax experienced by the square, pentagon and hexagonal pin profiles are 311.99 MPa, 226.70 MPa and 197.31 MPa, respectively, with the corresponding peak temperatures of 672 K, 676 K and 680 K. The tool durability indices are therefore estimated as 1.96 (611/311.99), 2.69 (609/226.70) and 3.08 (607/197.31), respectively, for the square, pentagon and hexagon pin profiles. Thus the hexagon pin profile depicts the maximum durability index and would be least susceptible to premature fracture for the welding conditions considered here [29]. The estimated values and the general trend of the tool durability indices provide a first step towards the fail-safe design of FSW tool following mechanics-based principle, which is currently absent. However, the further studies need to also consider the possible vibration of tool during actual FSW operation for the estimation of a more practical tool durability index.
Conclusions
Introduction Austenitic stainless steels are generally used where excellent corrosion resistance and good formability are required. The development of austenitic stainless steel with improved properties was initiated in 1960s and became widespread in the 1980s [1]. In general austenitic stainless steels contain nickel as an alloying angiotensin ii receptor blockers to stabilize the austenitic phase and provide corrosion resistance to some extent [2]. Earlier improvements were related to the increase in chromium, molybdenum and nickel contents [3]. Recently much interest has been expressed in raising the level of dissolved nitrogen in the steel. The later development of the so-called high nitrogen austenitic stainless steel (HNS) with nitrogen levels sometimes exceeding 0.5 wt% has resulted in austenitic stainless steel with an exceptional combination of strength, toughness and corrosion resistance. Nitrogen is one of the alloying elements which may be used to replace the Ni addition and has the additional benefits to increase the pitting corrosion resistance and enhance the strength levels of the steel. In fabricating the structural non magnetic material, welding is one of the most commonly used technique for joining high nitrogen austenitic stainless steels. During welding, it is essential to avoid nitrogen losses which could result in loss of mechanical properties and corrosion resistance. In order to reduce the risk of nitrogen induced porosity, the solubility of nitrogen in the weld metal has to be high enough to accommodate any increase in nitrogen concentration. The defects like porosity and solidification cracking can be overcome by the use of suitable filler wire which produces required amount of delta ferrite in the weld metal. Due to the high nitrogen content, welding requires special care to ensure that the nitrogen remains in the metal during welding [4]. Depending on service requirement, delta-ferrite content in stainless steel welds is often specified to ensure that weld metal contains a desired minimum and/or maximum ferrite level [5]. Nitrogen diffusion into the weld metal from the base metal (adjacent to the fusion line) at the elevated temperatures encountered during the weld thermal cycle could also play a role. If the nitrogen level exceeds the limit of solubility at any time during or prior to solidification in welding, the nitrogen bubbles can form in the liquid, thereby increasing the likelihood for nitrogen induced porosity [6]. In order to reduce the risk of nitrogen-induced porosity, the solubility of nitrogen in the weld metal has to be high enough to accommodate any increase in nitrogen concentration. As chromium and manganese are known to increase the solubility limit of nitrogen in austenitic stainless steel, the high levels of these elements are desired in the weld metal when filler wires for welding are selected. Another problem existing in welding a highly alloyed austenitic stainless steel is hot cracking. As a measure to minimize the hot cracking risk, one needs to choose a filler material with low impurity levels (e.g. S, P) in addition to focus on the least degree of segregation of the major alloying elements and the minimization of the level of intermetallic phase in the weld metal [7]. Nitrogen alloying can also play an important role in retarding the precipitation of intermetallic compounds [8], raising the ferrite/austenite transformation temperature and assisting the formation of austenite phase in heat affected zone of a weldment No matching filler wire is commercially available similar to the composition of the base metal (HNS). In the present work authors made an attempt to study the shielded metal arc welds of high nitrogen austenitic stainless steel using a nearest matching electrode of Cr-Mn-N type as it is presently available. Most of the researchers discussed the pitting corrosion resistance of this type of alloy, but the studies related to the welds are scarce. In view of the above, authors made an attempt to investigate the microstructural changes on pitting corrosion behaviour of weld metals of the high nitrogen stainless steel arc welds in 3.5% NaCl solution and to compare with that of the base metal, namely high nitrogen stainless steel (HNS).