Applications of duplex stainless steels are normally restricted to temperatures between room temperature and 275°C. Their sensitivity to embrittlement due to the formation of intermatellic phases, such as sigma, chi, and alpha prime restricts high temperature use, while the sensitivity of ferrite to cleavage fracture hinders possible cryogenic service.
Norberg [106] examined the applicability of duplex stainless steel SAF2205 above 300°C.
Charpy V-notch impact tests were performed on quench-annealed as well as cold-worked material after aging at 300 and 325°C for up to 30,000 hours. It is well recognised that duplex stainless steels exhibit a general tendency to embrittlement in the temperature range from 300 to 950°C. It was shown that the influence of aging on the impact energy was very small even after 30,000 hours aging at 300°C. However, the effect became pronounced after aging at 325°C, whereby the impact strength was 27 J at -100°C after aging for 30,000 hours. It was also shown that cold work reduced the upper-shelf energy to between one half and one third of the quench-annealed material. The Upper-shelf energy is described as the limit to which brittle to ductile behaviour is exhibited for a specific alloy. Norberg concluded that SAF 2205 can be used in the quench-annealed condition at 310oC for 20 years without the Charpy V-notch impact energy falling below 27 J at room temperature.
Unnikrishnan and Mallik [107] studied phase relationships associated with cold rolling and annealing in a U50 duplex stainless steel. Cold rolling and subsequent annealing in the temperature range from 800 to 1240°C resulted in a sequence of transformations. Annealing at temperatures between 800 and 975°C led to the formation of the sigma phase and at 1,000°C, recrystallisation was observed. Annealing at higher temperatures did not give rise to the sigma phase formation. The mechanical properties of the material treated up to 1,000°C are controlled by the presence of the sigma phase and the lack of recrystallisation.
Above 1,000°C, the effect of microstructure on the mechanical properties is dominated by the amount of ferrite and the associated microstructure and banding. In general, the tensile strength, yield strength and hardness decreased with increasing annealing temperature.
Solid solution hardening effects may also arise from the presence of substitutional elements due to composition balancing for suitable ferrite/austenite ratio. Table 2.5 shows the tensile properties and Charpy impact toughness values for Ferralium 255 and SAF 2205 duplex stainless steels.
Although tensile properties of duplex stainless steels are not anisotropic, impact properties show significant directionality. The impact toughness of duplex stainless steels is enhanced by the crack arresting feature of the tough austenite. In addition the introduction of nickel into ferrite improves toughness. Table 2.5 also shows Charpy impact energy values at room temperature.
Table 2.5: Mechanical properties of typical duplex stainless steels
Phase transformations in the high temperature region of the heat affected zones and the fusion zones not only influence toughness, but also affect the ductile to brittle transition temperature. Gooch [108] has stated that the heat affected zones will need to fulfil impact energy minima of about 27 J at 0oC and above, provided that an adequate austenite level is maintained.
The fusion zones toughness and ductile to brittleness transition temperature are of principal concern, which have been shown to be related to the ferrite/austenite balance. This
specimens. Toughness in the fusion zones of autogenous welds may be further reduced over that of the heat affected zones by the production of a coarse, columnar-shaped ferrite grain morphology. Flasche [110] examined the Charpy impact energy of Ferralium 255 gas tungsten arc weld metals produced with filler wire of matching composition, and reported a weld metal toughness of 78 J at room temperature. Testing at -20oC, the weld metal showed a toughness of only 16 J due to the highly ferritic nature of the fusion zone. Liljas and Qvarfort [111] investigated the effect of nitrogen on weldment properties in SAF 2205. They noted that both gas tungsten arc welding and pulsed tungsten arc welding processes gave high impact strengths in the order of 113 J in the as-welded condition, whereas post-weld heat treatment increased the impact energy to 227 J at -40°C. Hertzman et al. [112] reported a Charpy impact energy of 49 J at -20°C for weld metal produced by autogenous gas tungsten arc welding on SAF 2205. They stated that the weld metal showed a slightly lower impact energy level than the fusion boundary and heat affected zone region. Yasuda et al. [113]
obtained an impact energy of 50 J at -40°C in the ‘as welded’ condition for submerged arc welded SAF 2205, with impact energy values of about 100 J at 0°C.
Lundqvist et al. [114] reported that for SAF 2205 weld metals produced by gas tungsten arc welding with and without filler wire additions. Specimens in welded joints were taken both in the weld metal and at the fusion line. Fusion line specimens had the notch located with one half in the weld metal and one half in the heat affected zone. It was reported that gas tungsten arc welding welds with slightly lower ferrite content than the base metal showed slightly higher impact energies than the base metal. An increased ferrite content resulted in reduced impact toughness. Shielded metal arc welding and submerged arc welding weld metals induced the lowest impact energies in welded duplex stainless steels. They indicated that ferrite content in the weld metal from covered electrodes was lower than for the wire electrodes. Hoffmeister et al. [115] investigated the effect of weld metal composition and welding conditions on ferrite and Charpy V notch toughness of a 26Cr-5Ni-1.5Mo duplex stainless steel. Welding processes employed in this study were shielded metal arc welding, gas metal arc welding and submerged arc welding with filler wires of matching composition (generally containing high nickel content). They examined the impact toughness of the alloy at -40°C. Results showed increasing ferrite content in the weld metal considerably lowered the Charpy V notch toughness at values. A Charpy V notch value of about 25 J at -40°C in the half size test piece was achieved. This seems to indicate sufficient high toughness as compared to weld joint requirements of 40 J at room
temperature for full size specimens fabricated from SAF 2205 alloy. A typical time temperature transformation (TTT) diagram for SAF2205, is shown in Figure 2.16. Here, the curve corresponds to 27 J impact toughness indicating the rate of embrittlement at the different temperature regions [115].
Figure 2.16: Time temperature transformation (TTT) diagram for SAF2205, the curve corresponds to 27 J impact toughness indicating the rate of embrittlement at different temperature regions [115]