Current state of the welding of two-phase corrosion-resistant steels

Abstract

Austenitic-ferritic steels have already been studied fairly extensively [1] as part of an effort to conserve nickel and use these steels in place of anstenitic steels with an 18% chromium content and 10-12% nickel content. Interest in these steels has risen again in recent years in connection with their use as structural materials that not only have high strength, but most importantly are resistant to corrosion and corrosion cracking -including in media with a high concentration of hydrogen sulfide. It was noted in [2] and several other studies that austenitic-ferritic steels are widely used in the chemical, petrochemical, and gas industries, power generation, and the construction of offshore oil platforms. Their range of application could be broadened even further, since efficient methods exist for the production of steels with high contents of alloying elements. This applies in particular to chromium.Although a large number of grades of austenitic-ferritic steels with different degrees of alloying (Table 1) and different physicomechanical properties (Table 2) have been developed in different countries, a common approach can be taken to solving problems related to their welding: ensure that the metal of welds and heat-affected zones (HAZs) is resistant to the formation of embrittling phases (a-phase, carbonitride phase, etc.), that the proportions of ferrite and austenite in the HAZ vary as little as possible from the analogous proportions in the base metal, and that the weld is stable against the formation of cracks and voids.The steels being examined here can be divided into four groups (with respect to the alloying system): 20-22 % Cr-5 % Ni, with stabilization by titanium and niobium; 20-22% Cr-6% Ni-2% Mo, with stabilization by titanium; 23-25% Cr-6-8% Ni-3% Mo, with alloying by nitrogen, copper, and tungsten; 28% Cr-< 10% Ni-2% Mo, with alloying by nitrogen and copper.As a rule, two-phase steels have a low content of carbon (roughly 0.02-0.03%). When carbon content is higher, the steels must be stabilized by titanium and niobium.The phase diagram of these steels permks only an approximate evaluation of their phase composition as a function of the concentration of ferrite-forming (Creqv) and austenite-forming (Nieqv) elements. Ref'med phase diagrams of the weld metal can be used for relatively thin welds. When weld thickness increases to more than 10-12 mm and when welding is done in multiple passes, secondary heating begins to have a significant effect on phase redistribution both in the weld and in the HAZ.These changes in phase composition are due to the extremely low structural stability of austenite and ferrite. Martensitic transformations can occur in austenite, while the a-phase (FeCr), the a-phase (Fe36Cr12Mo), or secondary austenite can form in ferrite [3]. Thus, special attention should be given to problems connected with attaining a relatively stable phase composition. Figure 1 shows the microstructure of the metal of an HAZ and a weld of several two-phase steels differing in the contents of the...