METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 28A, FEBRUARY 1997—277 An Experimental and Theoretical Study of Heat-Affected Zone Austenite Reformation in Three Duplex Stainless Steels STAFFAN HERTZMAN, PAULO J. FERREIRA, and BENGT BROLUND Three duplex grades, one molybdenum-free, one 22Cr type, and one super duplex grade, have been subjected to weld simulation treatments, and the resulting microstructures have been quantified by automatic image analysis techniques. Substantial differences between the duplex grades were ob- served with an increased ability to reform austenite with increased alloying content. A theoretical model has been applied, based upon the paraequilibrium concept elaborated by Hillert, and the paraequilibrium compositions of individual phases were calculated as a function of temperature using the THERMOCALC database. A model based on Cahns theory of grain boundary nucleated reactions has also been utilized to calculate the kinetics of the reaction. By using this model, the grain size effects could be included in the treatment. The results of the calculations were compared with ex- perimental data, and the experimental results were reproduced using the same parameter set for the three materials, with the exception of the diffusion coefficient values which had to be adjusted. This adjustment has in a later study been verified experimentally. The results validate the model used and the physical relevance of using the paraequilibrium model. The appropriateness of a paraequilibrium approach is also supported by experimental evidence from weld metal compositions. It is shown that the nitrogen content of the alloys plays an important role, and a higher nitrogen content results in more efficient austenite reformation. This implies that the alloy nitrogen compositions should lie close to the upper specification limits for these materials and nitrogen losses should be avoided on welding since the material properties, both mechanical and corrosive, are strongly related to the austenite-ferrite phase ratio. I. BACKGROUND THE success of modern duplex stainless steels is to a large degree due to their weldability as well as their excel- lent corrosion resistance and mechanical properties. Weld- ability has been improved appreciably, compared to the earlier generation of duplex steels, by the introduction of nitrogen as an alloying element. The properties of duplex grades are dependent on the austenite-ferrite phase ratio, which in the base metal is designed to be approximately 1: 1. In the heat-affected zone (HAZ), however, the phase ra- tio is strongly dependent on the weld thermal cycle. High heat input will result in a good recovery of the phase ratio after high-temperature ferritization, whereas too low a heat input may lead to a limited austenite fraction with detri- mental consequences for the properties. The pitting corro- sion resistance and impact energy require a minimum de- gree of austenite reformation in order to avoid detrimental chromium-rich nitride precipitation occurring primarily in the interior of the ferrite grains. Excessive austenite for- mation, which is more likely to occur in overalloyed weld metal than the HAZ, may also be detrimental, the risk being an increased susceptibility to chloride-induced stress cor- rosion cracking. It must at the same time be borne in mind that an upper limit to heat input is set by the precipitation STAFFAN HERTZMAN, Head, and BENGT BROLUND, Group Leader, are with the Department of Stainless Steels and Casting Technology, Swedish Institute for Metals Research, S-114 28 Stockholm, Sweden. PAULO J. FERREIRA, Researcher, formerly with the Department of Stainless Steels and Casting Technology, Swedish Institute for Metals Research, is with the Department of Materials Science, Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted September 2, 1994. of intermetallic phases, and this risk is increased with in- creased alloy element level. [1,2] For all these reasons, it is important to gain an understanding of the rate-controlling mechanisms for the austenite reformation, and attempts have been made [3,4] to describe theoretically the kinetics and resulting austenite fractions, both on thermodynamic as well as morphological bases. Also, Ogawa and Kosaki, [5] Atamert and King, [6] and Mundt and Hoffmeister [7] have addressed the problem of modeling the austenite reforma- tion reaction, the latter under both isothermal and cooling conditions. Ameyama et al [8] have studied the crystallo- graphic relation between the Widmannsta ¨tten austenite and the ferrite and concluded that Widmannsta ¨tten precipitates are the result of separate nucleation events on the prior grain boundary austenite film or allotriomorph. The objective of the present work was twofold; first, to establish experimentally the austenite reformation in three duplex grades, one Mo-free, one conventional 22Cr, and one super duplex grade; and second, to analyze the results using a modified version of the method described previ- ously [3] to simulate more closely the paraequilibrium con- dition believed to control the phase transformation from ferrite to austenite in the HAZ region of nitrogen alloyed duplex stainless steels. II. EXPERIMENTAL Three duplex stainless steels of commercial grade were investigated with a chemical composition given in Table I. The 22Cr5Ni material was rolled 6-mm plate, the 23Cr4Ni was extruded tube (88.9  7.6 nm), and the 25Cr7Ni alloy was extruded bar of 15-mm diameter. Half-size Charpy V-notch blanks with dimensions 5 