An experimental and theoretical study of heat-affected zone austenite reformation in three duplex stainless steels
- PDF / 1,004,786 Bytes
- 9 Pages / 612 x 792 pts (letter) Page_size
- 82 Downloads / 210 Views
I.
BACKGROUND
THE success of modern duplex stainless steels is to a large degree due to their weldability as well as their excellent corrosion resistance and mechanical properties. Weldability 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 ratio 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 detrimental consequences for the properties. The pitting corrosion resistance and impact energy require a minimum degree of austenite reformation in order to avoid detrimental chromium-rich nitride precipitation occurring primarily in the interior of the ferrite grains. Excessive austenite formation, 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 corrosion cracking. It must at the same time be borne in mind that an upper limit to heat input is set by the precipitation
of intermetallic phases, and this risk is increased with increased 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 reformation reaction, the latter under both isothermal and cooling conditions. Ameyama et al[8] have studied the crystallographic 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 previously[3] to simulate more closely the paraequilibrium condition believed to control the phase transformation from ferrite to austenite in the HAZ region of nitrogen alloyed duplex stainless steels. II.
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 M
Data Loading...
