Effect of ferrite transformation on the tensile and stress corrosion properties of type 316 L stainless steel weld metal
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I.
INTRODUCTION
THE use of austenitic stainless steels as prime construction materials for liquid metal-cooled fast breeder reactors often necessitates the joining of two or more components by the process of welding. Hightemperature delta ferrite is deliberately retained in the weld metal to overcome the problem of hot cracking during weld metal solidification,t~l The use of these welded components at elevated temperature would result in the time-dependent degradation of the material, particularly of the weld metal, because the delta ferrite component of the duplex weld metal transforms to carbides/carbonitrides and intermetallic phases, such as the sigma phase, small quantities of which lead to large variations in the tensilet2,3j and corrosion properties, t4-71 No two investigators have reported matching results for the transformation kinetics of delta ferrite t3,81because of a large number of variables in a weld, such as chemical composition of the weld metal, size and shape of the delta ferfite, diffusion of chromium in the ferrite, service temperature, etc. The kinetics of the ferrite transformation as well as the type and amount of the precipitated phases mainly depend on the carbon content and the concentration of elements that promote sigma formation in the weld metal. [9'1~ The tendency toward sigma phase formation can be quantitatively determined using Hull's equation, by which the steel would be susceptible to sigma phase embrittlement if the "equivalent H. SHAIKH and H.S. KHATAK are Scientific Officers, J.B. GNANAMOORTHY is the Head of the Metallurgy Division, and P. RODRIGUEZ is Director, Indira Gandhi Centre for Atomic Research, Kalpakkam, India. S.K. SESHADRI, Associate Professor, is with the Department of Metallurgical Engineering, Indian Institute of Technology, Madras, India. Manuscript submitted April 16, 1992. METALLURGICALAND MATERIALSTRANSACTIONS A
chromium number" (ECN) exceeds approximately 17.8 wt pct. t~ll The transformation kinetics of the delta ferrite and the nucleation and growth of the sigma phase tt2j are faster when stainless steels contain low concentrations of carbon. The size and shape of the delta ferritetl~ also govern the transformation kinetics of the weld metal. Kokawa et al. t131 have reported that at 973 K, sigma precipitation occurs much earlier in the vermicular ferrite than in the lacy ferrite. Microstructurally, low-temperature aging (C C>R
tr > C > R tr > C
6 is delta ferrite C is carbide/carbonitride R is R phase ~r is sigma phase
VOLUME 26A, JULY 1995--1861
2000 hours indicated that on aging for these durations of time, the following transformation reaction occurred delta ferrite--~ sigma + carbide/carbonitride
which sigma phase precipitation is facilitated; and on (2) the time taken for critical amounts of sigma forming elements to diffuse from austenite matrix to the depleted ferrite. Optical micrographs in Figure 5(a) through (e) are for as-deposited weld metal and for weld metal aged for 20, 200, and 2000 hours, respectively. Figure 5(a) indicates disco
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