Ferrite number as a function of the larson-miller parameter for austenitic stainless weld metals after creep testing

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s considered that fully austenitic stainless steel welds are susceptible to weld deposit microfissuring or hot cracking upon weld solidification.[1–5] To decrease the hot cracking or microfissuring tendency, a minimum ferrite content in the weld metal is required for its beneficial effect in dissolving harmful elements that are sensitive to form low melting substances, such as sulfur, phosphorus, and boron.[6] Therefore, electrode manufacturers as well as consumers often rely on the ferrite content in as-deposited austenitic stainless steel weld metals as a specification in order to ensure that weldments contain a desired minimum (or maximum) ferrite level.[7] In 1974, the measurement of ferrite in nominally austenitic stainless steel weld metals was standardized by the American National Standards Institute/American Welding Society (ANSI/AWS) A4.2 specification in terms of magnetically determined Ferrite Number (FN), rather than the metallographicaly determined “volume percent ferrite.” Ferrite Number has been found to be very reproducible, which is the main advantage for its use in standardization. Ferrite Number approximates the “volume percent” at levels below 8 FN. Above this level, deviation occurs, where the FN value exceeds the actual volume percent ferrite. Weld metal FN can either be predicted using constitution diagrams,[8–10] or measured using instruments including the Magne Gage, Severn Gage, and Feritscope.[11–14] Ferrite in an austenitic stainless steel weld plays a dual role. On the one hand, it reduces the susceptibility of the weld to hot cracking, and, on the other hand, it negatively affects the creep properties for long-term service at elevated temperatures. In the temperature range between 650 °C and 1000 °C, ferrite will be transformed to hard and brittle sigma phase. The leaner austenitic grades free of -ferrite are slightly susceptible to sigma formation, but the higher-alloy grades

Y. CUI, Graduate Research Assistant, and CARL D. LUNDIN, Professor, are with the Department of Materials Science and Engineering, the University of Tennessee, Knoxville, TN 37996-2200. Contact e-mail: [email protected] Manuscript submitted April 25, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A

and those containing -ferrite are more prone to its formation. Sigma is frequently described as FeCr, although its composition can be quite complex and variable, ranging from B4A to BA4. Thus, FN in austenitic materials generally causes the most alarm when the weldment properties such as strength, toughness, corrosion resistance, and long-term service at elevated temperatures are being considered. The purpose of this work was to investigate the relationship between the FN and the LMP after creep testing. The LMP is the time-temperature parameter that relates the stress and temperature to the time to failure. LMP  (TF  460)(log tr  C), where C is a constant and often near 20 for various steels and other structural engineering metals, temperature (T) is expressed in degree Fahrenheit, and the time (tr) is expressed in ho