The Influence of Thermal Cycles on the Microstructure of Grade 92 Steel
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TRODUCTION
ENGINEERING steels that operate under high-temperature creep conditions typically contain alloying elements that, under controlled heat treatment conditions, lead to the formation of precipitates. During the development of new steels, heat treatment trials are carried out to define the temperature ranges for the normalizing and tempering processes, which will result in an appropriate type, size, and distribution of precipitates in order to optimize the mechanical properties. However, after the initial fabrication of the parent material, in most pressure boundary applications of these steels, fusion welding is also required. These fusion welding processes introduce thermal cycles in the base metal, with the region subject to these influences typically termed the ‘‘heat-affected zone’’ (HAZ). The specific thermal history of these processes modifies the
X. XU, G.D. WEST, and R.C. THOMSON are with the Department of Materials, Loughborough University, Loughborough, LE11 3TU, UK. Contact email: [email protected] J.A. SIEFERT and J.D. PARKER are with EPRI, Charlotte, NC 28262. Manuscript submitted October 28, 2016.
METALLURGICAL AND MATERIALS TRANSACTIONS A
microstructure within the HAZ and, thus, results in a variation in the corresponding mechanical properties. Experience has demonstrated that the modification of properties includes changes to creep strength, ductility and fracture resistance. In extreme cases, these changes in properties can lead to failure. For example, cracking in 0.5Cr0.5Mo0.25V low alloy steel welds was found to occur during the postweld heat treatment and in the HAZ region adjacent to the fusion line.[1] Research was carried out in order to determine specific thermal cycles and the resulting changes in the microstructure and properties of the HAZ and the magnitude and distribution of residual stresses in such low alloy steels. It was established that reheat cracking was the direct result of relaxation of residual stresses in microstructures with low creep ductility. The low ductility in the region immediately adjacent to the fusion line was a consequence of both the high-temperature cycles, which resulted in a large prior austenite grain size (PAGS) (>100 lm), and compositions that had relatively high levels of carbon and vanadium, which increased the high-temperature strength. One of the outcomes of this early research was that a generalized approach to describe HAZ microstructures was published for welds made from the low alloy ferritic and bainitic steels.[1,2] These descriptions have continued
to be used for steels that are within the family of 9 to 12 wt pct Cr, tempered martensitic steels. The classical description for the HAZ that is commonly applied to most steel grades considers that it is divided into four regions[3]: (1)
(2)
(3)
(4)
the coarse-grained HAZ region, in which the original matrix was fully reaustenitized with a relatively large prior austenite grain (PAG) size and complete dissolution of the pre-existing secondary precipitate particles with a peak temperatu
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