Correlation of microstructure and fracture properties in weld heat- affected zones of thermomechanically controlled proc
- PDF / 5,807,524 Bytes
- 14 Pages / 597 x 774 pts Page_size
- 15 Downloads / 207 Views
I.
INTRODUCTION
THERMOMECHANICALLY controlled processed (TMCP) steels have been developed in recent years to offer several inherent advantages over conventional normalized high-strength low-alloy (HSLA) steels. These steels show fine microstructures and excellent strength and fracture toughness even at low temperatures, thereby providing the opportunity to design offshore structures and vessels operating at such low temperatures. These advantages offered by TMCP steels, however, must be reconsidered carefully after a multipass arc welding procedure because of the possibility of very low toughness in the heat-affected zones (HAZs) of the welded joints.[l,2,3l In general, metallographic analyses of the HAZ microstructures reveal that there are four characteristic regions, i.e., a coarse-grained region, a fine-grained region, an intercritical region, and a subcritical region, determined by the peak temperature that the region was exposed to during the weld thermal cycle, t4,5,61 Among these regions, the coarse-grained region is prone to embrittlement and is thus called the local brittle zone (LBZ). The LBZ causes a serious fracture problem since it contains unfavorable microstructural features, such as coarse prior austenite grains, martensite islands, upper bainite, and microalloy precipitates, t5-131 The martensite islands govern the fracture toughness of the coarse-grained HAZ and cause a sudden drop of toughness because of the inherent brittle nature and crack susceptibility of martensite.t 10-13] SUNGHAK LEE, Associate Professor, and BYUNG CHUN KIM, Research Assistant, are with the Department of Materials Science and Engineering, Pohang Institute of Science and Technology, Pohang 790600, Korea, and jointly affiliated with the Iron and Steel Division of the Research Institute of Industrial Science and Technology. DONGIL KWON, Assistant Professor, is with the Department of Materials Science and Engineering, Changwon National University, Changwon 641-773, Korea. Manuscript submitted December 13, 1991. METALLURGICAL TRANSACTIONS A
It is also found that carbon equivalent (Ceq) and microalloying elements such as vanadium (V) and niobium (Nb) seriously affect fracture properties of the coarsegrained HAZ. [14,15,16]Vanadium and niobium are very often added to improve the strength and low-temperature toughness of HSLA steels by considerably reducing the carbon equivalent, particularly in TMCP steels microalloyed with elements which inhibit austenite recrystallization and grain growth. Typical TMCP schedules incorporating accelerated cooling include several important features. The steels are controlled rolled with a finishing temperature slightly above Ar3 to produce a fine deformed austenite microstructure, and the rolled plates are then rapidly cooled by water spray or mist. The final product has a fine ferrite-bainite microstructure, and the proportions of the two phases depend on the composition and processing conditions. However, under welding conditions, mechanical properties of the TMCP steels can be altered sign
Data Loading...
