The Behavior of Second Phase Particles in a Ti-Nb Microalloyed Steel during Welding Thermal Cycle

The behavior of second-phase particles in a microalloyed steel during weld thermal cycle has been investigated using analytical electron microscopy. All the particles in the base metal contains Nb and Ti, and the Nb weight fraction of the analyzed particles ranged from 0.22 to 0.85. During the heating stage of the weld thermal cycle, the small particles which were rich in Nb disappeared through complete dissolution and the large particles survived the weld thermal cycle with Nb weight fraction decreased. With a high cooling rate (t8/5 no greater than 60s), only heterogeneous re-precipitation occurred during cooling. With a low cooling rate (t8/5 = 120s), homogenous re-precipitation also occurred, producing a large number of small Nb-rich particles during cooling.

Microstructural and Mechanical Properties of the Intercritically Reheated Coarse Grained Heat Affected Zone (ICCGHAZ) of an API 5L X80 Pipeline Steel

The weld thermal cycle, depending on the welding process and steel composition can reduce the toughness of the HAZ when compared with the base metal. In the intercritically reheated coarse grained HAZ (ICCGHAZ) region, microstructural transformations from coarse austenite to bainite or martensite are liable to occur. Reheating into the dual phase field temperature and subsequent cooling can lead to the formation of “microphases” commonly referred to Martensite-Austenite (MA) constituent. Due to the C enrichment of the austenite, this region is regarded as local brittle zones (LBZ) and degradation of HAZ toughness can be attributed to the formation of local brittle zones (LBZ) at the ICCGHAZ. This work will discuss the characteristics of the ICCGHAZ of two API5LX80 steels produced by thermomechanical controlled process (TMCP) without accelerated cooling using a finishing rolling temperature in the dual phase field, where the main hardening mechanisms are grain refining and precipitation. Weld thermal cycle simulation, using a Gleeble 3800®, characterised by the peak temperature (Tp) of 800oC and the cooling time from 800 to 500oC (∆t800–500) were applied in order to obtain an ICCGHAZ equivalent to a 2.5, 3 and 4kJ/mm heat input. Charpy-V tests and metallographic analysis using optical and electron microscopy were carried out to evaluate the simulated zone. The results have shown that the ICCGHAZ presented a necklace microstructure at the prior austenite grain boundaries associated with the low impact energy and the presence of the MA microconstituent.

Creep Strength of Dissimilar Weld Using High B-9Cr Steel for A-USC Boiler

The research project aiming to commercialize 700°C class pulverized coal power system; advanced ultra-super critical (A-USC) pressure power generation has been conducted in Japan from 2008. In A-USC boilers, Ni base or Ni-Fe base alloys are used for high temperature parts at 650–700°C and advanced high Cr ferritic steels are planning to be used at the temperatures lower than 650°C. Because the high B-9Cr steel developed in National Institute for Materials Science (NIMS) has improved creep strength in weldments, it is one of the candidate materials for A-USC boilers. In the present paper, the creep tests of the dissimilar welds between high B-9Cr steels and Ni base alloys were conducted. In the heat affected zone (HAZ) of the high B-9Cr steels, fine-grained microstructures were not formed and grain size of the base metal was retained. Free boron on the grain boundaries is considered to affect the mechanisms of the α-γ transformation during weld thermal cycle. Consequently, the creep rupture lives of the dissimilar welds between high B-9Cr steels and Ni base alloys were 5–10 times longer than those of the conventional 9Cr steel welds at 650°C.