Recent Development in Controlling the Microstructure and Properties of Low Alloy Steel Weld Metals.

Abstract

(1) where, vTrs; 500/0 fracture appearance of transition temperature. weld metal and inclusions. The amount of AF increases in proportion to the carbon equivalent (CEQ) of weld metal27) as shown in Fig. 9. In other words, it is necessary to ensure appropriate hardenability to obtain good toughness. Harrison considers that supercooling is important for AF formation.11) However, whenheat becomes large, supercoolinput increases and T 800/500 ing decreases, which results in a decrease in AF and an increase in GBF as shown in Fig. 10.27) It is also reported that the AF width has a relation with toughness.28) The size of AF increases as shown in Fig. 11 Fig. 12(a).Secondary phases, such as martensite-austenite constituent (MA) and pearlite (P), increase with increasing carbon content.29) Austenite grain size decreases with carbon content (Fig. 12(b)), which seemingly promotes intergranular transformation. (1995). No. 10 ferrite with the second phase increases instead of GBF (Fig. 14).lo) However, toughness increases in proportion to nickel content regardless of the change in microstructurei0,34) ( Fig. 17(a) control the transformation temperature range by adding molybdenum and titanium at the same time so that AF is easily formed30) (Fig. 15).Ti:As is apparent in Figs. 8 (Fig. 16) boron.12,19,36,40-43) An exarnple is shown in Fig. 20.Critical oxygen content for transition from UB to AF is affected by welding method, cooling rate, and alloying elements, and it ranges widely from 150ppml9) to 200-250ppm30,36) and to 450ppm.41) A decrease in oxygen content lowers the start temperature of transformation,

On the Role of Non-metallic Inclusions in the Nucleation of Acicular Ferrite in Steels

“…The results of other authors 10,17,21,[23][24][25][26][27][28] also show the improvements of mechanical properties in weld metal and HAZ with an increased proportion of acicular ferrite in the microstructure. For instance, Garland and Kirkwood 24,25) found that additions of Mo-Nb or Mo-Ti improved the toughness due to AF formation in the welds of C-Mn steel.…”

“…Similarly, Peng et al 26) found that in submerged arc welds of X-70 line-pipe steel (0.087 % C, 1.5 % Mn and 0.014 % Ti), TiN inclusions nucleated AF which resulted in a high toughness. Even in fire resistant steels, Ohkita and Horii 10) found that AF steels are better than steels with ferrite-pearlite structures with respect to strength and toughness at high temperatures. Also, it has been found that the AF structures can result in superior stress-corrosion-cracking resistance, 29) which is needed in line-pipe steel grades.…”

“…In practice, each austenite grain after austenite-ferrite transformation includes two or more various microstructural constituents. The commonly found microstructures of ferrite in different steels are usually classified as any of the following shapes 6,10,14,15) : grain boundary allotriomorphs ferrite (GBA or GBF), intragranular polygonal ferrite (PF or IPF), Widmanstätten ferrite side plates (WF or FSP), intragranular acicular ferrite (AF or IAF), upper and lower bainite (B), martensite (M). The typical photographs of various morphology structures are shown in Fig.…”

“…The acicular ferrite arises when the low-carbon steel is cooled from the austenitic state at a rapid rate, to suppress the formation of GBA, WF and massive ferrite. [6][7][8][9][10][11][12][13] It is also a common constituent in several welded low-carbon steels. The rate of transformation is much higher than the formation of GBA or WF, but is lower than that for martensitic transformation.…”

“…This has basically been obtained by developing the acicular ferrite (AF) structures and several reviews have earlier been written on this subject. [6][7][8][9][10][11][12][13] Considerable information is now gathered on the role of individual impurity elements (S, O, and N mainly) and their interactions with alloying elements such as Mn, Al, Si, Ti, and Ce in obtaining fine microstructures. There is now considerable enthusiasm to extend these principles to the development of wrought steels with better mechanical properties through development of fine-grained AF structures.…”