Effects of B on microstructure and hardenability of resistance seam welded HSLA linepipe steel

Abstract

The effects of B on the microstructural and mechanical characteristics of resistance seam welded high strength low alloy linepipe steel were investigated at different regions, such as weld, heat affected zone (HAZ) and base. Because most of the solute B atoms were already precipitated as iron borocarbides and iron borides in as welded steels, the improved hardenability was not observed regardless of the observed regions. The amount of dissolved B precipitates increased with austenitising temperature, and the pre-existing B precipitates were fully dissolved at 950uC for 180 s. The segregation of solute B atoms into austenite grain boundaries was more marked at higher austenitising temperatures, and the segregated solute B atoms greatly increased the hardenability of the austenitised pipes, resulting in hard microstructures like bainite and martensite. The B addition also effectively compensated for the deteriorated hardness at the weld and HAZ of the tempered specimens, although B atoms were precipitated during tempering.

Elemental segregation during resistance spot welding of boron containing advanced high strength steels

“…Table 5 Weld nugget diameters and tensile peel strengths of resistance spot welds at I(max). (See Table 2 The embrittlement of welds resulting from the segregation of phosphorous to grain boundaries is reported to be reduced due to the addition of boron [13,15]. EPMA analysis indicated that with an addition of 27 ppm of boron, the phosphorous segregation at the grain boundaries in the fusion zone of Steel-CPB was reduced considerably.…”

“…The addition of boron to phosphorous containing steels is known to improve grain boundary cohesion by reducing phosphorous segregation, thereby reducing embrittlement [12,13]. Boron also improves the hardenability of steels, mainly by retarding the nucleation of polygonal ferrite at austenite grain boundaries [14][15][16]. Babu et al [14] report different weld sizes for two steels with nominally identical compositions, one containing 5 ppm boron by weight and the other 20-40 ppm boron by weight.…”

Toughness properties at multi-layer laser beam welding of high-strength steels

“…Even ferrite islands are still present in the structure. The identified nonmetallic inclusions [39] give hints that interlocking structures were formed. However, the number of inclusions might have been too small (below 10 to 26% [40]) to form acicular ferrite (AF).…”

Challenges and developments in pipeline weldability and mechanical properties

“…57,58 The other of great interest is to alter WM composition either through the use of newer filler metals or by metal powder additions in the WM. [59][60][61][62][63] Many of the basic microstructural principles that apply to the HAZ are also valid with respect to the WM. However, an additional important microstructure is acicular ferrite (AF), consisting of fine interlocking grains, which is associated with good toughness.…”