Effect of V Content and Heat Input on HAZ Softening of Deep-Sea Pipeline Steel

Abstract

In this paper, the welding thermal cycle process of deep-sea pipeline steel was investigated by welding thermal simulation. The microstructure evolution, crystallology and second-phase precipitation behavior of the soft zone of the heat-affected zone (HAZ) were characterized and analyzed by combining scanning electron microscopy, electron back-scattered diffraction, transmission electron microscopy and hardness testing. The results show that HAZ softening appeared in the fine-grained zone with a peak temperature of 900-1000 °C for deep-sea pipeline steel, the base metal microstructure of which was the polygonal ferrite and acicular ferrite. Using V microalloying and low welding heat input could effectively decrease the softening of the HAZ fine-grained region, which was achieved by reducing the effective grain size, increasing the proportion of the dislocation substructures, and precipitating the nanoscale second-phase particles.

Keywords: HAZ softening; effective grain size; heat input; second-phase precipitation; weld thermal simulation.

Figure 1

Base metal microstructure of experimental steel in thickness direction: (a–c) 1#-0.025V; (d–f) 2#-0.071V.

Figure 2

Schematic diagram of a hot welding simulation.

Figure 3

(a) Hardness distribution of HAZ in actual welded joint; (b) scanning electron micrographs (SEM) of microstructure in fine-grained HAZ.

Figure 4

Relationship between peak temperature and microhardness under 10 KJ/cm heat input of two steels.

Figure 5

SEM of different peak temperatures for 1# steel with 0.025% V under 10 KJ/cm heat input: (a) 1300 °C; (b) 1200 °C; (c) 1100 °C; (d) 1000 °C; (e) 900 °C; (f) 800 °C; (g) 700 °C; (h) 600 °C; (i) 500 °C.

Figure 6

SEM of different peak temperatures for 2#-0.071V steel under 10 KJ/cm heat input: (a) 1300 °C; (b) 1200 °C; (c) 1100 °C; (d) 1000 °C; (e) 900 °C; (f) 800 °C; (g) 700 °C; (h) 600 °C; (i) 500 °C.

Figure 7

EBSD maps of microstructure in fine-grained HAZ (950 °C) for experimental steel with different V contents under 10 KJ/cm heat input: (a) 1#—inverse pole figure; (b) 1#—grain boundary map; (c) 1#—local misorientation distribution map; (d) 2#—inverse pole figure; (e) 2#—grain boundary map; (f) 2#—local misorientation distribution map.

Figure 8

TEM maps of microstructure in fine-grained HAZ (950 °C) for experimental steel with different V contents under 10 KJ/cm heat input: (a) subgrain structures in 1# steel (b) dislocation structures in 1# steel; (c) subgrain structures in 2# steel (d) dislocation structures in 2# steel.

Figure 9

Precipitation and energy spectrum of experimental steel at fine-grained HAZ (950 °C) with different V contents under 10 KJ/cm heat input: (a–c) 1# steel with 0.025% V; (d–f) 2# steel with 0.071% V; (g–i) high-resolution photos of particle A; (j–l) high-resolution photos of particle B; (m,n) energy spectra of two types of precipitation.

Figure 10

HAZ microhardness of 2# steel with 0.071% V under different heat inputs.

Figure 11

SEM of 2# steel with 0.071% V: (a–c) peak temperature of 1200 °C under the heat input of 10 KJ/cm, 25 KJ/cm and 35 KJ/cm, respectively; (d–f) peak temperature of 950 °C under heat input of 10 KJ/cm, 25 KJ/cm and 35 KJ/cm, respectively; (g–i) peak temperature of 800 °C under heat input of 10 KJ/cm, 25 KJ/cm and 35 KJ/cm, respectively.

Figure 12

IPF of microstructure in fine-grained HAZ of 950 °C for 2# steel with 0.071% V under different heat inputs: (a) 10 KJ/cm; (b) 25 KJ/cm; (c) 35 KJ/cm. Grain boundary maps: (d) 10 KJ/cm; (e) 25 KJ/cm; (f) 35 KJ/cm. Local misorientation distribution maps: (g) 10 KJ/cm; (h) 25 KJ/cm; (i) 35 KJ/cm.

Figure 13

Second-phase precipitation and high-resolution diagrams of 2# steel with 0.071% V in fine-grained HAZ of 950 °C under different heat inputs: (a,d) 10 KJ/cm; (b,e) 25 KJ/cm; (c,f) 35 KJ/cm.