Effect of Laser Welding Process Parameters on Microstructure and Mechanical Properties on Butt Joint of New Hot-Rolled Nano-scale Precipitation-Strengthened Steel

doi:10.1007/s40195-014-0081-z

Abstract

Abstract:

A 4 kW fiber laser was chosen to weld the new hot-rolled nano-scale precipitation-strengthened steel with a thickness of 4.5 mm. The effect of laser power, defocusing distance, and welding speed on the welded joint appearance was examined, and the microstructure and mechanical properties on the typical butt joints were investigated. Results showed that increasing laser welding power may cause faster downward flow of molten metal to produce greater root humping. With the welding speed increasing, the average welding seam (WS) width decreased, and the average WS and heat-affected zone (HAZ) hardness increased. The microstructures of WS, fusion line, and coarse grain heat-affected zone were lath martensite, but the growth direction of the original austenite grain boundaries was significantly different. The microstructures of fine grain heat-affected zone were ferrite and martensite, and the microstructure of mixed grain heat-affected zone contained ferrite, massive M/A island, and a small amount of martensite. The micro-hardness values of WS, HAZ, and base metal (BM) were 358, 302, and 265 HV, respectively. The butt joint fracture at the BM far from the WS and the welded joint tensile strength are observed to follow proportional relationship with hardness.

Key words: Laser welding, Microstructure, Mechanical properties, Microalloyed steel

Min Zhang, Xiaonan Wang, Guangjiang Zhu, Changjun Chen, Jixin Hou, Shunhu Zhang, Hemin Jing. Effect of Laser Welding Process Parameters on Microstructure and Mechanical Properties on Butt Joint of New Hot-Rolled Nano-scale Precipitation-Strengthened Steel[J]. Acta Metallurgica Sinica (English Letters), 2014, 27(3): 521-529.

Figures/Tables 13

Table 1 Chemical compositions of experimental steel (wt%)

C	Si	Mn	P	S	Al	Ti	Nb	Fe
0.105	0.20	1.95	0.015	0.015	0.05	0.12	0.06	Bal.

Table 2 Mechanical properties of the experimental steel

Yield strength (MPa)	Tensile strength (MPa)	Elongation (%)
660	730	20

Table 3 Parameters used in the laser welding experiments No. 1–7

No.	Laser power P (kW)	Welding speed ν (m/min)	Defocusing distance ?f (mm)
1	2.5	1.2	-2
2	3.0	1.2	-2
3	3.5	1.2	-2
4	3.0	1.5	-2
5	3.0	1.8	-2
6	3.0	1.2	-3
7	3.0	1.2	0

Fig. 1 Typical surface positive and negative appearances (PA and NA) and cross section images of butt joints welded at v = 1.2 m/min, Δf = -2 mm, and P ranging from 2.5 to 3.5 kW: a PA at P of 2.5 kW; b NA at P of 2.5 kW; c cross section at P of 2.5 kW; d PA at P of 3.0 kW; e NA at P of 3.0 kW; f cross section at P of 3.0 kW; g PA at P of 3.0 kW; h the NA at P of 3.5 kW; i cross section at P of 3.5 kW

Fig. 2 Typical surface appearances and cross section images of butt joints welded at P = 3.0 kW, Δf = -2 mm, and v ranging from 1.2 to 1.8 m/min: a PA at v of 1.2 m/min; b NA at v of 1.2 m/min; c cross section at v of 1.2 m/min; d PA at v of 1.5 m/min; e NA at v of 1.5 m/min; f cross section at v of 1.5 m/min; g PA at v of 1.8 m/min; h NA at v of 1.8 m/min; i cross section at v of 1.8 m/min

Fig. 3 Typical surface appearances and cross section images of butt joints welded at P = 3.0 kW, v = 1.2 m/min, and Δf ranging from 0 mm to -3 mm: a PA at Δf of -3 mm; b NA at Δf of -3 mm; c cross section at Δf of -3 mm; d PA at Δf of -2 mm; e NA at Δf of -2 mm; f cross section at Δf of -2 mm; g PA at Δf of 0 mm; h NA at Δf of 0 mm; i cross section at Δf of 0 mm

Fig. 4 Microstructure of different zones in laser welding butt joint: a OM image of butt joint; b SEM image of WS; c SEM image of CG-HAZ; d SEM image of FG-HAZ; e SEM image of MG-HAZ; f SEM image of BM

Fig. 5 Schematic illustration showing CG-HAZ and FG-HAZ microstructure evolution induced by laser welding: a CG-HAZ; b FG-HAZ

Fig. 6 Epitaxial growth of WS near fusion line [25] a and SEM image of laser welding butt joint b

Fig. 7 Micro-hardness law of NPS steel welded joint at different defocusing distances (Δf) at a laser welding power (P) 3.0 kW and welding speed (v) of 1.2 m/min

Fig. 8 Effect of welding speed on the average hardness values of WS and HAZ

Fig. 9 Tensile stress–strain curve of the butt joint

Fig. 10 Fractured samples after tensile testing a and SEM image of a typical fracture surface b

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