Microcrack Formation During Gas Metal Arc Welding of High-Strength Fine-Grained Structural Steel

doi:10.1007/s40195-013-0011-5

Abstract

Abstract:

The recent development of high-performance-modified spray arc processes in gas metal arc welding due to modern digital control technology and inverter power sources enables a focused spray arc, which results in higher penetration depths and welding speed. However, microcracks occurred in the weld metal while approaching the process limits of the modified spray arc, represented by a 20-mm double layer DV-groove butt-weld. These cracks were detected in structural steel exhibiting a yield strength level of up to 960 MPa and are neither dependent on the used weld power source nor a consequence of the modified spray arc process itself. The metallographic and fractographic investigations of the rather exceptional fracture surface lead to the classification of the microcracks as hot cracks. The effects of certain welding parameters on the crack probability are clarified using a statistical design of experiment. However, these microcracks do not impact the design specification for toughness in the Charpy V-notch test (absorbed energy at -40 °C for the present material is 30 J).

Key words: High-strength low-alloy steel, Welding, Fracture, Design of experiments, Microcracking

Christoph Heinze, Thomas Michael, Andreas Pittner, Michael Rethmeier. Microcrack Formation During Gas Metal Arc Welding of High-Strength Fine-Grained Structural Steel[J]. Acta Metallurgica Sinica (English Letters), 2014, 27(1): 140-148.

Figures/Tables 15

Table 1 Chemical composition (wt%) of base and filler material, base material composition measured by spark emission spectroscopy, and filler material composition provided by manufacturer; yield strength level of both filler and base material is 960 MPa

Material	C	Si	Mn	Cu	P	S	Cr	Mo	Ni	Fe
Base material—test heat	0.16	0.31	0.94	–	0.011	0.001	0.55	0.30	1.00	Bal.
DIN EN 10025-6 (maximum values)	0.20	0.8	1.7	0.5	0.020	0.01	1.5	0.70	2.00	Bal.
Filler material—test heat	0.09	0.81	1.76	–	0.012	0.011	0.36	0.58	2.19	Bal.

Table 2 Factors of statistical design of experiment

Factor	Symbol	Level				Unit
Factor	Symbol	-1	0	1	Δ	Unit
Welding speed	v_w	35	40	45	5	cm/min
Wire feeding rate	R_WF	11	12	13	1	m/min
Groove angle	α	30	35	40	5	Deg.
Preheat temperature	T_p	50	75	100	25	°C

Fig. 1 Schematic of joint preparation

Fig. 2 Presentation of crack opening: a scheme of cross-section including crack position; b image of opened crack; c close-up of opened crack; d SEM overview of opened crack

Fig. 3 Microcrack development along the welding direction

Fig. 4 Number of microcracks in a sample plotted versus energy input per unit length

Fig. 5 Number of microcracks in a sample versus cooling time t8/5

Table 3 Overview about probabilities of single factors on number of microcracks

Factor	Bead 1	Bead 2	Whole sample
Welding speed	0.42	0.02	0.04
Wire feeding rate	0.21	0.38	0.28
Groove angle	0.58	0.50	0.49
Preheat temperature	0.78	0.32	0.40

Fig. 6 Two exemplary microcracks situated in the weld metal

Fig. 7 Preferred locations of the microcracks detected

Fig. 8 SEM images of etched weld metal cross-sections

Fig. 9 SEM images of weld metal cross-sections that show the microcrack surfaces

Fig. 10 Opened microcrack surface and similar surface appearance in underlying bulk

Fig. 11 Fracture surfaces of the Charpy V-notch sample after tested at -40 °C: a SEM image showing both fracture surface and microcrack surface; b Two measurement areas for AES indicated (1 fracture surface, 2 microcrack surface)

Fig. 12 Comparison of AES spectra: a fracture surface; b microcrack surface

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