Comparison of Microstructure and Residual Stress Between TIG and MAG Welding Using Low Transformation Temperature Welding Filler

doi:10.1007/s40195-017-0642-z

Abstract

Abstract:

A Cr-Ni type of low transformation temperature (LTT) welding filler was devised in the present study. The LTT weld microstructures of the tungsten inert gas (TIG) and metal active gas (MAG) weldings were investigated by using electron-backscattered diffraction and orientation imaging microscopy. The results showed that the LTT weld microstructures prepared by TIG and MAG weldings were primarily martensite with 17.5% and 8.0% retained austenite, respectively. The LTT weld metal using TIG welding had larger grain size than using MAG. In addition, based on the Taylor factor calculation, the weld metal using MAG welding was more competent in repressing fatigue crack initiation. Meanwhile, the high angle and coincidence site lattice grain boundaries were dominant in the LTT weld metal using MAG welding. Moreover, the hardness of the LTT weld metal using MAG welding was higher than that of using TIG. Based on heat input and phase transformation, finite element method was applied to analyzing the tensile residual stress (RS) reduction in welded joints prepared by both conventional and LTT welding fillers, respectively. The corresponding outcome confirmed that the LTT weld metal using MAG welding was more beneficial to tensile RS reduction.

Key words: Martensitic transformation, Taylor factor, Grain boundaries, Residual stress

Zhong-Yuan Feng, Xin-Jie Di, Shi-Pin Wu, Dong-Po Wang, Xiao-Qian Liu. Comparison of Microstructure and Residual Stress Between TIG and MAG Welding Using Low Transformation Temperature Welding Filler[J]. Acta Metallurgica Sinica (English Letters), 2018, 31(3): 263-272.

Figures/Tables 16

Table 1 Welding parameters for LTT welding filler using different welding methods

Welding method	Shielding gas	Average current (A)	Average voltage (V)	Welding speed (mm/min)	Heat input (kJ/mm)
TIG	Ar	120	12.0	50	1.73
MAG	80% Ar + 20% CO₂	290	26.4	300	1.53

Table 2 Chemical composition of LTT weld metals using different welding methods (wt%)

Welding method	C	Mn	Cr	Ni	Mo	Si	Fe
TIG	0.017	1.17	11.10	8.90	0.14	0.30	Bal.
MAG	0.025	1.04	11.95	9.23	0.14	0.39	Bal.

Fig. 1 Schematic drawing of cylindrical sample in surfacing joint

Fig. 2 Simulated welding thermal cycle for LTT weld samples

Fig. 3 Dilatometric curves of LTT weld metals using a TIG; b MAG weldings

Fig. 4 OM micrographs of LTT weld metals using a TIG; b MAG weldings

Fig. 5 Phase fraction of LTT weld metals using a TIG; b MAG weldings

Fig. 6 Comparison of grain size of LTT weld metals using different welding methods

Fig. 7 Taylor factor mapping of LTT weld metals using a TIG; b MAG weldings

Fig. 8 Misorientation angle distribution of LTT weld metals using different welding methods

Fig. 9 CSL grain boundaries of LTT weld metals using different welding methods

Fig. 10 Finite element model applied to simulate residual stress

Fig. 11 Temperature-dependent thermal physical properties of LTT weld metals [29, 30]

Table 3 Welding parameters of simulation cases considered in the FEA analysis

Case	Welding layer	Welding filler	Welding method	Average current (A)	Average voltage (V)	Welding speed (mm/min)
1	First	The conventional	MAG	260.0	24.5	300
1	Second	The conventional	MAG	290.0	26.4	300
2	First	The conventional	MAG	260.0	24.5	300
2	Second	The LTT	TIG	120.0	12.0	50
3	First	The conventional	MAG	260.0	24.5	300
3	Second	The LTT	MAG	290.0	26.4	300

Fig. 12 Longitudinal and transverse RS of the conventional and LTT welded joints

Fig. 13 Computed longitudinal RS, phase fraction and temperature as function of time at welded node: a the conventional; b LTT using TIG; c LTT using MAG

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