Effect of Nanoscale Cu-Riched Clusters on Strength and Impact Toughness in a Tempered Cu-Bearing HSLA Steel

doi:10.1007/s40195-021-01277-3

Abstract

Abstract:

The effect of Cu-riched clusters on strength and impact toughness in a tempered Cu-bearing high-strength low-alloy (HSLA) steel is investigated. With increasing the tempering temperature, it is found that the yield strength increases firstly, achieving the maximum value (~ 1053 MPa) at the tempering temperature of 450 °C, and then decreases significantly with the rise of tempering temperature. The tempering temperature-dependent yield strength is closely related to the precipitation of Cu-riched clusters. When tempering at 450 °C, the peak strength will be reached as the nanoscale Cu-riched clusters with small size and high number density will cause a strong precipitation strengthening (~ 492 MPa) due to the dislocation shearing mechanism. However, the Cu-riched clusters will coarsen with further increasing tempering temperature, resulting in obvious decrement of yield strength owing to the dislocation bypassing mechanism. Compared with the yield strength, the variation in impact energy displays an inverse tendency and the impact energy is only 7 J for the sample tempered at 450 °C. The fracture mode can be well explained by the competition between the cleavage fracture strength (σ_F) and “yield strength” (σ_Y). Although transgranular cleavage fracture can be found in samples tempered at 450 and 550 °C, the crack propagation along the lath boundaries is prevented in the sample tempered at 550 °C. The reason is that the number density of Cu-riched clusters at lath boundaries decreases and the segregation of Mo element at the lath boundaries is induced, which will increase the bonding energy.

Key words: High-strength low-alloy (HSLA) steel, Cu-riched cluster, Strengthening mechanism, Lath boundary, Impact energy

Yubin Du, Xiaofeng Hu, Yuanyuan Song, Yangpeng Zhang, Lijian Rong. Effect of Nanoscale Cu-Riched Clusters on Strength and Impact Toughness in a Tempered Cu-Bearing HSLA Steel[J]. Acta Metallurgica Sinica (English Letters), 2022, 35(4): 537-550.

Figures/Tables 18

Table 1 Chemical compositions of the experimental steel (wt%)

C	Ni	Mn	Mo	Cr	Si	S	P	Nb	Cu	Fe
0.041	4.01	1.00	0.56	0.95	0.22	0.005	0.007	0.05	1.40	Bal.

Fig. 1 Tempering temperature-dependent yield strength and Charpy impact energy for the as-quenched and tempered 1.4Cu steel

Fig. 2 SEM images for the as-quenched sample a and the sample tempered at 650 °C b

Fig. 3 Misorientation distribution of grain boundaries a and misorientation angle distribution maps b of the as-quenched 1.4Cu steel (The black lines and red lines denote HAGB (misorientation angle > 15°) and LAGB (misorientation angle 2-15°), respectively.)

Table 2 Percentage of HAGB and EGS of the as-quenched and tempered 1.4Cu steel

Tempering temperature (°C)	Percentage of HAGB (%)	EGS (μm)
As-quenched	41.5	1.96
450	41.6	2.00
550	41.9	2.05
650	40.6	2.00

Fig. 4 Bright-field TEM images for the as-quenched sample a and the samples tempered at 450 °C b, 550 °C c, 650 °C d, and the inset showing EDS mapping images d, respectively

Fig. 5 Cu atoms maps and the Cu-riched particles characterized by 5 at% Cu isoconcentration surface for the as-quenched sample a and the samples tempered at 450 °C b, 550 °C c, 650 °C d

Fig. 6 Radius and number density a and radius distribution b of Cu-riched clusters for the tempered 1.4Cu steel

Fig. 7 C atoms distribution map and Cu-riched particles characterized by 5 at% Cu isoconcentration surface a and one-dimensional (1-D) composition profiles obtained by a cylindrical probe (30 nm in diameter) through the C-riched interface for the 1.4Cu steel tempered at 550 °C b

Fig. 8 Equilibrium solubility of Cu [Cu]α-Fe a and diffusion coefficient of Cu $D_{{_{{\text{Cu}}} }}^{{\alpha - \text{Fe}}}$ in α-Fe b at different temperatures

Fig. 9 Mobility of Mo at different temperatures calculated by Thermo-Calc

Table 3 Dislocation density of the as-quenched and tempered 1.4Cu steel

Samples	As-quenched	450 °C	550 °C	650 °C
Dislocation density (m^-2)	10.7 × 10¹⁴	5.9 × 10¹⁴	4.2 × 10¹⁴	2.3 × 10¹⁴

Fig. 10 Contributions of various strengthening mechanisms to the yield strength of the as-quenched and tempered samples

Table 4 Contributions of each strengthening mechanism to the yield strength for the as-quenched and tempered samples (MPa)

Samples	σ₀	σ_s	σ_g	σ_dis	σ_cut	σ_bypass	$\sqrt {\left( {\sigma _{{\text{dis}}}^{{2}} + \sigma _{{_{{\text{Cu}}} }}^{{2}} } \right)} $	σ_y(cal)	σ_y(exp)
As-quenched	54	274	156	450	-	-	450	934	944
450 °C	54	223	156	334	492	-	595	1028	1053
550 °C	54	228	156	283	161	239	490	928	948
650 °C	54	239	156	209	-	120	241	690	719

Fig. 11 Impact fracture surfaces tested at - 50 °C for the as-quenched sample a and the samples tempered at 450 °C b, 550 °C c, 650 °C d (The insets are macro SEM fractographs.)

Fig. 12 Strain maps of the as-quenched sample a and the sample tempered at 450 °C b (The black lines and red lines denote the high-angle grain boundaries (misorientation angle > 15°) and low-angle grain boundaries (misorientation angle 2-15°), respectively.)

Fig. 13 SEM images of the cross-sectional areas adjacent to the impact fracture surfaces for the as-quenched 1.4Cu steel a and the samples tempered at 450 °C b, 550 °C c, and 650 °C d

Fig. 14 Schematic diagrams of the crack propagation for the samples tempered at 450 °C (a) and 550 °C (b)

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