Fatigue Properties Improvement of Low-Carbon Alloy Axle Steel by Induction Hardening and Shot Peening: A Prospective Comparison

doi:10.1007/s40195-021-01366-3

Abstract

Abstract:

Fatigue fracture is the major threat to the railway axle, which can be avoided or delayed by surface strengthening. In this study, a low-carbon alloy axle steel with two states was treated by surface induction hardening and shot peening, respectively, to reveal the mechanism of fatigue property improvement by microstructure characterization, microhardness measurement, residual stress analysis, roughness measurement, and rotary bending fatigue tests. The results indicate that both quenching and tempering treatment can effectively improve the fatigue properties of the modified axle steel. In addition, induction hardening can create an ideal hardened layer on the sample surface by phase transformation from the microstructure of ferrite and pearlite to martensite. By comparison, shot peening can modify the microstructure in surface layer by surface severe plastic deformation introducing a large number of dislocation and even cause grain refinement. Both induction hardening and shot peening create compressive residual stress into the surface layer of axle steel sample, which can effectively reduce the stress level applied to the metal surface during the rotary bending fatigue tests. On the whole, the contribution of induction hardening to the fatigue life of axle steel sample is better than that of the shot peening, and induction hardening shows obvious advantages in improving the fatigue life of axle steel.

Key words: Surface strengthening, Induction hardening, Shot peening, Axle steel, Fatigue property improvement

G. C. Chu, F. Z. Jin, X. J. Jin, Y. Zhang, Q. Wang, J. P. Hou, Z. F. Zhang. Fatigue Properties Improvement of Low-Carbon Alloy Axle Steel by Induction Hardening and Shot Peening: A Prospective Comparison[J]. Acta Metallurgica Sinica (English Letters), 2022, 35(8): 1343-1356.

Figures/Tables 24

Table 1 Chemical compositions of the modified low-carbon alloy steel (wt%)

C	Si	Mn	P	S	Cr	Ni	Mo	Nb	V
0.23	0.25	0.75	≤ 0.010	≤ 0.005	1.25	1.10	0.35	0.025	0.040

Table 2 Induction hardening parameters

Frequency (kHz)	Power (kW)	Heating time (s)	Cooling medium
250	50	15	Water

Table 3 Shot peening parameters

Shot size (mm)	Intensity (mmA)	Air pressure (MPa)	Time (s)	Coverage
0.2	0.13	0.2	16	> 100%

Fig. 1 Specimen shape and dimension of tensile samples

Fig. 2 Specimen shape and dimension of fatigue samples

Fig. 3 Microstructures of the as-received steels: OM images of N steel a, Q?+?T steel b; SEM images of N steel c, Q?+?T steel d

Fig. 4 Typical precipitates in the modified axle steel: a microstructure of N steel; b EDS of the marked precipitate in N steel; c microstructure of Q?+?T steel; d EDS of the marked precipitate in Q?+?T steel

Fig. 5 Tensile engineering stress-strain curves of the as-received steels

Table 4 Tensile properties of axle steels

Materials	Yield strength, σ_s (MPa)	Tensile strength, σ_b (MPa)	Elongation, δ (%)	Reduction of area, Ψ (%)
N steel	443	652	23.5	67.9
Q + T steel	857	932	21.3	66.8
Domestic EA4T	627	770	22.5	70.0
Imported EA4T	630	767	21.5	67.0

Fig. 6 Tensile fracture morphologies of N steel: a overall fracture morphology, corresponding region of b-d marked in a, respectively

Fig. 7 Tensile fracture morphologies of Q?+?T steel: a overall fracture morphology, corresponding region of b-d marked in a, respectively

Fig. 8 Surface morphologies of the samples treated by a induction hardening, b shot peening

Table 5 Surface roughness of the axle samples treated by induction hardening and shot peening

Surface roughness	Induction hardening		Shot peening
Surface roughness	Before treatment	After treatment (including grinding)	Before treatment	After treatment
R_a (μm)	0.186	0.196	0.179	1.528
R_z (μm)	1.686	1.718	1.732	13.966

Fig. 9 Microstructures of the induction hardened sample: a gradient microstructure in the surface layer; b microstructure of the fully hardened zone (5 μm from surface); c microstructure of transition zone (200 μm from surface); d microstructure of matrix (3 mm from surface)

Fig. 10 Microstructures of the shot peened sample: a gradient microstructure in the surface layer; b microstructure of the severe plastic deformation zone (2 μm from surface); c microstructure of transition zone (20 μm from surface); d microstructure of matrix (500 μm from surface)

Fig. 11 TEM images of a fully hardened zone, b severe plastic deformation zone

Fig. 12 Microhardness distribution in the surface layer of the induction hardened steel: a H-d curves; b H/Hm-d curve

Fig. 13 Microhardness distribution in the surface layer of the shot peened steel: a H-d curves; b H/Hm-d curve

Fig. 14 Residual stress curves of the axle steels treated by induction hardening and shot peening

Fig. 15 Number of cycles to failure of axle steels treated by different surface strengthening treatments

Fig. 16 Fatigue fracture morphologies of the induction hardened axle steel samples: a σa?=?600 MPa; b σa?=?650 MPa; c σa?=?700 Mpa

Fig. 17 Fatigue fracture morphologies of the shot peened axle steel samples: a σa?=?550 MPa; b σa?=?620 MPa; c σa?=?690 MPa

Fig. 18 Surface strengthening mechanism of induction hardening and shot peening

Table 6 Strengthening effect comparison of two surface strengthening technologies

Strengthening treatment	Original material	k (%)^a	Λ (mm)^b	σ_r (MPa)^c	d (mm)^d	R (μm)^e	N^f
Induction hardening	N steel	200	2.5	471	2.00	0.01	> 800
Shot peening	Q + T steel	20	0.4	492	0.25	0.349	2-8

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