Study on the Evaluation of Fatigue Limit and Harmless Crack Size of Double Shot Peening Treated Carburized Steel

2.1 Material and surface modification method

The material used is SCM822H, an alloy steel for mechanical structures. Table 1 shows the chemical composition of the material. The specimen shape (shape factor 𝑎=1.56) is shown in Fig. 1. The gas carburizing quenching tempering specimens (GCT) were carburized at 1,203 K for 4 hours, followed by oil quenching (Q), and tempered at 433 K for 2 hours. Subsequently, middle temperature tempering was conducted at 573 K for 2 hours, followed by DSP under the conditions shown in Table 2.

Table 1

Chemical compositions of used steel(mass%)

Fig. 1

Shape of specimen (unit : mm)

Table 2

DSP condition

The combinations of surface modification methods applied to each specimen are shown in Table 3. The base metal specimen (BM) is the GCT specimen, GCT1 is the specimen that done DSP on the BM. GCT2 is the specimen that was tempered at 573 K on the BM and then done DSP. GCT3 is the specimen that done DSP on the BM followed by tempering at 573 K.

Table 3

Surface refining method

2.2 Evaluation method of surface characteristics

Before experiment, the hardness distribution, amount of retained austenite (γ_R), and residual stress (σ_r) in the depth direction of the notch area were measured. The hardness measurements were conducted using a Vickers hardness tester with an indentation load of 2.9 kN. γ_R was obtained from the XRD data using the maximum values of 2𝜃 for each phase.^7,8) Residual stresses were measured in the depth direction using the hole drilling method.

2.3 Bending fatigue test method

Bending fatigue tests were conducted using a rotating bending fatigue testing machine of Ono type (Maximum bending moment 15 Nm). The applied stress was calculated by multiplying the nominal stress at the notch section by the shape factor α=1.56. The fatigue limit was defined as the maximum stress amplitude that could withstand for 107 loading cycles.

2.4 Evaluation of harmless crack size

The evaluation specimens are subjected to bending stress with a stress ratio of R=1, as shown in Fig. 2, with a width of W=100 mm and a thickness of t=10 mm. The specimens feature a semi-elliptical surface crack characterized by a crack depth (a) and crack length (2c) with a crack aspect ratio (As=a/c) of 1.0. The crack depth and crack surface are denoted as A and C, respectively.

Fig. 2

Schematic diagram of a finite plate with electric discharge machining crack

3.1 Effect of phase transformation on hardness

The surface carbon content of the GCT specimen was 0.83 mass%. Fig. 3 shows the distribution of γ_R content for BM, GCT1, GCT2, and GCT3 specimens. The surface γ_R content (γ_RS) of the BM was 17%, which decreased to 3.1% for the GCT1 specimen. This reduction indicates that 81.8% of γ_RS was transformed into deformation induced martensite due to the DSP. γ_RS in the GCT2 and GCT3 specimens tempered at 573 K decreased to 1.5% and 1.7%, respectively. This indicates that γ_R transformed into bainite in the temperature range of 503~543 K. This significant change highlights the effect of surface modification techniques on the microstructural properties of the specimens, ultimately influencing their mechanical behavior.

Fig. 3

Volume fraction of retained austenite

Fig. 4 shows the hardness distribution near the surface of each specimen. The surface hardness (depth 0.01 mm) of the BM, GCT1, GCT2, and GCT3 specimens was 695, 930, 676, and 707 HV, respectively. The hardness of the GCT1 specimen is 235 HV higher than that of the BM. This is attributed to the combined effects of the deformation induced martensite transformation of γ_R and the conventional work hardening by DSP. When comparing the hardness of the GCT2 and GCT3 specimens tempered at 573 K, the GCT3 specimen showed a hardness that was 31 HV higher.

Fig. 4

Distribution of Vickers hardness

3.2 Distribution of residual stress

Fig. 5 shows the distribution of residual stress for each specimen. The surface compressive residual stress (σ_rs) for the BM, GCT1, GCT2, and GCT3 specimens were 36 MPa, 1343 MPa, 1023 MPa, and 462 MPa, respectively. GCT3 showed a low σ_rs because surface plastic flow remained. The maximum compressive residual stress (σ_rmax) was determined considering the depth of fatigue cracks at Stage I of high-strength steels and the depth of fatigue cracks of the transition stage from Stage I to Stage II. Thst is, approximately 3 grain sizes (around 60 μm) is a maximum value. σ_rmax for the GCT1, GCT2, and GCT3 specimens were 1489 MPa, 1034 MPa, and 492 MPa, respectively. The depths for σ_rmax were 9 μm, 15 μm, and 14 μm, indicating that σ_rmax were introduced at nearly the surface, within one grain size (about 20 μm). Naturally, considering the temperature rise up to 573 K during operation, the GCT2 specimen, which done DSP after tempering at 573 K, exhibited a σ_rmax of 542 MPa, which is higher than that of the GCT3 specimen. Therefore, the GCT2 specimen, with its elevated compressive residual stress, is expected to show an improvement in fatigue limit compared to the GCT3 specimen.

Fig. 5

Distribution of residual stress

3.3 Fatigue limit

The S-N curves for the GCT1, GCT2, and GCT3 specimens are shown in Fig. 6. The bending fatigue limit for BM was 827 MPa.

Fig. 6

S-N curves of GCT specimens

However, the bending fatigue limit for the GCT1 specimen, which done after GCT, was significantly higher at 1,732 MPa. Considering the temperature rise to 573 K during operation, the bending fatigue limit for the GCT3 specimen was 1,045 MPa. In comparison, the bending fatigue limit for the GCT2 specimen, which done DSP after tempering at 573 K, was 1,357 MPa. This indicates that the bending fatigue limit of the GCT2 specimen is 30% higher than that of the GCT3 specimen.

Considering the temperature rise during operation, the differences in surface properties between the specimens that were tempered again at 573 K and those that were not were minimal. Therefore, to prevent bending fatigue failure of gears due to unexpected temperature increases during operation, the GCT2 specimen, which done DSP after tempering at 573 K, can be deemed effective.

Table 4 provides a comprehensive summary of the surface properties and fatigue limits obtained from the experiments. This shows surface hardness, maximum residual stress, yield stress, and bending fatigue limit for the various specimens (BM, GCT1, GCT2, and GCT3). The data presented in Table 4 facilitates a clear comparison of how different surface modification techniques and treatments influence the mechanical performance and durability of the specimens, highlighting the effectiveness of the DSP treatment in enhancing both surface characteristics and fatigue resistance.

Table 4

Refined surface characteristics and fatigue limit of GCT specimens

3.4 Evaluation of harmless crack size (a_hml)

Fig. 7(a)~(c) shows the relationship between ΔK_Tr and ΔK_th as a function of crack depth (𝑎) for the GCT1, GCT2, and GCT3 specimens with a crack aspect ratio of As=1.0. ΔK_th corresponding to crack length was evaluated using Eq. (1) when a semi-elliptical surface crack in a finite plate is subjected to bending stress with a stress ratio R.⁹⁾

Fig. 7

Evaluation of maximum crack size for GCT specimens that can be rendered harmless by DSP. (a) GCT1, (b) GCT2, (c) GCT3

Where Δσ_w is the fatigue limit of BM at 827 MPa, and ΔK_th(l) is the threshold stress intensity factor for long crack, given as 6.52 MPa$$ \sqrt{m}$$. 𝑎 is the semi-elliptical crack depth, while β is the shape correction factor from the Newman-Raju equation¹⁰⁾ in the crack depth and surface in the case of a finite plate subjected to bending fatigue stress.

ΔK_Tr is the sum of ΔK calculated using the Newman-Raju equation and K_r obtained from Eq. (2) of the compressive residual stress. The applied stress used in calculating ΔK is the fatigue limit (σ_w) of each specimen listed in Table 4. Additionally, the evaluation of K_r used the residual stresses shown in Fig. 5 for each specimen.¹¹⁾

Where G₀ ~ G₄ are the shape correction coefficients for the stress intensity factor according to API-RP579. a and c represent the depth and surface length of the semi-elliptical crack, respectively. W and t represent the plate width and plate thickness, respectively. σ₀ ~ σ₄ are coefficients obtained from the residual stress distribution using the fourth-order polynomial.

a_hml by DSP is determined by Eq. (3). That is, a_hml is determined the smaller crack size in the crack depth and the surface crack. In Fig. 7, all a_hml is shown as ● at the intersection points of ΔK_th(s) and

Since the hardness of BM before GCT used in this study is 440 HV, a threshold stress intensity factor of 6.52 MPa $$ \sqrt{m}$$ was applied.¹²⁾

(a) In GCT1, the intersection point of ΔK_th(s) and ΔK_Tr was obtained at depth A, and a_hml is a crack depth of 0.094 mm. (b) In GCT2, the intersection point was first obtained at surface C, and a_hml is a surface crack of 0.185 mm. Meanwhile, (c) In GCT3, the intersection point at depth A (0.036 mm) and surface C (0.037 mm) were nearly identical, resulting in almost no difference in a_hml. This means that cracks smaller than a_hml can be rendered harmless by DSP.