Investigation of S-N Test Data Scatter of Carburized 4320 Steel

Experimental analysis of bending fatigue strength of plain and notched case-hardened gear steels

“…As a result of this as well as subsequent analyses [4,5,6], besides the specimen's geometry and the loading type, several factors were identified as main causes of experimental differences in fatigue properties, such as the microstructure of the steels, the specimens size, the residual stress state, the case depth, the surface finish, the population of defects and the presence of intergranular oxidation. Specific techniques able to improve the endurance limit of case hardened gear steels introduce additional influencing factors, as for example reheating [5], which cause microstructure refinement, shot peening [6,7] or presetting [8], which increase compressive residual stresses.…”

“…Specific techniques able to improve the endurance limit of case hardened gear steels introduce additional influencing factors, as for example reheating [5], which cause microstructure refinement, shot peening [6,7] or presetting [8], which increase compressive residual stresses. Concerning the statistical scatter of fatigue test results, it has been tentatively interpreted on the basis of Fracture Mechanics calculations [6], where the scatter of material dependent parameters appearing in the Paris law was taken into account.…”

“…Three and four point or cantilever design plane bending tests are mainly carried out under load ratio close to zero R ≈ 0, being relevant to the teeth of a drive gear. Tests presented in refs [5,6,[9][10][11] were carried out on cantilever specimens with rounded corners to avoid edge effects during the carburization process, according to an improved geometry of the original specimen introduced by Brugger [12]. In fact, at specimen's corners the amount of adsorbed carbon during carburization is higher than at flat surfaces, resulting in different microstructure after quenching and tempering, eventually leading to a lower endurance limit [13].…”

“…The range of notch tip radii was from 0.5 to 7.62 mm. The experimental data concerning all plane bending tests taken from the literature [5,6,[9][10][11][14][15][16] are reported in Figure 1, where the net-section nominal bending stress amplitude (σ an , half the difference between maximum and minimum applied stresses) has been used to present the results (N f is the number of cycles to failure). Figure 1 reports also three test series investigated in the present work, namely A1, C1 and X1, which will be considered later on in the paper.…”

“…The specimens analysed in refs. [5,6,[9][10][11] were characterized by a net-section stress concentration factor equal to K t =1.3 and by surfaces chemically polished before the case-hardening treatment, consisting of gas-carburization, quenching and then tempering. The influence of different temperatures and holding times on fatigue properties was also investigated.…”