Ultrahigh cycle fatigue fracture mechanism of high-quality bearing steel obtained through different deoxidation methods

Abstract

The mechanism of oxide inclusions in fatigue crack initiation in the very-high cycle fatigue (VHCF) regime was clarified by subjecting bearing steels deoxidized by Al (Al-deoxidized steel) and Si (Si-deoxidized steel) to ultrasonic tension-compression fatigue tests (stress ratio, R = −1) and analyzing the characteristics of the detected inclusions. Results show that the main types of inclusions in Si-and Al-deoxidized steels are silicate and calcium aluminate, respectively. The content of calcium aluminate inclusions larger than 15 µm in Si-deoxidized steel is lower than that in Al-deoxidized steel, and the difference observed may be attributed to different inclusion generation processes during melting. Despite differences in their cleanliness and total oxygen contents, the Si-and Al-deoxidized steels show similar VHCF lives. The factors causing fatigue failure in these steels reveal distinct differences. Calcium aluminate inclusions are responsible for the cracks in Al-deoxidized steel. By comparison, most fatigue cracks in Si-deoxidized steel are triggered by the inhomogeneity of a steel matrix, which indicates that the damage mechanisms of the steel matrix can be a critical issue for this type of steel. A minor portion of the cracks in Si-deoxidized steel could be attributed to different types of inclusions. The mechanisms of fatigue fracture caused by calcium aluminate and silicate inclusions were further analyzed. Calcium aluminate inclusions first separate from the steel matrix and then trigger crack generation. Silicate inclusions and the steel matrix are closely combined in a fatigue process; thus, these inclusions have mild effects on the fatigue life of bearing steels. Si/Mn deoxidation is an effective method to produce high-quality bearing steel with a long fatigue life and good liquid steel fluidity.

Effect of Dynamic Soft Reduction Range and Amount on Central Segregation in Bloom and the Resulting Microstructure in the Rod of GCr15‐Bearing Steel

“…The total oxygen content of GCr15 steel produced by bloom continuous casting and rod hot‐rolling processes, which could represent the control level of nonmetallic inclusions and cleanliness, has been reduced below 6 × 10 −6 . [ 6,7 ] This phenomenon demonstrates that the control of inclusions in GCr15 steel could reach a good standard. Thus, the uniformity in the chemical composition and the distribution of carbides already play a more prominent role in affecting the internal quality of bearing materials.…”

Slag resistance mechanism of CaO·6Al2O3 refractory and its effect on inclusions of aluminum deoxidized steel

“…The formation of MgO⋅Al 2 O 3 in the slag can be concluded as two mechanisms: (i) during the smelting process, some of the (Mg), (O), and (Al 2 O 3 ) in the slag react to form MgO⋅Al 2 O 3 , such as the MgO⋅Al 2 O 3 in the slag layer of crucible without MgO (Figure 6A,C,D), seeing in the Equation (4); (ii) another way is that the reaction between the MgO in the refractories and the (Ca) in the slag produce Mg into the slag (Equation 5), after that the (Mg) reacting with (O) and (Al) or (Al 2 O 3 ) to produce the new phase MgO⋅Al 2 O 3 , similar to the first mechanism (Figure 6B,E). In addition, the reaction between Al 2 O 3 in the refractories and (MgO) in the slag will also produce MgO⋅Al 2 O 3 adhering on the surface of the refractories or entering the slag, as shown in Equation ( 6 7)- (9). In addition, the mechanism of MgO⋅Al 2 O 3 inclusions in Al 2 O 3 crucible is similar to that of CA 6 crucible.…”

“…During the deoxidation process, the aluminum reacts with the oxide, such as magnesium oxide and free oxygen elements in the molten steel, decreasing the content of oxygen. But at same time, it also can produce high melting point inclusions remained in the molten steel, such as alumina inclusions and spinel inclusion, causing great harm to the performance of the product 6–9 . At present, the problem of high melting point inclusions produced by aluminum deoxidation can only be adjusted slightly by some methods, such as calcium treatment and slag system optimization.…”

Microstructure-Based Fatigue Modeling with Residual Stresses: Effect of Inclusion Shape on Very High Cycle Fatigue Life

“…To reveal the effect of inclusions on fatigue properties, many studies were conducted via experiments and simulations [3][4][5][6]. A majority of them focused on the effect of inclusion size on fatigue life and it is proven that inclusion size indeed plays a significant role in determining the fatigue life of metals [7][8][9][10].…”