Evaluation Procedures of Nonmetallic Inclusions in Steel for Highly Reliable Bearings

Abstract

The improvement in rolling contact fatigue life is a key subject to increase the reliability of bearings. It has been well known that the fatigue properties are significantly affected by nonmetallic inclusions in steel. The rolling contact fatigue life may be divided into two types, according to the type of reliability requirements. One is “L10 life,” which represents general bearing performance. The other is “accidental short life,” where a bearing prematurely fails in its service period and its calculated life is hardly met. Since the probabilities of these failures are significantly different, the distribution densities of the corresponding life-limiting inclusions are also expected to be widely different. When nonmetallic inclusions are evaluated as an index of fatigue life, therefore, testing volume should be appropriately determined according to the concerned type of life. From this point of view, it is effective for the reliability assessment of bearing steel to combine the evaluations of large microscopic inclusions by statistics of extreme value and macroscopic inclusions in larger volume by a high-frequency ultrasonic test. It was found that the L10 life determined by thrust-type rolling-contact fatigue tests was well correlated with maximum micro-inclusion diameter predicted by the statistics of extreme value, where both oxide and sulfide played as life-limiting inclusions. Fifteen MHz ultrasonic testing was employed to evaluate the macroscopic inclusions. The combination of these two procedures was able to demonstrate the difference in reliability clearly, even when other cleanliness indices could not characterize the concerned materials.

Inclusion orientation dependent flaking process in rolling contact fatigue observed by laminography using ultrabright synchrotron radiation X‐ray

“…The steel was not a commercial bearing steel, but it was fabricated in our laboratory to intentionally contain a high concentration of sulfur to enable the observation of crack initiation from stringer‐type MnS inclusions. The deteriorative effect of MnS inclusion on RCF was reported for steel bearings 26,28,59 . The steel was forged from an ingot with a diameter of 70 mm, and inclusions had preferential alignment along the forging direction.…”

“…On the other hand, Kanetani and Ushida reported that cracks usually started forming from non-metallic inclusions rather than WBs. 12 The detrimental effect of inclusions in terms of RCF has been discussed, 17,19,20,[24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41] and it has been found that the size, shape, orientation, location, composition of inclusions, and interface conditions between inclusions and the surrounding matrix can be considered to be factors affecting the RCF life. The refinement of inclusions contributes to improving the RCF strength.…”

State of the Art in Control of Inclusions and Microalloying Elements in Tire Cord Steel and Saw Wire Steel

“…Moreover, processing properties of materials suffer from even more severe damage than the case of oxide inclusions, as shown in Figure 11 . [ 55 ] Therefore, it is essential to control the quantity and size of Ti‐containing inclusions in cord steel.…”

Effect of Sulphur Concentration on Precipitation Behaviors of MnS-containing Inclusions in GCr15 Bearing Steels after LF Refining

“…It was once reported that sulphur concentration must be controlled according to oxygen concentration in bearing steels. 18) Although there were many researches on the removal of oxide inclusions in bearing steels, [19][20][21][22][23] how much sulphur concentration should be controlled and how it effects on precipitation behaviors of sulphides are seldom reported for high quality GCr15 bearing steels. In order to decrease sulphur concentration and sulphide inclusions in the steel, desulphurization ability of refining slag is evaluated by analyzing samples sampled from industrialized production of GCr15 bearing steels during LF refining process in the present work.…”