Fatigue Properties Of High Strength Steels Research Articles

Fatigue cracking criterion of high-strength steels induced by inclusions under high-cycle fatigue

Fatigue properties of high-strength steels become more and more sensitive to inclusions with enhancing the ultimate tensile strength (UTS) because the inclusions often cause a relatively low fatigue strength and a large scatter of fatigue lives. In this work, four S–N curves and more than 200 fatigue fracture morphologies were comprehensively investigated with a special focus on the size and type of inclusions at the fatigue cracking origin in GCr15 steel with a wide strength range by different heat treatments after high-cycle fatigue (HCF). It is found that the percentage of fatigue failure induced by the inclusion including Al2O3 and TiN gradually increases with increasing the UTS, while the percentage of failure at sample surfaces decreases conversely and the fatigue strength first increases and then decreases. Besides, it is interestingly noted that the inclusion sizes at the cracking origin for TiN are smaller than that for Al2O3 because the stress concentration factor for TiN is larger than that for Al2O3 based on the finite element simulation. For the first time, a new fatigue cracking criterion including the isometric inclusion size line in the strength-toughness coordinate system with specific physical meaning was established to reveal the relationship among the UTS, fracture toughness, and the critical inclusion size considering different types of inclusions based on the fracture mechanics. And the critical inclusion size of Al2O3 is about 1.33 times of TiN. The fatigue cracking criterion could be used to judge whether fatigue fracture occurred at inclusions or not and provides a theoretical basis for controlling the scale of different inclusion types for high-strength steels. Our work may offer a new perspective on the critical inclusion size in terms of the inclusion types, which is of scientific interest and has great merit to industrial metallurgical control for anti-fatigue design.

Post-fire ultra-low cycle fatigue properties of high-strength steel via different cooling methods

To accurately evaluate the effect of cooling methods on the residual seismic capacity of Q690 high-strength steel (HSS) after fire, an experimental study was undertaken to investigate the post-fire ultra-low cycle fatigue properties of Q690 HSS. Accordingly, Q690 HSS was heated to 600, 700, 800, and 900 °C and cooled using three methods: air, water, and firefighting foam. Fatigue tests on Q690 HSS subjected to heating and cooling treatments were performed, and cyclic stress–strain curves of Q690 HSS were obtained for different temperatures and cooling methods. Based on the test results, the effects of the exposure temperature and cooling method on the ultra-low cycle fatigue properties of Q690 HSS were analysed. Moreover, the microstructure and fracture morphology of Q690 HSS were recorded and analysed. Finally, a method to predict the fatigue life of Q690 HSS subjected to different elevated temperatures and cooling methods was proposed.

Effects of inclusion types on the high-cycle fatigue properties of high-strength steel

The high-cycle fatigue fracture behavior induced by different types of inclusions for high-strength steels was investigated. It is found that fatigue cracking of high carbon chromium bearing steel mainly originated from the interface between the matrix and the oxide inclusions while from the inside for nitride inclusions in the fatigue failure origin, respectively. The two fatigue cracking modes induced by different types of inclusions often result in a great difference in the fatigue lives even though considering the stress amplitude as well as the inclusion size. Therefore, an inclusion type factor was established for the first time to unify the effects of the inclusion types on the fatigue properties and the modified stress intensity factor could well predict the fatigue lives. These results distinguish the differences of the inclusion types in the fatigue performance and the role in trace elements control in metallurgy for the high-strength steels.

10<sup>11</sup>-cycle Gigacycle Fatigue Properties of High-strength Steel

10<sup>11</sup>-cycle Gigacycle Fatigue Properties of High-strength Steel

Fatigue properties of high-strength steel in diesel oil

This paper describes the fatigue properties of chromium-molybdenum steel in air and diesel oil. 4-point bending fatigue tests were conducted at a stress ratio of R=0.05. The corrosion mark was not confirmed on the specimens tested in diesel oil, and the fatigue strength at Nf < 3×104 improved and that at Nf > 3×104 declined compared to in air. The fatigue crack initiation life accounted for about 90 % of the fatigue life Nf. The results of the elemental analysis of the specimen surface tested in diesel oil which were broken at Nf > 3×104, O, Si, and C had been concentrated. The reduction of fatigue strength at Nf > 3×104 is considered to be caused by some influence of deterioration of the diesel oil winhich progress with the fatigue test. On the other hand, the fatigue test at Nf < 3×104 is usually finished within one day, so diesel oil does not have the time to deteriorate. Then, a few fatigue tests were conducted in the deteriorated diesel oil at a relatively high stress range, in order to break the specimen at Nf < 3×104. As a result, fatigue life tends to shorten. Therefore, it was found that the diesel oil deterioration should be considered as one of the important factors to explain the decrease of fatigue strength.

OS2119 800MPaサイクル疲労強度評価

The giga-cycle fatigue properties of high-strength steel are important to maintain and manage the equipment. In the present study, the rotating bending test and the ultrasonic loading test up to 10^8 cycles are carried out using the 800MPa grade high strength steels in order to clarify the giga-cycle fatigue of high-strength steel. One of the several specimens failed due to internal fracture over 10^7 cycles in the ultrasonic loading test. It was clarified the origin of fracture on specimen which failed over 10^7 cycles is inclusions, which has large profile area, by fractography.

Notable size effects on very high cycle fatigue properties of high-strength steel

Very high cycle fatigue (VHCF) properties were compared between two types of specimens: enlarged specimens and our standard specimens. Fatigue tests were conducted by ultrasonic fatigue testing; the material used was commercial spring steel. All tests ended in internal fracture, with large-size effects observed, i.e., the enlarged specimens showed lower VHCF strength than the standard specimens. Most of the internal fracture origins were oxide-type inclusions that were larger in the enlarged specimens than in the standard specimens, indicating the size effect to be caused by the difference in oxide-type inclusion sizes at the origins of internal fractures. The large-size effect strongly urges the use of large specimens when conducting VHCF tests on high-strength steel. Moreover, the large-size effect implies that fatigue strength cannot in this case be determined using the conventional S– N curve approach, since the S– N curve depends on the specimen size. The evaluation of the VHCF strength thus needs two steps: an estimation of the maximal inclusion size, followed by an estimation of the VHCF strength based on the maximal inclusion size.

Specimen size effects on gigacycle fatigue properties of high-strength steel under ultrasonic fatigue testing

Ultrasonic fatigue testing was conducted for high-strength steel, employing enlarged specimens with a straight section. The results showed good agreement with conventional servohydraulic fatigue test results, demonstrating the validity of ultrasonic fatigue testing using these enlarged specimens under conditions where fish-eye fractures occur. When the results were compared with those using conventional specimens, specimen size effects were clearly observed: enlarged specimens showed lower fatigue strength due to appearance of larger inclusions in the fish-eye fracture origin.

Gigacycle Fatigue Properties of High-Strength Steels According to Inclusion and ODA Sizes

Gigacycle fatigue tests were conducted for several versions of JIS-SCM440 low-alloy and JIS-SUJ2 bearing steels using 20-kHz ultrasonic fatigue testing to elucidate the relationship of the inclusion size and type to fish-eye fracture properties. The total number of tested specimens was over 200. Most of the specimens revealed Al2O3 or (Cr, Fe)3C inclusion-originating types of fish-eye fractures, while TiN inclusions and the matrix also caused fish-eye fractures in some specimens. Based on these fatigue test results, 109-cycle fatigue limits were estimated according to inclusion size by resorting the obtained data points according to their inclusion sizes at the fish-eye fracture origin. The estimated fatigue limits revealed saturation when the inclusion sizes were smaller than 15 μm, while those fatigue limits depended on the inclusion sizes to the −1/6th power in the case of inclusions above 15 μm in size. The saturation of the 109-cycle fatigue limits was considered to be caused by the effects of the optically dark areas (ODAs). Moreover, the fatigue limits also depended on inclusion type. In comparing the Al2O3 and (Cr, Fe)3C inclusions, the key features causing the difference in the fatigue limits were likely to be bonding between the inclusion and the matrix, i.e., the (Cr, Fe)3C inclusions were tightly bonded to the matrix, unlike the Al2O3 inclusions, although both inclusions were of the hard type.

低合金鋼のギガサイクル疲労特性に及ぼす水素の影響

Gigacycle fatigue tests were conducted for hydrogen charged SCM440 low-alloy steel in order to investigate the effects of hydrogen on fatigue properties of high-strength steel. 200 and 550°C tempered samples were prepared and an ultrasonic fatigue testing machine was used for the fatigue tests. The base steel without hydrogen charging showed only fish-eye fractures in case of the 200°C tempered version, while the 550°C tempered version showed surface and fish-eye fractures in short and long life regions, respectively. On the other hand, the hydrogen charged steel of both versions showed only fish-eye fracture and the fatigue strength largely dropped. These results meant that the hydrogen effects were larger on the fish-eye fracture than on the surface fracture. However, no extension of ODAs was observed in the hydrogen charged steels, so the degradation of fatigue strength due to hydrogen had nothing to do with ODAs. Moreover, in this research, it was confirmed that the ultrasonic fatigue testing machine was effective in evaluating gigacycle fatigue properties of the hydrogen charged steel. The ultrasonic fatigue testing machine enabled us to finish the gigacycle fatigue test before the specimen lost a large amount of hydrogen due to emission, and no frequency effects were observed in case of fish-eye fracture even in the hydrogen charged steel.

On the fatigue properties of high-strength steels

The paper gives the results of an investigation of the optimal tensile strength level suitable for obtaining the maximum fatigue limit of a Mn-Cr-Ni-Mo steel produced by current technology with constant amount of alloying elements, but three different carbon contents, 0·21–0·41%, after conventional (CT) and thermomechanical treatment (TMT). The study was carried out in an effort to explain the considerable difference in the fatigue limit data of high strength steels published by different authors [1, 2, 3]. For a tensile strength of about 260 kp/mm2 the fatigue limits were found from 38 up to 116 kp/mm2. In addition to confirming the influence of the TMT on the tensile strength and ductility already known, it was found that the maximum of the fatigue properties for medium carbon content can be obtained in the tensile strength area of cca 220 kp/mm2. When the tensile strength is raised beyond this limit the fatigue limit falls. The values of the fatigue limit after CT as well as after TMT, but in comparison to the same tensile strength range, are very similar. The fatigue limits of about 90 kp/mm2 lie within the great range of other published data. Suggested causes for the decreased fatigue limit at high strength such as the method of testing, the shape of the test bar, the effect of ductility, the grain size and the level of internal stresses, are discussed and the idea is advanced that the main cause of the great differences is the increasing influence of the size of non-metallic inclusions as the strength increases. As a result, a preliminary hypothesis was made to explain the relation of the fatigue limit of steels to the size of non-metallic inclusions, referred to different limits of strength, based on the results of Stulen and co-workers [4]. By means of extrapolation the fatigue limit of a steel containing an inclusion of 0 μm diameter was also determined as a function of the tensile strength. This can be considered as the theoretical fatigue limit of the metal matrix itself, where the mechanism of the fatigue fracture is controlled by the fatigue phenomena in the matrix itself. Below this limit, on the contrary, the mechanism of the fatigue fracture is controlled by the presence of nonmetallic inclusions.

Low-Cycle Fatigue Properties of High-Strength Steel

Low-Cycle Fatigue Properties of High-Strength Steel

Fatigue properties of high-strength steels

Fatigue properties of high-strength steels