Fatigue. Effects of Prestraining on High Cycle Fatigue Strength of High Strength Low Alloy TRIP Steels.

Abstract

The effects of prestraining on high cycle fatigue strength of newly developed low alloy TRIP steels with different matrix structure and different retained austenite characteristics were investigated for the automotive applications. The prestraining to 10% in tension increased fatigue limit of the TRIP steels, especially in steel with polygonal ferrite matrix. It was considered that the polygonal ferrite matrix in the steel brought high fatigue limit mainly due to TRIP of the retained austenite and high compressive internal stress in the matrix, resulting from hard second phase on prestraining. On the other hand, the TRIP steel with bainitic ferrite lath matrix exhibited a small increment in fatigue limit after prestraining. This was supposed to be mainly owing to strain-induced martensite content increased by prestraining, with small effects of TRIP of retained austenite and compressive internal stress in the matrix. In addition, a very interesting finding was obtained that the internal stress is the most effective parameter among some parameters to increase the fatigue strength in the low alloy TRIP steels.

An Overview of Fatigue Strength of Case-Hardening TRIP-Aided Martensitic Steels

“…It is very difficult to compare the fatigue limits of TM steels with other AHSSs because fatigue tests are not conducted under the same fatigue mode and stress ratio. However, from comparison of several references [1,4,6,7,9,27,28,41,42,[44][45][46][47][48], we can conclude that TM steels possess the high fatigue limits compared to the conventional quenched and tempered martensitic steels, in the same In general, fatigue limits of smooth and notched specimens are principally controlled by fatigue crack initiation and propagation stages, respectively. Figure 10a shows a SEM image illustrating a fatigue crack initiated on the surface of a notched specimen of TM steel [13].…”

“…It is very difficult to compare the fatigue limits of TM steels with other AHSSs because fatigue tests are not conducted under the same fatigue mode and stress ratio. However, from comparison of several references [1,4,6,7,9,27,28,41,42,[44][45][46][47][48], we can conclude that TM steels possess the high fatigue limits compared to the conventional quenched and tempered martensitic steels, in the same way as several AHSSs such as TBF steels [1,4,6,7,9], Q&P steels [27,28] and nanostructured bainitic steels [41,42,[44][45][46][47][48][49].…”

“…The maximum roughness height (Rz, JIS B0601:2001) on the surface of both steels is less than 4 µm because of the use of the small shot particles (steel ball of 70 µm diameter) (Figure 11a,c). way as several AHSSs such as TBF steels [1,4,6,7,9], Q&P steels [27,28] and nanostructured bainitic steels [41,42,[44][45][46][47][48][49].…”

“…Recently, four types of transformation-induced plasticity (TRIP)-aided steels with different matrix structures and different chemical compositions, namely "TRIP-aided bainitic ferrite (TBF) steel" [1][2][3][4][5][6][7][8][9][10], "TRIP-aided martensitic (TM) steel" [11][12][13][14][15][16][17][18][19][20][21], "quenching and partitioning (Q&P) steel" [22][23][24][25][26][27][28][29][30][31], and "medium Mn TRIP-aided steel" [32][33][34][35][36][37][38][39] have been developed as third-generation advanced high-strength steels (AHSSs) for automotive sheet products. These steels possess high toughness, fatigue strength and delayed fracture strength, as well as excellent cold formability, especially in TM steels [7,11,[16][17][18][19][20].…”

Effects of Matrix Structure and Nitrogen Content on Fatigue Properties of Ultrahigh-Strength Low Alloy TRIP-Aided Steels

“…It has been reported that the behavior of fatigue crack propagation in TRIP-aided steels depends on (ⅰ) the plastic deformation resistance of the matrix and transformed martensite and (ⅱ) the relaxation of stress concentration due to the strain-induced transformation of the retained austen- ite. [21][22][23] Regarding (ⅰ) the plastic deformation resistance of the matrix, a high crack propagation resistance might be obtained in the TM and the TBM steels. This occurs because the TM steels with martensite matrix and the TBM steels with martensite-bainitic ferrite matrices exhibited higher TS than the TBF steels with bainitic ferrite matrix.…”