Mechanism of TiN Fracture During the Tensile Process of NM500 Wear-Resistant Steel
WU Xiang,ZUO Xiurong(),ZHAO Weiwei,WANG Zhongyang
Key Laboratory of Material Physics, Ministry of Education, Zhengzhou University, Zhengzhou 450052, China
Cite this article:
WU Xiang,ZUO Xiurong,ZHAO Weiwei,WANG Zhongyang. Mechanism of TiN Fracture During the Tensile Process of NM500 Wear-Resistant Steel. Acta Metall Sin, 2020, 56(2): 129-136.
Low-alloy high-strength martensitic wear-resistant steel has been widely used in the field of construction machinery due to its low cost and excellent mechanical properties. Microalloying elements, especially Ti, B and other elements, have been widely used to improve the performance of low carbon steel. However, addition of Ti will cause micron-sized Ti precipitates in the continuous casting process, causing cleavage fracture. Therefore, it is necessary to study the micron-sized TiN to reduce its influence on the toughness of the material. SEM, EDS, TEM and EBSD methods were combined with thermodynamic theory to study the precipitation rule of micron-sized TiN in NM500 wear-resistant steel, the fracture mechanism and the influence of matrix on the fracture mechanism. The results show that the tensile fracture mechanism of NM500 steel is mixed mode. There are two fracture morphology of micron-sized TiN on fracture surface: TiN is on the fracture surface, being on the tear ridge; TiN is at the bottom of a deep dimple. The Ti element in the steel precipitates at high temperature and forms a large number of micron-sized TiN. There are three kinds of fracture mechanisms in TiN when subjected to tensile stress: A single crack appears in TiN initiates and spreads to the matrix; A single crack appears in TiN initiates but stops at the matrix; A plurality of cracks are generated in the TiN, and the crack stops at the base, with the TiN shape being preserved intact. There are high strain zones and micron-sized TiN in NM500 steel, and the prior austenite grains are coarse. When the TiN cracks, the matrix has a poor ability to arrest the cracks, then the crack can extend on the substrate easily. When a plurality of TiN clusters are formed, the cracks are connected into one piece to be a weak band, leading to a poor plasticity to the steel.
Fig.1 SEM images of mixed mode fracture (a), multi-precipitates fracture (b), TiN at the fracture surface (c) and TiN at the bottom of the deep pit (d) in the tensile fracture of NM500 steel specimen
Fig.2 EDS analysis of precipitate in tensile fracture of NM500 steel specimen
Fig.3 SEM images of fracture profile (a), TiN on grain boundaries (b) and TiN among grain (c) after corrosion of NM500 steel specimen (PAGBs—prior austenitic grain boundaries)
Fig.4 SEM images of TiN precipitates on the uncorroded fracture profile of NM500 steel specimen(a) TiN of the cluster near the tensile fracture (b) TiN with a large crack (c) TiN with a big hole(d) TiN with several cracks (e) cluster-like TiN of different sizes (f) TiN with several small cracks(g) TiN with a small hole (h) TiN with two small cracks (i) TiN with a intact shape
Fig.5 TEM image of nano TiN in NM500 steel
Fig.6 Mass fraction of phases as a function of temperature in NM500 steel at thermodynamic equilibrium state (a) and the relationship between temperature and Ti content in austenite (b)
Fig.7 Schematics of changes in micron-sized TiN during tensile process of NM500 steel(a) TiN with a long and wide crack(b) TiN with a big hole(c) TiN with several cracks
Fig.8 Local misorientation distribution maps of a quarter of thickness (a) and center of thickness (b), and quantitative analysis of local misorientation in a quarter of thickness and center of thickness (c) (The blue color indicates misorientations less than 1°, green between 1° and 2°, yellow between 2° and 3°, orange between 3° and 4°, and red between 4° and 5°; the high-angle grain boundaries (＞15°) were delineated in black solid lines)
Fig.9 The image quality (IQ) maps of microstructure types of a quarter of thickness (a) and center of thickness (b), and quantitative analysis of distribution of misorientation angle of grains in a quarter of thickness and center of thickness (c) (θ means the angle of boundary, black line 15°≤θ≤50°, red line θ＞50°)
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