Effect of Nitrogen Addition on Microstructure and Mechanical Properties of As-Cast High-Chromium Cast Iron Containing Ni
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ORIGINAL ARTICLE
Effect of Nitrogen Addition on Microstructure and Mechanical Properties of As-Cast High-Chromium Cast Iron Containing Niobium Mervat M. Ibrahim1
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Khaled M. Ibrahim1
Received: 9 September 2019 / Accepted: 26 June 2020 The Indian Institute of Metals - IIM 2020
Abstract This work was conducted to study the effect of four different nitrogen additions on as-cast microstructure, hardness and impact toughness as well as abrasion wear resistance of high-Cr cast iron containing niobium. The results showed that as nitrogen content increased, the austenitic dendrites matrix became less elongated and the eutectic M7C3 carbides were gradually refined in size. EDS analysis demonstrated that N element preferentially accumulated in the NbC particles and substitute part of C to form Nb nitrides (NbN) or Nb cabonitrides Nb(C,N). Such nitrides or carbonitride particles were relatively hard compared to matrix and eutectic carbides resulting in an increase in hardness with increasing N content. The optimum toughness (12 J) was obtained for the iron containing 0.093 wt%N with an enhancement of 33%. The wear resistance increased with increasing N addition, in which the maximum wear resistance was reported for the alloy containing 0.15 wt% N. There was an enhancement in wear resistance of 47% by adding 0.15 wt%N to HCCI at low applied load (30 N), and it decreased to 17.5% at high applied load (60 N). Hence, the highest wear resistance and moderate toughness of HCCI could be achieved by adding 0.15 wt%N. Keywords High-chromium cast iron Microstructure Wear resistance Impact toughness Nitrogen and niobium
& Mervat M. Ibrahim [email protected] 1
Central Metallurgical Research and Development Institute, (CMRDI), P.O. Box 87, Helwan-El-Tebbin, Cairo, Egypt
1 Introduction High-Cr cast iron (HCCI) is widely used for manufacturing spare parts for mining, minerals and cement industry due to its excellent abrasion resistance and relatively low cost [1, 2]. The superior abrasion resistance of HCCI is related to its microstructure which contains a network of hard eutectic M7C3 carbides in a mainly soft austenitic matrix [3–6]. The presence of coarse M7C3 carbides in the matrix of HCCI leads to decreases in the toughness. Therefore, modification of structure and refinement of eutectic carbides are significantly important for enhancing toughness of HCCI and help to widen its application [7]. Enhancing impact toughness as well as wear resistance can be achieved by adding strong carbide-forming elements such as Nb, Ti, V, Mo and W [6–11]. These carbide-forming elements are added to refine the structure and precipitate other carbides with a high hardness and more favorable morphology. Due to the high cost of these alloying elements, researchers are just beginning to study the effect of adding nitrogen as an alloying element in HCCI alloys [12, 13]. Nitrogen is quite similar to carbon where it is considered as an interstitial element and may be kept in solid solution in matrix and precipitates as nitrides or carbonitr
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