Precipitation of aluminum nitride in a high strength maraging steel with low nitrogen content G. Jeanmaire a,b, ⁎, M. Dehmas a , A. Redjaïmia a , S. Puech b , G. Fribourg c a Institut Jean Lamour, UMR 7198 CNRS-Université de Lorraine, 54011 Nancy Cedex, France b Aubert & Duval, BP1, 63770 Les Ancizes, France c Snecma Gennevilliers, 171 Boulevard de Valmy-BP 31, 92702 Colombes, France abstract article info Article history: Received 11 August 2014 Received in revised form 31 October 2014 Accepted 1 November 2014 Available online 4 November 2014 Keywords: Maraging steel Aluminum nitride Precipitation Dissolution Quantification In the present work, aluminum nitride (AlN) precipitation was investigated in a X23NiCoCrMoAl13-6-3 maraging steel with low nitrogen content (wt.% N = 5.5 ppm). A reliable and robust automatic method by scan- ning electron microscopy observations coupled with energy dispersive X-ray spectroscopy was developed for the quantification of AlN precipitates. The first stage was to identify the solvus temperature and to develop a heat treatment able to dissolve the AlN precipitates. The experimental determination of equilibrium conditions and solvus temperature show good agreement with ThermoCalc® simulation. Then, from this AlN-free state, the cooling rate, isothermal holding time and temperature were the subject of an intensive investigation in the aus- tenite region of this maraging steel. In spite of the high temperatures used during heat treatments, the growth kinetic of the largest AlN precipitates (N 1 μm) is slow. The cooling rate has a major effect on the size and the num- ber density of AlN due to a higher driving force for nucleation at low temperatures. At last, quenching prior to iso- thermal annealing at high temperatures leads to fine and dense AlN precipitation, resulting from the martensite to austenite transformation. Experimental results will be discussed and compared with kinetic data obtained with the mobility database MobFe2 implemented in Dictra® software. © 2014 Elsevier Inc. All rights reserved. 1. Introduction At a time when kerosene costs more and more, airlines are seeking to minimize fuel consumption, and aircraft and engine manufacturers are putting strong efforts to develop competitive solutions for this pur- pose. Concerning the turbofan engines, one of the most efficient ways to realize this goal is to increase the bypass ratio, and to increase the torque transmitted from the turbine to the fan by the driveshaft. This can be done by improving the mechanical properties of the current ma- terials used for the turbine shaft. To this end, new maraging steels have been developed and patented. Their improved mechanical properties are coming not only from the aging of martensite, but also from the hardening induced by the co-precipitation of the intermetallic phase, B2-NiAl, and the secondary carbides M 2 C [1]. Since, nitrogen cannot be completely avoided in these maraging steels using current melting technologies such as VIM (Vacuum Induction Melting) or VAR (Vacuum Arc Melting), precipitation of nitrides may occur during solidification and/or heat treatment [2]. When the material is subjected to cyclic load- ing, several fracture surfaces of test pieces showed that cracking is initi- ated on AlN precipitates with a large size. These large precipitates affect the fatigue lifetime of any parts subjected to cyclic loading, particularly the driveshafts. Thus, one way of improvement of the fatigue lifetime goes through size reduction of these AlN precipitates. The aim of this paper is to meet this challenge. When the alloy contains aluminum, combined with the lack of tita- nium or niobium, AlN precipitation occurs in the austenitic or ferritic re- gions. Several investigations report a fine precipitation (≪1 μm) with a high number density of nitride particles for steels containing between 30 ppm and 300 ppm nitrogen [3]. This precipitation is known to have significant effects upon recrystallization and austenite grain growth [4]. The austenite grain size directly affects the mechanical and techno- logical properties including hot ductility and deep drawability [5,6]. Al- though aluminum nitrides are beneficial in restricting grain growth, they can cause embrittlement and induce cracking phenomena such as rock candy fracture during continuous casting and hot rolling [7–9]. Depending on the Al and N content as well as the thermomechanical processing route, AlN precipitates can adopt various morphologies: cu- boids, rods, large plates or spherical particles. A summary of the differ- ent morphologies of AlN in steels was made by Wilson and Gladman [10]. From a crystallographic point of view, most authors report a hex- agonal structure for the AlN precipitates. However, Massardier et al. [11] observed that the precipitation sequence can be more complex. In- deed, in the early stage of precipitation for low carbon aluminum-killed steel, the precipitation seems to start from a cluster of chromium atoms Materials Characterization 98 (2014) 193–201 ⁎ Corresponding author at: Institut Jean Lamour, UMR 7198 CNRS-Université de Lorraine, 54011 Nancy Cedex, France. E-mail address: guillaume.jeanmaire@univ-lorraine.fr (G. Jeanmaire). http://dx.doi.org/10.1016/j.matchar.2014.11.001 1044-5803/© 2014 Elsevier Inc. All rights reserved. Contents lists available at ScienceDirect Materials Characterization journal homepage: www.elsevier.com/locate/matchar