The effect of the nitrogen potential on the coarsening kinetics of VN precipitates
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I.
INTRODUCTION
THEwork on structural changes during nitriding of Fe-Ti 1'2 and Fe-V 3 alloys showed that the nitride precipitates are quite resistant to coarsening during annealing after nitriding. It was also demonstrated that the resistance to growth of TiN precipitates is affected by the furnace atmosphere. 2 In a recent publication4 it was reported that the thin platelets of molybdenum nitride precipitates coarsen at 600 ~ by a diffusion controlled process during annealing in vacuum following a hydrogen reduction treatment for two hours at 600 ~ The main equation for the nitriding of the present alloy as well as for the coarsening of the VN precipitates can be given in the following form: [V] + [N] = VN The same equation is also applicable to other iron alloys if V is replaced by the substitutional solum [M] by bearing in mind the stoichiometry of the nitride precipitate. [N] in the above equation is the nitrogen content in solution. This quantity can be kept constant by maintaining a constant nitrogen potential which can be defined by the following equation: r
=
PNH~
where PNH3 is the partial pressure of undissociated ammonia gas. Keeping a constant IN] content certainly imposes the composition constraint5 on the system during coarsening of VN precipitates. In the meantime a known [N] content allows one to calculate the [V] content in solution from the solubility product of VN according to the following equation: 6 Ks
= [pct V] fN [pct N]
The temperature dependence of Ks is given6 as log Ks-
7830 + 2.45 T
If we choose T = 600 ~ as the nitriding temperature, the value of the solubility product will be Ks = 3.1 • 10-7. O.E. ATASOY is Lecturer and Associate Professor, Metallurgical Engineering Department, Middle East Technical University, Ankara, Turkey. Manuscript submitted January 27, 1982. METALLURGICAL TRANSACTIONS A
Such a low value for Ks indicates that the value of IV] left in solution after nitriding is less than 0.1 pct. Then the activity coefficientfN in the equation for Ks can be approximated to unity6 when using that expression for the coarsening of VN precipitates during high temperature annealing.
II.
EXPERIMENTAL
The Fe-l.06 pct V alloy used in this work was supplied by BISRA as hot rolled bars of 12 mm diameter. The carbon content of the alloy was 0.01 pct. The bars were further hot rolled to about 1 mm thickness. About 20 mm long rectangular specimens were cut from the hot rolled strip and were then nitrided at 600 ~ in pure NH3 gas for four hours which was sufficient for the complete nitriding7 of the whole section. The specimens were mechanically ground on both faces to a thickness of 250/zm. This grinding operation also eliminated the Fe4N phase which forms at the surface and may extend to some 25 to 50/xm depth if nitriding is carried out in undiluted NH3 gas. The ground specimens were annealed in NH3/H: mixtures at 820 ~ for varying times. Both gases were metered by capillary flowmeters. The gas mixtures were analyzed at the outlet and at the inlet for NH3 content by the titrat
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