The document studied the effect of nitrogen on precipitation in an austenitic stainless steel used in orthopedic implants. It found that high nitrogen content led to precipitation of niobium nitrides in all annealing temperatures due to nitrogen's higher affinity for niobium than chromium. Two types of nitrides, chromium nitride and niobium nitride, were identified along with intermetallic phases Z and χ containing chromium, niobium, and molybdenum. The significant presence of nitrogen and niobium in the steel composition drove precipitation of these phases.

1. EFFECT OF NITROGEN ON PRECIPITATION OF THE AUSTENITIC STAINLESS
STEEL ISO 5832-9 – UTILIZED IN ORTHOPAEDIC IMPLANTS
Silva Neto, O. V. (1) and Tokimatsu, R. C. (1)
(1) Departamento de Engenharia Mecânica – Faculdade de Engenharia de Ilha Solteira - Universidade
Estadual Paulista; Fone: (18)37438138 - Avenida Brasil, 56 – CEP 15385-000 – Ilha Solteira-Brasil.
The austenitic stainless steels can contain in its microstructure precipitates or embrittling phases. The
amount, size, distribution and shapes of those phases influence the material properties strongly. The
main problem is related to carbides formation of the type M23C6 on grain boundary, reducing the
corrosion resistance locally and favouring the intergranular corrosion. The addition of elements having
larger affinity by carbon than chromium, together with carbon content decrease, reduces the sensitization
problem [1]. High contents of nitrogen are added to austenitics stainless steels with the purpose of
attributing larger mechanical and corrosion resistance [1, 2, 3]. However, the nitrogen can cause the
precipitation of phases in which it is not very soluble and delay the carbides precipitation, because it
reduces the diffusion coefficient of carbon [1].
In the present work it was studied the kinetics of precipitation of the ISO 5832-9 austenitic stainless
steels. The good properties attributed to this steel, used as orthopaedic implants, are originated of high
content of nitrogen (59,31Fe-22,60Cr-11,00Ni-0,29N) in him present. The studied material was
submitted to the thermal and mechanical treatments. The accomplished thermal treatment was the
annealing in 600, 700, 800 and 900ºC, in all the cases the samples were maintained in oven by 24 hours.
To obtain different deformation rates, stretched samples were made, resulting in conditions of 10 and
20% of deformation. Microstructural investigations were performed in a transmission electron
microscopy (TEM) of type Philips CM 120, 120 kV, provided with na EDX detector. Thin foil specimens
were prepared by electrochemical polishing in a solution of 5% perchloric acid and 95% of acetic acid.
In the cold worked conditions the samples contains a high density of dislocations (Fig. 1(a)). In the
annealed conditions showed intergranular precipitates forming at triple grain boundaries, as shown in
Figure 1(b). All the temperatures of annealing resulted in the high amount of coarse precipitates,
specially niobium nitride. The increase of the temperature showed more favorable of precipitation; the
800 and 900°C conditions was the more critical as to amount of inclusions and precipitate morphologies.
Two types of nitrides was found in the studies conditions; the cromium nitride Cr2N and niobium nitride
NbN (see the Figures 2(a and d) and 2(b and e)). The presence of N and of Nb in high amount, exerted
strong influence on the precipitation of the steel ISO 5832-9. In all the cases they were identified very
rude particles. The presence of chromium nitride of type Cr2N was confirmed in condition as received,
solubilized after hot rolling (Fig. 2(a)). The tetragonal Z-phase was found in all annealing temperatures.
This phase is a complex cromium-niobium nitride, which can be compound for other elements just as Fe,
Mn and Mo [4], Fig. 2(c and f). Other intermettalic phase present in the annealing samples was the phase
χ (Fe-Cr-Mo), its presence could be evidenced by the X-ray diffraction, as shown in Figure 3.
The great contents of N and Nb generated niobium nitrides precipitated in all the temperatures of
annealing. The precipitation of intermettalic phases Z and χ occured due the strong presence of Nb and
Mo in the compound of steel studied, respectively. The bigger affinity of Nb to the N, compared to the
Cr, resulted in small amounts of cromium nitride.
[1] PADILHA, A.F., GUEDES, L.C. Aços inoxidáveis austeníticos – microestrutura e propriedades.
São Paulo: Hemus Editora, 1994. p.170.
[2] ÖRNHAGEN, C. et al. Characterization of a nitrogen-rich austenitic stainless steel used for
osteosynthesis devices. Journal of Biomedical Materials Research, v.31, p.97-103, 1996.
[3] SPEIDEL, M.O., PEDRAZZOLI, R.M. High nitrogen stainless steels in chloride solutions.
Materials Performance, v.31, n.9, p.59-61, Sep. 1992.
[4.] JACK, D.H. and JACK, K.H. Structure of Z-Phase, NbCrN. p.790-792, 1972

2. Figure 1. (a) Transmission
electron micrograph of cold
worked material showing the
typical morphology of
dislocations.
(b) Optical micrograph –
precipitates at triple points
and at grain boundary.
(a) (b)
Cr2N NbN
Z
(a) (b) (c)
(d) (e) (f)
Figura 2. (a, b, c) Transmission electron micrograph. (d, e, f) Electron diffraction pattern.
(a) and (d) used to identify cromium nitride Cr2N, (b) and (e) to identify niobium nitride NbN and
(c) and (f) used to identify Z-phase.
Figura 3.
(a) Transmission
χ electron micrograph
and (b) Electron
diffraction pattern of
material annealed at
700 oC - used to
identify χ-phase.
(a) (b)