Boron segregation and precipitates were investigated using EFTEM. Chromium borides were identified. Comparison with published results suggests that segregation is mainly of non-equilibrium type. Non-uniform distribution of precipitates can be explained using nucleation theory if the segregation leads to grain-boundary boron concentration only slightly above the solubility limit.
The beneficial effect of boron on the mechanical properties of austenitic stainless steels is well documented [1,2]. A number of studies have confirmed that boron segregates to grain boundaries, in a variety of steels and nickel-base alloys (for example, [3,4,5]). Segregation of boron is known to occur via two mechanisms (for example, ):
Equilibrium segregation is caused by the high binding energy for boron at grain boundaries and other defects. This effect decreases with increasing temperature and depends strongly on the grain boundary structure.
Non-equilibrium segregation is caused by the dissociation of boron-vacancy complexes near the grain boundary which acts as a vacancy sink. In this case, the segregation effect increases as the starting temperature increases, and is believed to depend little on the nature of the grain boundary, as most are believed to be efficient vacancy sinks  (with the exception of twin boundaries, along which boron segregation is not observed).
The present study is concerned with segregation of boron and precipitation occurring in a modified type 310 austenitic stainless steel (20 wt% Ni, 25 wt% Cr), during quenching from high-temperature solution treatment. Although precipitation of borides or boro-carbides can be beneficial to creep properties, type 310 austenitic steel is primarily designed for use in highly corrosive environments. In this context, precipitation of borides or boro-carbides is undesirable. This is because, on one hand, the beneficial role of boron on hot workability is caused by boron in solid solution , and on the other hand, precipitation of chromium rich boro-carbides may result in sensitisation of the grain boundaries.
Precipitation of borides has not been systematically observed in boron containing steels [7,4], and it was therefore not evident that the results of the detailed investigation by Karlsson et al. [3,7,8,9] for a type 316 austenitic stainless steel (17 wt% Cr-13 wt% Ni) would apply to the type 310 investigated here.
All tubes had been solution-treated at 1433 K for 3 min, and cooled to 1333 K at about 10 K/s, then water-quenched (> 100 K/s).
For each tube, thin foils were prepared from three different areas, in some cases more than one foil of the same area was studied.
Microscopy was carried out in a FEI Tecnai F20-G2 fitted with a Gatan Imaging Filter. The acceleration voltage was 200 kV. Results are presented as jump ratio images (for a review of the EFTEM technique, see for example ). Details of the edges used, width of the energy window, etc. are given in Appendix. Composition profiles across the features of interest are shown that correspond to the length of selected areas in the images. These profiles are averaged over the width of the selected area. Being jump ratio images, the vertical axis of the profiles presented in figure 1, 3, 4 is unitless.
Four thin-foils were investigated for tube A. Of all the grain boundaries observed, only one was decorated with precipitates. One additional precipitate was observed at a triple point. Composition and morphology are presented in the next section, as they were found to be identical in all cases.
Precipitates were found in the four thin-foils observed for tube B, however on only a small fraction of the visible grain boundaries.
Figure 1 shows the composition maps for the triple point precipitate. The composition of other precipitates was identical. These maps clearly indicate a strong presence of chromium and possibly some iron. Maps for C, Mn, Mo did not reveal any re-distribution of these elements. Karlsson and Nordén  reported similar compositions for tetragonal M2B. Carbon enrichment was never observed in the precipitates.
Independent investigations  using energy dispersive X-ray analysis reported a subtitutional composition in agreement with the present results, with the exception of a significant Mo content: 73.5 Cr, 13.4 Fe, 1.5 Ni, 11.5 Mo.
Figure 2 shows a grain boundary decorated with precipitates in tube B. Localised, strong chromium depletion was systematically observed in the vicinity of the particles, as shown in figure 3.
The asymmetry in the chromium profile was observed for all precipitates, although with some variations in its amplitude. Similar observations have been reported in the case of M23C6 precipitation in 304, and attributed to the grain boundary migration occurring during growth  of the precipitate.
As discussed in the introduction, boron segregation can be of two types, which have different characteristics. In a study of type 316L, Karlsson et al. [3,8] reported mainly non-equilibrium segregation of boron for starting temperature (temperature from which the material is quenched) from 1348 to 1523 K, with cooling rates up to 530 K/s. Only lower starting temperatures (1073 K) and high cooling rates (600 K/s) resulted in equilibrium segregation.
Both values of cooling rates reported by KST fall clearly in a region where Karlsson et al. identified mainly non-equilibrium segregation. Independent experiments on the effect of starting temperature have confirmed the non-equilibrium nature of boron segregation in this steel .
Karlsson and Nordén  reported no precipitation for a cooling rate of 530 K/s and precipitates of only nm for 31 K/s. For a cooling rate of 13 K/s, the morphology and size of the precipitates were similar to those reported in the present study. This suggests that the precipitates observed in the present material most likely formed during the first part of the heat-treatment (1433 K to 1333 K at 10 K/s), rather than during the second part (water quenching from 1333 K). However, in contrast to the results reported in , the density of precipitates was not identical on all grain boundaries.
As non-equilibrium segregation is generally believed to affect similarly most grain boundaries, regardless of their degree of misorientation [3,13], it is not straightforward to explain the present precipitation behaviour. Karlsson et al.  suggested that grain boundary orientation might become the main factor controlling nucleation when the supersaturation in B is small. The present results support this hypothesis, particularly since boron segregation was observed on some grain boundaries otherwise precipitate free, as illustrated in figure 4.
To further assess the validity of this hypothesis, the nucleation rate
was estimated for the formation of M2B on grain
boundaries. Following the classical theory for nucleation
, the nucleation rate is given by:
The driving force for precipitation of M2B was calculated using the thermodynamic software MT-DATA  together with the NPL Plus and SGTE substances databases. It should be emphasised that the following discussion being qualitative, it would hold regardless of which exact boride (M2B, M3B2 or M5B3) is used in the calculation, a lower driving force would, for example, simply shift the curves to the right.
Figure 5 shows the nucleation rate on grain boundaries as a function of boron content, at 1433 K, for three different values of grain boundary energy. For bulk boron content (0.0027 wt%), M2B is not predicted to be stable. Clearly, at lower boron concentration, the nucleation rate sensitivity to the grain boundary energy is very large, while at higher boron concentrations, nucleation will happen on most grain boundaries with similar rates. This is entirely consistent with the observation made by Karlsson et al. that, for higher boron steels, precipitates are found with similar densities on all grain boundaries, while more irregularly distributed for lower boron content.
Note that the choice of 1433 K for the calculation is for simplicity. Given that significant growth occurs, nucleation must happen close to 1433 K in the temperature range 1333-1433 K, but cannot, in theory, happen at 1433 K where B segregation has not yet taken place. Choosing a different temperature would not affect this qualitative analysis in that the nucleation rate would remain very strongly dependent on at low supersaturation.
Comparison with published results suggests that the chromium borides form as a consequence of non-equilibrium segregation of boron to grain-boundaries, during the first part of the heat-treatment. Although this type of segregation is not affected by the nature of the grain boundaries, the non-uniform density of precipitation can be explained if the nucleation rate is considered: if the boron concentration is near the solubility limit, the nucleation rate is very sensitive to the grain boundary interfacial energy. The differences between grain boundaries blurs as the boron content is increased.
The observation of chromium borides on only a small fraction of grain boundaries can therefore be consistent with non-equilibrium segregation. In fact, segregation could be directly observed on some precipitate-free boundaries.
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