Valores institucionales

transition was associated with the appearance of large numbers of

streamers in close proximity to one another (Plate 161).

The graphitic areas at the ’grey' start (Plate 162) were

generally larger than those observed in Bl' or Al, but their shape was

similar, in general, to those illustrated in' Plate 40. At 63.5mm, the

large streamer 'colonies' shown in Plate 161 were observed. The

graphite within the streamers and the 'grey' areas at this distance

was mainly of the type illustrated in Plates 162 and 163 but isolated

flakes of what appeared to be 'A' type graphite were also noted.

The cell start comprised 'A' type flakes in a mixed ferritic/

pearlitic matrix. The ferrite was not confined to the cell centres

but tended to be more randomly distributed throughout the cell and,

flakes. In the ledeburitic areas, the graphite within the streamers

remained similar to that shown in Plates 162 and 163 though slightly

finer. At 89mm,the ledeburitic areas still predominated. In the

graphitic eutectic areas the 'A' type flakes had increased in size,

but remained associated with the mixed ferri tic/pearli tic matrix, whilst in the streamers networks of degenerate graphite flakes were

observed (Plate 164). Similar structures were seen at 91 and 127.1mm,

the graphitic eutectic areas comprising about 50% of the cross-section

of the casting at this distance.

At 177.8mm and 181mm, the streamer graphite exhibited open

' chain-like' structures similar to that in Plate 121. The more exten­

sive areas of graphitic eutectic contained mainly 'A' type flakes

in a pearlitic matrix (Plate 165) although small concentrations of 'D'

type flakes were also observed. The small amount of ferrite present

tended to be associated with individual graphite flakes.

Melt B 3 1 (3.65%C, 0.115%S, 0.090%Ti, 0.009%N)

The fracture (Plate 139) and the microstructural criteria

(Table 22) indicate that this casting exhibits a graphitic structure

over most of its length. The graphite at the streamer start was

similar, in size and form (at X16 magnification), to that shown in

Plate 164 (at X32 magnification), although, ittended to be more

obviously confined between the directionally aligned dendrite a r m s .

At 25.4mm,the graphite appeared much finer as shown in Plate 166.

The graphitic areas at the 'grey1 start took the form of broad

streamers similar in general appearance to that in Plate 147 but, the

internal graphitic structure was much finer. In general there were

areas degenerate flake-like forms of graphite, of the type shown in

Plate 150, were observed. Two quite extensive graphitic areas of an

unusual form were noted at 63.5mm (Plates 167 and 168). The graphite

in these areas appeared as a continuous network between the dendrite

arms with the internal structure of the primary dendrites appearing

to be graphitic also.

Two cells were identified at the cell start, the remaining

structure (about 75% of the cross section) being ledeburitic. The

cells contained compacted ’D 1 type flakes in a pearlitic matrix.

The pearlitic 'dendrite' arms tended to be surrounded by a dense

network of compacted flakes and, in some areas, the dendrite

arm extremities were again ferritic (Plate 169). By 127.lmm, the

graphitic cells had increased in size to occupy about 80% of the

cross-section^ the remaining ledeburite tending to outline the cell

boundaries. 'D' type flake graphite was identified in both the

streamers and in the cellular areas which also exhibited 'A' type

flakes (Plate 170). Slightly more ferrite was present at this

level than is suggested by Plate 170 and more was present at 177.8mm,

though ferrite remained a very minor component of the structure. At

177.8mm and 185mm,the areas of graphitic eutectic again constituted

about 80% of the cross-section. In these areas, the graphite mainly

resembled that shown in Plate 169 with.some 'A' type flakes compar­

able with the larger flakes shown in Plate 170. In the ledeburitic

areas large numbers of inclusions were noted especially associated

with the cementite; the graphite in the streamers appeared as net­

Melt C l 1 (3.60%C, 0.02%S, 0.12%Ti, 0.009%N)

The fracture (Plate 139) suggests that the lower third of the

casting has a white iron structure, the middle mottled and the upper

third has a cellular graphitic structure. Compared with Al' and

Bl', the fracture appears generally more graphitic. This is refl­

ected by the streamer, ‘g r e y ’ and cell starts (Table 22) which all

appear earlier than in Al' or Bl'.

The general form of the graphitic areas constituting the

'grey' start, and the size and type of graphite present, was the

same as that observed in B3'. The 'cell start1, an isolated cell

at 63.7mm, contained graphite very similar to that in the streamers

at the same distance as that shown in Plate 166. At 89mm,the

structure was still predominantly ledeburitic, with particularly

large pearlitic 'dendrites'. The graphite within the streamers was

again similar to, but coarser, in general, than that shown in

Plate 166, whilst the cellular areas exhibited a structure similar

to but containing less ferrite than in Plate 169. The graphite

in the cellular areas at 93mm was again like that in Plate 169 but

the amount of ferrite present had increased. At 127 ..lmm the

graphite in the ledeburitic areas was mesh type,(Plate 172) whilst

the graphitic eutectic areas exhibited small compacted flakes

(Plate 173). Even at this distance the ledeburitic areas con­

stituted 60% of the cross-section.

With increasing distance, the amount of ledeburite decreas­

ed until, at 177.8mm, it constituted only about 20% of the cross-

section. In the graphitic eutectic areas, flake-like forms similar

Plate 148, were observed. A typical area is shown in Plate 174.

In the ledeburitic areas, mesh graphite (ranging in form from

that shown in Plate 121 to that in Plate 172) was observed, togeth­

er with isolated compacted flakes of similar size and form to those

illustrated in Plate 148.

Melt C 2 ' (3.70%C, 0.05%S, 0.112%Ti, 0.010%N)

The resultant fracture had, generally, a more graphitic

appearance than C l 1 (Plate 139). In terms of the criteria listed

in Table 22 however, it appears similar, in terms of the degree

of graphitisation, to Cl* and B 2 1.

Streamer graphite was first observed at 12.7mm, the graphite

within the streamers being similar in general form to, but much

finer than, that shown in Plate 164. At the 'grey' start, a

variety of graphite forms were identified including a much finer

version of that shown in Plate 162, and forms similar, in shape

and size, to those in Plates .164 and 167. The graphitic areas

at the 'grey' start generally took the form of thickened

streamers, in common with most of the high nitrogen casts (Plate

157). However, triangular 'grey* areas, more common in the low

nitrogen melt programme (Plate 40) , were observed at 63.5mm and

In document Mujeres y relaciones de género en el poder azul: el caso de la Policía Metropolitana de Quito. (página 89-98)