4.08.3.3.1 Continuous cooling transformation diagram

The preparation of a CCT (continuous cooling trans­formation) diagram is essential to the microstructure

control of9Cr-ODS steels. Figure 12 exhibits a CCT diagram that was experimentally constructed for 9Cr-ODS steel.21 The minimum cooling rate for the matrix phase in order to fully transform to martensite is extremely higher in 9Cr-ODS steel (solid circular symbol) than in mechanically milled EM10 (open diamond symbol) that does not contain added Y2O3. Residual ferrite plays an important role in the process of continuous cooling transformation. The minimum cooling rate is known to increase with a decrease in the size ofprior austenite (g) grains. This smaller size of prior g grains provides more nucleation sites (grain boundaries) for a g-a-phase transformation, so that a higher cooling rate is required to enable steel with small prior g grains to fully transform to a. The presence of residual ferrite restricts the growth of g grains; the prior grain size of residual ferrite-containing steel is roughly 5 pm, thus increasing the minimum cooling rate to produce a full martensite matrix.

In steel that does not contain residual ferrite and the mechanically milled EM10, the size of the prior g grains is roughly 10 pm and 35 pm, respectively. The results shown in Figure 12 can be explained by the relationship between the size of prior g grains and the minimum cooling rate.21 As for the normal­izing heat treatment used in commercial furnaces, the cooling rate would be roughly 3000 °Ch~ , so that

Time from 800 °C, t (s)

Cladding tube At intermediate heat Cold rolling

treatment (pilger mill)

At final heat treatment

Figure 13 Cladding tube manufacturing process developed for 9Cr-ODS steel.

the matrix phase of 9Cr-ODS steel cladding consists of residual ferrite, martensite, and a small amount of transformed ferrite from the g-phase.

4.08.3.3.2 Manufacturing process

9Cr-ODS steels are promising materials to enable fast reactor fuel cladding to realize a high burnup of 200GWdt-1 at 700 °C, since they have superior radiation resistance and high temperature strength. Figure 13 shows a series of manufacturing processes of fuel cladding that is 8.5 mm in diameter by 0.5 mm in thickness by 2 m in length. The element powders and yttria powder are mechanically alloyed for 48 h in an argon gas atmosphere using an attrition type ball mill with a capacity of 10 kg batch. The mechanically alloyed powders are sealed in hollow-shaped cans and degassed at 400 °C in a 0.1 Pa vacuum for 2 h. The hollow shape of the bars is consolidated by hot — extrusion at an elevated temperature of 1150 °C to the dimensions of 32 mm in outer diameter, 5.5 mm in wall-thickness, and 4 m in length. After machining to the precise dimensions, claddings are produced at their final dimension (8.5 mm in outer diameter, 0.5 mm in thickness, and 2 m in length) by four-pass rolling with about a 50% reduction ratio on each pass by using a pilger mill.

Without heat treatment, it is too difficult to man­ufacture cladding for ODS steels by the cold-rolling process. Using the CCT diagram of 9Cr-0.13C-2W — 0.2Ti-0.35Y2O3, as shown in Figure 12, a cooling

rate of about 150 K h-1 was applied to the intermedi­ate heat treatment in order to induce the ferrite phase at room temperature without martensite transforma­tion. This phase has a lower degree of hardness. Hard­ened cladding due to the accumulation of cold deformation can be sufficiently softened by this inter­mediate heat treatment, and cold rolling can then be continued with the softened ferrite structure. Figure 14 represents the typical hardness change of 9Cr-ODS steel in the process of cladding manufacturing by repeated cold rolling and intermediate heat treatment. The elongated grain structure induced by the fourth cold rolling can ultimately be made into equi-axed grain structure by the final heat treatment, which
consists of normalizing at 1050 °C for 1 h, followed by tempering at 800 °C for 1 h.