**3. Effect of thermomechanical control processing on microstructure**

As it has been introduced in previous sections, the pioneer commercial 9-12Cr steels present an upper service temperature of 540°C, which was successfully increased in the late 1970s up to 595°C with the introduction of vanadium and niobium microalloying in the composition of the steel. This steel was used as a benchmark for the development of steels with upper-use temperatures of 600–620°C.

However, it is difficult keep pushing the higher operating temperature too much. Therefore, to continue to exploit the advantages of ferritic steels, oxide dispersionstrengthened (ODS) steels [43–46] were introduced. The first successful alloy was presented in the 1960s, and, since then, it has been an active research field. ODS steels are strengthened by small oxide particles, but the complicated and expensive manufacturing route avoided the full implantation as structural material in the current power plants.

Despite being around for about 40 years, the ODS steels are still in the development stage because of having mechanical property anisotropy [43, 45, 47–49]. Therefore, an alternative strategy to achieve a high number density of precipitates is needed. In this section, we present preliminary results that allow us to conclude that conventional thermomechanical control processing strategy is adequate to achieve dispersion-strengthened steels.

#### **3.1 Microstructure after conventional heat treatment**

Lath martensite is a particular microstructure that ensures microstructural stability. Furuhara and Miyamoto [50] described the variety of crystalline size in lath martensite structures. A hierarchy of lath martensite structure is clearly identified particularly in low-carbon steels. A prior austenite (γ) grain is divided into "packets," each of which consists of a group of martensite laths with the same

*Welding - Modern Topics*

sion strengthening.

**2.3 Creep tests**

about a creep mechanism.

**2.2 Creep and microstructural evolution**

The new environmental regulations and commercial needs of the industry are the driving force for the development of new heat-resistant steels that push forward the operational limits of current steels. In this framework, the high-Cr FM steels applied as structural materials in fossil-fired and in nuclear power plants need to implement the operating temperatures above 650°C [1–3, 8–12]. The mechanism responsible for creep strengthening in these steels is the solid-solution and disper-

In the particular case of the so-called 9Cr FM steels, the creep degradation is a consequence of the thermal evolution of their hierarchal martensitic microstructure constituted by prior austenite grains, martensitic packets, blocks, and laths [24]. The microstructural degradation during creep consists of the coarsening of the lath structure [12, 24]. Such coarsening is governed by the subgrain boundary formation and evolution inside the laths, which can be prevented at high temperatures, and virtually frozen, by the dispersion of proper precipitates. The precipitates pin boundary migration and dislocation motion, slowing down the degradation of the

There are two main actors for the microstructural stability driven by precipitation in 9Cr FM steels: The first one is the coarse M23C6 carbides located mainly at the grain boundaries either from the prior austenitic grains or from the blocks or martensite lath boundaries. The second one is the V- and Nb-rich MX carbonitrides. Contrary to M23C6, those MX precipitates are homogeneously distributed within martensite laths. Therefore, the ideal situation would consist of reducing the presence of the M23C6 carbides to the minimum since their fast coarsening induces crack formation at the particle-matrix interface and promoting the formation of MX carbonitrides (nanometric in size), since they will delay the lath coarsening as

Thermomechanical processing of 9Cr FM steels has been revealed as a promising tool to promote a high number density of MX carbonitrides [30–38]. TMT involves different steps that need to be optimized to produce the most favorable

The creep behavior of a material may be characterized by a number of different parameters which can be measured by performing the appropriate creep test. For metallic materials most creep tests are conducted in uniaxial tension with a dumbbell-shaped specimen similar to that used for tensile testing. The tests are carried out at a constant temperature and under either a constant load or stress. Applying a constant stress is more useful if the test is being employed to provide information

The conventional treatments (AR) and TMT considered in this work were carried out on 10 mm in length and 5 mm in diameter cylindrical samples using a DIL 805A/D plastodilatometer (TA instruments) as described elsewhere [39, 40]. Due to the limited amount of material available after the TMT is carried out in the plastodilatometer, the creep properties were investigated by means of the small punch creep test (SPCT) performed at 700°C as it has been previously reported [41, 42]. The SPCT samples were cut transversally, from cylindrical specimens, with a thickness of 600 μm and a diameter of 8 mm. Then, the disks were ground on both sides down to a final thickness of 500 μm. In the setup of the SPCT, the lower and upper dice are connected via a thread to ensure the clamping of the sample. The load is applied by a ceramic punch ball which is in contact with the sample. A plunger rod is used to transmit the dead weight load to

martensitic microstructure and hence reducing creep rates [25, 26].

mentioned above; it has been studied extensively [27–29].

precipitate microstructure for elevated-temperature strength.

**168**

parallel close-packed plane relationship in the Kurdjumov-Sachs (K-S) orientation relationship, denoted as "CP group" recently. In general, a packet is partitioned into several blocks, each of which contains laths of a single variant of the K-S relationship. Blocks and packets are mostly surrounded by high-angle boundaries, whereas lath boundaries inside a block are of low-angle type.

The microstructure resulting from conventional industrial heat treatment consists of tempered martensite, which presents elongated subgrains with an average size of 0.25–0.5 μ m (**Figure 3**). Two types of precipitates, M23C6 carbides rich in chromium and MX carbonitrides rich in V or Nb, are present in the microstructure. The size of M23C6 carbides is around 100–200 nm, and they are precipitated on subgrain boundaries and prior austenitic grain boundaries. The size of MX carbonitrides is much smaller than M23C6 carbides, 20–50 nm, and they are in the matrix [51]. The purpose of this work is to produce a dispersion of nanosized precipitates by a controlled TMT, bearing in mind that a high number density of fine MX precipitates (Nb-MX and V-MX) should display superior high-temperature performance.
