Effect of Long-Term Ageing at 600 °C on Microstructure of ZG1Cr10MoWVNbN Martensitic Heat-Resistant Steel

High-temperature ageing on the room temperature mechanical properties of tested steel

The room temperature mechanical properties of ZG1Cr10MoWVNbN heat-resistant steel before and after ageing are listed in Table 2. It can be seen from Table 2 that, although the room temperature mechanical properties of the tested steel after ageing were decreased, they were still in the acceptable range for material performance standards. The hardness of the tested steel was not significantly changed after long-term high-temperature ageing. However, the material strength was significantly changed. The yield strength (Rp0.2) and tensile strength (Rm) of the tested steel were still far higher than the strength value required by the material performance standards. In the plasticity index, the elongation was cut down by 29 % by ageing, and the contraction was cut down by 26 % as a result of ageing. The material toughness decreased markedly after ageing, and the material impact power was only 37 J. It decreased by 42 % as a result of ageing. Although the material properties were still within the range of the required standard values, the material properties were close to the minimum required value. This suggests that the material has obvious embrittlement at a high temperature after long-term ageing.

Microstructure analysis

Microstructure of ZG1Cr10MoWVNbN steel before and after ageing is shown in Figure 1. The matrix microstructure of materials is typically a martensite structure, and the lath martensite boundary can be clearly seen by metallography and TEM. As Figure 1(b) and (d) show, before ageing, the martensite laths were about 0.5 μm in width, and the microstructure of the steel was still a typical lath martensite structure after 17,000 hours of ageing treatment at 600 °C. However, the martensite lath become wider than the before ageing and the width was about 1 to 2 um. As shown by the arrow in Figure 1, even the “bamboo-like,” and the lath sectors became relatively vague. This is mainly because the phenomenon of recovery appeared in the steel during the process of long-term ageing, blurring the martensite lath boundaries.

Figure 1:

Effect of ageing time at 600°C on microstructure (a) The metallographic structures before ageing (b) The TEM photos before ageing (c) The metallographic structures after ageing (d) The TEM photos after ageing.

The organisational structure of the material was further observed by TEM. The lath martensite before ageing is shown in Figure 2(a). After the heat treatment, the carbide precipitation can be seen clearly in the martensite lath boundaries and within them. As shown in Figure 2(a), Area A was selected through the use of electron diffraction. The calibration of the electron diffraction spectrum was analysed and compared with the standard PDF card. It was determined that the carbides were Cr23C6 with the fcc structure . Cr23C6 is mainly distributed in the lath and lath boundaries, and its shape is a sheet or spherical. A large number of statistics show that the carbide is about 100 nm. Because the carbide in the strip boundary easily grows along the strip boundary, the morphologies of the carbides precipitated in the slab boundary are mainly irregular spherical and sheet-like, and these carbides have the tendency to form chains or nets. In Figure 2(c), a high density of dislocations can be observed in the interior of the slab, the dislocation density is reduced after ageing, and no high-density dislocation is found in some regions. As shown in Figure 2(d), in the long-term ageing process, due to the remarkable recovery process, the dislocation density decreased and the dislocation strengthening effect decreased, so the strength and toughness of the material decreased. Thus, after the heat treatment, the toughening mechanism is mainly the high dislocation density, which causes lath martensite strengthening, and the carbide precipitation causes precipitation strengthening. Therefore, the strength and toughness of the material are increased. Under 600 °C heat preservation and after 17,000 hours of long-term ageing, the matrix structure is still typical lath martensite, but the martensite has been significantly coarsened, and the dislocation density has been reduced. This is one of the reasons leading to the decline in the plasticity and toughness of the material. In addition, as shown in Figure 2(a), the B position inside the slab has carbide precipitation, which may be MX carbides.

Figure 2:

Effect of ageing time at 600°C on TEM microstructures (a) The TEM structure before ageing (b) The SADP in figure (a) A points (c) The dislocation structure before ageing (d) The TEM structure after ageing.

Figure 3 shows the TEM structure after 17,000 hours of ageing treatment at 600 °C. As shown in Figure 3(a), there are carbides precipitated in the inner and the strip of the lath martensite. As shown by the arrow in Figure 3(a), the electron diffraction was carried out, the electron diffraction spectrum was analysed and compared with the standard PDF card. It can be obtained that carbides were Cr23C6 with the fcc structure. This is the main precipitation strengthening phase in the martensitic heat-resistant steel. A large amount of Cr23C6 was precipitated in the lath martensite phase boundaries, and it was distributed in the form of chains or islands. The Cr23C6 were precipitated inside the lath through irregular spherulitic diffuse distribution. More Cr23C6 carbides precipitated in the lath boundary than that the lath, and the size of the carbide in the lath boundary was greater than that in the lath. The carbide in the lath boundary is about 250 nm and the carbide inside the lath is about 150 nm. The carbide particles obviously grew due to ageing, and they formed a chain or network [6]. It is also reported that M2C carbides precipitate along the slab in the initial stage of ageing. However, M2C carbides were not observed in this paper. It is possible that the M2C carbides were completely transformed into the larger M23C6 carbides after long-term ageing [7].

Figure 3:

Morphology of carbides in ZG1Cr10MoWVNbN steel after long-term ageing at 600°C (a) Precipitation phase morphology of chain (b) The SADP corresponding to (a) (c) Precipitation phase morphology of intragranular (d) The SADP corresponding to the Figure (c) A points.

As shown in Figure 3(c), fine MX carbides dispersed in the crystal were also found by TEM. As shown in Figure 3(d), selected – area electron diffraction (SAED) confirmed that the MX carbides are NbC with the fcc structure. These carbides are widely distributed in the lath, which are diffusely distributed in spherical particles or fine needles and have good strengthening effects on the matrix. The size of the carbides is at the nanometer level, and the carbides have a very strong hardening effect on the material.

The influence of carbide precipitation on the mechanical properties of the steel

The main strengthening mechanisms of heat resistant steel with 9 to 12 % Cr include: martensite phase strengthening, the precipitation strengthening of second phase particles, and the solid solution strengthening of alloy element [3]. Martensitic hardening is mainly due to the solid solution strengthening caused by the supersaturated solid solution of carbon. The dislocation strengthening was caused by the existence of high density dislocations, and the fine grain strengthening was caused by the substructure within the crystal particle.

In the heat-resistant steel strengthening factor, only crystalline structure refinement can improve both strength and toughness at the same time. Precipitation strengthening can reduce the toughness of the material and is related to the type, quantity, size and distribution of the precipitation. The results show that the precipitation of carbides along the martensite lath boundary was observed by TEM. Under 600 °C heat preservation and after 17,000 hours of long-term ageing, a large amount of Cr23C6 in martensite lath boundaries was significantly roughened, and the carbide accumulated along the lath boundaries and chainlike distribution. In particular, a large number of coarse carbides accumulated at the grain boundaries. The coarsening of the carbides in the slab leads to the decrease of the grain density of Cr23C6 and a decrease in the precipitation strengthening effect, which leads to micro cracks in the martensite boundary, increasing the tendency of intergranular fractures and leading to the brittleness of the material [8]. In addition, due to the coarsening of the Cr23C6 in the martensite, the amount of Cr around the Cr23C6 is reduced. This results in the desolventising of the alloy elements in the matrix and the weakening of the strengthening effect. Due to the long-term high-temperature ageing, the dislocation density inside the lath martensite was decreased. In particular, the coarsening of the Cr23C6 was found to lead to the desolventising of the matrix, which decreased the mechanical properties of ZG1Cr10MoWVNbN steel after ageing at room temperature. In addition, another study found that, in the process of long-term high-temperature ageing, high Cr martensitic heat-resistant steel will produce a brittle phase or impure elements at grain boundaries [9]. This is an important factor that leads to the decline of the mechanical properties of materials at room temperature. The MX (NbC) produced in the body of the slab was precipitated in spherical and needle plate shapes and was dispersively distributed. After long-term high-temperature ageing, if ageing continues, then the precipitated phases will continue to grow and increase. Because the carbide particles are very small (nanometer level) the fine dispersion distribution of MX plays a role in the precipitation strengthening of the second-phase particles, and it plays a role in hindering the dislocation of climbing. Thus, the strength and toughness of the heat-resistant steel are greatly improved. The MX is also the main strengthening phase of heat-resistant steel in the long-term high-temperature environment, and it plays a key role in improving the high-temperature creep properties of materials.