The Impact Toughness of a Nitride-strengthened

Martensitic Heat Resistant SteelW.F. Zhang1, 2, W. Yan1, W. Sha3, W. Wang1, Q.G. Zhou1, 2, Y.Y. Shan1, K.Yang1

1 Institute of Metal Research, Chinese Academy of Sciences, Shenyang 1100162 Graduate School of Chinese Academy of Sciences, Beijing 1000493School of Planning, Architecture & Civil Engineering, Queen’s University Belfast, Belfast BT9 5AG, UK.

AbstractThe nitride-strengthened martensitic heat resistant steel is precipitation strengthened only by nitrides. In the present work, the effect of nitride precipitation behavior on the impact toughness of an experimental steel was investigated. Nitrides could hardly be observed when the steel was tempered at 650°C. When the tempering temperature was increased to 700°C and 750°C, a large amount of nitrides were observed in the matrix. It was surprising to reveal that the impact energy of the half-size samples greatly increased from several Joules to nearly a hundred Joules. The ductile-brittle transition temperature (DBTT) was also discovered to decrease from room temperature to -50°C when the tempering temperature was increased from 650°C to 750°C. The nitride precipitation while increasing tempering temperature was revealed to be responsible for the improved impact toughness.

1. IntroductionThe efficiency of the power plants could be improved by enhancing the steam parameter. At present, the heat resistant steels for the high steam parameter of 650°C are being developed. This has put heat resistant steels such as T/P91, T/P92 and E211 out of consideration because of the loss of the microstructure stability during service at the high temperature [1]. More advanced steels should be developed to meet this requirement.

It is well accepted in heat resistant steels that highly stable microstructure will produce excellent creep strength. These heat resistant steels usually possess the typical microstructure of tempered lathy martensite dispersed with precipitates [2]. The precipitates are basically M23C6, where M is mainly Cr with substitution of Fe, and MX, the carbonitride of Nb, V or Ti. The MX type carbonitrides show much better stability than the M23C6 type carbide. Some recent work [3, 4] has shown that with increasing service time, the M23C6 carbide grew too fast to pin the dislocation movement and could not prevent grain boundaries or lath boundaries from migrating, resulting in premature fracture. In order to achieve microstructure with high stability, stable precipitates such as MX type carbonitrides were expected in heat resistant steels.

Corresponding authorEmail: yyshan@imr.ac.cn

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In addition to this initial tempered martensitic microstructure, long term microstructure stability should be paid enough attention to. Such coarse precipitates as Laves phase (Fe2W or Fe2Mo) and Z phase ((Cr,Nb)N) should be postponed, although they could only form after a long service time [5]. The formation of Laves phase and Z phase is thermally automatic process which cannot be avoided [6]. However, this process can be postponed by reducing the content of W, Mo and N.

Nitride-strengthened martensitic heat resistant steel is developed, based on the above ideas. In our previous work [4], the alloy design and the first results on mechanical properties of the nitride-strengthened martensitic steels have been reported. In the present work, we would like to present the excellent impact toughness of the steel after tempering.

2. ExperimentalThe chemical composition of the experimental steel is listed in Table 1. The steel was melted in a vacuum induction-melting furnace and then forged into square billet at 1150-900°C, to the dimension of 60×100×300 mm. After that, the steel was hot rolled to 7 mm thick plates.

Raw samples were cut from the plate perpendicular to the rolling direction, subject to normalizing at 980°C for 30 min, and then tempered at 650°C, 700°C and 750°C for 90 min to investigate the dependence of mechanical properties on tempering temperature. The tensile specimens had a gauge diameter of 5 mm and gauge length of 25 mm. The half size Charpy V-notched (CVN) impact specimens had a dimension of 5×10×55 mm.

The as tempered samples were tensile tested at both room temperature and 600°C. The as tempered half size Charpy V-notched specimens were subjected to impact test at room temperature, 0°C, -20°C, -40°C, -60°C, and -80°C.

The microstructure of the as tempered samples was observed under a scanning electron microscope (SEM) and a transmission electron microscope (TEM). The fractography analysis on the impact broken specimens was performed under SEM.

Table 1 chemical composition of the experimental steel, wt.%C Cr Mn W V Co Nb N Al

0.0050 8.63 1.06 1.53 0.19 1.47 0.062 0.033 <0.01

3. Results3.1 MicrostructureThe experimental steel normalized at 980°C for 30 min had full martensitic microstructure. After normalizing, the steel were tempered at 650°C, 700°C and 750°C for 90 min. Almost no precipitates were formed when the steel was tempered at 650°C, as illustrated in Fig. 1a. However, when the tempering temperature was increased to 700°C, the precipitates were noticed in the matrix, as shown in Fig. 1b. Finally, when the tempering temperature was

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increased to 750°C, Fig. 1c, the quantity of the precipitates increased promptly and the nano-sized precipitates were in a sharper and clearer shape than those tempered at 700°C.

Since the carbon content in the steel was decreased to such a low level of 0.0050% in wt.% (Table 1), it is very difficult to form carbides in the steel. This was also proved by the microstructure shown in Fig. 1 that no such big size carbides as Cr23C6 were observed under SEM. However, the nitrogen is at a high level in the steel. Therefore, it is reasonable to believe that the precipitates formed in the steel are nitrides of Nb and V, which are very fine and in the cubic shape shown in Fig. 2.

Fig. 1 The tempered microstructure of the experimental steel at (a) 650°C, (b) 700°C, (c)

750°C

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Fig. 2 TEM image of the specimen tempered at 750°C for 90 min, showing the MX type nitrides.

3.2 Mechanical propertiesThe precipitation behavior of nitrides is certain to affect the mechanical properties. The strength changing with the tempering temperature is shown in Fig. 3. It can be seen that the strength rapidly decreased with increasing tempering temperature, especially when tempered at 750°C. The room temperature yield strength nearly decreased by 100 MPa when the tempering temperature was raised from 650°C to 700°C and by 150 MPa from 700°C to 750°C. It is obvious that the room temperature yield strength decreased more quickly when the steel was tempered at 750°C. The commercial P92 steel was reported in literature to possess the yield strength of 345 MPa and the tensile strength of 390 MPa when tempered at 750°C [7]. Compared with this P92, the experimental steel tempered at 750°C still possessed relatively higher yield strength of 515 MPa and tensile strength of 625 MPa. It is proved that the experimental steel can have comparable room temperature strength with the P92 steel.

The high temperature yield strength of 600°C also decreased when the steel was tempered at 750°C, as indicated in Fig. 3. The yield strength of 600°C was reduced by 69 MPa when the tempering temperature was increased from 650°C to 700°C and by 86 MPa from 700°C to 750°C. However, the reduction amplitudes are similar. The experimental steel tempered at 750°C displayed high temperature yield strength of 307 MPa and tensile strength of 342 MPa, which are also comparable to those of the commercial P92.

Fig. 3 Strength of the steel tempered at different temperatures

3.3 DBTTFig. 4 demonstrated the toughness and the DBTT dependence on the tempering temperature. The CVN impact specimens tempered at 650°C could only absorb 18 J energy at room temperature and 2 J at -20°C, which indicated that the steel tempered at 650°C possessed a high DBTT of above room temperature. When the steel was tempered at 700°C, the CVN specimen could take in energy up to 86 J at room temperature but still

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decreased to 3.5 J at -20°C, which indicated that the steel tempered at 700°C presented a DBTT of about 0°C. However, when the tempering temperature was increased to 750°C, the steel could display not only good toughness of 96 J at room temperature, but also high toughness of 110 J at -20°C, 89 J at -40°C but 11 J at -60°C, which indicated that the steel tempered at 750°C possessed a low DBTT of around -50°C. These results clearly demonstrate that the DBTT of the steel would be greatly decreased by increasing the tempering temperature.

Fig. 4 Charpy value of the steel tested at different temperature

3.4 FractographyThe fractography analysis under SEM showed that the CVN specimens broken at low impact energy exhibited cleavage fracture characteristic, while those broken at high impact energy presented dimple fracture characteristic, as shown in Figs. 5-7. The CVN specimens tempered at 650°C gave brittle cleavage fracture at both room temperature and -20°C, as shown in Fig. 5. The CVN specimens tempered at 700°C displayed ductile dimple fracture at room temperature, but brittle cleavage fracture at -20°C, as shown in Fig. 6. The CVN specimens tempered at 750°C did not exhibit brittle cleavage fracture until the test temperature was decreased to -60°C. When the test temperature was above -40°C, the surface of the impact specimen showed ductile dimple fracture. In those dimples on the fracture surface, many big size particles could be seen.

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Fig. 5 Fractography of the broken impact specimen tempered at 650°C, (a) room temperature, (b) -20°C

Fig. 6 Fractography of the broken impact specimen tempered at 700°C, (a) room temperature, (b) -20°C

Fig. 7 Fractography of the broken impact specimen tempered at 750°C, (a) -40°C, (b) -

60°C.

4. Discussion4.1 Nitride precipitationAs mentioned in section 3.1, the precipitates formed in the matrix during tempering are MX type nitrides. From the response of the nitride precipitation to the tempering temperature, the conclusion could be reached that the nitrides precipitate from the matrix more rapidly when the temperature is as high as 750°C, which is widely accepted as the peak precipitation temperature of nitrides [8].

As shown in Figs. 1c and 2, nitrides of Nb and V are dispersed in the matrix, illustrating a desired martensitic microstructure strengthened by only thermally stable nitrides. Therefore, it is logical to speculate that the nitride-strengthened martensitic heat resistant steel should possess good long term creep strength due to the microstructure stability [9].

4.2 Effect of nitride precipitation on yield strength

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The strength of the experimental steel showed normal response to the increasing tempering temperature, i.e. the strength decreased with the increase of tempering temperature. However, it is noticeable that the room temperature yield strength decreased much quicker when tempering temperature was increased from 700°C to 750°C than from 650°C to 700°C, as described in section 3.2. The (more) complete precipitation of nitrides should be responsible for the quicker decrease. It is known that the formation of precipitates consumes dislocations. The nitride precipitation will decrease much the number of dislocations in the matrix, resulting in weakening of dislocation strengthening. On the other hand, the formation of nitrides consumed the dissolved nitrogen which could provide very strong solid solution strengthening. Therefore, although the nitride precipitation could produce precipitation strengthening, it is not enough to compensate for the loss of dislocation strengthening and nitrogen solid solution strengthening.

However, the high temperature yield strength of 600°C did not show an obviously accelerated decrease when tempering temperature was increase to 750°C. It could be interpreted from two views. The first one is that since the test was carried out at high temperature of 600°C, the dislocation was easier to move and annihilate. The advantage of high dislocation density was no longer obvious. The second view is that the more movable dislocation would become easier to reproduce due to nitride precipitation in the 750°C tempered specimens. Therefore, the precipitation strengthening would mainly compensate for the loss of nitrogen solid solution strengthening. Hence, accelerated reduction was not observed in the high temperature yield strength of 600°C.

4.3 Dependence of DBTT on tempering temperatureIt is shown in Fig. 4 that the DBTT decreased from above room temperature to -50°C when the tempering temperature increased from 650°C to 750°C. In order to reach a clear understanding on DBTT of the steel, it is very critical to take an investigation on the yield strength. That is the reason that a detailed discussion on the effect of tempering temperature on the yield strength has been made above.

It is widely believed that the cleavage fracture stress changes with the test temperature. Thus, it is easy to phenomenologically understand that the DBTT will be reduced with the decrease of yield strength when the tempering temperature was increased from 650°C to 750°C [10]. As discussed above, the nitride precipitation will lead to the decrease of yield strength. So, it actually can be interpreted that the DBTT decrease is really associated with the nitride precipitation. The nitride precipitation has improved the impact toughness and decreased the DBTT by toughening the steel.

5. ConclusionThe microstructure and the mechanical properties of a nitride-strengthened heat resistant steel were investigated in the present work. The following conclusions could be reached.1) The nitride precipitation in the steel reached its peak when the tempering temperature is

increased to 750°C. It is proved that tempering at 650°C or 700°C could not induce the nitride precipitation, at least not to effective levels.

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2) The steel could achieve a martensitic microstructure strengthened by only nitrides after tempering at 750°C. This microstructure is expected to possess good thermal stability and high creep strength. The steel tempered at 750°C could achieve comparable mechanical properties with the commercial P92 at both room temperature and 600°C.

3) The room temperature impact toughness of the steel was greatly enhanced from several J to nearly a hundred J by increasing tempering temperature from 650°C to 750°C.

4) The DBTT of the steel shows a great dependence on the tempering temperature. The DBTT of the steel could be reduced from above room temperature to -50°C when the tempering temperature was increased from 650°C to 750°C.

AcknowledgementThis work was financially supported by National Basic Research Program of China (No. 2010CB630800) and National Natural Science Foundation of China (No. 51001102).

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