Research Institute,Baoshan Iron & Steel Co.,Ltd.,Shanghai 201999,China

Abstract: The deformation behavior of 9Cr-3W-3Co heat-resistant steel at a high-temperature range of 1 060-1 260 ℃ and a strain rate of 0.3 s-1 was studied using a Gleeble 3800 heat-simulating test machine.The microstructure and precipitation phases of the steel at different temperatures were studied by optical microscopy,scanning electron microscopy,and transmission electron microscopy.The results show that due to its low melting point,coarse grain size,and the segregation of P,S,and Cu at the grain boundary,the thermoplasticity of 9Cr-3W-3Co steel is poor at temperatures higher than 1 200 ℃.The bulk ferrite phase was the main factor affecting the thermoplasticity at 1 100-1 200 ℃。

Key words: heat-resistant martensitic steel; 9Cr-3W-3Co; hot deformation; thermoplasticity

1 Introduction

Despite the extensive promotion of solar,wind,and water renewable energy sources and the increasing number of nuclear technologies in recent years,the global energy sector is still mostly based on fossil fuels,not only in Europe but throughout the world.In China,the generation of electricity continues to be based on coal,which is dictated by China’s economic conditions and natural resources.Coal is the most basic and inexpensive source of energy.However,its toxic gas emissions into the atmosphere is restricting further development of conventional power generation technologies.The development of ultra-supercritical units in fossil fuel power is directed toward high efficiency and low pollution.Ultra-supercritical units are inextricably linked to the development of materials technology and the need for materials that exhibit increased performance.LIU Z D et al.[1-3]and ABE et al.[4-5]developed 9Cr-3W-3Co series heat-resistant steels for use in 620-650 ℃ ultra-supercritical units,which are characterized by combined solid solution strengthening,precipitation strengthening,dislocation strengthening,and substructure strengthening mech-anisms.Compared with T/P91 and T/P92 steels,the creep rupture property and steam-oxidation corrosion resistance of 9Cr-3W-3Co steels are significantly improved.However,due to the addition of elements W,Co,and B,the large-scale industrialization of 9Cr-3W-3Co steel faces great challenges.In addition to the difficulty of smelting and casting,the pipe processing of this heat-resistant alloy,especially the hot working step,is a bottleneck.As such,the hot working properties of 9Cr-3W-3Co steel must be investigated.

Research on the hot deformation behavior of iron and steel materials is relatively mature,with previous litera-tures available regarding the hot deformation behavior of 9%-12%Cr martensitic heat-resistant steel[6-8].SHI R X et al.[7]studied the hot deformation behavior of P92 steel at a deformation temperature range of 900-1 250 ℃ and a strain rate of 0.1-10 s-1.YAN P et al.[8]used the compression method to study the hot working process of 9Cr-3W-3Co steel,and obtained the hot deformation activation energy,hot deformation equation,dynamic microstructure state diagram,and hot working diagram of the material.The authors recommended that the hot working of 9Cr-3W-3Co steel should be in the range of 1 150-1 200 ℃ and 0.3 s-1.The hot working process in industrial settings is complex,and includes the compression and drawing of the deformation modes.Therefore,in this study,the hot-drawing deformation behavior of 9Cr-3W-3Co steel was investigated with respect to the drawing deformation mode.

2 Experimental scheme

2.1 Test materials

The test materials were obtained from a 178-mm-diameter billet produced by Baosteel Special Steel,the chemical composition of which is shown in Table 1.For comparison,the commercial heat-resistant steel P92 was used.

2.2 Thermal simulation test

To prevent test differences obtained due to microstructural and composition segregation at different positions of the tube billet,all samples were obtained at the 1/2 radius of the tube billet.The hot tensile samples were heated at 1 060-1 260 ℃,then stretched at a deformation rate of 0.3 s-1until fracture,and then cooled rapidly.The reduction in the tensile fracture area was calculated after the test.

2.3 Microanalysis

After metallographic polishing,the microstruc-tures and sizes of the precipitates near the fracture surface were observed by optical microscopy,scanning electron microscopy (SEM),and transmission electron microscopy (TEM).

3 Test results and analysis

3.1 High-temperature phase diagram

The high-temperature phase diagram generated using Thermo-Calc software is shown in Fig.1,which reveals that the high-temperature phase com-position of 9Cr-3W-3Co steel is very complex.The main precipitates include the M23C6,M2B,Cr2B,M6C,Laves and Z phases.From the enlarged partial phase diagram,it is evident that 9Cr-3W-3Co steel enters a liquid-solid dual phase region above 1 255 ℃,which is about 150 ℃ lower than that of T/P91 steel.The austenite phase and a small amount of M2B occur in the range of 1 130-1 250 ℃,and the phase composition is relatively complex at tempera-tures less than 930 ℃.According to the phase diagram,the key reason that the high-temperature thermoplasticity is affected is related to the lower melting point of the alloy itself and the precipitation of M2B with a lower melting point.

3.2 Results of thermal simulation test

Fig.2 shows a plot of the area reduction rates of 9Cr-3W-3Co steel at different temperatures,which show that the area reduction rate of P92 steel is greater than 90% above 1 180 ℃,whereas the area reduction rate of 9Cr-3W-3Co steel moved to a lower zone at a temperature of above 1 120 ℃.This indicates that the hot plasticity in the hot drawing mode is obviously worse than that shown by the hot compression test results[8].

3.3 Metallographic structure

Figs.3-8 show the metallographic structures of longitudinal sections of the thermal simulation samples.A coarse grain structure over the whole fracture area was observed in the samples at temperatures higher than 1 220 ℃ and no high-temperature ferrite phase was evident on the fracture.Cracks that were initiating and propagating along the coarse grain boundary were observed.

When the temperature was reduced to 1 200 ℃,a massive high-temperature ferrite structure was observed,with coarse grains and many intergranular cracks observed near the fracture surface.A massive high-temperature ferrite phase occurred in the temperature range of 1 100-1 200 ℃,with the ferrite grain size becoming finer with decreases in temperature.Decreases in temperature also resulted in the increase of the area reduction rate.No ferrite phase was observed,with the grain size of austenite becoming finer at temperatures of 1 080 ℃ or lower,and the area reduction rate tended to be stable,reaching more than 90%.

3.4 SEM analysis

Figs.9 and 10 show the energy dispersive spectrometer(EDS) results of cracks at 1 200 ℃ and 1 100 ℃,respectively.The microstructure and EDS results are similar and are not shown for the temperatures of 1 180 ℃,1 160 ℃,1 140 ℃,and 1 120 ℃.The results show that the P,S,Ti,and Cu contents in the intergranular cracks were higher than those in the matrix at 1 200 ℃.The P,S,and Cu contents in the intergranular cracks were also higher than those in the matrix at 1 180 ℃.P and S were detected in the crack of the sample at 1 160 ℃.The intergranular characteristics of the cracks are not obvious at 1 140 ℃ and 1 120 ℃,and P and S were not detected in the cracks,although the element Cu was detected.MnS inclusions were observed in the cracks at 1 100 ℃,Cu was detected,but P was not.

3.5 TEM analysis

To determine whether there were low melting-point materials in the grain boundaries of the samples,the samples were analyzed at 1 200 ℃ and 1 100 ℃ by TEM.By comparing the TEM analysis results of samples with better (1 100 ℃) and poorer (1 200 ℃) thermoplasticity,it could be seen that the precipitates at a given temperature were W-rich intermetallic compounds of B,Nb,V,and Cu,with a lower Cr content in this phase,and a Fe content far lower than that of the matrix.No Co was detected in this phase,as shown in Figs.11 and 12.The contents of Cu and W at the martensite lath boundary and crystal are higher than those of the nominal composition.

In conclusion,the hot plasticity of 9Cr-3W-3Co steel was determined to be very poor at high temperatures.Based on the calculated phase diagram,metallographic analysis,and SEM/TEM analyses,the fundamental reason for the poor hot plasticity of 9Cr-3W-3Co steel in the high-temperature zone is its lower melting point (1 255 ℃),which leads to coarse grain and lower plasticity at temperatures of 1 200 ℃ and above.At the same time,although the overall P and S contents in the steel are very low,the plasticity of the material is further deteriorated by the increasing contents of P,S,and other segregating elements in the coarse grains in the smaller grain boundary area,with the appearance of high-temperature ferrite,which results in the lowest area reduction rate at 1 200 ℃.

4 Conclusions

(1) The phase diagram calculated for 9Cr-3W-3Co steel shows that the precipitation phases are complex,including M23C6,M2B,Cr2B,M6C,Laves and Z phases.The lowest liquidus temperature of 9Cr-3W-3Co steel is approximately 1 250 ℃.

(2) The area reduction of 9Cr-3W-3Co steel is initiated at temperatures above 1 120 ℃,which is called the first high-temperature brittle temperature zone.The high-temperature thermoplasticity of 9Cr-3W-3Co steel is much worse than that of P92 steel.

(3) Coarse grains and intergranular cracks were observed near the fracture surface at temperatures of 1 220 ℃ and above.When the temperature was reduced to 1 200 ℃,a large block of high-temperature ferrite phase and many intergranular cracks were observed near the fracture surface.

(4) The main cracks were initiated and propagated along grain boundaries at temperatures above 1 160 ℃,and the enrichment of elements P,S,and Cu was detected in the cracks.The intergranular characteristics of the cracks were not obvious in the temperature range of 1 100-1 140 ℃.

(5) The poor plasticity of 9Cr-3W-3Co steel at temperatures above 1 200 ℃ is related to the lower melting point of the alloy itself,the coarse grain size,and the segregation of P,S,Cu in the grain boundary.

(6) The massive ferrite phase was the main factor affecting the high-temperature plasticity of 9Cr-3W-3Co steel in the temperature range of 1 100-1 200 ℃.

Acknowledgement

The author is grateful to Luo Ming,Yu Min and Gao Jiaqiang for their support in calculating the phase diagram and performing the high-temperature thermal simulation test and TEM analysis.