Vladimir Skorobogatykh

Evolution of Laves-Phase Particles in a Low Carbon 9%Cr Martensitic Steel during Creep at 650°C

The evolution of microstructure in low carbon heat resistant steel of P92-type modified with 3%Co was examined during creep tests at 923K for 500, 1500, 6000 and 16000 hours. After tempering at 1023K (750°C), the steel was composed of martensite lath structure with numerous precipitates of MX-type carbonitrides and rare M23C6-type carbides. The structural changes during creep tests are characterized by an increase in the sizes of laths and second phase particles. Moreover, the Fe2W Laves-phase precipitates during long-term creep. The Laves-phase particles grow accordingly to power-law relationship of creep time.

Microstructure Evolution in a 9%Cr Heat Resistant Steel during Creep Tests

Dynamic structural changes during creep tests for about 103 hours at 600 and 650oC were investigated in a P92-type 9%Cr martensitic heat resistant steel. The structural changes are characterised by the development of relatively large equiaxed subgrains with relatively low dislocation densities in place of initial martensite laths. The coarsening of substructure was accompanied by a growth of second phase precipitates. The final grain/subgrain sizes and dislocation densities evolved after the creep tests were in rough correlation with applied stresses, i.e. larger (sub)grains developed under lower stresses. The structural mechanism responsible for microstructure evolution was considered as a kind of continuous dynamic recrystallization.

Effect of Long-Term Creep on Microstructure of a 9% Cr Heat Resistant Steel

The effect of long-term creep at 600°C under 137 MPa on the microstructure of a P92-type steel was investigated. The microstructure after tempering consisted of laths with an average thickness of 400 nm. Dispersion of secondary phases consists of M23C6 carbides with an average size of 85 nm located mainly on lath, block and prior austenite boundaries and MX carbonitrides with average size of 31 nm homogeniously distributed throughout. Creep with duration of 40738 hours led to coarsening of M23C6 carbides up to 182 nm. Precipitation of Laves phase with an average size of 290 nm took place in both grip and gauge portions of ruptured specimen. Vanadium-rich MX particles were replaced by particles of Z-phase with sizes of 97 and 48 nm after long-term creep and aging, respectively. The average misorientation of the lath boundaries was approximately 2° and scarcely varied during creep, while the mean lath thickness increased to 890 nm in gauge section of ruptured specimen and remained essentially unchanged in the grip section. Dislocation density decreased slightly under long-range aging and creep.

Structural Changes in a 9%Cr Creep Resistant Steel during Creep Test

Structural changes in a 9%Cr martensitic steel after 1%, 4% creep and creep rupture test at 650°C and stress of 118 MPa were examined. Heat treatment provided the formation of tempered martensite lath structure (TMLS) in the steel. The precipitations of second phase particles along block and lath boundaries provide effective stabilization of the TMSL under annealing/aging condition. This structure hardly changed under creep conditions in grip portion of crept sample. Significant coarsening of both the second phase particles and the martensite laths takes place in neck portion. In addition, the latter ones lose their original morphology and are replaced by large strain-induced subgrains. It should be noted that the increase of subgrain size is in almost direct proportion to the particle growth during the creep to 4% strain. The rapid growth of martesite laths followed by their evolution to deformation subgrains takes place within the tertiary creep regime.

Microstructure Evolution in a 3%Co Modified P911 Heat Resistant Steel under Creep Conditions

The microstructural changes in a 3%Co modified P911 heat resistant steel were examined under static annealing and creep at elevated temperatures. The quenched steel was tempered at temperatures ranging from 673 to 1073 K for 3 hours. The temperature dependence of hardness for the tempered samples exhibits the maximum at 723 – 823 K which is associated with the precipitations of fine carbides with an average size of about 20 nm. The transverse lath size of martensitic structure is  200 nm after air quenching and remains unchanged under tempering at temperatures below 800 K. An increase in tempering temperature to 1073 K resulted in hardness drop. Coagulation of carbides and growth of martensitic laths takes place at these temperatures. The creep tests were carried out at 873 and 923 K up to rupture, which occurred after about 4.5 × 103 hours. The structural changes in crept specimens were characterized by the development of coarse laths/subgrains. The mean transverse size of which was  0.67 and  1.3 m after the creep tests at 873 and 923 K, respectively. On the other hand, an average size of second phase particles of  165 nm was observed in the samples tested at both temperatures.