Valeriy Dudko

Microstructural changes in steel 10Kh9V2MFBR during creep for 40000 hours at 600°C

In this work, we have investigated microstructural changes in steel 10Kh9V2MFBR (analog of P02 steel) after long-term creep tests at a temperature of 600°C under an initial stress of 137 MPa. Time to rupture was found to be more than 40000 h. It has been established that, in the zone of grips and in the neck region of the sample, the size of the particles of the M23C6 carbides increases from 85 nm to 152 nm and 182 nm, respectively. In addition, large particles of the Laves phase with an average size of 295 nm are separated. The particles of these phases are located along high-angle boundaries. During prolonged aging and creep, the transformation of the M(C,N) particles enriched in V into the Z phase occurs. The average size of particles of the Z phase after prolonged ageing was 48 nm; after creep, it reached 97 nm. The size of M(C,N) particles enriched by Nb increases from 26 nm after tempering to 55 nm after prolonged aging and creep. It has been established that, in spite of an increase in the transverse size of the laths of tempered martensite from 0.4 to 0.9 µm in the neck of the sample, the misorientation of the lath boundaries does not increase. No recrystallization processes were found to develop in the steel during creep.

Effect of Tempering on Mechanical Properties and Microstructure of a 9% Cr Heat Resistant Steel

Effect of tempering temperature ranged from 400 to 720°C on mechanical properties and microstructure of a P92-type creep resistant steel was investigated. The hardness value of 400 HB, which was obtained after solution treatment, increased to 430 HB with increasing the tempering temperature to 525°С. Further increase in the tempering temperature resulted in gradual decrease in hardness, which approached a level of about 220 HB after tempering at 720°С. The equiaxed particles of MX-type carbonitrides with a size of about 30 nm were precipitated randomly after tempering under all conditions. At temperatures below 525°C, the tempered martensite lath structure (TMLS) was characterized by a random distribution of fine M3C-type carbides and MX-type carbonitrides. The precipitation of M23C6 was observed after tempering at T ≥ 525°C. At 525°C, the M23C6 carbides appeared as thin films on high-angle boundaries (HAB), while M23C6 particles having almost equiaxed shape and located on various boundaries including low-angle lath boundaries precipitate at higher temperatures.

Influence of the carbon content on the phase composition and mechanical properties of P92-type steel

The deformation behavior and the microstructure evolution under the creep of 10Kh9V2MFBR steel (Russian analog of the P92 steel) (in wt %, Fe–8.9% Cr–0.05% Si–0.2% Mn–1.9% W–0.5% Mo–0.25% V–0.07Nb–0.08% N–0.01% B) with the standard (0.1%) and lowered (0.018%) carbon contents have been investigated. After the heat treatment, which included normalizing at 1050°C and tempering at 720–750°C, carbides M23C6 and carbonitrides M(C,N) are formed in the 10Kh9V2MFBR steel, while in the 02Kh9V2MFBR steel (modified P92 steel), carbides M23C6, nitrides M2N, and carbonitrides M(C,N) as well as δ-ferrite (23%) were found. The measurements of hardness and tensile tests at room and elevated temper-atures did not reveal substantial distinctions in the short-term mechanical properties of both steels. The hardness of steels after tempering was 220 HB. At the same time, the creep characteristics of the steels were found to be different. A decrease in the carbon content leads to an increase in the long-term creep strength and creep limit at 650°C for short-term tests with time-to-fracture shorter than 103 h. The time to fracture of steels with various carbon contents is almost the same in long-term creep tests. Factor responsible for such effect of carbon on the creep strength are discussed.

The Formation of Fine-Grained Structure in S304H-Type Austenitic Stainless Steel during Hot-To-Warm Working

The dynamic process of grain evolution in a Super304H austenitic stainless steel was studied in compression tests. The tests were carried out to a strain of 0.7 at temperatures ranging from 700 to 1000°C and strain rate of 10-3s-1. In addition to single pass compression the multiple compressions with changing the loading direction in 90o and decreasing the temperature with step of 100°C from 1000 to 700°C in each pass were utilized to achieve large cumulative strains. Under multiple compression the values of flow stresses were lower than those at single-pass compressions under the same temperatures. The fraction of dynamically recrystallized grains decreased from 1.0 to almost zero with decreasing temperature in single-pass compressions. On the other hand, almost fully recrystallized structure developed under conditions of multiple compressions. The size of dynamically recrystallized grains decreased with decreasing the deformation temperature, approaching a submicrometer scale level at 700°C. The relationship between the deformation conditions and operating mechanisms of dynamic recrystallization is discussed in some details.

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 subgrain structure on the creep strength of alloy 1207

The effect of severe plastic deformation (SPD) on the creep resistance of the Al-6%Cu-0.48Mn-0.52Mg-0.3Sc-0.1Zr alloy has been examined in a temperature range of 125–180°C. It has been shown that SPD performed by the method of equal-channel angular pressing at 300°C to a true strain of ∼1 leads to the formation of a well-defined subgrain structure, which is retained upon solution treatment before quenching because of the presence in the alloy of ultradisperse Al3(Sc, Zr) particles with coherent boundaries. It was established that the creep strength at 125–150°C of the as-cast alloy and of the deformed material is approximately the same. At 180°C, the creep rate of the deformed aluminum alloy is almost an order of magnitude lower than that of the as-cast alloy. The reasons for the influence of the subgrain structure on the creep strength of the Al-Cu-Sc-Zr alloy are discussed.

Zener Pinning Pressure in Tempered Martensite Lath Structure

The Zener drag force exerted by M23C6 carbides, Fe2(W,Mo) Laves phase and M(C,N) particles for migration of different grain boundaries in P92-type and P911+3%Co heat-resistant steels was calculated. In particular, the prior austenite grain boundaries (PAGB), boundaries of packets and blocks, which are mainly high-angle boundaries (HAGB), were addressed. Zener pinning pressures were determined for each type of dispersoids separately taking into account that the M23C6 carbides, Fe2(W,Mo) Laves phase are inhomogeneously distributed such that they are mainly located at the boundaries, and the M(C,N) dispersoids are uniformly distributed throughout the metallic matrix. In the both steels, the pinning pressure from the second phase particles located at grain boundaries is about an order of magnitude higher than that caused by homogeneously distributed MX precipitates. In spite of numerous second phase particles precipitated during tempering, grain growth (although rather moderate) occurred during the creep tests of the studied materials. The driving pressure for grain boundary motion might be mostly associated with high dislocation density retained in the tempered martensite structure. The resulting pressure for grain growth in the P92-type steel under creep conditions at 600 and 650°C is somewhat higher than that for the P911 steel.

The Formation of Submicrometer Scale Grains in a Super304H Steel during Multiple Compressions at 700°C

The deformation behavior and the microstructure evolution in a 304-type austenitic stainless steel were studied in multiple forging tests at temperature of 700°C. The flow stresses increased to its maximum value with straining to about 1 and, then, slightly decreased resulting in a steady state deformation behavior at strains above 3. The structural changes were characterized by the development of a spatial net of deformation subboundaries, the misorientations of which increased to the values typical of conventional grain boundaries. The number of ultrafine grains increased with straining, leading to development of submicrocrystalline structure. The fraction of submicrocrystalline structure composed of ultrafine grains with an average size of about 300 nm exceeded 0.7 after straining to 2.

Dynamic polygonization in 9%Cr heat resistant steel

Structural changes in 9%Cr martensitic steel during creep were examined. The grip section of the crept specimen was characterised by a lath martensite structure, which hardly changed during the test. In contrast, quite different microstructure developed in the necking portion of the specimen. The structural changes were characterized by the evolution of relatively large equiaxed subgrains with remarkably lowered density of interior dislocations at places of initial martensite laths. The development of the well-defined subgrains in the necking portion was accompanied with a coarsening of second phase precipitations. The structural mechanism responsible for the microstructure evolution during creep is considered as a dynamic polygonization.

Microstructural changes in cast martensitic steel after creep at 620°C

Microstructural changes in the cast steel GX12CrMoWVNbN10-1-1 (Fe–0.11 C–0.31 Si–0.89 Mn–9.57 Cr–0.66 Ni–1.01 Mo–1.00 W–0.21 V–0.06 Nb–0.05 Cu–0.05 N in wt %) have been investigated after tests for long-term strength at a temperature of 620°C in the range of stresses of 120–160 MPa. Upon short-term creep (up to 5000 h), the tempered troostite structure and distribution of particles of proeutectoid constituents change insignificantly, except for the precipitation of particles of the Laves phase ∼100 nm in size along boundaries of laths, blocks, packets, and initial austenite grains. Upon long-term creep (to 10000 h), the tempered troostite partially transforms into the subgrain structure, which is accompanied by a decrease in the dislocation density from 6.4 × 1014 to 3.1 × 1013 m–2 and connected with growth of sizes of M23C6 carbides of 105–150 nm and particles of the Laves phase to 380 nm, due to the dissolution of these particles located along path boundaries. Upon long-term creep, the average size of V(C,N) particles increases from 45 to 64 nm (while Nb(C,N) particles increase from 48 to 87 nm), and the Nb content in V-enriched carbonitrides and the V content in Nb-enriched M(C,N) particles substantially decrease. No formation of the Z phase has been revealed. The combination of M(C,N) nanoparticles with the presence of W in the solid solution has been found to be responsible for the enhanced high-temperature strength of the steel.