Roman Mishnev

The Role of Microstructure in Creep Strength of 9-12%Cr Steels

Tempered martensite lath structure (TMLS) plays a vital role in creep resistance of high chromium martensitic steels. Under creep conditions the TMLS could be stabilized by three agents: (i) a dispersion of boundary M23C6 carbides and Laves phase; (ii) a dispersion of M(C,N) carbonitrides, which are homogeneously distributed within ferritic matrix; (iii) substitutional alloying element within ferritic matrix. The boundary particles exert a large Zener drag force which effectively hinders migration of low-and high-angle boundaries. A dispersion of M(C,N) carbonitrides both within ferritic matrix and lath boundaries provides the pinning of mobile dislocations. This process is responsible for reliving long-range elastic stress field originated from lath boundaries. In addition, M(C,N) carbonitrides provide high threshold stress. Substitutional elements as W and Mo effectively slowing down diffusion in ferritic matrix retard climb of lattice dislocation that also prevents the aforementioned knitting reaction. The suppression of knitting reaction between lattice dislocation and low-angle boundaries prevents their transformation to subboundaries by concurrent operation of all three agent types. Depletion of W and Mo from solid solution leads to the occurrence of static recovery and precipitation of Laves phase at boundaries under long-term aging. This process is responsible for creep strength breakdown. The strain-induced formation of Z-phase at the expense of V-rich M(C,N) carbonitrides highly facilitates this process. However, slow strain-induced coarsening of M23C6 carbides and M(C,N) carbonitrides provides the suppression of the knitting reaction between mobile lattice dislocations and intrinsic dislocations of lath boundaries and replacement of TMLS by subgrain structure. Ostwald ripening of boundary M23C6 carbides and Laves phase leads to rapid creep rate increase with strain in tertiary creep and premature rupture owing to the formation of subgrain structure replaced TMLS and further subgrain growth.

The Microstructural Criterion for Creep Strength Breakdown in a 10%Cr Martensitic Steel

A 10%Cr martensitic steel with 3%Co and 0.008%B tempered at 770°C exhibits no creep strength breakdown at a temperature of 650°C up to an extremely high rupture time of ∼4×104 h under an applied stress of 120 MPa. The minimum creep rate was ∼3×10-11 s-1. Microstructural characterization showed that superior creep resistance associated with a high stability of tempered martensite lath structure. Boundary M23(B⋅C)6 phase particles are highly stable against coarsening under long-term aging and creep conditions. These particles retain their orientation relationship with ferritic matrix unchanged under creep at a temperature of 650°C. As a result, no migration of lath boundaries and their transformation to subboundaries diminishing the long-range elastic stress fields take place. The role of M(C,N) carbonitrides in achieving extraordinary high creep strength consists in hindering the knitting reaction between mobile lattice dislocations and lath boundaries.

Dynamic strain aging behavior of 10Cr steel under low cycle fatigue at 650°C

The low cycle fatigue behavior of a 10Cr–2W–0.7Mo–3Co–NbV steel with 80 ppm of B additions was studied at elevated temperatures of 600 and 650°C. The steel after normalizing and tempering at 770°C was tested under fully reversed tension-compression loading with the total strain amplitude controlled from ±0.2 to ±1.0% at temperatures of 600 and 650°C. It was revealed that the steel exhibits a positive temperature dependence of both the cyclic strain hardening exponent n′ and the cyclic strength coefficient K ′ during cyclic loading at 650°C. It was suggested that dynamic strain aging causes fatigue resistance degradation through facilitating microcrack initiation.The low cycle fatigue behavior of a 10Cr–2W–0.7Mo–3Co–NbV steel with 80 ppm of B additions was studied at elevated temperatures of 600 and 650°C. The steel after normalizing and tempering at 770°C was tested under fully reversed tension-compression loading with the total strain amplitude controlled from ±0.2 to ±1.0% at temperatures of 600 and 650°C. It was revealed that the steel exhibits a positive temperature dependence of both the cyclic strain hardening exponent n′ and the cyclic strength coefficient K ′ during cyclic loading at 650°C. It was suggested that dynamic strain aging causes fatigue resistance degradation through facilitating microcrack initiation.

Microstructure Evolution during LCF of a 10%Cr Steel at Room Temperature

The influence of cyclic loading on microstructure and hardness of a 10%Cr steel with 3%Co and 0.008%B was examined at room temperature and total strain amplitudes of ±0.25% and ±0.6%. Low cycle fatigue (LCF) curves exhibit a stress peak after a few cycles. Hardening is attributed to an increase in dislocation density; no changes in lath size were observed. Then stress tends to decrease monotonically with number of cycles that is indicative for material softening. At εac =±0.25%, strain softening is attributed to decreasing dislocation density and lath coarsening under LCF, whereas at εac =±0.6%, the knitting reaction between dislocations comprising lath boundaries and trapped lattice dislocation leading to the transformation of lath boundaries to subboundaries is a reason for hardness decrease and strain-induced subgrain coarsening.

Superplastic Behavior of a Cu-Cr-Zr Alloy Subjected to ECAP

A Cu-0.87%Cr-0.06%Zr alloy was subjected to equal channel angular pressing (ECAP) at a temperature of 400 °C up to a total strain of ~ 12. This processing produced ultra-fine grained (UFG) structure with an average grain size of 0.6 μm and an average dislocation density of ~4×1014 m-2. Tensile tests were carried out in the temperature interval 450 – 650 °C at strain rates ranging from 2.8´10-4 to 0.55 s-1. The alloy exhibits superplastic behavior in the temperature interval 550 – 600 °C at strain rate over 5.5´10-3 s-1. The highest elongation-to-failure of ~300% was obtained at a temperature of 575 °C and a strain rate of 2.8´10-3 s-1 with the corresponding strain rate sensitivity of 0.32. It was shown the superplastic flow at the optimum conditions leads to limited grain growth in the gauge section. The grain size increases from 0.6 μm to 0.87 μm after testing, while dislocation density decreases insignificantly to ~1014 m-2.

Microstructure evolution in a Cu-Cr-Zr alloy during warm intense plastic straining

The effect of equal channel angular pressing at a temperature of 200 °C to a total strain of 12 on microstructure evolution and mechanical properties of a Cu-0.87wt.%Cr- 0.06wt.%Zr was investigated. New ultrafine grains resulted from gradual increase in the misorientations of strain-induced low-angle boundaries with increasing number of passes. Therefore, the development of ultrafine grains is considered as a kind of dynamic recrystallization. The equal channel angular pressing to a total strain of 12 resulted in the formation of almost equiaxed ultrafine grained structure with an average grain size of 0.5 dm and 0.7 dm in the solution treated and aged samples, respectively. At the same time, the fraction of ultrafine grains comprises 0.77 in the solution treated samples and 0.72 in the aged samples. Significant grain refinement led to the remarkable increase of the ultimate tensile strength up to 550 MPa.