V. Sagar Dwivedi

Characterization of a precrept modified 9Cr-1Mo steel through room temperature tensile and strain rate change tests

Abstract A Nb–V containing 9Cr–1Mo ferritic steel was allowed to age under creep conditions in the temperature range of 873–973 K. Its microstructure in normalized and tempered condition contained tempered martensite with well recognized laths. However, under high stress creep conditions at 973 K, cell walls and dislocation clusters are formed. A cellular structure develops instead with cell walls being formed at carbides. These microstructural features are reflected in their subsequent room temperature deformation behavior, which was studied through an analysis of strain rate change (SRC) tests and stress–strain curves. In the new analysis, Cottrell–Stokes (C–S) ratio and modified Haasen plots are derived to study the nature of thermal obstacles and component internal stresses. SRC results are used also to estimate long-range internal stresses, which are interpreted to be associated with cell walls, dislocation clusters etc. The nature of variation of activation area, long range stress, and work hardening rate as determined from room temperature tensile tests carry sufficient information to suggest a methodology for evaluating the damage state of the parent service exposed material.

Transient yielding in strain rate change tests

Strain rate change (SRC) tests, during a tensile test, are carried out frequently to determine values of activation area and proper interpretation of these results may provide considerable information on dislocation activities during deformation. However, transient load drop is generally observed during low to high SRC and this drop increases with increasing deformation. This effect may thus be exacerbated, at least in principle, by conducting SRC tests on pre-crept samples. In the present work the authors report these results and also discuss the nature of transients during low to high SRC with reference to overall deformation processes. The material used for this work was a modified 9Cr-1Mo steel containing niobium and vanadium. All SRC tests were carried out at room temperature on an Instron by instantaneously changing the cross-head speed by a factor of 10. Strain gauges were used to not only continuously monitor the overall strain but also to obtain the magnitude of strain accompanying the change in cross head speed.

STRENGTH, FRACTURE AND DEFORMATION BEHAVIOUR OF PORTLAND CEMENT PASTE

ABSTRACT The effect of strain rate on the strength and fracture toughness of cement paste has been studied at room temperature. At strain rates up to 10-3/s cement paste exhibits microductility, with rising strength and an activation volume of 1.5 - 2.0 × l04 a3. At strain rates above 10-3/s cement paste behaves in a brittle manner with constant strength and a marked fall in fracture toughness. This transition to brittleness is interpreted in terms of micro-shearing and slow crack growth. The critical strain rate corresponds to the fastest relaxation process found in a stress-relaxation test.

A new criterion for sound weldment

Abstract A new criterion for the occurrence of microcracks in weldments obtained by electrical resistance welding is proposed. The criterion is based on the free energy of a weldment system, which is proportional to a parameter R where R = grad h × Δh and grad h and Δh are the gradient and peak height respectively (with respect to the parent metal) of the hardness profile across the weld seam. Whenever the value of R for a weldment is greater than the critical value R c , the weldment is likely to contain microcracks. the Value of R c depends on the material and is found to be approximately 1000 (HV 25) mm −1 for a CMn steel.