B.K. Choudhary

Influence of temperature and strain rate on tensile work hardening behaviour of type 316 LN austenitic stainless steel

Abstract The analysis of tensile work hardening of 316 LN steel revealed three-stage behaviour. The anomalous variation in work hardening rate observed at intermediate temperatures is ascribed to dynamic strain ageing. Increased dynamic recovery at high temperatures resulted in a rapid decrease in work hardening rate.

An assessment of low cycle fatigue damage using magnetic Barkhausen emission in 9Cr-1Mo ferritic steel

A non-destructive, magnetic Barkhausen emission (MBE) technique has been used to assess various stages of low cycle fatigue (LCF) damage in 9Cr-1Mo ferritic steel. The initial decrease in the MBE peak height in the early stage of LCF cycling indicates the cyclic hardening stage, in which the formation of dislocation tangles reduces the mean free path of the domain wall movement. The increase in the MBE level again on further cycling indicates the progressive cyclic softening stage where the rearrangement of dislocation tangles into cells enhances the domain wall movement. The unaltered behaviour of MBE on continued cycling shows the saturation stage where the stabilization of dislocation substructure maintains the MBE level. Finally, a sharp increase in the MBE peak value identifies surface crack initiation and propagation, which is ascribed to the movement of additional reverse domains produced at the crack surfaces. This study establishes that the MBE technique can be used to assess the progressive degradation in the fatigue life of the ferritic steel components.

Relationship between time to reach Monkman–Grant ductility and rupture life

Based on continuum creep damage mechanics approach, we propose a new relationship between time to reach Monkman–Grant ductility and rupture life in terms of damage tolerance factor, and show the validity of this relationship for creep data on 9Cr–1Mo steel and AISI 304 stainless steel. Its implications to tertiary creep damage and engineering creep design are also discussed in this paper. 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Serrated yielding in 9Cr–1Mo ferritic steel

AbstractTensile tests were performed on specimens in quenched and tempered and thermally aged conditions over a wide temperature range (300–873 K) to assess the occurrence of serrated flow, a manifestation of dynamic strain aging (DSA), in 9Cr–1Mo ferritic steel, with an emphasis on the influence of prior thermal aging on serrated yielding. The alloy exhibited jerky/serrated flow in the load–elongation curves at intermediate temperatures. Types A, B, and C serrations were observed, depending on the test temperature and applied strain rate. The apparent activation energy of 83 kJ mol-1 measured for serrated flow suggests that diffusion of an interstitial solute such as carbon is responsible for dynamic strain aging in 9Cr–1Mo steel. Prior thermal aging at 793 K for 5000 h and at 873 K for 1000 and 5000 h resulted in a significant decrease in the height of serrations, i.e. the magnitude of the stress drop, as well as an increase in the critical strain for the onset of serrations. Both of these observations ...

Influence of prior thermal ageing on tensile deformation and fracture behaviour of forged thick section 9Cr-1Mo ferritic steel

Abstract Tensile tests were performed on specimens in quenched and tempered (Q+T) and thermally aged (TA) conditions over a wide temperature range (300–873 K) to assess the influence of prior thermal ageing on tensile deformation and fracture behaviour of forged thick section 9Cr–1Mo ferritic steel. Prior thermal ageing at 793 and 873 K for durations ranging from 10 to 5000 h did not cause a significant change in room temperature tensile properties. However, a marginal decrease in yield strength and reduction in area were observed for specimens aged for longer durations at 793 and 873 K. Prior thermal ageing at 793 K for 5000 h and at 873 K for 1000 and 5000 h produced significant reduction in strength values at intermediate temperatures (523–723 K) compared to that observed at high temperatures. At intermediate temperatures, the alloy in all heat treatment conditions exhibited serrated flow, a manifestation of dynamic strain ageing (DSA). The significant loss of strength in thermally aged conditions at intermediate temperatures has been attributed to reduced propensity to DSA. The elongation to fracture values at temperatures in the range 300–873 K were affected little by prior thermal ageing, whereas the reduction in area exhibited a decrease in the value with increasing thermal ageing. The fracture mode remained transgranular ductile at all test conditions investigated in the present study. However, specimens aged for longer durations exhibited chisel fracture at room and intermediate temperatures due to split in the martensite lath boundaries. The influence of thermal ageing on room temperature tensile properties of the forging remained similar to that reported for thin section 9Cr–1Mo steel.

Influence of strain rate and temperature on tensile properties of 9Cr–1Mo ferritic steel

AbstractTensile tests were performed on specimens in the normalised and tempered condition to assess the effects of temperature (300 – 873 K) and strain rate (6.33×10−5 − 6.33×10−3 s−1) on tensile deformation and fracture behaviour of 9Cr – 1Mo ferritic steel. At all strain rates, the yield and ultimate tensile strength values exhibited a gradual decrease up to intermediate temperatures followed by a rapid decrease at high temperatures. Elongation to fracture and reduction in area displayed a gradual decrease to a minimum at intermediate temperatures followed by a rapid increase at high temperatures. At intermediate temperatures, 9Cr –1Mo steel exhibited serrated flow, plateaus and peaks in strength values and work hardening rate, and negative strain rate sensitivity. The activation energy estimated from the strain to onset of serrations indicated diffusion of interstitial solutes such as carbon responsible for serrated flow in 9Cr – 1Mo steel. At high temperatures, a systematic decrease in yield and tens...

High temperature low cycle fatigue properties of a thick-section 9wt.%Cr-1wt.%Mo ferritic steel forging

Total-axial-strain-controlled fatigue tests have been conducted in air to ascertain the influence of temperature (723, 773 and 793 K) on the low cycle fatigue behaviour of thick-section (300 mm) 9Cr-1Mo tube plate forging (where the chromium and molybdenum contents are in approximate weight per cent). The alloy was tested in a simulated post-weld heat treatment codition. A symmetrical triangular waveform and a constant strain rate of 1 × 10−3 s−1 were employed for all the tests performed over strain amplitudes in the range from ±0.25% to ±1.00%. The crack initiation and propagation modes were studied. Deformation and damage mechanisms which influence the stress response and endurance have been identified. A reduction in fatigue life was observed at all the strain amplitudes with increasing temperatures. The temperature effect on life was more pronounced at low strain amplitudes. The reduction in fatigue life at elevated temperatures was attributed to the combined effects of increased inelastic strain and fatigue-oxidation interactions. Thick-section forged 9Cr-1Mo steel exhibited inferior fatigue resistance compared with the hot-rolled material either in normalized-plus-tempered or in simulated thick-section heat treatment conditions. The poor fatigue resistance of tube plate forging was ascribed to its coarse grain size. The cyclic stress response of tube plate forging varied as a complex function of temperature and strain amplitude. The cyclic stress-strain behaviour could be described by a power law relationship at 773 and 793 K. At 723 K, the alloy exhibited a two-slope cyclic stress-strain curve. The crack initiation and propagation modes remained transgranular at all the conditions investigated.

Effects of strain rate and temperature on tensile deformation and fracture behaviour of forged thick section 9Cr-1Mo ferritic steel

Abstract Tension tests have been conducted to evaluate the influence of strain rate (6.3 × 10−3 to 3.2 × 10−5 s−1) and temperature (300–873 K) on tensile deformation and fracture behaviour of 1000 mm diameter and 300 mm thick 9Cr-1Mo tube plate forging in simulated post weld heat treatment condition. Yield strength and ultimate tensile strength decreased gradually up to around 723 K. Beyond 723 K a rapid fall in both the strength values was observed. In the intermediate temperature range (523–673 K), the alloy exhibited higher yield and tensile strength values with decreasing strain rate, indicating negative strain rate sensitivity. In contrast, at temperatures of 723 K and above, the strength values decreased with decrease in strain rate. Serrated flow, a characteristic of dynamic strain ageing, was observed in the temperature range 523–673 K. The upper end temperature of serrated yielding decreased with decrease in strain rate. A detailed analysis of the dependence of critical strain for the onset of serrations on temperature and strain rate yielded an apparent activation energy of 83 kJ mol−1 for serrated flow at temperatures between 523 and 623 K. Ductility measured in terms of percentage elongation and reduction in area after fracture showed a gradual decrease up to about 673 K and a general increase at high temperatures at all the strain rates, exhibiting a ductility minima in the intermediate temperature range. The alloy has undergone predominantly transgranular ductile fracture at all strain rates and temperatures investigated. Yield and tensile strength of the forged tube plate material is consistently lower than the thin section bar material data. However, the strength values were still higher than the minimum values proposed in the ISO specification for the thin section material. Lower strength values of the forged tube plate material have been attributed to its coarse grain size compared to that of thin section bar material.

Influence of post-weld heat treatment on tensile properties of modified 9Cr–1Mo ferritic steel base metal

Abstract The paper presents the influence of post-weld heat treatment (PWHT) on tensile properties of modified 9Cr–1Mo ferritic steel base metal. Tensile tests at room and elevated temperatures (300–873 K) were performed on specimens in normalised and tempered condition as well as with additional PWHT (993 K for 1 h; 1013 K for 1 h and 1033 K for 1 h). The yield and ultimate tensile strengths decrease gradually up to intermediate temperatures followed by a rapid fall at high temperatures in all heat treatment conditions. At intermediate temperatures, the steel exhibited ductility minima, serrated flow, negative strain rate sensitivity on flow stress and peak in the average work hardening rate. The influence of additional PWHT is reflected in a systematic and gradual decrease in both the yield and tensile strength values with increasing PWHT temperature from 993 to 1033 K for 1 h. However, there has been no appreciable change in ductility values as well as the fracture mode in PWHT conditions compared with those observed in normalised and tempered condition. Comparison of strength values in PWHT conditions suggested that the strength values remained higher than the average values specified in the French Nuclear Design Code, RCC-MR.

Effect of thermal aging on the oxidation behaviour of 9wt.%Cr1wt.%Mo steel

The effect of thermal aging at 793 and 873 K of quenched-and-tempered thick-section 9wt.%Cr1wt.%Mo steel on its oxidation behaviour has been studied in air at 773 K for a maximum exposure of 2000 h. The oxidation rate was found to increase with increasing aging temperature. This increase has been attributed to the depletion of free chromium in the alloy matrix during aging with a corresponding enrichment of the chromium content of M23C6 at the boundaries of the lath martensite and proeutectoid ferrite present inherently in the microstructure. The enhancement of the oxidation rate of the quenched-and-tempered material upon thermal aging has been confirmed by acoustic emission (AE) tests. These showed more AE activity for an aged specimen during oxidation as well as during subsequent cooling, indicating thicker scale formation leading to more cracking and/or spalling. The influence of chromium depletion, due to thermal aging, on the oxide scale composition has also been confirmed by scanning electron microscopy and energy-dispersive X-ray analysis of the oxide-alloy interfaces.