Creep strength behavior of boron added P91 steel and its weld in the temperature range of 600–650 degree C

One of the promising ways for mitigation of Type IV cracking – a failure by cracking at the intercritical /fine grained heat affected zone, a life limiting problem in advanced 9–12 Cr ferritic steel weld like that of P91 is through modification of alloy composition by addition of boron. Addition of boron was observed to improve the microstructure at the weld zone and hence the creep strength. In the present work, boron (100 ppm with controlled nitrogen) added P91 steel after normalizing at 1050C and 1150C and tempered at 760C were studied for the creep behavior in the base metal and welded condition in the temperature range of 600–650C. Creep strength was characterized in terms of stress and temperature dependence of creep rate and rupture time. Weld creep life was reduced compared to the base metal with rupture occurring at the ICHAZ (Type IV crack). However at longer time (at lower stress levels) exposure creep crack moves from weld metal to HAZ (Type II crack). Rupture life was found to superior for the base and weld in the boron containing steel when higher normalizing temperature is used. Estimation of 10 5 h was attempted based on short term rupture data available and weld strength factors were calculated. Observed values are better for P91BH condition than the values for P91BL condition as well as those available for P91 in open literatur

A tuberculosis prevalence survey based on symptoms questioning and sputum examination

A sample survey was undertaken in Raichur district of Karnataka State to estimate the prevalence of bacteriologically positive pulmonary tuberculosis among symptomatics aged 15 years and above. A population of 72,448 persons was registered in a representative sample of 57 villages and 21 enumeration blocks. Of the 42,580 persons aged 15 years and above eligible for symptoms questioning, 40,657 (95.5%) were examined and 3,846 (9.5%) were found to be symptomatics and eligible for sputum examination. Sputum was collected from 3,685 (95.8%) of the 3,846 symptomatics, and subjected to bacteriological examination i.e., smear, culture and drug susceptibility. Certain important findings were as follows: (i) the number of symptomatics increased with increase in age, more often among males (11.9%) than among females (7.1%), (ii) the prevalence of tuberculosis, as assessed by smear and/or culture was 10.9 per 1,000 in population aged 15 years and above, (iii) the prevalence increased with age and was 3 times higher among males as compared to females, (iv) cough was found to be the predominant symptom among the symptomatics (87%) as well as among the cases detected (92%), (v) the prevalence rate based on smear examination of the sputum specimens, using the two microscopy methods (Ziehl-Neelsen and Fluorescence) was 7.6 per 1,000, (vi) culture examination of these specimens yielded 3.3 per 1,100 additional cases, (vii) both the microscopy methods were equally efficient in detecting smear positives, (vii) of the 355 culture positive cases, 17.7% were resistant to Streptomycin, 29.6% to Isoniazid and 7.6% to Rifampicin either alone or in combination with other drugs

Interesting relationships for creep deformation and damage and their applicability for 9Cr-1Mo ferritic steel

The paper presents the validity of several interesting relationships examined for better understanding of creep behaviour of 9Cr-1Mo ferritic steel. Creep rate-rupture time relationships of Monkman-Grant type have been found to be valid. Like stress dependence of creep rate and rupture life, both Monkman-Grant and modified Monkman-Grant relations (MGR and MMGR) exhibited distinct constant values of CMG and CMMG, respectively for low and high stress regimes. The validity of MGR and MMGR is a consequence of the creep deformation behaviour of 9Cr-1Mo ferritic steel obeying first order kinetics. On the basis of creep rate-rupture time relationships of Monkman-Grant type, several other relationships involving transient and tertiary creep parameters have been evolved and their applicability have been examined for the steel. Analogous to MGR and MMGR, a relationship involving transient creep parameters and the other involving tertiary creep parameters were found to be valid. Further, 9Cr-1Mo steel obeyed a recently introduced critical damage criterion interrelating time to reach Monkman-Grant ductility with rupture life, and the criterion depends only on creep damage tolerance factor. This unique relationship is evolved based on the seminal concept of time to reach Monkman-Grant ductility as the time at which the useful safe creep life is exhausted and damage attains a critical level. The important implications of this concept have been discussed

Monkman-Grant relation - analysis of first order kinetics of creep in AISI 304 stainless steel

The transient creep behaviour in AISI 304 Stainless Steel at 873 and 973 K for different test conditions obeyed first order kinetics and it was shown that the validity of Monkman-Grant relation (MGR) is a consequence of first order kinetics. The analysis is extended to tertiary creep regime and the results are found to obey first order kinetics for all test conditions. A new relationship between steady state and tertiary creep is proposed as epsilon over dot ((sic)(s))'.t(t)/epsilon(t) = constant (where t(t) and epsilon(t) are time spent in tertiary and the limiting tertiary creep strain respectively); alpha' is found to be unity for all test conditions implying the validity of first order kinetics. Further, this relation (for alpha' = 1) is found to be identical to modified Monkman - Grant relation (MMGR; m' = 1 where m' is the exponent for epsilon over dot (s) in MMGR) for the conditions satisfying f = 1/lambda and it is postulated that validity of MMGR is also a consequence of first order kinetics. When first order kinetics is applicable, a generalised relationship between steady state creep rate and rupture life is formulated and is valid for the results at all test conditions

A critical damage criterion for creeping solids

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Analysis of first order kinetics for tertiary creep in aisi 304 stainless steel

The results of constant load creep tests on AISI 304 stainless steel at 873 and 973 K for different test conditions were found to obey the first order kinetics for tertiary creep. Distinct master creep curves were obtained at 873 and 973 K with a separate set of constant values of K', epsilon(t), beta' and C-MG. A relationship between steady-state creep rate epsilon over dot(s) , time spent in tertiary creep t(t) and limiting tertiary creep strain epsilon(t) is formulated as epsilon over dot(s) . t(t)/epsilon(t) = constant and is found to be valid for all test conditions. Further, this relation is identical to the modified Monkman-Grant relation (MMGR) for the conditions satisfying f = 1/lambda; it is postulated that the validity of MMGR is a consequence of first order kinetics. Another important outcome of this study is a generalised relation of the form epsilon over dot(s) . t(r) = (epsilon(23).epsilon(f))(1/2); and this relation is compared with the relation proposed by Radhakrishnan, i.e. epsilon over dot(s) . t(r) = (epsilon(23)(2) .epsilon(f))(1/3). It is suggested that cavities act as vacancy sinks and accelerate dislocation climb controlled recovery process leading to tertiary creep. Copyright (C) 1996 Acta Metallurgica Inc

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

Creep deformation behaviour and kinetic aspects of 9Cr-1Mo ferritic steel

This paper presents the detailed investigations on creep behaviour of 9Cr-1Mo ferritic steel with an emphasis to understand and unify the different stages of creep deformation in the framework of first order kinetic approach. The different values of stress exponent and apparent activation energy observed for the two stress regimes have been rationalized by invoking the concept of resisting stress. The detailed analysis of results revealed that both transient and tertiary creep obeyed first order kinetics with separate values of transient and tertiary creep parameters in the respective stress regimes. The two stress regimes with different values of stress exponent are manifested as separate master creep curves for transient and steady state creep. Similarly, the analysis of tertiary creep also revealed distinct master creep curves relating steady state and tertiary creep in the respective stress regimes. The paper also focuses attention on two important relationships, one obtained between transient and steady state creep, and the other between steady state and tertiary creep. The useful implications of these relationships in understanding the existing creep rate-rupture life relationships of Monkman-Grant type are also highlighted in this paper

Transient creep behaviour of forged thick section 9Cr-1Mo ferritic steel

The results of constant load creep tests on quenched and tempered 9Cr-1Mo ferritic steel in the temperature range 773-873 K and in the stress range 60-275 MPa are analysed in the framework of first order kinetics for transient and steady state creep. The strain-time data was adequately described by the Garofalo relation, ε=ε0 +εT [(1-exp(-r×t)] +εs .t, where ε0 is the initial strain at time t=0, εT is the limiting transient creep strain, r is the rate of exhaustion of transient creep and εs is the steady state creep rate. The stress dependences of εs and r exhibited two slope behaviour. The results in both the stress regimes obeyed first order kinetics for transient creep, but with different values of β= 15 and 4.1 for low and high stress regimes respectively, where β is a constant and is the ratio of initial creep rate εi to εs (i.e. εi=β× εs ). Separate master creep curves relating transient and steady state creep were obtained in the respective stress regimes. Further, analogous to Monkman-Grant and modified Monkman-Grant relationships, the relationship between εs , εT and time for onset of steady state creep tos of the form εsα×tos / εT=constant was found to be valid; the value of αwas equal to unity