Development of process induced residual stress during flow forming of tubular 15-5 martensitic stainless steel

Flow forming is a near net shape process for manufacturing of dimensionally accurate hollow components such as shaft in gas turbines, that is currently at its development stage for aerospace industry. The process has several advantages such as reducing material wastage, extremely fast manufacturing time, and eliminating extra manufacturing processes such as machining. Due to the nature of this complicated cold deformation process, significant magnitude of residual stress is introduced into the component. Understanding the magnitude and distribution of residual stress is essential to tailor the flow forming process to achieve parts within dimensional tolerances and desired mechanical properties. The present research is aiming to explore the generation and evolution of residual stress at various stages of flow forming process in a tubular component made from martensitic 15Cr-5Ni stainless steel, using different techniques of neutron scattering, x-ray diffraction (XRD) and hole-drilling based on electronic speckle pattern interferometry (ESPI). Residual stress measurements were carried out in pre-formed and flow formed components at surface, near-surface and in the bulk of components using XRD, ESPI based hole-drilling and neutron diffraction techniques. These measurements were conducted at different levels of reduction in the thickness of the original part (i.e. after 20% and 40%), by applying identical forming parameters for all samples. The XRD results show significant change in hoop and axial residual stress levels with a reduction in the wall thickness. This is more pronounced for the axial component where the average stress switches from relatively high tensile (~ 450MPa) in the original part to significant compressive stress (~ -600MPa) in the formed part, after 20% of reduction. The bulk residual stress components measured in the middle of thickness of the parts, using neutron scattering, show a general increase in the magnitude of residual stress by higher level of deformation (i.e. reduction in the wall thickness). The measured bulk stress components through the thickness were tuned to tensile after reducing the wall thickness by 40%. The results of XRD and neutron diffraction stress measurements suggest that the residual stress along the length of the samples (i.e. axial direction) is consistent with ±800 MPa and ±400 MPa after 20% and 40% reduction by forming process, respectively. The results of ESPI based hole-drilling show tensile hoop residual stress (≈600 MPa) and an abrupt fluctuation (i.e. tension-compressive-tension) in the axial residual stress near the surface of the part following flow forming. The stresses measured by ESPI based hole-drilling are complementary to the results of the XRD on surface and neutron diffraction in the bulk to reconstruct the residual stress profile form the surface through to the bulk

Influence of thermal ageing on the creep behaviour of a P92 martensitic steel

It is well established that thermal ageing reduces the life of engineering components operating at elevated temperature. However, there is still not an accurate approach to estimate the change in the service life as a result of thermal ageing of the material. In this study, the effect of accelerated thermal ageing, for a thousand hours prior to creep testing at a temperature of 650 ℃ for a martensitic P92 steel has been compared to un-aged steels. The effect of thermal ageing on a primary-secondary stress dependent creep model has been explored, by conducting a set of uniaxial creep experiments on both un-aged and aged P92 steel. It was observed that thermal ageing enhanced the creep deformation for a narrow range of stresses, and systematically reduced the creep rupture life. These applied for conditions by up to a quarter of the original life. Results from creep crack growth tests for similarly aged material reduced incubation time and accelerated the crack growth rate. However, creep stress relaxation tests revealed no evidence of the influence of thermal ageing on rate of relaxation

A critical review of the current progress of plastic waste recycling technology in structural materials

One of the main environmentally threatening factors is plastic waste which generates in great quantity and causes severe damage to both inhabitants and the environment. Commonly, plastic waste generated on the land ends up in water bodies, resulting in detrimental solid impacts on the aquatics via poisoning and flooding the marine ecosystem. Exploring various approaches to convert plastic wastes into new products known as an efficient way to manage them and to enhance the sustainability of the environment, discussed in this article. Moreover, The limitation of the application of plastic waste for construction purposes is also considered. It is wind up that the usage of plastic waste for construction purposes will significantly rectify the sustainability of our environment and also be regarded as a trustworthy source of materials for applying in conventional materials such as concrete and asphalt

Creep deformation and stress relaxation of a martensitic P92 steel at 650 °C

This paper develops methods to predict creep stress relaxation in the presence of combined boundary conditions and explores the influence of primary-secondary stress dependent creep properties on predictions for a martensitic P92 steel at temperature of 650 °C. A series of forward creep and elastic follow-up experiments have been conducted. A summary is provided of empirical creep equations for forward creep and creep stress relaxation (elastic follow-up) tests, including the link to the experimental procedure. The creep stress relaxation tests have been performed with two rigs to give elastic follow-up factors of 1.17 and 1.7. Both time hardening and strain hardening approaches have been considered where the strain hardening model provided more accurate predictions compared to time hardening; except at relatively low stress levels. The difference between stress relaxations predicted using strain hardening and time hardening approaches are considerable. The model predicts the creep stress relaxation accurately in the early stage of relaxation, indicating that the majority of stress relaxation occurs where primary creep needs to be taken into account. This study highlights the importance of stress dependent creep model to predict stress relaxation, especially with high level of initial residual stresses

Bioactive and biodegradable polycaprolactone-based nanocomposite for bone repair applications

This study investigated the relationship between the structure and mechanical properties of polycaprolactone (PCL) nanocomposites reinforced with baghdadite, a newly introduced bioactive agent. The baghdadite nanoparticles were synthesised using the sol–gel method and incorporated into PCL films using the solvent casting technique. The results showed that adding baghdadite to PCL improved the nanocomposites’ tensile strength and elastic modulus, consistent with the results obtained from the prediction models of mechanical properties. The tensile strength increased from 16 to 21 MPa, and the elastic modulus enhanced from 149 to 194 MPa with fillers compared to test specimens without fillers. The thermal properties of the nanocomposites were also improved, with the degradation temperature increasing from 388 °C to 402 °C when 10% baghdadite was added to PCL. Furthermore, it was found that the nanocomposites containing baghdadite showed an apatite-like layer on their surfaces when exposed to simulated body solution (SBF) for 28 days, especially in the film containing 20% nanoparticles (PB20), which exhibited higher apatite density. The addition of baghdadite nanoparticles into pure PCL also improved the viability of MG63 cells, increasing the viability percentage on day five from 103 in PCL to 136 in PB20. Additionally, PB20 showed a favourable degradation rate in PBS solution, increasing mass loss from 2.63 to 4.08 per cent over four weeks. Overall, this study provides valuable insights into the structure–property relationships of biodegradable-bioactive nanocomposites, particularly those reinforced with new bioactive agents

Francisella tularensis human infections in a village of northwest Iran

Background: Recent seroepidemiological studies have suggested that tularemia could be an endemic bacterial zoonosis in Iran. Methods: From January 2016 to June 2018, disease cases characterized by fever, cervical lymphadenopathy and ocular involvement were reported in Youzband Village of Kaleybar County, in the East Azerbaijan Province, northwestern Iran. Diagnostic tests included Francisella tularensis serology (including tube agglutination test and ELISA), PCR, and culture. Results: Among 11 examined case-patients, the tularemia tube agglutination test was positive in ten and borderline in one. PCR detected the F. tularensis ISFtu2 elements and fopA gene in one rodent and a spring water sample from the same geographic area. Conclusions: Based on the clinical manifestations of the disease suggesting an oropharyngeal form of tularemia, serology results in case patients, and F. tularensis detection in the local fauna and aquatic environment, the water supply of the village was the likely source of the tularemia outbreak. Intervention such as dredging and chlorination of the main water storage tank of the village and training of villagers and health care workers in preventive measures and treatment of the illness helped control the infection