Modelling deformation-induced martensite transformation in high-carbon steels

The transformation behaviour of retained austenite in steels is known to differ according to chemical composition and other microstructural attributes. Earlier research indicated that austenite in high-carbon steels transforms into martensite only when the applied stress exceeds a critical value, contrary to low-carbon steels where transformation occurs in the early stages of deformation. Although transformation models have been proposed, most are optimised for low-carbon steels. Here, we propose physics-based models applied to high-carbon steels to overcome previous limitations. The models have fewer free parameters (4) compared to previous approaches (6), exhibiting improvements in the numerical and physical interpretation of the austenite transformation process. We envision the use of these models as tools for alloy design, also highlighting their scientific and technological value

Failure of femoral neck fracture fixation in younger patients

Background: Failure of femoral neck fracture fixation in younger patients frequently lead to significant morbidity. This study aims to evaluate the associated risk factors, to help achieve better outcomes.Methods: Records of 25 men and 6 women aged 23 to 50 (mean, 39.8) years who underwent fixation for femoral neck fractures using cancellous screws (n=29), a dynamic hip screw (n=1), or a locking plate (n=1) were reviewed. Patients with or without fixation failure were compared.Results: The mean follow-up period was 28.4 (range, 24–38) months. Of the 31 patients, 24 had bone union and 7 had fixation failure secondary to non-union (n=1), screw cut-out (n=4), and avascular necrosis (n=2). The Revised Trauma Score on admission was 4 in one patient with fixation failure and >6 in all other patients. Comparing patients with or without fixation failure, excess alcohol consumption (5/7 vs. 6/24, p=0.024), smoking (6/7 vs. 9/24, p=0.025), and operative time in minutes (57±16 vs 108±58, p=0.00004), fracture type of OTA 31 B2 or 31 B3 (7/7 vs. 14/24, p=0.027) were significant risk factors.Conclusions: Excess alcohol consumption, smoking, operative time >85 minutes, and displaced fractures were risk factors for femoral neck fixation failure

Understanding the Stability of Retained Austenite in High-Carbon Steels: Modelling and Alloy Design

In high-carbon steels, the austenite-to-martensite transformation under applied stress initiates at a critical value, contrary to low-carbon steels where the transformation occurs at the early stages of deformation. There are few models that account for the delayed transformation, because previous models were mostly developed from low-carbon steels that exhibit martensite transformation behaviour that is different from high-carbon steels. Also, few studies have been done on methods of tailoring chemical composition and processing to design steels that possess optimal austenite stability, but do not require too many expensive austenite stabilising elements such as nickel. This research aims to develop equations for modelling the critical stress and progress of deformation-induced martensite transformation in high-carbon steels. Another aim is to design a new high-manganese steel that is comparable to the high-nickel AISI 3310 carburising steel in terms of austenite stability and mechanical properties. The research investigated carburising-grade steels used in bearing applications. Various thermomechanical tests were conducted to investigate the influence of chemical composition and microstructure on retained austenite stability. Dilatometry experiments showed that the martensite-start temperature decreased with increasing grain size when the steels were austenitised below the Acm temperature, but increased with grain size above the Acm temperature. These observations were linked to the amount of carbon dissolved in austenite. Analytical models that describe the critical stress and progress of martensite transformation in high-carbon steels were developed. The critical stress model is a function of chemical composition, deformation temperature, and initial retained austenite fraction. Higher austenite stabiliser concentrations or deformation temperatures result in smaller magnitudes of the chemical driving force, leading to a higher critical stress predicted and represent higher austenite stability. A lower initial austenite fraction represents fewer martensitic nucleation sites, resulting in deformation-induced martensite transformation that occurs at a higher critical stress. The transformed amounts of austenite under applied stress depends on the critical stress for martensite transformation. The predicted results were in good agreement with experimental data. Model applications were demonstrated through examples that involve the determination of alloying combinations and service conditions that are suitable for desired range of critical stresses and austenite fractions predicted. The examples highlight the utility of these models as tools for alloy design. A new high-manganese carburised steel was developed based on thermodynamic modelling and literature-informed design criteria. Samples of the new steel and AISI 3310 steel were carburised and subjected to martensitic quench-and-temper heat treatments. The mechanical properties and austenite mechanical stability of the new steel were found to be not on par with the 3310 steel. The reasons include a lower bulk carbon content, and the presence of massive carbides and oxide inclusions in the new steel. Practical solutions to improve the low-pressure carburisation process of the new steel are suggested in Section 7.5.2. The possibility of designing new carburising steels with lower costs by tailoring composition and processing routes based on enhancing austenite stability was explored and demonstrated. Process improvement aspects identified from the carburisation and heat treatment of the new steel can be used to inform the processing of such steels in the future.This PhD project was funded by The Timken Company

Psychosomatic symptoms during South East Asian haze crisis are related to changes in cerebral hemodynamics.

ObjectivesForest fires in South Asia lead to widespread haze, where many healthy individuals develop psychosomatic symptoms. We investigated the effects of haze exposure on cerebral hemodynamics and new symptoms. We hypothesised that vasoactive substances present in the haze, would lead to vasodilation of cerebral vasculature, thereby altering cerebral hemodynamics, which in turn may account for new psychosomatic symptoms.MethodsSeventy-four healthy volunteers were recruited, and serial transcranial Doppler (TCD) ultrasonography was performed to record blood flow parameters of bilateral middle cerebral arteries (MCA). The first TCD was performed in an air-conditioned environment. It was repeated outdoors after the participants spent 30-minutes in the haze environment. The prevailing level of pollutant standards index (PSI) was recorded. Appropriate statistical analyses were performed to compare cerebral hemodynamics at baseline and after haze exposure in all participants. Subgroup analyses were then employed to compare the findings between symptomatic and asymptomatic participants.ResultsStudy participants' median age was 30 years (IQR 26-34), and new psychosomatic symptoms were reported by 35 (47.3%). There was a modest but significant decrease in pulsatility index (PI) and resistivity index (RI) in the left MCA after haze exposure (PI: p = 0.026; RI: p = 0.021). When compared to baseline parameters, haze exposure resulted in significantly lower mean PI (p = 0.001) and RI (p = 0.001) in symptomatic patients, but this difference was not present in asymptomatic patients (PI: p = 0.919; RI: p = 0.970).ConclusionHaze causes significant alterations in cerebral hemodynamics in susceptible individuals, probably responsible for various psychosomatic symptoms. The prognostic implications and health effects of haze require evaluation in a larger study