Mechanical and Mechatronics Engineering

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Browsing Mechanical and Mechatronics Engineering by Subject "22MnB5"

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    Characterizing the Constitutive Behaviour and Austenite Decomposition of an 1,800 MPa Press Hardenable Boron Steel
    (University of Waterloo, 2024-02-02) Lu, Stan
    The mechanical behaviour of a new 1,800 MPa strength grade of Al-Si coated, 37MnB5 press-hardenable steel, designated PHS 1800, was investigated under hot stamping conditions. Prior to mechanical testing, a novel heat treatment technique was developed to pre-alloy the Al-Si coating thereby preventing it from melting during austenitization. The alloy was held at 700°C for 7 – 10 minutes and then quenched to room temperature. This heat treatment technique prevented loss of DIC speckle patterns due to melting of the coating, thereby enabling digital image correlation (DIC) strain measurements of the test specimen at elevated temperatures, and was not observed to affect the constitutive behaviour of the material prior to necking. For constitutive testing, the pre-alloyed material was austenitized and then immediately deformed at strain rates varying between 0.01 and 1s-1 and temperatures between 500°C and 900°C. The experimental tensile test results were processed to obtain the material flow curves, such that the data could be used for finite element simulation of hot stamping processes. An identical series of experiments was also performed on the widely studied 1,500 MPa 22MnB5 hot stamping grade (designated PHS 1500) in order to validate the testing methods and to provide a baseline for assessment of the new PHS 1800 grade. The PHS 1500 results from the current research were compared to several results published in the literature and were found to be in good agreement, indicating the pre-alloying heat treatment and DIC techniques utilized in the current work were valid for the constitutive testing of PHS. Compared to PHS 1500, this new grade of PHS 1800 demonstrated approximately 20% higher flow stress at any given strain level, at the same temperature and strain rate test conditions. This relationship also holds true for both materials after quenching to a fully martensitic state, where the ultimate tensile strength of PHS 1800 is approximately 2,000 MPa and 25% higher than the tensile strength of PHS 1500. For the elevated temperature tensile tests, PHS 1800 exhibited similar strain rate and temperature sensitivities as PHS 1500, wherein higher strain rates or lower temperatures resulted in higher flow stresses. PHS 1800 also experienced necking at up to 31% lower strains than PHS 1500, with the strain to necking varying depending on the test conditions. A modified Norton-Hoff constitutive model, with a second-order polynomial equation to describe the variation in the strain hardening coefficient and strain rate coefficient with respect to temperature, was fit to the experimental results for both PHS 1500 and PHS 1800. An excellent fit (R-squared value of 0.971) was achieved overall at all temperatures and strain rates. A finite element model of the elevated temperature tensile test was created to evaluate the accuracy of the fit curves, and less than 12% error was observed in the predicted load versus displacement response. In addition to the mechanical behaviour of the material, the austenite decomposition characteristics of PHS 1800 were also investigated via constant cooling rate experiments. The effect of deformation on the austenite decomposition was investigated at three strain levels (0%, 10%, 20%), for three different cooling rates (5°C/s, 10°C/s, 50°C/s). It was found that higher cooling rates resulted in greater martensite phase fractions and higher microhardness values. Specimens cooled at 5°C/s and 10°C/s resulted in complex microstructures including ferrite, bainite, and martensite, while the 50°C/s specimens were fully martensitic. The range of hardness results (with no induced deformation) varied between 430HV to 570HV. When deformation was applied to PHS 1800 in its austenitic state, the fraction of ferrite and bainite phases increased substantially after quenching, resulting in a hardness as low as 320HV, a drop of up to 26%. For the specimens cooled at 50°C/s, induced deformation had no effect on the phase fraction or hardness values. Finally, the hardness and phase composition results of the constant cooling rate experiments were utilized to update an existing LS-DYNA PHS material model to enable the prediction of accurate phase change and hardness values for PHS 1800. The updated material model was capable of predicting the final hardness of PHS 1800 to less than 2% error at zero applied deformation, and to less than 14% error for the deformed specimens. The accuracy of the phase fraction predictions varied drastically, ranging from 8% to 56% difference compared to the experimental measurements, depending upon how the material parameters were calibrated.
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    Developing a Thermometallurgical Model and Furnace Optimization for Austenitization of Al-Si Coated 22MnB5 Steel in a Roller Hearth Furnace
    (University of Waterloo, 2019-01-25) Verma, Mohit
    Lightweighting of vehicles while preserving crash-worthiness, in order to satisfy stringent restrictions imposed by the government on the automotive industry, has become a sought after solution which can be realized via hot-forming die quenching (HFDQ). HFDQ is a process where boron-manganese steel blanks, a grade of ultra-high strength steels with a thin eutectic Al-Si coating, are heated beyond TAc3 to achieve a fully austenitic microstructure, a precursor for martensite. Heat treatment is performed using 30 to 40 meter long roller hearth furnaces, comprised of multiple heating zones, with two key objectives: (1) ensure complete austenitization of blanks and (2) transformation of the Al-Si coating into a protective Al-Si-Fe intermetallic coating. Blank heating rates are controlled by the roller speed and zone set-point temperatures, which are currently set by trial-and-error procedures. Therefore, a thorough understanding of the furnace parameters and the industrial objectives are essential. Patched blanks, with spatially varying thickness, leads to inhomogenous heating, making this relationship elusive. Previous furnace-based energy models only focused on simulating the sensible energy of the load with no explicit information about the latent energy associated with austenitization. Consequentially, the latent term had been incorporated into the sensible energy term thereby defining an effective specific heat. In order to realize how blank heating rate influences microstructural and Al-Si layer evolution, a model coupling heating and austenite kinetics is necessary. This integrated model serves as means for optimizing the heating process. In this work a thermometallurgical model is developed, combining a heat transfer submodel with two austenite kinetic submodels, an empirical first-order kinetics model and a constitutive kinetics model, via the latent heat of austenitization. The models simultaneously predict the heating and austenitization curves, for unpatched/patched blanks heated within a roller hearth furnace. Validation studies showed that the first-order kinetics model reliably estimated heating and transformation kinetics compared to the constitutive model. The validated models are then used to optimize the zone set-point temperatures, roller speed, and cycle length for a 12-zone roller hearth furnace whilst minimizing the cycle time in a deterministic setting. A gradient-based interior point method and hybrid scheme were used to assess the constrained multivariate minimization problem with two alternative austenitization constraints imposed: a soak-time based and explicitly modeled requirement. In both cases, the most savings in cycle time were achieved using the explicitly modeled phase fraction austenite constraint, with reductions of approximately 2 to 3 times from the nominal settings.
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    Resistance Spot Welding and In-Process Heat Treatment of Hot Stamped Boron Steel
    (University of Waterloo, 2016-03-15) Hou, Jeff Shao-Chun
    Escalating environmental concerns have prompted efforts to reduce vehicle weight and carbon emissions, resulting in increased application of advanced high strength steels (AHSS). 22MnB5 hot stamping grade AHSS, namely USIBOR 1500P, provide high strength to weight ratio allowing sheet thickness reduction to decrease weight, while maintaining high safety characteristics. Resistance spot welding (RSW) has been the predominant welding process for automotive assemblies. Welding hot-stamping AHSS has introduced new challenges for achieving acceptable welds. The added alloying elements and high hardenability characteristics resulting in low weldability and weld toughness complicates this initiative. The current study examines the effects of in-process weld tempering with secondary current pulse has on the weld toughness during RSW of USIBOR steels. RSW and weld tempering were tested on USIBOR at two different surface conditions; as-delivered and hot-stamped. Joint performance properties, micro-hardness map profiles, and failure modes of welds for both tempered and non-tempered conditions are detailed. Furthermore, a relationship between resulting joint performance and microstructural evolution is produced. The objective of this work is to optimize in-process tempering parameters, analyze metallurgical evolution of the weldments, and compare the effects on mechanical performance for both tempered and non-tempered welds.