Ultra-Rapid Laser Calorimetry for the Assessment of Crystallization in Low-Concentration Cryoprotectants

Cryoprotective agents (CPAs) are routinely used to vitrify, attain an amorphous glass state void of crystallization, and thereby cryopreserve biomaterials. Two vital characteristics of a CPA loaded system are the critical cooling and warming rates (CCR and CWR), the temperature rates needed to achieve and return from a vitrified state respectively. Due to the toxicity associated with CPAs, it is often desirable to use the lowest concentrations possible, driving up CWR and making it increasingly difficult to measure. This paper describes a novel method for assessing CWR between the 0.4×105-107 °C/min in microliter CPA loaded droplet systems with a new ultra-rapid laser calorimetric approach. Cooling was achieved by direct quenching in liquid nitrogen, while warming was achieved by the irradiation of plasmonic gold nanoparticle-loaded vitrified droplets by a high-power 1064 nm millisecond pulsed laser. We assume "apparent" vitrification is achieved provided ice is not visually apparent (i.e. opacity) upon imaging with a camera during cooling or highspeed camera during warming. Using this approach, we were able to investigate CWR in single CPA systems such as glycerol, PG, and Trehalose in water, and mixtures of glycerol-trehalose-water and propylene glycol-trehalose-water CPA at low concentrations (20-40 wt%). Further, an phenomenological model for determining the CCRs and CWRs of CPA was developed which allowed for predictions of CCR or CWR of single component CPA and mixtures, providing an avenue for optimizing CCR and CWR and perhaps future CPA cocktail discovery.

Microstructural and Tribological Characterization of API X52 Dual-Phase Steel

The effect of the morphology and the martensite volume fraction on the microhardness, the tensile, the friction and the wear behavior of API X52 dual phase (DP) steel has been investigated. Three different heat treatments were used to develop dual phase steel with different morphologies and with different amounts of martensite: Intermediate Quenching Treatment/Water (IQ); Step Quenching Treatment (SQ) and direct quenching (DQ). Tribological tests are conducted on DP steels using a ball-on-disc configuration under normal load of 5 N and at a sliding speed of 4 cm/s were used to study the friction and wear behavior of treated samples. Results show that the ferrite–martensite morphology has a great influence on the mechanical properties of dual phase steel. The steel subjected to (IQ) treatment attain superior mechanical properties compared to the SQ and the DQ treatments. On the other hand, it is also found that the friction coefficient and the wear rate (volume loss) decrease when the hardness and the martensite volume fraction increase. The steel with fine fibrous martensite provide good wear resistance.

Effect of Quenching Strategy and Nb-Mo Additions on Phase Transformations and Quenchability of High-Strength Boron Steels

The application of direct quenching after hot rolling of plates is being employed in the production of ultra-high-strength hot rolled plates. When heavy gauge plates are produced, the complexity involve in achieving high cooling rates in the plate core is increased and the formation of undesirable soft phases within martensite is common. In the current paper, both direct quenching and conventional quenching (DQ and CQ) processing routes were reproduced by dilatometry tests and continuous cooling transformation (CCT) diagrams were built for four different high-strength boron steels. The results indicate that the addition of Mo and Nb-Mo suppresses the ferritic region and considerably shifts the CCT diagram to lower transformation temperatures. The combination of DQ strategy and the Mo-alloying concept provides the best option to ensure hardenability and the formation of a fully martensitic microstructure, and to avoid the presence of soft phases in the center of thick plates.

Effect of Martensite Morphologies on Corrosion in 5% H2SO4 Solution of Borided X70 Dual Phase Steel

In the present investigation, some electrochemical properties of dual phase X70 steels with different martensite morphologies which have undergone boriding were studied. To obtain a variety of martensite morphologies, Direct Quenching (DQ), Intermediate Quenching (IQ) and Step Quenching (SQ) heat treatments were applied at an intercritical annealing temperature (IAT) of 760℃. The treatment (DQ) allowed the formation of fine martensite evenly distributed in the ferrite matrix. (IQ) treatment showed the formation of martensite along the ferrite / ferrite grain boundaries. In contrast, treatment (SQ) induced the formation of a banded morphology of martensite and ferrite. The realization of borides on X70 (DP) steel was carried out in a powder mixture containing 5% of B4C as source of boron, 5% of NaBF4 as activator and 90% of SiC as diluent at 950℃ for a period of time from 4 h. The corrosion behavior of X70 (DP) steel has been explored by the Tafel extrapolation method in a 5 wt. % H2SO4 solution. The corrosion resistance of steel which has undergone boriding (BDP) is higher than that of steel which has not undergone it (DP).

Toughness Property Control by Nb and Mo Additions in High-Strength Quenched and Tempered Boron Steels

The synergetic effect on hardenability by combining boron with other microalloying elements (such as Nb, Mo and Nb + Mo) is widely known for high-strength medium carbon steels produced by direct quenching and subsequent tempering treatment. The improvement of mechanical properties could be reached through optimization of different mechanisms, such as solid solution hardening, unit size refinement, strain hardening, fine precipitation hardening and the effect of carbon in solid solution. The current study proposes a procedure for evaluating the contribution of different microstructural aspects on Charpy impact toughness. First, the effect that austenite conditioning has on low-temperature transformation unit sizes and microstructural homogeneity was analysed for the different microalloying element combinations. A detailed crystallographic characterization of the tempered martensite was carried out using electron backscattered diffraction (EBSD) in order to quantify the effect of unit size refinement and dislocation density. The impact of heterogeneity and presence of carbides was also evaluated. The existing equations for impact transition temperature (ITT50%) predictions were extended from ferrite-pearlite and bainitic microstructures to tempered martensite microstructures. The results show that microstructural refinement is most beneficial to strength and toughness while unit size heterogeneity has a particularly negative effect on ductile-to-brittle transition behaviour. By properly balancing alloy concept and processing, steel having a yield strength above 900 MPa and low impact transition temperature could be obtained by direct quenching and tempering.

Improvement on Impact Toughness of Cold Formed S420 Steel by Direct Quenching

The aim of this work is to study the effect cold forming rate (CFR) on the mechanical properties and microstructure of a conventional TMCP and a direct-quenched steel in 420 MPa strength level. The microstructure was characterized using FESEM-EBSD. Tensile properties and Charpy-V impact toughness were determined. As the CFR increased, the yield and tensile strength raised quite linearly with both steels. Yield strength values increased from 450 MPa (as-rolled material) to 700 MPa (25 % CFR). However, tensile strength increased less compared to yield strength. Uniform elongation decreased linearly till about 10 % CFR and total elongation till about 15 – 20 % CFR. The impact values decreased quite linearly in -40 °C and -60 °C test temperature when the cold forming rate increased. In longitudinal direction (L-T) the impact values were at high level at -40 °C and -60 °C with both steels with all CFR. In transverse direction (T-L) the impact results were lower. Impact energies were enhanced by direct quenching compared to conventional steel in every CFR stage. EBSD results showed no major difference between steels in the grain sizes in generally. However, cold forming decreased the grain size and increased low-angle grain boundaries in correlation with increasing CFR. Small size of the coarsest grains (d90%) usually indicate better toughness, however in this case the impact values were decreased even with smaller grain size as cold deformation occurs. On the other hand, the strength level increased with forming rate. Therefore, a brief discussion of the microstructural features controlling the impact toughness is given.