Tensile strength, fracture toughness and impact properties were evaluated in compacted graphite (CG) cast iron with ferritic, pearlitic and ausferritic microstructures. Ultimate tensile strengths for the ferritic and pearlitic samples were 337 and 632 MPa respectively. The austempered samples showed a significant increment in the strength and recording values between 675 and 943 MPa. The fracture toughness test revealed that the stress-intensity factor K IC was 34?0 MPa m 1/2 for the ferritic CG iron, 39?7 MPa m 1/2 for the pearlitic and between 51?0 and 58?0 MPa m 1/2 for the austempered irons. On the other hand, CG iron with ferritic matrix exhibited the best impact properties with absorbed energy of 33?3 J. The absorbed energy of the pearlitic CG iron was the lowest, 14?3 J, while the austempered samples showed values between 17?2 and 28?4 J. Complementing these results, the critical crack size was also analysed.

Mechanical, electrochemical and magnetic properties of duplex stainless steel were analysed to evaluate its use as biomaterial, comparing the results with those obtained for austenitic stainless steel. Yield and ultimate tensile strengths are almost twice in duplex stainless steel, being the values 870 MPa and 564 MPa, respectively. The electrochemical test revealed that this material has lower susceptibility to localised corrosion because of its greater passive range, 1 V from the open circuit potential, while the austenitic stainless steel exhibited a passive region of 0.370 V. Both steels behave as soft magnetic materials, however, duplex stainless steel has higher magnetic saturation and remanence, while austenitic stainless steel is more prone to heating when exposed to a magnetic field.

Countergravity low pressure casting (CLA) was performed to enhance the properties of ASTM F745 stainless steel (SS), which is usually used as biomaterial. The macro-and microstructures were compared with those obtained by the conventional process of investment casting (IC). The SS cast by CLA (SSCLA) exhibited a smaller size of solidification cell and finer dendritic microstructure. The average of its dendritic primary spacing was 110?4 mm, while for the same steel cast by IC (SSIC), it was 186?7 mm. The density of non-metallic inclusions d I in the SSCLA was 717 I mm 22 , being the majority of them smaller than 1?5 mm. In the case of SSIC, d I was 852 I mm 22 , with a size distribution of up to 8 mm. The SSCLA showed a higher breakdown potential than the SSIC, the values being 0?300 and 0?210 V(saturated calomel electrode) respectively, which means a higher resistance to suffer localised corrosion. Finely, the CLA process also allowed obtaining better mechanical properties.

No abstract

Background: Body fluids are highly corrosive as they contain chlorides and hydroxides ions, as well as salts, bacteria, proteins and dissolved oxygen. The pH of the body is usually around 7.4, although this value can vary in a range of 4 to 9 after surgery or because of haematomas, inflammations and infections. ASTM F745 (type 316L) stainless steel has been used for load bearing partial and total joint replacements and post trauma reconstructive surgeries. However, long exposure to the aggressive effect of chloride ion present in the human body, may increase the susceptibility to suffer localized corrosion. Although UNS S32750 has greater corrosion resistance to chloride ion, its magnetic characteristics inhibit its use in implantable devices. Nevertheless, this stainless steel could be used in temporary implants and orthodontic appliances such as brackets, wire arches and bands, due to its high resistance to corrosion, the greater mechanical resistance and the high capacity of plastic forming. Objectives: The objective was to evaluate the susceptibility to localized corrosion in simulated body fluid, in the pH range of 4 to 9. Another objective was to evaluate the cytotoxicity of Cr and Ni present in the chemical deposition of both stainless steels. Cytocompatibility was also analysed by seeding cells on the surfaces of both stainless steels. Methods: Cyclic polariation test was performed to evaluate the susceptibility to localized corrosion in 0.9 wt% NaCl aqueous solution, at pH between 4 and 9, maintained at 37°C. For cytotoxicity evaluation, neutral red, MTT and collagen assays were performed using UMR-106 cell line. Cytocompatibility was analysed by seeding UMR-106 cells on the surfaces of both stainless steels. Results: F745-SS was more susceptible to suffer localized corrosion than UNS S32750. Although it showed a tendency to develop transpassive reactions at low pH, galvanostatic tests did not reveal the onset of localized corrosion. The results from the cytotoxicity assays indicated that no adverse effects were observed. UMR-106 osteoblastic cells showed high viability, however, it was observed a slight reduction in the collagen production. The cytocompatibility was also satisfactory, since the cells seeded on the surfaces had adequate proliferation. Conclusion: F745-SS is more susceptible to suffer localized corrosion than UNS S32750 in the pH range between 4 and 9. UNS S32750 showed an extensive passive region, however, transpassive reactions were observed at lower pH. On the other hand, no cytotoxic effects were promoted by both stainless steels, although a slight reduction in collagen production was observed. Cells seeded on F745-SS and UNS S32750 surfaces had an acceptable proliferation, without evidence of changes in their morphology.