A Multi-Method Analysis of Carburized Materials: Accurately Measuring the Carbon Gradient and Comparisons to Physical Characteristics

Carburization is a common method of hardening steel surfaces to be wear-resistant for a wide range of mechanical processes. One critical characteristic of the carburization process is the increase in carbon content that leads to the formation of martensite in the surface layer. Combustion and spark-OES are two common methods for determination of carbon in steels. However, these techniques do not effectively separate carbon from near surface contaminants, carburized layers, and base material composition. Careful consideration of glow discharge spectroscopy as a method of precisely characterizing carbon concentration in surface layers as part of a production process should be evaluated in terms of how the resulting data align with other common analytical and metallurgical measurements. When used together, glow discharge spectroscopy, optical microscopy, and microhardness testing are all useful, complementary techniques for characterizing the elemental composition, visually observable changes in material composition, and changes in surface hardness throughout the hardened case, respectively. Close agreement between related measurements can be used to support the use of each of these techniques as part of a strong quality program for heat treatment facilities.

Improving the Tribological Properties and Biocompatibility of Zr-Based Bulk Metallic Glass for Potential Biomedical Applications

Zr-based bulk metallic glasses (Zr-BMGs) are potentially the next generation of metallic biomaterials for orthopaedic fixation devices and joint implants owing to their attractive bulk material properties. However, their poor tribological properties and long-term biocompatibility present major concerns for orthopaedic applications. To this end, a novel surface modification technology, based on ceramic conversion treatment (CCT) in an oxidising medium between the glass transition temperature and the crystallisation temperature, has been developed to convert the surface of commercially available Zr44Ti11Cu10Ni11Be25 (Vitreloy 1b) BMG into ceramic layers. The engineered surfaces were fully characterised by in-situ X-ray diffraction, glow-discharge optical emission spectroscopy, scanning electron microscopy, transmission electron microscopy, and scanning transmission electron microscopy. The mechanical, chemical, and tribological properties were evaluated respectively by nano-indentation, electrochemical corrosion testing, tribological testing and the potential biocompatibility assessed by a cell proliferation assay. The results have demonstrated that after CCT at 350 °C for 40 h and at 380 °C for 4.5 h the original surfaces were converted into to a uniform 35–55-nm-thick oxide layer (with significantly reduced Ni and Cu concentration) followed by a 200–400-nm-thick oxygen-diffusion hardened case. The surface nano hardness was increased from 7.75 ± 0.36 to 18.32 ± 0.21 GPa, the coefficient of friction reduced from 0.5–0.6 to 0.1–0.2 and the wear resistance improved by more than 60 times. After 24 h of contact, SAOS-2 human osteoblast-like cells had increased surface coverage from 18% for the untreated surface to 46% and 54% for the 350 °C/40 h and 380 °C/4.5 h treated surfaces, respectively. The significantly improved tribological properties and biocompatibility have shown the potential of the ceramic conversion treated Zr-BMG for orthopaedic applications.

X-ray Determination of Compressive Residual Stresses in Spring Steel Generated by High-Speed Water Quenching

Automotive components manufacturers use the 5160 steel in leaf and coil springs. The industrial heat treatment process consists in austenitizing followed by the oil quenching and tempering process. Typically, compressive residual stresses are induced by shot peening on the surface of automotive springs to bestow compressive residual stresses that improve the fatigue resistance and increase the service life of the parts after heat treatment. In this work, a high-speed quenching was used to achieve compressive residual stresses on the surface of AISI/SAE 5160 steel samples by producing high thermal gradients and interrupting the cooling in order to generate a case-core microstructure. A special laboratory equipment was designed and built, which uses water as the quenching media in a high-speed water chamber. The severity of the cooling was characterized with embedded thermocouples to obtain the cooling curves at different depths from the surface. Samples were cooled for various times to produce different hardened case depths. The microstructure of specimens was observed with a scanning electron microscope (SEM). X-ray diffraction (XRD) was used to estimate the magnitude of residual stresses on the surface of the specimens. Compressive residual stresses at the surface and sub-surface of about −700 MPa were obtained.

Effect of Different Corroding Media on the Corrosion Rate of Locally Produced Thermo Mechanically Treated (TMT) High Strength Structural Steel Bar

The corrosion behavior/rate of TMT steel bars in different corroding media such as rain water, storage atmosphere and concrete ponded in 10% sodium chloride solution has been investigated for a corrosion duration of 40 weeks. The corrosion rate of TMT steel bars corroded in the three media was measured at an interval of 4 weeks. Both macroscopy and microscopy were carried out to reveal different zones such as hardened case, soft core and transition zone in the cross-section and microstructure in these zones respectively of the TMT steel bars in the as received condition and after corrosion of 40 weeks duration. It was found that the corrosion rate of the TMT steel bars in rain water decreases gradually with corrosion duration and reaches a minimum value at 24 weeks of duration after which the corrosion rate increases. The corrosion rate of TMT steel bars in storage atmosphere initially increases and after reaching a maximum value at 16 weeks duration it starts decreasing. The corrosion rate of the TMT steel bars in concrete with 10% NaCl solution decreases very slowly and after reaching minimum value at 32 weeks of duration it starts increasing gradually. It was also found that the corrosion rate of the TMT steel bar in rain water is much higher than that of the TMT bar in storage atmosphere and the corrosion rate of the steel bars in concrete with NaCl solution is the lowest of all the media.

Ultrasonic Testing for the Hardened Layer Depth of Induction Quenched 20CrMo Axle

The interface between the surface hardened layer and the base layer would produce while the 20CrMo axis is treated with high frequency induction quenching method; therefore, the hardened case depth can be measured by high frequency ultrasonic wave based on the echo technique. The test results compared with those from the metallographic method show good qualitative agreements.

Residual Stress Distribution in Hardened Case Layer of Cr-Mo Steel after Carburizing and Quenching

The hollow circular cylinder specimen of Cr-Mo steel with 0.20 mass% C was carburized in carrier gas and quenched in oil bath. After quenching, the surface residual stress distributions in the radial, axial and hoop directions of the specimen were measured experimentally by x-ray as a function of the distance from the carburized surface. The case depth of the quenched specimen was about 0.8 mm. Diffraction from Fe-211 by Cr-Kα radiation was used to minimize the effect of carbon content gradient on the nonlinearity of 2θ-sin2ψ, because effective x-ray penetration depth was about 5.8 μm at sin2ψ=0. The hardened case layer was gradually removed by electrolytic polishing. X-ray stress measurement was repeated on the polished surface from the carburized surface to the interior. The ψ-splitting was not observed on the carburized surface. The 2θ-sin2ψ diagrams were found to shift from low to high angles in inverse proportion to carbon content. The residual stresses in the hardened case layer were compressive. The maximum compressive residual stresses on the hollow circle and periphery surfaces were about −559 and −544 MPa at the depth of 0.2 and 0.3 mm, respectively. On the other hand, the tensile residual stress was not observed. The full widths at half-maximum intensity of Fe-211 diffraction peaks in the hardened case layer were wider than that of the interior of the specimen. Martensitic transformation of the carbon-rich layer leaded to the broadening of diffraction peaks. Therefore the interior of carburized specimen were deformed elastically to balance the existence of the surface compressive residual stresses.

PARAMETERIZED FINITE ELEMENT MODELING OF THE SURFACE SYSTEMS WITH COATINGS WHICH CONSIDERS THE CRACKING OF THE COATINGS AND INFLUENCES OF THE CASE-HARDENING OF THE SUBSTRATE

Accurately predicting the failure of multilayered surface systems, including coatings on tools or products, is of significance for all of the parties concerned within the chain of design, manufacturing and use of a product. Previous modeling work has, however, been focused largely on the effect of individual parameters rather than on the performance of a multilayered system as a whole. Design and manufacture of multilayered surface systems, currently, still relies largely on experiments and failure tests. A parameterized approach which considers geometrical, material, interfacial and loading variables, processing history, thermal effects, surface-failure modeling, etc. has therefore been developed to address the situation in order to be able to improve the efficiency and accuracy of the analysis and design of multilayered coating-systems. Material property values for the hardened case of the substrate are described with a function of the hardened depth and defined with a field method. Initial residual stresses calculated using a newly developed theoretical model are incorporated into the model as initial stress conditions. Thermo-mechanical coupled modeling is incorporated into the model so as to be able to consider temperature effects. These are associated with a cohesive-element modeling approach, which has been used to predict indentation-induced crack initiation and propagation within the coating layer. The comparison of experimental results with those of numerical modeling affords excellent agreement. The parameterized modeling method developed allows for the parameters to be changed easily during a series analysis. Combined with the capability of the prediction of cracking of the coatings, the developed method/model provides an efficient way for investigating the effects of these parameters on the behavior of multilayered systems, which is demonstrated by the analysis of three cases of the coated tool steels (H11): (i) a substrate without being pre-heat-treated; and (ii) two substrates with a shallow and a deep hardened-case, respectively, (both are treated by plasma-nitriding). The results showed that the case-hardening of a substrate has a significant influence on the performance of the surface system with coating, especially on its load-bearing capacity and the cracking of the coating.