ABSTRACT
This research consisted of implementing and evaluating an empirical mathematical model to reproduce analytically the dilatometric behavior of ASTM 420A and ASTM 440C martensitic stainless steels, widely used for manufacturing surgical tools. Martensitic stainless steels can be subdivided into three subgroups: low-carbon, medium-carbon and high-carbon steels. The microstructure of each group is also characteristic as needlelike martensitic; very fine martensitic; and ultra-fine martensitic containing carbides. The proposed method was based on experimental data obtained from the dilatometric testing of the steel samples applying low heating rates. It was possible to determine the formation of phase fields near the equilibrium conditions. The method, being based on empirical data, ensured a greater approximation to the experimental values, verifying that it can be applied as a useful tool in the evaluation of industrial heat treatments for surgical tools.
O presente trabalho consistiu em implementar e avaliar um modelo matemático empírico que reproduz analiticamente o comportamento dilatométrico dos aços inoxidáveis martensíticos ASTM 420A e ASTM 440C, utilizados em ferramental cirúrgico. Aços inoxidáveis martensíticos podem ser subdivididos em três subgrupos, ou seja, baixo carbono; médio carbono e alto carbono. A microestrutura de cada grupo é caracterizada por martensita em forma de agulha; martensita fina e martensita ultra-fina contendo carbetos. A elaboração do método matemático se baseou em dados extraídos de ensaios dilatométricos sob baixas taxas de aquecimento. Foi possível determinar a formação dos campos de fase próximos às condições de equilíbrio. Os resultados obtidos garantiram boa aproximação com os valores experimentais, evidenciando que o modelo aplicado é um instrumento útil na avaliação dos tratamentos térmicos industriais para ferramental cirúrgico.
ABSTRACT
The purpose of this study was to evaluate the metal-ceramic bond strength (MCBS) of 6 metal-ceramic pairs (2 Ni-Cr alloys and 1 Pd-Ag alloy with 2 dental ceramics) and correlate the MCBS values with the differences between the coefficients of linear thermal expansion (CTEs) of the metals and ceramics. Verabond (VB) Ni-Cr-Be alloy, Verabond II (VB2), Ni-Cr alloy, Pors-on 4 (P), Pd-Ag alloy, and IPS (I) and Duceram (D) ceramics were used for the MCBS test and dilatometric test. Forty-eight ceramic rings were built around metallic rods (3.0 mm in diameter and 70.0 mm in length) made from the evaluated alloys. The rods were subsequently embedded in gypsum cast in order to perform a tensile load test, which enabled calculating the CMBS. Five specimens (2.0 mm in diameter and 12.0 mm in length) of each material were made for the dilatometric test. The chromel-alumel thermocouple required for the test was welded into the metal test specimens and inserted into the ceramics. ANOVA and Tukey's test revealed significant differences (p=0.01) for the MCBS test results (MPa), with PI showing higher MCBS (67.72) than the other pairs, which did not present any significant differences. The CTE (10-6 oC-1) differences were: VBI (0.54), VBD (1.33), VB2I (-0.14), VB2D (0.63), PI (1.84) and PD (2.62). Pearson's correlation test (r=0.17) was performed to evaluate of correlation between MCBS and CTE differences. Within the limitations of this study and based on the obtained results, there was no correlation between MCBS and CTE differences for the evaluated metal-ceramic pairs.