Introduction: The mechanical properties and corrosion resistance of a material are dependent on its microstructure and can be modifi ed by phase transformation. When a phase transformation occurs in a material it usually forms at least one new phase, with physical-chemical characteristics that differ from the original phase. Moreover, most phase transformations do not occur instantly. This paper presents an evaluation of the phase transformation of martensitic stainless steels ASTM 420A and ASTM 440C when submitted to different thermal processes. Methods: Dilatometry tests were performed with several continuous heating and cooling rates in order to obtain the profi les of the continuous heating transformation (CHT) and continuous cooling transformation (CCT) diagrams for these two types of steel. Also, the temperature ranges for the formation of the different phases (ferrite and carbides; ferrite; austenite and carbides; non-homogeneous and homogeneous austenite phases) were identifi ed. Rockwell hardness (HRC) tests were performed on all thermally treated steels. Anodic and cathodic potential dynamic polarization measurements were carried out through immersion in enzymatic detergent as an electrolyte for different samples submitted to the thermal processes in order to select the best routes for the heat treatment and to recommend steels for the manufacture of surgical tools. Results: The martensitic transformation temperature tends to increase with increasing temperature for the initiation of cooling. The 440C steel had a higher hardness value than the 420A steel at the austenitizing temperature of 1100 °C. Above the austenitizing temperature of 1100 °C, the material does not form martensite at the cooling rate used, which explains the sharp decline in the hardness values. Conclusion: The study reported herein achieved its proposed objectives, successfully investigating the issues and indicating solutions to the industrial problems addressed, which are frequently encountered in the manufacture of surgical instruments.

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.Keywords Dilatometric test, Mathematical modeling, Phase transformation, Martensitic stainless steels, Surgical tools. Modelamento matemático de ensaio dilatométrico em aços inoxidáveis martensíticos 420A e 440C utilizados em ferramental cirúrgico