Resumen:
Tool, machinery and structural steels are the type of Fe-C alloy, which is the material most used in the manufacture of tools, machine parts and equipment for the construction, in addition to work elements that allow various industrial activities in which high-strength and durability is required due to the corrosion and wear effects. In this sense, there are common steels that are useful for different manufacturing applications to improve their working efficiency and lifetime through the study of the mechanical behavior when they are subjected to a heat treatment and thermochemical process to be viable operationally and they can meet the requirements of the engineering industry.
For this reason, the need to improve the materials used daily in the engineering industry, construction, agriculture, chemical, food, among others, so in this research three types of AISI carbon steels (D2, cold work tooling, H13, hot work tooling and 1045 structural materials) were subjected to a heat treatment followed by a thermochemical nitriding process using two different salt mixtures: NaCNO + Na2CO3 or NaNO3 + KNO3 in order to improve their wear resistance and corrosion protection. Additionally, two heat treatments such as quenching and tempering were combined for a better performance. Furthermore, the nitriding times used in this research varied from 60-120 min in terms of two temperatures 600 and 650 °C.
The nitrided products were structurally characterized by using optical microscopy (OM), scanning electron microscopy (SEM) and X-ray diffraction analysis (XRD), while the nitride hardness surface was determined by Vicker test (Hv). Additionally, the evaluation of corrosion resistance was performed by electrochemical techniques; anodic polarization-DC current and electrochemical impedance spectroscopy, EIS, AC current. The results indicate that the application of a suitable thermochemical treatment for a tool and structural steels can significantly increase the lifetime of the steel-materials. The best nitriding time was found to be 120 min, in which different thicknesses of the nitrided layer was obtained: 52 m in D2 steel, 44 m for H13 steel and 21 m in 1045 steel. The X-ray analysis of the nitrided products showed that it is possible to produce hard compounds on the surface of nitrided steels.
Furthermore, the SEM images showed that some steel samples are composed by two layers, an oxide layer followed by a nitride coating. In some cases the results of the analysis, showed that the nitrided layer is not observed on steels, an oxide compounds of hematite, magnetite and
cohenite were formed at high temperatures. Finally, the corrosion tests indicated that the nitriding layer is useful to decrease the corrosion rate providing corrosion resistance of the industrial steels.
Descripción:
Se logró nitrurar exitosamente los tres tipos de acero AISI D2, H13 y 1045 mediante el método de sales fundidas con una mezcla de sales de NaCNO+Na2CO3
La mezcla de sales de NaNO3+KNO3 no posee las propiedades necesarias para nitrurar los aceros D2, H13 y 1045 ya que no se lleva a cabo la reacción de disociación del nitrógeno.
Se obtuvo un incremento en la dureza superficial de los tres tipos de materiales de 3-5 veces en comparación con los aceros sin tratamiento.
Las propiedades mecánicas se asocian a los cambios micro-estructurales durante el T.T. y la nitruración con sales.
Se optimizó el proceso de nitruración con sales en base al análisis factorial y estableciendo las mejores condiciones experimentales.
El proceso de nitruración con sales fundidas en comparación con otros métodos (gaseoso y iónico) representa un método optimizado de bajo costo que tiene impacto en la materia prima y equipos de nitruración, además de ser un proceso no tóxico y por lo tanto amigable con el medio ambiente.
Se obtuvieron recubrimientos con espesores homogéneos capaces de actuar como barreras anticorrosivas en los tres tipos de aceros.
Los recubrimientos obtenidos a partir de la mezcla de sales de NaCNO+Na2+CO3 favorecieron a un mejor desempeño frente a la corrosión en un medio de H2SO4+NaCl, que los recubrimientos obtenidos con la mezcla de sales de NaNO3+KNO3.