Desenvolvimento e avaliação de um sistema de controle de temperatura para aplicação em ensaios de tração de amostras de aços avançados de alta resistência em temperaturas criogênicas

Abstract

Advanced High Strength Steels (AHSS) are widely used in the manufacturing of automotive body parts, involving high-productivity cutting and stamping operations. The combination of high mechanical strength and plasticity in AHSS is promoted by its microstructure, consisting of a highly ductile ferritic matrix and a second phase of high hardness dispersed in the ferritic matrix. This second phase can be martensite, bainite, or a mixture of both, depending on the steel class. Among AHSS, Dual Phase (DP), Complex Phase (CP), and Transformation Induced Plasticity (TRIP) steels stand out. Due to the presence of the second phase of high hardness, the occurrence of cracks initiated at cutting edges during the punching operation is common, leading to defects during the stamping process. A possible solution to mitigate this problem is being studied at the UFRN Manufacturing Laboratory, hypothesizing the reduction of work hardening at the cutting edges and its implications through nitrogen-assisted punching. In this context, knowledge of the ductile-to-brittle transition temperatures of these AHSS is of great importance. Thus, this work aims to develop and evaluate a temperature control system for use in tensile tests of high-strength advanced steels at cryogenic temperatures, with the purpose of determining the ductile-to-brittle transition temperatures of the AHSS under study at UFRN. To achieve this, a test bench was developed, where a subsize specimen of an AHSS used for tensile tests was positioned. Three liquid nitrogen (LN2) discharge nozzles were placed in the middle of the specimen for cooling at different sub-zero temperatures. The release of LN2 flow to the nozzles was controlled by a solenoid valve managed by a control system composed of a microcontroller and firmware that controls the actuator of the system to maintain the specimen temperature around the chosen reference value in each test, the Setpoint. The temperature of the specimen was measured using a type K thermocouple attached to the sample. The results showed that, even employing a simple ON/OFF control system, it was feasible to control the specimen temperature, although variations around the reference value were observed. However, the test provided indicative data that there is room for improvement and refinement of the system, suggesting possible enhancements that can be implemented in the future.