THE EFFECT OF TEMPERING TEMPERATURE VARIATION IN DEEP CRYOGENIC TREATMENT ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF AISI 4340 STEEL

• Main Author: Al Syaukhani, Imam
• Format: Final Project
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/85025
• Institution: Institut Teknologi Bandung

AISI 4340 steel is widely used in the industry due to its ease of manufacture, good weldability, and low production costs. Deep cryogenic treatment is a process that enhances the mechanical properties of steel, being cost-effective, energy-efficient, non-damaging to processing equipment, and non-polluting. This process transforms the retained austenite microstructure into martensite, thereby increasing the strength, hardness, and toughness of the steel. However, the enhancement of mechanical properties through deep cryogenic treatment is influenced by the austenization and tempering process parameters, as well as the cooling media. Hence, research can be conducted to study the effects of these heat treatment parameters on deep cryogenic treatment. This study performed the deep cryogenic treatment process with varying tempering temperatures in the conventional heat treatment process of AISI 4340 steel. The experiment started with initial characterizations including Vickers hardness test, tensile test, Charpy impact test, metallographic characterization, and chemical composition analysis using optical emission spectroscopy on the as-received samples. Then, the heat treatment was conducted with austenization at 900 °C for 30 minutes, oil quenching, and tempering for 1 hour at varying temperatures of 150 °C, 200 °C, and 250 °C in a muffle furnace, followed by deep cryogenic treatment at -196 °C for 1 day. The experimental samples were divided based on whether they underwent deep cryogenic treatment with varying tempering temperatures. After heat treatment, characterization was performed using Vickers hardness test, tensile test, Charpy impact test, and metallographic analysis with an optical microscope. The experimental results showed that variations in tempering temperature during deep cryogenic treatment affected the steel's microstructure by determining the decomposition or transformation of retained austenite and the formation of carbides. The mechanical properties of the steel exhibited increased hardness and yield strength at 200 °C, which decreased at 250 °C, with the highest hardness and yield strength being 599.8 HV and 1322 MPa, respectively. Tensile strength and impact energy decreased with increasing tempering temperature, with the highest values at 150 °C being 1836.5 MPa and 22.81 J/cm². Elongation percentage increased with rising tempering temperature, peaking at 250 °C with 15.84%. Deep cryogenic treatment resulted in martensite predominantly tempered martensite and finer carbides, and enhanced hardness, tensile strength, and yield strength, but decreased elongation percentage and impact energy.