THE EFFECT OF DEEP CRYOGENIC TREATMENT SOAKING TIME IN CONVENTIONAL HEAT TREATMENT METHOD ON MICROSTRUCTURE AND MECHANICAL PROPERTIES AISI 4340 STEEL

• Main Author: Reidita Artha Putri, Putu
• Format: Final Project
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/82727
• Institution: Institut Teknologi Bandung

The demand from various industrial sectors to produce high quality steel continues to increase along with the growth of the construction sector, automotive, and other industrial applications. Heat treatment becomes one of the important methods to obtain the desired mechanical properties. In the processing of AISI 4340 steel, heat treatment is involved to improve the mechanical properties. One of the heat treatments to increase the hardness and strength of AISI 4340 steel is by rapid cooling from the austenite phase to obtain a hard martensitic structure. However, there are conditions that the austenite phase does not fully transform into martensite. One solution to reduce the presence of retained austenite is to conduct cooling at - 196°C, commonly known as deep cryogenic treatment (DCT). In this study, variation of the DCT soaking time was carried out to observe the microstructure and mechanical properties. A series of DCT experiments were conducted to study the effect of soaking time on the microstructure and mechanical properties. The austenization treatment was carried out at 900°C with a holding time of 30 minutes and followed by quenching using oil. After that, the specimens were soaked with liquid nitrogen at -196°C with a variation of soaking time of 6, 12, and 24 hours. The tempering process was carried out at a temperature of 250°C with a holding time of 1 hour and cooled in free air. Characterization was carried out by tensile testing, vickers microhardness, and metallography using an optical microscope. From the characterization data obtained, the influence on the microstructure and mechanical properties was determined. The results indicated that with an increase in the duration of the DCT soaking time, the retained austenite will decrease. The tempering treatment will result in the precipitation of cementite and transition carbide precipitates. An increasing hardness trend was obtained with a maximum value of 517 HV at 24 hours soaking time with tempering treatment. An increasing tensile strength trend was also obtained with a maximum value of 1606.11 MPa at the soaking time of 24 hours with tempering treatment. The highest yield strength was obtained at 24 hours soaking time with tempering treatment which is 1280 MPa, while the highest %elongation was obtained at 12 hours immersion time with tempering treatment which is 15.84%.