STUDY OF THE EFFECT OF SECOND STAGE AUSTENITIZATION TEMPERATURE ON DOUBLE QUENCHING TEMPERING AND DEEP CRYOGENIC TREATMENT ON MICROSTRUCTURE AND MECHANICAL PROPERTIES OF AISI 4340 STEEL

• Main Author: Athifah Riezkie Aldhiada, Ainu
• Format: Final Project
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/38735
• Institution: Institut Teknologi Bandung

AISI 4340 steel is classified as medium carbon steel which widely used in many industries such as aircraft, automotive, and manufacturing industry. Conventional Heat Treatment (CHT) is used to increase the hardness of the steel in general. However, the hardness obtained by this method has not reached its optimum value, so an additional method is required. Deep Cryogenic Treatment (DCT) is known as an additional method that can increase the hardness. However, DCT method can transform all retained austenite into martensitic structure and it will decrease the toughness of the steel. The improvement of toughness can be achieved by applying Double Quenching Tempering (DQT) method to obtain austenite grain refinement. In this study, the effect of CHT, DQT, and DQT-DCT method on mechanical properties of AISI 4340 steel will be learned. CHT method is performed by heating the steel at 1000°C for 60 minutes, followed by oil quenching, then tempered at 300°C for 60 minutes. DQT and DQT-DCT method are performed by heating the steel twice with 60 minutes each. The first heating stage was carried out at 1000°C and the second heating stage was done at 950°C, 900°C, and 850°C, then followed by tempering process at 300°C for 60 minutes. For DQT-DCT method, the process was followed by immersing the steel in a liquid nitrogen for 48 hours. The final tempering process was carried out at 300°C for 60 minutes. A series of experiments were conducted to determine the effect of different heat treatment on mechanical properties, microstructure, and fractures of AISI 4340 steel. The tests consisted of impact test, hardness test, metallographic observation using OM with nital and picric acid etchant, XRD, and SEM. The results of the experiment show that the decrease of second stage austenitization temperature leads to refinement of the austenite grain. The smallest grain is 11,00 ?m obtained at second stage austenitization temperature of 850°C. The decrease of second stage austenitization can increase the toughness of the steel. Toughness value reaches its highest value at 35,272 J and was achieved by DQT method at second stage austenitization temperature of 850°C. Combination of DQT and DCT method generally can increase the toughness of the steel than CHT method. The increase of hardness can be attributed to the higher second stage austenitization temperature. Hardness value reaches its peak at 541,5 HV and was obtained using DQT-DCT method at second stage austenitization temperature of 950°C. DQT and DQT-DCT method can increase the hardness up tp 27,56% and 33,27% than CHT method. All three methods produce transgranular fracture with quasi-cleavage brittle pattern.