The research aims to investigate the effect of the cooling medium on the hardness characteristic and microstructure of the 42CrMo4 steel due to hardening treatment at a temperature of 830°C and holding time of 30 minutes. Various oil such as SAE-10W40, SAE-20W50, SAE-40, and used oil was used in the cooling medium. The changes in product size, hardness, and microstructure have been carefully assessed. The results indicated that the viscosity of the coolant medium strongly influenced the cooling rate of the cooling medium, hardness, and microstructure. SAE-10W40 oil and SAE-20W50 oil only needed 2 hours to return within room temperatures before quenching, whereas SAE-40 oil and used oil took 3 hours. The sample size did not change after hardening-quenching. However, there was a residual carbon layer on the sample surfaces. Quenching caused the changes of microstructure from pearlite and ferrite to ultrafine double phase, consisting of martensite and austenite, which were unable to transform during rapid cooling. The highest hardness value was achieved by the treated product, which was quenched in SAE-10W40, which had 54.59 HRC. The high hardness was attributed to the content of 95% martensite. However, used-oil caused in similar hardness as SAE-20W50.

D2 high chromium tool steel is widely used to produce tools and components that work at significant dynamic loads, such as dies, punch and rollers. The steel must have a good combination of strength and toughness, which heat treatment can obtain. Therefore, this study discusses the effect of normalising, hardening, and tempering on the microstructure and hardness of D2 tools. Normalising and hardening were carried out respectively at 1020°C for 30 minutes, followed by rapid cooling using oil. Tempering was realised by reheating the quenched sample at 250°C and 400°C with variations in holding time of 15 minutes, 30 minutes, and 45 minutes. The hardness of the specimens was measured using a Rockwell hardness tester, whereas the microstructure was observed with an optical microscope. The results indicated that the microstructure changes to martensite and carbide after quenching, while the microstructure becomes tempered martensite and carbide after tempering. Normalising and hardening have almost no impact on hardness, and the increase in temperature and holding time causes a decrease in hardness. The reduction in hardness is noticeable for steels tempered to 400 °C and held for 45 minutes.

Baja AISI 1045 banyak digunakan di berbagai industri karena harganya yang lebih murah dan memiliki kemampuan yang tinggi untuk dikeraskan (hardenability) dengan perlakuan panas. Sifat material setelah perlakuan panas sangat dipengaruhi oleh laju pendingin. Oleh karena itu penelitian ini membahas studi eksperimental kekerasan dan struktur mikro baja karbon AISI 1045 setelah hardening-quenching dan tempering. Hardening dilakukan pada suhu 850°C selama 30 menit, kemudian dilanjutkan dengan pendinginan cepat (quenching) menggunakan air, oli isorapid dan oli SAE20W50. Tempering dilakukan dengan memanaskan kembali sampel yang telah di-quenching menggunakan air pada suhu 350°C dengan waktu tahan 30 menit dan 60 menit, kemudian dilanjutkan dengan pendinginan di udara bebas. Kekerasan benda uji diukur menggunakan alat uji kekerasan Rockwell dan struktur mikro diamati dengan mikroskop optik. Hasil uji kekerasan menunjukkan quenching dengan air menghasilkan kekerasan tertinggi sebesar 61,24 HRC, dengan struktur mikro martensit dan setelah tempering berubah menjadi struktur martensit temper. Waktu tahan tempering yang lebih lama menghasilkan kekerasan yang lebih rendah.