PENGARUH SUHU TEMPER TERHADAP KEKERASAN PERMUKAAN, KETANGGUHAN IMPAK, LAJU KEAUSAN, DAN LAJU KOROSI BAJA PADUAN HQ 809

WILDHAN IHSAN FAUZI, Dr. Ir. Viktor Malau, DEA

2018 | Skripsi | S1 TEKNIK MESIN

Baja HQ 809 merupakan machinery steel produk PT. Tira Austenite. Produk ini berbasis baja paduan rendah dengan kode AISI 4140 yang telah melewati proses pre-harderning berupa normalizing, sehingga baja HQ 809 memiliki sifat mekanis yang baik. Salah satu proses untuk meningkatkan kekerasan baja yang umum dilakukan adalah quenching yang diikuti dengan tempering. Penelitian ini bertujuan untuk mengetahui pengaruh suhu temper terhadap kekerasan permukaan, ketangguhan impak, laju keausan, dan laju korosi baja paduan HQ 809. Selain itu, pengaruh suhu temper terhadap struktur mikro juga diteliti pada varian suhu tertentu. Perlakuan panas yang dilakukan pada spesimen uji raw material HQ 809 adalah pemanasan sampai suhu austenisasi 950 derajat celcius di dalam furnace dengan laju pemanasan sebesar 10 derajat celcius/menit dimulai dari suhu ruangan. Lalu spesimen ditahan pada suhu tersebut selama dua jam dan kemudian langsung dicelupkan ke dalam bak berisi media pendingin oli. Setelah itu spesimen diperlakukan proses tempering pada variasi suhu 150, 200, 250, 300, 350, 400, 450, 500, 550, dan 600 derajat celcius yang ditahan selama dua jam. Spesimen yang telah diperlakukan panas selanjutnya diuji kekerasan permukaan, kekuatan impak, keausan permukaan, uji struktur mikro dan uji SEM, dan pengujian korosi. Media korosi menggunakan campuran NaCl 3%. Metode pengujian korosi pun menggunakan Tafel untuk mencari laju korosi permukaan. Hasil penelitian menunjukkan bahwa perlakuan panas quenching pada material baja HQ 809 meningkatkan kekerasan permukaan hingga 370 kgf/mm2 dari kekerasan raw material dengan nilai 270 kgf/mm2. Kekerasan akan menurun seiring dengan meningkatnya suhu tempering. Perlakuan panas quenching pada material baja HQ 809 akan menurunkan nilai keausan spesifik hingga 3,81 x10^-9 mm3/kg.mm dari keausan spesifik raw material dengan nilai sebesar 6,83 x10^-9 mm3/kg.mm. Ketahanan aus menurun seiring dengan meningkatnya suhu tempering. Quenching pada material baja HQ 809 menurunkan ketangguhan impak hingga 139,02 kJ/mm2 dari ketangguhan impak raw material sebesar 410,99 kJ/mm2. Ketangguhan impak meningkat seiring dengan meningkatnya suhu tempering. Quenching pada material baja HQ 809 menurunkan corrosion rate hingga 15,62 mpy. Apabila proses quenching diikuti dengan proses tempering, maka corrosion rate menurun seiring dengan meningkatnya suhu temper dari 22,71 mpy pada raw material ke 7,81 mpy. Pengamatan Struktur mikro material baja HQ 809 menghasilkan butiran yang semakin besar apabila suhu tempering semakin tinggi. Perubahan sifat mekanis pada baja HQ 809 terjadi akibat meningkatnya jumlah martensite akibat dari perlakuan quenching. Jumlah fraksi fasa lunak meningkat seiring dengan meningkatnya suhu temper. Keywords: quenching, tempering, HQ 809 steel, hardness, wear, impact strain, corrosion rate, microstructure

HQ 809 low-alloy steel is one of machinery steel products from PT. Tira Austenite. This product is based on the low-alloy steel coded AISI 4140 which has gone through a pre-hardening process normalizing, causing the HQ 809 steel to obtain great quality of mechanical properties. A well known method of increasing the hardness of low-alloy steel is by quenching followed by tempering. Therefore, this research aims to observe the effect of tempering temperature to HQ 809 surface hardness, wear resistance, impact strain rate, corrosion rate as well as its physical characteristics. The first heat treatment process done to the raw HQ 809 test specimen was to heat it up to its austenitizing temperature of 950 celcius degree with a heating rate of 10 celcius degree/minute. The specimen was then held at temperature of 950 celcius degree for two hours until it was rapidly quenched into oil media. Tempering came up next, with the temperature range of 150, 200, 250, 300, 350, 400, 450, 500, 550, and 600 celcius degree, which were then held at their respective temperatures for two hours. The heat treated specimens were then tested for their surface hardness, surface wear, impact test, corrosion rate and resistance, as well as microstructure and SEM. The corrosion media used was 3% NaCl mixture, and the Tafel method was used to determine the rate of corrosion. The results show that quenching HQ 809 low-alloy steel increases surface hardness up to 370 kgf/mm2 from the hardness of the raw material with a value of 270 kgf/mm2. The hardness value decreases with the increase of the following tempering temperature. Quenching HQ 809 low-alloy steel also decreases the specific abrasion up to 3,81 x10^-9 mm3/kg.mm from the specific abrasion of the raw material with a value of 6,83 x10^-9 mm3/kg.mm and the wear resistance value decreases with the increase of the following tempering temperature. Quenching HQ 809 low-alloy steel also decreases the impact strain resistance to 139,02 kJ/mm2 from the impact strain resistance of raw material 410,99 kJ/mm2 and the impact strain resistance value increases with the increase of the following tempering temperature. As for the corrosion test, quenching HQ 809 decreases the corrosion rate to 15,62 mpy, and quenching followed by tempering results in an decrease in corrosion rate as the tempering temperature increases from 22,71 mpy for raw material into 7,82 mpy. The change in HQ 809 microstructure as the tempering temperature increases is the increase of carbide grain size structures on the surface. Changes in mechanical properties in HQ 809 steel occur due to increased number of martensite due to quenching treatment. The number of soft phase fractions increases with increasing tempering temperature. Keywords: quenching, tempering, HQ 809 steel, hardness, wear, impact strain, corrosion rate, microstructure

Kata Kunci : quenching, tempering, HQ 809 steel, hardness, wear, impact strain, corrosion rate, microstructure