Nitriding 135 steel is a modification of AISI 4140 specifically designed to be nitrided. It is a Chrome-Moly alloy steel with additions of aluminum typically used for nitrided parts requiring high surface hardness and heat resistance. Nitriding 135 is usually supplied in the pre-hardened condition, about 26/30 HRC, ready for nitriding. Further heat treatment is generally not required but will develop high core strength. Nitriding 135 parts also retain case and core properties at temperatures up to 1000°F and typically show less distortion in heat treating than similar parts fabricated from alloys requiring quenching and tempering to achieve similar surface hardness.