The hardness of artificial ageing at 170°c for 4 hours of specimen is the best of hardness (113 BH.). The hardness of artificial ageing increase more than the natural ageing in shorter times. The artificial ageing tend to increase too when the proper time. At 2 hours of specimen, the hardness is decrease. I think that had error because quenching isn't rapidly ,may be cause during bring the specimen out beak and hold in air is too long.