Not yet reached the period of limitation effects on heat treatment and quenching, since organizations pearlite coarse grains combined organization will inevitably lead to large ferrite is also very thick, all the weeks when heated to produce coarse pearlite austenite in the pearlite grain boundaries are soon transformed into austenite, but coarse ferrite into austenite is difficult, and very few reasons for ferritic carbon content, the amount of carbon dissolved in 727 ℃ maximum for 0.0218%. It takes the shape of the ferrite after quenching mesh retained after quenching martensite + mesh ferrite, tempered ferrite will not change, stay on the same cause hardness specimen vary widely. For example: GB5787 hex flange bolts M6x40, mechanical performance requirements of 8.8, according to GB3098.1-2000, part of the technical parameters: hardness HRC22-32, the minimum guaranteed load of 16100N, observed under a microscope in the same field have Two organizations, two durometer hardness measured value (Figure 1) heat treatment and mechanical properties vary greatly, hardness HRC27-29, local HRC18-20, the minimum guaranteed will be pulled off when the load is greater than 12000N. Some hardness qualified individual products, some hardness unqualified, qualified the thread, screw unit failure, severe uneven hardness. Such products are not re-heat treatment and in the matter, only stored for a period of time to re-heat-treated to make it natural aging.