4：Heat Treatment 

Heat treatment is to strengthen the strength, hardness and toughness of bolts according to the needs of products.

Good fastener must not be too hard -- easy to break, but not too soft -- so slippery that it won't lock.

Therefore, heat treatment must to be one of the important keys to make the bolt have the characteristics of rigidity outside and flexibility inside.  

4.1 Heat Treatment Worksho precess control.

Strictly monitor and control whether the parameters of each process in the room are within the working permissible range.

Dephosphorization：Phosphorus will decrease the strength of the whole material and the quality of surface treatment during heat treatment.

Equipment process parameter point inspection record.

Product hardness confirmed to meet the process requirements.

Product wedge load confirmed to meet the process requirements.

The concentration of dephosphorization solution meets the requirements.

4.2 Metallographic test：

Decarbonization is not allowed.

The core is evenly organized.

4.3 Process ：

Product classification -Trail -Conditions to determine -Plan -Preparing -Incoming -Rinse -Dephosphorization -Check product -Incoming -Rinse -Drying -Preheating -carbon level control -No.1 Quenching heating zone Heat to 80% above operating temperature -No.2 Quenching heating zone is heated to operating temperature -No.3 Quenching heating zone -No.4&5 Quenching heating zone operating temperature carburizing -No.6 Quenching heating zone -Oil quenching tank-Drying deoiling -Rinse -Drying -Preheating -Tempering heating No.1 zone -No.2 zone -No.3 zone -No.4 zone -Discharge (Transfer to quality control process )-Clean the empty barrels -Check the empty barrels-Transfer -Check again