STUDY OF HEAT TREATMENT PROCESS IN HEX BOLT

MECHANICAL ENGINEERING

ABSTRACT In selecting the type of steel, attention should be givento the deoxidation practice for the grades used for fastenermanufacturing. A number of factors should be considered such as heat treated property requirements, heat treat conditions,fastener size and steel availability, to name a few.Silicon, for example acts as a ferrite strengthener and therefore in the absence of aluminum, produces steel with somewhat greater hardenability. Silicon killed steel tends to have coarse (large) austenitic grains. For the same carbon grade and heat treatment conditions with and without aluminum,complete transformation of the fastener core during heat treatment can take place in a larger section using a coarse grain steel. Silicon killed, fine grain has both silicon added as the deoxidizer followed by the addition of aluminum for grain size control Austenitic grain size is not usually a factor for consideration in cold forming, but has a significant effect in subsequent fastener heat treatment. Aluminum not only deoxidizes the steel, but also refines the grain size (aluminum killed steel). Aluminum also reacts with nitrogen in the steel to form aluminum nitride particles that precipitate both at the grain boundaries and within the austenitic grains thus restricting the size of the grains; even when the steel is reheated for carburizing or neutral hardening, hence the term fine grain.In the two types where silicon is added, the silicon content can have several ranges with the most common being 0.15% to 0.30%. When aluminum is added to these steels for grain size control, the aluminum content is generally in the 0.015% to 0.030% range. The aluminum content in fully aluminum killed steels is generally 0.015% to 0.055%, somewhat higher on average since the aluminum must both deoxidize and control grain size at the same time.The disadvantage of silicon killed steels is reduced ductilityand tool life during cold heading because of its ferrite strengthening characteristic. Aluminum killed steels are usually more formable and hence provide somewhat improved tool life (e.g., heading operations), but show reduced heat treat response, particularly in larger size fasteners. For this reason the recommended maximum diameter for oil quenched aluminum killed carbon grades is typically 4.8 mm (0.190").

This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.