Delta Ferrite in Heat Treated Bolts - Characterization and Consequences

In the process of mechanical forming bolts, the raw material must be phosphate coating to reduce friction and increase the tool life. However, when this screw undergo heat treatment of quenching and tempering so that these screws can acquire the mechanical strength the element phosphorus diffuses into the austenite and generates the formation of a brittle phase rich in phosphorus.

Bolts with high strength according ISO 898 Part 1 class12.9 compulsorily must be free of white superficial layer rich in phosphorus to ensure that requisite before process of quenching and tempering parts must undergo alkaline cleaning to ensure the exemption superficial phosphates.

This paper cover the formation of delta ferrite in bolts of low / medium  carbon steels heat treated in neutral atmosphere with phosphorus.  Presence of delta ferrite in this kind of bolts cause some problems like brittleness and micro crack whose can suddenly fail.