There are usually two types of aging in mild steel: quench aging and strain aging, both of which are caused by interstitial elements, mainly due to redistribution of carbon, nitrogen, and oxygen.
Quench aging is a phenomenon in which the properties of steel change over time after rapid cooling from high temperatures. The carbon content, degree of deoxidation, and nitrogen content in steel have a great influence on quenching aging. Low-carbon steel, deoxidation of boiling steel, and high nitrogen content of steel quenching aging is the most obvious, carbon content of about 0.3% For medium-carbon steel, quenching aging caused by the change in performance has been greatly reduced. The carbon content of about 0.6% of high-carbon steel is virtually no age-hardening effect.
Strain aging is a phenomenon in which the performance after cold working deformation changes with time. The effect of carbon and nitrogen on strain aging is similar to the effect on quenching aging, and phosphorus also promotes strain aging. The yield point of low carbon steel that disappeared due to cold deformation gradually recovers with time. Strain aging is more complicated than quenching aging. If the steel is quenched and then cold worked, its strain aging will be accelerated regardless of room temperature or slightly higher temperature.
The aging of carbon steel often brings great harm to industrial production. For example, after boiling steel is welded, small cracks appear in the heat-affected zone of the welded joint due to aging, which seriously affects the safety of the welded structure. However, due to the development of modern metallurgical technology and its application in industrial production, especially oxygen converter steelmaking can obtain lower nitrogen and oxygen content, the aging problem has been reduced.