Aging

Change in the structure of a metal material.

The cause of aging is considered to be precipitations from supersaturated solid solutions. Aging is always connected to changes in mechanical properties, with a distinction being made between natural and artificial aging.

Natural aging occurs spontaneously with certain materials in the as-cast condition or following cold forming. By contrast, aging may also be induced by heat treatment. This is known as artificial aging and is performed in order to accelerate and stabilize changes in structure and properties. During aging processes, the hardness increases which is why this is referred to as hardening.

When generally referring to aging, this usually means strain aging which is of special importance for metals such as steel. The responsible substances for this kind of aging are carbon, phosphorus, oxygen and, preferably, the dissolved, unbound nitrogen. This aging is characterized by an increase in tensile strength and yield strength along with a decrease in toughness during storage or after cold forming (strain) of the steel.

The process which is of great importance for many high-strength welding steels is shown in Fig. 1. Due to the maximum N content of 0.02% in the steel, only the section on the far left is of interest. It can be seen that all steels follow the course indicated by the arrow after hot forming. Above the A-B line, all N atoms are uniformly dissolved in the iron as interstitial solid solutions , underneath this line, they should migrate to the grain boundaries where they form the compound Fe4N (iron nitride) in a ratio of four Fe atoms + one N atom.

This process is called precipitation. The smaller, more mobile C atoms are able to substantially reach the grain boundaries during normal cooling, even at higher temperatures; the more inert N atoms are not. They remain dissolved in the iron until cooled to room temperature, even though there is no room for them. Such a “supersaturated solution” will endeavor to convert to a stable state over time with the N atoms slowly migrating in the direction of the grain boundaries. However, they will not find any lattice vacancies where they could find a place and remain. As a result, the deformability of the steel is reduced, i.e. the toughness descreases.

  • Fig. 1: Iron/nitrogen binary diagram
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