What Is Precipitation Hardening?

Precipitation hardening refers to a heat treatment process in which a solute atom segregation region of a supersaturated solid solution and / or the particles that are dissolved out of the metal are dispersed in the matrix, resulting in hardening.

Precipitation hardening (precipitation strengthening) refers to the

The precipitation hardening mechanism is because the second phase particles in the metal material precipitate from the supersaturated solid solution and cause strain, which causes the strengthening of the metal lattice. The greatest strengthening is caused before the visible second phase particles are formed, this phase is called the incubation phase of precipitation. At this stage, the atoms that are to be formed to form the second phase tend to accumulate in groups. They maintain a continuous coherent connection with the mother phase. At this time, the largest strain occurs, which results in the greatest strengthening.

Precipitation hardening has two effects. Eliminate the stress of martensite, increase toughness, plasticity and corrosion resistance. The hardening effect is increased by the precipitation of intermetallic compounds.

Precipitation hardening stainless steel is a steel type developed by the American Iron and Steel Company in the 1940s. After precipitation hardening heat treatment, it has high strength, better plasticity and corrosion resistance than other stainless steels.

Precipitation hardening stainless steel can be divided into 3 types of martensite type, semi-austenite type and austenite type according to the metallographic structure of its matrix.

(1) Martensite type

Martensitic precipitation hardening stainless steel is usually supplied in the martensitic state and undergoes a simple aging treatment for precipitation hardening. The properties of martensitic precipitation hardening stainless steel can be obtained through the combined action of one or two of the martensite formation and precipitation hardening mechanisms. It is the most widely used steel in precipitation hardening stainless steel.

(2) Half austenite type

The matrix of semi-austenitic stainless steel is austenite and contains 5% ~ 20% ferrite. Before hardening, the austenite is transformed into martensite by aging treatment. Semi-austenitic stainless steel can be processed into various products, but is mainly used for flat-rolled sheet and strip. This precipitation-hardened stainless steel is generally not used in valve products.

(3) Austenitic type

Austenitic stainless steel is supplied in the austenitic state, which is rarely used ^[2]

Precipitation strengthening is still the most effective method for developing ultra-high-strength alloys. It is strengthened by precipitating second-phase precipitation particles that are dispersed and can hinder dislocation movement. It has been nearly a century since Wilm first discovered aging hardening in aluminum alloys in 1911. Since then, precipitation strengthening mechanisms and precipitation kinetics have become research subjects. However, it was not until 1940 that Mott and Nabarro proposed the dislocation theory that the basic principles of age hardening were really understood. As a milestone in this research, Oorwan derived its equilibrium equation in 1948, which is considered to be the basis of the theory of dispersion strengthening or strengthening alloys with undeformed particles. In 1963 Kelly and Nicholson summarized the early theory of precipitation hardening. In 1971, Brown and Ham summarized the research progress on the understanding of the precipitation mechanism, and focused on the ways in which dislocations interact with the precipitate. Ardell has comprehensively summarized the relevant research work up to the 1980s, and has focused on the statistical problems of dislocation-particle interaction and aging hardening mechanism. Although Ardell advocated theoretical innovation, the precipitation since then Hardening research is not very active. In recent years, special studies or partial studies on the mechanism of hardening have largely continued the ideas of Ardell in the past. Martin and Bratland et al. Summarized the progress of precipitation kinetics.

Quantifying precipitation hardening is a challenging topic because it requires comprehensive knowledge of precipitation strengthening mechanisms and precipitation growth / roughening dynamics. Many scholars have attempted to reproduce experimental observations through computer simulations based on reinforcement mechanisms and kinetic theory. However, no theoretical innovation has been seen in recent years ^[3] .