Hardening precipitation

Furthermore, the 6150 alloy has the highest ductility (by a narrow margin), which would give it a slight edge in the selection process. 

Alloy Designation/ Quenching Medium As- Quenched Tempered at 54 PC (lOOCPF) Tempered at 595°C (IIOtTF) Tempered at 65(TC (120(TF)  

The strength and hardness of some metal alloys may be enhanced by the formation of extremely small, uniformly dispersed particles of a second phase within the original phase matrix this must be accomplished by phase transformations that are induced precipitation by appropriate heat treatments. The process is called precipitation hardening because 

Precipitation hardening and the treating of steel to form tempered martensite are totally different phenomena, even though the heat treatment procedures are similar therefore, the processes should not be confused. The principal difference lies in the mechanisms by which hardening and strengthening are achieved. These should become apparent with the following explanation of precipitation hardening. 

Two requisite features must be displayed by the phase diagrams of alloy systems for precipitation hardening an appreciable maximum solubility of one component in the other, on the order of several percent and a solubility limit that rapidly decreases in concentration of the major component with temperature reduction. Both of these conditions are satisfied by this hypothetical phase diagram (Figiue 11.22). The maximum solubility corresponds to the composition at point M. In addition, the solubility limit boundary between the a and a + /3 phase fields diminishes from this maximum concentration to a very low B content in A at point N. Furthermore, the composition of a precipitation-hardenable alloy must be less than the maximum solubility. These conditions are necessary but not sufficient for precipitation hardening to occur in an alloy system. An additional requirement is discussed in what follows. 

---

The enhanced strength and corrosion properties of duplex stainless steels depend on maintaining equal amounts of the austenite and ferrite phases. The welding thermal cycle can dismpt this balance therefore, proper weld-parameter and filler metal selection is essential. Precipitation-hardened stainless steels derive their additional strength from alloy precipitates in an austenitic or martensitic stainless steel matrix. To obtain weld properties neat those of the base metal, these steels are heat treated after welding. 

Both sohd-solution hardening and precipitation hardening can be accounted for by internal strains generated by inserting either solute atoms or particles in an elastic matrix (11). The degree of elastic misfit, 5, produced by the difference, Ai , between the lattice parameter, of the pure matrix and a, the lattice parameter of the solute atom is given by... 

Precipitation Hardening. With the exception of ferritic steels, which can be hardened either by the martensitic transformation or by eutectoid decomposition, most heat-treatable alloys are of the precipitation-hardening type. During heat treatment of these alloys, a controlled dispersion of submicroscopic particles is formed in the microstmeture. The final properties depend on the manner in which particles are dispersed, and on particle size and stabiUty. Because precipitation-hardening alloys can retain strength at temperatures above those at which martensitic steels become unstable, these alloys become an important, in fact pre-eminent, class of high temperature materials. 

Precipitation hardening consists of solutioning, quenching, and aging. Solutioning entails heating above the solvus temperature in order to form a homogeneous soHd solution. 

Fig. 1. Development of wrought nickel alloys (see Tables 5 and 6), where ( ) represent soHd solution material and (C3) represent precipitation-hardenable...

Several of the Al—Li alloys developed in the 1980s contain both magnesium and copper. No quaternary Al—Cu—Li—Mg phase has been found in the alloys. The S -phase in addition to 5 and provides precipitation hardening. 

In Ni—P electroless deposits, there can be as much as 10% by weight of phosphoms. The amount depends on the added complexing agents and the pH. The Ni—P deposits are fine-grained supersaturated soHd solutions, which may be precipitation hardened by heat treatment to form dispersed Ni P particles in a nickel matrix. 

Because these alloys are precipitation hardenable, they can be customized for specific requirements across a wide range of property combinations. Advances in composition control, processing, and recycling technology have broadened the capabiUties and expanded the range of appHcation. Data sheets pubhshed by the manufacturers and others (41) give compositions, properties, and typical appHcations. 

---