Precipitation-hardenable steels

HE is the loss of ductility of materials containing hydrogen, which occurs in high-strength steels, primarily quenched and tempered and precipitation-hardened steels, with tensile strengths greater than about 1034 MPa. There are two types of HE (9, 91). 

Precipitation hardening steels These are steels with high toughness, high strength and having optimum creep properties. Aluminum, molybdenum, copper and titanium additions are made for intermetallic strengthening. 

Corrosion-resistant steels should be passivated. No further finish is required to provide corrosion resistance to 300 series steel where tarnish, rust, or surface stain would be objectionable, the 400 series and precipitation hardening steels should be given additional protection by a suitable plating or, after passivation, receive one coat of zinc chromate primer followed by a suitable top coat. 

Many of the mechanical properties of materials are improved by various heat treatments. Unfortunately, such properties as hardness and strength are often achieved at the expense of corrosion resistance. For example, the hardness and strength of martensitic steels are counterbalanced by a lower corrosion resistance than for the ferritic and austenitic steels. The very high strengths achieved for precipitation-hardened steels are due to the secondary precipitates formed during the solution heat treating and aging process. Precipitates with electrochemical properties distinctly different from those of the matrix have a deleterious effect on corrosion. 

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. 

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. 

Table 3.12. Examples of Precipitation Hardening Stainless Steels...

With the precipitation hardening types, high strengths can be obtained with good toughness. A feature of these steels is that the ductile-brittle transition is less sharp, although low impact values are obtained at very low temperatures. Properties for a typical example are shown in Tables 3.17 and Fig. 3.13. 

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