Precipitation and embrittlement

Under various circumstances, the different stainless steel types can suffer undesirable precipitation reactions which

lead to a decrease in both corrosion resistance and toughness. Figure 30 gives a general overview of the

characteristic critical temperature ranges for the different steel types.

475°C embrittlement

If martensitic, ferritic or ferritic-austenitic steels are heat treated or used in the temperature range 350-550°C, a

serious decrease in toughness will be observed after shorter or longer times. The phenomenon is encountered in

alloys containing from 15 to 75 % chromium and the origin of this embrittlement is the spinodal decomposition of

the matrix into two phases of body-centered cubic structure, a and a´. The former is very rich in iron and the

latter very rich in chromium. This type of embrittlement is is usually denoted 475°C embrittlement.

Carbide and nitride precipitation

If ferritic steels are heated to temperatures above approximately 950°C, they suffer precipitation of chromium

carbides and chromium nitrides during the subsequent cooling, and this causes a decrease in both toughness and

corrosion resistance. This type of precipitation can be reduced or eliminated by decreasing the levels of carbon and

nitrogen to very low levels and at the same time stabilizing the steel by additions of titanium as in 18Cr-2Mo-Ti.

Carbide and nitride precipitation in the austenitic and ferritic-austenitic steels occurs in the temperature range 550-

800°C. Chromium-rich precipitates form in the grain boundaries and can cause intergranular corrosion and, in

extreme cases, even a decrease in toughness. However, after only short times in the critical temperature range, e.g.

in the heat affected zone adjacent to welds, the risk of precipitation is very small for the low-carbon steels.

Intermetallic phases

In the temperature range 700-900°C, iron alloys with a chromium content above about 17% form intermetallic

phases such as sigma phase, chi phase and Laves phase. These phases are often collectively called “sigma phase”

and all have the common features of a high chromium content and brittleness. This means that a large amount of

the precipitated phase leads to a drop in toughness and a decrease in resistance to certain types of corrosion. The

size of the deterioration in properties is to some extent dependent on which of the phases that actually is present.

Alloying with molybdenum and silicon promotes the formation of intermetallic phases, so the majority of ferritic,

ferritic-austenitic and austenitic steels show some propensity to form "sigma phase". Intermetallic phases form

most readily from highly-alloyed ferrite. In ferritic and ferritic-austenitic steels, intermetallic phases therefore form

readily but are on the other hand relatively easy to dissolve on annealing. In the austenitic steels, it is the highly

alloyed grades which are particularly susceptible to intermetallic phase formation. Austenitic steels which have low

chromium content and do not contain molybdenum require long times to form intermetallics and are therefore

considerably less sensitive to the precipitation of these phases.

 

Finally, it should be noted that all types of precipitates can be dissolved on annealing. Re-tempering martensitic

steels and annealing and quenching ferritic, ferritic-austenitic or austenitic steels restores the structure. Relatively

long times or high temperatures may be required for the dissolution of intermetallic phases in highly alloyed steels.