In the linepipe industry the steels and the corrosion resistant alloys are some- times characterised by their microstructure. In the following a brief description of steel microstructure is given, although for a pipeline engineer it is still mechanical properties and weldability that are the key issues.

Carbon steel microstructure

Linepipe steel is basically iron alloyed with small amounts of strengthening elements, primarily carbon and manganese. In molten condition and while above 750°C the iron atoms arrange themselves in a characteristic lattice, termed body- centred cubic. While in this state the steel phase is referred to as gamma phase or austenite. At lower temperatures the atom positions are changed to that of face- centred cubic lattice, resulting in a new phase called either alpha-phase or ferrite. Depending on the carbon content of the steel, some of the austenite is trans- formed into alternating layers of ferrite and cementite (a carbon rich constituent). This aggregate is termed pearlite (because of its mother of pearl-like visual appearance at high magnification), and the overall microstructure is referred to as ferrite-pearlite. Depending on how fast the temperature is lowered during the phase transformation from austenite to ferrite-pearlite it is possible to form other phases such as martensite and bainite. The latter can be further divided into upper and lower bainite. These phases causes internal stresses in the lattice, making the steel strong, hard and less ductile. Hardness and ductility may be controlled by a heat treatment called tempering. If the resulting grain size in the steel is too coarse (due to grain growth during hot forming while the steel is austenitic) it is possible to improve the situation by a normalising heat treatment, consisting of austenitising the steel, and repeating the phase transformation to ferrite-pearlite in a controlled manner.

Stainless steel microstructure

Stainless steel is characterised as steels with more than 12% chromium added as an alloying element. For low carbon steels, 12% chromium results in a martensitic microstructure. Increasing the chromium content to 17%, for example, allows the steel microstructure to be ferritic. If and when nickel is also added, the micro- structure is changed to austenite. In general the corrosion resistance improves when going from martensitic through ferritic to austenitic stainless steels, at the expense of strength. Specially balanced alloys of iron, chromium and nickel use the best of each world, resulting in a duplex microstructure consisting of both austenite and ferrite, with good corrosion resistance and good mechanical properties. In austenitic and duplex stainless steel brittle phases, named sigma-, chi- and Laves phases, may form at grain- and phase boundaries during undue heat treatment or abusive welding conditions. In addition to being brittle, these phases also reduce corrosion resistance.

Nickel alloys

All alloys with nickel as the major element will be austenitic. In addition to the normal gamma phase, the atoms may compile in specially oriented arrays; gamma′ and gamma′′. These structures improve both the mechanical properties and the corrosion resistance.


Steel Microstructure and Corrosion Resistance

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