Grain Structures

The grain structure of a material will influence its weldability, its mechanical properties and in-service performance. The type and number of grain structures present in a material will be primarily influenced by three factors: (1) the elements in the material, (2) the temperatures reached during welding and/or post-weld heat treatment and (3) the cooling rates produced. Single or multiple grain structures may be present in a material in its final state.

Austenite

Austenite is the high temperature form of Fe (pure iron) found in C, C- Mn* and alloy steels which exists above 723°C. The temperature at which the steels are fully austenitic depends on carbon content, e.g. low carbon < 0.1%C – over 910°C, 0.8%C about 730°C.

The cooling rate from the austenite region determines the hardness of the steel at room temperature. Very slow cooling produces very soft steels; medium cooling rates produce soft to medium steels; fast cooling can produce very hard and brittle steels depending on the carbon content and thickness of the steel.

Ferrite

Ferrite is essentially pure iron at room temperature, it contains either very little or no carbon. This grain structure is formed from the austenite region by holding at a temperature which depends on the carbon content of the steel, e.g. 910°C for low carbon steel. Ferrite is very soft and ductile and has low tensile strength but has good machining properties, with a curie temperature/point of approximately 770°C.

Pearlite

Pearlite forms from the austenite region under slow cooling and consists of plates of ferrite and cementite, it is harder than ferrite because of the layers of hard cementite it contains. Pearlite is the most frequently encountered grain structure in a constructional steel.

Bainite

Bainite forms from the austenite region when the cooling rate is too fast for pearlite to form, it is harder and usually tougher than pearlite. often forms in the HAZ area of C-Mn steel welds.

Martensite

Martensite is a very hard and brittle grain structure but it can be tempered in order to improve toughness. It is formed from the austenite region by quenching or very fast cooling. This grain structure can only be formed in plain steels when sufficient carbon exists, usually over 0.3%. For alloy steels this figure may be much lower because other alloys in the steel – especially chromium – also have an influence. Unless specifically designed into the steel, the presence of martensite should be avoided.