The Widmannstett structure was first discovered in the study of iron-nickel meteorites. In technology, this change in the crystal lattice of metals and their alloys is undesirable, as it leads to a deterioration in physical and mechanical properties. To assess this structure, a scale normalized by the state standard is used.
Widmannstett structure - what is it?
In 1808, during the study of meteorites, the Austrian mineral scientist Alois von Widmannstetten discovered a new type of metallographic structure of alloys, which was named after the name of its discoverer. The term "Widmannstätten figures" was used to characterize the patterns that arise in the manufacture of thin sections of most iron meteorites. It was later discovered that this phenomenon is often found in the industrial manufacture of metal structures.
The Widmannstett structure of steel is a ferrite-pearlite structure in which its components (ferrite and perlite) are arranged in the form of geometrically ordered figures (plates, needles, squares, rhombuses polygons). This anomaly is associated with the formation of large grains in primary austenite. Lamellar forms are pre-eutectoid ferrite, released in excess inside austenitic grains, needle-shaped - secondary cementite. The plates are located along the planes of the austenite lattice. Below is a photo of the vidmanstett structure.
Appearance conditions
From the point of view of physics, the occurrence of such changes is due to the fact that they provide the lowest value of the surface energy of interaction of particles at the phase boundary. Most often, this phenomenon is observed during steel overheating, however, it does not always receive this structure.
Factors contributing to its formation are:
- significant enlargement of austenitic grains;
- high metal cooling rate;
- the presence of manganese, chromium and molybdenum.
This steel structure can also occur without overheating as a result of a high cooling rate when passing through the temperature range Ar 3 -Ac 1 . Most often, the Widmanstättian structure is observed in such technological processes as welding (in the weld metal and the adjacent superheat), casting, heat treatment, and artificial aging of alloys, which are used to increase their strength characteristics. In the latter case, this occurs as a result of the separation of new phases from supersaturated solutions.
The properties
The formation of the vidmanstett structure begins at a cooling rate after overheating above 100 ° C / min. If the alloy has a coarse-grained structure, then this change in the crystal lattice is also detected at lower cooling rates. The thickness of the ferrite plates may vary in the range of 1-8 μm.
The coarse-grained and lamellar structure of the Widmannstadt structure determines the deterioration of the mechanical characteristics of metals and alloys. During steel overheating, excess ferrite released at the grain boundaries “cuts through” perlite, which leads to a change in properties.
Influence on technological characteristics of structures
In addition to the change in the crystal lattice indicated above, another undesirable phenomenon may occur - burning of the metal. In this case, ferrous films of iron are formed along the grain boundaries, the connection between the grains is broken. The formation of a vidmanstett structure leads to the following negative consequences:
- reduction in toughness and strength;
- tendency to brittle fracture;
- poor resistance to dynamic loads.
Therefore, this process is undesirable for structural materials and unacceptable for the manufacture of industrial products for critical purposes.
To improve the structure of the steel, heat treatment is performed (complete annealing or normalization). Burned material cannot be fixed.
To prevent the formation of a Widmanstätten structure during welding, those that have hereditary fine grains are used as base metals, that is, when heated to high temperatures, they do not significantly increase grain. Alloying additives are also used that reduce the steel's ability to overheat. The most common occurrence of this undesirable effect is observed in gas welding. For an arc, this is less common.
Structure assessment
The degree of change in the crystal lattice during the formation of the Widmannstadt structure is regulated according to GOST 5640-68. To evaluate it, microsections of 3 × 4 cm are made. Cutting of samples should be carried out by the cold method (mechanical or other). The development of this structure in a metal is characterized by an increase in the number and size of acicular ferrite formations, as well as by the size of the austenitic grain.
The assessment is made in two rows and 6 points in each of them. The first row is used for low-carbon steels (carbon content <0.3%), the second - for medium-carbon steels (C <0.5%). Evaluation on a point scale is made by comparing with metal samples, at least 3 points in the microsection of the studied structure.