Widmanstatten Structure in steel deteriorates its properties and formed due to improper heat treatment. For studying the effect of heat treatment, we used AISI 1050 steel, heated at 880oC, and used forced air-cooling method. Then after metallography the microstructure and hardness of these steels were observed by metallurgical microscope and Rockwell hardness tester, respectively.
Experimental results have shown that relatively faster cooling yields widmanstatten structure with higher hardness and smaller impact strength. By varying parameters, different hardness values and DBTT can be achieved which can make these steels useful in certain low-temperature engineering applications.
Before Proceeding to main Post, Follow
- Development of steel microstructure in case of equilibrium cooling – Annealing Steel
- Effect of non-equilibrium cooling on Phase transformation – TTT Diagram of Steel
Introduction
A microstructure results when steels are cooled at a critical rate from extremely high temperatures. It consists of ferrite and pearlite and has a cross-hatched appearance due to the ferrite having formed along certain crystallographic planes.
In German, Widmanstatten ferrite is also known as an over-heated structure. You get that particularly well expressed in hypo eutectoid plain carbon steel that is cooled down from a temperature above (overheated) the A3 temperature, where the pure γ-phase (austenite) starts to transform into primary ferrite and austenite.
For understanding martensitic transformation, have a look into this writing, “Effect of Austenitizing temperature on Martensitic Transformation”.
The few main factors responsible for the formation of widmanstatten structure in steel are grain size of austenite, chemical composition,
Mechanism
Consider having a composition in the hypoeutectoid region with a composition closer to high carbon i.e. more than 0.45 % Carbon. Heat up a sample to such high temperature that all structure converts to the pure austenite phase. After giving some soaking time, cool the structure at a slow cooling rate. At the extremely slow cooling rate, when (α + γ) region is hit, ferrite will start developing as shown in the image. Since the structure is heated and grains are coarsened due to overheating at such high temperature, so ferrite crystals will form on grain boundaries and triple points which are low in number due to coarsen microstructure. This results in the formation of ferrite crystals within grains resultantly becoming longish.
The longish needle-like ferritic structure is formed mainly due to the coherency difference between the alpha and gamma phases of steel. Due to complete incoherency within grains, ferrite structure grows within grain in one direction giving us a needle-like structure. Now, this ferrite formation is occurring at the expense of omitting carbon in neighbor resultantly shifting towards high carbon region in the neighbor. When we follow the phase diagram, as we cool down from A3 temperature line and alpha is forming, we are slowly moving towards A1 line and eutectoid carbon percentage. At A1, we have eutectoid carbon in the remaining region which gives us messy pearlite that may not even show a clear Zebra pattern.
In cast steels, the austenite grains formed during solidification are relatively coarse, which lead to a large ferrite grain size on further cooling to room temperature. In addition, some ferrite often forms as Widmanstatten structure, primarily due to large austenite grain size, which adversely affects the impact properties of the steel. In hot worked steels also, when finishing temperature is relatively high, austenite grains get sufficient time to grow, leading to a coarse grain structure. In welded steels, the welded seams have a structure very similar to cast structure and grain coarsening occurs in heat affected zone. According to literature, the Widmanstatten structure is undesirable because of its negative influence on the impact strength of steel.
In order to rectify these undesirable microstructural features, steels are normally fully annealed. On heating above Ac3 temperature, with nucleation and growth of fresh austenite grains, the austenite grain size is refixed at a lower value.
Widmanstatten steel structures do possess certain useful applications and their study is also important with respect to weldments. They are used in low temperature applications because their DBTT temperature is lower than normal steel structures of same composition making them ductile at low temperatures (in range of -30 oC). In this temperature range, their toughness is also increased. This is because of contraction in ferrite producing internal forces to form small crystals of ferrite by breaking the long ferritic plates. Hence toughness increases.
Microstructure of Widmanstatten Structure in steel
Widmanstatten structure in steel Properties
The sample with forced air-cooling yields widmanstatten ferrite only whereas as the other sample which was cooled in air possessed mixture of normal granular ferrite and relatively smaller amount of widmanstatten ferrite, the reason is obviously the cooling rate. Relatively fast cooling in first sample gives no time for more nucleation sites of ferrite and due to carbon concentration gradient and coherency difference the ferrite nucleus began to grow inside the grain yielding Widmanstatten ferrite. Whereas the other sample was subjected to slower cooling which give time for more nucleation of ferrite leading to normal granular ferrite.
Hardness of the sample with predominantly widmanstatten structure should possess higher hardness due to higher phase boundary area. Dislocations are hindered at short distances due to which hardness is increased. According to research, hardness of sample with widmanstatten ferrite is in range of 430 HV to 510 HV which is surely greater than hardness of our sample i.e. 20.5 HRC (241 HV), so it is proved experimentally.
In case of impact strength, it decreases due to micro cracks generated at the tips of widmanstatten ferrite because of which it is not preferred in routine applications but in special applications.
Summary
Steel with widmanstatten ferrite structure was successfully prepared and the effect of cooling on its transformation was observed in this experiment. It is concluded that widmanstatten structure forms by relatively high rate normalizing yielding higher hardness and smaller impact strength. By varying composition, cooling rate and grain size of austenite, different hardness values and DBTT can be achieved which can make these steels useful in certain engineering applications.
