How would you expect an increase in the austenite grain size to affect the hardenability of a steel alloy?

How would you expect an increase in the austenite grain size to affect the hardenability of a steel alloy?

The hardenability increases with increasing austenite grain size, because the grain boundary area is decreasing. This means that the sites for the nucleation of ferrite and pearlite are being reduced in number, with the result that these transformations are slowed down, and the hardenability is therefore increased.

How does austenite grain size affect martensite formation?

The austenite grain size influences the martensitic transformation through the nuclei density provided by the grain boundary area and through the strengthening of the austenite phase as the transformation progresses.

How does alloying increase hardenability?

Most metallic alloying elements slow down the formation of pearlite, ferrite and bainite, therefore they increase a steel’s hardenability.

How does alloying affect hardenability?

Alloying elements. The main alloying elements which affect hardenability are carbon, boron and a group of elements including Cr, Mn, Mo, Si and Ni. Carbon controls the hardness of the martensite. Increasing the carbon content increases the hardness of steels up to about 0.6wt%.

Why coarse grained austenite is not desired for hardening?

As martensite forms easily in coarse austenite grained steel, the hardenability is increased of this steel, though this method to improve hardenability is not resorted to as the coarse grained austenite results in coarse martensite, which makes the steel little less hard and more brittle, that is, even in hardened …

What is austenite grain size?

From optical microscope observations, the average sizes of the austenite grains were 12.8 μm (ASTM 10) at a heating temperature of 800 °C, 30.8 μm (ASTM 7.4) at a heating temperature of 900 °C and 66.8 μm (ASTM 5.2) at a heating temperature of 1000 °C.

What is the grain size of martensite?

(1) The morphology of martensite transformed from the austenite with grain sizes ranging from 35µm to 750nm was all lenticular type containing midrib.

Which element increases hardenability?

Hardenability is appreciably enhanced by manganese, chromium, molybdenum, and very small additions of boron (0.001–0.003%). On the contrary, nickel and silicon have mild influence on increase of hardenability of steels.

What is hardenability and how is it affected by alloy content?

The hardenability of a metal alloy is the depth to which a material is hardened after putting it through a heat treatment process. It should not be confused with hardness, which is a measure of a sample’s resistance to indentation or scratching.

What is the grain size of austenite?

At the end of the intercritical annealing stage, the austenite grain size is of the order of 1–2 μm, which is generally much smaller than in the case of initially fully austenitic microstructures that are then transformed to bainite.

Why is it important to have fine austenite grains?

Hence, finer the austenite grain size, finer is the resulting ferrite grain size, and better are the mechanical properties of the steel. Thus, it is important to have fine austenite grains. Austenite normally does not occur at room temperature, except in some alloys steels.

What is the importance of austenitic grain size of steel?

Importance of Austenitic Grain-Size of Steels: The size of austenitic grains is the most important structural characteristic of heated steel. The austenitic grain size strongly effects its own transformation behaviour and the mechanical properties of the micro-structures formed from austenite.

What are the disadvantages of austenite?

The co-segregation of impurities like Sb, P, Sn, As along the large angle grain boundaries of austenite grains weakens the adhesion at these boundaries to cause fracture along them. Such intergranular fracture is quite brittle. The impact resistance is low, even in slowly cooled steels having coarse austenitic grain size.