Boron and Hardenability of Steel | Metallurgy

In this article we will discuss about:- 1. Effect of Boron on Hardenability 2. Mechanism of Boron Hardenability 3. Hardenability Curve for Boron Steels 4. Effect of Alloying Elements.

Effect of Boron on Hardenability:

Boron (atomic radius 0.94 A), being the largest size interstitial solid solution forming element in iron has very limited solid solubility in γ – Fe, ~ 0.001% at 910°C to increase with temperature upto a maximum of about 0.005% at eutectic temperature, and essentially zero in α— Fe. Because of low solubility, it remains probably, segregated at the austenite grain boundaries.

Boron is nowadays being added only for one reason, i.e., to increase the hardenability of the steels.

Some unique features of boron hardenability are:

1. Large Increase of Hardenability at Extremely Low Boron Content:

Boron content of 0.0005% to 0.003% has relatively large and optimum effect, and beyond this, decreases the hardenability as austenite may be unable to keep it in dissolved state, and boron may form borocarbide, Fe₂₃ (C, B)₆ at austenite grain boundaries, which above a critical size act as nucleation sites for ferrite and pearlite.

The optimum level of boron to have highest effect on hardenability, has not been agreed to, but is thought to be most probable between 0.001% to 0.002%, and as per Kapadia (Fig. 4.26), it is 0.001% boron.

As with other alloying elements, boron too can be effective in increasing hardenability if it is in “free” from, i.e., must be in solution in austenite. Boron has high affinity for oxygen and nitrogen to combine readily to form B₂O₃ and BN respectively, during steel making. These compounds, if formed do not decompose and dissolve in austenite at the highest austenitising temperatures used.

Thus, during steel making, boron is protected:

(i) By proper deoxidation practice with the addition of Al, Si, or other deoxidisers,

(ii) By addition of strong nitride formers, like Ti, Zr, etc. prior to the addition of boron, or add more boron to take care of existing nitrogen. Normally, the free boron aimed at is slightly more than the optimum level desired. Boron is added as component of complex ferro-alloy containing Al, Mn, Si, Ti, Zr etc.

2. As Grain Size Becomes Finer, the Hardenability Effect of Boron Increases:

Boron probably, segregates to grain boundaries of austenite and thus, suppresses the nucleation of proeutectoid ferrite, which means, boron diminishes the hardenability effect of gain size, or it may be said that boron has larger hardenability effect in fine-grained steels than in coarse-grained steels.

The discrepancy about optimum boron content for maximum hardenability could be understood by the fact that it depends on the amount of grain boundary area. If the amount of boron level is too small to segregate to suppress the nucleation of ferrite, (because of larger grain area of fine grains), marked increase in hardenability cannot occur, but by increasing the austenitising temperature, austenite grains coarsen to decrease the grain boundary area. Now the above boron level becomes effective to cause mark increase in hardenability.

3. High Austenitising Temperatures Reduces the Hardenability Effect of Boron:

Boron hardenability is reported to be maximum, when boron steel is austenitised in the range 845-985°C but normally around 900°C, but then decreases with the increase of temperature, because the amount of boron dissolved increases. As higher is amount of boron in solution, more readily the borocarbide precipitates. That is why, the case-hardenability of boron steels is better if quenched from 925°C.

If the austenitising temperatures are very high (~ 1100°C), then a phenomenon occurs called boron ‘fade’ particularly in nitride bearing steels. There is the partial or the total loss of the boron hardenability and is permanent. The prolonged high temperatures break the nitrogen protection provided by elements like Al, V, or Nb fester than if protected by Ti, Zr. The free boron is permanently lost to nitrogen, and the hardenability decreases.

4. Increased Boron Hardenability is in Hypoeutectoid Steels, no Effect on Eutectoid Steel, but has Decreased Hardenability in Hypereutectiod Steels:

Even in hypoeutectoid steels, the highest effect of boron on hardenability occurs at low carbon content ≈ 0.10%, then the boron multiplying factor continuously decreases to become unity at eutectoid carbon content (no effect) and becomes less than unity beyond in hyper- eutectoid steels, as illustrated in Fig. 4.27. It is apparent that boron is most effective in increasing the hardenability of low carbon steels.

Mechanism of Boron Hardenability:

Boron segregates to grain boundaries of austenite and suppresses the nucleation of ferrite, but has no effect on the kinetics of pearlite reaction. There are two schools of thought about this suppression. According to one, the segregated boron lowers sufficiently its grain boundary energy. It thus, reduces the effectiveness of these boundaries as nucleation sites for ferrite, or upper bainite. According to second, very fine precipitates of borocarbides, Fe₂₃ (C, B)₆ form at the grain boundaries of austenite.

These borocarbides precipitates are coherent with at least one of the austenite grain. Nucleation of ferrite on the coherent side of precipitate is prevented again due to very low interfacial energy, and so ferrite cannot precipitate. But, after attaining a critical size of the borocarbide precipitates, these act as nuclei for cementite or pearlite.

Hardenability Curve for Boron Steels:

The Jominy curve of a boron steel is different as compared to boron-free steel, because here the hardness falls much more steeply around the critical Jominy distance. This explains that boron decreases only the nucleation rate, whereas other alloying elements effect the nucleation as well as growth rates (Fig. 4.28). This rapid fall in hardness as a function of the distance may not be desirable in components. Thus, boron is preferred in steels for sections, which are through-hardened.

Boron is mostly used in C-Mn steels containing 0.15 to 0.4% carbon and 0.80 to 1.65% Mn. These are cheaper steels, and if used as replacement steel leads to large savings in cost, apart from having better cold forming properties, equal, or better machinability and least tendency to quench-cracking.

Effect of Alloying Elements on Boron Hardenability:

On an average, the boron hardenability decreases with the increase in the alloying elements, even at a fixed carbon content. Boron has no retarding effect on pearlitic transformation, but almost all alloying elements decrease the eutectoid carbon content, and thus at the same carbon content, the alloying elements decrease the boron factor.

However, molybdenum is an exception, as its presence increases the boron effect.

Effect on M_s Temperature:

As boron does not lower the M_s temperature, more severe quenching can be used in boron steel without danger of quench cracking (as is well known that decrease of M_s temperature increases danger of quench cracking).