Hardenability of Steel: 4 Factors | Metallurgy

The following points highlight the four main factors affecting the hardenability of steel. The factors are: 1. Grain Size 2. Austenitising Temperature and Time 3. Carbon Content of the Steel 4. Alloying Elements in Steel.

Factor # 1. Grain Size:

Grain boundaries are the preferential nucleation sites for ferrite and pearlite. If austenitic grain size is large, the grain boundary area decreases. This means that nucleation sites are being reduced in number. Thus these transformations are slowed down. Thus, hardenability of a steel increases as its grain size increases. Grange gave the Fig. 4.21 correlating grain size and hardenable diameter (for 90% martensite, water quenched). If hardenable diameter and the grain size is indicated by a cross, then the hardenable diameter at any other grain size can be obtained by drawing parallel to diagonal lines as illustrated by dashed line.

This method of increasing hardenability by increasing the grain size of the steel is normally avoided as the coarse grains have deleterious effect like increased brittlness, more tendency to quench-cracks, etc.

Factor # 2. Austenitising Temperature and Time:

The beneficial effects of the alloying elements in steel in increasing hardenability is realised only when the alloying elements are made to go into solution and form homogeneous austenite by heating the steel to proper austenitising temperature and enough time at this temperature. A too high a temperature and/or too long austenitising time may coarsen the austenite grains (which too results in increased hardenability) with its resultant bad effects.

Proper choice should be made after examining the size of the part, microstructure of incoming steel, amount and type of alloying elements in steel, etc. Normally, the effect, if proper austenitising temperature and time are used, is not substantial, however Fig. 4.22 indicates large effect on Jominy curve by using different austenitising temperatures.

Factor # 3. Carbon Content of the Steel:

Carbon fixes the maximum attainable hardness on quenching. It also increases the hardenability of the steel as it stabilises austenite resulting in shifting the CCT curve to the right as its content increases up to 0.77%, but beyond that hardenability decreases.

This is because the austenitising temperatures used for hypereutectoid steels have undissolved carbides in them, which invariably act as nuclei for the pearlitic transformation to shift the CCT curves towards left (with increasing carbon), and thus, decrease hardenability. Pure Fe-C steels have very poor hardenability.

For example, D_I for 0.77% carbon and ASTM grain size 8 is only 0.28 inch i.e. in an ideal quench such carbon steel can harden up 1/4″ diameter. Fortunately, commercial steels always have manganese and other elements by the process of making steel economically, which increase their hardenability.

Factor # 4. Alloying Elements in Steel:

Most metallic alloying elements (except cobalt) slow down the ferrite and pearlite reactions, i.e., increase the stability of austenite resulting in shifting the CCT curves towards right. This means, the hardenability is increased (provided alloying elements are dissolved in austenite). Undissolved inclusions such as carbides, nitrides, nonmetallic inclusions, and even heterogeneity of austenite decrease the hardenability.

For example, presence of undissolved alloy carbides not only depletes the austenite of the alloying elements but also the carbon present as carbide. Their presence in dissolved state in austenite would have increased hardenability. These as carbides decrease the grain growth, which otherwise (dissolved in austenite) would have increased the hardenability.

Most of these inclusions also help in pearlite formation, a factor which decreases the hardenability. Cobalt increases the nucleation and growth of pearlite, thus shifts the CCT curve towards left to decrease the hardenability. However, quantitative assessment of effects of different elements is needed.

Although, alloying elements shift the CCT diagrams towards right, the effect of an element on pearlitic and bainitic region is not normally same. A cooling rate which may be sufficient to suppress the pearlitic reaction may not be large enough to prevent bainite formation. In such a case, the critical cooling rate for ≈ 100% martensite formation is governed by the bainite reaction, and the non-martensitic product shall be bainite.

This results in two types of hardenabilities:

(i) Pearlitic hardenability

(ii) Bainitic hardenability.

Pearlitic hardenability is based upon where the remaining 50% non-martensitic product is ferrite-pearlite, whereas, the bainitic hardenability has bainite as the non-martensitic product. As a clear distinction is very important as this 50% non-martensitic product have vast influence on the mechanical properties, but it is not easily distinguishable in the Jominy test, but is in CCT diagrams.

Hardenability is sharply increased by manganese (most potent except boron), chromium, molybdenum and very small additions of boron (0.003 – 0.005%). Nickel and silicon have smaller effects. The hardenability is specially increased, when several alloying elements are added to steel (instead of one in large amount).

Alloying elements effect the hardenability in a complex manner, and there is no simple correlation to explain the effects over the whole range of composition of steels. Alloying elements affect the kinetics of austenite decomposition in a different and complex manner. Elements like Cr and Mo make bainite as the main non-martensitic product.

Also, the effect of all the alloying elements is not separable in that, for the same amount of addition of an alloying element, the effect on the hardenability would, in general, depend upon the presence, or the absence of certain other alloying elements in the steel. In spite of these problems, some correlation between effects of alloying elements on hardenability has been found based on average effects derived from a large amount of empirical data.

One of the very important aims of adding alloying elements is to increase the hardenability of the steels, so that large section—a meter, or more in diameter like shafts and rotors, could be induced with high strengths. To harden thoroughly, such sections, alloying elements in appropriate proportions have to be added. Higher concentration than necessary for complete hardening of the required section not only makes it expensive as alloying elements are usually much more expensive than iron, but these are scarce also.

Thus, these elements should be used effectively in heat treatment. Carbon in added just enough to give the needed martensitic hardness. Carbon does markedly increases hardenability (Fig. 4.32), but its increased concen­tration seriously decreases toughness, fabrication-ability, and greatly increases danger to distortion and quench-cracking and welding-cracks.

Manganese is a very potent element in increasing the hardenability (see figure 4.24), particularly of plain carbon steels, and as its content increases from 0.6% to 1.40%, hardenability increases substantially. Chromium and molybdenum are cheaper effective alloying elements per unit of increased hardenability.

Boron has tremendous potentials in lower carbon content steels in increasing hardenability substantially, when present in amounts as small as 0.001%. If toughness and other mechanical properties are not very adversely decreased, an increase of grain diameter from 0.02 mm to 0.125 mm increases hardenability by 50%. Fig. 4.33 illustrates steels with different hardenabilities but with same carbon content.

Many machine parts and tools perform well with low hardenability, where high surface wear resistance and tough core is desired. A shallow hardening steel such as WI give remarkable good performance, as such steels can Stand up to impact stresses extremely well. Moreover, in such shallow hardening steels on quenching, the surface is under compressive internal stresses, while core is under tensile stresses. This additional advantage resist fatigue crack propagation in steels.

Steels, which are to be welded should not have high hardenability. At some distance each side of weld, austenitisation takes place, which may transform to martensite (if hardenability of the steel is high), which is hard and brittle and stresses may be created to cause cracks. Steels for bridges, buildings and ships have moderate hardenabilities.