PROPERTIES OF STEEL


Effect of chemical elements on the mechanical properties of steel


The effects of the alloying elements in steel are listed here. The influence of chemical elements on the properties of steel are not only dependent on the type of element added to the steel but also the amount of the alloying element that is added. For example, Si can cause low galvanizing thickness at both low concentration or high concentration but in a mid-region will cause the zinc layer to be thick. This page intends not to go into such detail but to merely summarize the primary effect that an alloying element has on the properties of steel.

Silicon, Manganese, Phosphorus and Sulphur as well as carbon occur in all unalloyed steel.

1. Manganese (Mn):

Found in many commercial steels the effect of Manganese on steel is as follow:

Manganese is one of the elements found in many commercial steels with the primary purpose of increasing the hardenability and strength of the steel. The effect of manganese on the hardenability as well as on the strength of the steel is to a lesser degree than that of carbon. The increased hardenability is due to the cooling rate which is decreased by manganese during the hardening process. Apart from the effect on hardening and strength it also fulfils the role of a mild deoxidiser.

Combining it with sulphur, Manganese Sulphide (MnS) will form to increase the machinability of the steel, including the weldability and forgeability. Brittleness is prevented by this combination, which leads to a better surface finish of the steel.

2. Carbon (C):

The effect of carbon on steel is described below.

The most important chemical element found in steel is carbon. The strength and hardenability is directly linked with the carbon content, when the carbon content increases the strength and hardenability will also increase. The opposite is also true for ductility, forgeability and machinability which will decrease as soon as the amount of carbon increases, this is also true for the weldability properties of the steel.

3. Sulphur (S):

Even though the effect of sulphur on steel is negative at certain stages, any sulphur content less than 0.05% has a positive effect on steel grades.

Sulphur contains a few disadvantages when the content is higher than 0.05%. Brittleness is caused and the weldability decreases as the sulphur content increases to more than 0.05%. However, the content of sulphur in free cutting or free machining steel can contain up to 0.40% sulphur. Free machining steel also know as resulfurized steel has improved machinability properties, but is not recommended for welding. Higher sulphur content will lower the weldability and increases the risk of welding cracks forming. If it is combined with low carbon and low manganese content the surface quality might be harmed.

4. Phosphorus (P):

Effect of phosphorus will have various effects on steel depending on concentration.

The maximum amount of phosphorus in higher grade steel is between 0.03 to 0.05% due to the fact that is detrimental. Up to 0.10% of phosphorus in low-alloy high-strength steels will increase the strength as well as improve the steel's resistance against corrosion. The possibility of brittlement increases when the content in hardened steel is too high. Even though the strength and hardness is improved, the ductility and toughness decreases.

The machinability is improved in free-cutting steel, but weld brittle and/or weld cracks can occur during welding if the phosphorus content is more than 0.04%. Phosphorus also affects the thickness of the zinc layer when galvanising steel. More information can be found on this when looking up the Scandalin curve.

5. Silicon (Si):

As described below the effect of silicon on steel is the same as manganese in some instances, but to a smaller degree. This and other effects which silicon has on steel is described in this section.

Silicon is the same as Manganese in the sense that all steel grades contains silicon and even though it is less than manganese, it still increases the strength and hardness. The strengthening factor is due to the silicon which dissolves in the iron.

It plays an important role as a deoxidiser in order to prevent defects and/or damages. In a case where silicon is mainly used as a deoxidiser for rolled steel the content will be kept to a small percentage. Electrical steels contain a high amount of silicon as the electrical conductivity is reduced by its presence.

When it comes to galvanising steel that contains a higher percentage silicon than 0.04% it directly influences the coating's thickness as well as appearance. The coating will be thicker while the surface finish will be darker due to the coating consisting out of zinc and iron. The corrosion protection is not affected by the zinc-iron combination.

6. Vanadium (V):

The effect of vanadium on steel is very positive and important due to the following:

Only a small percentage of vanadium is required as the effectiveness of this element is very high. The primary function being that of a grain refiner. The moment steel with higher than 0.05% of vanadium goes through thermal stress relieve process it will become embrittled. Even though hardenability is increased by vanadium, as soon as it exceeds 0.05% the hardenability decreases due to the forming of carbide. The positive effect of the formation of carbide is that wear resistance is higher and the temperature strength is increased. In hardened and tempered steels the rate against which the grain grows during heat treatment is lower due to the presence of vanadium. The softening on tempering is decreased which has a positive effect on the secondary hardness of high speed steels. Therefore, vanadium is primarily used in high speed and hot forming steel.

7. Aluminium (Al):

Aluminium has the following effects on steel:

Aluminium mainly functions as a deoxidiser and a grain refiner. The steel grade's toughness is increased due to the grain refiner function of this chemical element. Increasing the scale resistance, aluminium, is also useful in ferritic heat resisting steel grades. The combination of aluminium and nitrogen lowers the possibility of strain ageing.

8. Chromium (Cr):

By far the most powerful element is chromium and the effect it has on steel cannot be ignored or overlooked.

The hardenability of steel is first and foremost increased by Chromium, the most powerful element. Corrosion resistance of steel as well as the yield strength in increased with higher percentage of chromium content. Only 1% is required to improve the newtons per square millimetre of the tensile strength with 8 - 100 N/mm2.

9. Molybdenum (Mo):

The effect of molybdenum on steel includes the following:

Tensile strength, heat resistance and weldability of steel is increased by the molybdenum, but as soon as the content is too high the forgeability is negatively influenced. The maximum amount present in steel is 1%.

10. Nickel (Ni):

In this chapter the effect of nickel on steel is explained.

At the lower temperatures Nickel is used to increase the toughness of the steel. The weldability of the steel is not decreased by the presence of this element. The nickel drastically increases the notch toughness of the steel. Higher nickel content (>8%) will change the phase of the solid steel from ferritic to austenitic stainless steels therefor contain a fair amount of nickel.

11. Boron (B):

The effect of boron on steel is very important and should be studied carefully to ensure that it doesn't become irrelevant as different factors plays a role in its success.

The most important effect and the purpose of boron in steel is to drastically improve the hardenability. Thee biggest advantage of boron is that a small amount can be added to get the same result as other elements required in large amount in terms of added hardenability. During the heat treatment process boron, a replacement for other elements, is added to increase the hardenability of medium carbon steel. The cutting performance for high-speed steels is increased but at the expense of the forging quality. It is also possible that the content of boron can be too high which decreases hardenability, toughness as well as cause embrittlement. The percentage carbon present in the steel also plays a role in the hardenability effect of boron. As boron's effect on hardenability increases the amount of carbon should proportionally be decreased.

When boron is added to steel, precaution must be taken to ensure that it does not react with oxygen or nitrogen as the combination of boron with either one of the two will make the boron useless.