Just The Right Steel

Most people think steel is just steel. However there are literally hundreds of “recipes” for steel. When you fabricate a safe, it is important that you choose the right steel for the right part of the safe. Choosing the correct steel will make the safe more burglar proof than choosing the wrong steel.

Making the right steel

Steel is an alloy of iron and carbon plus other elements that are used to change the attributes of the final material so they are right for the particular application.

So what are the main attributes and why are they important?

Strength is a measure of how well steel can resist being deformed from its original shape. Typically, this is measured by the tensile strength or the resistance to being pulled apart.

Hardenability is a way to indicate steel’s potential to be hardened by thermal treatment. Hardness is resistance to penetration. The depth of hardening is an important factor in steel toughness.

Ductility is a measure of steel’s ability to undergo significant deformation before it ruptures. This may be expressed as percent elongation from a tensile test.

Brittleness is a property of a steel exhibited by failure without undergoing any deformation on application of stress.

Toughness is the ability of steel to absorb energy and deform without fracturing. Toughness requires a balance of strength and ductility.

So what elements go into iron to make steel and what do they do?

1. Carbon (C)

Carbon is the most important element in steel, it is essential in steels which have to be hardened. The degree of carbon controls the hardness and strength of the material, as well as response to heat treatment (hardenability). Ductility, forgeability and machinability will decrease as the amount of carbon increases.

2. Manganese (Mn)

Manganese is the second most important element after Carbon in steel. It has effects similar to those of carbon. It increases the hardenability and tensile strength but decreases ductility. These two elements are used in combination to obtain a material with the desired properties.

3. Phosphorus (P)

Although it increases the tensile strength of steel and improves machinability it is it can also have an embrittling effect. It will also improve the steel's resistance against corrosion. Even though the strength and hardness is improved, the ductility and toughness decreases.

4. Sulphur (S)

Sulphur improves machinability but lowers transverse ductility and notched impact toughness. 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.

5. Silicon (Si)

Silicon dissolves in iron and tends to strengthen it. Silicon increases strength and hardness but to a lesser extent than manganese. The resulting decrease in ductility can present cracking problems.

What We Need For Safes

The Safe Box

So for a good quality safe we want steel for the body that is hard enough to thwart drilling, but not hard enough to become brittle and crack when hit with a hammer.

In our fireproof safe wall construction we use two sheets of 4mm carbon steel that sandwich a fireproof layer of gypsum. This forms a significant barrier for would be thieves.

The Safe Door

The door is a different matter and requires a different formulation of steel. We need to ensure that would be thieves find it incredibly difficult to penetrate the outer layer of the door to gain access to the locking mechanism. For this reason we double the thickness of the outer steel to 8mm and increase the hardness by increasing the Carbon content by 50% and reducing the Silicon content. This forms a very hard steel, but one which is less susceptible to cracking if hit with a hammer and cold chisel.

We have found that this combination of steels results in a safe that has significant burglar proof attributes.

Typical Chemical Analysis of Our Steel

  Carbon    Silicone    Manganese    Phosphorus    Sulphur    Yield Strength    Tensile Strength  
Body Steel 4mm 821432110310410
Door Steel 8mm 151472211279425

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