Recycled steel is becoming more popular as consumers look for green building alternatives to traditional materials.
However, as a homeowner or builder, you’ll want to be sure that this material meets your construction requirements in terms of strength, durability, and so on. This is why it’s essential to know the properties of recycled steel before using it in any project.
Strength, durability, availability, and versatility are some of the properties that people cite as important for any construction material. Materials with these features are often selected for use in long-lasting structures that withstand wear and tear from the elements for many years.
You might know that recycled steel is a more sustainable building material than virgin steel, but does it possess other important physical and chemical properties we rely upon in construction?
In this article, I’ll discuss the properties of recycled steel that make it suitable for construction. From strength and durability to hardness and ductility, you’ll know it all. Keep reading!
Steel is the main metal used in construction due to its excellent strength, durability, and versatility.
Steel’s strength depends partly on its carbon content and contains up to 2.1 percent carbon by weight. The presence of carbon atoms in steel’s crystal lattice deforms its structure, making it harder and stronger.
Steel recycling involves melting steel scrap in a furnace before casting the molten metal into molds, where it cools and solidifies to create recycled steel. The process preserves the atomic composition of the material, allowing it to retain its strength and durability.
It’s worth noting that steel retains its structural and chemical properties even when recycled many times, so it has excellent strength and durability, just like its virgin counterpart.
The strength and durability of steel can be quantified in various ways by different lab techniques. Let’s look at some of the different ways that the strength of steel is described.
Tensile strength is the maximum weight per unit area a material can support without failing when pulled or stretched.
Components of construction projects are often designed to resist large tensile forces due to the heavy weight of the structure that must be supported, wind pressure, seismic waves, and other external forces.
Materials used in applications in construction where tensile forces must be resisted must have excellent tensile strength. This is where recycled steel shines.
According to Sciencing, steel-iron-nickel alloy has a tensile strength of 1,460 Mpa, and stainless steel has a tensile strength of up to 1,600 Mpa.
These tensile strength numbers are impressively strong and make steel ideal for construction.
Since recycling steel doesn’t change its atomic composition, recycled steel has just as high tensile strength as virgin steel made from iron ore.
A material’s yield strength is the stress at which it deforms plastically. In other words, it’s the stress magnitude that facilitates a material’s transition from elastic to plastic, causing permanent deformation.
Materials like metals have higher yield strength than, say, plastic because of the stronger bonds between their atoms.
Yield strength is crucial in construction as it ensures structures can perform optimally within the elastic region under normal conditions.
The two main types of recycled steel have yield strengths as follows:
- Steel-iron-nickel alloy: 1,420 Mpa.
- Stainless steel: 1,560 Mpa.
These high yield strengths in recycled steel mean it can withstand high stress due to loads like those from concrete floors and external forces such as wind shear without undergoing permanent deformation.
Fatigue failure is the failure of a material at a stress value less than the maximum value of stress it can withstand under repeated or fluctuating loads. Fatigue resistance, sometimes called fatigue strength or fatigue limit, refers to the maximum stress a material can withstand for a specific number of cycles without failure occurring.
According to the University of New South Wales, fatigue resistance is directly proportional to a material’s tensile strength. Therefore, the higher the tensile strength of a material, the better its fatigue resistance.
As mentioned above, recycled steel has tensile strengths of 1,460 and 1,600 Mpa for steel-iron-nickel alloy and stainless steel, respectively. This ensures excellent fatigue resistance in both types of recycled steel.
Impact toughness is steel’s ability to deform plastically while absorbing energy before fracturing in response to a suddenly applied force.
According to Iowa State University, impact toughness incorporates ductility and strength. Therefore, a material with high ductility and strength, like steel, has high impact toughness.
Steel’s impact toughness varies according to the following factors:
- Temperature: Toughness reduces with a decrease in temperature.
- Strain rate (loading rate): Although steel may have excellent toughness under static loads, it may fail under dynamic impacts and loads.
- Notch effect: The presence of a notch can cause complex multiaxial stress distribution on the steel’s surface. Although steel has excellent impact toughness under uniaxial stress, it might not withstand the simultaneous plastic and elastic deformations due to the presence of a notch.
According to the Journal of Scientific Reports, you can achieve ultrahigh Charpy impact toughness in recycled steel by hot rolling and alloying it in austenite or ferrite temperature regions. Doing so results in impact toughness as high as 450J.
Hardness and wear resistance refer to the same thing in steel. It is the material’s ability to withstand localized deformation from cutting, scratching, indentation, or bending. The higher the hardness, the better the material is for construction.
It’s worth mentioning that steel’s hardness and hardenability are different.
While hardness refers to steel’s ability to withstand plastic deformation forces, hardenability is its ability to harden due to quenching and tempering.
There are two main ways to measure steel’s hardness and wear resistance:
- Mohs Hardness: This is the most common method that uses the Mohs hardness scale. It determines the metal’s hardness based on surface wear by scratching one material with another. According to the Mohs hardness scale, steel has a hardness of 4.0, which also applies to recycled steel.
- Rockwell hardness: This uses Rockwell hardness measurements, which determine steel’s hardness based on the indentation created when a certain amount of force is applied to it. Since the Rockwell scale correlates linearly with a material’s tensile strength, it’s the best method for accurate results. Steel (including recycled steel) has a hardness rating between 64 and 68 based on Rockwell hardness.
We all know that steel rusts because we’ve seen the familiar flaky orange stuff that peels away from the surface of unprotected steel left out in the open for too long. Left to run its course, the rusting process will cause failure of the steel, which will eventually crumble away to dust given enough time.
But not all steel is the same. Although mild steel and high-carbon steel are susceptible to rust, if you add chromium during the manufacturing process, you get stainless steel, which is far more corrosion-resistant than standard mild steel or high-carbon steel.
Steel alloyed with chromium to form stainless steel is resistant to corrosion due to the formation of an invisible, thin layer of oxides on its surface.
Chromium has a higher oxidation potential than iron, and reacts with oxygen before the iron does, forming a thin layer of chromium oxide on the stainless steel’s surface.
The oxide layer acts as a barrier between the environment and the metal, protecting it from oxidation and corrosion.
Steel can only corrode in the presence of moisture and oxygen. Therefore, a barrier of non-reactive metal oxide between the metal and the environment eliminates oxygen and moisture, protecting the underlying metal from corrosion.
So, if corrosion resistance is important, recycled stainless steel is a great option to prevent rusting, although it is much more expensive (stainless steel is around 3-4 times as expensive as carbon steel at the time of writing).
A cheaper alternative is painting, which protects steel from corrosion by providing a physical barrier between the metal and the elements. However, you’ll need to look after this protective layer with repeated re-painting.
Weldability, also referred to as joinability, is a metal’s capacity to be welded without the risk of cracking or distortion under fabrication conditions. Cracking and distortion can hinder a metal’s application for the intended purpose.
The weldability of recycled steel depends on its composition, especially its carbon content, but the presence of other alloying materials, such as vanadium and chromium, also influences weldability.
The higher the proportion of carbon and other elements in the steel alloy, the lower the weldability. This is why stainless steel, which contains 10.5% chromium and often around 8% nickel, is more difficult to weld.
On the other hand, low alloy steels, such as the high-strength low-alloy (HSLA) steels developed in the 1970s and which contain 0.05-0.25% carbon and much smaller amounts of other alloying materials, have much better weldability.
Recycled steel with carbon content between 0.15 and 0.3 percent has excellent weldability properties. These steels don’t deform, crack, or fracture due to welding.
An increase in steel’s carbon content reduces its weldability and vice versa. Higher carbon content increases hardness, making the steel more prone to cracking when welded.
Although most types of recycled steel are magnetic, there is variation in its magnetic properties based on its makeup.
Recycled steel falls into two categories based on its magnetic properties:
- Ferromagnetic recycled steel: This is the category that most recycled steel falls into. Ferromagnetic metals are strongly attracted to magnets’ north and south poles. Mild steel and HSLA steel are examples of ferromagnetic steel.
- Paramagnetic recycled steel: This category of steel exhibits very weak magnetic properties (only weakly attracted to a single `magnetic pole) and is often used in applications that require “non-magnetic” properties. Austenitic stainless steel is an example of such non-magnetic steel. The austenite structure at the crystalline level is more random than in ferritic or martensitic steels and prevents the electrons from aligning their rotation in a consistent direction, thereby preventing the material from exhibiting magnetic properties.
Ductility is a metal’s ability to be stretched or drawn without breaking, and formability is a metal’s capacity to withstand plastic deformation to a given shape without developing defects.
Ductility and formability in recycled steel vary based on their alloying components. Recycled steel with high carbon content has low ductility and formability and vice versa. This is because the presence of carbon or other alloying materials deform the crystalline structure of the metal at the atomic level, causing it to become harder and more brittle.
This explains why mild steel with a lower (0.05 to 0.25%) carbon content is more ductile and formable than its stronger high-carbon steel cousin.
The above properties of recycled steel make it a suitable eco-friendly material for residential and commercial construction.
Recycled steel is strong and durable enough to stand the test of time. It’s also ductile, formable, and weldable, making it easy to work with.
Choose mild recycled steel if you want an option with excellent weldability, ductility, and formability. Mild steel has a low carbon content, making it easier to work with on construction projects, especially when it comes to welding.