Over 1,085 million tons of steel are recycled worldwide annually, making steel the most recycled material in use today.
The recycling process produces different types of recycled steel that contribute to green building initiatives. Besides its sustainability, recycled steel brings economic prosperity to areas with recycling plants.
Construction-based steel is recycled because it is cheaper to produce steel from scrap than from iron ore. Recycled steel is also more eco-friendly and minimizes the industry’s adverse environmental impact.
This is why 99 percent of structural steel is recycled. As well as saving money, doing so diverts waste from landfills, conserves energy, and reduces greenhouse gas emissions. These are the different ways recycled steel contributes toward sustainable construction.
In the rest of this article, I’ll discuss the different types of recycled steel and how to tell them apart. I’ll also discuss the recycling process itself, so keep reading to learn more.
So, what are the different types of steel scraps?
There are two basic types of steel scrap for recycling — ferrous and non-ferrous.
Ferrous steel scrap is the most common.
Most people prefer ferrous steel, which has excellent tensile strength and durability. These properties make it ideal for construction, industrial containers, and large-scale piping.
The above features explain why the U.S. manufactures the highest amount of ferrous metal daily.
Ferrous scrap is magnetic because it contains iron. It’s commonly used in refrigerators, cars, ovens, cast iron skillets, and other household items.
Non-ferrous steel scrap is mostly nonmagnetic and contains little or no iron. It has a higher commercial value than ferrous metals due to its lightweight nature and resistance to corrosion.
Non-ferrous scrap is also easy to fabricate in terms of machinability, welding, and casting.
Most stainless steel products are considered non-ferrous because they contain they contain a large proportion of austenite, which is non-ferromagnetic.
Non-ferrous metals, including copper and aluminum, are suitable for making plumbing materials, electronics, coins, and jewelry.
The table below summarizes some of the key differences often found between ferrous and non-ferrous metals:
|Ferrous Metals||Non-Ferrous Metals|
|Excellent tensile strength||Typically, lower tensile strength|
|Low corrosion resistance||Typically, better corrosion resistance|
|High density||Often lower density|
|Usually silver in color||Multiple colors like yellow gold and pale reddish|
Table 1: Common differences between most ferrous and non-ferrous metals
There are four primary grades of steel at scrap yards, as discussed below:
This is the most popular steel scrap grade in most yards because it accounts for 90 percent of steel production.
To be considered carbon steel, steel scrap must have a carbon content between 0.05 and 1.5%.
Carbon-grade steels are further classified into the following:
- Mild-carbon steel: It contains the lowest carbon content, typically between 0.05 and 0.25%. They’re inexpensive and versatile, making them ideal for construction and manufacturing.
- Medium-carbon steels: Their carbon content ranges from 0.3 to 0.6%. The hardness and strength of medium carbon steels are higher than steel with lower carbon contents. They are well-suited to forming cutting blades, but their welding performance is less good.
- High-carbon steel: They contain between 0.6 and 1.5 percent carbon content. This grade is more brittle than the rest due to its high carbon content.
Alloy steels contain other metals like chromium, aluminum, copper, and nickel.
The quantity of alloying elements in these grades varies between 1 and 50 percent.
The variation in the quantity of alloying elements creates two categories — low and high alloy steels.
Although the exact definition of low and high-alloy steels is somewhat debatable, a rule of thumb defines a cut-off based on the proportion of alloy elements they contain.
Steel with an alloy element content below 8% is considered low steel alloy. Anything above 8% is considered a high steel alloy.
Stainless steel is the most widely known grade of steel. It’s used in construction and manufacturing due to its excellent corrosion resistance and strength.
Most stainless steels contain 10% to 20% chromium combined with other elements like carbon, iron, nickel, manganese, and silicon. These components are responsible for the grade’s high resistance to corrosion because they create a surface oxide film that prevents oxidation of the metal beneath.
Stainless steel is also suitable for places where hygiene is important. As a result, it’s commonly used for worktop surfaces in food manufacturing, hospitals, and scientific laboratories.
This steel grade is highly resistant to heat mutation and stress. This feature is attributed to the grade’s molybdenum, vanadium, tungsten, and cobalt combination. These components are responsible for tool steels’ excellent physical and chemical properties.
Tool steels also have a higher resistance to wear and abrasion, making them suitable for cutting tools such as drill bits and saw blades. They’re also ideal in places that require heavy tools, such as the automotive industry.
Now that you know the different types of steel scraps and their grades, let’s explore the recycling process.
Steel recycling is the process of collecting, processing, and repurposing steel scraps for reuse. But why is steel recycled?
Steel is recycled to reduce production costs, energy consumption, and carbon dioxide emissions compared to primary steel production.
Manufacturing virgin steel is an energy-intensive process that consumes approximately 6.0 MJ per ton (2,000 lb) of steel produced. The process emits about 1.4 tons (2,800 lb) of carbon per ton (2,000 lb) of steel produced.
Another benefit of steel recycling is reducing landfill waste, particularly from construction and demolition projects. Moreover, recycling steel is an eco-friendly process that aids in the following:
- Prevention of environmental degradation through opencast mining.
- Elimination of greenhouse gases like nitrogen oxide and sulfur dioxide.
- Preservation of natural resources like iron ore, coal, and limestone.
The steel recycling process comprises the following steps.
The first step in steel recycling is collecting steel scrap from various sources.
The following are the primary categories of steel scrap sources:
- Home scraps come from in-house steel projects in steel-producing mills. They include steel rejects and trimmings. In most cases, they’re usually channeled back into the furnace for reprocessing.
- Prompt (Industrial) scraps come from manufacturing activities like automobile production.
- Obsolete scraps incorporate the steel waste from products that have reached the end of their service life, like old cars, household waste, individual appliances, and buildings.
This step involves sorting and separating the scrap into ferrous and non-ferrous categories.
The two streams are further categorized by grade, composition, and size.
In most cases, scrappers use magnets to sort and separate the different types of steel scrap.
Scraps that stick to the magnet are ferrous, while those that don’t are non-ferrous.
It’s worth mentioning that while all steel is recyclable, scrap may contain non-recyclable content that can affect the final product’s quality. Therefore, ensuring all scraps are contaminant-free before recycling is always advisable.
Once at the recycling plant, steel scraps undergo compaction to reduce their volume and improve loading capacity.
The next step is shredding the scrap into smaller, manageable pieces. Recycling smaller steel pieces is easier since they occupy less space and require less energy to melt.
The shredded steel scraps are transferred into a furnace heated to approximately 3,500°C (6,332°F) for smelting.
Once the scrap is melted, the molten steel is refined to remove impurities and create a product of the desired grade.
Companies may also add alloying elements like carbon, chromium, and silicon to improve the steel’s properties.
The melted and refined steel scrap is then cast into slabs or ingots before being left to cool and solidify.
The products cool while taking the shape of the molds, which are usually plates or bars, depending on the intended use.
The cooled recycled steel can be further processed for specific applications or stored in warehouses before transportation.
The last step in steel recycling is quality control testing to ensure the products meet the required standards and specifications.
Tests include tensile strength tests, hardness tests, and microstructure analysis with metallurgical microscopes.
Destructive tests like ductility, fatigue, and corrosion resistance are also conducted to ensure the steel is safe and reliable.
You can identify the type of steel in the following ways:
Most steel products have a number label that indicates the grade of steel used. You can find the number on the metal’s surface or the wrapping.
Steel products are labeled based on AES and ASTM systems.
The AES system has four digits; the first two refer to the type of steel.
The ASTM system also incorporates a number and one or more letters. The letters indicate the type of steel based on the following coding:
- A: Axle steel
- S: Carbon steel
- SS: Stainless steel
- W: Low alloy steel
- CS: Low-carbon chromium
A chip test will help you view the inside of the metal. Use a chisel and a hammer to create a small chip to see the inside of the sample.
A bright gray interior confirms you have stainless steel, while a dull gray interior indicates carbon steel.
You can also identify the type of steel by performing a magnetic test.
The test involves placing a magnet on the product and observing its reaction. If the magnet sticks to the product, you have ferrous metals like carbon and alloy steel.
On the other hand, if it doesn’t stick, the product is made from a non-ferrous metal like aluminum or stainless steel.
Finally, you can identify the type of steel by dropping small amounts of phosphoric acid on the metal surface.
The acid will react differently based on the type of steel:
- Carbon steel and cast iron: High corrosion and bubbling.
- Low-chromium stainless steels: High corrosion resistance. However, you’ll see some bubbling.
Steel recycling is a sustainable approach to preserving natural resources and the environment.
After reading this article, you have learned about the different types of recycled steel, how to identify them, and the recycling process. This knowledge should help you choose the right materials for your purposes.
Finally, remember to adhere to all safety guidelines when handling recycled steel, especially when using chemicals or using a chisel to remove chips for testing.