Galvannealed Steel: Process, Methods, Properties, and Uses
Galvannealed steel, which is made by a sort of fusion of the galvanizing and annealing processes, is valued for its corrosion resistance, paintability, and formability. Galvannealed steel offers good corrosion resistance, good paintability, and weldability. The improved paintability of galvannealed steel is due to its rougher surface. The zinc-iron layer is harder and stronger than the zinc layer on ordinary galvanized steel, but also more brittle. Galvannealed steel is often used in applications where both corrosion protection and a painted finish are required, such as for the production of parts and panels in the automotive industry, as well as in the manufacturing of appliances and HVAC (heating, ventilation, and air conditioning) systems.
This article will describe galvannealed steel in detail and explore its production, properties, and appearance.
What Is Galvannealed Steel?
Galvannealed steel is a zinc-coated steel that has undergone a combination of galvanizing and annealing. Galvanizing involves coating steel with a layer of zinc to protect it from corrosion, while annealing involves heating the steel to a specific temperature and then allowing it to cool slowly to relieve internal stresses and improve its mechanical properties.
What Does Galvannealed Steel Look Like?
Galvannealed steel typically has a matte gray or matte silver finish. The surface is not as shiny as traditional galvanized steel; instead, it has a more subdued and uniform appearance. The matte finish of galvannealed steel is a result of the annealing process, which allows the zinc coating to alloy with the surface of the steel. This forms a zinc-iron alloy layer, giving the steel its characteristic appearance. The surface is also slightly rough and grainy, but it is not very pronounced.
How Does Galvannealed Steel Differ From Regular Steel?
Galvannealed steel differs from regular steel in that it has a matte-finish zinc coating, which enhances paint adhesion. This coating is more absorbent than the standard galvanized finish, making galvannealed steel better suited for applications requiring effective paint bonding. The zinc-iron coating also provides the steel with a corrosion-resistant layer that helps to protect the steel underneath.
What Is the History of Galvannealed Steel?
The history of galvannealed steel is closely tied to the broader history of galvanized steel, with both materials sharing a common origin in the process of coating steel with zinc to prevent corrosion. The roots of galvanizing can be traced back to more than 300 years ago when a chemist envisioned immersing iron into molten zinc, resulting in a shiny, silver coating. This visionary idea became a reality in 1742 when French chemist Melouin submitted a paper to the French Royal Academy outlining a method for producing a zinc coating. This involved immersing iron into molten zinc, resulting in the formation of a protective zinc layer on the iron surface.
The term "galvanizing" was coined in 1836 when Sorel in France patented a process of coating steel by cleaning it and then dipping it into molten zinc, calling it “galvanizing.” The process was recognized for its protective properties, and the application expanded from household utensils to various industrial uses. The significance of galvanizing was further emphasized in the 19th century with experiments related to electrical phenomena conducted by scientists like Luigi Galvani and Michael Faraday.
The history of galvannealed steel specifically is intertwined with the evolution of galvanizing. The galvanizing process, which initially involved simply dipping steel in molten zinc, saw advancements such as cleaning the steel with water and sulfuric acid before the zinc coating. This improvement, patented by Sorel in 1836, contributed to making the iron even more resistant to oxidation.
How Does the Galvannealing Steel Process Work?
Galvanneal shares similarities with other galvanized steels, particularly hot-dip galvanizing. In the galvannealing process, the steel substrate is dipped into a zinc liquid bath, causing zinc to adhere to the steel’s surface. This mirrors the hot-dip galvanizing method. The galvanizing process establishes a metallurgical bond between the base steel and the zinc coating, forming a barrier that becomes part of the steel. Galvanized coatings exhibit adhesion to the underlying steel at least ten times greater than other coatings. In the galvanization process, a reaction occurs between the molten zinc and the iron in the steel, leading to the formation of several layers of zinc-iron alloy.
Subsequently, while the zinc is still in a liquid form, the galvanized steel passes through a high-volume, low-pressure air knife to remove the excess coating before it solidifies, resulting in a thinner coating compared to the zinc coating of standard galvanized steel. After this, the zinc-coated steel immediately enters the annealing furnace after exiting the zinc bath, reaching an annealing temperature between 450 and 550 °C to allow diffusion between the iron in the steel and the zinc coating. This results in a zinc-iron alloy layer that forms on the surface. The steel is then cooled and subjected to a heat treatment to produce a matte finish. Finally, the galvannealed steel is cut to the desired dimensions. This entire process enhances paint adhesion, making it ideal for applications requiring a well-bonded paint finish.
What Is Galvannealed Steel Made Of?
Galvannealed steel is regular steel that has been coated with zinc to make it corrosion-resistant. The steel itself is usually low-carbon steel but may cover a range of compositions, with carbon (less than 2%), manganese (1%), small amounts of phosphorus, silicon, oxygen, and sulfur, and the balance iron. The zinc-coated metal undergoes an annealing process which differentiates it from galvanized steel. The annealing step diffuses iron from the substrate into the zinc, which results in a zinc-iron alloy coating.
How Long Does Galvannealing Steel Take?
Galvannealing steel can take a few hours or even up to a day or two to complete. The first step, the hot-dip galvanizing step, typically takes a few minutes to dip prepared steel into the molten zinc to form the zinc coating. However, if you take into account the preparation of the steel, the hot-dip galvanizing step can take up to about half a day to complete from start to finish. After this step is complete, it’s time to move on to the annealing step where the zinc coating and iron of the substrate fuse to form a zinc-iron coating. The annealing diffuses the iron throughout the thickness of the zinc coating. The annealing process typically takes a few seconds to complete. The main factors that influence the duration of the annealing process are the metal being annealed and the cooling time.
What Are the Properties of Galvannealed Steel?
The properties of galvannealed steel are outlined in Table 1 below:
| Property | Description |
|---|---|
Property Density | Description 7.00-8.05 g/cc |
Property Coating composition | Description Zinc-iron alloy (90% Zn - 10% Fe) |
Property Appearance | Description Gray matte |
Property Surface finish | Description Matte, rough |
Property Paint adhesion | Description Excellent |
Property Weldability | Description Good (better for spot welding than galvanized) |
Property Edge protection | Description Good |
Property Corrosion resistance | Description Good (less than galvanized steel) |
Property Formability | Description Good |
Property Heat resistance | Description Moderate |
Table Credit: https://eziil.com/galvanneal-vs-galvanized
How Can Galvannealed Steel Avoid Rust?
Galvannealed steel avoids rusting through its corrosion-resistant zinc-iron alloy coating, which acts as a protective barrier. The alloy layer, formed during the galvannealing process, provides corrosion resistance by preventing direct contact between the steel surface and environmental elements.
Is Galvannealed Steel Magnetic?
Yes, galvannealed steel is magnetic. This is because the base steel layer is magnetic. Like galvanized steel, galvannealed steel is covered by a thin layer of zinc, or in this case, a zinc-iron alloy, which does not influence the magnetic properties of the steel underneath.
Is Galvannealed Steel Weldable?
Yes, galvannealed steel is weldable. Its zinc-iron alloy layer, formed during the galvannealing process, contributes to good weldability compared to galvanized steel. This property makes galvannealed steel easy to join and fabricate, particularly advantageous in industries with significant welding operations like automotive manufacturing and construction. The alloy layer not only helps prevent weld spatter but also provides an electrically conductive surface for creating strong and reliable weld joints.
Is Metal Sheet the Most Common Form of Galvannealed Steel?
Yes, galvannealed steel is often produced and supplied in sheet form. This is very useful for applications such as: roofing, automotive panels, and more.
Why Use Galvannealed Steel?
Galvannealed steel is favored for its enhanced paint adhesion compared to galvanized steel, which makes it well-suited for applications requiring a durable and well-bonded paint finish. The zinc-iron alloy layer provides good corrosion resistance, which enhances the metal’s durability. It also offers excellent abrasion resistance. Additionally, galvannealed steel exhibits good weldability and formability. All these factors make galvannealed steel popular for many applications, such as: automotive parts, construction components, waste disposal chute systems, and more.
What Industries Use Galvannealed Steel?
Galvannealed steel is used in industries such as: automotive manufacturing, construction, appliance production, agricultural applications, waste disposal chute systems, electrical enclosures, and general manufacturing.
What Are the Advantages of Galvannealed Steel?
Galvannealed steel comes with a lot of advantages over uncoated steel and other coated materials. Some of its advantages are listed here:
- It is specifically intended to be painted. Its matte zinc-iron coating offers superior paint adherence compared to standard galvanized steel, thanks to its increased absorbency.
- The zinc-iron coating of galvannealed steel is harder and stronger than that of regular galvanized steel. This coating provides extra scratch resistance.
- It offers better weldability and formability than other types of galvanized steel.
- Galvannealed steel offers better corrosion resistance than uncoated carbon steel.
What Are the Disadvantages of Galvannealed Steel?
While galvannealed steel offers some notable advantages, it’s also important to consider its disadvantages. Some of these disadvantages are listed below:
- Due to its thinner coating, galvannealed steel is less corrosion-resistant than galvanized steel.
- It is more expensive to produce galvannealed steel than regular galvanized steel due to the added annealing step in the manufacturing process.
- Although harder than galvanized steel, galvannealed tends to be more brittle. This can limit its applications, especially if flexibility is required.
Can Galvannealed Steel Be Used in Form Milling?
Yes, galvannealed steel can be used in form milling. Compared to galvanized steel, galvannealed steel offers certain advantages in machining processes. Galvannealed steel is more machinable than galvanized steel because it has a thinner and more uniform coating. The annealing process also improves the steel's overall machinability.
However, excessive heat during the milling process can have negative effects on the zinc coating, potentially compromising its corrosion resistance and releasing harmful zinc oxide fumes.
Is Galvannealed Steel Worth It?
Yes, galvannealed steel is worth considering for applications that demand a balance of corrosion resistance, paintability, and machinability.
Are Galvannealed Steels Expensive?
Yes, galvannealed steel is more expensive than galvanized steel because of the extra step (annealing) involved in the manufacturing process. And since galvanized steel and untreated structural steel are more or less priced the same, galvannealed steel is also more expensive than regular steel. Its market value is also considered to be high due to its added rust and corrosion resistance, formability, and weldability.
What Is the Difference Between Galvannealed and Galvanized Steel?
The difference between galvannealed and galvanized steel is the galvanizing process for both coated steels begins with the steel substrate being immersed in a bath of molten zinc. However, galvannealed steel undergoes an additional step: it is heated in an annealing furnace, creating a zinc-iron alloy that enhances durability and paintability.
In terms of coating appearance, galvanized steel has a polished, spangled surface with a thicker pure zinc layer, while galvannealed steel is coated with a matte zinc-iron alloy layer. Galvanized steel is suitable for applications like: HVAC systems, agriculture, and industrial equipment. In contrast, galvannealed steel finds use in: the automotive industry, appliance manufacturing, and electrical systems.
Galvannealed steel is generally more expensive than galvanized steel due to the additional manufacturing step required to produce galvanneal. The decision between the two depends on the specific application’s requirements, with galvannealed steel being chosen for its superior paint adhesion and formability, and galvanized steel for its cost-effectiveness in certain industrial and outdoor applications.
Summary
This article presented galvannealed steel, explained it, and discussed its various properties and applications. To learn more about galvannealed steel, contact a Xometry representative.
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