Galvanized steel is produced by coating carbon steel with a protective layer of zinc. To form galvanized wire rope, carbon steel wires are immersed in molten zinc and then cooled, forming galvanized steel wire. The newly formed galvanized steel wire is then fed into a stranding machine, where it is helically stranded to produce galvanized steel wire rope. Due to its excellent breaking strength, durability, and relatively low cost compared to its stainless steel counterpart, galvanized wire rope is a common choice in construction, agriculture, and manufacturing industries.
To characterize galvanized steel wire rope's break strength, let's look at Sava's 3375 cable. With a diameter of 3/8 inches, it boasts a breaking strength of 14,400 lbs.
Galvanized steel's durability is thanks to its aforementioned zinc coating. This coating acts as a barrier, protecting galvanized steel's vulnerable carbon core from wear and tear, moisture, and other corrosive elements that are often present in the final application's environment.
While galvanized steel is water-resistant, it can corrode in saltwater due to chloride ion reactions. This reaction causes the coating to erode, thus exposing the underlying carbon steel. Stainless steel, which is naturally resistant to corrosion due to its lower carbon content, offers greater resistance to saltwater. However, due to its chemical makeup and a less demanding manufacturing process, galvanized steel is more cost-effective than stainless steel, making it more suitable for applications where harmful environmental factors are limited.
How Galvanized Steel Wire Rope is Made
Manufacturing galvanized steel begins with selecting the appropriate steel substrate, either high-carbon steel or low-carbon steel. The chemical composition of the steel is key as it affects the zinc coating's effectiveness. The zinc coating itself is applied primarily through three methods: hot-dip galvanizing, electrogalvanizing, or mechanical galvanizing. Finally, after coating, the steel wires are helically stranded into unique wire rope constructions.
The Chemical Composition of Galvanized Steel
We've mentioned that galvanized steel is produced by coating carbon steel with a protective layer of zinc. However, there are many varieties of carbon steel, including high-carbon steel and low-carbon steel, and the selection of the carbon steel influences the galvanized steel result. High-carbon steel is defined as steel with a carbon content of 0.6-1.0%, while low-carbon steel, which contains up to 0.25-0.3% carbon. Galvanized steel generally consists of low-carbon steel as the substrate, due to its better formability and ease of coating with zinc compared to high-carbon steel.
In addition to selecting the low-carbon steel, the chemical composition of the carbon steel must be considered to properly apply zinc coating.
Composition of Carbon Steel to be Galvanized
| Element | Composition |
|---|---|
| Carbon | Less than 0.25% |
| Phosphorus | Less than 0.05% |
| Manganese | Less than 1.3% |
| Silicon | 0–0.04% or 0.15–0.25% |
Zinc Coating Properties
The zinc coating's primary function is to protect the carbon steel core from oxidation and other environmental damage. However, it's important to consider potential hydrogen embrittlement in high-strength steel, which can occur if the galvanization process is not carefully controlled. This can reduce the ductility and load-bearing capacity of the wire rope, highlighting the need for precise manufacturing practices.
Methods of Galvanization
Hot-Dip Galvanizing
- Process: The steel wire is immersed in a bath of molten zinc.
- Advantages: Provides a thick, robust coating that offers excellent corrosion resistance.
- Applications: Ideal for outdoor applications where the wire rope is exposed to natural environments.
Electrogalvanizing
- Process: Zinc is applied to the steel wire through an electrolytic process.
- Advantages: Produces a thinner, more uniform coating with a smooth finish.
- Applications: Ideal for indoor applications where the wire rope is limited to natural environments, such as electrical equipment, and consumer goods.
The Electrogalvanizing Process
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Electrogalvanizing in an alkaline zinc bath
Mechanical Galvanizing
- Process: Zinc powder is mechanically bonded to the steel wire.
- Advantages: Provides a durable coating with good adhesion without using heat.
- Applications: Used in environments where thermal processes are not suitable. Mechanical galvanizing is often used for bolts or other zinc-plated mechanical cable fittings.
Zinc-Plated Steel Cable Eyelets
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Carl Stahl Sava Industries
Stranding Galvanized Steel Wire Rope in Three Steps
1. Wire Drawing
The manufacturing process of galvanized steel wire rope begins with wire drawing. Steel billets are transformed into thin wires by pulling them through a series of dies. This process reduces the size of the wire to the diameter determined by the end result wire rope.
2. Galvanization Process
Following the wire drawing, the next step is galvanization. As described earlier in the article, the drawn steel wires are immersed in a bath of molten zinc, ensuring a continuous zinc coating.
3. Stranding Techniques
Various stranding configurations are employed to create wire rope, each offering distinct benefits:
- 1x19 (19 wires): A single strand with 19 wires, offering superior strength and moderate flexibility.
- 3x7 (21 wires): Composed of three 1x7 strands, this configuration provides high flexibility in small diameters.
- 7x7 (49 wires): Made from seven 1x7 strands, it balances abrasion resistance and flexibility across various diameters.
- 7x19 (133 wires): Consisting of seven 1x19 strands, this configuration combines strength and flexibility, suitable for a wide range of applications.
1x19 Galvanized Wire Rope
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Carl Stahl Sava Industries
Cable Coatings for Galvanized Wire Rope
An optional manufacturing step we have yet to mention is applying a natural plastic coating to the finished galvanized steel wire rope. Coatings such as FEP, nylon, and vinyl, each with their unique trade-offs, provide an additional barrier for galvanized steel wire rope. These plastic coatings enhance the cable's resistance to abrasion, chemical attack, and fatigue, making it more durable in demanding applications. For example, orange vinyl-coated galvanized steel wire ropes are often used as pull cords for operating safety switches that stop machines in emergencies.
Galvanized Wire Rope: Common Applications
Suspension Bridges
Galvanized wire rope is commonly used in suspension bridges, enduring constant exposure to rain, UV light, and wind. The zinc coating protects the steel core from corrosion, ensuring long-term durability and safety while under outdoor conditions.
Lifting Mechanisms
The characteristics of galvanized wire rope are ideal for elevators and lifting mechanisms. Its high breaking strength and excellent strength-to-weight ratio support the safe lifting of heavy loads, providing consistent performance in vertical transportation systems. This reliability is vital for safety and efficiency in applications where lifting and lowering heavy objects are routine.
Galvanized Wire Rope: Common Misconceptions
The first misconception is that galvanized steel cannot rust. While the zinc coating provides corrosion resistance to the carbon steel core, it is not invcincible to oxidation. Over time, corrosive factors can damage the zinc coating, exposing the carbon steel core which leads to potential rust.
Another misconception is that all galvanized wire rope is the same. Various factors, such as coating thickness and wire rope construction, significantly influence the suitability for specific applications.
- Zinc Coating Thickness: The thickness of the zinc coating varies. Hot-dip galvanization provides a thicker, more durable coating but results in a rougher exterior. Electrogalvanization offers a thinner, smoother coating, better suited for indoor applications where visual appeal matters.
- Construction Type: Construction type refers to the number of helically stranded wires within the wire rope. The wire rope's particular construction impacts its flexibility and cost. If your application calls for the wire rope to be flexible, requiring a higher wire count, this will then increase the cost. Conversely, for applications without such requirements, choosing a wire rope with a lower wire count is more cost-effective while maintaining project integrity.
Understanding these nuances helps in choosing the right galvanized wire rope for each unique application.
Still have a few questions? We recommend talking to our cable experts®. This is the surest way to determine the material best suited to your project. For over 50 years now, Sava has been producing the galvanized wire rope the whole world uses.
