How Does steel clad Work?

Cladding can enhance a metal’s corrosion-resistant properties. The process involves the deposition of a corrosion-resistant alloy (CRA) layer over a base metal with weaker properties. The most common CRA is stainless steel. Along with its corrosion-resistant properties, stainless steel also offers the benefits of increased strength, temperature resistance, and cost-efficiency.  

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From pipeline safety to architectural improvement, stainless steel cladding provides benefits across wide-ranging of applications. In this article, we’ll examine the ideal stainless steel types and take a closer look at the cladding process. 

Stainless Steel Cladding

Consider oil and gas pipelines: most use carbon steel. Their operating conditions, however, can expose pipelines to harsh chemicals and environments that may adversely affect carbon steel. Stainless steel cladding adds a thin layer of stainless steel over the carbon steel surface to increase its durability. Further advantages of stainless steel cladding include:

• Heightened resistance to corrosion and rust to extend the operational life
• Improved mechanical strength and thermal resistance to prevent failure
• Excellent weldability and polished finish
• Greater cost-efficiency than more exotic cupronickel alloys 
• Ideal for sanitary application.

With many stainless steel types to choose from, cladding is possible for an array of unique applications. Because it enhances strength and provides the highest resistance against corrosion (due to its ferrite and austenite composition), duplex stainless steel is commonly used—especially when cladding pressure vessels and tanks. Common stainless steel grades, such as 304 and 316, can be used to clad components that are used outdoors. 

Things To Note When Stainless Steel Cladding 

There are several ways to perform stainless steel cladding, including hot roll bonding, cold roll bonding, explosive bonding, welding, and more. Hot roll bonding is a commonly used method where the plates of the base metal are placed on or between the plates of the stainless steel cladding layer. 

The weld cladding process is another effective method. When welding, a layer of stainless steel is deposited over the carbon steel surface in single or multiple passes. GTAW or GMAW is popular with this method; the choice of weld process depends on factors like material use, accessibility, weld positioning, and cost-effectiveness. 

In each case, the cladding process should meet all design and material standards specific to the industry, especially for critical applications like pressure vessels or pipelines. For instance, API 5LD covers the specification for CRA-clad steel pipes used for natural gas or petroleum pipeline systems. Listed below are standards specific to stainless steel cladding:

• ASTM A263
• ASME SA-263
• ASTM A264
• ASME SA-264.

Additional stainless steel cladding design considerations include:

• The thickness of the clad layer should be at least half the thickness of the base metal 
• Cladding thickness should be between 1.5 to 5mm; although above 5 mm is also possible 
• The recommended thickness for stainless steel cladding on a carbon steel base is 3 mm. 

The Importance of Selecting the Right Stainless Steel Cladding Process

For cladding pipelines, pressure vessels, and structural components, stainless steel has become the top choice across industries because it can resist corrosion and high temperature while providing enhanced strength and a clean finish. However, you must select the right cladding process to achieve optimal results. 
Orbital GTAW is always a good option for weld cladding. The cladding process may require flexibility for complex geometries in cramped, uncomfortable spaces. Additionally, welders can be exposed to harmful fumes such as hexavalent chromium when working with stainless steel. Orbital GTAW eliminates these issues while delivering the highest quality stainless steel cladding for diverse applications.  

While many people can identify a cladded product by looking at it, not many people know how the process works. At Materion we have been cladding since and have a deep understanding of the process. In this article we'll dive into how clad products are made.

While there are many process variations of cladding, we specialize in cold roll bonding (CRB). In this method, cladding is achieved at room temperature using pressure at the bonding mill. This method differs from hot roll bonding (HRB) which uses heat above the metals’ recrystallization temperature along with pressure to join the two metals together. CRB's advantages over HRB include:

1. Lower cost due to simplified equipment.
2. Superior control of microstructure because the operating temperature is below recrystallization temperature.
3. Ability to join dissimilar metals since brittle intermetallic compounds will not be formed due to lower operating temperatures (Fig 1).

Figure 1. Metal alloy combinations in cold roll bond contrasted against electroplating and electron beam welding.   

Materion can produce inlay clad, overlay clad, and Dovetail Clad® materials. See Table 1 for details on each version. Regardless of the cladding method, there are three key steps to cladding metals: cleaning, bonding, and sintering.

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Table 1. Review of Clad Types Offered by Materion  

CLEANING

The key to getting good clad metal starts well before the bonding mill.  Surface preparation is important and often needs to be done in two steps. First, a degreasing step is applied to remove any grease or debris that may be on the metal. If grease or dust gets between the two layers, it can cause delamination or blisters in subsequent steps. Once the metal is clean it then needs to be brushed. Brushing removes the oxide layer and adds texture to the metal. Both are critical to achieve a good bond.

BONDING

When the metals are clean enough, they are brought to the bonding mill which is similar to a rolling mill. Here, the material undergoes severe plastic deformation through a 60-75% thickness reduction in one pass (Fig 2). The texture applied at the brushing step reduces the overall pressure needed to achieve this reduction.

At the roll bite, two things are happening simultaneously. First, heat is being generated due to the high reductions of the metals. Second, the metal is being stretched linearly, exposing new, unoxidized metal surfaces at the interface. These conditions allow for what is known as a green bond to form between the two metals because there is enough heat and pressure generated to overcome the activation energy for the two unoxidized surfaces to adhere together.

Figure 2. Bonded material leaving the roll bite on the right shows significant reduction compared to unbonded material on the left.  

This green bond is a mechanical bond formed between the two metals. Generally, the higher the reduction the metals undergo, the stronger the green bond will be. However, there is a limit to the amount of cold working a metal can undergo before it becomes too brittle to work with. These characteristics must be balanced when designing the bond parameters.

While it may seem like a simple process, there are many other parameters that must be controlled to get a perfect bond. These include speed, lubrication of the rolls, front and back tension, temperature control to avoid overheating, temper of the metals, etc.

SINTERING

The green bond created at bonding is relatively strong, but not strong enough to withstand the highly technical applications our customers use clad metals for. To achieve our customers’ requirements, the clad metal must be sintered. This drives diffusion between the two metals and creates a true metallurgical bond but keeps the individual layers intact (Fig 3). After this step, the material can then be handled like any other strip metal.

Figure 3. Fully sintered clad showing three distinct layers.

Understanding how clad metals are made is essential for appreciating their benefits. In combining two or more dissimilar metals, the resulting material can deliver properties from all those materials. Learning more about how the metals are combined could provide the solution to your materials challenges. In the next article, we will explore the numerous benefits of clad metals and their applications across various industries.