Fusion cladding provides protection for metallic components by depositing a layer of material, normally using an arc welding process, to provide a corrosion- or erosion-resistant surface.
The technique is well-established and has been in use for decades on applications as diverse as turbine blades, high performance steam valves, bearing surfaces and sub-sea components.
A wide range of surfacing metals is in use and these include copper, nickel and cobalt based alloys and many stainless steels. Cost savings are impressive: fully cladding a carbon steel component with nickel 625, as opposed to producing it in solid alloy, can reduce costs by as much as 60%.
|Fig 1. Basic cladding concept using the GTAW process. The clad layer is typically between 2 and 20 mm thick.|
The effectiveness of a clad layer depends entirely on being able to deposit reliably and repeatably the alloy defined in the weld specification. The procedure clearly needs to take into account the dilution which might take place with the substrate material. Often overlooked however is the need to ensure that exclusion of air and thus oxygen is prevented during welding. Unless oxygen is excluded, essential elements such as chromium, included for example in a stainless steel deposit for corrosion resistance, can be lost by oxidation during the cladding operation.
Even the superalloys containing cobalt and nickel can oxidise during fusion welding where temperatures typically exceed 3,000oC. Elements including chromium, cobalt and nickel in cladding alloys are crucial in protecting against corrosion and erosion in use and if lost by oxidation the protection can be significantly reduced.
Welds carried out on most metals with inadequate inert gas coverage oxidise. The effect is even noticeable with many stainless steels. To some, the discolouration due to oxidation is an inconvenient feature that can be removed after welding, but this may be difficult and, in any case, costly, especially if access is restricted. Unfortunately, any oxidation can result directly in a reduction in corrosion resistance and in some cases loss of mechanical strength.
Effective protection is essential and this is achieved by surrounding the joint with an inert gas such as argon or helium.
Protection of Metal Surfaces
|Fig 2. Externally clad ball valve where protection against oxidation of the cladding alloy is required.|
When low current arc welding processes are used to deposit thin surface layers, the inert gas from the torch is usually adequate to protect against oxidation. As the thermal output increases however, the weld deposit remains at a high temperature after the torch has passed and additional inert gas coverage is required whilst the temperature falls. This can be achieved using a Trailing Shield® to protect the solidified weld metal and associated heat-affected zones until temperature falls below 400°C.
The Trailing Shield® concept, developed to provide inert gas protection behind the welding torch was introduced over 50 years ago. It has been redesigned recently by Huntingdon Fusion Techniques Ltd to take advantage of technical developments in materials. This UK company is established as the leading international supplier of Trailing Shields® having sold well over 5,000 units worldwide.
Argweld® Trailing Shields®, made in Wales by Huntingdon Fusion Techniques HFT®, have been designed specifically for use with GTAW (TIG) or PAW (plasma) torches and provide a high level of additional inert gas shielding to supplement that supplied by the basic torch.
There are models for internal and external curved applications down to 25 mm diameter.
The welding torch is mounted on the leading end of the shield and inert gas fed through one or more ports behind the fusion zone. A seal between the shield and the work is assured through the use of flexible, pre-formed and easily replaceable silicone skirts. Turbulence inside the cavity is avoided by passing the gas through a fine mesh filter above the fusion zone.
|Fig. 3. The range of Trailing Shields® is extensive and can accommodate curved surfaces in addition to plane cladding applications. Images courtesy Huntingdon Fusion Techniques,HFT®.|
Protection of complex three-dimensional products
Weld cladding is used extensively to protect exposed parts of valves. Access is often restricted and under these circumstances there has been an increased use of flexible enclosures where an entire valve together with robot-controlled welding torch can be accommodated and total protection against oxidation assured.
|Fig. 4. Typical valve section illustrates the dimensional complexity and the problems of access for a welding torch. Enclosing the entire operation inside an inert gas-filled enclosure is used increasingly to provide protection against oxidation.|
The original standards for welding enclosures were based on vacuum systems and were established by aerospace companies and this industry is still the largest single user. Because of the high pressure differential expected between the inside and outside of a vacuum enclosure it is necessary that they were designed to withstand the high stresses involved.
The main advantages of such enclosures were fast purge times, fast removal of contamination and quick turnaround of parts. The disadvantages arise from them being heavy, expensive and the fact that they occupy considerable floor space. Contemporary flexible enclosures overcome all the disadvantages of metal boxes.
The flexible enclosures now available overcome many of the disadvantages of alternative systems, not least a significantly lower cost. They exploit recent advances in material specifications and sealing techniques.
Ultra-violet stabilised engineering polymers are used throughout during manufacture. Entry points provide for equipment loading, operators gloves and service panels for welding torches.
Size-for-size a flexible enclosure costs less than 10% of a metal glove box and only 2% that of a vacuum system.
Flexible enclosures are now used by leading manufacturing companies across the globe. The aerospace, automotive, biochemical, medical, food and beverage, semiconductor and nuclear sectors all take advantage of the low cost and ease of use. The concept is being used globally for weld deposition incorporating multi-axis robots and all welding equipment. Enclosures up to 9 cu m volume are routinely available but larger models can be manufactured.
|Fig. 5. Flexible Welding Enclosures® manufactured for the Cranfield Welding Engineering Research Centre, UK. It illustrates some singular features of flexible enclosures such as options for multiple operator and large equipment access locations.|
Control over the oxygen level inside these enclosures can be effected by continuous monitoring using instruments such as the PurgEye® series.
|Fig. 6. PurgEye® Weld Purge Monitor® designed specifically for use with welding enclosures can be used to detect an oxygen content as low as 10 ppm.|
The Argweld® PurgEye® 500 Desk incorporates a unique, fast response, long life sensor with little maintenance requirement. The instrument comes complete with an integral pump to deliver regular flow of exhausting weld purge gas to the sensor to ensure consistent measurements and readings. Software is included for computer interfacing, data acquisition, storage and printing of results and graphs for Quality Management Control purposes.
The need to avoid oxidation of elements during weld cladding is often overlooked. This can have significant effects since loss by oxidation can reduce resistance to corrosion and erosion but can also affect mechanical properties of the clad layer. Use of commercially available equipment such as trailing shields and flexible enclosures can eliminate oxidation by ensuring that a protective inert gas environment is provided.
- High-temperature oxidation of alloys. Wood G.C. Vol 2, 1970.
- High‐Temperature Oxidation of Metals. Sneha Samal. IntechOpen Limited UK. September 2016.
- Weld Overlay - Influence of Welding Process and Parameters on Dilution and Corrosion Resistance. Paper presented at Stainless Steel World America 2010, Houston, Texas. October 2010. Kumar V et al.
- Hot Corrosion of Inconel 625 Overlay Weld Cladding in Smelting Off-Gas Environment. Zarani A.M.et al. Metallurgical and Materials Transactions Vol 44, may 2014.
- Avoiding loss of corrosion resistance when welding stainless steels. Metalworking World Magazine. November 2014.
- Introduction to Stainless Steels. Welding Knowledge - Part 2. Huntingdon Fusion Techniques, HFT®, UK.
By Michael Fletcher, PhD. Metallurgy
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