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Hardfacing Explained - Part I

Date: 08-10-2021

Hardfacing Explained


What is Hardfacing?

Hardfacing is the deposition of hard wear resistant materials on worn or new component surfaces that are subject to wear in service. The abrasive and harsh nature of the mechanical processes of moving and transporting ores or dirt in the mining and construction industry often cause a lot of wear and tear to equipment and machinery, in fact enough to significantly reduce their lifetime and productivity. Metal parts in contact with mineral ores or soils often fail their intended lifespan not because they breakdown but because they wear too fast due to high abrasive nature and impact stresses which cause progressive deterioration.

Welding is an economic method used to apply hardfacing to improve the wear resistance of various components by the deposition of a hardfacing layer on the parts exposed to wear and tear. Welding a hardfacing protective layer or layers on components and tools subjected to aggressive wear is the most common industry method. However, there are other available methods.

 

Applying Hardfacing

Hardfacing is more than just applying a protective layer to a base material and hoping for the best… it is a process that needs careful consideration of all the applications parameters so you achieve the best outcome. Why spend countless hours on a cheaper 'multi-purpose' product that will barely provide double wear life? For any successful hardfacing outcome there is never one product that suits all. If there is a consistent environment provided, you must analyse the 'what, where, with and how' so you have the correct product to achieve the best outcome. The team at Alphaweld are experienced in this area and are always ready to assist.

The prime factor to consider is whether it is an abrasion application or impact application, or even both? That’s your starting point. Globally there are many manufacturers who will sell you anything just to get you to use their product and it may not necessarily be the right one. On the flip side however, there are manufacturers such as Corodur® that have excellent products designed for very specific applications.

Firstly, rebuilding may be required to restore the part to its original condition. This means that a filler has to be applied to bring the damaged worn part back to original state or to as near as possible. The filler material must have similar or uniform characteristics as the base, material compatibility is important as well for its success to eliminate the risk of cold cracking and provide a uniform operating temperature.

Buffer layer or layers need to be applied in some applications to overcome problems of incompatibility between substrate and cladding. It is designed to provide a good stable base between the parent metal and the hardfacing layer and avoid shrinkage cracks from the hardfacing to the base metal. Great care must be undertaken when choosing the filler metal for the buffer layer. If the differences in elasticity or expansion between the base metal, buffer and cladding are too great, this can cause the application to fail prematurely.

The top layer is the hardfacing layer and fundamentally if this is not harder than the parent metal, it will either not work, get torn off by impact or rapidly worn off by abrasion. This layer may involve depositing one or several layers of weld metal. Some hardfacing products are designed to be applied single layer only, while others can be applied as multiple layers. Factors of consideration are the environmental conditions, product toughness and resistance to impact or high temperatures and corrosion. 

As well as rebuilding there is also preventive hardfacing where proactive hardfacing techniques are applied in the production of a brand new metal component or applied to equipment before it is used to preserve the integrity of the equipment and to minimise the need of rebuilding layer when replacing the hardfacing. In this case, the need for a buffer layer may also be less relevant.

Hardfacing Application

To achieve the best possible outcome of wear, several aspects need to be considered…

  1. Is it abrasion factors, impact factors or both? Manganese steels are often used in applications involving repeated shocks (impact), whereas tungsten carbide alloys are better at resisting abrasion.
  2. The mechanical properties of the base material is known. E.g. composition and manufacturing method. This information allows us to understand its susceptibility to wear and the welding conditions required during repairs or hardfacing.
  3. The external element. What causes wear of the substrate or what characterises the dynamic and physical properties? Its hardness, shape and texture determine the damage it will cause depending on the pressure, speed and angle of contact with the material. The sharper and harder the abrasive, the higher the abrasion rate.
  4. Are thermal (heat) cycle loads involved?
  5. The application. What conditions are you applying the protective layer or layers in and what equipment do you have?
  6. Are the skills available to do it?

Transfer Techniques  

The application of hardfacing in welding is either by Gas Shielded Metal Arc Welding (GMAW), Shielded Metal Arc Welding (SMAW), Gas Tungsten Arc Welding (GTAW) or Manual Metal Arc (MMA). Self shielded welding, also known as open arc in 'MIG/MAG' or 'stick' is a common procedure that is particularly used for hardfacing solutions in outdoor welding such as onsite or in a draught.

Submerged arc hardfacing or 'Subarc' as it is known is generally done in-house or in production and is a technique where the molten metal is generated by an electric arc between wire and the base beneath a 'blanket' of powdered flux. The arc is not highly visible and the welding flame is mostly absorbed by the flux. The procedure is generally restricted to flat welding positions on plate or pipe but has an advantage of a very high deposition rate. Semi automated welding equipment is often required.

Welding GMAW wires produce either spray or globular transfer of molten metal across the welding arc. Spray transfer is a dispersion of fine molten metal drops and can be characterised as a smooth-sounding transfer. These wires are desirable in joining applications in which you require good penetration. 

Ball transfer wires disperse larger molten metal drops or balls. This type of transfer promotes low penetration and dilution and is suitable for hardfacing. It has a noisier arc that produces an audible crackling sound and generally has a higher spatter level than spray transfer wires. Welding parameters such as electrical stickout, gas (if any), amperage, and voltage can affect the size of the ball and its transfer. Gasless, or open arc, wires all have a globular or ball transfer. 

 

What type of gas is used in GMAW Hardfacing? 

Low penetration and dilution are the major objectives in hardfacing, so pure argon and mixtures of argon with oxygen or carbon dioxide for shielded wires generally will produce the desired result. You can also use pure carbon dioxide, but you'll get more spatter than you would with an argon mixture. Self shielded wires are available and are very user friendly.

 

Conclusion

For any successful hardfacing outcome there is never any one product that suits all hardfacing applications. For example silica sand (quartz) has a Mohs hardness of 7.0, so it wouldn't make sense to use chrome carbide (65RHC) that has a Mohs of 6.5-7.0. Using the correct product and technique will always pay for itself many times over.

Our experts at Alphaweld stand ready to assist should you be facing any challenges with hardfacing. We stock a large range of specialist hardfacing products to suit any hardfacing application.

Call us today on (08) 9456 8000 or email sales@alphaweld.com.au if we can assist.