• Frequently asked questions: Hardfacing

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Frequently asked questions: Hardfacing

Understand what hardfacing is, and how it is used.

The following answers to frequently asked questions can assist you in selecting hardfacing products that are most appropriate for your application.

Every industry is exposed to some form of destructive wear of its in-service parts, components, or equipment. Due to the impact of wear, metal parts regularly fail at their intended use, through loss of dimension and functionality, which can result in unplanned downtime.

Hardfacing, also known as hard surfacing, surface welding, and cladding, offers a solution for resurfacing worn-out parts instead of replacing them, as well as for protecting new parts.

“Hardfacing” is the deposition of a surface layer by welding, which is harder than the base material, its purpose is to give wear resistance.

The properties usually sought through the application of hardfacing are greater wear resistance from abrasion, impact, adhesion (metal-to-metal), heat, corrosion or a combination of these factors.

A wide range of hardfacing alloys are available to fit the needs of practically any metal part. Some alloys are very hard, while others are softer with hard abrasion-resistant particles dispersed throughout. Certain alloys are designed to build a part up to a required dimension, while others are designed to be a final overlay that protects the work surface.

Through hardfacing, a filler metal is used to bond to the equipment’s base metal to achieve specific dimensions or wear properties, such as impact, adhesive wear or abrasion resistance. Hardfacing can be applied through a range of technologies, such as welding processes, thermal spraying, spray-fuse or similar processes, used predominantly to reduce wear.

Hardfacing provides the most economical means to minimise wear and increase the service life of re-surfaced parts, by up to 300% compared to non-surfaced parts.

Since its inception in 1966, Welding Alloys has become a recognised leader in the development of premium hardfacing and speciality alloys. We offer hundreds of hardfacing products, suitable for a range of processes to repair and/or prevent wear of in-service parts.

The main reasons businesses use hardfacing include:

  1. Cost Savings – Hardfacing/hard surfacing of worn metal parts can result in a new or better-than-new condition of the parts, at between 25 – 75% of the replacement cost. Also, savings can be achieved through less expensive base metals being used.
  2. Extended Equipment Life – Surfacing of parts through hardfacing can extend their service life by between 30 – 300% when compared to non-surfaced parts.
  3. Increased Operating Efficiency – Longer-lasting parts can reduce downtime, resulting in extended operational uptime and fewer plant shutdowns, associated with the replacement of worn parts.
  4. Lower Spare Part Inventories – Fewer replacement parts are required. Instead of keeping an extensive spare part inventory, worn-out parts can be rebuilt.
  5. Positive environmental impact – Resurfacing of existing parts, rather than replacement, results in lower energy requirements, when compared to the energy needed to produce new parts. This reduces CO2 impact on our planet.
  1. Rebuilding or build-up

“Rebuilding” is the restoration of a part to its initial dimensions when its geometry has been changed by wear. Normally, a homogeneous filler metal is used: its chemical composition and mechanical characteristics are similar or identical to those of the base metal.

In some cases, however, a heterogeneous alloy could be used, provided its characteristics are compatible with those of the substrate.

The three major factors in choosing a suitable filler metal for rebuilding are:

  • The risk of cold cracking: both the preheating temperature and the interpass temperature need to be defined (typically determined by base material type).
  • The service temperature and, therefore, the differences in thermal expansion between the filler metal and the base metal.
  • Compatibility between the rebuilding filler metal and any subsequent surfacing.
  1. Buffer layer

Also known as the “sub-layer” or ”metallic transition”, a buffer layer is used when necessary to overcome problems of incompatibility between substrate and cladding.

Why use a buffer layer?

  • To provide a good foundation between the base metal and the hardfacing.
  • To avoid the propagation of shrinkage cracks from the hardfacing to the base metal.

Care must be taken when choosing the filler metal for the buffer layer. If differences in elasticity or thermal expansion between the base metal, buffer and cladding are too great; excessive stresses may be generated at the weld joints. This may cause it to fail prematurely.

  1. Hardfacing or overlay

Hardfacing is the deposition of a surface layer by welding, which is harder than the base material. Its purpose is to give wear resistance. Hardfaced layers may also be characterised by the following properties:

  • Soundness (cracks are acceptable in some cases).
  • Toughness, depending on the need to resist impact.
  • Resistance to environmental stresses such as corrosion and high temperatures.

Hardfacing may involve depositing one or several layers of weld metal. Some types are designed to be applied in one layer only, while others can be applied in multiple layers. “Preventive hardfacing” is the application of hardfacing to a brand new component. In this case, the nature of the base metal may be less relevant, apart from cost considerations. “Remedial hardfacing” involves reconstitution of an already worn part, so compatibility with the material of the part needs to be considered.

For an illustration of these processes, please refer to page 12 of our Fundamentals of Hardfacing by Arc Welding booklet.

Increasingly, semi-automatic and automatic welding procedures are used for hardfacing. The following popular welding processes are used. Please note, these FAQs do not cover processes such as thermal spraying, laser etc.

Gas Tungsten Arc Welding Process (TIG)

In the TIG process, an electric arc is produced between a refractory tungsten electrode and the part. A metallic filler wire may or may not be used.

The weld pool is protected from oxidation by an inert atmosphere (often argon).

Shielded Metal Arc Welding Process 

The consumable electrode is composed of a solid core wire and a flux covering. An electric arc creates a weld pool between the electrode core and the part. The slag produced by the fusion of the coating protects the molten metal against oxidation and can contribute to the deposits’ chemical analysis.

Tubular electrode 

A tubular electrode consists of a thin steel tube filled with a powder mixture. This type of electrode is only used for hardfacing applications. A uniform electric arc is formed between the tube wall and the part. This results in lower dilution and wider deposits compared with a conventional coated electrode.

This type of electrode is less susceptible to moisture pickup than standard electrodes

Gas Shielded Metal or Flux-Cored Arc Welding Process

The molten metal is obtained by creating an electric arc between a wire electrode (solid or tubular cored) and the base metal. Flux cored wires:

  • Improve fusion characteristics
  • Protect the molten metal against excessive oxidation
  • Offer a wider range of alloys that can be deposited

Depending on the protective gas used, the terms Metal Inert Gas (MIG) and Metal Active Gas (MAG) are often used. This procedure is easy to automate.

Self-shielded / Open Arc Process

This process is similar to MIG/MAG, but it has the advantage of not requiring the use of a protective gas.

It is usually used in the following cases:

  • Working conditions unsuitable for other welding procedures (outdoor welding, draughts etc.).
  • Exposure to the atmosphere has no negative effect on deposit performance.
  • This procedure is particularly used for hardfacing welding (excellent hardness and wear-resistance characteristics).

Submerged Arc Welding Process

The molten metal is generated by an electric arc between a wire and the part, beneath a “blanket” of powdered flux. The electric arc is not visible and the welding fumes are mostly absorbed by the flux layer. The procedure’s configuration and the use of powder flux restrict its application to flat welding positions on plates and rolls. The submerged arc welding procedure provides very high deposit rates.

For more information, please refer to our Fundamentals of Hardfacing by Arc Welding booklet.

Hardfacing technology continues to evolve and there is a wide range of equipment, power sources and consumables to choose from and a range of factors to consider.

Welding Equipment 

  • The availability and types of welding equipment, the size of the power source and whether the amperage range is adequate for hardfacing. Which weld position will be used and can the component be adjusted to allow welding in the flat weld position?

Welding Process 

  • Does your chosen equipment require SMAW stick welding, FCAW-S or FCAW-G semi-automatic welding or SAW welding?

Hardfacing Consumable choice

  • The availability, quality and performance of hardfacing materials are key considerations, as well as determining the SMAW rod or wire diameter that will be best for your application. If wire welding is used, also determine which wire process is best for your application such as FCAW-G gas-shielded wire, FCAW-S self-shielded wire or SAW submerged arc wire.

If you require assistance in selecting and ordering the most suitable welding consumable, please contact Welding Alloys

Operator skill level 

  • The skill of the operator should be taken into account when selecting the arc welding process to be used. Welding processes range from manual, semi-automated to fully automated and each requires different skill levels.

Physical environment 

  • Whether the welding location is indoors or outdoors, accessibility and any specific conditions need to be considered. To ensure appropriate health and safety standards, ventilation and fume extraction equipment should be adequate.

Welded component/part

  • The component shape, area size and features will need to be considered. Determine the welding position that will be used and can the component be adjusted to allow welding in the flat weld position?

Weld deposit requirements 

  • Additional factors that will impact preparation include previously welded components, painted finishes, oil or dirt.
  • Determine if there are specified machining requirements and what finish is desired.
  • Confirm the desired deposit thickness or final dimensions
  • Do you have specific production targets that determine a target deposition rate?
  • Will welding beads be left without grinding or machining?

The choice of welding material will depend upon three main factors:

  1. Base Metal

The base metal of the equipment to be hardfaced is an important consideration when selecting a hardfacing filler metal. Read more about which base metals can be hardfaced and key considerations.

  1. Type of Wear

A primary consideration in selecting the final hardfacing layers is the type of wear encountered in service. In many, if not most cases, the wear is the result of a combination of two or more of the phenomena described in this section. Read more about the different wear types.

  1. Arc Welding Method

The choice of the most suitable arc welding method depends on the size and number of components, available positioning equipment and frequency of hardfacing. Increasingly, semi-automatic and automatic welding procedures are used for hardfacing. Read more about the available processes.

Control of dilution is essential when surfacing as dilution affects the chemical composition of the deposit, hardness and quality. During welding, some of the base metal dissolves into the weld pool, diluting it. The deposition rate is an important consideration when evaluating the overall economics of hardfacing and it is important to select the best-suited welding procedure.

Dilution can be defined as the proportion of base material in the resulting weld metal and, for a single bead deposit, it is usually taken to be the ratio of the cross-sectional area of melted base material to the total cross-sectional area of the fusion zone, as demonstrated by DuPont and Marder (1996).

Control of dilution is essential when surfacing as it affects the chemical composition of the deposit, hardness and quality. During welding, some of the base metal dissolves into the weld pool, thereby diluting it.

Dilution is calculated as follows: % dilution = B/ (A + B) x 100

During surfacing operations, dilution should be limited to optimise deposit characteristics, whilst ensuring a good fusion with the substrate. How can dilution be controlled?

Select the right welding procedure, particularly heat input.

Welding sequence: an overlap of weld passes, of about 50%, provides good dilution control. Multi-pass surfacing results in lower dilution than single-pass surfacing.

For a better understanding of dilution control, please refer to the illustration on page 20 of our Fundamentals of Hardfacing by Arc Welding booklet.

The deposition rate expresses the amount of usable weld metal that will be deposited in one hour of actual arc-on time.

  • This is the rate at which the weld metal can be deposited by a given electrode or welding wire, typically expressed in lbs/hr or kg/hr.
  • It is based on continuous production, not allowing time for stops/starts/cleaning or inserting new electrodes.
  • Deposition rate is directly proportional to the welding current being used.
  • On a constant current machine, increasing the amperage increases the deposition rate. With a constant voltage machine, increasing the wire feed speed increases the deposition rate.

With each welding process, the typical deposit rate and the dilution percentage will vary. To find out more, please refer to the summary table on page 18 of our Fundamentals of Hardfacing by Arc Welding booklet.

The base metals most frequently hardfaced, include:

  • Stainless steels
  • Manganese steels
  • Carbon and alloy steels
  • Cast irons
  • Nickel based alloys
  • Copper-based alloys

Key base metal considerations

The base material of the equipment to be hardfaced is an important consideration when selecting a hardfacing filler metal.

Low-alloy and higher-carbon steels

A significant proportion of manufacturing equipment is produced using low-alloy and higher-carbon steels. Carbon and low-alloy steels with a carbon content of below 1% can be hardfaced without using a buffer layer, whilst high-carbon alloys may require a special buffer layer. Base materials that contain higher amounts of carbon and/or alloy content also tend to be more brittle and may require pre- and/or post-heating, or stress relief to prevent cracking, the same applies to thicker base materials.

Austenitic manganese steels are used in equipment for their abrasion and impact-resistant properties and are suitable for hardfacing. Austenitic manganese steels also don’t require preheating unless the temperature of the part is less than 10 °C, in order to remove moisture within the steel. However, austenitic manganese steels may become brittle during the welding process. During hardfacing, the base metal temperature should remain below 260 °C. Should this temperature be exceeded for prolonged periods, it will increase the metal’s brittleness and reduce the abrasion resistance of the steel. The time-temperature reaction described above is accelerated in austenitic manganese steels with high carbon and low manganese content.

For all materials, it is important to pre-clean the part prior to the hardfacing process. Wipe the part free of all contaminants and, if necessary, remove old hardfacing layers, as well as any cracks, by carbon arc or plasma gouging, or through grinding.

Determining the wear type can be challenging but is an important factor in determining which welding product and process should be used.

Wear can be described as a process of gradual removal of material from surfaces of solids subject to contact and sliding where damage to the contact surfaces occurs, as a result of wear. Typically, worn parts don’t fail from a single type of wear, but from a combination of modes, such as abrasion and impact.

The wear categories listed below, are not an exhaustive list, however, these represent the most common wear phenomena and their estimated percentage contribution to total wear:

A: Wear mechanisms 

The study of interacting surfaces in relative motion and its effect on friction and wear is referred to as “Tribology”. To achieve the best possible characterisation of wear mechanisms in metals, three elements have to be understood:

  1. The base material, or substrate, is characterised by its chemical composition and its production method (rolled, forged, cast), i.e. its mechanical properties. Component geometry also plays a fundamental role. This information allows us to understand its susceptibility to wear and the welding conditions required during repairing, rebuilding, and/or hardfacing.
  2. The external element (abrasive) which causes wear of the substrate is characterised by its dynamic and physical properties. Its hardness, shape, and texture determine the level of damage it will cause, depending on the pressure, speed, and angle of contact with the substrate.
  3. The environment in which the wear occurs is an essential factor in choosing the ideal welding solution. Operational conditions such as temperature, pressure and humidity should be characterised as much as possible.

B: Different types of wear

  1. Low and moderate stress abrasion/low impact

This type of wear is the result of particles rubbing/sliding on the substrate. As the pressure from these abrasives is very low, they don’t change size and don’t break up. Since the angle of attack of these particles is very low, the term “micromachining” is sometimes used.

The following terms are used in the field:

  • “Low-stress abrasion”, where two bodies are involved, the abrasive and the substrate.
  • “Moderate stress abrasion”, where three bodies are involved, two surfaces moving against each other with an abrasive between them.

The sharper and harder the abrasive, the higher the abrasion rate. As there is no impact, substrate ductility is not an issue. As long as the hardness of the base material is higher than that of the external element, wear or abrasion will be very low. Hardfaced parts, heat-treated steel plates and ceramic components are used to resist wear in these situations (e.g., 400 HB).

  1. High-stress abrasion/under-pressure abrasion 

Abrasion under high pressure occurs in equipment where the abrasive is compressed between two surfaces. The abrasive is then broken into many pieces. Due to the high pressure, the wear to the surface manifests itself in the form of chipping, possibly gouging, detachment of hard phases (carbides, borides etc.) or plastic deformation of the matrix.

The surfacing solution should therefore be an optimised balance between yield, ductility and hardness. A typical example where this type of wear can occur is in a coal crusher.

  1. Severe abrasion (gouging)/high impact

The term “gouging abrasion” is also used. This denotes a combination of low, moderate and high abrasion combined with impact. This type of wear results in large chips and scratches. It may be accompanied by plastic deformation. A solution to gouging requires the use of ductile materials that resist shock (force applied to a single point of contact) and impact (force applied to multiple points of contact). Manganese steels are often used in applications involving repeated shock, whereas titanium carbide alloys are ideal at resisting impacts. A typical example of a part subject to this type of wear is a crusher hammer.

  1. Adhesion/friction 

When two metal bodies rub against each other and material is transferred from one substrate to the other, this is known as “adhesive wear”. This type of wear occurs under conditions of high temperature, high pressure and friction. Contact between uneven surfaces, accompanied by relative movement, results in the microfusion of asperities that are immediately sheared off. Any unevenness may not be visible to the naked eye, as this wear mechanism occurs at the microscopic level. The rate of adhesive wear depends on several factors: the force acting between the two surfaces, relative speed, temperature of the working environment, surface condition, and surface friction coefficients. The type of material used also has an influence. The use of materials with identical crystallographic structures tends to increase the risk of adhesion. Some examples of parts where adhesion or friction occurs are continuous casting rollers, shears and rolling bearings.

  1. Erosion (similar to abrasion)

This type of wear occurs when solid particles or drops of liquid strike a surface at high speed. The rate of wear depends on the angle of attack of the external element and on the speed at which it is projected. The physical properties of the substrate determine the rate of wear by erosion. At low angles of attack (less than 30°), erosion occurs due to micromachining comparable to low or moderate-stress abrasion. The rate of wear depends directly on the substrate’s hardness. At a higher angle of attack (30 to 90°), the erosive particles will deform or even chip the substrate. It then becomes necessary to use materials that are capable of absorbing the energy released by the impact without deforming or cracking. A typical example of where this type of wear can occur is on sludge dewatering equipment.

  1. Cavitation 

Cavitation occurs when highly turbulent liquids are in contact with a solid surface. Cavities are formed in the liquid and implode, creating wear. The term “cavitation erosion” is also used. Repeated cavitation results in cyclic loads, wear, and base metal fatigue. Fatigue cracks then result in component failure. Under such stresses, materials offering high toughness show greater resistance to this type of wear as they dissipate the energy released by the implosion of the cavities. A typical example of equipment where cavitation can occur is hydroelectric turbine blades.

  1. Thermal fatigue 

This type of fatigue refers to wear generated by thermal cycle loads on the base metal. When a part is repeatedly heated and cooled, expansion and contraction occur. These processes lead to surface cracking known as “thermal fatigue cracking”. Typical examples of where this type of wear can occur include forging tools and hot rolling rollers.

  1. Fretting 

The types of wear mentioned previously result in a continuous loss of material. “Fretting” is caused when there is a recurrent rolling or sliding action between two components. Under such conditions, a sudden loss of material, in the form of pitting or chipping, will be observed. Parts rolling or sliding under high pressure are subjected to heavy mechanical loads. Cracks may appear and propagate under load, and may even cause spalling or gouging. Typical examples of parts where fretting occurs include gear teeth, rails, and roller presses.

  1. Corrosion 

Wear by corrosion is a vast and complex topic. Cladding solutions are often used to mitigate the effects of corrosion. Austenitic stainless steels (300 series) and nickel based alloys are preferred. In welding qualification tests, this type of surfacing must meet certain requirements, particularly crack-free 180° bending. Hardfacing does not require this type of test. For hardfacing applications, corrosion is not a major issue. Typical examples of parts subject to corrosion include paper screw conveyors or continuous casting rolls.

  1. Combined wear 

In some applications, the equipment may be subjected to several stresses at once. This results in a combination of different types of wear occurring. For example, corrosion and/or high temperatures may be combined with other types of wear, known as secondary factors.

The main categories can be broken down into two groups:

Group 1: Iron-based with less than 20% alloying

Low-alloyed steels

These filler metals contain a maximum of 0.2% carbon and hardness after welding does not exceed 250HV. They are produced for use in the rebuilding of parts prior to hardfacing. They provide a metallurgical transition between the soft base metal and the hardfacing. The deposited metal has good mechanical properties and resists compression well. Their composition, however, means that these filler metals respond poorly to wear.

Welding Alloys’ products include HARDFACE BUF-O

Medium alloyed steels

The most commonly used filler metals are those that deposit a martensitic-bainitic structure. These are low-cost filler metals with alloying additions to give wear resistance. As well as carbon, they may contain:

  • Carbide-forming elements, such as chromium and molybdenum.
  • Elements that refine the structure, such as manganese.

Weld deposit hardness may vary from 250 to 700HV. It is useful to note that deposits with hardness less than 300HV are easy to machine, whilst surfacing exceeding 50HRC is usually impossible to machine.

The harder the deposit, the greater its resistance to abrasion under low or moderate stresses. Such materials are frequently found in earthmoving and agricultural activities.

Welding Alloys’ products include ROBODUR K 250-G, ROBODUR K 600-G, HARDFACE T-G, HARDFACE L-G

Martensitic stainless steels

Martensitic stainless steels, with over 12% Cr, offer good resistance to wear from thermal fatigue and corrosion. These grades are ideal for applications where there is hot metal-to-metal wear. Martensitic stainless steels are widely used in steel making and forging for casting, rolling and forming operations. The addition of elements such as nitrogen and cobalt increases the resistance of these alloys to high temperatures and corrosion.

Nitrogen reduces the segregation of chromium carbides at the grain boundaries and provides improved resistance to pitting corrosion (PREN=Cr+3.3Mo+16N). Cobalt gives the deposit improved resistance to high temperatures and, therefore, to both thermal fatigue and high-temperature corrosion.

When surfacing a low or medium alloy base metal with martensitic stainless steels, it is advantageous to apply a special buffer layer over-alloyed in chromium (~ 17%) to guarantee metallurgical soundness and to avoid cracking in service.

Welding Alloys’ products include CHROMECORE 430-G, CHROMECORE 434N-S, CHROMECORE 414DN-S.

Tool steels

Tool steels are used for high-temperature forming in repeated cycles. They must withstand a temperature range of 500-600 °C without softening. Elements such as molybdenum, vanadium, titanium, and tungsten are added to ensure this.

Forging tools, such as knives, closed dies, hammers and mandrels, are made from these steels, or surfaced with them. They exhibit good resistance to the combined effects of thermal fatigue, plastic deformation and fretting. Further down, we see that other, more highly alloyed solutions are available, based on cobalt and nickel alloys (STELLOY).

Welding Alloys’ products include ROBOTOOL 46-G, HARDFACE WLC-G, HARDFACE AR-G

Austenitic manganese steels

Steels with 12 to 14% Mn have a soft austenitic structure (hardness ~ 200HV), with the ability to surface work harden when the part is subjected to high impact. Hardnesses of around 500HV can be achieved.

When cracks form in service, the lifetime of the surfacing is not necessarily compromised. In fact, this type of deposit shows high resistance to crack propagation.

14% Mn grades contain about 1% carbon. This results in embrittlement if the cooling rate is too slow, due to precipitation of carbides at the grain boundaries. Welded components are often solution-treated at 1000 °C to give a purely austenitic structure.

Unfortunately, solution annealing is not always possible. Excessive interpass temperatures and overly slow cooling must be avoided. Cored wires are ideally suited to achieve this, combining metallurgical soundness with productivity.

When surfacing with 14% Mn steel on a non or low-alloy substrate, the use of an austenitic stainless buffer layer (307 or 312) is highly advisable. This avoids any risk of creating a martensitic heat-affected zone. Without this intermediate layer, a brittle zone would form leading, under high impact, to spalling of the surfacing.

Welding Alloys’ products include HARDFACE NM14-O

Group 2: Iron-based with over 20% alloying

Austenitic Chromium-Manganese steels

As with 14% Mn steels, austenitic chromium-manganese deposits are work hardening. However, because of their high alloy content, these products can be applied directly to non or low-alloy substrates; with no risk of forming a martensitic structure at the interface. This type of alloy is often used in a buffer layer before depositing a 14% Mn alloy.

It should also be noted that the presence of chromium means flame-cutting cannot be used on this alloy.

Welding Alloys’ products include HARDFACE 19 9 6-G,  HARDFACE AP-G

Tool steels

Thanks to alloying with cobalt, chromium and molybdenum, HARDFACE DCO filler metal is a superalloy offering performance very similar to cobalt based alloys. It is the perfect solution for high-temperature stresses (500-600 °C).

HARDFACE DCO-G

Chromium cast irons

These deposits are composed of hard phases in a matrix whose structure depends on the composition of the filler metal: martensitic, bainitic or austenitic. They are mainly used to resist wear by abrasion. In the case of low or moderate abrasion, deposits with an austenitic matrix are normally used. However, a martensitic matrix is a better solution for high abrasion under pressure.

The size of the hard phases (carbides, borides) and their distribution in the matrix have a direct influence on the deposit’s resistance to abrasion. For example, for the same hardness, a surfacing with bigger and closely spaced carbides will tend to give better results than one with smaller particles.

For applications involving severe abrasion under impact, a deposit containing titanium carbides provides the perfect solution. The fine regular distribution of hard phases provides excellent resistance to combined stresses.

Welding Alloys’ products include HARDFACE HC-O, HARDFACE TIC-O, HARDFACE BN-O

Group 3: Non-ferrous alloys, Cobalt or Nickel based

Cobalt based alloys

Cobalt based filler metals are mainly alloyed with carbon, chromium and tungsten, also sometimes with nickel and molybdenum. These alloys are especially suited to applications involving high temperatures (up to 800 °C), retaining high hardnesses over time. Chromium provides a protective layer and, therefore, plays an anti-oxidation role. As in iron-based alloy, chromium, tungsten and molybdenum combine with carbon to create hard carbides.

The lower the carbon content, the better the resistance to cracking. Grade 21 STELLOY is largely insensitive to cracking and offers good impact characteristics. STELLOY 6, being harder, offers improved resistance to abrasion at both high and low temperatures, but is less crack-resistant.

These alloys are ideal for wear caused by metal-to-metal friction at high temperatures and in the presence of abrasives. Their low coefficient of friction, and their self-polishing tendency, make them highly scratch-resistant and help maintain an excellent surface quality.

To avoid cracking, any welding operation with this type of filler metal requires preheating. In most cases, grade 6 STELLOY filler metals are welded using a preheating temperature of around 350 °C, followed by slow cooling under thermal insulation.

Welding Alloys’ products include STELLOY 21-G, STELLOY 6-G

Nickel based alloys

The nickel based alloys most commonly used for hardfacing contain chromium, boron and carbon. They contain multiple hard phases (chromium carbides and borides) in a nickel-chromium matrix. This structure provides them with good resistance to oxidation (up to ~ 950 °C) and enables them to maintain their hardness up to 500 °C.

Resistance to low or moderate abrasion is good irrespective of the process temperature and improves in proportion to carbon content. However, this type of alloy offers poor resistance to heavy abrasion under pressure. In addition, severe abrasion combined with heavy impact will degrade the surfacing.

These alloys are primarily used for applications involving abrasion and corrosion at high temperatures. Examples of parts where nickel based alloys provide an ideal solution include valves, valve seats or spiral conveyor screws.

Other nickel based alloys exist which are particularly resistant to high-temperature stresses and thermal shock. The addition of chromium, molybdenum, tungsten and cobalt provides them with the ideal properties to use as a solution on open forge hammers.

Welding Alloys’ products include: STELLOY C-G, STELLOY Ni520-G

Group 4: Tungsten carbide

Tungsten carbide provides extreme resistance to abrasive wear. Surfaced layers containing a dispersion of tungsten carbide are produced using a cored wire filled with up to 60% of tungsten carbide grains, 100 – 250 microns in size. These pass directly through the welding arc without melting, in contrast to the carbides formed by precipitation in iron and cobalt based hardfacing alloys. The wire sheath melts to form the matrix of the deposit. Mild steel, stainless steel and nickel based matrices are available.

To ensure a good distribution of grains and good abrasion resistance, it is essential to use a low heat input. Welding parameters that are too high would result in the carbides dropping to the bottom of the weld pool.

Welding Alloys’ products include: HARDFACE NICARBW, HARDFACE STAINCARBW, HARDFACE STEELCARBW

This will depend on the type of hardfacing alloys used and while many assume that all cracks are bad news, transverse cracks, also called check-cracks are common in some hardfacing applications. For example, many chromium carbide alloys do check-crack when cooled to moderate temperatures; which is normal and to be expected. However, austenitic and martensitic alloy families should not crack when applied with the correct welding procedures.

Check-cracking, also known as checking transverse cracks, occurs in the metal carbide families in a direction perpendicular to the weld. The primary reason for this is that the weld metal significantly overmatches the strength of the base metal and when the weld metal cools it shrinks longitudinally

For projects where the parent metal is hard or brittle, consider a buffer layer of a softer, tougher weld metal from the austenitic alloy family.

This is a process by which a coating of chromium carbide is applied to the surface of components, to form a metallic bond with the substrate, which is resistant to impact and abrasion. Chromium carbide hardfacing is done with iron-based alloys that contain high amounts of chromium (>18%) and carbon (>3%) and form hard carbides (chromium carbides) that resist abrasion. Often, there is check-cracking to relieve stress.

Abrasion resistance increases proportionally as the amount of carbon and chromium increases, although the carbon content has a greater influence. Elements can be added to form other carbides or borides to increase wear resistance for high-temperature applications and the alloys are limited to two or three layers.

The low friction coefficient also makes chromium carbide hardfacing ideal for applications requiring a material with good slip.

Chromium carbide hardfacing alloys are available in a range of hardness levels, which are dictated by the alloying of chrome carbide in the weld material, used in the hardfacing process. Typical hardness values range from 40HRC to 65HRC and hardfacing material can be applied by different processes including metal spraying or weld depositing.

Chrome carbide hardfacing deposits offer a cost-effective solution to solving wear problems.

Complex carbides are generally associated with chromium carbide deposits that have the addition of niobium, molybdenum, tungsten, or vanadium. The addition of these elements and carbon results in the formation of their own carbides and/or a combination with the present chromium carbides to increase the alloy’s overall abrasion resistance. They can have all of these elements or just one or two. They are used for severe abrasion or high-heat applications.

As a general principle, all the parts to be hardfaced should at least be brought to room temperature. In general, the industries that require hardfacing mainly use non-alloy, low alloy, high alloy and manganese steels, as base metals.

When the material being welded has a high carbon or alloy content, preheating will be required to prevent cracking. Cracking can result because welding introduces a sudden high temperature to the parts, causing a thermal shock. In such cases, it is necessary to select higher preheat and interpass temperatures based on the base metal chemistry and the hardfacing products used.

However, where an austenitic 11-14% manganese steel is used, preheating must be avoided, as temperatures above 150 °C during welding have a significant risk of embrittlement.

The preheating temperatures required for welding base metals vary according to their carbon equivalent (Ceq), which directly influences their weldability:

  • Low Carbon Equivalent (Ceq < 0.35):
      • Weldability: Good
      • Preheating: Light preheating is required, suggesting minimal preparation due to the metal’s good weldability.
      • Postheating: Not required, indicating no additional heat treatment is necessary after welding.
  • Moderate Carbon Equivalent (0.35 < Ceq < 0.6):
      • Weldability: Acceptable
      • Preheating: Moderate preheating between 150 to 250°C is necessary to avoid potential issues during welding.
      • Postheating: Preferable, implying that post-weld heat treatment could benefit the final weld quality and structural integrity.
  • High Carbon Equivalent (Ceq > 0.6):
      • Weldability: Precautions are required due to the increased risk of welding defects.
      • Preheating: Intensive preheating above 250°C is needed to ensure safe and effective welding.
      • Postheating: Required, emphasising the necessity of post-weld heat treatment to mitigate stresses and potential cracking

As hardfaced layers are not ductile, shrinkage cracks frequently appear. To minimise cracking, the nature of the filler metal also needs to be considered. In certain cases, even if the C-Mn base metal has a Ceq < 0.35, the use of a cobalt based alloy (STELLOY 6) requires a minimum preheat of 300-350 °C. In addition, to avoid cracking in the deposited metal, slow cooling is required (typically less than 50 °C per hour). Always consult the manufacturers’ guidelines to prevent cracking and spalling. [/av_toggle] [av_toggle title='When is a cobalt based or nickel based hardfacing alloy used?' tags='' custom_id='when-is-a-cobalt-based-or-nickel-based-hardfacing-alloy-used' av_uid='av-lnx1ylr0-1-1-1-4' sc_version='1.0'] Cobalt and nickel based alloys are used in a range of industries such as chemicals, petrochemicals, power generation production, pulp and paper, fluid handling and transportation, agriculture and food production. Cobalt based alloys

Welding Alloys manufactures a range of cobalt based cored wires designed to perform in harsh environments, primarily those involving mechanical and chemical degradation and contributing wear factors such as heat.

Available welding processes include SAW, SMAW, and GMAW. Cobalt based alloys offer anti-galling, high-temperature hardness and resistance to cavitation erosion. They also offer good bonding with stainless steel and weldable alloys.

Cobalt alloys can contain various types of carbides and perform well in severely-abrasive, high-temperature environments. For certain applications, they also provide good corrosion resistance. Deposit hardness ranges from 25HRC to 55HRC and work hardening alloys are also available.

Nickel based alloys

Welding Alloys manufactures several nickel based wires that are specifically developed for low and medium-carbon steels, cast irons and stainless steels. Our alloys offer abrasion and impact resistance and maintain excellent levels of resistance at high temperatures, with resistance to corrosion, galling and pitting. Nickel based alloys can contain chromium borides that are resistant to abrasion.

For assistance with selecting the most suitable hardfacing product for your wear resistance requirements, please contact welding alloys.

Hardfacing may involve depositing one or several layers of weld metal. While certain types are designed to be applied in one layer only, others can be applied without limit.

For example, wire having high boron content achieves extremely high hardness from the first layer but can’t be applied in several layers due to risk of spalling (e.g. HARDFACE BN-O, HARDFACE BNC-O). This is due to the brittle nature of the carbides, which can lead to check-cracking when multiple layers are applied. If stress is allowed to build, separation or spalling can occur between the parent metal or buffer and the hardfacing deposit.

Depending on wear performance requirements, service life expectations and application, many hardfacing products can typically be applied in several layers unless otherwise specified by the manufacturer. Low and medium alloyed steels such as ROBODUR K 600-G, HARDFACE L-O  and chromium carbide deposits such as HARDFACE HC-O, HARDFACE CNV-O can generally be applied in multiple layers.

Yes, hardfacing of cast iron is regularly undertaken to add metal material to a surface exposed to wear. Hardfacing is commonly used on rolling mill rollers, tracks, feeders, crushing jaws, ploughshares, excavator shovels, buckets, digger teeth and surfaces exposed to abrasion.

For example, when a digger bucket is exposed to surfaces which cause wear to the metal of the bucket’s construction, metal can be deposited onto the teeth and at the bottom of the bucket through welding.

Nickel and nickel-iron products are not affected by the carbon content of the parent metal and remain ductile, making them suitable for rebuilding cast iron. Due to the limited thermal expansion of nickel, it is also less prone to cracking. It is recommended to use the minimum recommended current settings when welding cast iron with nickel to minimise heat stress.

For assistance with selecting the most suitable hardfacing product for your wear resistance requirements, please contact welding alloys.

When it comes to hardfacing products, not all products are equal, and quality and performance depend on a range of factors such as:

  • the alloy types used
  • the alloy content percentage
  • how the products are produced; and
  • quality management processes

Welding Alloys only produces quality hardfacing products that may be more expensive than cheaper, unbranded alternatives. However, our data and results have proven that the benefits of the quality and performance of our products outweigh the additional expense.

The past two decades have seen a surge in low-cost/low-quality hardfacing products containing a limited range of low-cost alloys and in very small percentages, produced in poorly controlled environments.

This results in limited wear life of hardfaced parts, leaving users disappointed in the wear performance. Sadly, this is giving the efficacy of hardfacing processes a bad reputation and has raised questions about the worth or value of hardfacing.

To illustrate some of the differences more clearly, the better-quality hardfacing products typically contain 40-60% alloys offering very advanced wear protection.  However, poor-quality products can contain as little as 4% alloys.

For assistance with selecting the most suitable hardfacing product for your wear resistance requirements, please contact welding alloys.

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