Dilution rate refers to the percentage of base metal that melts and mixes into the deposited hardfacing layer during the welding process.

During PTA hardfacing, a small portion of the substrate material is intentionally melted to create a metallurgical bond between the overlay and the base metal. The extent of this mixing determines the dilution rate.

Simply put:

•  High dilution = More base metal mixed into the overlay.
•  Low dilution = Overlay composition remains closer to the original alloy powder chemistry.

Since hardfacing alloys are carefully engineered for specific wear and corrosion resistance properties, maintaining a low dilution rate is often crucial for achieving the desired performance.

 

How Is Dilution Rate Calculated?

Dilution rate is commonly calculated using cross-sectional metallographic analysis.

The formula is:

Dilution Rate (%) = A ÷ (A + B) × 100

Where:

•  A = Area of melted base metal
•  B = Area of deposited hardfacing material

After preparing a metallographic cross-section, image analysis software is typically used to measure these areas accurately.

 

Typical Dilution Rates of Different Hardfacing Processes

Different welding and cladding technologies produce significantly different dilution levels.

Process	Typical Dilution Rate
SMAW Hardfacing	15% – 35%
MIG/GMAW Hardfacing	10% – 25%
TIG Hardfacing	5% – 15%
PTA Hardfacing	3% – 10%
Laser Cladding	1% – 5%

PTA hardfacing is widely recognized for offering one of the lowest dilution rates among arc welding processes while maintaining excellent deposition efficiency.

 

Why Does Dilution Rate Matter?

1. It Directly Affects Chemical Composition

Hardfacing powders are designed with precise chemical compositions to achieve specific properties.

Examples include:Stellite 6、Stellite 12、Inconel 625、Ni60、Tungsten Carbide Reinforced Alloys

These materials contain carefully balanced elements such as:Chromium (Cr)、Nickel (Ni)、

Cobalt (Co)、Tungsten (W)、Molybdenum (Mo)、Carbides

Excessive dilution introduces unwanted elements from the substrate, especially iron (Fe), which alters the overlay chemistry.

As a result:

•  Hardness decreases
•  Wear resistance declines
•  Corrosion resistance deteriorates

 

2. It Influences Hardness

Overlay hardness is highly dependent on maintaining the intended alloy composition.

For example, a Stellite 6 overlay may achieve:40–45 HRC

However, if dilution becomes excessive, hardness can drop significantly due to increased iron content from the substrate.

Lower hardness often translates directly into shorter service life under abrasive conditions.

 

3. It Reduces Wear Resistance

Many hardfacing alloys rely on hard phases such as:

•  Tungsten Carbides (WC)
•  Chromium Carbides
•  Borides

These hard phases provide excellent resistance to:

•  Abrasive wear
•  Erosive wear
•  Metal-to-metal wear

When dilution increases, the concentration of these strengthening phases decreases, reducing the overall wear resistance of the coating.

 

4. It Impacts Corrosion Resistance

For critical applications such as:Valve sealing surfaces、Chemical processing equipment、

Offshore components、Oil and gas systems

Nickel-based and cobalt-based alloys are commonly used to provide corrosion protection.

Excessive dilution increases the iron content within the overlay, weakening its resistance to:

•  Chloride attack
•  Acidic environments
•  Seawater exposure
•  High-temperature oxidation

This can dramatically reduce component lifespan in aggressive operating environments.

 

Why PTA Hardfacing Achieves Low Dilution

Precise Heat Input Control

PTA welding uses a highly concentrated plasma arc with independently fed alloy powder.

Unlike conventional wire-fed welding processes, the heat input can be precisely controlled, minimizing substrate melting.

This results in:

•  Reduced penetration depth
•  Smaller heat-affected zones
•  Lower dilution rates

 

Independent Powder Feeding System

The powder is injected directly into the plasma arc and melted before entering the weld pool.

Because the deposited material is not dependent on melting a filler wire, the process offers superior control over overlay composition.

Benefits include:

•  Consistent chemistry
•  Uniform deposits
•  Improved process stability

 

High Energy Density

The plasma arc can reach temperatures exceeding 10,000°C.

This concentrated energy allows efficient powder melting while minimizing unnecessary melting of the substrate.

As a result, PTA hardfacing delivers:

•  Excellent metallurgical bonding
•  Minimal dilution
•  High-quality overlays

 

What Is the Ideal Dilution Rate for PTA Hardfacing?

The optimal dilution level depends on the application.

Wear-Resistant Hardfacing

Recommended dilution:3% – 8%

Applications:Crusher components、Mining equipment、Screw conveyors、Earthmoving parts

 

Corrosion-Resistant Overlays

Recommended dilution:2% – 5%

Applications:Valves、Pumps、Chemical processing equipment、Offshore components

 

Dimensional Restoration and Repair

Recommended dilution:5% – 10%

Applications:Shaft rebuilding、Roll repair、Mold refurbishment

 

How to Reduce Dilution in PTA Hardfacing

Optimize Welding Current

Excessive current increases penetration depth and substrate melting.

Maintaining the proper current level is one of the most effective ways to control dilution.

 

Maintain Stable Travel Speed

Inconsistent travel speed can cause variations in weld pool size and penetration.

Automated systems help maintain uniform dilution across the entire component.

 

Use Advanced PTA Equipment