Bronze

An in depth guide

Bronze is an alloy comprising primarily of copper and about 12% tin. Other metals such as zinc, manganese, nickel, or aluminium are also commonly added to produce a range of alloys that are much harder than unalloyed copper. Other non-metals, or metalloids as they are also known, such as arsenic, phosphorus or silicon may also be added to create alloys with added benefits such as increased stiffness, ductility, or improved machinability. Bronze is non-magnetic.

The earliest known bronze artifacts can be dated back to the 5th millennium BCE, when during the 'Bronze Age', it was the hardest known metal. Historically, bronze was made by adding arsenic to copper to create 'arsenic bronze'. By 4500 BCE, tin was the main alloying element in bronze which led to a stronger alloy that was easier to cast without emitting toxic fumes as was the case with arsenic use. Tin remains the primary alloying metal in modern bronze composition.

Bronze has good corrosion resistance, good strength, good resistance to shock loading and very good machining properties which make it particularly suitable for bearings, bushings, wear plates, pump valves, gears, worm wheels and marine propellers.

Phosphor bronze is widely used in the sand casting method of bell making, the bells are carefully machined and polished to produce the exact note required. Phosphor bronze is also often used as a substitute for brass when making musical instruments as it has greater resilience, leading to a longer sustain and a broader tonal spectrum. Bronze is widely used in architecture and sculpture due to its attractive and distinctive colour, corrosion resistance and strength.

Over time, bronze can lose its lustre and become tarnished; this is noticeable as a slight green patina on the surface of the metal. Patina can be removed by using soap and warm water applied gently with a cloth to rub off the dirt and grime, then gently dry the metal and buff to restore to its former condition. Proprietary bronze cleaners are available commercially, however, these can be quite abrasive and may damage the surface if not used with care. A gentler method for deeper cleaning of bronze is to make a paste of equal parts of lemon juice and baking soda and rub the mixture over the surface to be cleaned with your fingers, a soft toothbrush or a soft, lint free cloth, leave the paste to work for several minutes before rinsing with clean water and drying well; repeat process if necessary for stubborn marks. As with all cleaning of metals, it is best to start with the least aggressive process and work through the methods as required.

Bronze is fully recyclable without losing any of its physical and mechanical properties.

We stock and supply PB1 grade and SAE 660 grade bronze in round bar and tube profiles. The tube and round bar are both supplied oversize to accommodate machining back to the listed dimensions.

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  PB1 Grade

PB1 grade bronze conforms to BS1400 standard. This grade of bronze also known as 'Phosphor bronze' due to the addition of phosphor used in production to improve its wear resistance and stiffness and is widely used for bearings; particularly those used for heavy loads at high speeds due to its excellent fatigue resistance. PB1 also has excellent formability, superior spring qualities and solderability making it perfect for the manufacture of worm gears, springs, valve bodies, gears, and bushes.

PB1 or phosphor bronze has a relatively high tin content which improves its strength and corrosion resistance, especially to seawater and chloride environments, making it the perfect choice for use in the aerospace, marine and chemical industries.

Chemical Composition of PB1 (% weight)

Sn P Zn Ni Pb Al Cu
10.2 – 11.5 0.6 – 1.2 0.05 max 0.10 max 0.25 max 0.005 max Balance

Physical and mechanical properties of PB1 phosphor bronze

Property Value Unit
Density 8800 Kg/m3
Melting point 1000 °C
Modulus of elasticity 100 GPa
Electrical resistance 0.17 µΩm
Thermal conductivity 52 W/m°k
Thermal expansion 18 µm/m-k
Yield strength 190 N/mm²
Proof stress 170 N/mm²
Shear modulus 41 GPa
Hardness Vickers 104 Vickers - HV
Elongation 8 %
Tensile strength 340 N/mm²

Machinability

PB1 Phosphor bronze has very good machining qualities and is free cutting but very tough; the use of tipped tools is recommended. Bronze gets hot when drilling and turning, therefore, the use of a coolant is recommended. Machining externally can be performed at fairly fast speeds, slightly slower is recommended for drilling and boring, and very slow for reaming as the bronze may 'grab' causing the reamer to become embedded.

Formability

PB1 can be cold formed by using 'die' forming techniques. This process will increase the hardness, yield strength and tensile strength of the metal.

Ductility

PB1 has good ductility, making it suitable for engineering projects that need to accommodate stress concentrations.

Cutting

Phosphor bronze can be successfully cut using machines/ saw cutting and flame cutting techniques. The use of coolant is recommended to prolong the life of tools.

Welding

PB1 is not suitable for welding processes but can be successfully brazed and soldered.

Annealing

PB1 can be annealed/ stress relieved by heating to 500°C-700°C for one hour per inch thickness and left to air cool. Rapid cooling or quenching techniques are not recommended as this can result in fractures and crumbling.

Hardening

This bronze grade is not suitable for heat treatments but can be work hardened.

Tempering

Not suitable for this grade of bronze.

Corrosion resistance

PB1 has excellent corrosion resistance in harsh marine environments and chloride environments. Bronze will not rust.

Casting

This grade of Bronze can be used for casting and injection moulding processes, the casting characteristics are stated in the following table.

Casting Characteristics of PB1

Casting attribute level
Casting yield High
Drossing Low
Effect on section size Medium
Fluidity Medium
Gassing Medium
Pattern makers shrinkage (inches per inch) 7/32
Shrinkage in Solidification Low

Forging

Phosphor Bronze is not suitable for forging; even at 'dull red' temperatures it is prone to crumbling. PB1 can be cold worked and annealed / stress relieved as required.

Brazing

PB1 is classified as having 'good' brazing qualities. Brazing temperatures are within the hot-short range for this grade of bronze, so care must be taken to not place too much stress on the part during the brazing process.

Soldering

Excellent results are achieved with soldering PB1.

Process Rating
Workability cold Fair
Machinability Good
Weldability- Gas Not suitable
Weldability- Arc Not suitable
Weldability- resistance Not suitable
Brazability Good
Solderability Excellent

We stock and supply PB1 in round rod and round tube profiles. The products are supplied oversized to allow for machining down to the stated stock size.

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 SAE 660 (C93200)

SAE 660 grade bronze is also referred to as bearing bronze. This grade is one of the most commonly used bronzes for general engineering components. It is malleable and has strong resistance to abrasion and corrosion. This grade is supplied as a continuously cast, semi-finished product that is free from shrinkage, cavities, pores, blowholes, and segregations, it has a smooth surface with close dimensional tolerances with no hard casting skin. This grade has lower tensile strength than phosphor bronze.

This Grade of bronze is not suitable for heat treating.

The properties of SAE 660 make it particularly suitable for the manufacture of; Marine pumps, valves, bearings, bushings, trunnion bearings, thrust washers, forging press toggle bearings, thrust washers, and machine parts.

Chemical Composition of SAE 660 (% weight)

Cr Pb Zn Ni P Sn Cu
0.0 - 0.10 6.00 - 7.50 4.00 - 6.00 0.0 - 0.10 0.0 - 0.15 6.30 - 7.50 Balance

Physical and mechanical properties of SAE 660 bronze

Property Value Unit
Density 8910 Kg/m3
Melting point 977 °C
Modulus of elasticity 99.97 GPa
Electrical resistance 14.2 µΩm
Thermal conductivity 58.2 W/m°k
Thermal expansion 18 µm/m-k
Yield strength 140 N/mm²
Proof stress 138 N/mm²
Shear modulus 38 GPa
Hardness 83 Vickers - HV
Elongation 10 %
Tensile strength 240 N/mm²

Machinability

Bearing bronze has excellent machinability. The use of coolant/lubricant is recommended while working as the metal will get very hot. It is also recommended to use tipped machine bits and tools to get the best results and finish for your project. Bronze can be prone to 'grabbing', so slower speeds should be used during some operations, particularly reaming. Bearing bronze can be machined using the following processes; CNC machining, boring, reaming, thread cutting, rotary cutting and grinding.

Formability

SAE 660 can be cold formed; this process will increase the hardness, yield strength, and tensile strength of the metal.

Ductility

Bearing bronze has fair ductility.

Cutting

SAE 660 can be successfully cut using machines/ saw cutting and is suitable for suitable for flame cutting. The use of coolant is recommended during mechanical cutting to prolong tool life.

Welding

SAE 660 grade is not suitable for welding processes but can be successfully brazed and soldered.

Annealing

Annealing / stress relieving can be achieved by gently heating the metal to 260°C for one hour per inch of wall thickness and slow cool. Bronze should not be fast cooled or quenched as it is susceptible to crumbling or fracturing if not slowly air cooled.

Caution should be taken when heating SAE 660 as it has a higher lead content than other bronzes; the use of fume extraction equipment and suitable PPE is strongly recommended.

Hardening

Bearing bronze is not suitable for heat treatments but it can be successfully work hardened, however, care should be taken to not over work the piece as it may become brittle.

Tempering

Bearing Bronze is not suitable for tempering.

Corrosion resistance

Very good corrosion resistance, even in chloride and sea water environments. Bronze will not rust, surface discolouration, known as patina, can be easily removed by simple cleaning techniques to maintain a pleasing finish.

Forging

Bearing Bronze is not suitable for forging as it is prone to crumble, even at 'dull red' temperatures. It is more successful to cold work and re-anneal/ stress relieve as needed.

Brazing

The results of brazing are good with this grade, however, as brazing temperatures are performed within the hot-short range, excess strain must be avoided during the brazing process.

Soldering

SAE 660 responds with excellent results to the soldering process.

Process Rating
Workability SAE 660 is recommended for cold working
Machinability very good
Weldability - Gas Not recommended
Weldability - Arc Not recommended
Weldability - Resistance Not recommended
Brazability Good
Solderability Excellent

We stock and supply SAE free machining bronze in round rod and round tube profiles. The products are supplied oversized to allow for machining down to the stated stock size.

Brazing/ soldering consumables

Brazing/ soldering PB1 & SAE 660 bronze to Consumables most commonly used.
Bronze Sifphosphor bronze No. 8 & Sifsilcopper flux/ silver solder No 39 & silver solder flux
Corten Sifbronze 1 &sifbronze flux/silver solder No 39 & silver solder flux
Galvanised mild steel Sifbronze 1 & sifbronze flux/ silver solder No 39 & silver solder flux
Brass Sifphosphor bronze No. 8 & Sifsilcopper flux/silver solder No 39 & silver solder flux
Copper Sifphosphor bronze No. 8 & Sifsilcopper flux
Stainless steel silver solder No 39 & silver solder flux
Mild steel Sifbronze 1 & sifbronze flux/ silver solder No 39 & silver solder flux
Cast iron C2 silicon bronze brazing rod
Aluminium Sifmig 328 / Sifphosphor bronze no 8 & alu braze No 12 flux
Weld/joining method suitability
Gas Not suitable
Arc Not suitable
Resistance Not suitable
Brazing Very good
Soldering excellent
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