Chrome Bronze

What Is Chrome Bronze?

Chrome bronze is a copper alloy containing 0.4% to 1.1% Cr. Chrome bronze can be strengthened by quenching-aging or quenching-cold deformation-aging treatment. At the eutectic temperature of 1072℃, the maximum solubility of chromium in copper is 0.65%. With the decrease of temperature, the solid solubility drops sharply, and the aging treatment after solid solution precipitates Cr particle phase.

The addition of chromium, on the one hand, significantly increases the recrystallization temperature and thermal strength of the alloy; on the other hand, the conductivity of copper is slightly reduced. The conductivity of the solution-treated chromium bronze rod is 45% IACS, which rises to 80% IACS after aging treatment. The softening temperature of aged chromium bronze is 400°C, which is twice that of cold-worked copper.

This alloy can be used in the as-cast state and the deformed state. When Al and Mg are added as alloying elements of chromium bronze, a thin and dense oxide film that is firmly bonded to the base metal can be formed on the surface of the Cu-Cr alloy, which improves the high-temperature oxidation resistance and heat resistance of the alloy. The content of Mg and Mg in the alloy is usually not more than 0.3%.

The Chinese name is chrome bronze. It has good electrical and thermal conductivity. It can be used for forming, welding, cutting and grinding performance. Motor commutator, slip ring, high temperature switch

The chemical composition of chromium bronze, see the table below
Table 1 Chemical composition of chromium bronze (mass fraction, not more than) (%)

Grade	Cr	Cu	Impurity
			Mg	Al	Zr	Si	Pb	P	Fe	Ni	Total impurities
QCr0.5	0.4~l.1	margin	—	—	—	—	—	—	0.1	0.05	0.5
QCr0.5-0.2-0.1	0.4~l.0	margin	0.1~0.25	0.1-0.25	—	—	—	—	—	—	0.5
QCr0.6-0.4-0.05	0.4~0.8	margin	0.04~0.08	—	0.3~0.6	0.05	—	0.01	0.05	—	0.5
C18150	0.5~1.5	margin	—	—	0.05~0.25	—	—	—	—	—	—
Cl8200	0.6~1.2	margin	—	—	—	0.1	0.05	—	0.1	—	0.5
C18400	0.40~1.2	margin	—	—	—	0.1	—	0.05	0.15	—	0.5
C18500	0.40~1.0	margin	—	—	—	—	0.015	0.04	—	—	—

Physical properties

(1) Thermal performance

Liquidus temperature: 1075~1080℃;
Solidus temperature: 1070~1073℃;
Specific heat capacity: 385J/(kg.℃) at 20℃;
Thermal conductivity: TBOO condition, 171W/(m.℃) at 20℃; THO4 condition, 324W/(m.℃) at 20℃.
Linear expansion coefficient: QCr0.5 alloy 20~100℃ average linear expansion coefficient α1=17.64*10-6/℃.

(2) Quality characteristics

The density of QCr0.5 alloy is 8890kg/m3 at 20℃.

(3) Electrical performance

Electrical conductivity: TBOO state (solid solution state), 40% IACS at 20°C; THO4 state (solid solution-cold working-aging state), 80% IACS at 20°C.
Resistivity: THO4 state (solid solution-cold working-aging state), 0.0216uΩ.cm at 20°C.
Temperature coefficient of resistance: 0.0033/℃ for 20~100℃.

Chemical properties

Corrosion resistance: the corrosion resistance is similar to pure copper, and the electrical corrosion resistance is better than pure copper.
Oxidation resistance: the alloy has good high temperature oxidation resistance, and the high temperature oxidation performance of chrome bronze is shown in the following table

High temperature oxidation performance of chrome bronze

alloy	The average mass increase value of the sample at different temperatures/mg. (cm.h)
	500℃	600℃	700℃	800℃
Pure copper	0.58	1.4	2.47	4.5
QCr0.5	0.5	0.7	2.04	4
QCr0.5-0.2-0.1	0.23	0.57	0.8	1.21

 

Mechanical properties

Table C18200, C18400 and C18500 chromium bronze alloy typical mechanical properties [1]

status	σb/MPa	σs/MPa	δ/%	HRB
TB00 (Solid solution treatment)	235	130	40	16
TF00 (Solid Solution-Aging)	350	250	22	59
TD04 (Solid solution-cold working to full hard)	365	350	6	66
TH04 (Aging after TD04)	460	405	14	79
TF00 (Solid Solution-Aging)	400	290	25	70
TF00 (Solid Solution-Aging)	385	275	30	68
TH08 (Solid solution-cold working-aging to elasticity)	510	505	5	-
TB00 (Solid solution treatment)	595	530	14	-
TF00 (Solid Solution-Aging)	310	97	40	-
TD04 (Solid solution-cold working to full hard)	485	380	21	70
TH04 (Aging after TD04)	395	385	11	65
TH03 (Solid solution-cold working to 3/4 hard-aging), cold working 6%	530	450	16	82
TF00 (Solid Solution-Aging)	530	460	19	83
TF00 (Solid Solution-Aging)	495	450	18	80
TF00 (Solid Solution-Aging)	485	450	18	75
TF00 (Solid Solution-Aging)	450	380	18	70
060 (softening annealing)	380	295	25	68
TD04 (Solid solution-cold working to full hard)	375	105	50	59
TH04 (aging after TD04), 28% cold working	405	395	21	67
	475	435	26	84

①Extension 0.5% under load;

②Aging at 500℃ for 3h;

③Aging at 450℃ for 3h.

 

Process performance

(1) Melting and casting process

Chrome bronze alloys are usually smelted in an intermediate frequency induction furnace. The molten pool is covered with a solvent composed of 60% to 70% borax and 30% to 40% glass. It can also be covered with calcined charcoal and phosphorus deoxidized copper. Chromium is added in the form of Cu-Cr master alloy or metallic chromium. Semi-continuous casting is carried out under soot cover, and the pouring temperature is 1300~1360℃.

(2) Formability

Chromium bronze alloy has good cold and hot workability, and can be subjected to hot working such as extrusion, hot rolling, forging (solution treatment after forging is required), and the hot working temperature is 820~930℃. In the state of solution, annealing or proper drawing, cold working such as drawing, cold rolling, upsetting, swaging or bending can be carried out.

(3) Welding performance

The chromium bronze alloy can be soldered, silver soldered and brazed, and it is easy to perform gas shielded arc welding. It uses electronics for good heat dissipation. The beam welding effect is good. Welding and brazing will reduce the performance of the material after heat treatment. This type of welding is usually used in a soft state, followed by necessary heat treatment. The soft soldering performance is good. Oxyacetylene welding, shielded metal arc welding, resistance spot welding and resistance seam welding are not recommended.

(4) Cutting and grinding performance

The machinability of chromium bronze alloy is free-cutting brass HPb63-320%. Use mineral oil containing 20% ​​lead oil as a cooling lubricant when cutting.

Hot working specification

Thermal processing temperature range: 800~925℃;
Solution treatment: 980~1000℃, 10~30min, water quenching;
Aging treatment: 425~500℃, 2~4h, air cooling.

main feature

Chrome bronze has high strength and hardness at room temperature and below 400°C, good electrical and thermal conductivity, and good processing and forming properties.

main application

Chrome bronze is widely used in high-temperature conductive wear-resistant parts of electrical equipment. The main uses are: motor commutator, slip ring, high temperature switch, welding machine electrode, roller, clamper, brake disc, disc and other parts requiring high thermal conductivity, electrical conductivity, and high thermal strength in bimetallic form.

Process conditions

Test conditions

Melting equipment: 0.5 t medium frequency electromagnetic induction electric furnace.
Raw materials: 99.99% pure copper, 99.95% pure chromium, 99.98% pure cobalt, 48% cerium mixed rare earth.
Auxiliary materials: 10% phosphorous-copper alloy, pure metal magnesium, calcium carbide, borax, charcoal, broken glass.

Process key analysis

1 Furnace and tool preparation

The smelting furnace is a 250kg induction electric furnace. After the furnace is built (the lining material is 98% quartz sand + 2% boric acid), paint is applied, and then it is thoroughly dried.
Melting tools: bell jars, pressure bleaching, stirring rods and other tools to apply paint. All kinds of smelting tools in contact with metal must be painted and dried thoroughly, and the pouring bag must be thoroughly dried after the paint is painted.

Inspection molds in front of the furnace: air content inspection molds (standard specimens), angle inspection molds (standard specimens), and Kiel specimen molds (standard specimens) are painted and dried thoroughly.

2 charge preparation

The pure copper of the metal furnace charge is cut into blocks of 100mm×100mm~200mm (depending on the caliber of the furnace). Too large blocks will easily cause the phenomenon of shed material and prolong the smelting time, and it is prone to oxidation loss.

Pure chromium is brittle and has a high melting point (1857℃). It is broken into granules below 10mm, which is easy to melt quickly.
The pure cobalt plate has a relatively high melting point (1490℃) and is broken into small pieces of 20 mm×20 mm, which are easy to melt.

Mixed rare earths have a low melting point and are easy to burn. They are cut into 50mm×50mm size blocks or granules. When cutting rare earths, they should be cut intermittently and slowly to prevent burning, and stored in kerosene after cutting. The auxiliary charge of calcium carbide is broken into granules below 10mm. The calcium carbide is easily damp and should be placed in a dry environment. The damp calcium carbide (surface powder) must not be added to the furnace. The charcoal lumpiness can be broken into about 20mm particles. Borax should be agglomerated after removing the crystal water at a temperature of 880℃, and the agglomeration should be controlled at about 50mm. The degree of broken glass is not more than 100mm.

Excessively large metal charge is not conducive to alloy melting, and sometimes the phenomenon of shed material occurs. However, if the metal charge is too small, it is easy to oxidize during the preheating process. Auxiliary charge (covering agent) with a small lumpiness can quickly spread and cover the liquid metal, and it is not conducive to fast covering the liquid metal when the lump is too large.

3 preheating of charge and process equipment

3.1 Metal charge

Pure copper is ready to be used when it is preheated to 500~600 ℃. It can be preheated with an open flame or a preheating furnace. If it is preheated with an open flame, the time should not exceed 30 minutes, otherwise oxidation will occur. When the pure chromium particles are preheated to 200~300 ℃, they are wrapped with red copper skin, and the mass of each package is not more than 1.0 kg. Pure cobalt preheating adopts slow heating, and it is best to use a resistance furnace to preheat to 100-200 ℃ with red copper skin, because pure cobalt preheating at a higher temperature (above 300 ℃) will react with oxygen in the air. loss. Mixed rare earths are easy to oxidize and burn, so they can be preheated on the stove after being wrapped in red copper skin, but the preheating temperature should be checked at all times to prevent overheating and burning.

3.2 Auxiliary charge

Calcium carbide and charcoal (dried in advance) do not need to be preheated. The cullet is preheated to 50~100 ℃ for use. Borax is used after removing the crystal water. The gas content sample and the corner sample mold are preheated to 40~60 ℃ for use. When the pouring ladle is preheated to above 800 ℃, it will be used.

3.4 Smelting

After all the preparations are ready, the furnace crucible is preheated to 500 ℃ or higher, and the material is added for smelting (all the charge must not be added in a cold state). First put calcium carbide, charcoal, broken glass and borax at the bottom of the crucible (calcium carbide: charcoal: broken glass: borax=40:40:15:5), the thickness should cover more than 100 mm of liquid metal, and then add four-fifths The preheated pure copper (the other one-fifth adjusts the temperature of the liquid metal in the later stage of smelting) quickly melts. When the pure copper is melted into a liquid state, it naturally settles at the bottom of the crucible, and the covering agent floats on top of the liquid metal, completely covering the copper liquid. Note Observe the surface of the liquid metal. When the covering agent is less than 100mm, add it to more than 100mm. This process is maintained until the end of smelting.

When the smelting temperature reaches 1200～1250℃, add two-thirds of the phosphorous copper (the total added amount accounts for 0.3% to 0.5% of the mass of the copper liquid) with a bell jar to pre-deoxidize, and then add the preheated chromium and cobalt to the One third away from the bottom of the copper liquid, the density of chromium (7.19g/cm3) is relatively small, just add it directly with an ordinary bell; the density of cobalt (8.9 g/cm3) is almost the same as that of copper, and it is added to prevent sedimentation in the crucible. The bottom is difficult to melt. Use a bell jar with a sealing device to add in batches. In order to make the high melting point chromium (1857 ℃) and cobalt (1490 ℃) quickly melt in the liquid metal and reduce oxidation, use chromium and cobalt The bell jar moves slowly and circularly in the liquid metal until the chromium and cobalt metals are completely melted in the copper liquid;

Then use the remaining one-fifth of the pure copper to adjust the temperature of the liquid metal. When the temperature is adjusted to about 1200 ℃, add the remaining one-third of the phosphorous copper for secondary deoxidation, and then add it in 2 to 3 times with a bell jar Mixed rare earths.

Finally, use a bell jar to press pure magnesium (the total added amount accounts for 0.1%~0.3% of the mass of the copper liquid) into the bottom one-fifth of the crucible, slowly stir refining, exhaust, and clear the slag. After completing the above steps, let it stand for 5 ~10min, let the gas and oxidized slag in the liquid metal float fully, and inspect the quality when the temperature of the liquid metal reaches 1180~1200℃.

Compared with beryllium bronze

A. High stress relaxation resistance, good thermal stability, wide aging range, high yield, and long-term use at high temperatures (100°C-250°C). Upright performance is particularly good.
B. Good conductivity, conductivity ≥80%IACS.
C. Good corrosion resistance.
D. Good electroplating performance.
E. Non-toxic, suitable for sanitary ware molds and food molds.

Application example:

Mold core, food mold, sanitary ware mold, injection nozzle of injection molding machine, etc.

A trace amount (≤0.01) of zirconium is added to the chromium bronze. It has good electrical conductivity, thermal conductivity, high hardness, wear resistance, explosion resistance, crack resistance and softening temperature, low loss during use, and low total welding cost. It is suitable for welding electrodes and related parts of welding machines.

Beryllium bronze is divided into processed beryllium bronze and cast beryllium bronze. Commonly used cast beryllium bronzes include Cu-2Be-0.5Co-0.3Si, Cu-2.6Be-0.5Co-0.3Si, Cu-0.5Be-2.5Co and so on. The beryllium content of the processed beryllium bronze is controlled below 2%, and the domestic beryllium copper is added with 0.3% nickel or 0.3% cobalt.

Commonly used processing beryllium bronzes are: Cu-2Be-0.3Ni, Cu-1.9Be-0.3Ni-0.2Ti and so on. Cast beryllium bronze is used for explosion-proof tools, various molds, bearings, bushes, bushings, gears and various electrodes. Processed beryllium bronze is mainly used for various high-grade elastic components, especially various components that require good conductivity, corrosion resistance, wear resistance, cold resistance, and non-magnetic components. It is widely used as diaphragms, diaphragms, bellows, and micro switches. Wait.