Hardening of Copper Alloys

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Copper alloys can be hardened through heat treatment, which is categorized into two main types: those softened by high-temperature quenching and then hardened by lower-temperature treatments, and those hardened by quenching from high temperatures through martensitic reactions. Alloys that harden during low-to-intermediate temperature treatments following solution quenching include precipitation hardening, spinodal hardening, and order hardening. Quench-hardened alloys mainly consist of aluminum bronzes, nickel-aluminum bronzes, and certain copper-zinc alloys. Typically, quench-hardened alloys are tempered to enhance toughness and ductility while reducing hardness, similar to the treatment of alloy steels.

Low-Temperature-Hardening Alloys

To provide a clear comparison, Table 1 outlines examples of various low-temperature hardening alloys, detailing their typical heat treatments and achievable property levels.

Table 1. Heat Treatment of Low-Temperature Hardening Alloys

Alloy	Solution -Treating Temperature (°C)	Ageing Treatment Temperature, Time	Hardness (H)
Precipitation hardening
C15000	980	500-550, 3	30 HRB
C17000, C17200, C17300	760-800	300-350, 1-3	35-44 HRC
C17500, C17600	900-950	455-490, 1-4	95-98 HRC
C18000 (b), C81540	900-930	425-540, 2-3	92-96 HRB
C18200, C18400, C18500, C81500	980-1000	425-500, 2-4	68 HRB
C94700	775-800	305-325, 5	180 HB
C99400	885	482, 1	170 HB
Spinodal hardening
C71900	900-950	425-760, 1-2	86 HRC
C72800	815-845	350-360, 4	32 HRC

(a) Solution treating is followed by water quenching.
(b) Alloy C18000 (81540) must be double aged, typically 3h at 540°C followed by 3h at 425°C.

Precipitation Hardening Alloys

Copper alloys undergoing precipitation hardening are primarily used in electrical and heat conduction applications. The heat treatment must be carefully designed to achieve the necessary mechanical strength and electrical conductivity, relying on the effectiveness of the solution quench and the control of the precipitation treatment. Terms like "age hardening" are often used interchangeably with "precipitation."

Copper alloys are hardened through elevated temperature treatment rather than ambient temperature ageing, with electrical conductivity increasing over time until reaching a maximum in the fully precipitated condition. The optimal treatment typically occurs at temperatures and durations just beyond the hardness-ageing peak, and cold working prior to ageing enhances final hardness.

In lower-strength wrought alloys such as C18200 (Cu-Cr) and C15000 (Cu-Zr), some heat-treated hardness may be sacrificed for improved conductivity, with final hardness enhanced through cold working. Alloy C18000 (Cu-Ni-Si-Cr) requires two distinct precipitation treatments to achieve maximum electrical conductivity and hardness.

When precipitation hardening is performed at the mill, further treatment after fabrication is usually unnecessary. However, stress relief may be beneficial for complex shapes like cantilever-type springs to eliminate induced stresses.

Transformation Hardening

Transformation hardening strengthens specific alloys by inducing a phase change to a harder and stronger form. Two-phase aluminum bronzes and some manganese bronzes undergo quench-and-temper treatments to enhance strength without significantly compromising ductility. These alloys are hardened by rapid cooling from a high temperature, producing a martensitic structure, and then tempered at a lower temperature to stabilize the structure while restoring some ductility and toughness.

Two-Phase Aluminum Bronzes

Binary copper-aluminum alloys exhibit two stable phases at room temperature with aluminum content between 9.5% and 16%. With additional elements like iron (1% to 5%), the aluminum content adjusts to 8% to 14%. Quenching and tempering strengthen these alloys, with transformations occurring at temperatures between 815°C and 1010°C. Rapid quenching produces a hard, brittle structure known as martensitic beta, achieved through both oil and water quenching methods.

Tempering for 2 hours at temperatures from 595°C to 650°C induces reprecipitation of fine alpha in a tempered beta-martensite structure, enhancing ductility while reducing hardness.

Nickel-aluminum bronzes respond similarly to quench-and-temper treatments. Alloys like C95500 and C63000 achieve higher hardness but are more prone to quench cracking in heavy or complex sections, making oil quenching preferable.

Cast two-phase aluminum bronzes are often normalized by heating to 815°C, furnace cooling to about 550°C, and then air cooling. This treatment improves machinability and yields uniform hardness.

Spinodal Hardening Alloys

Alloys that harden through spinodal decomposition follow a treatment similar to precipitation hardening. A high-temperature solution treatment followed by quenching generates a soft and ductile spinodal structure. This material can be cold worked or formed, and a subsequent lower-temperature spinodal treatment, known as ageing, enhances hardness and strength.

Spinodal-hardening alloys primarily consist of copper-nickel alloys with chromium or tin additions. The hardening mechanism involves a miscibility gap in the solid solution leading to fine-scale chemical segregation in the alpha crystal matrix. Since no crystallographic changes occur, these alloys maintain excellent dimensional stability during hardening.

Order Hardening Alloys

Certain alloys, nearly saturated with an alloying element dissolved in the alpha phase, undergo an ordering reaction when highly cold-worked material is annealed at low temperatures. Copper alloys like C61500, C63800, C68800, and C69000 exhibit this behavior. Strengthening arises from short-range ordering of solute atoms, which impedes dislocation motion through the crystals.

The low-temperature order-annealing treatment also serves as a stress-relieving process, increasing yield strength by reducing stress concentrations at dislocation pileup sites. Thus, order-annealed alloys demonstrate improved stress-relaxation characteristics.

Order annealing typically occurs for short durations at temperatures between 150°C and 400°C, requiring no special protective atmosphere. This process is often performed after the final fabrication step to maximize stress-relieving benefits, especially where resistance to stress relaxation is critical.

Quench Hardening and Tempering

Quench hardening and tempering are primarily applied to aluminum bronze and nickel-aluminum bronze alloys, with occasional applications for some cast manganese bronze alloys containing zinc equivalents of 37% to 41%. Aluminum bronzes with 9% to 11.5% Al and nickel-aluminum bronzes with 8.5% to 11.5% Al effectively respond to quench hardening through martensitic reactions. Alloys with higher aluminum content are prone to quench cracking, while those with lower aluminum content often lack sufficient high-temperature beta phase for adequate response to quench treatments.