Uneven hardness distribution after heat treatment of phosphor bronze is often caused by uneven heating, insufficient cooling, or internal component segregation. Addressing this issue requires systematic adjustments from three perspectives: process parameter optimization, equipment improvement, and material pretreatment.
Uniformity during the heating phase is fundamental to hardness distribution. Unstable furnace temperatures or excessively high charging density can lead to localized overheating or underheating of the phosphor bronze, resulting in hardness variations. For example, when furnace temperature deviations exceed ±10°C, the degree of austenitization is uneven, and the martensite conversion rate after quenching varies significantly. In this case, leaks in the furnace should be repaired through a leak test, and the temperature measuring instrument should be calibrated to within ±1°C. Furthermore, the distance between workpieces during furnace loading should be at least twice their thickness to prevent heat conduction obstruction. For large phosphor bronze castings, a staged heating method can be used: preheating at a low temperature to relieve stress before gradually heating to the solution temperature to minimize structural distortion caused by thermal stress.
Uniformity during the cooling phase directly impacts hardness consistency. Insufficient agitation of the quenching medium or poor workpiece movement can lead to the formation of soft spots in localized areas due to differential cooling rates. Taking water quenching as an example, if the water vapor film remains on the workpiece surface for too long, the cooling rate in that area will decrease, which can easily lead to the formation of a mixed structure of troostite and martensite, resulting in reduced hardness. Improvement measures include: using a propeller-type stirring device to ensure the medium flow rate is no less than 0.5 m/s; the workpiece should be introduced into the water at a 45° angle to the medium flow direction to prevent destabilization of the vapor film; and for workpieces with complex shapes, a special fixture can be designed to rotate the workpiece in the medium to promote uniform cooling. Furthermore, the medium concentration and cooling performance should be regularly tested, and aging media should be replaced promptly to avoid hardness fluctuations caused by medium failure.
Material pretreatment is crucial to reducing compositional segregation. The presence of network carbides or large blocks of free ferrite in the as-cast structure of phosphor bronze can significantly hinder uniform phase transformation during quenching. High-temperature homogenization annealing, heating the phosphor bronze to 50-80°C below the solidus and holding it for 4-6 hours, dissolves the carbides and eliminates dendritic segregation, providing a uniform matrix for subsequent heat treatment. For forged phosphor bronze, the final forging temperature must be controlled above 650°C to avoid residual stress concentration caused by uneven deformation, which in turn affects the uniformity of the quenched structure.
Matching process parameters is key to addressing uneven hardness. Excessively high solution treatment temperatures can lead to grain coarsening and reduce hardenability; too low temperatures can lead to incomplete austenitization, leaving residual undissolved phases that can become soft spots. The solution temperature should be adjusted according to the specific phosphor bronze grade. For example, C5191 phosphor bronze should be held at 900-950°C for 1-2 hours to ensure complete dissolution of alloying elements. During the aging stage, temperature and time must be precisely controlled to avoid coarsening of precipitates due to excessive aging, which can reduce hardness uniformity.
Equipment upgrades and maintenance are also crucial. Using a vacuum quenching furnace can eliminate scale and decarburization layers, preventing surface hardness loss. A dual-channel monitoring system with an infrared thermometer and thermocouples provides real-time feedback on furnace and workpiece temperatures, ensuring a controlled heating process. For high-frequency hardening, the induction coil design must be optimized to ensure a uniform gap between the induction coil and the workpiece surface. The spray angle and nozzle layout must also be adjusted to ensure even coverage of the cooling medium.
By controlling heating uniformity, optimizing the cooling system, strengthening material pretreatment, precisely matching process parameters, and upgrading and maintaining equipment, the uneven hardness distribution of phosphor bronze after heat treatment can be systematically addressed, improving product performance stability.
