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Powder metallurgy plungers offer the advantages of high material utilization and easy formation of complex shapes, but their mechanical properties are highly dependent on sintering process parameters. Systematic research into the effects of sintering temperature, holding time, ramp rates, sintering atmosphere, compacting density, and post-processing (tempering, hot isostatic pressing) on ​​density, porosity, microstructure, and interfacial bonding is crucial for improving plunger strength, hardness, fatigue life, and wear resistance.

First, sintering temperature is the core factor controlling densification. As temperature increases, the diffusion rate significantly increases, leading to the growth of necks between particles and the reduction of pores, thereby increasing density and enhancing tensile and compressive strength. However, excessively high temperatures can lead to grain coarsening, precipitation of brittle phases, or matrix softening, which in turn reduces impact toughness and fatigue properties. Therefore, there is typically an optimal temperature window that strikes a balance between strength and toughness.

Second, holding time has a cumulative effect on micropore convergence. Short holding times are insufficient to complete neck growth and pore closure, limiting part strength. Excessively long holding times promote grain growth and may cause compositional segregation. In actual production, an appropriate holding time must be selected based on powder composition and particle size to achieve high density while suppressing coarse grains.

Third, the sintering atmosphere and reducing environment influence surface oxide layer removal and interface cleanliness. Active atmospheres (hydrogen or reducing gases) facilitate deoxidation, improve particle contact, and reduce interfacial brittle phases, while inert atmospheres are suitable for systems that are susceptible to oxidation but do not require high deoxidation requirements. Nitrogen-containing or nitriding processes can generate hardened phases on the surface to improve wear resistance, but these should be controlled to avoid embrittlement.

Fourth, the initial compaction density and particle size distribution directly determine the distribution of residual porosity after sintering. A high initial density reduces the amount of densification required during sintering, thereby achieving better mechanical properties at lower temperatures. Particle morphology and size also influence the neck formation rate and pore connectivity.

Fifth, the heating and cooling rates and cooling methods significantly influence the stress field and phase transformation behavior. Slow heating facilitates uniform debinding and gas escape, avoiding slag inclusion in pores. Rapid cooling or quenching produces a fine-grained structure and higher strength, but may introduce thermal stresses that lead to cracking risks. Hot isostatic pressing (HIP) or subsequent heat treatments such as tempering and nitriding can further seal residual porosity and improve surface hardness and fatigue life.

Finally, process parameters often interact with each other. For example, extending the holding time at a lower sintering temperature can partially compensate for insufficient densification, but it can also exacerbate grain heterogeneity. Experimentally, it is recommended to use orthogonal experiments or response surface methodology to establish a multi-parameter-performance relationship model. This model, combined with microstructure characterization, porosity measurement, hardness, and fatigue testing, can form a process optimization curve.

In summary, improving the mechanical properties of powder metallurgy plungers depends on achieving a balance between temperature, time, atmosphere, initial density, and post-treatment. Through appropriate parameter design and a combination of multi-stage heat treatments, it is possible to maintain density while suppressing grain growth, achieving high strength, high wear resistance, and excellent fatigue performance, meeting the requirements for long plunger life under complex operating conditions.