Knowledge

I. Introduction

In the field of heat treatment of metallic materials, quenching is an extremely crucial and commonly used process. It involves heating the metal material to an appropriate temperature and maintaining it for a certain period of time, followed by cooling at an appropriate rate, thereby altering the internal microstructure of the material and significantly enhancing its mechanical properties, such as hardness, strength, and wear resistance. As an advanced form of quenching, ultrafine-grained quenching has attracted extensive attention in the field of materials science in recent years. Through specific technological means, it enables the metal material to obtain an ultrafine-grained microstructure after quenching, further endowing the material with superior comprehensive performance. It has demonstrated great application potential in numerous fields with extremely high requirements for material performance, such as aerospace, automotive manufacturing, and mold industry.

 

II. Principles of Ultrafine Grain Quenching

 

(1) The Relationship between Grain Refinement and Material Properties

The properties of metallic materials largely depend on their internal microstructure, especially the size of grains. According to the Hall-Petch formula, the yield strength of a material is inversely proportional to the square root of the grain size, meaning that the finer the grains, the higher the strength of the material. This is because smaller grains imply an increase in the grain boundary area, and grain boundaries act as obstacles to dislocation movement, effectively preventing dislocation slip and diffusion, thereby enhancing the material's resistance to deformation. Additionally, in the fracture process of fine-grained materials, crack propagation needs to cross more grain boundaries, consuming more energy, and thus they also possess higher toughness and fatigue resistance.

 

(2) Mechanism of Ultrafine Grain Formation during Quenching Process

1. Phase transformation-driven grain refinement: During the quenching process, metallic materials undergo a transformation from high-temperature phases (such as austenite) to low-temperature phases (such as martensite, bainite, etc.). During this phase transformation, the formation of the new phase is often accompanied by the re-nucleation and growth of grains. By precisely controlling parameters such as quenching temperature and cooling rate, the nucleation of the new phase can be promoted within extremely small regions, thereby forming ultra-fine grain structures. For instance, under rapid cooling conditions, the transformation of austenite to martensite can be completed in an instant, and the size of martensite plates or laths can be controlled within a very fine range.

 

2. Second-phase particle inhibition of grain growth: Introducing appropriate second-phase particles (such as carbides, nitrides, etc.) into metallic materials, these particles can be distributed on the grain boundaries, playing a role in pinning the grain boundaries and effectively inhibiting the growth of grains during heating and cooling processes. By rationally controlling the size, quantity, and distribution of second-phase particles, ultrafine grains can be achieved during quenching. For instance, in some alloy steels, through proper heat treatment processes, carbides are uniformly and finely distributed in the matrix, thereby hindering the growth of austenite grains during quenching and obtaining fine quenched structures.

 

III. Methods for Achieving Ultrafine Grain Quenching

 

(1) Rapid cooling quenching method

The rapid cooling quenching method is to increase the cooling rate, enabling the metal material to quickly pass through the phase transformation temperature zone during the quenching process and inhibit the growth of grains. Commonly used rapid cooling media include water, oil, and some new polymer quenching agents. For example, in the water quenching process, water's high thermal conductivity can quickly remove heat from the metal surface, allowing the material to cool rapidly and achieve ultra-fine grain refinement. However, water quenching is prone to causing workpiece deformation and cracking. Therefore, for some complex-shaped or workpieces with strict deformation requirements, oil quenching or polymer quenching agents need to be used. Oil quenching has a relatively slower cooling rate but can reduce the tendency of workpiece deformation and cracking; polymer quenching agents can adjust their cooling characteristics as needed to achieve optimized quenching for different materials and workpieces.

 

The process flow is as follows: First, the parts are heated at a relatively fast speed to a temperature above Ac3, then a short period of heat preservation is carried out, followed by rapid cooling. This series of steps of heating, heat preservation and cooling is repeated multiple times. Because each time the parts undergo a heating process, the austenite crystals will be refined once, so after completing 4 such cycles, the grain size of 45 steel can be successfully refined from the original grade 6 to grade 12.

 

(2) Deformation heat treatment method

Shape deformation heat treatment is a process method that combines plastic deformation with heat treatment. Before quenching, appropriate plastic deformation (such as rolling, forging, drawing, etc.) is applied to the metal material, which can introduce a large number of dislocations and defects into the material. These dislocations and defects can serve as nucleation cores for new phases during the subsequent quenching process, promoting the formation of fine grains. At the same time, plastic deformation can also break up the original coarse grains in the material, creating favorable conditions for grain ultra-fine refinement. For example, in the heat treatment of some aluminum alloys, cold rolling deformation is first applied to the material, followed by quenching treatment, which can obtain a fine recrystallized grain structure, significantly improving the strength and toughness of the material.

The process is as follows: First, heat the steel to a temperature slightly above Ac3 to achieve austenitization; then carry out hot rolling to cause intense deformation of the austenite; next, conduct an appropriate period of isothermal holding to promote the initiation of recrystallization in the deformed austenite; finally, perform quenching before the grains start to grow.

 

(3) Microalloying Method

Microalloying is a method that involves adding a small amount of alloying elements (such as niobium, vanadium, titanium, etc.) to metallic materials. These alloying elements can form stable second-phase particles with carbon, nitrogen and other elements. During the quenching process, these second-phase particles can effectively inhibit the growth of grains, achieving ultrafine grain size. For instance, when a trace amount of niobium is added to low-carbon steel, niobium can form fine niobium carbide particles with carbon. These particles are distributed along the grain boundaries during austenitization, preventing the coalescence and growth of grains.

 

Vigor has a professional supply chain of high-level heat treatment and surface treatment for castings, forgings, and our CNC-machined parts. If you have any questions or parts that need to be developed, please feel free to contact us at info@castings-forging.com