Cemented carbide grain growth is a core microscopic change during the sintering process of cemented carbide. This article popularly explains the core knowledge of cemented carbide grain growth, including the fundamental driving force of grain growth, the principle of grain boundary movement, how temperature and time affect grain size, the factors that restrict grain growth, and the differences between normal and abnormal grain growth.

What’s the Driving Force of Grain Growth?

Cemented carbide is composed of numerous fine grains with different orientations. The interface between adjacent grains is defined as the grain boundary. Compared with the internal structure of grains, atoms at grain boundaries are arranged disorderly and irregularly, carrying extra energy known as interface energy. In short, the more grain boundaries there are, the finer the grains of cemented carbide will be.

All substances in nature tend to develop toward a stable state with lower energy. Under the high-temperature environment of sintering, cemented carbide spontaneously reduces the number of grain boundaries and promotes the merging and growth of fine carbide grains. This process reduces the overall energy of the material, decreases the number of grains, increases the average grain size, and stabilizes the material structure.

Figure 1. SEM microstructure of cemented carbide at different sintering temperatures

The microstructure of cemented carbide sintered at different temperatures shows obvious changes. Grains are fine and dense at low sintering temperatures, and become significantly coarser and fewer in number as the temperature rises. Grain growth only changes the size and quantity of carbide grains, without altering the material phase and inherent properties of cemented carbide.

Why do Large Grains Move Automatically to Swallow Small Ones?

Grain boundaries become movable at high temperatures and automatically move toward the center of their curved surface. Atoms spontaneously migrate from the convex high-energy side to the concave low-energy side, driving the grain boundary to move toward the curvature center.

This process leads to a typical phenomenon: large grains swallow small grains. The more obvious the carbide grain boundary curvature, the stronger the driving force for grain boundary movement. There is a regular pattern in microstructure evolution: grains with fewer than six sides tend to shrink and disappear, grains with more than six sides keep growing, and hexagonal grains maintain a relatively stable state.

Figure 2. Grain microstructure coarsening evolution with sintering time (5–120 min) and temperature (1123, 1223 K)

Both experimental observations and simulation results verify that higher sintering temperature and longer holding time accelerate the disappearance of fine carbide grains. Large grains continuously absorb adjacent small grains, leading to a steady increase in the overall grain size.

This spontaneous grain growth process continues as long as the temperature is sufficiently high. It explains why grains still coarsen continuously even when cemented carbide achieves full densification under prolonged high-temperature holding.

How Do Temperature and Time Determine the Rate of Grain Growth?

Temperature acts as the dominant factor for carbide grain growth.

The rate of grain growth depends entirely on atomic activity. Higher temperature greatly increases the frequency of atomic jumping and migration, and accelerates grain boundary movement, resulting in coarser grains within the same period. A small temperature rise can double the carbide grain growth rate, which is why sintering temperature requires precise control in production.

Grain growth proceeds fastest in the early sintering stage and gradually slows down over time. In general, temperature determines the growth rate of grains, and holding time determines the final growth degree.

Figure 3. Variation of squared grain diameter d2 with holding time (a) and Arrhenius relationship of ln k versus 1/T (b, activation energy Q ≈ 148 kJ/mol)

Advanced sintering technologies such as rapid sintering and two-step sintering are widely used to inhibit excessive grain coarsening. Their core principle is to shorten the high-temperature holding time, which ensures material densification while limiting grain growth.

Raw Powder and Milling Process Determine the Initial Grain Size

The final grain size of finished products is determined by both sintering parameters and raw powder conditions. In cemented carbide production, prolonged milling produces finer and more uniform powder, which contributes to finer grains under the same sintering conditions.

To put it simply, sintering temperature controls how fast carbide grains grow, and raw powder fineness controls the initial grain size. Fine raw powder delivers better fine-grain effects, while coarse powder hardly forms ultra-fine grains even at reduced sintering temperatures.

Figure 4. Average grain size of WC-Co cemented carbide varies with sintering temperature and milling time (A1–A3)

Accordingly, the conventional method for preparing fine-grain cemented carbide is to obtain ultra-fine and uniform powder through sufficient milling, combined with low-temperature and short-time sintering, so as to control grain size from raw material processing to sintering.

What Factors Restrict Grain Boundary Movement and Prevent Excessive Grain Growth?

Pinning Effect

Uniformly distributed fine second-phase particles inside cemented carbide can block and pin moving grain boundaries, which is defined as the pinning effect. Grain boundaries have to bypass or drag particles to continue moving and merging grains, which greatly increases movement resistance and slows down or even stops grain growth.

More and finer particles provide stronger pinning effects, setting a maximum limit for carbide grain growth. Grain size stops increasing and remains fine when the pinning force of particles balances the driving force of grain growth.

Figure 5. Phase-field simulation of single grain growth with different second-phase particle contents (0–0.20%)

Simulation results show that grains grow rapidly without particle pinning, and the growth amplitude decreases significantly with the increase of particle content. Dispersed carbide and oxide particles are commonly added in industry to prepare fine-grain cemented carbide with high temperature resistance and stable performance.

Other Factors for Inhibiting Grain Coarsening

Solute Atoms

Solute atoms in cemented carbide can hinder carbide grain boundary movement. These atoms gather at grain boundaries and move along with boundaries, producing a drag force to reduce grain boundary mobility. This phenomenon is known as solute drag.

Residual Pores

Micro residual pores that fail to discharge completely in the early sintering stage can also pin grain boundaries and restrain grain growth. However, rapidly moving grain boundaries may break away from pores, trapping pores inside grains. These internal pores are difficult to eliminate and will reduce the densification of cemented carbide.

Additives for Auxiliary Inhibition of Grain Coarsening

Special additives used in production have dual functions. Part of the additives dissolve in the material matrix to produce solute drag, and the rest precipitate into fine particles to pin grain boundaries. The dual effects effectively inhibit grain coarsening, optimize grain size distribution, and reduce coarse carbide grains in cemented carbide.

Figure 6. SEM microstructure and carbide grain size distribution of cemented carbide with different additive contents

Differences Between Normal and Abnormal Grain Growth

Normal Grain Growth

Under conventional sintering conditions, all grains of cemented carbide grow uniformly at a similar speed with a consistent and stable size distribution. This is normal grain growth, which produces a uniform microstructure and stable mechanical properties, meeting the optimal production requirements.

Abnormal Grain Growth

Abnormal grain growth is an uncontrolled growth behavior. A small number of grains gain growth advantages during sintering, rapidly swallow surrounding fine grains, and form oversized coarse grains, forming a bimodal structure mixed with coarse and fine grains.

Figure 7. TEM, SAED, and HRTEM images of coarse grains (a–c) and fine grains (d–f) in cemented carbide

Microscopic detection confirms the typical characteristics of abnormal growth: micron-scale coarse grains and nano-scale fine grains coexist in the same cemented carbide material with a huge size gap.

Abnormal grain growth destroys the microstructure uniformity of cemented carbide, reduces material strength and toughness. Coarse carbide grains easily induce crack initiation, shortening the service life of cemented carbide tools and molds.

Sumário

Cemented carbide grain growth is a spontaneous microstructure evolution process driven by energy reduction under high sintering temperature, which is manifested as grain boundary migration and grain merging coarsening. In actual production, optimizing raw material pretreatment, sintering temperature curve, and doping formula can balance the relationship between material densification and grain growth, avoid coarse grain defects, and prepare fine-grain cemented carbide products with a uniform microstructure and excellent comprehensive performance.