In the field of metal processing, the quality of forgings plays a decisive role in the performance of the final product. Among the many factors affecting forging quality, grain structure inheritance, though seemingly a subtle microstructural issue, can pose significant hidden risks to mechanical performance. This article provides an in-depth discussion of the formation mechanism of grain inheritance, its adverse effects on performance, and a range of proven solutions, helping readers gain a comprehensive understanding of this complex yet critical metallurgical issue with forgings.
Grain inheritance commonly occurs in certain forgings, especially martensitic steel die forgings, when both the heating temperature during forging and the finishing forging temperature are relatively high. After forging, coarse austenite grains cool to room temperature and undergo phase transformation. Within a single austenite grain, many smaller grains are formed. These smaller grains share nearly the same spatial orientation as the original austenite grain, giving the appearance that the coarse grain has been subdivided.
However, this subdivision is only superficial. In essence, these smaller grains still retain the characteristics of the original coarse grain. When the material is reheated for normalizing, these small grains revert to the original austenite grains with nearly unchanged orientation. The extent of this reversion depends on the normalizing temperature. Upon cooling after normalizing, the austenite grains are once again subdivided into smaller grains. Although the grain size appears refined after normalizing, many of these fine grains maintain the same crystallographic orientation as the original coarse grain, meaning that the characteristics of the coarse grain still persist.
The inheritance of coarse grains has a highly detrimental impact on the final heat treatment results and service performance of forgings, particularly under overload and impact conditions. The presence of coarse grains makes the material more prone to crack initiation and fracture when subjected to impact loads. This is because coarse grain boundaries are regions of stress concentration, where defects are more likely to form under external forces, thereby reducing toughness and strength.
Moreover, during subsequent machining and service, inherited coarse grain structures can increase machining difficulty, degrade surface quality, and even lead to premature fatigue failure. The degree and stubbornness of grain inheritance vary among different materials, making it essential to adopt material-specific solutions.
To address the challenge of grain inheritance, researchers and engineers have developed a variety of effective methods through long-term study and industrial practice. These approaches range from improvements in conventional heat treatment processes to specialized treatments tailored to specific materials. Together, they provide practical pathways for enhancing forging quality and performance.
For most general structural steels, a single normalizing treatment is usually sufficient to refine the grains and eliminate the original grain boundaries. Normalizing is a common heat treatment process in which steel is heated to a specified temperature and then air-cooled, causing microstructural transformation that refines grains and relieves internal stresses.
During normalizing, austenite grains re-nucleate and grow. Since nucleation sites and growth orientations are random, the original coarse grain structure is disrupted, resulting in a fine and uniform grain structure that improves mechanical properties.
Some steels, such as 18CrNiWA and 20Cr2NiA, still exhibit grain inheritance after a single normalizing treatment, with original grain boundaries remaining visible. For these materials, double or multiple normalizing treatments are required, and in some cases high-temperature normalizing or repeated high-temperature normalizing is necessary.
Each cycle of normalizing heating and cooling causes small grains to revert to austenite grains or a single austenite grain to decompose into multiple smaller grains. Although their crystallographic orientations may not be completely random, repeated normalizing gradually destroys the inherited orientation relationship, ultimately producing truly fine grains.
For example, martensitic stainless steels such as 3Cr3Mo3VNb can be treated by annealing or isothermal annealing. Through phase transformation and recrystallization, grains are refined and coarse grain inheritance is effectively eliminated.
For steel forgings such as 18Cr2Ni4WA and 20Cr2Ni4A, performing a high-temperature tempering treatment after cooling to room temperature is another effective solution. During high-temperature tempering, the precipitation of highly dispersed particles and the recrystallization of the α-phase matrix promote the formation of austenite grains around these precipitates during subsequent normalizing.
This process disrupts the original crystallographic orientation, enabling true and sufficient grain refinement after phase transformation. The key to this method lies in precise control of tempering temperature and time, as well as the heating and cooling parameters of the subsequent normalizing process.
In die forging, once the billet size is fixed, the degree of deformation is essentially determined. In contrast, open forging allows for increased deformation through repeated upsetting and drawing, and the finishing forging temperature can be relatively lower, resulting in finer grains after forging.
With greater deformation, a large amount of residual strain and internal stress is introduced. When reheated to a temperature slightly above the transformation point during normalizing, the α→γ phase transformation occurs in a disordered manner, disrupting the original orientation relationship and achieving true grain refinement. Therefore, proper control of deformation degree and finishing forging temperature is crucial in preventing grain inheritance.
For large low-alloy steel forgings such as 30Cr2Ni4MoV, coarse grains often originate from the inheritance of non-equilibrium structures (bainite/martensite). A triple normalizing process can achieve grain refinement through multi-stage austenite reconstruction.
The typical process involves heating the forging at a rate of ≤150 °C/h to 50–100 °C above Ac₃ (e.g., 870–920 °C), holding for a time calculated as t = k × D (where k = 0.8–1.5 min/mm and D is the effective thickness), followed by air cooling combined with spray cooling to 300 ± 10 °C. This stage fully austenitizes the material and breaks the original grain orientation.
Next, the forging is reheated to Ac₃ + 80–120 °C and water quenched, using transformation stress to induce recrystallization and eliminate residual austenite inheritance. Finally, isothermal normalizing is carried out in the Ac₁–Ac₃ two-phase region, followed by tempering at 560–600 °C, stabilizing the grain size to ASTM grade 5 or finer.
In practice, this process has shown remarkable results. For example, in a nuclear power rotor application, ultrasonic inspection pass rates increased from 65% to 92%.
For medium- to high-hardenability steels such as 26Cr2Ni4MoV, conventional normalizing is insufficient to eliminate structural inheritance. High-temperature normalizing is required to trigger austenite recrystallization.
The forging is heated to 150–200 °C above Ac₃ (e.g., 1050–1100 °C), with holding time calculated as section size × 1.2 h per 100 mm. At such high temperatures, the migration energy barrier of austenite grain boundaries is reduced, and internal stresses generated by volumetric expansion drive recrystallization. Newly formed grains have no orientation relationship with the original coarse grains.
In one turbine shaft forging, high-temperature normalizing at 1040 °C for 8 h increased grain size from ASTM grade 2 to 4.5, while eliminating 80% of banded segregation. When combined with gradient cooling—air cooling to 650 °C followed by furnace cooling—secondary grain coarsening can be avoided. This method is particularly effective for ultra-large forgings with diameters exceeding 2000 mm.
For martensitic steels with a strong tendency toward structural inheritance, such as 2Cr12NiMo1W1V, rapid heating in the critical temperature range can effectively block grain inheritance.
Coarse-grained forgings are rapidly heated at a rate of ≥370 °C/h to Ac₁ + 30–50 °C (e.g., 750–780 °C), held briefly (10–15 min/mm), and then oil quenched. Rapid heating suppresses the nucleation of lamellar austenite and promotes spheroidal austenite-dominated transformation. Subsequent subcritical annealing at 690 ± 10 °C for 4 h ensures uniform carbide precipitation.
In an aerospace valve disc application, this approach reduced mixed-grain structures from 35% to below 5%, achieving ASTM grain size grade 5. This process requires precise control of heating rates, with temperature accuracy within ±5 °C.
Low-alloy steels such as 45 steel can achieve grain refinement of 1–2 grades by normalizing at 920–950 °C with a cooling rate exceeding 200 °C/h, while reducing energy consumption by 40% compared with annealing.
Bainitic steels such as 17CrNiMo6 require annealing at 680–720 °C for 6 h to fully decompose non-equilibrium structures into pearlite and ferrite, completely eliminating the basis for inheritance. In one wind power gearbox application, grain uniformity improved by 60% after annealing.
For stubborn coarse grains, a combined process of “normalizing + annealing + secondary normalizing” can be used. For example, a heavy-duty crankshaft treated in this way saw its grain size improve from ASTM grade 1 to grade 6. During initial furnace charging, rapid heating at 80–120 °C/h to 650 °C avoids prolonged holding that could cause carbide aggregation. Holding for 2–4 h at 50 °C below Ac₁ (e.g., 680 °C) promotes dislocation rearrangement and subgrain formation.
By alternating mist cooling and air cooling, the temperature difference between surface and core can be controlled within 100 °C. In a nuclear power tube sheet application, grain size deviation was reduced from ±2 grades to ±0.5 grades.
When applying these methods in practice, several key factors must be considered. First, material characteristics and process requirements vary, so process selection must be based on chemical composition, performance requirements, and actual production conditions. For temperature-sensitive materials, heating and cooling rates must be strictly controlled to prevent new defects.
Second, equipment accuracy and stability are critical. Furnace temperature control precision, as well as cooling medium flow rate and temperature control, directly affect treatment outcomes. Regular maintenance and calibration are essential.
Finally, process parameter optimization is vital. Through experimentation and data analysis, parameters should be continuously adjusted to achieve optimal grain refinement.
Grain structure inheritance is a critical factor affecting forging performance. However, by applying appropriate heat treatment processes and controlling deformation during forging, this issue can be effectively addressed. From single normalizing for general structural steels to multiple or high-temperature normalizing for special materials; from triple normalizing for large low-alloy forgings to high-temperature normalizing with gradient cooling for medium- to high-hardenability steels; and from rapid critical-range heating for martensitic steels to specialized treatments for low-alloy and bainitic steels, each method provides an effective solution for grain refinement in specific materials.
In practice, these methods must be carefully selected and optimized according to actual conditions to ensure forging quality and performance. With ongoing advances in materials science and heat treatment technology, more efficient and energy-saving processes are expected to emerge, offering stronger support for the continued development of the metal processing industry.
