After forging, alloy steel is normalized, and the grain size is found to be too large. What impact does multiple normalizing have on the material microstructure?

In the world of alloy steel heat treatment, normalizing is considered a crucial step. Its purpose is to break down the coarse grains left from casting or forging through air cooling after austenitization, paving the way for subsequent quenching and tempering. However, when you open the furnace door full of anticipation, only to find that the grain size is still coarse, or even facing the vicious cycle of “the more normalizing, the worse it gets,” this is not only a failure of the process, but also a severe warning about the evolution of the microstructure.

Regarding your two crucial questions—”the impact of multiple normalizing on the microstructure” and “whether it will form an unchangeable microstructure”—the answer is not a simple “yes” or “no,” but a perilous struggle between high temperature, diffusion, and phase transformation.

I. The “Failure” of the Initial Normalizing: Why Doesn’t the Grain Refine?

To understand the consequences of multiple normalisations, we must first diagnose why the initial normalisation was “ineffective.” Alloy steels (especially those containing Mn, Cr, Ni, and Ti) are highly temperature-sensitive.

1. The “Greed” of Austenite: If the heating temperature is too high (e.g., exceeding Ac3 by 150-200℃) or the high-temperature holding time is too long, austenite grains will rapidly devour the boundaries like a greedy snake, growing rapidly. This coarsening is particularly pronounced in steels containing titanium and vanadium, because the carbides of these elements dissolve easily at high temperatures, losing their “pinching” effect that hinders grain boundary migration.

2. The “Lukewarm” Cooling: Normalizing relies on air cooling, with a cooling rate much lower than quenching. For large-section forgings or high-alloy steels (e.g., 12Cr2Ni4A), air cooling may not be sufficient to suppress grain growth before pearlite transformation; in some regions with high alloy element content (e.g., Mn segregation zones), coarse proeutectoid ferrite or network carbides may preferentially grow.

3. The “Curse” of Critical Deformation: This is the most insidious killer. If the deformation after forging falls precisely within the “critical deformation zone” (approximately 5%-10%), the recrystallization driving force is extremely weak, and the grains will inherit the coarse state from the heating process, forming what is known as “critical deformation coarse grains.” This structure is extremely stubborn, and a single normalising process is often ineffective.

After forging, alloy steel is normalized, and the grain size is found to be too large. What impact does multiple normalizing have on the material microstructure (1)

II. Multiple Normalizing: “Refinement” or “Destruction”?

When coarse grains are discovered, an engineer’s instinctive reaction is “do it again.” However, multiple normalizations are a double-edged sword, and their effects must be considered from both sides:

1. Positive Effects: Breaking the Inherited Structure, Reshaping Uniformity

For coarse grains caused by uneven heating or slight overheating, secondary normalizing can often revive them. The first normalising step may not have eliminated compositional segregation caused by casting or forging, leaving residual alloy-rich and alloy-poor regions in the microstructure. In a second heating process, using a static recrystallisation mechanism, the low-melting-point alloy zone melts and recrystallises first, followed by the high-melting-point alloy zone. This effectively breaks down coarse austenite grains, transforming them into a fine, mixed ferrite-pearlite microstructure. Experiments show that for alloy steels such as 18CrNi4A, secondary normalizing at around 860℃ can refine the grain size of the coarse-grained surface region from hundreds of micrometres to 25-30 micrometres, significantly improving the strength-toughness balance.

2. Negative Effects: Oxidation, Stress, and “False Refinement”

However, if process parameters are out of control, multiple normalizing processes can be disastrous:

• Accumulation of oxidation and decarburization: Each heating process is like “peeling” the workpiece surface. Repeated high-temperature exposure leads to a sharp decrease in surface carbon content, forming soft spots, and the oxide scale pressed into the surface can become a fatigue crack initiation point.

• Graphitisation Risk: For high-carbon, high-alloy steels (such as tool steels), repeated exposure to high temperatures (close to Ac1) will cause cementite to decompose into graphite, forming an irreversible graphitised structure, rendering the steel unusable.

• Uneven Solidification of the Structure: If the cooling rate is not adequately controlled, repeated normalizing can cause the segregation of alloying elements to exhibit a periodic “banded distribution.” The initially chaotic segregation is “organized” into regular bands. This banded structure can lead to uneven hardness during subsequent quenching and may even become stress concentration zones under stress, which is more dangerous than random coarse grains.

After forging, alloy steel is normalized, and the grain size is found to be too large. What impact does multiple normalizing have on the material microstructure (2)

III. Core Panic: Will it form an “irreversible structure”?

This is the most chilling question. In metallurgy, there is indeed a situation in which structural stability reaches its limit, to the point that a phase transformation cannot occur within the conventional heat-treatment window.

1. The “Martensite Trap” in High-Alloy Steels

For high-alloy steels (such as high-chromium steel and austenitic stainless steel), if the normalising cooling rate is too fast (e.g., spray normalising), austenite may transform directly into high-hardness martensite or bainite. Once formed, these structures are complicated to dissolve during subsequent normalizing heating (usually below or near Ac3) due to the substantial diffusion hindrance imposed by the alloying elements, resulting in a very low austenite nucleation rate. The result is that, even after normalizing, it remains “unaffected,” retaining large martensite lath bundles, and even after high-temperature tempering, it still maintains its framework. This is a typical example of a “structure that cannot be completely transformed.”

2. Severe “Inherited Grains” and Texture

Under specific hot-deformation conditions (such as low temperature and small deformation), highly preferred-oriented “texture-rich large grains” or “coarse-grained nuclei” can form due to insufficient dynamic recovery. These coarse grains have extremely low dislocation density, serrated grain boundaries, and are rich in aggregated carbides. Repeated conventional normalising often only refines the periphery, while the core remains stubbornly resistant to recrystallisation. This mixed microstructure of “coarse core + fine periphery” may appear normal under metallographic examination, but under stress, the coarse core grains become a highway for rapid crack propagation.

3. Stable Carbonitrides

If the steel contains strong carbide-forming elements (Ti, Nb, V) and the normalising temperature is insufficient to dissolve them completely, these tiny carbonitrides will pin grain boundaries for a long time. Although this sounds like grain refinement, if the distribution is highly uneven, it can lead to incomplete austenitization in local areas, leaving behind free ferrite or special carbide aggregates after cooling. These microstructures are also “unyielding” anomalies in subsequent quenching and tempering processes.

IV. Breaking the Deadlock: How to Scientifically “Rework”?

Since repeated normalizing carries risks and may result in “irreversible” grain growth, the correct approach must be precise and ruthless:

1. Heat-increasing normalizing method: If the coarse grains are due to undissolved carbides, avoid repeating the same temperature. Increase the normalising temperature (e.g., 150-200℃ above Ac3) to forcefully dissolve the carbonitrides that hinder grain growth, utilise the rapid diffusion at high temperatures to homogenise the composition, and then rapidly air-cool (or even use forced air) to obtain bainite or sorbite, utilizing the phase transformation shear force to break up the grains.

2. Annealing as a buffer: If the microstructure is extremely heterogeneous after normalizing (e.g., severe banding or Widmanstätten structure), perform a full annealing first. Slow furnace cooling can maximize the elimination of internal stress and allow carbon atoms to diffuse, entirely reducing unevenness. After the microstructure is homogenised, perform another normalising step, which often yields remarkable results.

3. Controlling the critical zone: Strictly avoid the 5%-10% critical deformation range. For large forgings, the deformation in the final forging must be sufficiently large (>15-20%) to trigger adequate dynamic recrystallization, or multi-directional forging (drawing + upsetting) can be used to disrupt grain orientation.

4. The combination of normalising and tempering: When simple normalising is insufficient, a quenching and tempering pretreatment of “high-temperature normalising + high-temperature tempering” is used. Carbide precipitation during tempering disrupts the original austenite memory, creating entirely new nucleation sites for recrystallisation during the subsequent normalising process, thus completely severing the chain of “microstructure inheritance.”

Conclusion

The coarse grains in forged alloy steel are a manifestation of the material’s “disorder” at high temperatures. Multiple normalising processes are not simply a “Ctrl+Z” undo operation; they can be either a “furnace” for rebirth through recrystallisation or a “purgatory” leading to oxidation, segregation, solidification, or even the formation of abnormally stable phases.

Will it form an unchangeable microstructure? Yes! When segregation is exceptionally severe, the texture is significantly developed, or highly stable martensite unexpectedly forms, conventional normalising will be ineffective. In such cases, a decisive upgrade to the process is necessary, using higher temperatures, longer holding times, or annealing to “reset” the microscopic world. Remember, the essence of heat treatment lies not in simply piling up cycles, but in absolute control of temperature, time, and cooling rate. When dealing with coarse grains, either “melt” them at high temperatures or “refine” them through annealing; avoid meaningless cycling at the same temperature, as this only prolongs the steel’s lifespan, not saves it.