English Views: 0 Author: Eternal Industry Tech Team Publish Time: 2026-07-24 Origin: Site
To understand this issue, one must first grasp the fundamental difference between austenitic stainless steel and other types of steel. Stainless steel is classified into ferritic, austenitic and martensitic types.
In heat treatment, there is a term known as ‘austenitisation’. For materials such as 40Cr, 42CrMo and 45 steel, the equilibrium microstructure at room temperature is pearlite, sorbite or troostite; however, when heated to high temperatures, the microstructure transforms entirely into austenite (i.e. a phase transformation occurs). During cooling, the austenite then decomposes and transforms into other microstructures. The grain size and microstructural morphology of these steels can be altered through heating and cooling.
Austenitic stainless steel, however, is different. It is austenitic at room temperature and remains austenitic when heated to high temperatures; there is no phase transformation involving a ‘change from one crystal structure to another’. To put it plainly, its crystal structure remains unchanged from start to finish.
Conventional methods for grain refinement rely on re-nucleation during phase transformations—where the old phase disappears and a new phase nucleates and grows simultaneously at multiple locations, thereby ‘dividing’ coarse grains into fine ones.
Austenitic stainless steel lacks this mechanism. During solution treatment, it merely dissolves carbides and alloying elements into the austenitic matrix, homogenising the composition, and rapidly cools the material to ‘lock’ these elements within the solid solution. As no new phases are nucleated during this process, it is naturally impossible to refine the grain size through phase transformation.
Not only does solution treatment fail to refine the grain size, but it may actually lead to grain growth. Research data indicates that when the solution treatment temperature exceeds 1100 °C, austenitic grains gradually grow; at 1200 °C, grain size can increase to over 100 μm. Case studies show that for a certain 00Cr17Ni14Mo2 forging, the austenitic grain size, which was Grade 6.2 in the as-forged condition, actually increased to Grade 5.4 following solution treatment.
The purpose of solution treatment for austenitic stainless steel is not to increase hardness or strength, but to prevent intergranular corrosion.
The solubility of carbon in austenite is very low, at approximately 0.02%–0.03%, whilst the carbon content of ordinary austenitic stainless steel is often higher than this value, meaning the carbon is in a supersaturated state. When the steel is held within the sensitisation temperature range (approximately 427 °C to 816 °C), the supersaturated carbon diffuses towards the grain boundaries, where it combines with chromium to form chromium carbides (primarily of the Cr₂₃C₆ type) that precipitate at the grain boundaries. As chromium diffuses slowly within the grains and cannot reach the grain boundaries in time, ‘chromium-depleted zones’ with a chromium content below 12 per cent form on either side of the grain boundaries. The corrosion resistance of these zones is significantly reduced, leading to intergranular corrosion in corrosive media.
The solution treatment process involves heating the steel to a sufficiently high temperature (typically 1000 °C to 1150 °C) to allow the carbides to dissolve completely, followed by rapid cooling (e.g. water quenching). This ‘locks’ the carbon and alloying elements within the austenite grains, preventing their precipitation at the grain boundaries and thereby ensuring that the chromium content near the grain boundaries remains sufficiently high to resist intergranular corrosion.
Since solution treatment cannot refine the grain size, what determines the grain size of austenitic stainless steel?
The answer is simple: forging (pressure working).
After austenitic stainless steel undergoes thermoplastic deformation such as forging, the grains are fragmented and elongated, and a large number of dislocations and deformation bands are generated within the grains, storing a high amount of deformation energy. Upon subsequent heating (which may be the residual heat after forging or the heating process during solution treatment), these deformed structures undergo recrystallisation—forming new, strain-free grains within the deformed matrix, thereby refining the originally coarse microstructure.
Research indicates that the extent of recrystallisation depends on several factors: the higher the strain energy stored, the lower the recrystallisation temperature; the finer the grains of the original microstructure, the more readily recrystallisation occurs; and raising the recrystallisation temperature can significantly accelerate the recrystallisation process. This is why the forging ratio and forging process are crucial to the final grain size of austenitic stainless steel—increasing the forging ratio is an effective means of refining the grains in austenitic stainless steel.
In the case of austenitic stainless steel, the matter is not quite so straightforward.
From the perspective of strength, the finer the grain, the greater the number of grain boundaries; as grain boundaries act as barriers that impede dislocation movement and prevent deformation, a fine grain size does indeed enhance strength. However, in terms of corrosion resistance and radiation resistance, excessively fine grains are actually detrimental—a high number of grain boundaries implies a greater number of potential weak interfaces. Generally speaking, it is ideal for the grain size of austenitic stainless steel to be controlled within grades 3 to 5. If the grain is too fine (e.g. grades 8 to 9), this is detrimental to resistance to intergranular corrosion and radiation; if it is too coarse (e.g. grades 1 to 2), the strength is insufficient. The standards of some valve manufacturers specify that the grain size of austenitic stainless steel forgings must fall within the range of grades 5 to 7.
The fundamental reason why solution treatment of austenitic stainless steel cannot alter grain size is that it does not involve a phase transformation—the material remains austenitic at both high temperatures and room temperature, and there is no mechanism for grain refinement via phase transformation and re-nucleation. The purpose of solution treatment is to dissolve carbides, prevent precipitation at grain boundaries, and resist intergranular corrosion. The method that truly refines grain size is forging—achieved by driving recrystallisation through the energy stored during deformation. As for grain size, maintaining it at grades 3 to 5 is a reasonable choice that balances strength, corrosion resistance and radiation resistance.
