Why does stainless steel need solution aging heat treatment?

Austenitic stainless steel needs solution treatment to soften. The process involves heating the stainless steel forging to about 950-1150°C and keeping it at this temperature for a period of time. This allows carbides and various alloying elements to dissolve evenly into the austenite. The steel is then rapidly quenched to prevent the precipitation of carbon and other elements, and finally a pure austenite structure is obtained. This process is called solution treatment.

Purpose of solution treatment:

1. Uniform composition: Solution treatment ensures that the microstructure and composition of the steel remain consistent. This is crucial for raw materials because different rolling temperatures and cooling rates during hot rolling can lead to inconsistent structures. High temperatures promote atomic migration, dissolve the σ phase, and obtain a uniform single-phase structure after rapid cooling.
2. Eliminate work hardening: This treatment helps eliminate work hardening, thereby facilitating further cold working. It can restore the distorted lattice and recrystallize the elongated and fragmented grains, thereby reducing internal stress. As a result, the tensile strength of forged stainless steel will decrease, while the elongation will increase.
3. Restoring corrosion resistance: Cold working causes carbide precipitation and lattice defects, which reduce the corrosion resistance of stainless steel. Solution treatment can restore this corrosion resistance to its original state.

The key factors in the solution treatment of stainless steel forgings include temperature, holding time and cooling rate. The solution temperature mainly depends on the chemical composition. Generally, for steels with a large number of alloying elements and contents (especially manganese, molybdenum, nickel and silicon), the solution temperature must be increased to ensure complete dissolution and thus effective softening.

However, for stabilized steels such as 1Cr18Ni9Ti, higher solution temperatures cause the stabilizing element carbides to dissolve into austenite, resulting in the precipitation of chromium carbides (Cr23C6) at the grain boundaries during the subsequent cooling process, thereby initiating intergranular corrosion. To prevent this, lower solution temperatures are usually used.

Stainless steel is often considered to have rust-proof properties, but in fact it is also acid-resistant. This property stems from the formation of a chromium-rich oxide layer (passivation film) on its surface. There is a relative relationship between rust-proof properties and corrosion-resistant properties.

Experimental evidence shows that as the chromium content increases, the corrosion resistance of steel increases in both weak media (such as air and water) and oxidizing environments (such as nitric acid). Once the chromium content reaches a certain threshold, the corrosion resistance changes significantly – from rust-prone to highly corrosion-resistant.

Stainless steel aging treatment:

Aging treatment is the process of solution treating a metal or alloy part (such as low carbon steel) and then holding it at elevated or room temperature for a period of time. This process affects the properties, shape, and size of the part over time. Generally, aging causes an increase in hardness and strength, while reducing plasticity and toughness.

High carbon steel reaches high hardness immediately after quenching, but has low plasticity. In contrast, aluminum alloys may not reach peak strength or hardness immediately after quenching; instead, their plasticity may increase. Over time (for example, 4-6 days at room temperature), these alloys will significantly increase in strength and hardness, while plasticity may decrease. This phenomenon of the properties of the quenched alloy changing over time is called aging. Aging at room temperature is called natural aging, while aging at elevated temperatures is called artificial aging.

Aging is an intentional process whereby a material is kept at room temperature or above for an extended period of time to produce an aging effect.