how to harden SUS 316 stainless steel?

Its hardening needs to be achieved through the following methods, and the specific selection needs to be combined with the application scenario (such as whether overall hardening, surface wear resistance or corrosion resistance is required):

1. Cold work hardening (main means)

Principle:
Through cold working processes such as cold rolling, cold drawing, and cold forming, the material undergoes plastic deformation, resulting in dislocation proliferation, grain distortion, and work hardening (strain hardening), thereby improving hardness and strength.

Features:
Significant hardness improvement: The hardness of annealed SUS 316 is about HB 150-180, which can be increased to HB 200-250 after cold working (depending on the amount of processing, such as the hardness can reach HB 230 or more when the cold rolling deformation is 30%).

Synchronous improvement in strength: The tensile strength is increased from about 520 MPa to 700-900 MPa, but the plasticity decreases (the elongation is reduced from 40% to less than 20%).
No impact on corrosion resistance: Cold working has little effect on the corrosion resistance of the austenite matrix, and is suitable for scenarios that require both strength and corrosion resistance (such as chemical equipment and medical equipment).

2. Surface hardening treatment (local strengthening)
If only surface wear resistance is required (such as bearings and seals), the surface hardness can be improved through the following processes while retaining the toughness and corrosion resistance of the matrix:

1. Nitriding/nitrocarburizing
In an environment of 500-600℃, nitrogen atoms are infiltrated into the surface to form a hardened nitride layer (such as γ'-Fe4N, ε-Fe3N), with a hardness of up to HV 800-1200.

Advantages: The temperature is lower than the sensitization range (450-850℃), no carbides are precipitated, and the corrosion resistance remains basically unchanged.

2. Carburizing
Carbon atoms are infiltrated at high temperatures (900-1050℃) to form a hardened layer, but the surface corrosion resistance may decrease due to the precipitation of carbides, and solid solution treatment is required.
3. Physical Vapor Deposition (PVD) / Chemical Vapor Deposition (CVD)
Deposit hard coatings (such as TiN, TiC, CrN) with hardness up to HV 2000-3000 and thickness of 1-10μm, suitable for precision parts.

3. Heat treatment strengthening (limited application)
SUS 316 cannot be hardened by quenching (no martensitic phase transformation), but can be assisted by the following heat treatments:

1. Solution treatment + cold working
First heat to 1050-1100℃ to completely dissolve the carbides, quickly water cool (solution treatment) to obtain a single austenite structure, and then cold work for a more uniform hardening effect.

2. Stabilization treatment (for steels containing titanium/niobium)
Steels containing stabilizing elements such as SUS 321 can fix carbon through titanium/niobium, but SUS 316 does not contain stabilizing elements, so this method is not applicable.

Cold work hardening is reversible. If it is subsequently heated to the recrystallization temperature (about 800°C or above), the hardness will decrease due to grain recovery.
The above method can effectively improve the hardness of SUS 316 stainless steel without reducing the core corrosion resistance. The most appropriate process should be selected according to the working conditions (such as load, environment, temperature).