Stainless steel 302 AISI - Z12CN18-09

Strengths and weaknesses of the chemical composition of AISI 302

AISI 302 is an alloy mainly composed of chromium and nickel, with a relatively high carbon content (0.15 % max) for an austenitic steel. While it allows high mechanical strength through work hardening, this carbon content, although limited compared with a tool steel, is still high enough to risk carbide formation; the alloy therefore becomes vulnerable to intergranular corrosion. As noted, this is why 304—very low carbon—was developed and has supplanted 302 for welded structures.

Its maximum manganese content (2.0 %) increases steel strength, improves wear resistance and acts as a de-oxidizer during manufacturing. Chromium (17.0 % to 19.0 %) forms a protective oxide layer that enhances corrosion resistance.

However, this resistance varies with the environment: it performs poorly in seawater, which can cause pitting corrosion, due to the absence of molybdenum—a feature present in AISI 316. It nevertheless offers good heat resistance, up to 1697 °F in continuous service and 1598 °F intermittently. Corrosion resistance can be improved by polishing, which removes micro-cracks, areas where corrosive agents can “catch”. But it can decrease after cold forming: work hardening of 302 can induce a local phase transformation and part of the austenite may convert to martensite, which is less corrosion-resistant. It also introduces residual stresses, areas that promote stress-corrosion cracking.

Using 302: Heat treatment, machining and forming

AISI 302 belongs to the family of austenitic stainless steels; it therefore cannot be hardened by quenching, because it does not form martensite upon cooling. However, it can be annealed between 1850 °F and 2048 °F, followed by rapid cooling. This step is imperative to preserve corrosion resistance by preventing carbide precipitation. The method depends on the thickness of the part: air cooling is sufficient for thin sections, whereas water quenching is necessary for thicker sections to ensure a sufficient cooling rate at the core.

With a high work-hardening rate, it is suitable for forming (bending, deep drawing…) and cold heading. However, as mentioned, the material work-hardens strongly, leading to increased hardness but also decreased corrosion resistance.

Because of its high work-hardening rate and its low thermal conductivity (about 16.2 W/m·K at 212 °F) preventing heat from dissipating during cutting, 302 has poor machinability. It is therefore not used for automatic turning, unlike AISI 303 which contains sulfur to break chips, greatly improving machinability.

Mechanical properties

AISI 302 stainless steel offers a high tensile strength after work hardening, notably thanks to its slightly higher carbon content than AISI 304. It ranges between 515 MPa and 2200 MPa, depending on the treatment condition (annealed or work-hardened), and can be even higher for very small-diameter wires. Its elongation at break ranges from about 60 % in the annealed state, in which it is very ductile, to only a few percent in the most work-hardened state; its hardness ranges between 150 HB (annealed) and 400 HB (work-hardened).

Weldability and forging

Because of its relatively high carbon content, 302 becomes sensitized during welding, which leads to chromium carbide precipitation, the disappearance of the work-hardened structure, and vulnerability to intergranular corrosion in the heat-affected zone. Its weldability is therefore poor, even very poor for applications where corrosion resistance is important.

To counterbalance these shortcomings, it is recommended to use a filler metal with very low carbon (such as 308L) to dilute the carbon in the weld pool. Finally, a solution-annealing treatment is advised after welding to redissolve the carbides.

Open-die forging and closed-die forging are possible for this steel, which also offers good malleability. The material must first be heated slowly to about 2102 °F to 2192 °F, a range in which it is fully austenitic and very ductile. Forging can then be carried out between 2192 °F and 1652 °F to avoid cracking, with rapid cooling in water (thick parts) or in air (thin parts).

Industrial applications of a spring steel

AISI 302 stainless steel is used in the aerospace, automotive and appliance industries. It is called “spring steel” because of its mechanical properties suited to this type of application, which is also the most common. It is therefore found in:

• Compression springs: designed to be compressed, such as helical springs in suspensions or shock absorbers.
• Extension springs: designed to be stretched, typically helical springs fitted with hooks for return systems.
• Torsion springs: which work in rotation, like those found in clothes pegs or hinge mechanisms.
• Flat springs: such as Belleville washers or retaining clips.
• But also fasteners, staples and other non-critical parts