A Comprehensive Guide to Stainless and Acid-Resisting Steels

A Comprehensive Guide to Stainless and Acid-Resisting Steels

Stainless steel refers broadly to steels that resist corrosion in air, steam, water, and other mild corrosive media. In more aggressive chemical environments—acid, alkali, salts—steels with higher alloying content and enhanced corrosion resistance are sometimes called acid-resisting steels (or “acid steels”). In practice, steels employed for mild corrosion resistance are often denoted “stainless,” while those engineered for harsher chemical media are called “acid-resisting steels.” Because of differences in alloy composition, a stainless steel may not always survive in strong chemical environments; conversely, a true acid-resisting steel will typically also have the “stainless” property. The key to corrosion resistance lies in the alloying elements present.

Classification by Microstructure

Based on metallographic structure, common stainless / corrosion-resistant steels are classified as:

1. Austenitic Stainless Steels
   The matrix is mainly face-centered cubic (γ, austenite). These steels are typically nonmagnetic and are strengthened by cold working (though heavy cold work may introduce slight magnetic behavior). In the U.S. steel systems they appear in the 200 and 300 series (e.g. 304).

2. Ferritic stainless steels
   Matrix is body-centered cubic (α, ferrite). These steels are magnetic, generally not hardenable by heat treatment, but may gain slight strength from cold work. Common designations include 430, 446, etc.

3. Martensitic stainless steels
   The matrix is martensite (often body-centered tetragonal or modified). These are magnetic and can be heat treated to adjust strength and hardness. Typical designations include 410, 420, 440, etc.

4. Duplex (Austenite + Ferrite) stainless steels
   These combine austenite and ferrite phases. Usually at least ~15% of the minor phase is present. Duplex steels are magnetic, can be cold worked, and offer higher strength than simple austenitic steels. They also tend to have better resistance to stress corrosion cracking and chloride attack compared to purely austenitic grades.

5. Precipitation-hardening (PH) stainless steels
   These steels have a base microstructure (often austenite or martensite) but can be strengthened by precipitating secondary phases under heat treatment. Examples include 17-4PH, 15-5PH, etc.

6. High-alloy / Superalloy steels (low iron fraction)
   In extreme environments, steels may have alloy contents so high that iron comprises less than half of the material. These are sometimes classed among “acid-resisting steels” rather than “ordinary stainless steels.”

In general, among these types, austenitic steels offer the best general corrosion resistance. Ferritic steels are used in environments with mild corrosion demands. Where high strength or hardness is required in a corrosive environment, martensitic or precipitation-hardening steels may be chosen.

Thickness & Tolerance Considerations

• During rolling, the steel plate may have slight thickness variation: the middle often ends up slightly thicker than the edges due to deformation of the roll under heat and load. In national / industrial measurement practices, thickness is often gauged at the head-middle location.

• Tolerance arises from manufacturing and customer requirements. Steel sheets may be sold in “loose tolerance” or “tight tolerance” grades, depending on allowable deviation in thickness.