AUSTENITIC STAINLESS STEELS.

Austenitic stainless steels have sufficient alloying to stabilize austenite at room temperature. These steels being austenitic are nonmagnetic. Austenitic stainless steels have excellent low-temperature toughness, weldability, and corrosion resistance.

On the other hand, they have relatively low yield strength and can only be strengthened by cold working the steel, by precipitation hardening, or by interstitial or substitutional solid solution strengthening.

The table on pp. 15.1–15.4 of the ASM Metals Handbook, Desktop Edition, 1985, lists the composition limits of the austenitic stainless steels. In general, the 3xx series are iron – chromium–nickel alloys that contain 16–26% chromium and 6–22% nickel. The popular type 304 austenitic stainless steel contains 18–20% Cr and 8–12% Ni and is often referred to as ‘‘18-8’’ stainless steel for the chromium and nickel content. There are many compositional variations of austenitic stainless steels. The following list summarizes these variations:

201 Low nickel replaced with manganese and nitrogen

202 Higher Mn than 201

205 Higher Mn and N than 202

301 Lower Ni and Cr to increase work-hardening ability

302 General-purpose 18–8 stainless steel

302B Scaling resistance improved with Si

303 Enhanced machinability with a S addition

303Se Improved machined surfaces with a selenium addition

304 Popular 18–8 stainless steel, lower C than 302

304L Low-carbon 304 for improved corrosion resistance

304LN Low-carbon 304 with nitrogen added for strength

304H Higher carbon 304

304Cu Copper added for improved cold working

304N Nitrogen added for strength

305 Higher Ni for reduced work hardening

308 Higher Cr and Ni for weldability

309 High Cr and Ni for heat resistance

309S Lower carbon 309

309Cb Niobium (columbium) added

310 Higher Cr and Ni than 309 for improved heat resistance

310S Lower carbon 310

310Cb Niobium (columbium) added

314 Higher Si for improved heat resistance

316 Mo added for improved corrosion resistance

316F Higher S and P for machinability

316L Lower C for improved corrosion resistance and weldability

316LN Lower C and higher nitrogen (for strength)

316H Higher carbon 316

316N Nitrogen added for strength

316Ti Titanium added

316Cb Niobium (columbium) added

317 Higher Cr and Mo for improved corrosion resistance

317L Low-carbon 317 for improved weldability

321 Titanium added to minimize Cr carbide precipitation

330 High Ni to minimize carburization and improve thermal shock

347 Nb and Ta added to minimize Cr carbide precipitation

347H Higher carbon 347

348 Ta and Co added for restricted nuclear applications

348H Higher carbon 348

384 Higher Ni for decreased work hardening

The limiting of carbon is important in austenitic stainless steels. When heated, carbon forms chromium carbide that precipitates on the austenite grain boundaries and produces a condition known as sensitization. Because the chromium is tied up as carbide, the chromium adjacent to the boundaries will be depleted in chromium and corrosion can take place.

Sensitization is reversible by heating the steel to temperatures between 1040 and 1150 C followed by rapid cooling to room temperature. The high temperature dissolves the carbides and the rapid cooling prevents reprecipitation of the carbides.

Bruce L. Bramfitt

International Steel Group, Inc.

Research Laboratories

Bethlehem, Pennsylvania

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