type stainless stels2

Ferritic Stainless Steels:

This group of alloys generally containing only chromium, with the balance mostly Fe. These alloys are somewhat less ductile than the austenitic types and again are not hardenable by heat treatment. These are plain chromium stainless steels with varying chromium content between 12 and 18%, but with low carbon content.

Basic properties:

- moderate to good corrosion resistance increasing with chromium content

- not hardenable by heat treatment and always used in the annealed condition magnetic

- weldability is poor

- formability not as good as the austenitics

Common uses

computer floppy disk hubs (430)

automotive trim (430)

automotive exhausts (409)

colliery equipment (3CR12)

hot water tanks (444)

Martensitic Stainless Steels:

Martensitic stainless steels were the first stainless steels commercially developed (as cutlery) and have relatively high carbon content (0.1 - 1.2%) compared to other stainless steels. They are plain chromium steels containing between 12 and 18% chromium.

Basic properties

moderate corrosion resistance

can be hardened by heat treatment and therefore high strength and hardness levels

can be achieved

poor weldability

magnetic

Common uses

Knife blades

surgical instruments

shafts

spindles

pins

Precipitation-Hardening Steels:

These steels have been formulated so that they can be supplied in a solution treated condition, (in which they are machinable) and can be hardened, after fabrication, in a single low temperature "aging" process.

Basic properties

Moderate to good corrosion resistance

very high strength

good weldability

magnetic

Common uses

Shafts for pumps and valves.

Duplex Stainless Steels:

These are stainless steels containing relatively high chromium (between 18 and 28%) and moderate amounts of nickel (between 4.5 and 8%). The nickel content is insufficient to generate a fully austenitic structure and the resulting combination of ferritic and austenitic structures is called duplex. Most duplex steels contain molybdenum in a range of 2.5 - 4%.

Basic properties:

high resistance to stress corrosion cracking

increased resistance to chloride ion attack

higher tensile and yield strength than austenitic or ferritic steels

good weldability and formability

Common uses

marine applications, particularly at slightly elevated temperatures

desalination plant

heat exchangers

petrochemical plant

Chloride Stress Corrosion Cracking (CSCC):

Austenitic stainless steels may be susceptible to chloride stress corrosion cracking (CSCC). The standard 304/304L and 316/316L grades are most susceptible. Increasing nickel content above 18 to 20% or the use of duplex, or ferritic stainless steels improves resistance to CSCC. High residual or applied stresses, temperatures above 65-71C (150-160F) and chlorides increase the likelihood of CSCC. Crevices and wet/dry locations such as liquid vapor interfaces and wet insulation are particularly likely to initiate CSCC in susceptible alloys. Initiation may occur in several weeks, in 1-2 years or after 7-10 years in service.

Methods of minimizing chloride stress corrosion cracking:

Improved design.

Examples: Fill or seal crevices, paint under insulation, keep tensile stresses below the yield strength, shot peen, provide galvanic or cathodic protection.

Select a higher nickel content austenitic alloy.

Examples: Alloy 330, 904L.

Select a ferritic stainless steel if the lower corrosion resistance is acceptable.

Examples: 439, 26Cr 1Mo, 18Cr 2Mo

Select a duplex stainless steel.

Examples: 329, 2205.

Evaluate stress relief.

Note! Stress relief treatments above 425C (800F) may sensitize stainless steel to intergranular corrosion.

Thermal conductivity

All stainless steels have a much lower conductivity than that of carbon (mild) steel (plain chromium grades approximately 1/3 and austenitic grades approximately 1/4). This must be kept in mind for any operation which involves high temperature, e.g. effects during welding (control of heat input), longer times required for heating to attain a uniform temperature for hot working.

Expansion coefficient

Plain chromium grades have an expansion coefficient similar to carbon (mild) steels, but that of the austenitic grades is about 11/2 times higher. The combination of high expansion and low thermal conductivity means that precautions must be taken to avoid adverse effects, e.g. during welding use low heat input, dissipate heat by use of copper backing bars and use adequate jigging. This factor must also be considered in components which use a mixture of materials, e.g. a heat exchanger with a mild steel shell and austenitic grade tubes.