Stainless Steel - Properties and Applications of Ferritic Grade Stainless Steel

New generation general-purpose ferritic grade AWM 404GP™ can be used to replace the workhorse austenitic grade 304 in most applications. it is a corrosion resistant ferritic stainless steel with excellent strength, toughness, fabrication characteristics and weldability. The general corrosion resistance of AWM 404GP™ is at least as good as grade 304, with better resistance to atmospheric corrosion, stress corrosion cracking and sensitization to intergranular corrosion. Grade AWM 404GP™ compared to grade 304.

Ferritic Grade AWM 404GP Stainless Steel

Grade AWM 404GP™ contains 21% of chromium and 0.5% of copper, which give it excellent resistance to corrosion in many environments. AWM 404GP™ has at least the corrosion resistance of grade 304, and is increasingly replacing 304 in sheet metal applications. many users of AWM 404GP™ make significant savings from the ease of fabrication – ferritic grades give less tool & machine wear than 304. Better quality products, with crisper, neater bends are achievable with AWM 404GP™. AWM 404GP™ is magnetic, which does not affect the excellent corrosion resistance given by the high chromium content. Thermal expansion is lower, so there is less thermal distortion of panels in fabrication and in service.

Chemcial Composition of Ferritic Stainless Steel

The chemical composition of different grades of ferritic stainless steel are summarised in the following table.

Table 1. Chemical composition of different grades of ferritic stainless steel

Mechanical Properties of Ferritic Stainless Steel

The mechanical properties of different grades of ferritic stainless steel are summarised in the following table.

Table 2. Mechanical properties of ferritic stainless steel

Physical Properties of Ferritic Stainless Steel

The physical properties of different grades of ferritic stainless steel are summarised in the following table.

Table 3. Physical properties of ferritic stainless steel

Applications of Ferritic Stainless Steel

Benchwork, cold-water tanks, whitegoods, refrigeration cabinets, chemical and food processing, water treatment plant, street furniture, electrical cabinets. AWM 404GP™ can be substituted for 304 in most applications except structures and pressure vessels.

Toughness of Ferritic Stainless Steel AWM 404GP

Toughness AWM 404GP™ has the same ferritic microstructure as carbon steel, so undergoes a ductile-to-brittle fracture mode transition at low temperatures. AWM 404GP™ is tough in the welded condition down to 0°C, and can be used for non-structural purposes at lower temperatures.

Heat Treatment of Ferritic Stainless Steel AWM 404GP

Heat Treatment AWM 404GP™ is not hardenable by heat treatment. Heat treatment is rarely required in sheet metal applications, but AWM 404GP™ can be annealed at 820 – 900°C, rapid air cool.

Corrosion Resistance of Ferritic Stainless Steel AWM 404GP

Corrosion Resistance The chemical composition of AWM 404GP™ gives it a higher Pitting Resistance equivalent (PRe) than 304, which indicates at least equivalent pitting corrosion resistance. Pitting is the form of corrosion stainless steels are often least resistant to. laboratory tests and service experience have not identified circumstances in which the corrosion resistance of AWM 404GP™ is inferior to 304, except pitting propagation rates, which are 3 – 4 times faster. unfortunately, once initiated, pitting corrosion propagation is rapid for both grades, and practical selection of stainless steel aims to avoid pitting corrosion rather than achieve low propagation rates.

Figure 1. Resistance to the initiation of pitting corrosion in artificial seawater at 30°C.
Shows equivalent resistance to pitting corrosion initiation for the general purpose grades 304 and AWM 404GP™

Atmospheric Corrosion of Ferritic Stainless Steel AWM 404GP

As a ferritic grade, AWM 404GP™ does not stress corrosion crack in drinking water, unlike 304 which may stress corrosion crack above about 50°C. The titanium in AWM 404GP™ prevents sensitization to intergranular corrosion by welding.

Figure 2. Test Conditions: Marine Industrial atmosphere at Chiba, Tokyo, Japan (10m from a bay bigger than Melbourne Bay) Samples polished to #600 grit. Exposed for 12 months.
Note: these grades would not normally be specified for these aggressive conditions, which require 316 or 445M2.

High Temperature Applications of Ferritic Stainless Steel AWM 404GP

The high chromium content of AWM 404GP™ gives excellent oxidation and sulphidation resistance, and similar grades are used extensively in automotive muffler systems. AWM 404GP™ can be used in air up to 980°C for continuous service, 1035°C for intermittent service. The strength of ferritic grades above about 600°C is generally less than that of austenitic grades. Where later ambient temperature toughness and corrosion resistance are important, the service temperature should be restricted to 400°C to avoid precipitation reactions.

Machinability of Ferritic Stainless Steel AWM 404GP

Machinability AWM 404GP™ is easier than 304 to machine due to lower work hardening rate and better thermal properties. Cleaner cuts and flatter panels are obtained, with better cutting tool life.

Weldability of Ferritic Stainless Steel AWM 404GP

Weldability AWM 404GP™ can be welded by the common methods (TiG, miG, spot, seam), without preheat, post heat or post weld heat treatment. Heat input needs to be 10 – 30% higher than for grade 316 as heat is conducted away from the weld pool more quickly by AWM 404GP™. Manual metal arc welding (mmaW or stick welding) is not recommended – AWM 404GP™ is used in light gauges, not easy to stick weld. AWM 404GP™ can be TiG welded without filler metal, or with 308l(Si) or 309l(Si) filler metal. Avoid introducing carbon into the weld by degreasing before welding – degreasing is as important with AWM 404GP™ as it is with aluminium. Gas shielding must be excellent – use argon, or argon plus helium or up to 3% CO₂. Never use nitrogen or hydrogen in welding or backing gases. For best pickling after welding, ensure oxidation is minimised and use the pickling paste for longer than with 304.

Cleanability of Ferritic Stainless Steel AWM 404GP

Cleanability AWM 404GP™ is excellent for food equipment, where cleanability for the removal of bacteria is vital. AWM 404GP™ resists corrosion by food and cleaning chemicals, and harbours very low levels of bacteria after cleaning. Fabrication and Formability as a ferritic stainless steel, AWM 404GP™ behaves like carbon steel (G300). Bends are neater, with low springback and forming loads. Wear on cutting and forming tools is typically 3 – 5 times better. Cutting tool clearances are similar to carbon steel. The minimum radius for bending is 1t. Deep drawability is better than 304, but stretch capability is lower. Please consult Austral Wright Metals for deep drawing applications.

This information has been sourced, reviewed and adapted from materials provided by Austral Wright Metals - Ferrous, Non-Ferrous and High Performance Alloys.

For more information on this source, please visit Austral Wright Metals - Ferrous, Non-Ferrous and High Performance Alloys.