Stainless Steel 347

Alloy 347 is a columbium-stabilized austenitic stainless steel with good overall corrosion resistance and slightly better performance under strong oxidizing conditions than 321 (UNS S32100). It offers excellent resistance to intergranular corrosion after exposure to the chromium carbide precipitation temperature range of 800–1500°F (427–816°C). This alloy also provides good oxidation resistance and creep strength up to 1500°F (816°C), along with good low-temperature toughness. Alloy 347H (UNS S3409) is the high-carbon version (0.04–0.10% C) of the alloy, developed to provide improved creep resistance and higher strength at temperatures above 1000°F (537°C).

In most cases, the carbon content allows dual certification. Alloy 347 cannot be hardened by heat treatment, only by cold working. It can be easily welded and processed using standard shop fabrication practices.

Corrosion Resistance

Alloy 347 provides good overall corrosion resistance, comparable to that of Type 304. It was developed specifically for use in the chromium carbide precipitation range of 800–1500°F (427–816°C) where unstabilized alloys like 304 are prone to intergranular attack. Within this temperature range, Alloy 347 outperforms Alloy 321 in general corrosion resistance. Additionally, Alloy 347 performs slightly better than 321 in strongly oxidizing environments up to 1500°F (816°C). The alloy can be used in nitric acid solutions, most dilute organic acids at moderate temperatures, and in pure phosphoric acid at lower temperatures or in up to 10% diluted solutions at higher temperatures.

Alloy 347 resists polythionic acid stress corrosion cracking in hydrocarbon service. It is also suitable for use in chloride- and fluoride-free caustic solutions at moderate temperatures. However, Alloy 347 performs poorly in chloride solutions, even at low concentrations, and is not recommended for use in sulfuric acid environments.

Physical Properties

Mechanical Properties

Welding

Alloy 904L can be readily welded using most standard methods. Austenitic plate materials have a homogeneous microstructure with a uniform distribution of alloying elements. During solidification post-welding, some redistribution of elements such as molybdenum, chromium, and nickel occurs. These segregations remain in the cast structure of the weld and may affect corrosion resistance in specific environments. Segregation is less pronounced in 904L, which is typically welded with matching filler metal or even without filler metal in some cases.

Heat Treatment

Hot working temperatures between 1562°F and 2102°F (850–1150°C) are recommended. Normally, hot forming should be followed by solution annealing and quenching, but for 904L, if hot forming ends above 2012°F (1100°C) and the material is immediately quenched, post-treatment may not be required. It is critical that the entire workpiece be quenched from above 2012°F (1100°C). If the component is only partially heated or cooled below 2012°F (1100°C), or if cooling is too slow, then solution annealing and quenching must always follow. 904L should be solution-treated at 1940–2084°F (1060–1140°C).