347 347H 1.4550 Heat Treatment

The annealing temperature range for Alloys 321 and 347 is 1800 to 2000°F (928 to 1093°C). While the primary purpose of annealing is to obtain softness and high ductility, these steel may also be stress relief annealed within the carbide precipitation range 800 to 1500°F (427 to 816°C), without any danger of subsequent intergranular corrosion.

Relieving strains by annealing for only a few hours in the 800 to 1500°F (427 to 816°C) range will not cause any noticeable lowering in the general corrosion resistance, although prolonged heating within this range does tend to lower the general corrosion resistance to some extent. As emphasized, however, annealing in the 800 to 1500°F (427 to 816°C) temperature range does not result in a susceptibility to intergranular attack.

For maximum ductility, the higher annealing range of 1800 to 2000°F (928 to 1093°C) is recommended.

When fabricating chromium-nickel stainless steel tube into equipment requiring the maximum protection against carbide precipitation obtainable through use of a stabilized grade, it is essential to recognize that there is a difference between the stabilizing ability of columbium and titanium. For these reasons, the degree of stabilization and of resulting protection may be less pronounced when Alloy 321 is employed.

When maximum corrosion resistance is called for, it may be necessary with Alloy 321 to employ a corrective remedy which is known as a stabilizing anneal. It consists of heating to 1550 to 1650°F (843 to 899°C) for up to five hours depending on thickness. This range is above that within which chromium carbides are formed and is sufficiently high to cause dissociation and solution of any that may have been previously developed.

Furthermore, it is the temperature at which titanium combines with carbon to form harmless titanium carbides. The result is that chromium is restored to solid solution and carbon is forced into combination with titanium as harmless carbides.

This additional treatment is required less often for the columbium-stabilized Alloy 347.

When heat treatments are done in an oxidizing atmosphere, the oxide should be removed after annealing in a descaling solution such as a mixture of nitric and hydrofluoric acids. These acids should be thoroughly rinsed off the surface after cleaning.

These alloys cannot be hardened by heat treatment.

Cleaning
Despite their corrosion resistance, stainless steel need care in fabrication and during use to maintain their surface appearance even under normal conditions of service.

In welding, inert gas processes are used. Scale or slag that forms from welding processes is removed with a stainless steel wire brush. Carbon steel wire brushes will leave carbon steel particles in the surface which will eventually produce surface rusting. For more severe applications, welded areas should be treated with a descaling solution such as a mixture of nitric and hydrofluoric acids to remove the heat tint, and these acids should be subsequently washed off.

For material exposed to inland, light industrial, or milder service, minimum maintenance is required. Only sheltered areas need occasional washing with a stream of pressurized water. In heavy industrial areas, frequent washing is advisable to remove dirt deposits which might eventually cause corrosion and impair the surface appearance of the stainless steel.

Design can aid cleanability. Equipment with rounded corners, fillets, and absence of crevices facilitates cleaning as do smooth ground welds and polished surfaces.

SA 213 TP 347 347H