Heat Treatment of 310S Stainless Steel

The primary reason for annealing these alloys is to produce a recrystalized microstructure with a uniform grain size and for dissolving detrimental chromium carbide precipitates. To ensure complete annealing, pieces should be held in the range 2050-2150°F (1120-1175°C) for approximately 30 minutes (time at temperature) per inch of section thickness. This is a general recommendation only – specific cases may require further investigation. When properly annealed, these grades are primarily austenitic at room temperature. Some small quantities of ferrite may be present.

Oxide scale formation is inevitable during air annealing of Alloys 309/309S and 310/310S. The scale that forms is generally rich in chromium and relatively adherent. The annealing scale generally must be removed prior to further processing or service. There are two typical methods for removing scale – mechanical and chemical. A combination of surface blasting prior to chemical scale removal is generally effective at removing all but the most tightly adherent scale. Silica sand or glass beads are a good choice for the blasting media. Iron or steel shot can also be used but will lead to embedded free iron in the surface which may then result in surface rusting or discoloration unless the surface is subsequently pickled.

Chemical removal of scale is generally performed with mixed nitric-hydrofluoric acids. The proper bath makeup and process temperature combination depends on the situation. A typical pickling bath used consists of 5-15% HNO3 (65% initial strength) and 1/2 -3% HF in aqueous solution. Higher concentrations of hydrofluoric acid lead to more aggressive scale removal.

Bath temperature generally range from ambient to about 140°F (50°C). Higher temperature result in faster descaling but may attack grain boundaries aggressively, resulting in surface grooving. Acid pickling must be followed with a thorough water wash to remove all traces of pickling acids. Drying should then be used to avoid spotting and staining.

As noted, Alloys 309/309S and 310/310S consist solely of austenite at room temperature – they cannot be hardened through heat treatment. Higher mechanical strengths are attainable via cold or warm working, but these grades are generally not available in such conditions. The higher tensile strength and yield strength obtainable through cold working not followed by full annealing are not stable at the higher temperatures at which these alloys are often used. Creep properties in particular may be adversely affected by the use of cold worked material at elevated temperatures.

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