The Quality of 304 Stainless Steel Surface: Heat Treatment and Acid Pickling

The quality of the surface of 304 stainless steel is intricately linked to the post-heat treatment acid pickling process. If the preceding heat treating steel phase leaves uneven surface oxidation or an inconsistent structure, mere acid pickling fails to enhance surface smoothness and uniformity. Hence, meticulous attention to pre-treatment surface cleansing or heating is imperative.

In instances where the thickness of surface oxide varies across stainless steel plates, disparities emerge in the underlying base metal's surface smoothness between thick and thin oxide areas. Moreover, during pickling acid for stainless steel, the dissolution of surface oxide differs from the extent of acid corrosion on the base metal beneath the oxide, resulting in surface unevenness across the steel plate.

Consequently, during heat treatment heating, it's essential to uniformly form surface oxide. To meet this demand, several considerations must be made:

Surface Contamination Control: During steel heat treating, if oil adheres to the workpiece surface, varying thicknesses and compositions of oxide develop between oil-affected and unaffected areas, potentially leading to carburization. Areas where the base metal is carburized beneath the oxide will suffer severe acid corrosion. Initial oil droplets emitted during heavy oil burner ignition pose significant risks if they adhere to the workpiece. Even fingerprints from operators can cause interference. Hence, operators should avoid direct contact with stainless steel parts and prevent workpieces from accumulating fresh oil stains. Clean gloves must be worn during operations. If lubricating oil from cold working adheres to the workpiece surface, it must be thoroughly degreased with trichloroethylene or caustic soda solution, followed by warm water rinse before heat treatment.

Foreign Material Contamination: The presence of impurities, particularly organic substances or ash, on the workpiece surface affects oxide formation during heating.

Variations in Furnace Atmosphere: Discrepancies in furnace atmosphere across different regions lead to varying oxide formation, contributing to post-acid pickling unevenness. Hence, consistent atmosphere distribution throughout the furnace during heating is crucial, necessitating the consideration of atmosphere circulation.

Residual Oxide Before Heat Treatment: If residual oxide persists on localized areas of the workpiece before steel heat treatment, differences in oxide thickness and composition emerge post-heating, resulting in uneven surfaces after acid pickling. Therefore, equal attention must be given to intermediate heat treatments and acid pickling, not just the final heat treatment.

Direct Flame Contact: Oxide formation differs between areas directly contacted by gas or oil flames and those that are not. Hence, during heating, workpieces should not directly touch flame outlets.

Impact of Surface Smoothness on Oxide Formation: Variances in surface smoothness lead to differing oxide formations even with identical heating. For instance, areas cleaned of local defects versus those left uncleansed exhibit different oxide formation, causing post-acid pickling surface irregularities.

In essence, achieving impeccable surface quality in 304 stainless steel demands a symphony of meticulous processes, from thorough pre-treatment surface cleansing to uniform heat treatment and precise acid pickling, each step orchestrated to perfection.