Treatment after fabrication of stainless steels is often needed for many reasons ranging from demand for a specific surface finish, specific mechanical properties and minimizing residual stresses to securing corrosion properties. A final cleaning process is often required when imperfections have occurred during manufacturing and restoration of the surface quality is needed. Several processes are used to achieve the very highest final results.
For a variety of reasons, some kind of treatment of stainless steels is often required after fabrication. The process environment or the application may demand a specific surface finish, a minimum of residual stresses, a desired microstructure, or certain mechanical properties. To secure the corrosion properties of a stainless steel component, pickling of the welds and removal of weld spatter are often required. In applications where a high fatigue load is expected, annealing to relieve residual stresses might be necessary.
Most of the commonly encountered corrosion problems, in environments where the stainless steel grade normally performs well, can be traced to inadequate cleaning of the surface following fabrication. For best performance, it is essential to prevent or remove all fabrication-related defects.
Surface defects and imperfections introduced during manufacturing operations may drastically disturb the self-healing process of stainless steel’s passive film and reduce resistance to several types of local corrosion. This means that a final cleaning process will often be required to restore an acceptable surface quality with regards to hygiene and corrosion.
The extent and methods of treatment will be determined by the corrosiveness of the environment, the inherent corrosion resistance of the steel grade, hygienic requirements, and/or aesthetic considerations.
Post-fabrication treatments can be broadly divided into cleaning procedures and heat treatment processes.
Cleaning procedures
Different chemical and mechanical methods, and sometimes a combination of both, can be used to remove different types of contamination. Generally, cleaning based on chemical methods produces superior results, since most effective mechanical methods tend to produce a rougher surface. While chemical cleaning methods reduce the risk of surface contamination, local regulations regarding environmental and industrial safety, as well as waste disposal problems, may limit their applications.
Grinding and polishing
Coarse-grit grinding removes deep defects like weld undercut and deep scratches. The grinding wheel or belt must be new or previously used only on stainless steel. Grinding should not heat the surface to such high temperatures that a yellow oxide layer forms. Coarse grinding should be followed by grinding with successively finer grit. If surface requirements are very exacting, polishing may be necessary as a final step.
Sand blasting
Sand and grit blasting can be used to remove high-temperature oxides as well as iron contamination. However, care must be taken to ensure that the sand (preferably of olivine type) or grit is perfectly clean and not previously used for carbon steel. Sand blasting can easily embed contaminants like dirt or iron particles that can cause rust discolorations on the stainless steel.
Shot peening
Shot peening may remove oxide, but is primarily used to give compressive surface stresses.
Brushing
For the removal of superficial heat tint, surface contamination, and dirt, brushing using stainless steel or nylon brushes often provides a satisfactory result. These methods do not cause any serious roughening of the surface, but do not remove any chromium-depleted layer beneath the oxide. They may leave a residual surface oxide, which is not visible to the naked eye but can nevertheless impair corrosion resistance.
Degreasing
Organic contaminants like cutting oils, drawing lubricants, or crayon marks prevent the stainless steel surface from being wetted during chemical cleaning with acids. These contaminants must be removed with a non-chlorinated solvent prior to final use or before any chemical cleaning treatment.
Acid pickling
Pickling creates the most corrosion-resistant surface of all cleaning methods. It uses strong acids that remove oxide scale and the underlying chromium-depleted layer. Pickling normally involves using an acid mixture containing nitric acid and hydrofluoric acid. These acid mixtures are hazardous and must be handled with due care and disposed of correctly. Agents containing chloride, such as hydrochloric acid, must be avoided, since there is an obvious risk of pitting corrosion.
As pickling dissolves the stainless steel surface, it must be carefully controlled. It produces clean surfaces with a dull gray, matte finish that passivate spontaneously. Pickling can be performed by immersing the stainless part in a bath or by spraying solution on the part while passing it through a spray box. Proprietary pickling products can also be applied locally in spray, gel, or paste forms.
Passivation
Chemical passivation is rarely needed for improved corrosion resistance and is not required if the stainless steel has been properly pickled. On the other hand, passivation is an effective way to clean stainless steel that has not been pickled. Although there are many passivation options available, the overwhelming choice is still nitric acid based solutions.
Electropolishing
Electropolishing is an electrochemical process that removes embedded iron, heat tint, and non-metallic inclusions. It smoothens the surface and leaves a shiny appearance. It is often used in applications where extreme cleanliness is important, for example in the pharmaceutical, semiconductor, and dairy industries.
Heat treatment
Heat treatment is not without risk, as the material properties can easily deteriorate if it is done in an incorrect and uncontrolled way. The wrong combination of temperature, time, and cooling rate may impair mechanical properties, impact toughness, and reduce corrosion resistance. In most cases, it is therefore usually safer to avoid heat treatment altogether, but there are certain situations when it is justified and it is often driven by code or application requirements. The main reasons for heat treatment are to restore the microstructure to improve properties, relax residual stresses to reduce risks of fatigue and stress corrosion cracking, and improve dimensional stability.
Solution annealing
Solution annealing softens stainless steel after cold working. Because it is conducted at high temperatures, annealing in air produces a surface oxide scale that must be removed by descaling and pickling to restore the surface corrosion resistance. The annealing temperature is normally 750 to 1,200 °C, depending on steel grade and the purpose of the annealing. Cooling should normally be as fast as possible – air cooling is normally fast enough but water quenching can be necessary for certain grades.
Stress relieving
Stress relief treatments reduce residual stresses that may develop during forming and welding, and lower the risk of distortion or stress corrosion cracking. Stress relief is performed at temperatures below those used for solution annealing, and may not always require the descaling and pickling needed after full annealing.
There are several procedures (temperature/time intervals) to perform stress relieving, each with its pros and cons. When selecting heat treatment, the steel grade’s susceptibility to precipitation of detrimental phases has to be considered. Furthermore, the shape of the work piece has to be taken into account, as large differences in thicknesses can give rise to new residual stresses during rapid cooling from high temperatures.
Glossary
Passivation
(1) A reduction of the anodic reaction rate of an electrode involved in corrosion, for example due to the presence of a passive film.
(2) Chemical treatment to improve the passive layer on stainless steel. Normally not needed if the steel has been properly pickled.