Stainless Steel Knowledge Centre

The definite process for a grade of stainless steel differs in the later stages. How a grade of steel get into shape, worked and finished plays a very important role in determining how it will look and performs.most steel grades share common starting steps.

These are:

STEP 1: MELTING PROCESS

Manufacturing stainless steel starts with melting the scrap metals and additives in an electric arc furnace (EAF). Using high-power electrodes, the electric arc furnace heats the metals over many hours to create a molten, fluid mixture.stainless steel is 100% recyclable and so many stainless orders contain as much as 60% recycled steel. This helps to not only control costs but reduce the impact on environmental.

STEP 2: REMOVING THE CARBON CONTENT

Carbon helps to refine the hardness and strength of iron. However, too much of carbon quantity can create problems—such as carbide precipitation during welding.Before casting molten stainless steel, calibration and reduction of carbon content to the proper level are essential.There are two ways foundries control carbon content. The first is through Argon Oxygen Decarburization (AOD) or Second through Vacuum Oxygen Decarburization.

STEP 3: TUNING PROCESS

After reducing the carbon content, there is final balancing and homogenization of temperature and chemistry. This ensures that the metal meets requirements for the grade that has been intended and that the steel’s composition is consistent throughout the batch. Samples are tested and analyzed. Adjustments are then made until the mixture match the required standard.

STEP 4: FORMING OR CASTING

With the molten steel created, the foundry should now create the primitive shape used to cool and work the steel. The exact shape and dimensions will depend on the final product that is produced.Some of the Common shapes include:

• Slabs
• Rods
• Blooms
• Billets
• Tubes

Forms are then marked with an identifier to track the batch through the various processes to follow. From here steps will start to differ depending on the intended grade and final product or function. Depending on the grade or format ordered, stainless steel might go through some of these steps multiple times to give it the desired characteristics. Following are the most common steps:

• Hot Rolling
• Cold Rolling
• Annealing
• De - scaling or Pickling
• Cutting
• Finishing

Everybody prefers stainless steel because of its corrosion-resistant properties. The resistance levels and weakness of a stainless depend largely on its metallurgic composition and how it is made.Let’s know what makes stainless so resistant.

Stainless Steel grades may differ in their metallurgic composition and alloying or the process of manufacturing, But the trait of the passivation layer is the same among them. A layer of chromium oxide forms when the surface of the metal is exposed to the oxygen. This layer is too thin to observe directly and in most of the circumstances, it remains clear. This allows the finished texture of a stainless steel product to provide a variety of appearance options from high gloss to even matte. This passivation layer is self-generating and it serves to protect the metal. While it might become colourless under extreme temperatures, it will still be working

Which stainless steel corrodes, it happens only through sustained contact with liquids or harmful chemicals. Most stainless steels respond in a particular behaviour to a range of chemicals.Exposure to the following may increase the corrosion risks.

ACIDS

Stainless steel is resistant to acidic corrosion. But the exact resistance levels generally depend on the steel in use, the concentration levels, and types of acid, and temperature. For example:

• Grades 316 and 317 offer resistance to phosphoric acid & sulphuric acid at most at concentrations.
• Types 304L and 430 is resistant to nitric acid.
• Grade 904 offers resistance to high concentrations of sulfuric acid.

BASES

Stainless steel offers great resistance to bases in both high concentrations & high-temperature environment. whereas stronger bases will cause cracking. Due care must be taken with chloride solutions like sodium hypochlorite.

ORGANICS

Resistance to organics differs to an extent. 300-series steels perform best with these solutions. Definite recommendations vary based on environmental factors such as temperature and levels of oxygen.

Stainless steel corrosion can take on different forms. Following are the ways to avoid them.

1. GALLING

When stainless steel parts are forced together Galling takes place. It can be seen in nuts, bolts, and other fasteners. It is not visible until the parts are disassembled. With threaded parts and lubrication, one can reduce galling.

2. CREVICE CORROSION

It occurs when a crevice between the stainless steel and some other material allows chlorides to prevent optimum oxygen levels to regenerate the layer of steel’s oxide. It is found near or generally within the gap between two metals, it can also occur between metallic and some non-metallic surfaces.Welding, proper drainage and using of proper gaskets can help to prevent it, as all these things help to decrease access to crevices or null them.

3. WELD DECAY

It is also known as the intergranular corrosion, it results from heating of stainless steel to a specific temperature range— generally between 550-degree celsius and 850-degree celsius. It causes chromium to precipitate from the stainless steel and thus reduces the ability for the passivation layer to come over it.low carbon steels such as 304L and 316L when welding can help to reduce the impact of the decay of the weld. Titanium or Niobium stabilize the steel and reduce precipitation during the welding process. Post -weld heat treatment is utmost important to minimize corrosion.

4. GALVANIC

It is also known as bimetallic corrosion, It occurs when stainless steel is used with other metals especially in an environment that is filled with moist. Rain, condensation or other moisture sources may cause one metal to corrode at an accelerated rate. Characteristics of galvanic corrosion vary based on metals used, temperatures and other factors.

5. PITTING

This is generally a result of exposure to chemicals in the environment or poor aeration. It is common in steels with manufacturing defects. Ample oxygen availability may help to reduce the rate of pitting. Steels having molybdenum also have higher abilities to resist pitting.

1. ELECTROLYTES

While stainless works great with dissimilar metals, one may encounter problems in moist environments. If it is not properly grounded, moisture can create an electrolytic cell that can drastically speed up degradation and leads to corrosion. Smaller items like fasteners, hinges, and handles must be inspected regularly.

2. APPLYING TREATMENTS

One should consider applying a treatment of passivation or wax paste after the installation process. Passivation treatments help remove free iron and other contamination that may find its way to your stainless through handling, fabrication, or environmental exposure during installation. Wax paste adds a layer of protection and further improves the performance of the passivation layer.

3. CLEANING METHODS

One can clean stainless with warm water. Using stainless steel in areas with drainage can help to speed up the process of maintenance —especially in food preparation environments.

Always take appropriate precautions when cleaning your steel to protect yourself and also the metal. The following tips may help ease the cleaning process:

• Do not use an abrasive on stainless steel.
• Always use appropriate safety gear like Goggles, gloves, and other protective gears.
• Use cleaners in a ventilated environment.
• Always add water to the acid and not acid to water.

Care & maintenance can improve the appearance and increase the life of a stainless steel product. There are many treatments for this. But Passivation is one of the best treatments to remove contaminants. But if you need absolute control, Pickling provides a best way of scaling, applying spot treatments, removing contaminants, and restores the surface of stainless steel. It is Available as paste and gel. This treatment helps in maximizing the corrosion resistance of welded or heat treated steels.

There are various forms of Pickling solutions, but all the solutions use acid to accomplish its goals. Pickling basically removes thin layer of chromium-depleted steel so to clean and restore the exposed surface and gives it the same characteristics as that of the raw metal. Some of the basic common components of pickling solutions include nitric hydrofluoric acids & hydrochloric with varying levels of concentration.Most pickling products use alternative ingredients to improve performance, protect steel, or increase safety.

Some of the common uses of Pickling solutions are:.

• Removing of the mill scaling
• Removing stains and impurities
• Removing of oxides from welding

Pickling is available in a varying forms. pickling solutions are best for full submersion and spot treatment for the bathing of liquors to thick pastes.Other most common uses for stainless pickling is to remove discolouration from the welding. By the applying of paste or gel to the welds, a better cleaner appearance can be attained and corrosion resistance can be restored. But one of the most important things to be noted is that pickling treatment removes the depleted layer of chromium so that the pickled surface may not match in finish and visual brightness to the untreated surface. One should consider combining both pickling and passivation to ensure an even appearance and to get strong passivation layer.

Pickling solutions are highly acidic in nature, One must be careful during both the pickling process and also while disposing of any resulting product. The acids can also create create fumes.

You shoul always consider consulting the MSDS data and manufacturer recommendations before using the pickling solution because concentration levels and ingredients can vary between the products. pickling liquors are most commonly used in baths for the submersion of fabricated stainless steel items. Most products require this treatment for half hour or maximum to a period of an hour.

One of the most important thing is to thoroughly clean the surface you wish to pickle because acids are not designed to remove oils, greases, and other similar contaminants so one must always consider cleaning it before Pickling treatment to get the best out of it.