Common Material Grades of Stainless Steel Casting

Stainless steel casting is a special type of casting that is mainly used for its resistance to corrosion. All material grades in this casting method contain at least 10.5% chromium. This element has excellent chemistry with oxygen, and will produce a coating of chromium oxide on the component’s surface.

Chromium is the constant deciding element; however, others are also added to enhance the corrosion-resistance of components. The most common are molybdenum and nickel.

Stainless steel castings come in various types – martensitic, austenitic, ferritic, and duplex. They earned their names from the crystal form of the steels, which will establish their metallurgical behavior.

• Martensitic Grades

These are the first stainless steel casting grades that were developed commercially. They have a higher carbon content with a percentage of 0.1% to 1.2% compared to the other material grades.

Martensitic grades contain chromium about 12% to 18%. They can be hardened through tempering and quenching like basic carbon steels. They are magnetic and mainly used in general engineering, cutlery, and aerospace.

• Austenitic Grades

Austenitic grades are non-magnetic. The basic structure of these castings is 18% chromium and 8% nickel. When nickel is mixed with stainless steel in adequate amounts, the crystal form will change into “austenite.” This will improve their corrosion-resistance features. This will also change their form from ferritic to austenitic.

These type of grades are the most common among the materials covering at least 70% of production processes. Also, they cannot be hardened through heat treatment.

• Ferritic Grades

Ferritic grades are basic stainless steel castings that contain chromium content between 10.5% and 18%. They have low carbon content. They cannot be hardened by heat treatment. They are also magnetic.

Ferritic grades have excellent formability and ductility. However, they have a weak high-temperature resistance compared to the austenitic grades.

• Duplex Grades

Duplex grades contain higher chromium content compared to others. They typically have around 18% to 28% of chromium. They have moderate levels of nickel with an average between 4.5% and 8%.

The combination of impressive mechanical features and high chromium content of these casting material grades can be understood through their chemical structure and equal microstructure of balanced portions of austenite and ferrite.

Stainless Steel Production and Processing

Stainless steels are typically made from elements found on earth. These will include chromium, nickel, nitrogen, manganese, iron ore, carbon, and silicon. The different amounts of these elements will determine the features of the final alloy. For example, nitrogen will improve corrosion resistance and ductility, which makes it useful in duplex stainless steels.

Stainless steels go through different stages during the manufacturing process:

Stage 1: Melting and Casting

The first step is to melt the raw materials together using an electric furnace. This stage will typically need about eight to twelve hours to complete. After the melting process, the fused steel will be casted into a semi-finished state.

Step 2: Forming

Stage 2 involves the semi-finished steels undergoing a series of formation process. It will start with hot rolling, where the steels are heated and will pass through big rolls.

Step 3: Heat Treatment

After forming the stainless steel, several types usually go through a hardening stage. Steel will undergo heating and cooling under managed conditions for the relief of internal stresses and to make it soft.

Meanwhile, other metals undergo heat treatment to make them stronger. This process is known as age hardening and will require the utmost control, for even the smallest changes, like the specified time, cooling rate, and temperature can significantly affect the features.

Step 4: Descaling

The heat treatment will cause a buildup or formation of scale on the steel. The scale can be taken out through various methods. Among these are pickling and electrocleaning.

The pickling method will involve a nitro-hydrofluoric acid bath for descaling. The electrocleaning process will involve applying electric current to the surface with the use of phosphoric acid and cathode.

The heat treatment and descaling stages will be done at different times depending on the material being worked on, i.e. austenitic, martensitic, and ferritic.

Step 5: Cutting

Cutting is usually needed to achieve the preferred blank size or shape to cut the part to its final size. Cutting can be done in various methods including mechanical cutting, flame cutting, and plasma cutting.

Step 6: Finishing

The surface finish is an essential requirement for stainless steel, particularly for applications wherein the appearance is also a priority. Some finishes make it easier to clean stainless steel, which is useful in sanitary applications. A smooth surface achieved through polishing provides more resistance to corrosion. Meanwhile, a rough finish is often useful in lubrication applications.

Considerations When Ordering Stainless Steel Investment Castings

I. The Primary Design

Take note of these tips to maximize the full casting process and minimize expenses as much as possible:

Draft:

These are the tapers on the vertical faces of a pattern. They’re useful in removing a pattern from a shape without causing damage to the walls.

Cores:

These are the hollow gaps within the casting. They can be used instead of the draft, depending on the application.

Section Thickness:

The preferred strength of a casting will determine its minimum thickness. A stainless steel casting must have a minimum thickness of 0.76 millimeters.

Parting Line:

Parting will facilitate the production of patterns and molds. Those patterns that have straight parting lines in a single plane are usually simpler and cheaper to make.

Patterns

Manufacturers are going to use wax patterns to produce investments or molds. A casting will depend on its pattern, particularly when it comes to smooth finishes. Establishing excellent communication about the pattern equipment for production and design will help avoid any confusions and will give you a clear idea of the expenses.

Specifications

The specifications are a series of guidelines that should be met for both steel and non-steel investment casting processes. These guidelines will ensure that metallurgical standards are met. To avoid any future misunderstandings, unnecessary costs, and other problems, these requirements must be observed.

Quantity

It’s important to specify the number of parts needed as this will determine which tooling type is required for the application. This is also important as some manufacturers only cater to high-volume production, while others accept short-term projects.

Product Integrity

It’s almost impossible to find a product that is defect-free. Before going into production, discuss the criteria with your manufacture the details on the acceptance and rejection of the final products.

Weight

Take note that the heavier the stainless steel castings are, the more expensive they become. Being specific with weigh-related details when discussing the quote will provide a better cost estimate.

Testing and Inspection

The investment castings should be tested and inspected by the manufacturer. This is to ensure that all specifications are met, including physical features, surface finishes, dimensional accuracy, etc. Even before production starts, you must come with an agreement with the manufacturer regarding the requirements for testing and inspection.

Machining

This process involves the removal of unnecessary components from a casting to guarantee dimensional accuracy. It’s ideal to allocate a particular amount of the casting for machining allowance. This is to create a compact tolerance.

II. Production and Delivery

Make sure to discuss all details regarding the production and delivery of your products. This is to manage your expectations, and at the same time, come up with the best arrangement for both parties.