Stainless Steel-304 material's usage in CNC fabrication

Basic introduction:

304 stainless steel material's usage in CNC fabrication is a common material among stainless steels, with a density of 7.93 g/cm³. It is also called 18/8 stainless steel in the industry, which means it contains more than 18% chromium and more than 8% nickel. Since the nickel (Ni) contained is more corrosion-resistant and heat-resistant than steel containing chromium (Cr), it has very good physical properties, such as high temperature resistance of 800℃, good processing performance, high toughness, etc. It is widely used in the manufacture of equipment and parts that require good comprehensive performance (corrosion resistance and formability), and is often used in industry, furniture decoration industry, food and medical industry, etc.

304 stainless steel is a grade of stainless steel produced according to American ASTM standards.

304 stainless steel grade description:

Common marking methods on the market include 06Cr19Ni10 and SUS304.

• Among them, 06Cr19Ni10 is a stainless steel grade produced according to national standards;

• 304 is a stainless steel grade produced according to the American ASTM standard;

• SUS304 is a stainless steel grade produced according to Japanese standards.

Current plate thickness:

304 stainless steel chemical composition:

For 304 stainless steel, the Ni element in its composition is very important, which directly determines the corrosion resistance and value of 304 stainless steel.

The most important elements in 304 are Ni and Cr, but they are not limited to these two elements. The specific requirements are specified by the product standards.

The industry generally believes that as long as the Ni content is greater than 8% and the Cr content is greater than 18%, it can be considered 304 stainless steel. This is why the industry calls this type of stainless steel 18/8 stainless steel.

Physical properties:

Surface treatment:

• Russ

• Sandblasting

• Plastic spraying

• Silkscreen

• Laser marking

Application areas:

304 stainless steel is the most widely used chromium-nickel stainless steel. As a widely used steel  in CNC fabrication, it has good corrosion resistance, heat resistance, low temperature strength and mechanical properties; it has good hot workability such as stamping and bending, and no heat treatment hardening phenomenon (operating temperature -196℃～800℃); it has good processing performance and weldability.

Suitable for food processing, storage and transportation. Plate heat exchangers, bellows, household items (category 1 and 2 tableware, cabinets, indoor pipelines, water heaters, boilers, bathtubs), auto parts (windshield wipers, mufflers, molded products), medical equipment, building materials, chemistry, food industry, agriculture, ship parts, etc. 304 stainless steel with strictly controlled content can also be called food grade 304 stainless steel .

Most of the use requirements are to maintain the original appearance of the building for a long time. When determining the type of stainless steel to be selected, the main considerations are the required aesthetic standards, the corrosiveness of the local atmosphere, and the cleaning system to be adopted. However, other applications are increasingly seeking structural integrity or impermeability. For example, the roofs and side walls of industrial buildings. In these applications, the owner's construction cost may be more important than aesthetics, and the surface may not be very clean. The use of 304 stainless steel in a dry indoor environment works quite well. However, in rural and urban areas, if you want to maintain its appearance outdoors, you need to clean it frequently. In seriously polluted industrial areas and coastal areas, the surface will be very dirty and even rusty.

However, to achieve the aesthetic effect in the outdoor environment, nickel-containing stainless steel is required. Therefore, 304 stainless steel is widely used in curtain walls, side walls, roofs and other construction purposes, but in the corrosive industrial or marine atmosphere, 316 stainless steel is preferred.  Stainless steel sliding doors, people have fully realized the advantages of using stainless steel in structural applications. There are several design criteria that include 304 and 316 stainless steel. Because the "duplex" stainless steel 2205 has integrated good atmospheric corrosion resistance with high tensile strength and elastic strength, this steel is also included in the European criteria.

In fact, stainless steel is manufactured in all standard metal shapes and sizes, and there are many special shapes. The most common products are made of thin plates and strips, and medium and thick plates are also used to produce special products, such as hot-rolled structural steel and extruded structural steel. There are also round, oval, square, rectangular and hexagonal welded or seamless steel pipes and other forms of products, including profiles, bars, wires and castings.

Reasons for rusting of 304 stainless steel:

There may be several reasons for the rust of 304 stainless steel:

Chloride ion

Chloride ions are widely present, such as salt/sweat/seawater/sea breeze/soil, etc. Stainless steel corrodes very quickly in an environment with chloride ions, even more than ordinary low-carbon steel. Therefore, there are requirements for the use environment of stainless steel, and it needs to be wiped frequently to remove dust and keep it clean and dry.

Solution treatment

The alloy elements are not dissolved into the matrix, resulting in low alloy content in the matrix structure and poor corrosion resistance.

Intergranular corrosion

This material  in CNC fabrication without titanium and niobium has a tendency to intergranular corrosion. Adding titanium and niobium, combined with stabilization treatment, can reduce intergranular corrosion. Stainless steel is a high-alloy steel that can resist corrosion in air or chemical corrosive media. It has a beautiful surface and good corrosion resistance. It does not need to undergo surface treatment such as color plating, but exerts the inherent surface properties of stainless steel. It is a kind of steel used in many aspects and is usually called stainless steel. Representative performance includes high-alloy steels such as 13 chromium steel and 18-8 chromium-nickel steel. From a metallographic point of view, because stainless steel contains chromium, a very thin chromium film is formed on the surface. This film isolates the oxygen that invades the steel and plays a corrosion-resistant role. In order to maintain the inherent corrosion resistance of stainless steel, the steel must contain more than 12% chromium. It is used in occasions where welding is required. The lower carbon content minimizes the precipitation of carbides in the heat-affected zone near the weld, and the precipitation of carbides may cause intergranular corrosion (welding erosion) of stainless steel in certain environments.

Surface marks

The surface mark phenomenon refers to the line or dot-shaped pit defects that occur on the surface of 304 stainless steel after deep processing or grinding of processed products.

Cause: There are foreign objects on the surface and mold surface, and there are scars on the surface of the edge pad.

Countermeasures: Remove foreign matter and scars before processing, and remove orange peel by sufficient grinding.

The orange peel phenomenon refers to the phenomenon that the surface of 304 stainless steel molded products produces an orange peel-like shape during grinding or other conditions.

Reason: Insufficient grinding, the greater the processing, the more serious the orange peel, the coarse grains of the raw materials, and the softening by heat treatment.

Countermeasures: Grind deep-processed products thoroughly, select appropriate heat treatment conditions, and control the grain size of raw materials.

※ Some data comes from the Internet and is for reference only