Overview of 303 Stainless Steel for Precision Machining

Stainless steel is one of the best and most commonly used materials for precision machined parts. There’s a seemingly endless variety of stainless steel alloys, which makes it difficult to choose the best one for your parts.

Spex uses 303 stainless steel to manufacture tons of different parts. 303 is a free machining alloy, meaning that its properties make it a great choice for precision machining.

We’ll provide you with an overview of the material and help you decide if it’s the right choice for your project.

What is 303 stainless steel?

303 stainless steel was developed specifically to increase machinability while maintaining the positive aspects of stainless steel. This alloy has the benefits of stainless steel: good strength and high resistance to corrosion.

Stainless steel is a hard metal, so it’s more difficult and expensive to machine parts. 304 stainless steel is a general purpose alloy, and other elements are added to increase or decrease certain properties of the metal. To increase the machinability, sulfur is added to 303.

To classify different alloys, there are families of stainless steel, and specific grades. 303, and other 300 grade stainless steels, are in the austenitic family. This is the most widely used family across the world.

The austenitic stainless steels are made with austenite base, which is a combination of carbon and a nonmagnetic form of iron.

Composition

Element	Percentage
Iron (Fe)	66.5 - 70.5%
Chromium (Cr)	17 - 19%
Nickel (Ni)	8 - 10%
Manganese (Mn)	2%
Silicon (Si)	1%
Copper (Cu)	1%
Nitrogen (N)	0.10%
Carbon (C)	0.10%
Phosphorous (P)	0.045%
Sulfur (S)	0.015 - 0.35%

Common applications

As we mentioned, SS303 is very common–especially for machined parts. 303 stainless steel is used to machine:

Nuts
Bolts
Screws
Fasteners
Shafts
Aircraft fittings
Electrical components
Gears
Automotive parts
Aerospace components
Medical instruments and other parts.

Because of the added sulfur, this alloy loses some of its hardness and corrosion resistant properties. Parts aren’t designed for marine environments, and shouldn’t be used constantly at working temperatures above 1400°F.

303 also isn’t recommended for parts that need to be welded. This alloy also can’t be heat treated, but it can be cold worked to increase the hardness and tensile strength.

Machinability

Free machining alloys like SS303 are ideal for parts that require extensive machining. The metal produces better chips while it’s being cut. That means there aren’t long strings of metal coming off, and tight tolerances are easier to achieve. Because the metal is softer, the cutting tools last longer, making it less expensive to machine.

303 has a machining cost factor of 2.7 when compared to standard steel. The machining cost factor is about half of 304 and 316 stainless steel.

Corrosion resistance

303 has relatively good resistance to mild environments, but it’s significantly less than 304 stainless steel. When 303 is in a moist environment, it forms rust and is subject to rapid-pitting corrosion.

Also, most austenitic stainless steels will start cracking in chloride environments above 140°F.

Heat resistance

Stainless steel usually has good heat resistance. 303 grade doesn’t have the best heat resistance, but it can withstand mild heat fairly well. 303 has a higher carbon content so it’s also vulnerable to sensitisation, which is a negative effect of high and changing temperatures.

You can visit our material page to compare the different options for your next CNC machining project. If you have any other questions about choosing the right materials, reach out to our team.