SUS303 is frequently used as the primary material, although SUS304, SUS316 and SUS316F are also widely used. However, SUS303 tends to be the most used.
Stainless Steel Machining Parts
In our work, we often encounter users' stainless steel machining parts designs. The material is often labeled “SUS304.
In reality, the material used for most 300 series stainless steel machining products tends to be SUS303. So why do users prefer to label it as 304? My bold guess would be that perhaps users are more familiar with 304.
Many people assume that the type of stainless steel is 304, but if it is used for machining parts, processing 304 can be quite challenging.
Why unnecessarily increase production costs due to familiarity with a certain degree?
Next, we will compare these two materials.
This is the chemical composition of SUS304:
| Element Name | Standard range |
| W | ≤0.025 |
| Mn | ≤1.78 |
| Yes | ≤0.3 |
| P | ≤0.027 |
| s | ≤0.02 |
| No | ≤8.1 |
| Mo | ≤0.13 |
| Ass | ≤1.96 |
| Cr | ≤18.2 |
And this is the composition of SUS303:
| Element Name | Standard range |
| W | ≤0.04 |
| Mn | ≤2.08 |
| Yes | ≤0.3 |
| P | ≤0.027 |
| s | ≤1.76 |
| No | ≤8.14 |
| Mo | ≤0.13 |
| Ass | ≤1.96 |
| Cr | ≤18.02 |
When comparing these two tables, we see the biggest difference: SUS303 contains significantly higher levels of manganese, sulfur and copper, and slightly higher carbon. This formulation aims to improve its machinability, while the primary rust resistance elements, nickel and chromium, remain essentially the same.
Therefore, in terms of rust resistance, 303 and 304 are virtually identical. Their main difference is in their applications:
303 is used for machining, while 304 is used for cold heading.
So when we suggest replacing 304 with 303, it's for these reasons. We recommend that users do not adhere to their prejudices.
Of course, if there are other considerations that require the use of 304, that's a completely different question.
