A surface finish chart for stainless steel tubing usually means maximum Ra (roughness average) values. The most widely used table is ASME BPE’s SF designations, SF1 to SF6, for hygienic process tubing. This page gives those values, explains what ASTM A270 does and does not specify, and lists what to put on an order.
Ra limits are from ASME BPE-2024 Table SF-2.4.1-1. Finish definitions are from ASTM A270/A270M-24 Section 13.
What Ra means
Ra is the arithmetic average roughness of the surface profile, reported in microinches (µin) or micrometres (µm). 1 µm = 39.37 µin, so 0.51 µm is 20 µin. Both ASME BPE and ASTM A270 require Ra to be measured in accordance with ASME B46.1.
ASME BPE surface finish chart (Ra max)
| Designation | Process | Ra max, µin | Ra max, µm |
|---|---|---|---|
| SF0 | — | No finish requirement | No finish requirement |
| SF1 | Mechanically polished | 20 | 0.51 |
| SF2 | Mechanically polished | 25 | 0.64 |
| SF3 | Mechanically polished | 30 | 0.76 |
| SF4 | Electropolished | 15 | 0.38 |
| SF5 | Electropolished | 20 | 0.51 |
| SF6 | Electropolished | 25 | 0.64 |
BPE notes to the table:
- Readings are taken across the lay wherever possible.
- No single Ra reading may exceed the maximum in the table.
- “Mechanically polished” includes any other finishing method that meets the Ra maximum.
- Mechanical buffing as a final polishing finish is unacceptable, and flash electropolishing is not permitted.
ASTM A270 finishes: grit numbers are not Ra values
ASTM A270 lists these finishes for sanitary tube:
| A270 finish | Definition |
|---|---|
| Mill finish | No additional polishing or smoothing operations |
| Finish No. 80, 120, 180, 240 | Ground finish produced with abrasive media impregnated with No. 80, 120, 180 or 240 grit |
| Electropolished finish | Bright reflective finish produced by electropolishing |
| Maximum Ra finish | The customer specifies a maximum Ra on the inside, outside or both surfaces |
A270 defines the numbered finishes by grit only and gives no Ra value for them. Charts that convert grit numbers to Ra are not from the standard, and the result depends on the polishing process. If roughness matters, specify a maximum Ra. For circumferential (rotary) polishing A270 requires Ra to be measured in the longitudinal direction; for longitudinal polishing the measurement is agreed between buyer and manufacturer, with ASME B46.1 used to settle disputes.
What to put on the order
- Which surface: inside (ID), outside (OD) or both.
- The finish: an ASME BPE designation (for example SF1 or SF4), or a maximum Ra value in µin or µm.
- Mechanically polished or electropolished.
- For A270 pharmaceutical tubing (Supplementary Requirement S2), the finish must be stated on the purchase order.
- Whether passivation to ASTM A967 is required after polishing.
Related guides: ASME BPE tubing guide, ASTM A270 guide, how to specify sanitary tubing and passivation to A967/A380. TeCarve supplies sanitary stainless steel tube with the ID and OD finish stated on the order.
“BA” is a heat-treatment condition, not a roughness grade; see bright annealed (BA) tube.
Frequently asked questions
What is the Ra of an ASME BPE SF1 finish?
SF1 is a mechanically polished finish with a maximum Ra of 20 µin (0.51 µm), per ASME BPE-2024 Table SF-2.4.1-1.
What is the Ra of an electropolished SF4 finish?
SF4 is electropolished with a maximum Ra of 15 µin (0.38 µm).
What Ra does a No. 180 grit finish give?
ASTM A270 defines Finish No. 180 only by the abrasive grit used and gives no Ra value. If roughness matters, specify a maximum Ra.
How do you convert Ra from µm to µin?
Multiply micrometres by 39.37. For example, 0.51 µm is about 20 µin.
