Valve trim is the set of replaceable internal parts in contact with the process fluid — typically the stem, seat rings, wedge or disc facings and backseat bushing. API 600 assigns standard trim numbers: Trim 1 is 13Cr (F6), Trim 5 adds Stellite hard-facing, Trim 10 is 316 stainless, and so on.
Body material gets the attention, but trim does the work: the seating surfaces take every closure impact, the stem takes every torque cycle, and when a valve fails to seal it is almost always trim, not body, that wore out. Because these parts are renewable, standards treat them as a defined, orderable set — and API 600's trim number table turns a paragraph of metallurgy into two digits.
The numbers you will actually meet: Trim 1 — 13% chromium (F6/410) throughout, the refinery default for general hydrocarbon and steam service; Trim 5 — 13Cr with Stellite-faced seating surfaces for erosive or frequent-cycling duty; Trim 8 — 13Cr stem with Stellite seats and hard-faced wedge, the workhorse steam trim; Trim 10 / 12 — full 316 for corrosive service; Trim 16 — F316 with Stellite facing, combining corrosion resistance and wear life. Matching trim hardness to the service — not maximising it — is the engineering: Stellite everywhere doubles cost and buys nothing in clean, mild duty.
Specify trim by API 600 number or by explicit part materials. VTXFLOW stocks 13Cr, Stellite-faced and 316 trims across gate, globe and check valve production, documented per part on the material certificate.