Post-weld heat treatment (PWHT) requirements for carbon and low alloy steel pressure vessels are set out in ASME Section VIII Division 1, paragraph UCS-56, across eleven material-specific tables (UCS-56-1 through UCS-56-11). Each table gives a minimum holding temperature, a holding time formula based on nominal thickness, and a thickness threshold above which PWHT becomes mandatory. This guide walks through ten worked examples spanning eight different P-Number material groups and a range of thicknesses, using the same logic behind this site's free PWHT Requirement Calculator.
PWHT Basics: Temperature, Time, and Mandatory Thickness
Every UCS-56 table answers three questions for its material group:
- Minimum holding temperature — the lowest temperature the weld must reach and be held at during PWHT (some tables also specify a maximum).
- Minimum holding time — how long to hold at that temperature, almost always a function of nominal thickness (including corrosion allowance, per UW-40(f)).
- Mandatory-PWHT threshold — the nominal thickness above which PWHT is required by default, absent a specific exemption in that table's General Notes.
All ten examples below use the same three-step process: identify the P-Number, look up the base temperature and time formula, then check the thickness against the mandatory threshold.
Examples 1–2: P-No. 1 Carbon Steel — Thin vs. Thick
P-No. 1 (Gr. Nos. 1, 2, 3) covers ordinary carbon steel — the most common pressure vessel material. Table UCS-56-1 specifies 1100°F (595°C) minimum, with time = 1 hr/in. (15 min minimum) up to 2 in. (50 mm), then 2 hr plus 15 min for each additional inch. PWHT is mandatory above 1½ in. (38 mm) unconditionally, and in the 1¼–1½ in. (32–38 mm) band unless preheat ≥ 200°F (95°C) is applied.
Example 1 — 25 mm carbon steel shell
- P-Number: P-No. 1
- Nominal thickness: 25 mm (0.984 in.)
25 mm is below the 32 mm threshold, so PWHT is not mandatory by thickness alone. Holding time: max(0.984 × 60, 15) = 59 minutes.
Example 2 — 45 mm carbon steel shell
- P-Number: P-No. 1
- Nominal thickness: 45 mm (1.772 in.)
45 mm exceeds the 38 mm threshold, so PWHT is mandatory regardless of preheat. Holding time: max(1.772 × 60, 15) = 106 minutes.
Example 3: P-No. 3 (C-Mo) Thin-Wall Pipe
P-No. 3 (Gr. Nos. 1, 2, 3) covers carbon-molybdenum and manganese-molybdenum-nickel steels. Table UCS-56-2 specifies the same 1100°F (595°C) and time formula as P-No. 1, but the mandatory threshold is different and group-dependent: Gr. No. 3 material is mandatory at all thicknesses, while Gr. Nos. 1 and 2 become mandatory above 5/8 in. (16 mm).
Example 3 — 10 mm C-Mo pipe
- P-Number: P-No. 3
- Nominal thickness: 10 mm (0.394 in.)
For Gr. Nos. 1/2, 10 mm is below the 16 mm threshold, so PWHT is not mandatory by thickness alone (Gr. No. 3 material would still require it at any thickness). Holding time: max(0.394 × 60, 15) = 24 minutes.
Example 4: P-No. 4 (1¼Cr-½Mo) Shell
P-No. 4 (Gr. Nos. 1, 2) covers 1¼Cr-½Mo and ½Cr-½Mo low alloy steels — common in elevated-temperature service. Table UCS-56-3 specifies 1200°F (650°C) minimum, time = 1 hr/in. (15 min minimum) up to 5 in. (125 mm). PWHT is mandatory on essentially all P-No. 4 Gr. 1/2 material, with only narrow exemptions for small circumferential butt welds under specific carbon and preheat conditions.
Example 4 — 30 mm 1¼Cr-½Mo shell
- P-Number: P-No. 4
- Nominal thickness: 30 mm (1.181 in.)
Holding time: max(1.181 × 60, 15) = 71 minutes. Mandatory by default (no exemption modeled here).
Examples 5–6: P-No. 5A (2¼Cr-1Mo) — Normal vs. Alternate Temperature
P-Nos. 5A, 5B, 5C (Gr. No. 1) cover 2¼Cr-1Mo and similar chromium-molybdenum steels. Table UCS-56-4 specifies 1250°F (675°C) minimum, same 5 in. (125 mm) time threshold as P-No. 4. General Note (c) offers a rarely-used but important alternative: when 1250°F is impractical, PWHT may instead be run at 1200°F (650°C) minimum, at the cost of a much longer holding time — 4× the normal time for thickness over 2 in. (50 mm), or the greater of 4 hr / 4 hr-per-inch for thickness up to 2 in.
Example 5 — 60 mm 2¼Cr-1Mo, normal temperature
- P-Number: P-No. 5A
- Nominal thickness: 60 mm (2.362 in.)
Holding time: max(2.362 × 60, 15) = 142 minutes.
Example 6 — Same 60 mm shell, alternate temperature
60 mm exceeds the 2 in. (50 mm) breakpoint in General Note (c), so the alternate holding time is the normal time (142 min) × 4 = 568 minutes — nearly 9.5 hours, versus 142 minutes (~2.4 hours) at the standard 1250°F. This trade-off is why the alternate temperature is only used when 1250°F genuinely cannot be achieved in the available furnace.
Examples 7–8: P-No. 9A — Below vs. Above the 16 mm Threshold
P-No. 9A (Gr. No. 1) covers low-alloy nickel-chromium-molybdenum steels. Table UCS-56-5 specifies 1100°F (595°C) minimum, with a simpler time formula than P-1/P-3/P-4/P-5A: 1 hr minimum, plus 15 min for each additional inch over 1 in. (25 mm). The mandatory threshold is the same 5/8 in. (16 mm) break point as P-No. 3 Gr. 1/2.
Example 7 — 12 mm, below threshold
- P-Number: P-No. 9A
- Nominal thickness: 12 mm (0.472 in.)
12 mm ≤ 16 mm, so not mandatory by thickness alone. Since thickness ≤ 1 in., holding time is the flat 60-minute minimum.
Example 8 — 20 mm, above threshold
- P-Number: P-No. 9A
- Nominal thickness: 20 mm (0.787 in.)
20 mm exceeds 16 mm, so PWHT is mandatory. Thickness is still ≤ 1 in., so the holding time is unchanged at 60 minutes — the time formula and the mandatory threshold are independent checks.
Example 9: P-No. 10B — Always Mandatory
P-No. 10B (Gr. No. 1) is one of only two groups in this set (with P-5A) where PWHT is mandatory at every thickness — Table UCS-56-8's General Note (a) states this plainly, with no thickness-based exemption at all.
Example 9 — 8 mm P-No. 10B, thin section
- P-Number: P-No. 10B
- Nominal thickness: 8 mm (0.315 in.)
Even at just 8 mm — thinner than every other mandatory example in this guide — PWHT is still required, because P-No. 10B has no thickness exemption. Holding time is the flat 60-minute minimum (≤ 1 in.).
Example 10: P-No. 15E (Grade 91) — Thick Section
P-No. 15E Group No. 1 covers Grade 91 (9Cr-1Mo-V) steel, widely used in high-temperature power and process piping. Table UCS-56-11 specifies the highest base temperature in this set — 1300°F (705°C) — with a distinct time formula: 1 hr/in. (2 min/mm), 30 min minimum, up to 5 in. (125 mm), then 5 hr plus 15 min per additional inch. Grade 91 also carries extensive additional notes on filler-metal-dependent maximum temperatures and overheating remediation not modeled in a simple thickness lookup.
Example 10 — 75 mm Grade 91 weld
- P-Number: P-No. 15E
- Nominal thickness: 75 mm (2.953 in.)
75 mm is under the 5 in. (125 mm) breakpoint, so time = max(2.953 × 60, 30) = 177 minutes.
Comparing All Ten Examples
| # | P-Number | Thickness | Holding Temp | Holding Time | Mandatory? |
|---|---|---|---|---|---|
| 1 | P-1 | 25 mm | 1100°F (595°C) | 59 min | No |
| 2 | P-1 | 45 mm | 1100°F (595°C) | 106 min | Yes |
| 3 | P-3 (Gr.1/2) | 10 mm | 1100°F (595°C) | 24 min | No |
| 4 | P-4 | 30 mm | 1200°F (650°C) | 71 min | Yes |
| 5 | P-5A | 60 mm | 1250°F (675°C) | 142 min | Yes |
| 6 | P-5A (alt.) | 60 mm | 1200°F (650°C) | 568 min | Yes |
| 7 | P-9A | 12 mm | 1100°F (595°C) | 60 min | No |
| 8 | P-9A | 20 mm | 1100°F (595°C) | 60 min | Yes |
| 9 | P-10B | 8 mm | 1100°F (595°C) | 60 min | Yes (always) |
| 10 | P-15E | 75 mm | 1300°F (705°C) | 177 min | Yes |
A few patterns stand out: holding temperature ranges from 1100°F for the most common carbon/low-alloy groups up to 1300°F for Grade 91; the mandatory threshold varies from "never exempt regardless of thickness" (P-10B, P-5A) down to specific mm break points (16 mm, 32 mm, 38 mm) depending on the group; and the 4× alternate-temperature option for P-5A (Example 6) shows just how steep the time penalty is for avoiding the higher standard temperature.
⚠ What This Guide and Calculator Don't Cover
Every example above applies each Table's general default rule only. Actual PWHT applicability also depends on: the specific Group Number within a P-Number (not just Gr. 1/2/3 as one input), carbon content, whether qualifying preheat was applied, whether PWHT is imposed as a separate UW-2 service requirement regardless of thickness, and narrow exemptions for small groove/ fillet welds, tube-to-tubesheet joints, studs, and weld overlay listed in each table's General Notes. None of these are modeled here. Always confirm the applicable exemption against the current Code edition and your specific joint design before omitting PWHT on an actual project.
✓ Verified Against ASME BPVC.VIII.1-2023
All temperatures, time formulas, and mandatory-thickness thresholds in this guide were checked directly against Tables UCS-56-1 through UCS-56-11 in the 2023 edition of ASME BPVC.VIII.1. Two thickness-logic errors and one time-calculation error found in an earlier version of this calculator's source code — an inverted mandatory check for P-No. 1 and P-No. 10C above 38 mm, and a stray rounding offset in the P-1/P-3 holding-time formula — have been corrected in the current calculator.
Try It Yourself
Reproduce any of the ten examples above, then try your own material and thickness combination:
- Open the PWHT Requirement Calculator.
- Select your P-Number from the dropdown.
- Enter your nominal thickness (including corrosion allowance) and select mm or inch.
- Click Calculate and compare your result to the tables above.
- For P-No. 5A/5B/5C, try checking "Use alternate temperature" to see the 4× time trade-off.
Check PWHT Requirements for Your Material Right Now
Free, browser-based, no account required. Holding temperature, holding time, and mandatory-PWHT threshold for 10 P-Number material groups per ASME Section VIII Division 1, UCS-56.
Open the PWHT Requirement Calculator →