13.1 B31.3 Preheat and Postweld Heat Treatment Rules & Tables
Key Takeaways
- Under ASME B31.3 Table 330.1.1, P-No. 1 carbon steel requires only 50°F (10°C) unless thickness exceeds 1 in. (25 mm) AND carbon exceeds 0.30% (then 200°F/95°C); alloy preheat is keyed to specified minimum tensile strength — P-No. 3 needs 200°F (95°C) above 1/2 in. (13 mm) or 65 ksi SMTS, P-No. 4 needs 250°F (120°C) for all materials, and P-Nos. 5A/5B need 300°F (150°C) at or below 60 ksi and 400°F (200°C) above it.
- Para 330.1.4 requires the preheat zone to be at or above the specified minimum temperature in all directions from the point of welding for the larger of 3 in. (75 mm) or 1.5 times the greater nominal thickness, and 1 in. (25 mm) for tack welds.
- Table 331.1.1 holds P-Nos. 1 and 3 at 1,100°F to 1,200°F (595°C to 650°C), P-No. 4 at 1,200°F to 1,300°F (650°C to 705°C), and P-Nos. 5A and 5B at 1,250°F to 1,400°F (675°C to 760°C), all at 1 hr/in. of control thickness with a 15-minute minimum.
- Para 331.1.4 caps heating and cooling above 600°F (315°C) at 600°F/hr divided by one-half the maximum material thickness in inches, and in no case above 600°F/hr; B31.3 sets no general lower rate floor.
- Para 331.2.6 requires local PWHT to heat a full circumferential band with the weld centred, at least three times the wall thickness of the thickest part wide, extending two times the run pipe thickness beyond nozzle and attachment welds.
13.1 B31.3 Preheat and Postweld Heat Treatment Rules & Tables
In process piping fabrication, thermal conditioning — preheat and postweld heat treatment (PWHT) — is the metallurgical control that keeps welded joints out of the hydrogen cracking and brittle fracture regimes. Under ASME B31.3 Chapter V (Fabrication, Assembly, and Erection), preheat is governed by Para 330 and PWHT by Para 331.
For the AWS endorsement, this is high-value territory for a specific reason: B31.3's preheat table and B31.1's preheat table are structurally identical, with the same P-Number rows, the same thickness breakpoints, and the same temperatures. Learn one and you have both — 16 B31.3 items and 26 B31.1 items partly covered by a single memorization.
ASME B31.3 Para 330 & Table 330.1.1: Preheat Rules
Preheating heats the base metal surrounding the joint before and during welding, thermal cutting, or tacking. Under Para 330.1, the requirements apply to all types of welding, including tack welds, repair welds, and seal welds on threaded joints, and the preheating rules of ASME B31P may be used as an alternate. The engineering purposes are the familiar three:
- Slowing the Cooling Rate: Retarding the quench transforms austenite into ductile ferrite, pearlite, or bainite rather than hard martensite.
- Promoting Hydrogen Diffusion: Allowing diffusible hydrogen to escape the weld pool and HAZ before it collects at microstructural defects and initiates delayed cold cracking.
- Reducing Shrinkage Stress: Decreasing the thermal gradient between the deposit and the adjacent cold parent metal.
Preheat Zone and Verification (Paras 330.1.3 and 330.1.4)
Para 330.1.4 sets the geometry precisely: the preheat zone shall be at or above the specified minimum temperature in all directions from the point of welding for a distance of the larger of 3 in. (75 mm) or 1.5 times the greater nominal thickness. For tack welds, the base metal temperature shall be at or above the specified minimum for a distance of not less than 1 in. (25 mm) in all directions from the point of welding.
Para 330.1.3 permits temperature-indicating crayons, thermocouple pyrometers, or other suitable means, verified prior to and maintained during welding, and allows thermocouples to be attached directly to pressure-containing parts by the low energy capacitor discharge method in accordance with Para 328.7.
Table 330.1.1 Minimum Preheat Temperatures
Thickness in the table is the nominal thickness of the thicker component, and composition may be based on ladle or product analysis or on specification limits. Higher minimum preheats may be required by the WPS or the engineering design; Para 330.1.2 requires preheat for an unlisted material to be specified in the WPS.
| Base Metal P-No. | Base Metal Group | Greater Material Thickness | Additional Limits | Required Minimum Temperature |
|---|---|---|---|---|
| 1 | Carbon steel | $< 1\text{ in.}$ ($25\text{ mm}$) | None | 50°F (10°C) |
| 1 | Carbon steel | $\ge 1\text{ in.}$ ($25\text{ mm}$) | $%\text{C} \le 0.30$ | 50°F (10°C) |
| 1 | Carbon steel | $\ge 1\text{ in.}$ ($25\text{ mm}$) | $%\text{C} > 0.30$ | 200°F (95°C) |
| 3 | Alloy steel, $\text{Cr} \le 1/2%$ | $\le 1/2\text{ in.}$ ($13\text{ mm}$) | $\text{SMTS} \le 65\text{ ksi}$ ($450\text{ MPa}$) | 50°F (10°C) |
| 3 | Alloy steel, $\text{Cr} \le 1/2%$ | $> 1/2\text{ in.}$ ($13\text{ mm}$) | $\text{SMTS} \le 65\text{ ksi}$ | 200°F (95°C) |
| 3 | Alloy steel, $\text{Cr} \le 1/2%$ | All | $\text{SMTS} > 65\text{ ksi}$ | 200°F (95°C) |
| 4 | Alloy steel, $1/2% < \text{Cr} \le 2%$ | All | None | 250°F (120°C) |
| 5A | Alloy steel | All | $\text{SMTS} \le 60\text{ ksi}$ ($414\text{ MPa}$) | 300°F (150°C) |
| 5A | Alloy steel | All | $\text{SMTS} > 60\text{ ksi}$ | 400°F (200°C) |
| 5B | Alloy steel | All | $\text{SMTS} \le 60\text{ ksi}$ | 300°F (150°C) |
| 5B | Alloy steel | All | $\text{SMTS} > 60\text{ ksi}$ | 400°F (200°C) |
| 5B | Alloy steel | $> 1/2\text{ in.}$ ($13\text{ mm}$) | $%\text{Cr} > 6.0$ | 400°F (200°C) |
| 6 | Martensitic stainless steel | All | None | 400°F (200°C), max interpass 600°F (315°C) |
| 9A | Nickel alloy steel | All | None | 250°F (120°C) |
| 9B | Nickel alloy steel | All | None | 300°F (150°C) |
| 10I | 27Cr steel | All | None | 300°F (150°C), interpass held 300–450°F |
| 15E | 9Cr–1Mo–V CSEF steel | All | None | 400°F (200°C) |
| All other materials | — | — | None | 50°F (10°C) |
[!IMPORTANT] The P-No. 1 carbon-content gate. B31.3 keys carbon steel preheat to carbon content, not to tensile strength. A 1.125 in. wall A106-B spool with 0.25% carbon stays at the 50°F (10°C) baseline, because thickness alone does not trigger the higher value; only material that is both at or above 1 in. and above 0.30% carbon reaches 200°F (95°C). Options built on "over 1 in. therefore 175°F" or "SMTS over 71 ksi therefore 175°F" are distractors — 175 °F does not appear in the current Table 330.1.1 at all.
[!NOTE] Grade 91 is P-No. 15E. Section IX groups 9Cr–1Mo–V creep-strength-enhanced ferritic steels (A335 Grade P91) under P-No. 15E, and Table 330.1.1 carries a dedicated 15E row at 400°F. Table 330.1.1 has no P-No. 5C row.
Interruption of Welding (Para 330.2.2)
Para 330.2.1 resolves dissimilar joints: when welding two different P-No. materials, the preheat temperature shall be the higher temperature shown in Table 330.1.1.
Para 330.2.2 then mirrors B31.1's interruption rule with one important difference — B31.3 includes P-No. 15E in the list. After welding commences, the minimum preheat temperature shall be maintained until any required PWHT is performed on P-Nos. 3, 4, 5A, 5B, 6, and 15E, except when all of the following are satisfied:
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| PARA 330.2.2 EXCEPTION — ALL CONDITIONS MUST BE SATISFIED |
+-----------------------------------------------------------------------------------------+
| (a) At least 10 mm (3/8 in.) thickness of weld is deposited, OR 25% of the welding |
| groove is filled, WHICHEVER IS LESS, with the weld supported to prevent |
| overstressing if the weldment is moved or loaded. The surface condition before |
| cooling should be smooth and free of sharp discontinuities. |
| (b) For P-Nos. 3, 4, and 5A materials, the weld is allowed to cool slowly to room |
| temperature. |
| (c) For P-Nos. 5B, 6, and 15E materials, the weld is subjected to an intermediate heat |
| treatment with a controlled rate of cooling. |
| (d) The cooled weld is visually examined before welding resumes. |
| (e) The required preheat is reapplied before welding resumes. |
+-----------------------------------------------------------------------------------------+
Austenitic Stainless Steel (P-No. 8)
P-No. 8 austenitic stainless steels fall under "all other materials" at the 50°F (10°C) baseline. Elevated preheat is counterproductive: holding between roughly 800°F and 1,500°F precipitates chromium carbides along grain boundaries (sensitization), and the high thermal expansion and low conductivity of austenitic grades increase solidification cracking risk when welding on already-hot metal. Fabrication specifications commonly impose a maximum interpass temperature; B31.3 itself imposes interpass maxima only where the table says so (600°F for P-No. 6, a 300–450°F band for P-No. 10I).
ASME B31.3 Para 331 & Table 331.1.1: Postweld Heat Treatment
Para 331.1.1 requires PWHT in accordance with the material groupings and ranges of Table 331.1.1, except as provided in Table 331.1.2 (alternate lower temperatures) and Table 331.1.3 (exemptions); the PWHT rules in ASME B31P may be used as an alternate. The PWHT to be used after production welding shall be specified in the WPS and used in qualifying the welding procedure.
Table 331.1.1 Holding Temperatures and Soak Times
| P-No. and Group No. | Alloy Description | Holding Temperature Range | Soak, Control Thickness $\le 2\text{ in.}$ | Soak, Control Thickness $> 2\text{ in.}$ |
|---|---|---|---|---|
| P-No. 1, Gr. 1–3 | Carbon steels | 1,100–1,200°F (595–650°C) | 1 hr/in., 15 min minimum | 2 hr plus 15 min per additional inch |
| P-No. 3, Gr. 1–2 | C-1/2Mo, 1/2Cr-1/2Mo | 1,100–1,200°F (595–650°C) | 1 hr/in., 15 min minimum | 2 hr plus 15 min per additional inch |
| P-No. 4, Gr. 1–2 | 1-1/4Cr-1/2Mo (A335 P11) | 1,200–1,300°F (650–705°C) | 1 hr/in., 15 min minimum | 2 hr plus 15 min per additional inch |
| P-No. 5A, Gr. 1 | 2-1/4Cr-1Mo (A335 P22) | 1,250–1,400°F (675–760°C) | 1 hr/in., 15 min minimum | 2 hr plus 15 min per additional inch |
| P-No. 5B, Gr. 1 | 5Cr-1/2Mo, 9Cr-1Mo (P5, P9) | 1,250–1,400°F (675–760°C) | 1 hr/in., 15 min minimum | 2 hr plus 15 min per additional inch |
| P-No. 6, Gr. 1–3 | Martensitic stainless | 1,400–1,475°F (760–800°C) | 1 hr/in., 15 min minimum | 2 hr plus 15 min per additional inch |
| P-No. 7, Gr. 1–2 | Ferritic stainless | 1,350–1,425°F (730–775°C) | 1 hr/in., 15 min minimum | 2 hr plus 15 min per additional inch |
| P-No. 8, Gr. 1–4 | Austenitic stainless | PWHT not required unless required by the WPS | — | — |
| P-No. 9A / 9B, Gr. 1 | Nickel alloy steels | 1,100–1,200°F (595–650°C) | 1 hr/in., 15 min minimum | 2 hr plus 15 min per additional inch |
| P-No. 15E, Gr. 1 | 9Cr-1Mo-V CSEF (Grade 91) | 1,300–1,425°F (705–775°C) | 1 hr/in., 30 min minimum | 1 hr/in. up to 5 in., plus 15 min per additional inch over 5 in. |
| All other materials | — | PWHT as required by the WPS | Per WPS | Per WPS |
[!IMPORTANT] The 15-minute minimum is universal in the main block. P-Nos. 1, 3, 4, 5A, 5B, 6, 7, 9A, and 9B all use "1 hr/in., 15 min minimum." Only P-No. 15E differs, at 30 minutes. An option that assigns a "1 hour minimum" or "2 hour minimum" to P-No. 4 or P-No. 5A is wrong. Likewise, P-Nos. 5A and 5B start at 1,250°F, not 1,300°F.
Para 331.1.6(c) requires the material temperature during PWHT to be within the Table 331.1.1 range, although the designer may extend the range as permitted by Table 331.1.2 provided the lower transformation temperature is not exceeded. Table 331.1.2 is the alternate schedule for P-Nos. 1 and 3 only: a 50°F (30°C) decrease requires 2 hours, 100°F (55°C) requires 4 hours, 150°F (85°C) requires 10 hours, and 200°F (110°C) requires 20 hours — and a decrease of more than 100°F is allowable only for P-No. 1, Group Nos. 1 and 2.
Para 331.2.3 handles dissimilar joints: between dissimilar ferritic metals, PWHT shall be at the higher of the Table 331.1.1 ranges; joints combining ferritic and austenitic components and filler metals shall be treated as required for the ferritic material unless the engineering design says otherwise.
Control Thickness (Para 331.1.3)
Control thickness is the lesser of (1) the thickness of the weld, or (2) the thickness of the materials being joined at the weld (or of the pressure-containing material where a nonpressure-containing part is attached). Weld thickness itself is defined by joint type: groove welds use the thicker abutting end after preparation including I.D. machining; fillet welds use the throat thickness; partial penetration welds use the groove depth; repair welds use the cavity depth; and branch welds are computed from the figures referenced in Para 331.1.3(b)(5). Separately, nominal material thickness — used in Table 331.1.3 — is the thicker of the materials being joined at the weld.
Table 331.1.3: Exemptions From Mandatory PWHT
Table 331.1.3 is organized as P-Number, control thickness, weld type, and a column of additional limitations required for exemption — and every limitation in the row must be satisfied. Four blanket notes govern the whole table: no exemption is permitted for a PWHT required by the designer or the WPS; exemptions may be based on actual chemistry; single-layer or single-pass welds may be exempted provided the WPS was qualified using single-pass welds with +10% heat input and all other exemption conditions are met; and additional exemptions apply to welds made under Para 328.7.
| P-No. | Control Thickness for Exemption | Representative Additional Limitations |
|---|---|---|
| 1, all Groups | All control thicknesses, all weld types | A preheat of 200°F (95°C) is applied prior to welding on any nominal material thickness over 1 in. (25 mm); multiple layer welds are used when nominal material thickness exceeds 3/16 in. (5 mm) |
| 3, Gr. 1–2 | $\le 5/8\text{ in.}$ ($16\text{ mm}$), all weld types | Preheat of 200°F (95°C) prior to welding on any nominal material thickness over 1/2 in. (13 mm); specified carbon content of base materials $\le 0.25%$; multiple layer welds when nominal thickness exceeds 3/16 in. |
| 4, Gr. 1 | $\le 5/8\text{ in.}$ ($16\text{ mm}$) by weld type (groove; socket and fillet; seal and non-load-carrying attachments) | Mandatory preheat applied; specified carbon content $\le 0.15%$; fillet or socket weld throat $\le 1/2\text{ in.}$ (13 mm); multiple layer welds above 3/16 in. |
| 5A, Gr. 1 | $\le 5/8\text{ in.}$ ($16\text{ mm}$) by weld type | Mandatory preheat applied; carbon $\le 0.15%$; throat limits on socket and fillet welds; multiple layer welds above 3/16 in. |
| 8, 10H, 10I | All | PWHT neither required nor prohibited (10I limited to nominal thickness $\le 1/2\text{ in.}$) |
| 9A | All | Pipe carbon $\le 0.15%$; nominal material thickness $\le 1/2\text{ in.}$ (13 mm); mandatory preheat applied |
| 9B | All | Nominal material thickness $\le 5/8\text{ in.}$ (16 mm) and the WPS qualified on material of equal or greater thickness than the production weld |
| 7, Gr. 2 · 15E · 62 | — | No exemptions from PWHT |
[!WARNING] P-No. 15E has no exemption. Grade 91 piping is postweld heat treated regardless of thickness or weld type. Conversely, do not assume P-Nos. 4 and 5A have no exemptions — they do, but only up to a 5/8 in. control thickness and only with the carbon, preheat, throat, and multiple-layer limitations satisfied together.
Heating and Cooling Rate Regulations (Para 331.1.4)
Para 331.1.4 states the limit in one sentence: Above 315°C (600°F), the rate of heating and cooling shall not exceed 335°C/h (600°F/hr) divided by one-half the maximum material thickness in inches at the weld, but in no case shall the rate exceed 335°C/h (600°F/hr).
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| B31.3 PARA 331.1.4 RATE LIMIT ABOVE 600°F (315°C) |
+---------------------------------------------------------------------------------------------------+
| Max Rate = 600 degF/hr / (0.5 x t) == 1,200 / t, never more than 600 degF/hr |
| |
| t = 0.5 in.: 1,200 / 0.5 = 2,400 degF/hr --> CAPPED at 600 degF/hr |
| t = 2.0 in.: 1,200 / 2.0 = 600 degF/hr --> 600 degF/hr (at the cap) |
| t = 3.0 in.: 1,200 / 3.0 = 400 degF/hr |
| t = 8.0 in.: 1,200 / 8.0 = 150 degF/hr |
| |
| Below 600 degF (315 degC): Para 331.1.4 imposes no rate limit. |
+---------------------------------------------------------------------------------------------------+
[!IMPORTANT] Two persistent errors. The divisor is one-half the maximum material thickness, so the working form is $1{,}200/t$, not $600/t$. And B31.3 sets no general 100°F/hr floor — that language belongs to Section VIII. B31.3 does impose specific cooling controls through Table 331.1.1 notes, for example a maximum of 100°F/hr above 1,200°F for P-Nos. 7 and 10I and a mandated 300–600°F/hr cooling rate for P-No. 11A, but these are per-material notes, not a general minimum.
Local Heat Treatment (Para 331.2.6)
Furnace treatment of an entire piping system is rarely practical in the field. Para 331.2.6 permits local PWHT by heating a circumferential band around the entire component with the weld located in the centre of the band, with the dimensions expressed as multiples of thickness:
- Girth welds: the width of the band heated to the specified temperature range shall be at least three times the wall thickness at the weld of the thickest part being joined.
- Nozzle and attachment welds: the heated band shall extend beyond the weld on each side at least two times the run pipe thickness and shall extend completely around the run pipe.
<----------- HEATED BAND >= 3t, FULL 360 deg ----------->
+--------------------+--------------+--------------------+
| | \ WELD / | |
| BASE PIPE | \ / | BASE PIPE |
| | ======== | |
+--------------------+--------------+--------------------+
Para 331.2.6 adds three controls an inspector should verify: thermocouple placement guidance in AWS D10.10 Sections 5, 6, and 8; special consideration for thermocouple placement near large heat sinks such as valves or fittings and when joining parts of different thickness; and the hard limit that no part of the material subjected to the heat source shall exceed the lower transformation temperature except as permitted by Para 331.2.1. Particular care applies when the PWHT temperature sits close to the lower transformation temperature, as it does for P-No. 15E. Local heating may not be used for austenitizing heat treatments.
Para 331.2.5 covers the related partial-furnace case: where an assembly will not fit in the furnace, it may be heat treated in more than one heat provided there is at least 1 ft (300 mm) of overlap between successive heats and the parts outside the furnace are protected from harmful gradients — and again, not for austenitizing treatments of ferritic materials.
Real-World Inspection Scenarios & High-Frequency Exam Traps
Scenario: The 1.125-inch Wall Carbon Steel Spool
A contractor welds a 14 in. Schedule 100 A106-B pipe spool (nominal wall 1.125 in., P-No. 1, material test report carbon content 0.24%). The site QA manager, reading the thickness row only, issues a 200°F preheat instruction and stops work when the crew strikes an arc at 60°F metal temperature.
The CWI Audit Finding: The QA manager has misread the row. Table 330.1.1 raises P-No. 1 to 200°F (95°C) only when the greater material thickness is at or above 1 in. and the carbon content exceeds 0.30%. At 0.24% carbon the second condition fails, so the required minimum is 50°F (10°C), which the 60°F metal temperature already satisfies. The crew may weld. Note what the CWI must still verify: the 50°F minimum has to hold across the full preheat zone — the larger of 3 in. or $1.5 \times 1.125 = 1.69\text{ in.}$, so 3 in. — in all directions from the arc, and the WPS may still impose a higher preheat than the Code minimum, in which case the WPS governs.
Scenario: The Grade 91 Soak Window
A fabricator PWHTs a Grade 91 (P-No. 15E) spool with a 0.400 in. control thickness and programs 1 hr/in., giving 24 minutes.
The CWI Audit Finding: Nonconformance. Table 331.1.1 gives P-No. 15E a 30-minute minimum soak, not the 15 minutes that applies to the other P-Numbers, so the correct hold is 30 minutes. The holding temperature range is 1,300–1,425°F (705–775°C), and there is no thickness exemption available — Table 331.1.3 lists "no exemptions from PWHT" for P-No. 15E.
Common Exam Traps to Avoid
- The Carbon-vs-Tensile Mix-Up: B31.3 P-No. 1 is gated on carbon content; P-Nos. 3, 5A, and 5B are gated on specified minimum tensile strength (65 ksi for P-No. 3, 60 ksi for P-Nos. 5A/5B).
- The Phantom 175°F: 175°F does not appear in the current Table 330.1.1. The carbon-steel escalation value is 200°F (95°C).
- The P-No. 4 Overthink: P-No. 4 preheat is 250°F for all materials, all thicknesses, with no additional limits.
- The Rate-Formula Halving Error: $600/t$ gives half the allowable rate. Use $1{,}200/t$, capped at 600°F/hr, and remember there is no general 100°F/hr floor.
- The Soak-Minimum Trap: 15 minutes for the main block; 30 minutes only for P-No. 15E.
- The Band-Width Unit Error: Local PWHT band width is 3× the wall thickness of the thickest part for girth welds, and 2× the run pipe thickness beyond each side for nozzle and attachment welds — not a fixed 1 in. offset.
- The Preheat Zone Distance: 3 in. (75 mm) or 1.5 × the greater nominal thickness, whichever is larger, in all directions — with 1 in. (25 mm) for tack welds. B31.1 Para 131.1 uses the identical figures.
Under ASME B31.3 Table 330.1.1, what is the mandatory minimum preheat temperature for welding P-No. 1 carbon steel piping with a nominal wall thickness of 1.25 in. (32 mm) and a specified maximum carbon content of 0.25%?
During postweld heat treatment of a P-No. 1 piping component with a maximum material thickness of 3.0 inches at the weld, what is the maximum permissible rate of heating and cooling above 600°F (316°C) under ASME B31.3 Para 331.1.4?
According to ASME B31.3 Para 331.2.6, what is the minimum width of the band heated to the specified temperature range when locally postweld heat treating a girth butt weld between a 0.75 in. wall pipe and a 1.25 in. wall fitting?