11.2 B31.1 Postweld Heat Treatment (PWHT) Rules, Temperatures & Holding Times

11.2 B31.1 Postweld Heat Treatment (PWHT) Rules, Temperatures & Holding Times

Postweld heat treatment (PWHT) is a controlled thermal operation performed after welding to relieve locked-in residual stresses and restore ductility to the weld deposit and heat-affected zone (HAZ). Under ASME B31.1 Power Piping, PWHT is governed by Paragraph 132 and Table 132. Section IX tells you how PWHT affects procedure qualification ranges (QW-407); B31.1 tells you whether PWHT is mandatory for production piping, the soak temperatures and times, the thermal cycle limits, and the conditions under which PWHT may be omitted.

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Metallurgical Purpose of Postweld Heat Treatment

During fusion welding, molten weld metal solidifies and contracts inside the rigid restraint of the surrounding pipe, locking yield-level tensile residual stress into the root, cap, and adjacent parent metal. In power generation service — high pressure, elevated temperature, cyclic thermal fatigue, and aggressive boiler feedwater chemistry — un-stress-relieved joints are prone to brittle fracture, stress corrosion cracking, and premature creep damage.

Subcritical PWHT (heating below the lower transformation temperature $A_1$) achieves three outcomes:

1. Thermal Stress Relaxation: At soak temperature the yield strength of the steel drops sharply. Locked-in elastic strain relaxes by localized creep, pulling peak residual tensile stress down from near yield toward a low baseline.
2. Tempering of Hard Microstructures: Rapid weld cooling leaves portions of the HAZ as untempered martensite or bainite. PWHT precipitates fine carbides from the supersaturated matrix, converting brittle martensite into tempered martensite with far better notch toughness.
3. Hydrogen Effusion: Residual diffusible hydrogen is driven out of the lattice, removing the delayed-cracking risk that would otherwise persist through hydrostatic testing and early service.

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Table 132 Holding Temperatures and Soak Times

Para 132.1 requires that, except as provided in Paras 132.2 and 132.3, all welds in the P-Numbers listed in Table 132 receive PWHT within the temperature range specified in that table. Welds in materials not listed in Table 132 are heat treated in accordance with the WPS.

[!IMPORTANT] Three numbers candidates routinely get wrong. (1) P-No. 3 holds in the same 1,100–1,200°F band as P-No. 1 — it is not a wider band. (2) P-Nos. 5A and 5B both start at 1,250°F, not 1,300°F or 1,350°F. (3) The absolute minimum soak is 15 minutes for every P-Number in the main block, including P-No. 4 and P-No. 5A; only P-No. 15E carries a different minimum (30 minutes). Options that assign a "1 hour minimum" to P-No. 4 or P-No. 5A are distractors.

+-----------------------------------------------------------------------------------------+
|                   B31.1 TABLE 132 HOLDING TIME CALCULATION                              |
+-----------------------------------------------------------------------------------------+
| 1. Control thickness t <= 2.0 in. (50 mm):                                              |
|    Holding Time = t (inches) x 1.0 hour/inch, subject to the table minimum              |
|    - Example (P-1, t = 0.50 in.): 0.50 x 60 min = 30 minutes                            |
|    - Example (P-1, t = 0.1875 in.): 0.1875 x 60 min = 11.25 min --> HOLD 15 MIN         |
|    - Example (P-15E, t = 0.25 in.): 0.25 x 60 min = 15 min --> HOLD 30 MIN              |
|                                                                                         |
| 2. Control thickness t > 2.0 in. (50 mm):                                               |
|    Holding Time = 2.0 hours + 15 minutes for each additional inch (or fraction) over 2" |
|    - Example (P-1, t = 3.0 in.): 2.0 hr + (1 x 15 min) = 2 hr 15 min (135 minutes)      |
|    - Example (P-4, t = 4.0 in.): 2.0 hr + (2 x 15 min) = 2 hr 30 min (150 minutes)      |
+-----------------------------------------------------------------------------------------+

Table 132.1: Alternate Lower-Temperature PWHT for P-Nos. 1 and 3

When it is impractical to PWHT at the Table 132 range, B31.1 permits carbon and low-alloy steels to be held lower for longer in accordance with Table 132.1:

Two conditions control this table: the listed times apply to thicknesses up to 1 in., with 15 min/in. added for control thicknesses over 1 in.; and a decrease of more than 100°F below the specified minimum is allowable only for P-No. 1, Group Nos. 1 and 2 materials.

Upper Limit and Dissimilar Joints (Para 132.2)

The upper limit of the Table 132 range is a recommended value that may be exceeded, provided the actual temperature does not exceed the lower critical temperature of either material. When parts of two different P-Numbers are joined, PWHT shall be that specified for the material requiring the higher PWHT temperature; and when a nonpressure part is welded to a pressure part, the maximum PWHT temperature shall not exceed the maximum acceptable for the pressure-retaining part.

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Control Thickness: What Actually Drives the Soak (Para 132.4)

Para 132.4.1 defines control thickness as the lesser of:

• (A) the thickness of the weld; or
• (B) the thicker of the materials being joined at the weld, or the thickness of the pressure-containing material when a nonpressure-containing part is being attached.

Para 132.4.2 then defines "thickness of the weld" by joint type — for groove welds (girth and longitudinal), the thicker of the two abutting ends after weld preparation including I.D. machining; for fillet welds, the throat; for partial penetration welds, the depth of the groove; for repair welds, the depth of the cavity; and for branch connections, the groove depth plus the throat of any cover fillet.

[!NOTE] Control thickness is a "lesser of" test, and that matters commercially. A 3/8 in. throat attachment fillet welded onto a 2 in. wall header has a control thickness of 3/8 in., not 2 in. The exemption tables and the soak time are both read against that 3/8 in.

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Exemptions From Mandatory PWHT (Para 132.3 and Table 132.2)

B31.1 concentrates its PWHT exemptions in Table 132.2, and Para 132.3.1 adds three blanket exemptions unless the qualified WPS or the designer requires otherwise: welds in nonferrous materials; welds exempted in Table 132 or Table 132.2; and welds subjected to temperatures above the lower critical temperature during fabrication, provided the WPS was qualified with PWHT at that range. Para 132.3.2 allows the exemptions to be judged on actual ladle or product analysis rather than specification maxima — a frequently useful concession on carbon content.

For P-No. 1 carbon steel, B31.1 provides two routes, and every condition inside a route must be met:

• Route 1 — the 3/4 in. route. PWHT is not mandatory when the control thickness is 3/4 in. (19 mm) or less, and a minimum preheat of 200°F (95°C) is applied when the nominal material thickness of either base metal exceeds 1 in. (25 mm).
• Route 2 — the low-hardenability route to 1-1/2 in. For P-No. 1 material over 3/4 in. (19 mm) but not more than 1-1/2 in. (38 mm), PWHT is not mandatory provided all of the following hold: the carbon equivalent CE is less than 0.50; a minimum preheat of 250°F (121°C) is applied; and the maximum weld deposit thickness of each weld pass does not exceed 1/4 in. (6 mm).

$\text{CE} = \text{C} + \frac{\text{Mn} + \text{Si}}{6} + \frac{\text{Cr} + \text{Mo} + \text{V}}{5} + \frac{\text{Ni} + \text{Cu}}{15}$

If analysis for the last two terms is unavailable, B31.1 permits substituting 0.1% for them, giving the short form $\text{CE} = \text{C} + (\text{Mn} + \text{Si})/6 + 0.1$.

For the low-alloy P-Numbers, the exemption conditions are narrower and are organized by weld type rather than by a single thickness. Read the table rather than a rule of thumb, but know the shape of it:

• P-No. 3 exemptions turn on a control thickness limit, a specified carbon content of 0.25% or less, and applied preheat.
• P-No. 4 and P-No. 5A exemptions exist for pipe welds and attachment welds at a 1/2 in. (13 mm) nominal material thickness limit with a specified carbon content of 0.15% or less, for seal welds with a throat of 3/8 in. (9 mm) or less, for socket welds and slip-on flange welds with a throat and pipe wall of 1/2 in. (13 mm) or less and carbon of 0.15% or less, and for non-load-carrying attachment welds welded by SMAW (low-hydrogen electrodes) or GTAW with a throat of 3/16 in. or less.
• P-No. 15E and other CSEF materials are the strict case: mandatory PWHT with no general thickness exemption.

[!WARNING] Do not memorize "P-No. 5 has no exemptions" or "P-No. 4 is limited to NPS 4." Neither statement is in the Code. P-No. 4 and P-No. 5A both carry weld-type exemptions in Table 132.2, and none of them is written as a nominal pipe size limit. This is an open-book exam: tab Table 132.2 and read the row.

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Heating and Cooling Rate Restrictions Above 600°F (Para 132.5)

Uncontrolled heating or cooling creates through-wall thermal gradients that can distort the component or initiate secondary cracking. Para 132.5 states the limit in one sentence:

Above 600°F (315°C), the rate of heating and cooling shall not exceed 600°F (335°C) per hour divided by one-half the maximum thickness of material in inches at the weld, but in no case shall the rate exceed 600°F (335°C) per hour.

$\text{Max Rate} \left(^\circ\text{F/hr}\right) = \min\!\left(\frac{600}{0.5\,t},\; 600\right) = \min\!\left(\frac{1{,}200}{t},\; 600\right)$

+-----------------------------------------------------------------------------------------+
|                  B31.1 PARA 132.5 RATE LIMIT ABOVE 600°F (315°C)                        |
+-----------------------------------------------------------------------------------------+
|   Max Rate = 600 degF/hr / (0.5 x t)  ==  1,200 / t   [capped at 600 degF/hr]           |
|                                                                                         |
| - t = 1.0 in.:  1,200 / 1.0 = 1,200 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 = 6.0 in.:  1,200 / 6.0 =   200 degF/hr                                             |
| - t = 8.0 in.:  1,200 / 8.0 =   150 degF/hr                                             |
|                                                                                         |
| Below 600 degF (315 degC): no rate limit is imposed by Para 132.5.                      |
+-----------------------------------------------------------------------------------------+

[!IMPORTANT] Two errors to avoid. First, the divisor is one-half the maximum thickness, not the thickness — dividing by $t$ alone halves every answer. Second, B31.1 Para 132.5 has no lower rate floor. The "need not be less than 100°F/hr" language belongs to ASME Section VIII, Division 1; importing it into B31.1 produces a wrong answer on heavy sections. B31.1 does impose specific cooling rate controls on a few materials through Table 132 notes — for example, 100°F/hr maximum above 1,200°F for P-No. 7 and P-No. 10I — but those are note-driven exceptions, not a general floor.

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Local Band Heating vs. Full Furnace PWHT

Furnace Heating (Para 132.6)

Heating an assembly in a furnace should be used when practical, because it eliminates differential gradients between adjacent components. Size or shape frequently makes it impossible in field erection, which is why local heating exists.

Local Heating (Para 132.7)

Welds may be locally heat treated by heating a circumferential band around the entire component with the weld located in the centre of the band. The dimensional requirement is stated in multiples of thickness, not in inches:

• Girth welds: the width of the band heated to the PWHT temperature shall be at least three times the wall thickness at the weld of the thickest part being joined.
• Nozzle and attachment welds: the band heated to the PWHT temperature shall extend beyond the nozzle or attachment weld on each side at least two times the header thickness, and shall extend completely around the header.

$\text{Girth Weld Heated Band Width} \ge 3 \times t_{\text{thickest part}}$

                 <----------- HEATED BAND >= 3t, FULL 360 deg ----------->
                +--------------------+--------------+--------------------+
                |                    | \  WELD  /   |                    |
                |     BASE PIPE      |  \      /    |     BASE PIPE      |
                |                    |  ========    |                    |
                +--------------------+--------------+--------------------+
                          Weld centred in the band; insulation extends
                          beyond the band to soften the gradient.

Spot heating one side of a pipe is never acceptable — the band must encircle the component so that differential expansion does not bow the line.

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Real-World Field Inspection Scenario & Exam Traps

Field Scenario: The Over-Heated Grade 91 Header

A field heat treatment contractor sets ceramic resistance pads on a 16 in. Schedule 160 SA-335 Grade P91 main steam pipe ($t = 1.594\text{ in.}$) and programs a soak of 1,460°F (793°C) for 2 hours, reasoning that "hotter is better for stress relief." The CWI audits the strip chart during the soak.

The CWI's Intervention: Stop work and raise a nonconformance. Grade 91 is P-No. 15E, and Table 132 fixes its holding range at 1,350–1,425°F (730–775°C). Although Para 132.2(A) allows the upper limit of a Table 132 range to be exceeded, it does so only while the actual temperature stays below the lower critical temperature of the material — and for Grade 91 that margin is very small, which is precisely why the table caps it at 1,425°F and why Table 132 attaches filler-metal Ni+Mn and temperature-excursion conditions to this P-Number. Heating to 1,460°F risks partial re-austenitization; on cooling the affected volume forms untempered martensite and loses creep-rupture strength. The correct soak is 1,350–1,425°F for 1 hr/in. of control thickness (1.594 hr here, comfortably above the 30-minute minimum), with the heating and cooling rate above 600°F limited to $1{,}200 / 1.594 = 753^\circ\text{F/hr}$, capped at 600°F/hr.

Common Exam Traps to Avoid

• The 1-Hour Minimum Myth: Table 132's minimum soak is 15 minutes for P-Nos. 1, 3, 4, 5A, 5B, 6, 7, 9A, and 9B alike. Only P-No. 15E uses 30 minutes. For a 0.125 in. control thickness, $1\text{ hr/in.} \times 0.125 = 7.5$ minutes, so you hold 15 minutes.
• The Rate-Formula Halving Error: $600 \div t$ gives half the allowable rate. The divisor is $0.5t$, so the working form is $1{,}200 \div t$, capped at 600°F/hr.
• The 100°F/hr Floor That Isn't: B31.1 Para 132.5 sets no minimum rate. Do not import Section VIII's floor.
• The 600°F Threshold: Rate control applies only above 600°F (315°C). Below that, still-air cooling is unrestricted by Para 132.5.
• The Band-Width Unit Error: Local PWHT band width is expressed as a multiple of thickness ($3t$ for girth welds), not as a fixed 1 in. or 2 in. offset from the weld toe.
• The Control Thickness Direction: Control thickness is the lesser of the weld thickness and the base metal thickness. Candidates who take the greater value will over-soak and mis-apply exemptions.