5.3 Post-Weld Heat Treatment (PWHT) & Impact Testing (UCS-56, UCS-66)

Post-Weld Heat Treatment (PWHT) & Impact Testing (UCS-56, UCS-66)

In pressure vessel engineering, welding introduces severe residual tensile stresses (approaching the yield strength of the material) and creates hardened, notch-sensitive microstructures within the Heat-Affected Zone (HAZ). To prevent catastrophic brittle fracture and environmental cracking (such as Caustic Cracking or Amine Stress Corrosion Cracking per API RP 571), ASME Section VIII Division 1 enforces rigorous rules for Post-Weld Heat Treatment (PWHT per UCS-56) and Impact Testing Exemption (UCS-66).

An API 510 Inspector must master the thermal parameters governing PWHT as well as the multi-step protocol for establishing the Minimum Design Metal Temperature (MDMT).

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1. Post-Weld Heat Treatment Requirements (ASME UCS-56)

Post-Weld Heat Treatment (PWHT)—also called stress relieving—involves heating a completed welded vessel or sub-assembly to a controlled elevated temperature below the lower transformation temperature ($A_1$), soaking for a specified holding period, and cooling at a controlled rate.

Primary Metallurgical Objectives of PWHT:

1. Relieve Residual Stresses: Thermally relaxes weld-induced residual tensile stresses to less than 15–20% of yield strength.
2. Temper Hardened Microstructures: Softens brittle martensitic/bainitic phases in the HAZ, restoring ductility and notch toughness.
3. Hydrogen Bake-Out: Accelerates the diffusion of trapped atomic hydrogen out of the weldment, mitigating Delayed Hydrogen-Assisted Cracking.
4. Dimensional Stability: Ensures dimensional accuracy during subsequent machining operations.

+-----------------------------------------------------------------------------------------+
|                        ASME UCS-56 PWHT THERMAL CYCLE ARCHITECTURE                      |
|                                                                                         |
|   Temperature (°F)                                                                      |
|                                                                                         |
|   1,100°F |------------------+==============================+-----------------------    |
|           |                  |   SOAK PERIOD (HOLDING TIME) |                      \    |
|           |                  |   1 hr/in (up to 2")         |                       \   |
|           |                  |   + 15 min/in (over 2")      |                        \  |
|     800°F |-----+------------+                              +------------+----------+-- |
|           |    / Controlled Heating                                       \ Controlled  |
|           |   /  Rate: Max 400/t °F/hr                                     \ Cooling    |
|           |  /   (Max 400°F/hr, Min 100°F/hr)                               \ Rate:     |
|           | /                                                                \ 400/t    |
|   Ambient |+                                                                  +----->   |
|           +-------------------------------------------------------------------------+   |
|             Unrestricted Rate | <--- CONTROLLED ZONE ABOVE 800°F ---> | Unrestricted    |
|             (Below 800°F)     |                                       | (Below 800°F)   |
+-----------------------------------------------------------------------------------------+

Master PWHT Rules for Carbon Steels (P-No. 1, Groups 1, 2, 3)

2. Alternative Lower Holding Temperatures (Table UCS-56.1)

When metallurgical, service, or structural constraints prevent heating a carbon steel vessel to 1,100°F, ASME Section VIII paragraph UCS-56.1 permits PWHT at lower holding temperatures, provided the holding time is proportionately increased:

Table UCS-56.1 Holding Temperature & Time Multipliers

[!WARNING] Service Exclusion Trap: Lower holding temperatures under Table UCS-56.1 are prohibited for vessels in lethal substance service or vessels where PWHT is mandated for environmental cracking resistance (such as Caustic, Amine, or Carbonate stress corrosion cracking per API RP 571 / NACE SP0472), because stress relief kinetics at 950°F are insufficient to reduce residual stresses below cracking thresholds.

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3. Low-Temperature Impact Testing Exemption (ASME UCS-66)

Brittle fracture is a rapid, catastrophic failure mechanism that occurs without plastic deformation at temperatures below a metal's ductile-to-brittle transition temperature. ASME Section VIII Division 1 paragraph UCS-66 establishes a comprehensive screening methodology to determine whether carbon and low-alloy steel vessels require Charpy V-notch impact testing per UG-84 or are exempt down to a calculated Minimum Design Metal Temperature (MDMT).

+-----------------------------------------------------------------------------------------+
|                     ASME UCS-66 MATERIAL EXEMPTION CURVES (A, B, C, D)                  |
|                                                                                         |
|   MDMT (°F)                                                                             |
|   +120 |                                                                                |
|        |        [CURVE A: Lowest Toughness] ---> As-rolled, coarse grain (SA-283, 285)  |
|    +80 |       /                                                                        |
|        |      /    [CURVE B: Moderate Toughness] -> As-rolled SA-516, SA-106, WCB cast |
|    +40 |     /    /                                                                     |
|        |    /    /    [CURVE C: Good Toughness] ---> SA-516 Gr 55/60 if not normalized       |
|      0 |   /    /    /                                                                  |
|        |  /    /    /    [CURVE D: Superior Toughness] -> All normalized SA-516 grades |
|    -40 | /    /    /    /                                                               |
|        |/    /    /    /                                                                |
|    -80 +----+----+----+--------------------------------------------------------------> |
|        0.5  1.0  1.5  2.0    Governing Thickness (inches)                               |
+-----------------------------------------------------------------------------------------+

Material Assignments to UCS-66 Curves

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4. Multi-Step Procedure to Establish Component MDMT

To determine the final allowable MDMT without mandatory Charpy impact testing, the inspector executes a structured four-step evaluation:

+-----------------------------------------------------------------------------------------+
|                    COMPLETE UCS-66 / UCS-68 MDMT CALCULATION WORKFLOW                   |
|                                                                                         |
|   [STEP 1: GOVERNING THICKNESS (t_g)] ---> Identify thickness of shell, head, or nozzle |
|                                            welded joint                                 |
|                                                  |                                      |
|                                                  v                                      |
|   [STEP 2: BASELINE MDMT (UCS-66)]   ---> Assign material to Curve A, B, C, or D; read  |
|                                            baseline MDMT from Figure UCS-66 at t_g      |
|                                                  |                                      |
|                                                  v                                      |
|   [STEP 3: COINCIDENT RATIO (UCS-66.1)]-> Calculate Ratio = t_required / (t_actual * E*)|
|                                            Determine Temperature Reduction from Fig 66.1|
|                                                  |                                      |
|                                                  v                                      |
|   [STEP 4: PWHT BONUS REDUCTION]     ---> If PWHT is performed voluntarily (NOT code    |
|   (UCS-68(c))                              mandated), subtract an additional -30°F      |
|                                                  |                                      |
|                                                  v                                      |
|   [FINAL MDMT COMPUTATION]           ---> MDMT_final = Baseline_MDMT - Reduction - Bonus|
|                                            (Exemption floor capped at -155°F)           |
+-----------------------------------------------------------------------------------------+

Step 1: Determine Governing Thickness ($t_g$)

• For welded flat plates and shells: $t_g = $ nominal plate thickness.
• For formed heads: $t_g = $ thinnest point after forming.
• For welded attachments and nozzles: $t_g = $ governing weld throat or nozzle neck thickness per UCS-66(a).

Step 2: Read Baseline MDMT from Figure UCS-66

Enter Figure UCS-66 with $t_g$ on the horizontal axis, move vertically to the assigned Curve (A, B, C, or D), and read the baseline temperature on the vertical axis.

Step 3: Determine Temperature Reduction from Figure UCS-66.1

If the component operates at a stress level below its allowable limit, it possesses excess wall thickness, which significantly reduces the driving force for brittle fracture. The inspector calculates the Coincident Stress Ratio ($R$):

$R = \frac{t_{\text{required}}}{t_{\text{actual}} \times E^*}$

Where:

• $t_{\text{required}} = $ Minimum required thickness calculated per ASME design formulas (using $E = 1.0$).
• $t_{\text{actual}} = $ Nominal thickness minus corrosion allowance ($t_{\text{nom}} - CA$).
• $E^* = $ Weld joint efficiency of the component ($1.0$ for seamless components).

Using Ratio $R$, enter Figure UCS-66.1 to find the allowable Temperature Reduction ($T_{\text{reduction}}$):

• If $R = 1.00$: Temperature Reduction $= 0^\circ\text{F}$.
• If $R = 0.80$: Temperature Reduction $\approx 20^\circ\text{F}$ (11°C).
• If $R = 0.70$: Temperature Reduction $\approx 32^\circ\text{F}$ (18°C).
• If $R \le 0.35$: Component qualifies for MDMT down to $-155^\circ\text{F}$ without impact testing.

Step 4: Apply UCS-68(c) Bonus PWHT Reduction

Under ASME UCS-68(c):

If Post-Weld Heat Treatment is performed when it is NOT mandated by code rules (e.g., for P-No. 1 carbon steels with thickness $\le 1.25\text{ in.}$ where PWHT is optional), a bonus temperature reduction of 30°F (17°C) is subtracted from the MDMT!

$\mathbf{MDMT_{\text{final}} = MDMT_{\text{baseline}} - T_{\text{reduction}} - 30^\circ\text{F} \quad \text{(if UCS-68(c) applies)}}$

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5. Charpy V-Notch Impact Testing Fundamentals (ASME UG-84)

When a component operates colder than its exempt MDMT, Charpy V-Notch (CVN) impact testing per UG-84 becomes mandatory:

• Test Specimens: Three standard full-size specimens ($10\text{ mm} \times 10\text{ mm} \times 55\text{ mm}$) machined transverse to the rolling/welding direction.
• Testing Locations: Tested across the weld metal and the Heat-Affected Zone (HAZ).
• Acceptance Criteria: Must meet minimum absorbed energy (foot-pounds) based on specified minimum yield strength (SMYS) and thickness per Table UG-84.1 (typically 15 ft-lbs to 20 ft-lbs for standard carbon steels).

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6. Comprehensive Step-by-Step MDMT Calculation Problem

Problem Statement

An in-service vertical vessel shell is fabricated from SA-516 Gr 70 (as-rolled, not normalized) plate.

• Nominal Shell Thickness ($t_{\text{nom}}$): $1.00\text{ in.}$
• Corrosion Allowance ($CA$): $0.125\text{ in.}$
• Design Pressure ($P$): $200\text{ psig}$
• Inside Radius ($R$): $36.0\text{ in.}$
• Allowable Stress ($S$): $20,000\text{ psi}$
• Joint Type: Type 1 butt weld, spot radiographed ($E = 0.85$).
• PWHT Status: PWHT was voluntarily performed during fabrication (not code mandated for $1.00\text{ in.}$ P-No. 1).

Calculate the final allowable Minimum Design Metal Temperature (MDMT) without mandatory Charpy impact testing.

Step 1: Determine Governing Thickness ($t_g$) and Material Curve

• Governing thickness $t_g = 1.00\text{ in.}$
• SA-516 Gr 70 as-rolled is assigned to Curve B.

Step 2: Read Baseline MDMT from Figure UCS-66

From ASME Figure UCS-66, for Curve B at $t_g = 1.00\text{ in.}$, the baseline MDMT is: $\mathbf{MDMT_{\text{baseline}} = +31^\circ\text{F}}$

Step 3: Compute Coincident Stress Ratio ($R$) and Figure UCS-66.1 Reduction

Calculate required shell thickness ($t_{\text{req}}$) using UG-27(c)(1) with $E = 1.0$: $t_{\text{req}} = \frac{P \cdot R}{S \cdot 1.0 - 0.6 \cdot P} = \frac{200 \times 36.0}{(20,000 \times 1.0) - (0.6 \times 200)} = \frac{7,200}{19,880} = 0.3622\text{ in.}$

Calculate actual corroded thickness ($t_{\text{actual}}$): $t_{\text{actual}} = t_{\text{nom}} - CA = 1.00 - 0.125 = 0.875\text{ in.}$

Compute Ratio ($R$): $R = \frac{t_{\text{req}}}{t_{\text{actual}} \times E^*} = \frac{0.3622}{0.875 \times 1.0} = \mathbf{0.414}$

From Figure UCS-66.1, for Ratio $R = 0.414$, the temperature reduction is: $T_{\text{reduction}} = (1 - R) \times 100^\circ\text{F} \approx (1 - 0.414) \times 100 = \mathbf{58.6^\circ\text{F} \approx 59^\circ\text{F}}$

Step 4: Apply UCS-68(c) Bonus PWHT Reduction

Because PWHT was performed on $1.00\text{ in.}$ P-No. 1 carbon steel (which is not mandated by UCS-56): $\text{Bonus Reduction} = \mathbf{30^\circ\text{F}}$

Step 5: Compute Final MDMT

$MDMT_{\text{final}} = MDMT_{\text{baseline}} - T_{\text{reduction}} - \text{Bonus}$ $MDMT_{\text{final}} = +31^\circ\text{F} - 59^\circ\text{F} - 30^\circ\text{F} = \mathbf{-58^\circ\text{F} (-50^\circ\text{C})}$

Conclusion: The vessel can safely operate down to -58°F (-50°C) without requiring Charpy impact testing.