Fab PWHT

Module from the Pressure Vessel Engineering Curriculum curriculum.

Module 7 - Fabrication, Welding, and PWHT · Learning Objectives · 1. Classify weld joints A, B, C, D per UW-3 and identify RT requirements · 2. Apply PWHT per UCS-56 and Table UCS-56-1 (CS) or UHA-32 (SS) · 3. Apply hardness limits for sour service (NACE MR0175 HB <= 200 for CS) · 4. Specify PMI (positive material identification) for alloy materials · 5. Monitor weld repair limits (UW-38) and require re-PWHT at repairs · Weld Joint Categories (UW-3)

Cat	Location	Typical Joint	Efficiency Range	NDE	Notes
A	Longitudinal in shell/nozzle + head joint to cylinder (if non-circ)	Butt	0.70-1.00	RT level per spec	Highest stress
B	Circumferential shell joint or shell-to-head (hemis/ellip)	Butt	0.70-1.00	RT per spec	Next highest
C	Flange face to shell or head	Butt/corner	0.70-1.00	VT at minimum	Flange area
D	Nozzle neck to shell/head	Butt/corner/fillet	Varies	Per UW-11	Reinforced per UG-37
PWHT Triggers (UCS-56 Table UCS-56-1 - selected rows)
P-No. Material	Thickness Trigger (in)	Hold Temp (F)	Hold Time/in	Min Hold	Notes
P-1 CS (<0.35 C)	>1.5	1100-1200	1 hr/in	15 min	Most CS
P-1 CS (>0.35 C or alloy added)	>0.75	1100-1200	1 hr/in	15 min	Sour service common
P-3 (C-Mn-Si)	>0.625	1100-1200	1 hr/in	30 min	Enhanced
P-4 (1.25Cr-0.5Mo)	All	1250-1300	1 hr/in	2 hr	Always PWHT
P-5A (2.25Cr-1Mo)	All	1275-1375	1 hr/in	2 hr	Always
P-5B (9Cr or V-added)	All	1400-1450	1 hr/in	2 hr	Narrow window

Source: Pressure_Vessel_Engineering_Curriculum_v1.xlsx · Sheet: Fab PWHT

© 2026 Inflection Point Engineering, LLC. All rights reserved. The content of this page — including calculation methods, reference data, written analysis, interactive tools, and source code — is the intellectual property of Inflection Point Engineering, LLC and is protected under applicable copyright, trademark, and trade secret laws. Unauthorized reproduction, redistribution, modification, or derivative use in whole or in part is prohibited without prior written consent.

Disclaimer. This material is provided for informational and educational purposes only and does not constitute professional engineering advice. Calculations, reference data, and methodologies are based on published standards and accepted engineering practice but are not a substitute for engineering judgment, site-specific analysis, or review by a licensed Professional Engineer. Inflection Point Engineering, LLC makes no warranties, express or implied, regarding the accuracy, completeness, or fitness for a particular purpose of any content presented here, and shall not be liable for any direct, indirect, incidental, or consequential damages arising from its use. Users assume all risk associated with applying this content to real-world design, operations, or decisions.