{"id":4216,"date":"2026-05-25T02:53:03","date_gmt":"2026-05-25T02:53:03","guid":{"rendered":"https:\/\/zxweldingrobot.com\/?p=4216"},"modified":"2026-05-26T07:22:07","modified_gmt":"2026-05-26T07:22:07","slug":"post-weld-heat-treatment","status":"publish","type":"post","link":"https:\/\/zxweldingrobot.com\/pt\/blog\/post-weld-heat-treatment\/","title":{"rendered":"Tratamento T\u00e9rmico P\u00f3s-Soldagem (PWHT): Conformidade e Integra\u00e7\u00e3o Rob\u00f3tica da Se\u00e7\u00e3o VIII da ASME"},"content":{"rendered":"

Post Weld Heat Treatment (PWHT): The Engineer’s Complete Guide to Code Compliance, Process Parameters & Inspection<\/p>\n

Most engineers know post weld heat treatment exists. Fewer know exactly when their code mandates it – and that distinction matters more than the treatment itself. A fabricator who applies PWHT to every carbon steel weld wastes time and money. One who skips it on a P91 steam line creates a brittle fracture waiting to happen.This guide cuts through the ambiguity. You will find specific ASME thickness thresholds (including the nuanced 38mm rule), a verified temperature and hold-time table across six material grades, a step-by-step equipment guide with soak-band dimensions, and inspection acceptance criteria with hardness limits. Practical power generation case studies, drawn from Zhouxiang\u2019s field projects, show how inter-pass temperature control connects directly to PWHT effectiveness.<\/p>\n

What Is Post Weld Heat Treatment?<\/h2>\n

\"What<\/p>\n

Post weld heat treatment (PWHT) is a controlled thermal process applied to a welded assembly after welding is complete. The assembly is heated to a specified temperature – always below the lower critical transformation temperature (Ac1, approximately 720-730C for carbon steel) – held at that temperature for a defined period, then cooled at a controlled rate.<\/p>\n

The distinction matters: PWHT does not re-melt the weld or alter the weld geometry. It operates entirely in the solid state, allowing the steel\u2019s atomic lattice to relax, redistribute locked-in stresses, and in some materials, temper a hard microstructure into a tougher one. The welding process deposits heat in a highly concentrated, non-uniform pattern – heat treatment after welding corrects the metallurgical consequences of that asymmetry. It is an engineering precision task, not a simple \u201cheat and cool\u201d operation.<\/p>\n

\n
\n
Temperature Range<\/div>\n
595 \u2013 770\u00a0\u00b0C<\/div>\n<\/div>\n
\n
Hold Time<\/div>\n
1 hr \/ 25\u00a0mm (min 30 min)<\/div>\n<\/div>\n
\n
Heating Rate<\/div>\n
Max 400\u00b0F\/hr \u00f7 wall thickness<\/div>\n<\/div>\n
\n
Max Temp Differential<\/div>\n
139\u00a0\u00b0C (250\u00a0\u00b0F) during soak<\/div>\n<\/div>\n
\n
Process Standard<\/div>\n
ASME UCS-56 \/ B31.1 \/ B31.3<\/div>\n<\/div>\n<\/div>\n

Residual stress reduction is the primary goal of post welding heat treatment. Secondary goals include improving toughness and ductility in the heat-affected zone (HAZ), softening hard martensitic microstructures, and driving out diffusible hydrogen that accumulated during welding. All three objectives reduce the risk of in-service cracking.<\/p>\n

<\/p>\n

Why Welding Creates Residual Stress<\/h2>\n

\"Why<\/p>\n

When a welding arc heats metal to 1,500C or higher, the surrounding base material stays relatively cool. Rapid cooling from welding temperatures then forces the hot weld metal to contract against the restraint of the cooler surrounding structure. The welded joint cannot contract freely – the surrounding material holds it. The result: the weld metal and the HAZ are placed in a state of residual tensile stress that can approach the yield strength of the material. For a mild carbon steel with a yield strength of 250MPa, the locked-in residual stress can reach 200-250MPa even in a perfectly made weld.<\/p>\n

Two failure mechanisms are directly amplified by high residual stress:<\/p>\n