By Ravi Kumar Thammana | IWE | ASNT Level III | CEO, Trinity NDT WeldSolutions Pvt. Ltd., Bangalore Published: March 2026 | Reading Time: 8 minutes

Here is the complete blog post — written in the authentic voice of a 30-year welding engineering veteran:

When Is PWHT Mandatory for Carbon Steel Welds? A Welding Engineer’s Practical Guide

By Ravi Kumar Thammana | IWE | ASNT Level III | CEO, Trinity NDT WeldSolutions Pvt. Ltd., Bangalore Published: March 2026 | Reading Time: 8 minutes

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I want to start with a story.

A few years ago, a fabrication shop in Pune called us in a panic. They had completed a pressure vessel for a petrochemical client — P265GH carbon steel, 38mm wall thickness, fully welded, hydrotest passed, NDT cleared. The vessel was sitting in their yard, ready for dispatch.

Their client’s inspector arrived and asked one question:

“Where is your PWHT record?”

The fabricator had not performed PWHT. Nobody had told them it was required. The WPS didn’t mention it. The shop supervisor assumed that because the material was ordinary carbon steel — not chrome-moly, not stainless — PWHT wasn’t needed.

That vessel had to be stress-relieved before dispatch. Two weeks of delay. ₹4 lakh in additional cost. A near-miss on a critical delivery.

In 25 years of welding engineering, I have seen this mistake more times than I can count. And it almost always comes from the same assumption:

“It’s just carbon steel. PWHT is for exotic materials.”

That assumption is wrong. Dangerously wrong in some applications.

Let me explain exactly when PWHT is mandatory — and why — in plain engineering language.

What Is PWHT and What Does It Actually Do?

Post Weld Heat Treatment (PWHT) is a controlled thermal process in which a completed weld joint is heated to a specified temperature, held at that temperature for a specified time, and then cooled at a controlled rate.

For carbon steel, the typical PWHT temperature range is 595°C to 650°C (1100°F to 1200°F) — well below the lower critical transformation temperature (Ac1), so no metallurgical phase transformation occurs. This is purely a stress-relief operation.

What it achieves:

1. Residual Stress Relief Welding generates intense, localised heat followed by rapid cooling. The surrounding cold metal restrains the contracting weld metal, creating residual tensile stresses — often approaching the yield strength of the material. These stresses are invisible, unmeasurable by conventional NDT, and do not cause immediate failure. But combined with service loads and certain environments (especially hydrogen and wet H2S), they become a primary driver of cracking. PWHT reduces these residual stresses by 85–90%.

2. Hydrogen Diffusion Welding processes — particularly SMAW (stick welding) and FCAW — introduce atomic hydrogen into the weld metal and HAZ. Hydrogen causes delayed cracking (also called cold cracking or hydrogen-induced cracking), which can occur hours or even days after welding is complete. Elevated temperature accelerates hydrogen diffusion out of the joint. PWHT at 200–250°C (dehydrogenation heat treatment) or full PWHT at 595°C+ removes this risk.

3. HAZ Softening and Tempering The heat-affected zone of carbon steel welds, particularly in thicker sections or higher-carbon materials, can contain hard martensitic and bainitic microstructures. These are brittle and susceptible to stress corrosion cracking. PWHT tempers these hard zones, improving toughness and ductility.

4. Dimensional Stability For components that will be machined after welding, PWHT reduces distortion and improves dimensional accuracy.

When Does the Code Say PWHT Is Mandatory?

This is where most fabricators get confused — because the answer is not a single rule. It depends on which code governs your job, the material, the wall thickness, and the service conditions. Let me go through each major scenario.

1. Wall Thickness — The Universal Trigger

Regardless of service conditions, every major fabrication code mandates PWHT above a certain wall thickness. For carbon steel:

Code	Mandatory PWHT Wall Thickness Threshold
ASME BPVC Section VIII Div.1	> 38 mm (1.5 inches) for P-No.1 carbon steel
ASME B31.1 (Power Piping)	> 19 mm (0.75 inches) for P-No.1
ASME B31.3 (Process Piping)	> 19 mm for normal fluid service; always for Category M fluids
EN 13445 (Pressure Vessels)	> 35 mm for carbon-manganese steels
AWS D1.1 (Structural Welding)	Not thickness-based — preheat is primary; PWHT engineer-specified

Let me emphasise B31.1 and B31.3 here — the threshold is 19mm, not 38mm. I have seen fabricators apply the Section VIII 38mm rule to their B31.1 piping jobs. That is incorrect and non-compliant. Always check the specific code governing your job.

2. Carbon Equivalent (CE) — The Often-Ignored Trigger

Carbon equivalent is a formula that combines the effect of carbon and other alloying elements on hardenability. High CE materials are more susceptible to hydrogen cracking, harder HAZ formation, and brittle fracture.

The most common formula (IIW):

CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15

For carbon steel, when CE exceeds 0.45, both preheat and — in thicker sections — PWHT become critical. Many “standard” carbon steels like IS 2062 Grade B or ASTM A516 Grade 70 can have CE values of 0.42–0.48 depending on the heat. Always get the material test certificate (MTC) and calculate CE before writing your WPS.

3. Sour Service / Hydrogen-Containing Environments — Non-Negotiable

This is where PWHT transitions from a code requirement to a metallurgical necessity.

NACE MR0175 / ISO 15156 — the governing standard for equipment exposed to wet H2S (sour service in oil and gas) — mandates strict hardness limits: maximum 22 HRC (248 HBW) anywhere in the weld metal, HAZ, or base metal.

In practice, for carbon steel welds in sour service, achieving sub-22 HRC in the HAZ is extremely difficult without PWHT — especially in wall thicknesses above 12–15mm. The rapid cooling rates in the HAZ during welding promote hard microstructures, regardless of preheat.

For any carbon steel component that will be in contact with produced fluids, process streams containing H2S, or any wet sour environment — treat PWHT as mandatory, regardless of thickness. The consequence of getting this wrong is not a delayed delivery — it is catastrophic cracking, leaks, and potential fatality.

4. Impact Testing Requirements

When a design requires Charpy impact testing (toughness qualification) at low temperatures — such as for pressure vessels operating below 0°C, or LNG-related applications — PWHT is typically required as part of the WPS qualification to achieve the required toughness values in the HAZ.

If your WPS was qualified without PWHT and your job now requires impact testing, your WPS is not valid for that application. A new WPS with PWHT must be qualified.

5. Specific Materials — Even at Low Thickness

Certain carbon and low-alloy steels require PWHT at much lower thicknesses due to their chemistry:

P-No.1 Group 2 (higher carbon content, e.g. A516 Gr.70 thick plates, A105 flanges) — some specifications require PWHT above 16mm
Chrome-Moly steels (P-No.4, P-No.5) — PWHT is always mandatory regardless of thickness, though technically these are low-alloy steels, not plain carbon steels
High-carbon content repair welds on castings or forgings — always require PWHT

Common Misconceptions I Encounter Every Week

“Our material is A36 structural steel — PWHT is not needed.” Correct for general structural work under AWS D1.1. Incorrect if the same structure carries cyclic loads, operates in a corrosive environment, or has specific code requirements imposed by the end-user.

“We did preheat — that means we don’t need PWHT.” Preheat and PWHT serve different purposes. Preheat controls cooling rate during welding and reduces hydrogen cracking risk. PWHT relieves residual stresses after welding is complete. In thick sections and sour service, you need both — they are not interchangeable.

“The weld passed UT and RT — there are no defects, so PWHT is unnecessary.” RT and UT detect existing defects. They cannot detect residual stress magnitude. A weld can be completely clean on RT and UT and still fail in service due to stress corrosion cracking if PWHT was not performed. These are different failure mechanisms.

“PWHT will distort our component.” Possible — but manageable with proper fixturing, controlled heating rates, and engineering. The distortion risk from PWHT is far smaller than the integrity risk from skipping it in a mandatory application.

Practical Checklist — Should Your Carbon Steel Weld Be PWHT’d?

Work through this list before finalising your WPS:

☑ Identify your governing code — ASME VIII, B31.1, B31.3, EN 13445, AWS D1.1, or client spec ☑ Check wall thickness against code mandatory threshold (19mm for B31.1/B31.3; 38mm for Section VIII Div.1) ☑ Calculate Carbon Equivalent (CE) from MTC — above 0.45 warrants serious PWHT consideration ☑ Confirm service environment — any H2S, wet sour, hydrogen-containing service = PWHT mandatory ☑ Check design temperature — sub-zero service requires impact testing, which typically requires PWHT qualification ☑ Review client specification — client specs often impose PWHT at lower thresholds than code minimum (always check) ☑ Review repair weld requirements — repair welds on existing equipment often have stricter PWHT requirements than original fabrication

If you answer YES to any of the above — PWHT is required. Do not proceed without it.

PWHT Parameters for Carbon Steel — Quick Reference

When PWHT is required for P-No.1 carbon steel per ASME BPVC:

Parameter	Requirement
Temperature Range	595°C – 650°C (1100°F – 1200°F)
Minimum Holding Time	1 hour per 25mm of thickness (minimum 15 min for thickness < 13mm)
Heating Rate above 315°C	Max 220°C/hour (56°C × 25/thickness, not to exceed 220°C/hour)
Cooling Rate above 315°C	Max 280°C/hour (56°C × 25/thickness, not to exceed 280°C/hour)
Below 315°C	Cool in still air
Thermocouple Placement	Per ASME Section V — thermocouples at hottest and coldest points of component
Documentation	Time-temperature chart signed by Level III / Responsible Welding Engineer

Final Thought

PWHT is not a bureaucratic formality. It is not something you do to satisfy an inspector and forget about. It is a critical metallurgical process that directly determines whether your weld joint will survive its intended service life — or fail prematurely, sometimes catastrophically.

The 30-year-old pressure vessel still in service. The pipeline that has never leaked. The aircraft component that has never cracked. These outcomes are not accidents. They are the result of engineering decisions made correctly at the fabrication stage — including the decision to perform PWHT when the code, the material, and the service conditions demand it.

When in doubt — perform PWHT. The cost of a stress-relief cycle is a fraction of the cost of a failure investigation, a recall, or a tragedy.

Ravi Kumar Thammana is the CEO and co-founder of Trinity NDT WeldSolutions Pvt. Ltd., Bangalore — India’s only NADCAP and NABL dual-accredited NDT and welding centre. He holds ASNT Level III certifications in UT, RT, MT, PT, VT, and ET; NAS 410 Level III for aerospace NDT; and is an International Welding Engineer (IWE) certified by the International Institute of Welding. With over 25 years in welding engineering, NDT, and quality assurance across aerospace, oil & gas, pressure vessels, and structural fabrication, he has qualified over 16,000 NDT and welding professionals across 45 countries.

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