Heat Treatment & PWHT Requirements

QC Notes Level-22

Heat Treatment & PWHT Requirements

✅ Introduction

PWHT is required where there is a risk of environmentally-assisted cracking. PWHT of C-Mn steels is typically carried out at approximately 600°C, for one hour per 25mm of thickness.

Equipment used for the PWHT should be suitable according to ISO 17663.
The ASME B31. 3 code provides guidelines for piping fabrication in the process industry, including post-weld heat treatment (PWHT) requirements. Section 331.1. 1 outlines PWHT temperatures for various P-Number and group numbers, exemptions, and altered holding times.
The post weld heat treatment is often usually after the union by welding of two material base such us carbon steels or alloy steels. Post weld heat treatment (PWHT) can be apply to one complete piece or locally, by applying only the heat treatment to the welded joint area.
The need of PWHT is mostly due to residual stresses and microstructural changes that occur after welding. During the welding process, a high-temperature gradient is experienced between the weld metal and parent metal. Codes like ASME specify the specific requirements for PWHT of materials.

✅ PWHT (POST WELD HEAT TREATMENT)

Applied to some welded steels in order toensure that the properties of weldment will be suitable for intended applications often called Stress Relief:

Intermediate Stress Relief = 600*C for 1 hour
Dehydrogenation Heat Treatment = 350*C for 4 hours
Specified PWHT area extending at least 6 times the thickness of the material
being welded but not less than 25mm from the edge of the weld, which is greater
If PWHT not perform in Furnace, insulation shall be applied 300mm on either side of the weld, insulation shall be remove before the temperature has cooled to 150*C
Minimum soaking time shall be 1 hour per 25mm of thickness

✅ PWHT BENEFITS

To improve the resistance of the joint to brittle fracture
To improve the resistance of the joint to stress corrosion cracking
To enable welded joints to be machined to accurate dimensional tolerance

✅ PWHT HEAT SOURCES

Permanent or Semi-permanent furnace using gas or oil or electric heater
Electrical Resistance Heater
Induction Heater

✅ THERMOCOUPLE LOCATION IN FURNACE

Thinnest major component
Thickest member
Top of the vessel
Bottom of the vessel
Temperature recorder shall be calibrated every 3 months or can be extended to 12 months with aramco approval
Machine surfaces, flange faces, threaded bolt holes, threads, etc. shall be protected from oxidation it shall be coated with deoxiluminite or other suitable materials
After completion of PWHT attachment areas shall be ground smooth, if specified by inspector areas shall be examined by MT or PT after grinding

12- SAMSS-008 – Enlargement of bolt holes shall be done by reaming or drilling only. Flame cutting or burning are not allowed (PIP STS05130, para. 6.7.3)

SAIC-W-2068 Welding Inspection on Vessel Fabrication

✅ Document required at welding station:

All weld joint shall be marked for identification by a weld number & welder symbol and made with suitable weather proof marking material.
The markings shall be placed in location such that they will be easily observed and remain visible for a time suitable for SAPID.

WELDING – ASME SEC.IX

A weld – is a union of materials produced by heat and or pressure
A joint – is a configuration of members

Types of Common Welds

Butt weld
Fillet weld
Spot weld
Seam weld
Edge weld

Types of Common Joints

Butt joints
Tee joints
Lap joints
Open corner joints
Close corner joints

Weld Preparation

Single bevel butt weld
Single vee butt weld
Single J butt weld
Single U butt weld
Double bevel butt weld
Double vee butt weld
- Normally made on thinner materials when access from both sides are restricted
Double J butt weld
Double U butt weld