Introduction
Post Weld Heat Treatment (PWHT) is a controlled process that involves heating a welded component to a specific temperature, holding it for a set duration, and then cooling it at a controlled rate. The purpose of PWHT is to reduce residual stresses, enhance mechanical properties, and improve the metallurgical structure of the welded joint. The calculation of a PWHT cycle is crucial to ensure compliance with international standards and to achieve optimal results.
Key Factors in PWHT Cycle Calculation
The PWHT cycle consists of three main stages: Heating, Holding (Soaking), and Cooling. Each stage must be carefully calculated to meet the material and design requirements.
1. Heating Rate Calculation
The heating rate is determined based on the material thickness and type. Excessive heating rates can lead to thermal shock, while slow rates may cause unnecessary delays.
Formula for Heating Rate:
HR = \frac{T_{final} – T_{initial}}{t_{heating}}
HR = Heating Rate (°C/hour)
T_final = Target PWHT Temperature (°C)
T_initial = Starting Temperature (°C)
t_heating = Time required for heating (hours)
2. Holding (Soaking) Time Calculation
The soaking time is based on the thickness of the thickest weld section. Industry standards provide guidance on the required soaking time per unit thickness.
Formula for Holding Time:
HT = k \times t_{thickness}
HT = Holding Time (hours)
k = Soaking factor (typically 1 hour per 25 mm of thickness)
t_thickness = Maximum wall thickness (mm)
Example:
If the component thickness is 50 mm and the soaking factor is 1 hour per 25 mm, then:
HT = 1 \times (50/25) = 2\text{ hours}
3. Cooling Rate Calculation
Cooling is critical to avoid distortion and cracking. The cooling rate depends on material type and must follow controlled cooling guidelines.
Formula for Cooling Rate:
CR = \frac{T_{final} – T_{ambient}}{t_{cooling}}
CR = Cooling Rate (°C/hour)
T_final = PWHT Temperature (°C)
T_ambient = Room Temperature (°C)
t_cooling = Cooling Time (hours)
PWHT Cycle Example Calculation
Material: P91 Alloy Steel
Thickness: 50 mm
PWHT Temperature: 750°C
Ambient Temperature: 30°C
1. Heating Rate Calculation
HR = \frac{750 – 30}{3} = 240°C/hour
2. Holding Time Calculation
HT = 1 \times (50/25) = 2\text{ hours}
3. Cooling Rate Calculation
CR = \frac{750 – 30}{4} = 180°C/hour
Standards Governing PWHT Cycles:
International codes and standards provide guidelines for PWHT cycle calculations. Some key standards include:
ASME Section VIII, Division 1 & 2 – Specifies PWHT requirements for pressure vessels.
ASME B31.3 – Covers PWHT for process piping.
AWS D1.1 – Structural welding code guidelines for PWHT.
API 510 & 570 – PWHT procedures for refinery equipment and pipelines.
ISO 15614 & EN 13445 – Guidelines for heat treatment of welds.
Conclusion:
The correct calculation of a PWHT cycle is essential for ensuring weld integrity, stress relief, and compliance with industry standards. At PWHT Tech India, we specialize in providing expert guidance on PWHT cycle calculations, ensuring precision, safety, and efficiency in heat treatment processes. Contact us for customized PWHT solutions tailored to your project needs.