Post-weld heat treatment is one of the most demanding applications for a bogie hearth furnace — and one where the cost of getting it wrong is measured in failed pressure vessels, rejected heat exchangers, and scrapped heavy fabrications worth hundreds of thousands of dollars.
MONTE INTELLIGENCE has supplied bogie hearth furnaces for PWHT applications to fabricators in China, Southeast Asia, and the Middle East. These furnaces handle weldments ranging from 5-tonne pressure vessel shells to 80-tonne reactor columns. This article covers the furnace design requirements, the procedural discipline, and the code compliance documentation that separate a successful PWHT furnace from a liability.
PWHT is required by construction codes — ASME Section VIII for pressure vessels, ASME B31.3 for process piping, AWS D1.1 for structural welding — when the base metal thickness exceeds specified limits, when the service environment involves hydrogen or stress corrosion, or when the design specification calls for it regardless of code requirements. The purpose of PWHT is to reduce residual stresses from welding, temper the heat-affected zone microstructure, and in some cases, reduce the risk of hydrogen-induced cracking.
The thermal cycle for PWHT has three phases that the furnace must execute precisely. First, the heating phase — the furnace must raise the workpiece temperature from ambient to the soak temperature at a controlled rate. ASME Section VIII specifies a maximum heating rate of 222°C per hour divided by the thickness in inches, up to a maximum of 222°C per hour, above 315°C. For a 50 mm (2-inch) thick weldment, that means a maximum heating rate of 111°C per hour above 315°C.
Second, the soaking phase — the workpiece must be held at the specified soak temperature for a minimum time. ASME Section VIII specifies a minimum soak time of one hour per 25 mm (1 inch) of thickness, with a minimum of 30 minutes. The soak temperature depends on the base material. For P-No. 1 carbon steels, the minimum soak temperature is 593°C (1100°F). For P-No. 4 Cr-Mo steels, it ranges from 675-730°C depending on the chromium content.
Third, the cooling phase — the workpiece must be cooled from the soak temperature to below 315°C at a controlled rate. The maximum cooling rate is 278°C per hour divided by thickness in inches, up to 278°C per hour maximum, above 315°C. Below 315°C, the workpiece may be cooled in still air.
These heating and cooling rate requirements are what make PWHT furnace design challenging. For the 80-tonne reactor column mentioned above, with weld thickness of 100 mm, the maximum heating rate above 315°C is just 56°C per hour. The total PWHT cycle — heat from ambient to 620°C, soak for 4 hours, cool to 315°C — takes 28-32 hours. The furnace must maintain temperature uniformity across the entire length and cross-section of the workpiece throughout that entire cycle.
Temperature uniformity is the furnace performance parameter that determines PWHT quality. ASME Section VIII requires that the temperature difference between any two points on the workpiece during the soak period shall not exceed 65°C (150°F) for most materials. For a 12-meter-long reactor column in a bogie hearth furnace, achieving that uniformity requires careful burner placement, recirculation fan design, and control zone division.
We typically divide large PWHT bogie hearth furnaces into 4 to 8 independently controlled temperature zones, each with its own burner or heating element, its own thermocouple input, and its own PID controller. The zone controllers communicate with a central supervisory controller that coordinates the setpoint ramping to maintain the specified heating and cooling rates while keeping inter-zone temperature differences within the allowed limits.
Thermocouple placement and attachment is the measurement link in the control chain. Code requires thermocouples to be attached to the workpiece, not floating in the furnace atmosphere. For thick sections, thermocouples should be attached at the weld location because that is where the temperature is most critical. Attachment methods include capacitive discharge welding (preferred for permanent thermocouples), hose clamps (for temporary thermocouples on smaller parts), and wire ties (for awkward geometries).
The number of thermocouples required depends on the workpiece size and the code requirements. ASME Section VIII requires a minimum of one thermocouple for the first 3 meters of workpiece length and one additional thermocouple for each additional 3 meters, with a minimum of three total. A 10-meter vessel requires four thermocouples. Each thermocouple must be connected to a calibrated recorder that prints or logs the temperature throughout the cycle.
Calibration is the documentary foundation of PWHT quality assurance. Every thermocouple used for PWHT must be calibrated against a traceable standard within the preceding 12 months. The temperature recorder must be calibrated within the preceding 6 months. The furnace itself should undergo a temperature uniformity survey (TUS) annually, per AMS 2750 or an equivalent standard, to verify that the furnace achieves the required uniformity under loaded conditions.
Load configuration affects temperature uniformity as much as furnace design does. A workpiece placed close to the furnace wall may see a different temperature than one placed in the center. A workpiece that blocks the recirculation airflow can create a cold spot downstream. The PWHT specification should include a loading diagram that addresses these concerns, and the bogie car should have marked locations for workpiece supports to ensure consistent loading from one cycle to the next.
MONTE INTELLIGENCE bogie hearth PWHT furnaces are designed with these code requirements in mind. Our standard design includes multi-zone temperature control, high-volume recirculation fans (typically 3-6 recirculations per minute), calibrated thermocouple inputs, and data logging systems that generate the required code documentation automatically.
For a PWHT furnace proposal specific to your fabrication requirements, contact helenxu@cnlymonte.com.
