Post Weld Heat Treatment Machine Specification

Post weld heat treatment (PWHT) is an integral part of welding and fabrication industry. PWHT helps temper metal by relieving residual stresses and thus reduce brittle fracture, corrosion, and stress cracking risks.

PWHT is often required by fabrication codes and standards for carbon steel components, although local supercritical PWHT above its critical transformation temperature would lead to undesirable temperature gradients across through thickness temperatures.

Temperature Range

PWHT (Post Weld Heat Treatment) is a process where metal is heated and soaked for a certain time to ease internal stresses induced by welding, which could otherwise cause cracking, fatigue or stress corrosion failure of material. PWHT is often required in construction codes and helps meet requirements for certain grades or thicknesses of steel.

Libratherm offers Single and Multi-Zone PID Ramp Soak controllers to accurately regulate the PWHT process. Both controllers accept both grounded/ungrounded thermocouples directly, providing analogue/SSR outputs with PID control action for ramping, soaking, cooling steps to achieve precise heating rates. They may operate alone or cascade mode (Master-Slave); furthermore an optional MODBUS over RS-485 interface provides access to SCADA, PLCs or data logging systems.

The required soaking times of different steels and weld procedures vary considerably, for instance resistance welding requires different soak times than arc welding; and this soaking period also depends on factors like desired service life and likely failure mechanism of welds. When heated to this temperature for extended periods, HAZs and welds become part of what’s known as the soak band or temperature gradient control band – whether axially or along pipe thickness.

Heating Time

PWHT can significantly enhance the mechanical properties of structural steels, decreasing hardness and increasing ductility while relieving weld-induced stresses, protecting against corrosion and increasing fatigue resistance of welded structures. Unfortunately, however, heat treatment requires specialized equipment and skilled operators whose cost may increase along with any delays that result.

Timing of PWHT cycles varies based on material and service needs, and requires careful consideration of temperature tolerances, heating rates and times at temperatures. Failure to control this process properly could result in distortion, temper embrittlement and over-softening effects which could prove disastrous for an assembly’s functionality.

Heating time is critical to reaching an acceptable PWHT level, and can be controlled using different types of induction machines with various power levels. MSM currently maintains three PWHT ovens to give us greater control and accelerate processes. Induction PWHT machines feature air cooling circuits to prevent metal from overheating as well as large LCD touch screens that make operation simpler; additionally they record and print up to six temperature curves allowing us to record them and print up 6 temperature curves easily! Moreover, their heater sizes and powers can be adjusted to suit specific parts and meet desired temperature requirements.

Temperature Recorder

Temperature recorders are widely used by commercial facilities to track shipments, while aquariums often employ them to ensure the health of their fish, which require very narrow temperature ranges for proper growth. Scientists may also use temperature recorders to monitor river and stream temperatures in order to assess environmental health issues.

Data loggers come in various forms and sizes, yet each serves the same basic function: connecting to temperature transducers to record data on strip charts for later analysis. Most modern data loggers support both thermocouples and resistance temperature detectors for ultimate flexibility.

Temperature recorders must ensure accurate measurement. This information typically appears as either “FS” or “rdg”, where “FS” represents total scale error while “rdg” displays reading precision.

Considerations should also include how long-term data collection takes place and therefore which temperature recorder has the capability of collecting it, since this can generate large amounts of information over time. It would be prudent to choose one with high memory capacity.

Make sure that the temperature recorder can restart collection in case of power-on failure, in order to minimize errors during measurement. Some models offer this capability while others do not.

Compensating Cable

Air-cooled PWHT machines should be selected so as to prevent their coils from overheating during heating. In general, the larger and higher power level a heated part is, and vice versa, the better this choice should be considered.

Stress relief is an integral component of carbon steel parts that may experience high stresses during welding or postweld heat treatment, and may reduce distortion or cracking during service.

The triple cable set consists of two lengths of heat and oil resistant, flame retardant (HOFR), double insulated copper cables equipped with 300 amp camlock connectors on one end and compensating cable fitted with thermocouple plug and socket at the other, to connect multiple heating elements to one control output from a heat treatment unit for accurate zonal temperature regulation.