Post Weld Heat Treatment Services ( PWHT ) is defined as one of heat treatments done after welding/machining to improve the Chemical mechanical properties of weldment / machined surfaces. In concept, PWHT covers many different potential treatments. However, in steel fabrication, most common procedure used is Stress Relieving.
Post-Weld Heat Treatment/PWHT purpose
The primary reasons that components are required to be subjected to PWHT within the ASME Code rules are that PWHT reduces residual stresses and tempers hardened microstructures. PWHT will achieve both of these results but might not positively benefit the overall properties of the weldment if not properly controlled, particularly in terms of the toughness in the heat-affected zone. When toughness is a requirement, the Codes will impose additional controls on the PWHT, such as time at temperature controls. PWHT done to meet the Code requirements is typically performed at subcritical temperatures.
Residual stresses: can contribute to increases in the susceptibility to corrosion mechanisms and to fatigue. Because residual stresses cannot exceed the yield strength of the materials, an immediate benefit of increasing the temperature of the material during PWHT is a corresponding drop in the yield strength of the material and thus a reduction of the maximum residual stresses in the weldment. In order to further reduce the residual stresses, the weldment will need to be held for Longer periods at the elevated temperatures (this reduction occurs by relaxation-recrystallization or primary creep mechanisms).
Although the reduction of residual stresses is a benefit of PWHT, most of the rules for PWHT specified in the Codes are targeted at the hardened microstructures. This is because the applications that might require a reduction in residual stresses are not addressed specifically in the Codes.
Phase diagram of an iron-carbon alloying system
Post-weld treatment acts as a tempering process by reducing the hardness of the heat-affected zone and the weld metal. Tempering is a heat treatment whereby the material is heated to a temperature below the lower critical temperature (often assumed to be approximately 1340ºF [725ºC] for carbon steels). The PWHT for carbon steels is generally done in the range of 1100-1200ºF (600-650ºC), although some Codes specify only the minimum temperature of 1100ºF (600ºC). A secondary effect of tempering is to allow some additional transformation of the martensitic grain structure into ferrite, but the main objective is tempering the martensite. The result can be increased ductility and toughness in addition to reduced hardness. If the tempering temperature is too high or held too long, some corresponding reduction in the toughness can result.
NITS has a talented crew that uses diesel fuel as source of heating for stress relieving for pressure vessels with the sole objective of reinforcing process and component integrity and quality. The experience heat treaters from NITS have the experience, equipment, and expertise to develop custom configuration for your particular process. Our heating processes include low-Range, Mid-Range & High-Range Temperature Heating.
- Permanent Low Thermal Mass Furnaces for heat treatment by Oil / Gas / Electrical mode of heating. Types of furnaces comes under the category are Bogie Hearth Furnace, Top Hat Furnace, Fixed Hearth Furnace, Pit type Furnace, Box Furnace, Roller Hearth Furnace etc.
- Temporary Low Thermal Mass Furnace for Stress Relieving, Normalizing, Solution Annealing etc.
Heat treatment Equipment:
- High velocity oil & gas fired burner system provides capacity from 5,00,000 kcal/hr up to max. 8,000,000 kcal / hr.
- Heat treatment low voltage power source of 40,50,75,100,125 kva capacity.
- Heat Treatment Distribution Panel.
- Flexible Ceramic Pads operating at 45V, 60V, 80V, 230 V.
- Electric Channel Elements.
- Programmer Controller, 03 Point, 06 Point, 12 Point Inlet.
- Thermocouple attachment unit.
- Twin / Trinary Heat Module for Portable Operation.
- Thermocouple (Simplex / Duplex, Wire type)
Oil/Gas Fired High Velocity Burner Equipment:
The Indotherm Oil / Gas high velocity burner systems provide from 5,00,000 kcal/hr up to 8,000,000 Kcal/hr ( 2000,000 Btu/hr to 32,000,000 Btu/hr ). For On Site Heat Treatment from a flame supervised, automatic / manually controlled, nozzle mix high velocity burner which will operate on all commercially available fuels.