T/P91 alloy steel is widely used in high-temperature and high-pressure piping systems, especially in power and petrochemical industries. While its welding and heat treatment processes are well-documented, there remains some confusion and inconsistency in practical applications, particularly regarding the selection of post-weld heat treatment (PWHT) temperatures and durations. This article offers a practical perspective on how to make informed decisions in these areas.
A common issue in some projects is the tendency to select PWHT temperatures based solely on general specifications or to use the upper limit of recommended temperature ranges to meet hardness requirements. However, this method can be risky. Since T/P91 is typically delivered in a normalized and tempered state, applying a PWHT temperature higher than its original tempering temperature can damage the steel’s microstructure, reducing its mechanical integrity.
Instead, it is essential to refer to the actual tempering temperature provided by the steel supplier. For example, Vallourec Mannesmann specifies a normalizing temperature of 1040–1080°C and a tempering range of 750–780°C for its T/P91 steel. Therefore, the PWHT should be carefully controlled around 750°C, ensuring the steel retains its designed strength and toughness.
Key Considerations for Welding and Heat Treatment of T/P91 Steel
T/P91 has now achieved domestic production. However, normalizing and tempering processes vary between manufacturers, making it crucial to fully understand the manufacturer’s actual tempering temperature when determining post-weld heat treatment temperatures. Furthermore, when selecting welding consumables, due to differences in production processes and alloying element additions between different manufacturers, it’s also important to refer to their recommended heat treatment parameters. Second, martensitic transformation should be fully considered during post-weld cooling and post-heat treatment. T/P91 is a martensitic heat-resistant steel. After welding and heat treatment, the goal is to achieve a stable lath-shaped tempered martensite structure, so the martensitic transformation process is crucial. According to the CCT curve provided by Vallourec Mannesmann Tube, T/P91’s Ms temperature (martensitic transformation start temperature) is approximately 400°C, and its Mf temperature (martensitic transformation end temperature) is 100°C. When the austenite temperature cools below Ms, phase transformation begins immediately, requiring no incubation period. The transformation is extremely rapid, and once cooling ceases, the transformation also ceases. In other words, martensitic transformation requires continuous cooling and is completed below the Mf temperature. Therefore, post-weld cooling should be continuous, slow cooling to below 100°C. Precisely because of this characteristic, the power industry standard DL/T869-2021, “Technical Code for Welding of Thermal Power Plants,” explicitly requires that the welding temperature be lowered to 80-100°C and held at this temperature for 1-2 hours after T/P91 welding. In contrast, the relevant pipeline welding specifications for T/P91 in the petrochemical industry do not specify a holding time in this temperature range. Therefore, it is recommended to refer to the power industry standard DL/T869-2021 for guidance.
DL/T 869–2021
- b) After welding, post-weld heat treatment (PWHT) or re-welding shall be carried out.
E.2.6
For re-welding, the surface of the weld must be inspected to ensure there are no cracks, and preheating must be carried out as specified.
E.2.7
If post-weld heat treatment is not carried out immediately after welding, the weldment should be slowly cooled to a temperature of 80°C–100°C and held for 1–2 hours, after which it should be insulated or processed.
Post-weld heat treatment temperature should be 300°C–350°C, and the holding time should be 2 hours.
E.2.8
If the post-weld heat treatment is carried out after welding is completed, the weld should be cooled to 80°C–100°C, held for 1–2 hours, and then treated.
In addition to complying with the requirements of DL/T 819 for post-weld heat treatment, the following regulations must also be observed:
- a)
For welded joints heated using a resistance heating furnace or resistance heating blanket for post-weld heat treatment, except for one thermocouple installed for temperature control, each group of heating devices must have at least one thermocouple installed for temperature measurement. - b)
For water or steam pipelines with a diameter greater than or equal to 273 mm, sectional heating must be carried out, and the placement of the heating devices must comply with the provisions of DL/T 819. - c)
For internal furnace heat treatment, the holding time should be calculated as 4 min/mm (wall thickness). - d)
For manually welded joints using SMAW with a wall thickness of not less than 76 mm, the holding time for post-weld heat treatment should not be less than 2 hours.
For ceramic heater heating, refer to process table E.1.
PWHT Timing & Storage Guidelines for T/P91
Regarding post-heat treatment, given the reversibility of martensitic transformation and temper brittleness, the specification does not recommend it be performed arbitrarily. Unless specifically necessary, post-heating should be performed after the martensitic transformation has fully completed, and the post-heating temperature should be below the martensitic transformation onset temperature.
However, Vallourec Mannesmann Tube does not require post-heat treatment in its welding process. If post-heat treatment is not performed after welding, the material must be stored. Storage should not exceed one week, and the components must be kept dry. During prefabrication at a domestic factory, microcracks on the surface of T/P91 steel were not promptly treated with post-weld heat treatment. Therefore, it is recommended to schedule the process appropriately and perform post-weld heat treatment promptly as required by the specification. If immediate heat treatment is not possible under special circumstances, surface inspection of the weld is recommended after post-heating is completed but before heat treatment is performed.
It’s important to note that the welding material standard stipulates that the combined Mn and Ni content of welding materials matching T/P91 base metal should not exceed 1.5%. This requirement is also related to the martensitic transformation. Excessive Ni and Mn content not only lowers the Ac1 temperature but also causes a decrease in the martensitic transformation temperature, Ms, which in turn further reduces the Mf transformation temperature. This means that the weld joint must be cooled to a lower temperature to complete the austenitic transformation, which is detrimental to preventing cold cracking in the weld joint. Third, the heat treatment duration should fully consider the cumulative time required for repairs. T/P91 steel is typically heat-treated directly after welding, with nondestructive testing performed after heat treatment. If repairs occur, further heat treatment is required. The requirement in NB/T47014 that “the test piece holding time must not be less than 80% of the cumulative time of the weld during the manufacturing process” must be fully considered. Therefore, when formulating the welding procedure qualification, the specific repairs should be fully considered to ensure that the performance of the welded joint meets the standard.
6.1.4 Post-Weld Heat Treatment (PWHT) Evaluation Requirements
6.1.4.1
If the post-weld heat treatment specification is changed, the welding procedure qualification must be re-evaluated.
6.1.4.2
When impact testing is required, except for fillet welds, tack welds, and socket welds, if the post-weld heat treatment holding temperature or holding time is changed beyond the specified range, the welding procedure qualification must be re-evaluated.
The PWHT of test pieces shall be consistent with that of the actual weld during fabrication.
If PWHT is performed at a lower temperature than the actual process, the holding time of the test piece shall not be less than the accumulated holding time during actual fabrication.
Conclusion
According to NB/T 47014, any change in post-weld heat treatment (PWHT) parameters, such as holding temperature or time, requires requalification of the welding procedure. For impact-tested joints, PWHT must closely replicate actual production conditions. If the PWHT temperature is reduced, the holding time must be increased to ensure equivalent treatment. Additionally, if multiple PWHT cycles are used, their combined duration must be at least 80% of the time required for a single continuous cycle. This ensures the mechanical properties and structural integrity of the welded joint are maintained.
