What Is a Thermal Break?

Post-heating refers to post-weld heating to prevent rapid cooling. Post-heating has two purposes: one is to promote the diffusion of hydrogen in the weld as soon as possible to avoid hydrogen-induced cracks; the other is to appropriately slow down the residual stress of the weld joint and prevent the occurrence of cold or reheat cracks. Post-weld heat treatment and post-heat are two different concepts. The processes and purposes of the two are different and cannot be replaced.

At present, many standards require that after the welding of metal materials in thermal power plants is completed or because the process is interrupted, post-weld heat treatment is required in a timely manner. However, for power plant piping manufacturers or power construction units, for various objective reasons, post-weld heat treatment cannot be performed immediately. To further ensure welding quality and avoid cracks, post-heat treatment is required. On the other hand, after the welding is interrupted, the welding cannot be performed again for a short period of time, or it is economically unreasonable to keep the heat until it is re-welded, and a post-heat treatment is also required.

Post-heating has two purposes: one is to promote the diffusion of hydrogen in the weld as soon as possible to avoid hydrogen-induced cracks; the other is to appropriately slow down the residual stress of the weld joint and prevent the occurrence of cold or reheat cracks.

In terms of purpose, the post-heating process includes the stopping of welding work (welding interruption and welding completion), and all subsequent heat treatment forms, including post-weld heat treatment. However, the difference between post-heating and post-weld heat treatment is temperature. Post-heating generally occurs in a relatively low temperature range; and post-weld heat treatment generally requires a higher temperature, that is, 30 to 50 ° C below Ac1. Therefore, post-weld heat treatment and post-heat are two different concepts. The processes and purposes of the two are different and cannot be replaced by each other. ^[1]

The role of post-heating is to eliminate the welding

The electric power standard stipulates that the post-heating construction sequence for martensitic hot-strength steel welded joints is: after welding, the weldment is at 80 to 120 , and the heat is maintained for 1 to 2 hours to complete martensitic transformation, and then heated to 300 to 400 , The holding time is 2 to 4 h [2]. However, some boiler manufacturers and foreign standards require that after welding is completed, the temperature should be constant at 350 , after holding for 2 to 4 hours, and then cooled to room temperature, and the martensite transformation is performed at room temperature. According to different regulations, different post-heating processes are used for different welds of P92 steel. After completion, they are inspected to determine the effect of different post-heating methods on the mechanical properties of the welds. ^[2]

The raw material of a power plant pipe is A335P92, and the specification is ID 368 mm × 41 mm. There are six sections, and each section is 250 mm in length. Proof materials such as material books are complete. Every two sections form a weld joint, and a steel seal is stamped on each section as a logo.

The substrate of argon arc welding wire is ThermanitMTS 616, specification OD2. 4 mm, manual arc welding electrode fill selection Thermanit MTS 616 specification OD 3 2 mm, submerged arc automatic welding wire using Thermanit MTS 616 specification OD 2 4 mm, submerged arc welding flux selected MARATHON 543. The above welding consumables have been re-inspected, and their performance is reliable.

In order to avoid experimental errors caused by different placement times, three groups of welders apply welding at the same time. The three groups of welders are all skilled welders, and the welder's license is within the validity period. Reliable performance of each welding machine used ^[3]

1 Welding

Each section of pipe is beveled on one side and aligned as required to form three welds, which are welded by different welders at the same time. The three groups of samples are labeled GYSY01, GYSY02, and GYSY03, respectively. The welding process, preheating, heat tracing, and interlayer temperature all meet relevant requirements.

2 Post-heating process

The three groups of samples were completed on time, and the time difference did not exceed 5 min. After welding is completed, for GYSY01, no post-heat treatment is adopted, that is, it is directly placed in still air and slowly cooled. For GYSY02, after the welding is completed, heat up to 350 , after constant temperature for 2 hours, the post-heating process is completed, and the weld joint is slowly cooled. For GYSY03, after welding is completed, the temperature is first reduced to 90 to complete the martensitic transformation, and the temperature is maintained for 2 hours. The temperature was raised to 350 , and the temperature was maintained for 2 h. The post-heating process was completed.

When GYSY02 and GYSY03 are post-heated, the thermocouple position is reasonably arranged and there are records of RBI. After the post-heating was completed, UT inspected each welding joint, and no welding defect was detected.

3 Intermediate processing

In order to reflect the influence of different forms of heat on the quality of the weld joint, after the temperature of the three weld joints dropped to room temperature, they were exposed to the outdoor environment and kept for 30 days. During the outdoor period in winter, the daily maximum temperature does not exceed 5 , and a snowfall occurs.

4 Post-weld heat treatment

After the retention time has elapsed, the three samples are heat treated as a whole. The three samples are processed in the same furnace. The heat treatment process is as follows: temperature rising and lowering speed 120 / h, constant temperature 760 , constant temperature time 4 h.

5 Post thermal inspection

After UT inspection, micro cracks appeared in GYSY01, and the welding quality of GYSY02 and GYSY03 was qualified without cracks.

6 Mechanical inspection

Three welds were sampled and subjected to mechanical inspection. According to the relevant regulations, the results of the mechanical property inspection of the above samples all meet the requirements of weld joint performance and can be judged as qualified ^[2]

1) Under severe environment, micro-cracks may occur on steels that have a tendency to cold crack, such as P92 steel, without post-heat treatment.

2) The main role of post-heating is to avoid hydrogen-induced cracking, so post-heating cannot be used instead of post-weld heat treatment.

3) In the conventional power plant material testing method, it is impossible to determine whether the weld joint has been subjected to post-weld heat treatment, but only to determine whether a post-weld heat treatment has been performed.

4) Without post-heat treatment, it may be judged to be qualified by conventional power plant inspection methods.

5) Post-heat treatment before martensitic transformation, or post-heat treatment after transformation, both have the same effect, and can avoid hydrogen-induced cracking to a certain extent.

6) From the perspective of economic benefits, if the martensite transformation is completed before the post-heating, it is necessary to lower the temperature to 90 and hold it for 2 hours to complete the martensitic transformation, and then raise the temperature to 350 and hold it for 2 hours for hydrogen elimination treatment. . In actual production, for small-caliber and large-wall-thick pipes, the phenomenon of temperature drop and slow temperature rise may occur.

After testing, in the summer, after the welding is completed, the natural cooling from 280 to 90 takes 7 to 8 hours, or longer. This process requires real-time monitoring. With this process sequence, the post-heating process can reach 16 to 18 hours. It is not necessary to eliminate hydrogen before completing martensitic transformation. It only needs to be heated to 350 after the welding is completed, and the temperature can be maintained for 2 hours. ^[3]

1) If the welding is completed or the interruption time is relatively long, and the post-weld heat treatment cannot be performed immediately, the weld joints with a tendency to cold crack need to be immediately post heat treated, otherwise delayed cracks or cold cracks may occur.

2) The main purpose of post-heating is to eliminate hydrogen, by slowing the cooling rate of the weld and accelerating the escape of diffused hydrogen to prevent the occurrence of hydrogen-induced cracks. It is only a temporary measure and cannot effectively reduce the welding participation stress Therefore, post-heating cannot be used instead of heat treatment.

3) Theoretically, whether or not post-heating is performed, the martensitic steel must have a martensitic transformation process (after 2 h below 100 ) after the welding is completed and before the welding heat treatment. Otherwise, the metallographic structure of the weld may not meet the requirements.

4) From the analysis of experimental results, the martensite transformation is completed before the post-heating or the martensite transformation is performed after the post-heating, both of which meet the requirements.

5) The process sequence of martensite transformation after the post-heat treatment is completed, the production efficiency is significantly improved, and the cost is reduced to a certain extent. It is recommended to popularize it ^[2] .