Vietnam's largest desalination pipe plan-Pit-type annealing furnace
Intergranular corrosion of duplex stainless steel pipe after welding and its annealing treatment
Duplex stainless steel is widely used in the field of seawater desalination, petrochemical industry and other fields because of its combination of austenitic stainless steel toughness and ferritic stainless steel strength and resistance to chloride stress corrosion cracking. However, during the welding process of duplex stainless steel, due to the local high temperature and subsequent cooling, may lead to sensitisation in the heat affected zone (HAZ) or weld seam, that is, the precipitation of carbides (mainly M23C6) at the grain boundaries. These carbides deplete the chromium near the grain boundaries, leading to chromium depletion at the grain boundaries, which reduces the corrosion resistance of the area and makes it susceptible to intergranular corrosion in corrosive media. Salt spray test failure rate of 31% is a direct reflection of intergranular corrosion, which seriously affects product quality and service life.
In order to eliminate or mitigate intergranular corrosion, the welded duplex stainless steel tube needs to be solution annealed. The purpose of solution annealing is to make the precipitation of carbide re-dissolved into the matrix, restore the chromium-poor region of the grain boundary, thereby restoring the corrosion resistance of the material. The annealing process parameters (e.g. temperature, holding time, cooling rate) are critical to the final organisation and properties. Conventional annealing furnaces may suffer from poor temperature uniformity and imprecise atmosphere control, resulting in poor annealing results and failure to completely solve the intergranular corrosion problem.
Pit-type annealing furnace
The pit-type annealing furnace used in this case is a cyclic heat treatment equipment specially designed for heat treatment of long pipe fittings. Its unique structure and mode of operation give it significant advantages in the treatment of duplex stainless steel tubes:
1. Vertical loading and heating uniformity: The pit-type furnace adopts vertical loading mode, and the pipe fittings are suspended vertically in the furnace chamber, which can effectively avoid the deformation of the pipe fittings due to self-weight at high temperature. At the same time, the furnace chamber is usually designed with multiple heating zones, through the precise control of the heating power of each zone, to ensure the uniformity of the temperature field inside the furnace, so as to ensure that the pipe fittings are heated uniformly along the entire length, to avoid overheating or underheating, which is crucial for preventing intergranular corrosion from occurring again.
2. Furnace structure and sealing: pit-type furnace body is usually buried in the ground or semi-underground, the furnace cover can be lifted, good sealing. This structure is conducive to the stable control of the furnace atmosphere, reduce the penetration of outside air, for the subsequent protective atmosphere annealing provides a good foundation.
3. Applicability: Pit-type annealing furnace is especially suitable for annealing, normalising and quenching of long shafts, rods, pipe fittings and other long and thin workpieces, and can meet the demand of large-size pipe fittings of seawater desalination plants.
Fully automatic nitrogen-hydrogen mixed atmosphere (oxygen content <10ppm)
In order to completely solve the problem of intergranular corrosion of duplex stainless steel tubes and obtain bright annealing effect, the control of furnace atmosphere is the key. In this case, a fully automated nitrogen-hydrogen mixed atmosphere is used, and the oxygen content is controlled at a very low level of <10 ppm, which is decisive for the prevention of oxidation and intergranular corrosion:
1. Advantages of Nitrogen-Hydrogen Mixed Atmosphere: Nitrogen (N2) acts as a carrier for the protective atmosphere, and hydrogen (H2) acts as a reducing gas.
. Hydrogen is a reducing gas. Hydrogen reduces oxides on the metal surface, preventing oxidation of the workpiece at high temperatures and resulting in a bright surface. At the same time, the mixing ratio of nitrogen and hydrogen can be precisely adjusted according to the specific requirements of the process, in order to achieve the best protection effect. 2.
2. Oxygen content control: Oxygen content is the key index affecting the annealing effect. ＜An ultra-low oxygen content of <10ppm means that the atmosphere in the furnace is of extremely high purity, which prevents surface oxidation and intergranular corrosion of the workpiece from occurring to the maximum extent possible. This requires state-of-the-art gas purification equipment (e.g. palladium catalytic deaerator, molecular sieve dryer) and a precise gas flow control system to ensure that the nitrogen-hydrogen gas mixture entering the furnace is pure and stable.
3. Fully automated control: The atmosphere control system is fully automated and includes gas proportioning, flow control, dew point monitoring and on-line analysis of the oxygen content. The system automatically adjusts the gas parameters according to the preset programme, and alarms and handles abnormal situations.
ensures the stability and consistency of the annealing process.
32-Point Temperature Recorder + Cloud Data Traceability
In response to the high number of non-conformities in IATF audits due to manual recording of furnace temperatures, the solution introduces an advanced temperature recording and data traceability system to achieve digital management of the heat treatment process:
1. 32-point temperature logger: 32 high-precision temperature sensors (thermocouples) are arranged in the furnace chamber for real-time, continuous monitoring of the temperature at different locations in the furnace. These sensors capture subtle changes in the furnace temperature field and provide detailed data for precise control and optimisation of the annealing process. Compared to traditional manual recording, the 32-point logger provides more comprehensive and accurate temperature data, effectively avoiding missing data and human error.
2. Cloud data traceability: All temperature data are uploaded to the cloud server for storage and management in real time through the network. The cloud data traceability system has powerful data storage, query, analysis and visualisation functions. This means:
Real-time monitoring: Managers can view real-time curves and historical data of furnace temperatures anytime, anywhere via the Internet.
Data analysis: The system can analyse historical data to identify temperature fluctuation patterns, optimise process parameters and improve product consistency.
Audit Traceability: All heat treatment data can be traced back to meet the strict audit requirements of quality management systems such as IATF. In the event of a quality problem, it can be quickly traced back to the specific batch and process parameters, and the cause can be analysed and improved. This completely solves the problems of irregularities and non-conformities caused by manual recording, and improves the quality management level and international competitiveness of the enterprise.
The combined application of these technologies has enabled this desalination pipe plant in Vietnam to reach the international leading level in terms of product corrosion resistance, production efficiency and quality management.