**1. Overview**
SA335-P91 is a martensitic heat-resistant steel with a composition of wCr=9%, wMo=1%, wV=0.2%, wNb=0.08%, and wN=0.05%. Its microstructure consists of low-carbon tempered martensite. Through the use of micro-alloying control and grain refinement techniques, P91 has been developed into a fine-grained steel. This not only enhances the impact toughness of the material but also improves its high-temperature creep strength, making it ideal for high-temperature applications.
One of the key differences in welding P91 compared to other low-alloy heat-resistant steels is that the weakest point in the weld joint is not the fusion zone but the weld metal itself. The weld metal tends to have lower toughness and higher hardness, which requires special attention during the welding process.
**2. Process Principle**
(1) Since SA335-P91 is a fine-grained steel, maintaining a controlled interlayer temperature is crucial. If the temperature becomes too high, it can increase t8/5, leading to grain growth and a loss of mechanical properties. Therefore, strict control of interlayer temperature is necessary to prevent grain coarsening and ensure proper weld quality.
(2) Heat treatment parameters such as heating width, insulation thickness, and dwell time significantly affect the weld's toughness. Increasing these values and extending the constant temperature period can improve the tempering effect, thereby enhancing the weld’s toughness.
**3. Welding Process**
(1) The root pass is typically done using double-layer TIG welding, while subsequent layers are completed using multi-pass welding. A 3.2mm diameter electrode is used, with each layer kept below 3mm in thickness. Proper control of welding current and speed is essential. Increasing the welding speed and using a wide weaving technique with thin layers helps reduce bead height and improve weld quality.
(2) During welding, the technician uses a far-infrared thermometer to monitor the interlayer temperature. The temperature between weld layers should be maintained below 300°C. If the temperature exceeds this, welding must be paused until it drops to 230°C. After each layer, the weld bead thickness is measured with a vernier caliper, ensuring it does not exceed 3mm. Fillet welds between the groove and the bead are strictly avoided.
**4. Welding Precautions**
The choice of welding current depends on the electrode type. For electrodes with a flux coating, a lower current is preferred to reduce heat input. However, this may cause tungsten inclusion due to the high melting point of the coating. Regardless of the electrode used, it is important to maintain good molten pool fluidity and ensure full penetration, especially at the root of the groove. Smaller, more controlled operations are recommended for better results.
**5. Heat Treatment Process**
Post-weld heat treatment is carried out using a DKPC-12360-12 heat treatment machine, which uses ceramic resistors mounted on tracks. Insulation is provided by aluminum silicate wool, and K-type armored thermocouples are used with compensation wires and an automatic temperature recorder.
(1) Preheating is performed electrically, with four thermocouples placed at specific points along the weld. The preheating temperature is set to 150°C, and after 30 minutes, the bottom weld is started. During arc welding, the interlayer temperature must stay between 200–300°C. An over-temperature alarm is set at 260°C, and welding is stopped if this threshold is exceeded.
(2) During post-weld heat treatment, thermocouples must be tightly secured to the weld. They are wrapped with 20# wire to prevent loosening due to thermal expansion. The heater must be cleaned before installation to ensure good contact with the surface. Additional rope heaters are used for complex joints like elbows or tees to ensure even heating.
(3) The insulation thickness is increased to 100mm, and the heating width is adjusted accordingly. This helps achieve a more uniform tempering effect and improves weld toughness.
(4) Heat treatment parameters are adjusted based on the specific application. While increasing dwell time and heating width can enhance toughness, excessive increases may lead to softening of the base material. Careful control is essential to achieve optimal results.
**6. Heat Treatment Process Precautions**
(1) Thermocouples must be positioned 20mm from the edge of the weld groove during preheating. Once the preheating temperature is reached, a 30-minute hold is required before starting the weld. Interlayer temperatures are closely monitored, and the system is designed to delay temperature readings by about 30°C.
(2) Accurate temperature measurement is critical. Thermocouples and recorders must be calibrated regularly, especially after 200 hours of use. Temperature check points are set at 200°C, 400°C, 600°C, and 800°C to ensure reliable readings.
(3) When connecting compensation wires, polarity must be correct. The junctions between the thermocouple and the machine must be at the same ambient temperature to avoid errors. Reliable connections are made using terminal blocks rather than direct screwing to prevent resistance-related issues.
**7. Conclusion**
By following the described welding and heat treatment procedures, the weld hardness of SA335-P91 meets the requirements specified in DL/T438-2009 (180–270 HBW). It is clear that the heat treatment process plays a vital role in achieving the desired mechanical properties. Modern practices should move beyond simple thickness-based guidelines and consider the location and conditions of the weld. This approach ensures better control of weld hardness and overall welding quality for P91 steel.
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