When welding stainless steel pipes, in order to reduce the tendency of thermal cracking, the microstructure design of the weld generally contains a small amount of ferrite, and the control of the ferrite content is mainly realized by the alloy composition. Austenitic stainless steel welds contain more than 3% ferrite, which can effectively avoid solidification cracks, but too much ferrite will have an adverse effect on performance.
1. Effect of ferrite on corrosion performance of stainless steel pipe
The sensitization tendency of the weld metal is lower than that of the base metal. There is ferrite in the weld, and its Cr content is higher than that of austenite, and the diffusion rate of Cr in ferrite is much faster than that of austenite. However, M23C6 carbides tend to precipitate at the pitted ferrite-austenite interface. Instead of precipitation at the relatively flat austenite-austenite interface, the rapid diffusion of chromium in the ferrite can overcome the problem of chromium deficiency at the grain boundaries. These factors greatly limit the sensitization of austenitic stainless steel welds containing ferrite to prevent intergranular corrosion of stainless steel water pipes.
Austenitic stainless steel weld metals containing a small amount of ferrite are corrosion resistant and free of ferrite in most corrosive environments, corresponding to the base metal. However, among organic acids, these media will selectively corrode ferrite. When the amount of ferrite in the weld metal is greater than 5FN, corrosion will proceed along the ferrite network, resulting in serious damage to the weld.
2. Stainless steel used at high temperature
The ferrite formed in the austenitic stainless steel weld metal acts as a second phase strengthening. Compared with the base metal and heat-affected zone, the yield strength is significantly improved, while the ductility is comparable. Increasing the ferrite content in the weld metal can significantly increase the room temperature strength, but the increase in high temperature strength is small. Ferrite is a harmful phase of stainless steel water pipes in high temperature environment.
(1) Creep damage of ferrite
The continuous ferrite network in the weld metal, which promotes rapid creep damage due to crack initiation earlier than the ferrite-austenite interface. At this time, the ferrite content should be lower than 5FN to prevent the formation of continuous ferrite network
(2) 475°C embrittlement of ferrite
Fe-Cr alloys with a Cr content of 15% to 70% will produce severe embrittlement of Cr-rich ferrite and iron-rich ferrite when heated to 425°C-550°C. The ferrite phase of the austenitic stainless steel weld metal is essentially ferrite (Cr20-30%, Ni4%-5%) embedded in the austenite matrix. Therefore, if it stays in the temperature range of 425℃~550℃ for a long time (about 5000h), the ferrite phase will be embrittled, the hardness will increase, and the plasticity and toughness will be damaged. Short-term heating above 550 °C can eliminate this embrittlement phenomenon, but long-term heating will cause embrittlement of the sigma phase.
3. Low temperature performance
At low temperatures, austenitic stainless steels exhibit good strength, ductility and toughness; whereas ferritic undergoes a brittle transformation. Therefore, the use of stainless steel water pipes at low temperatures requires lower ferrite content in the welds.
A small amount of ferrite in the weld of austenitic stainless steel will reduce the low temperature toughness. When the ferrite content in the weld reaches FN=10, the low temperature toughness will decrease by 50%.
The low temperature toughness of stainless steel pipe is not only related to the ferrite content, but also related to the cleanliness of the weld. The test shows that the oxygen content of the deposited metal of the alkaline low-hydrogen electrode is lower than that of the titanium-calcium electrode, and the weld toughness is also better. The oxygen content of deposited metal in inert gas shielded welding is lower than that of slag shielded welding, and the weld toughness is also better. The deposited metal of ultra-low carbon welding materials has less intergranular precipitation and better welding toughness.
When heat treatment is required after welding or in service at high temperature, the ferrite number of weld metal shall not exceed 10FN measured before post-weld heat treatment. When the stainless steel pipe is used in a low temperature environment below -100°C, the low temperature impact test of the weld is carried out in accordance with relevant regulations; when it is used in a low temperature environment below -196°C, the ferrite content of the weld is lower than 5FN. When the stainless steel pipe is used in a non-magnetic or special corrosive environment, it needs to be controlled according to the design requirements.
1. Effect of ferrite on corrosion performance of stainless steel pipe
The sensitization tendency of the weld metal is lower than that of the base metal. There is ferrite in the weld, and its Cr content is higher than that of austenite, and the diffusion rate of Cr in ferrite is much faster than that of austenite. However, M23C6 carbides tend to precipitate at the pitted ferrite-austenite interface. Instead of precipitation at the relatively flat austenite-austenite interface, the rapid diffusion of chromium in the ferrite can overcome the problem of chromium deficiency at the grain boundaries. These factors greatly limit the sensitization of austenitic stainless steel welds containing ferrite to prevent intergranular corrosion of stainless steel water pipes.
Austenitic stainless steel weld metals containing a small amount of ferrite are corrosion resistant and free of ferrite in most corrosive environments, corresponding to the base metal. However, among organic acids, these media will selectively corrode ferrite. When the amount of ferrite in the weld metal is greater than 5FN, corrosion will proceed along the ferrite network, resulting in serious damage to the weld.
2. Stainless steel used at high temperature
The ferrite formed in the austenitic stainless steel weld metal acts as a second phase strengthening. Compared with the base metal and heat-affected zone, the yield strength is significantly improved, while the ductility is comparable. Increasing the ferrite content in the weld metal can significantly increase the room temperature strength, but the increase in high temperature strength is small. Ferrite is a harmful phase of stainless steel water pipes in high temperature environment.
(1) Creep damage of ferrite
The continuous ferrite network in the weld metal, which promotes rapid creep damage due to crack initiation earlier than the ferrite-austenite interface. At this time, the ferrite content should be lower than 5FN to prevent the formation of continuous ferrite network
(2) 475°C embrittlement of ferrite
Fe-Cr alloys with a Cr content of 15% to 70% will produce severe embrittlement of Cr-rich ferrite and iron-rich ferrite when heated to 425°C-550°C. The ferrite phase of the austenitic stainless steel weld metal is essentially ferrite (Cr20-30%, Ni4%-5%) embedded in the austenite matrix. Therefore, if it stays in the temperature range of 425℃~550℃ for a long time (about 5000h), the ferrite phase will be embrittled, the hardness will increase, and the plasticity and toughness will be damaged. Short-term heating above 550 °C can eliminate this embrittlement phenomenon, but long-term heating will cause embrittlement of the sigma phase.
3. Low temperature performance
At low temperatures, austenitic stainless steels exhibit good strength, ductility and toughness; whereas ferritic undergoes a brittle transformation. Therefore, the use of stainless steel water pipes at low temperatures requires lower ferrite content in the welds.
A small amount of ferrite in the weld of austenitic stainless steel will reduce the low temperature toughness. When the ferrite content in the weld reaches FN=10, the low temperature toughness will decrease by 50%.
The low temperature toughness of stainless steel pipe is not only related to the ferrite content, but also related to the cleanliness of the weld. The test shows that the oxygen content of the deposited metal of the alkaline low-hydrogen electrode is lower than that of the titanium-calcium electrode, and the weld toughness is also better. The oxygen content of deposited metal in inert gas shielded welding is lower than that of slag shielded welding, and the weld toughness is also better. The deposited metal of ultra-low carbon welding materials has less intergranular precipitation and better welding toughness.
When heat treatment is required after welding or in service at high temperature, the ferrite number of weld metal shall not exceed 10FN measured before post-weld heat treatment. When the stainless steel pipe is used in a low temperature environment below -100°C, the low temperature impact test of the weld is carried out in accordance with relevant regulations; when it is used in a low temperature environment below -196°C, the ferrite content of the weld is lower than 5FN. When the stainless steel pipe is used in a non-magnetic or special corrosive environment, it needs to be controlled according to the design requirements.
