1. Forth a question
The unified technical regulations generally include that “austenitic stainless steel vessels are used in the environment that may cause intergranular corrosion, and solid solution or stabilization treatment shall be carried out after welding“.
It is reasonable to put forward such requirements.
However, even if the designer puts forward this clause in the technical requirements of the drawing, requiring the manufacturer to conduct post welding heat treatment of stainless steel vessels (such as heat exchangers), it is usually difficult to meet the ideal requirements put forward by the designer due to the difficulty in controlling the actual heat treatment process parameters and other unexpected difficulties.
In fact, most of the stainless steel equipment in service is used after welding.
This prompts us to think: intergranular corrosion is the most common corrosion form of austenitic stainless steel.
What is the mechanism of intergranular corrosion?
Under what medium environment can intergranular corrosion be caused?
What are the main methods to prevent and control intergranular corrosion?
Are austenitic stainless steel vessels used in environments that may cause intergranular corrosion subject to heat treatment after welding?
This article refers to relevant standards, specifications and monographs, and discusses personal opinions in combination with production practice.
2. Mechanism of intergranular corrosion
Intergranular corrosion is a common local corrosion.
The corrosion develops along the grain boundary of metal or alloy or its adjacent area, and the grain corrosion is very slight.
This corrosion is called intergranular corrosion, which greatly weakens the bonding force between grains.
Severe intergranular corrosion can make the metal lose strength and ductility and break under normal load.
The modern intergranular corrosion theory mainly includes the theory of poor chromium and the theory of selective dissolution of grain boundary impurities.
2.1 Lean chromium theory
The intergranular corrosion of commonly used austenitic stainless steel in oxidizing or weakly oxidizing medium is mostly caused by improper heating during processing or use.
Improper heating means that steel will be sensitive to intergranular corrosion when it is heated or slowly cooled through the temperature range of 450-850 ℃.
Therefore, this temperature is a dangerous temperature for austenitic stainless steel.
The stainless steel material has been solution treated when it leaves the factory.
The so-called solution treatment is to heat the steel to 1050 ~ 1150 ℃ and then quench it to obtain a homogeneous solid solution.
Austenitic steel contains a small amount of carbon, and the solid solubility of carbon in austenite decreases with the decrease of temperature.
For example, for 0Cr18Ni9Ti, the solid solubility of carbon is about 0.2% at 1100 ℃, and about 0.02% at 500-700 ℃.
Therefore, carbon in solution treated steel is supersaturated.
When the steel is heated or cooled through 450-850 ℃, carbon can form (Fe, Cr) 23C6 and precipitate from austenite and distribute on the grain boundary.
The chromium content of (Fe, Cr) 23C6 is much higher than that of austenitic matrix.
Its precipitation naturally consumes a large amount of chromium near the grain boundary, and the consumed chromium cannot be timely replenished from the grain through diffusion.
Because the diffusion speed of chromium is very slow, the chromium content near the grain boundary is lower than the limit necessary for passivation (i.e., 12% Cr), forming a chromium poor region.
Therefore, the passive state is damaged, and the potential of the region near the grain boundary decreases.
However, the grain itself still maintains a passive state with a high potential.
The grain and the grain boundary form an active passive micro galvanic battery.
The battery has an area ratio of a large cathode to a small anode, which leads to corrosion in the grain boundary region.
2.2 Theory of selective dissolution of grain boundary impurities
In production practice, we also learned that austenitic stainless steel can also produce intergranular corrosion in strong oxidizing medium (such as concentrated nitric acid), but the corrosion situation is different from that in oxidizing or weak oxidizing medium.
It usually occurs on the steel that has been treated by solid solution, and generally does not occur on the steel that has been sensitized.
When the phosphorus impurity in the solid solution reaches 100ppm or the silicon impurity is 1000-2000ppm, they will segregate on the grain boundary.
These impurities will dissolve under the action of strong oxidizing medium, resulting in intergranular corrosion.
When the steel is sensitized, because carbon can form (MP) 23C6 with phosphorus, or because the first segregation of carbon restricts the diffusion of phosphorus to the grain boundary, the segregation of impurities at the grain boundary will be eliminated or reduced in both cases, thus eliminating or weakening the sensitivity of the steel to intergranular corrosion.
The above two theories to explain the mechanism of intergranular corrosion are applicable to the structure state of a certain alloy and a certain medium, and are not mutually exclusive but complementary.
Most of the most common intergranular corrosion of stainless steel in production practice occurs in weak oxidizing or oxidizing media, so the vast majority of corrosion cases can be explained by the chromium poor theory.
3. Medium environment causing intergranular corrosion
There are two main types of media causing intergranular corrosion of austenitic stainless steel.
One is oxidizing or weak oxidizing medium, and the other is strong oxidizing medium, such as concentrated nitric acid.
The first type is common.
The following is a list of common medium environments that cause intergranular corrosion of austenitic stainless steel.
3.1 Common medium causing intergranular corrosion of austenitic stainless steel
In the “corrosion data chart” prepared by G A. Nelson lists the common mediums causing intergranular corrosion of austenitic stainless steel: acetic acid, acetic acid + salicylic acid, ammonium nitrate, ammonium sulfate, chromic acid, copper sulfate, fatty acid, formic acid, iron sulfate, hydrofluoric acid + iron sulfate, lactic acid, nitric acid, nitric acid + hydrochloric acid, oxalic acid, phosphoric acid, seawater, salt mist, sodium bisulfate, sodium hypochlorite, sulfur dioxide (wet), sulfuric acid, sulfuric acid + copper sulfate, Sulfuric acid + ferrous sulfate, sulfuric acid + methanol, sulfuric acid + nitric acid, sulfite, phthalic acid, sodium hydroxide + sodium sulfide.
3.2 Intergranular corrosion tendency test
When austenitic stainless steel is used in an environment that may cause intergranular corrosion, the intergranular corrosion tendency test shall be conducted according to GB4334.1 ～ GB4334 test method for intergranular corrosion of stainless steel.
The selection and qualification requirements of test methods for intergranular corrosion tendency of austenitic stainless steel shall meet the following requirements:
(1) Austenitic stainless steel and special stainless steel for concentrated nitric acid used in nitric acid with temperature greater than or equal to 60 ℃ and concentration greater than or equal to 5% shall be tested according to GB4334.3 test method for 65% nitric acid corrosion of stainless steel.
The average corrosion rate in five cycles or three cycles shall not be greater than 0.6g/ m2 h (or equivalent to 0.6mm / a).
The sample state may be in use or sensitized.
(2) Chromium nickel austenitic stainless steel (such as 0Cr18Ni10Ti, 0Cr18Ni9, 00Cr19Ni10 and similar steels): General requirements: according to GB4334. 5 sulfuric acid copper sulfate corrosion test method for stainless steel, after bending test, there shall be no intergranular corrosion crack on the surface of the sample.
Higher requirements: the average corrosion rate shall not be greater than 1. 1g / m2 h according to GB4334. 2 sulfuric acid ferric sulfate corrosion test method for stainless steel.
(3) Molybdenum containing austenitic stainless steel (such as 0Cr18Ni12Mo2Ti, 00Cr17Ni14Mo2 and similar steels): General requirements: according to GB4334.5 sulfuric acid copper sulfate corrosion test method for stainless steel, after bending test, there shall be no intergranular corrosion crack on the surface of the sample.
Higher requirements: according to GB4334.4 nitric acid hydrofluoric acid corrosion test method for stainless steel, the corrosion ratio shall not be greater than 1.5.
The average corrosion rate shall not be greater than 1.1g/m2 h according to GB4334.2 test method for sulfuric acid ferric sulfate.
(4) If the medium has special requirements, intergranular corrosion tests other than those specified above can be carried out, and corresponding qualification requirements shall be specified.
4. Measures to prevent and control intergranular corrosion
According to the corrosion mechanism, the measures to prevent and control the intergranular corrosion of austenitic stainless steel are as follows:
(1) Ultra low carbon stainless steel is used to reduce the carbon content to below 0.03%.
For example, 00Cr17Ni14Mo2 is selected to prevent the formation of (Fe, Cr) 23C6 in the steel and the occurrence of chromium poor zone, so as to prevent intergranular corrosion.
Generally, for parts with low strength, low stress and good plasticity, 0Cr18Ni9 can be selected from the economic point of view.
(2) The stabilized stainless steel is the stainless steel containing titanium and niobium in the steel (i.e. the stabilized stainless steel we often say).
When smelting the steel, a certain amount of titanium and niobium are added, and their affinity with carbon is large, so that tic or NBC is formed in the steel.
Moreover, the solid solubility of tic or NBC is much smaller than that of (Fe, Cr) 23C6, and it is almost insoluble in austenite at the solid solution temperature.
In this way, although (Fe, Cr) 23C6 does not precipitate on the grain boundary when passing through the sensitization temperature, the tendency of intergranular corrosion of austenitic stainless steel is largely eliminated.
For example, 1Cr18Ni9Ti, 1Cr18Ni9Nb and other steels can work in the range of 500-700 ℃ without intergranular corrosion.
(3) When the austenitic stainless steel is electrically welded, the temperature of the arc pool is as high as 1300 ℃, and the temperature on both sides of the weld decreases with the increase of the distance, in which there is a sensitization temperature zone.
The austenitic stainless steel shall be heated and cooled slowly within the sensitization temperature range as far as possible.
In case of intergranular corrosion tendency, the unstable stainless steel shall be heated to 1000-1120 ℃ for 1-2 minutes per millimeter, and then quenched;
It is suitable to heat the stabilized stainless steel to 950 ～ 1050 ℃.
The steel after solution treatment shall be prevented from being heated at the sensitization temperature, otherwise chromium carbide will be precipitated along the grain boundary again.
(4) When the correct welding method is selected for welding, if the operation is unskilled or the welding material is too thick, the longer the welding time, the more chances to stay in the sensitized temperature zone, resulting in the sensitivity of the base metal on both sides of the weld to intergranular corrosion.
In order to reduce the sensitivity of welded joints, the input of line energy should be minimized during welding.
Generally, the input line energy of argon arc welding is lower than that of electric arc welding, so argon arc welding shall be used for welding and welding repair.
Ultra low carbon stainless steel or stainless steel containing Ti and Nb stabilizing elements shall be selected for welding parts, and ultra low carbon welding rod or Nb containing welding rod shall be selected for welding rod.
When argon arc welding is adopted, in order to avoid overheating of the welding joint, the operation shall be fast and the base metal on both sides of the weld shall be cooled quickly after welding to minimize the stay time in the sensitization temperature range.
5. Post welding treatment
The post weld heat treatment is not always emphasized in the weld area.
Generally, the solid solution treatment shall be carried out in the range of 1100-1150 ℃ for a certain time and then quenched.
The cooling within the temperature range of 925-540 ℃ shall be completed within three minutes, and the rapid cooling shall be continued to below 425 ℃;
The stabilization treatment shall be air cooled after several hours of heat preservation within the temperature range of 850-880 ℃.
The expected effect of post welding heat treatment is closely related to the key process parameters of the whole process of heat treatment (such as furnace temperature, temperature rise rate, temperature difference of various parts of the workpiece during the temperature rise, furnace atmosphere, holding time, temperature difference of various parts during the heat preservation, cooling rate, furnace temperature, etc.).
For austenitic stainless steel vessels that may cause intergranular corrosion, the solution treatment or stabilization treatment of general parts can be realized.
However, the post weld heat treatment of the welds of the whole vessel (mostly heat exchanger) will face many difficulties.
This kind of treatment is not local post weld heat treatment, but the post weld heat treatment of the whole welded parts or the whole vessel.
Due to the complex structure and shape of most chemical vessels (such as our commonly used shell and tube heat exchanger).
If post welding solid solution or stabilization treatment is required for the weld area of the whole shell and tube heat exchanger, the above key process parameters can not be controlled at all, let alone the quality of post welding heat treatment.
Even the treatment is often self defeating, not only the weld structure has not been improved, but the base metal structure has been unnecessarily deteriorated.
Therefore, more than 90% of the austenitic stainless steel chemical vessels used in the intergranular corrosion environment are still used in the post welding state, rather than in the post welding heat treatment state.
Chromium nickel austenitic stainless steel is the most commonly used corrosion-resistant material, and intergranular corrosion is the most common failure form of chromium nickel austenitic stainless steel vessels.
Intergranular corrosion greatly weakens the bonding force between grains, and in severe cases, the mechanical strength can be completely lost.
The surface of the stainless steel subjected to this corrosion is still very bright, but it can not withstand gentle knocking and will break into fine particles.
Because intergranular corrosion is not easy to check, it causes sudden damage to equipment, which is very harmful and should be paid enough attention to.
Chromium nickel austenitic stainless steel vessels are basically formed by welding, and the two sides of the welded joint are intergranular corrosion sensitized areas, which are always subject to corrosion damage before the base metal.
Through post weld heat treatment, the ability of resistance to intergranular corrosion in the weld zone is improved to the same level as that of the base metal.
This is our goal and our original intention of post weld heat treatment.
However, in practice, there are many factors to be considered, such as: the overall structure and shape of the weldment are complex, and it is difficult to guarantee the process parameters of post welding heat treatment.
Therefore, in fact, most of the in-service chromium nickel austenitic stainless steels are used after welding.
Whether the weld zone of chromium nickel austenitic stainless steel vessel used for intergranular corrosion resistance is subject to solid solution treatment or stabilization treatment can not be generalized.
The structural shape of the vessel should be specifically analyzed to determine whether the effect of heat treatment can be ensured.
Otherwise, even if we put forward the requirements of post weld heat treatment, it will often backfire, not only failing to achieve the desired effect, but also affecting the structure of the base metal.
In order to improve the intergranular corrosion resistance of chromium nickel austenitic stainless steel vessels, it is necessary to select ultra-low carbon stainless steel and stabilized stainless steel according to the specific corrosion environment and corrosion mechanism, select the correct welding method during welding, and properly combine the above-mentioned prevention and control measures to achieve good results.
It is not possible to rely solely on the solid solution or stabilization treatment after welding.