If you choose the wrong material, the expansion joint is a time bomb
Two days ago, a power station customer came back for repair. The reason is very simple: 304 material was used to remove the flue gas containing chloride ions. As a result, the bellows was stress corroded and cracked in less than three months. And guess what? When you take it apart, the crack extends from the trough to the crest of the bellows, and the whole pipe looks like shredded paper. This is not an isolated case-I've seen too many projects end up reworking several times the cost for "saving that little money on materials".
Many people come up and ask "What material is the metal bellows expansion joint made of?"-it seems that just pick one random. However, the reality is that material selection directly determines how high temperature, how strong corrosion and how frequent displacement the expansion energy saving can withstand. Don't just look at the price, first find out exactly what your working condition requires.
Five kinds of mainstream materials, each temper you have to find out
304 stainless steelWith the lowest cost and the best toughness, it can cover 80% of conventional working conditions. For water, steam and air media, the upper temperature limit is 450℃-304 is sufficient for steam pipelines, but don't exceed this temperature, otherwise the risk of intergranular corrosion will soar.
316LWith the addition of molybdenum element, the resistance to chloride ion corrosion is obviously improved. In humid and acidic environments such as desulfurization and chemical industry, 316L is the bottom line. Remember, it's the "bottom line"-if you encounter high chlorine concentrations in wet smoke, 316L just barely copes.
321 stainless steel, containing titanium stabilizing element, specially to deal with intergranular corrosion. When no heat treatment is done after welding or the working temperature fluctuates intermittently between 400-800℃, 321 is more stable than 304 and 316L. In some areas of hot air ducts and power station boilers in the cement industry, 321 is a good choice for money.
Inconel 625 nickel-based alloyIt can withstand high temperatures above 700 ℃ and strong corrosion, and the price is 5-8 times that of 304. High-temperature flue of cement industry, tail flue of power station boiler-the temperature in these places is always 600-800℃, and there are sulfur and chlorine sandwiched in the flue gas. Don't use Inconel 625? The bellows will soon fail due to high temperature oxidation or creep, and it will be too late to regret it.
Duplex stainless steel 2205With outstanding resistance to stress corrosion cracking, seawater or chemical medium containing chloride is the first choice. The fatigue life of air-cooled island vacuum pipe, seawater heat exchanger pipe, 2205 is several times that of ordinary stainless steel.
Medium and Temperature: Two Iron Laws of Material Selection
To put it bluntly, material selection is to compare the media composition with the temperature range. Steam pipes, especially superheated steam, 304 or 316L can handle most working conditions, but don't think about it when the temperature exceeds 450℃-honestly cut to 321 or Inconel 625.
When encountering sulfur and chlorine-containing flue gas, such as wet flue gas after desulfurization, 316L is the starting point. More severe environments require duplex steel or nickel-based alloys. Someone asked: Why can't you use 304? Because chloride ions can cause stress corrosion cracking of 304 above 60℃, can you afford this probability?
The temperature of hot air ducts in cement industry is often between 600-800℃. At this time, when you look through the parameter tables of 304, 316L and 321, you will see "long-term use is not recommended". Go straight to Inconel 625, don't hesitate. Otherwise, the corrugated pipe will fail by high-temperature oxidation and creep, and one will be scrapped in three months. The labor cost of replacing the pipe will be more expensive than the material price difference.
Special scenarios: vacuum, high-frequency fatigue, thick-walled parts
In addition to regular working conditions, there are several types of scenarios that need to be weighed separately.
Vacuum environment— — For example, the double hinge expansion joint of air-cooled island vacuum pipeline, the wall thickness and layer number of corrugated pipe should be designed strongly, and 316L or Inconel 625 are commonly used in materials to ensure pressure stability. Once the bellows is unstable in vacuum, the vacuum degree of the whole system collapses instantly, and the consequences are more serious than leakage.
High frequency fatigue cycle— — The peak shaving unit of the power station starts and stops frequently, and the bellows may have to expand and contract dozens of times a day. Nickel-based alloys, although expensive, have an order of magnitude more cycles than stainless steel. To put it bluntly, ordinary stainless steel may crack after carrying it 30,000 times, but Inconel 625 can reach 300,000 times. You calculate the cost of replacement over the lifecycle of the equipment, who is more cost effective?
Large diameter thick wall expansion joint— — For example, the double straight pipe bypass pressure balance expansion joint has large wall thickness, high weldability and heat treatment requirements. The performance difference between 321 and 316L on thick-walled parts is obvious: 321 has better high temperature strength and 316L has stronger corrosion resistance. Don't get confused. If you don't do solution treatment after welding, 316L thick-walled parts are prone to sensitization in the heat-affected zone, and intergranular corrosion will come to your door.
Don't just focus on bellows-connections and deflectors are just as deadly
Many people only stare at the bellows itself, but ignore the material matching between the connector and the guide tube.
The function of the guide tube is to guide the flow of the medium and protect the inside of the bellows. If the material of the guide tube is not corrosion-resistant, it is perforated first, and the medium directly impacts the bellows-no matter how expensive your 316L bellows is, it is useless. I have seen a case: a carbon steel guide tube was matched with 316L bellows. As a result, the guide tube rotted out for half a year, and the high-temperature flue gas was directly injected into the bellows. A month later, the bellows was also scrapped. The cost of rework is several times higher than directly selecting the right material.
Structural parts such as tie rods, nuts and end pipes should be selected in the same grade or higher as bellows. A single spot of corrosion can drag down the entire system. The expansion joint is a whole, and any short board will become a breakthrough of failure.
In reality: There is no best, only the best match
There is no best material for expansion joints, only the best match. 304 can solve 80% of the normal working conditions, and the remaining 20% of the harsh environment-go to 316L when you should go to 316L, and don't hesitate to change to Inconel when you should go to 316L.
When selecting materials, pull out the data of your own media composition, temperature range, pressure fluctuation frequency and installation space, and match them with the description of the product list of this site (such as high-temperature axial expansion joint, corrugated expansion joint for power station industry and metal corrugated expansion joint for cement industry). If you are really unsure, you should refer to the case when looking for the manufacturer-don't pat your head to determine the material, which is the bottom line of equipment life and safety.
What is the material of metal bellows expansion joint? The answer is hidden in your own operating parameters. Pull out the data, compare it and choose it, and it will be right.