High strength stainless steel used in aircraft applications

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We usually call tensile strength higher than 800MPa, yield strength higher than 500MPa stainless steel is high strength stainless steel, yield strength higher than 1380MPa stainless steel is called ultra high strength stainless steel. The development of the aviation industry has proved that the improvement of aircraft and aero-engine performance largely depends on metal materials. Due to the high strength, high toughness, high stress corrosion cracking resistance and good impact resistance of steel, some key structural components of aircraft such as landing gear, girder, high stress joints, fasteners and other high strength stainless steel are still used.

High strength stainless steel mainly includes Martensite precipitation hardening stainless steel and semi – Austenite precipitation hardening stainless steel. The strength of martensite precipitation hardening stainless steel is achieved by martensite transformation and precipitation hardening treatment, the advantage is high strength, at the same time due to low carbon, high chromium, high molybdenum and/or high copper, its corrosion resistance is generally not less than 18Cr-8Ni austenitic stainless steel; Free cutting, good welding ability, do not need local annealing after welding, heat treatment process is relatively simple. The main disadvantage is that even in annealed state, its structure is still low carbon martensite, so it is difficult to conduct deep deformation cold working. The typical steel grade is 17-4PH and PH13-8Mo, used for the manufacture of high strength corrosion resistant bearing components, such as engine bearing parts, fasteners, etc. working at 400℃. PH13-8Mo is widely used in aeronautical bearing corrosion resistant medium temperature structural parts.

The semi-Austenite precipitation hardened stainless steel can be machined, coldly deformed and welded in Austenite state, and then the martensite transformation and precipitation hardening can be controlled by adjusting aging to obtain different strengths and toughness coordination. The steel has good corrosion resistance and thermal strength, especially stress corrosion resistance, and is especially suitable for the manufacture of parts used below 540℃. The disadvantage is that the heat treatment process is complex, the heat treatment temperature control requirements are very accurate (±5℃); The work hardening tendency of steel is large, and many intermediate annealing times are often needed for deep deformation cold working. Typical grades are 17-7PH, PH15-7Mo, etc. This kind of steel is mainly used in the aviation industry to work at 400℃ below the corrosion bearing structure, such as all kinds of pipes, pipe joints, springs, fasteners, etc.

 

Aircraft landing gear

The materials used for the construction of aircraft landing gear are 30CrMnSiNi2A, 4340, 300M, Aermet100 and other aircraft landing gear and fasteners with higher requirements are mostly made of precipitation hardened stainless steel, such as 17-4PH for THE landing gear of F-15 aircraft, 15-5pH for the landing gear of B-767 aircraft. PH13-8mo steel has the potential to replace 17-4PH, 15-5PH, 17-7PH, PH15-7Mo and other steels due to its better stress corrosion resistance than precipitation hardened stainless steel of the same grade.

The plane bearing

German FAG company developed the nitrogen-added martensite stainless steel Cronidur30 (0.31%C-0.38%N-15% Cr-L %Mo), which is produced by PESR process of electroslag remelting under high pressure nitrogen atmosphere. It is a high temperature stainless steel with high nitrogen completely hardened, which is more resistant to corrosion than SUS440. It is not suitable for high DN value (D: bearing inner diameter/mm, N: shaft revolution/arin) because of its characteristics of full hardening type, the same Cronidur30 can satisfy the residual compressive stress and fracture toughness value of DN4 million at the same time through high-frequency quenching. But the tempering temperature is lower than 15O℃, it can not withstand the rise in bearing temperature caused by thermal shock after engine shutdown.

Aircraft bearing structural components

High-strength stainless steel in aircraft bearing structure is mainly 15-5PH, 17-4PH, PH13-8Mo, etc., including hatch cover latch, high-strength bolt, spring and other parts. Civil aircraft use such high-strength stainless steel for wing spars, such as 15-5PH steel for Boeing 737-600 wing spars; Type A340-300 wing SPAR PH13-8Mo steel. Ph13-8Mo is used for parts requiring high strength and toughness, especially for transverse performance, such as fuselage frames. More recently, Custom465 has been tested due to increased toughness and stress corrosion resistance. Custom465 was developed by Carpenter on the basis of Custom450 and Custom455 for the manufacture of aircraft flap guides, slat guides, transmissions, engine mounts, etc. The steel is currently included in the MMPDS-02, AMS5936 and ASTM A564 technical specifications. HSL180 high strength stainless steel (0.21C-12.5Cr-1.0Ni-15.5Co-2.0Mo) is used to manufacture the aircraft structure, which has the same strength of 1800MPa as low alloy steel such as 4340 and the same corrosion resistance and toughness as precipitation hardened stainless steel such as SUS630.

 

Why is duplex stainless steel used in nuclear power plant cooling water systems?

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As a clean energy source, nuclear power is a major contributor to reducing carbon emissions worldwide. The cooling water piping system is the key to the safe operation of a nuclear power plant. It consists of thousands of feet of pipes of various diameters and sizes. It provides a reliable water supply for the cooling of plant equipment. The non-safety piping system must provide enough cooling water to cool the plant, while the safety system must provide enough cooling water to bring the reactor under control and safely shut it down in case of an emergency.

These pipe materials must be resistant to cooling water corrosion throughout the service life of the equipment. Depending on the plant’s location, the type of cooling water can range from relatively clean fresh water to contaminated seawater. Experience has shown that as systems age, a variety of corrosion problems and varying degrees of corrosion can occur, damaging the system and preventing it from providing the required cooling water.

Problems with cooling water piping often involve materials and their interactions with cooling water. Leakage from fouling (plugging) and corrosion of the system are the most common problems, including sediment accumulation, Marine biological attachment (biofouling), accumulation of corrosion products, and blockage of foreign matter. Leakage is usually caused by microbial corrosion (MIC), which is very corrosive corrosion caused by certain microorganisms in water. This form of corrosion occurs frequently in carbon steel and low-alloyed stainless steel.

Stainless steel has long been considered a viable option for building new water supply piping systems and for repairing or replacing existing carbon steel systems. The stainless steel commonly used in piping upgrade solutions is 304L, 316L, or 6%-Mo stainless steel. 316L and 6% Mo stainless steel yo big differences in performance and price. If the cooling medium is untreated water, which is highly corrosive and carries a risk of microbial corrosion, 304L and 316L are not suitable choices. As a result, nuclear plants have had to upgrade to 6%-Mo stainless steel or accept the high maintenance costs of carbon steel systems. Some nuclear power plants still use carbon steel lining pipes because of the lower initial cost. According to ASTM A240，Industrial water supply piping systems are often made of stainless steel below:

Grades UNS C N Cr Ni Mo Cu
304L S30403 0.03 / 18.0-20.0 8.0-12.0 / /
316L S31603 0.03 / 16.0-18.0 10.0-14.0 2.0-3.0 /
6%Mo N08367 0.03 0.18-0.25 20.0-22.0 23.0-25.0 6.0-7.0 0.75
2205 S32205 0.03 0.14-0.2 22.0-23.0 4.5-6.5 3.0-3.5 /

The 2205 duplex stainless steel proved to be an excellent choice. Duke Power’s Catawba nuclear power plant in South Carolina is the first nuclear power plant to use 2205 (UNS S32205) dual-phase stainless steel in its systems. This grade contains approximately 3.2% molybdenum and has improved corrosion resistance and significantly better microbial corrosion resistance than 304L and 316L stainless steels.

The carbon steel lining piping on the overground portion of the piping system conveying the supply water to the cooling tower of the main condenser was replaced with 2205 duplex stainless steel piping.

The new replacement 2205 duplex stainless steel pipe was installed in 2002. The pipe is 60 meters long, 76.2 cm and 91.4 cm in diameter, and the wall thickness of the pipe is 0.95 cm. The system specified in accordance with ASME B31.1 Power piping, which is one of the management codes for the safe use of power plant piping systems and is widely used in the world. After 500 days of service, the system was thoroughly inspected. No scaling or corrosion was found during the inspection. 2205 duplex stainless steel performed very well. 2205 stainless steel piping has been performing well for more than a decade since its installation. Based on this experience, Duke Power has used 2205 duplex stainless steel pipes in other parts of its system.

Internal of 2205 pipe after 500 days use.

 

Designers of nuclear power plant water systems now have one more option when it comes to choosing piping materials for corrosion-resistant cooling water. The successful application of 2205 duplex stainless steel can reduce maintenance costs, reduce downtime and ensure the operation safety of nuclear power plants.

What’s DSS?

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DSS, the abbreviation of Duplex stainless steel, is a classification of stainless steels composed of two steel with the center one composed of either austenitize or ferric. These are also known as duplex steels since their chemical structure features two distinct phases, both of which are usually represented by martensite respectively. These steels are very useful in applications requiring extreme toughness since the two phases can be applied together at high temperatures and pressures. The duplex stainless steel is able to obtain sufficient hardness in both its austenitic and martensite phases due to the presence of significant amounts of residual austenite. The commonly used DSS grades are S31803, S32750 and SS32550.

The duplex stainless steel grades

Type UNS Sweden German France Japan
Low alloy UN23（SAF2304） SS232（SAF2304） W.Nr.1.4362 UR35N DP11
Medium alloy UNS S31500

UNS S31803

 SS2376(3RE60)

SS2377(SAF2205)

 W.Nr.1.4417

W.Nr.1.4462

 UR45N DP1

DP8
High alloy UNS S32900

UNS S31260

 SS2324(10RE51) W.Nr.1.4460

W.Nr.1.4501

   329J1

329J2L
Super duplex UNS S32750

UNS S32550

 SS2328(SAF2507) W.Nr.1.4410

W.Nr.1.4507

 UR47N+

UR52N+

  

 

Apart from the alloy itself, another important factor that contributes to its corrosion resistance is the nickel content. Nickel is commonly found in higher percentages in most alloys, which makes it an extremely useful component. In comparison to nickel, which is often used in high-performance alloys for its electrical conductivity and ability to form good-quality alloys, nickel is not as frequently used in making high-quality duplex stainless steel. One of the most interesting aspects of nickel alloys is its corrosion resistance ability, which makes it the best alternative for high-performance materials. When mixed with the steel, nickel produces a more stable alloy, which can increase the alloy’s wear-ability and mechanical strength.

Another significant property of this alloy is its high resistance to thermal expansion. It exhibits a high level of thermal expansion resistance despite the expansion resistance ability of austenitic stainless steels, due to its superior mechanical properties. This property gives it an excellent corrosion protection capability, especially during the tempering/stain removal cycle. The excellent corrosion resistance feature of duplex stainless steel enables it to stand up against a wide range of chemicals. It also has high levels of resistance towards oil, grease and other liquids with a high viscosity level.

Apart from the above features, duplex stainless steel is also popular because of its high strength and durability. Its high strength rating of up to 300Kg is made possible through its ability to make use of two-directional mandrel rolls. It is comprised of a hard carbon fiber rolled into strips that are interlaced on both sides and formed into a bar with a mandrel. A further feature that makes it an excellent alloy is that its surface is completely smooth with no ridges.

One of the most important factors that contribute to the durability of duplex stainless steels is their low rate of pitting corrosion resistance. These steels exhibit a low rate of formation of crystalline grains inside the hot alloy. They can be used to build both large and small structures in different industries. Due to their resistance to crystalline grains, they are highly valued by the construction industry.

The mechanical properties of duplex stainless steel offer a number of benefits that make them an excellent choice for a wide range of applications. These properties allow these steels to be used for a variety of applications including precision engineering component building, heat exchangers and sheet metal fabrication. Some other important properties of this type of alloy include high heat tolerance, low density and excellent corrosion resistance. They also offer a number of mechanical properties that contribute to the overall properties of the alloy. These include extreme hardness, toughness, chemical resistance and creep resistance.

Nickel Austenitic stainless steel grades

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Nickel is known to be an expensive alloying element and is essential in some applications where both stress corrosion resistance and austenite structure are required. For example, creep resistance is important in high temperature environments, where austenitic stainless steels are needed. Similar to the traditional austenitic stainless steels, the twin boundary is a significant feature of the nickel-rich austenitic stainless steels because of the lower stacking fault energy. Austenitic stainless steels are prone to stress corrosion cracking (SCC). However, the stress corrosion resistance is greatly improved when the nickel content exceeds 20%. The effect of nickel on the stress intensity of stress corrosion threshold (105℃, 22% NaCl aqueous solution) in Fe-Ni-Cr alloys containing 16%~21% chromium is studied. Nickel-rich austenitic stainless steel (NiASS) can be considered as a separate class of stainless steel. In fact, the stress corrosion resistance of biphasic and ferrite stainless steels is comparable to that of biphasic and ferrite stainless steels when the nickel content exceeds 30%. Several limited grades of nickel-rich austenitic stainless steels are listed in the table below. Super austenitic stainless steels 254SMO and 654SMO are designed specifically for the oil and gas industry. Typical applications are seawater cooling, pulp bleaching, and hydraulic and instrument piping equipment.

 

Ni-Austenitic stainless steels grades

Alloy C Si Mn Cr Ni Mo W Co Cu Nb N
254SMo 0.01 0.8 1.0 20 18 6.1 0.7 0.2
654SMo 0.01 3.5 24 22 7.3 0.5 0.5
Sanicro 25 0.1 0.2 0.5 22.5 25 3.6 3.5 3.0 0.5 0.23
Sanicro 28 0.02 0.6 2.0 27 31 3.5 1.0
Alloy 800 0.07 0.6 0.6 20.5 30.5
353MA 0.05 1.6 1.5 25 35 0.16
Alloy 825 0.03 0.5 0.8 20 38.5 2.6
Alloy 625 0.03 0.5 0.5 21 Bal 8.5
Alloy 690 0.02 0.5 0.5 30 60
Alloy 600 0.05 0.4 0.8 16.5 Bal 0.5

SANICRO 25, a 22Cr-25Ni alloy, is designed for use in boilers up to 700 °C. It is a material suitable for superheaters and reheaters due to its good creep fracture strength and high temperature corrosion resistance,. In fact, the creep fracture strength of SANICRO 25 is superior to that of most austenitic stainless steels in the range of 600~750℃. In a highly corrosive acidic environment, The Sanicro 28 is usually the best choice. It is used in high-intensity drilling Wells with tubing, casing and acid gas lining, and other applications include heaters, pump systems, and pumps and containers in wet phosphoric acid plants and super phosphoric acid plants.

Alloy 800 is often used in the environment range from 550 to 1100℃, which requires excellent creep resistance, good high-temperature corrosion resistance and high-temperature strength of materials. These alloys are also used in the inlet and outlet ports of the production of ammonia, methanol and civil gas, as well as in the furnace tubes used in the production of vinyl chloride and ethylene. Other applications include heat exchange tubes and radiation tubes for fluidized combustion beds and parts of heat treatment furnaces, such as muffler tubes and protective sleeves for thermocouples.

The 25Cr-35Ni alloy 353Ma is designed for use in cracking furnaces and reforming tubes where synthetic gases are treated in environments where carburizing and nitrogen absorption are potentially problematic. Although there are other alternatives that contain more chromium, 353 MA is the best choice. One reason is that it contains the element Ce, which helps form a very stable surface oxide layer.

Alloy 690 contains 60 percent nickel and is used mainly in the piping of steam generators in nuclear power plants. The operating temperature is 365℃, at which the stress corrosion crack between grains is a potential problem. Under given service conditions, alloy 690 is almost free from corrosion, making it the preferred alloy.

It is interesting to note that nickel-rich Austenitic stainless steel 254SMO is also used for art. “God, Over the Rainbow” sculpture by Carl Milles was installed in 1995 on the south coast of the Nak Strand in Stockholm. The sculpture is about 23m high and is a famous scenic spot where a large number of sailors pass by every day. The surrounding seawater contains salt, chloride is very easy to cause surface corrosion, high strength super austenitic stainless steel 254SMO is very suitable for this environment.

When Stainless steel bellows used in shell heat exchanger

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The bellows tube heat exchanger is an upgrade based on a straight (bright) tube heat exchanger. The design of the crest and trough of the wave inherits the advantages of the tubular heat exchanger such as durability and safety, and at the same time overcomes the defects such as poor heat transfer capacity and easy scaling. The principle is to improve the total heat transfer coefficient so as to reduce the required heat transfer area, which can save materials and reduce weight under the same heat transfer effect.

Because the bellows body is processed by cold pressing of bright pipe billet, it is generally believed that the bellows body can be strengthened after forming. The external pressure stability experiment shows that the instability of the corrugated heat exchange tube under external pressure first occurs in the straight pipe section, and the corrugated tube will be unstable only if the external pressure continues to rise. This indicates that the stability of the corrugated section is better than that of the straight section and that the critical pressure of the corrugated section is higher than that of the straight section.

Experiments show that the ripple of buckling deformation occurred in the wave trough, especially local single wave trough, generally not more than two troughs instability at the same time, it shows that the stability of the wave crest is better than trough but sometimes also can appear the opposite, in the cold pressing mark process, both trough and the wall thickness of the straight section is constant, cold after the tube is actually shorter.

The existence of wave peaks and troughs in the bellows increases the effect of radial heat exchange convection in the tubes, as shown in Fig below:

Radial convection has a great influence on the total heat transfer coefficient, which is the fundamental reason for the low price and lightweight of the double tube plate bellows heat exchanger. The heat exchange area of the tube body surface of the bellows and the straight tube is large at the same length, but this change is far less than the contribution of changing the coefficient value. It can be clearly seen that the flow velocity of the straight (light) tube is significantly reduced when it is close to the tube wall.

The shell heat exchanger with bellows can make the fluid speed and direction of constant change to form turbulence compared with a straight tube exchanger, making exchange heat with the wall, the boundary effect that affects heat transfer will no longer exist. The total heat transfer coefficient can be increased by 2 ~ 3 times, and the actual operation can even reach 5 times, and the weight is light, which is the reason why the price of bellows heat exchanger is lower than that of the straight tube heat exchanger. According to calculation and practical experience, the total heat transfer coefficient of 1 mm thick bellows is 10% lower than that of 0.5 mm thick bellows. The operation data of hundreds of bellows heat exchangers show that the wall thickness (almost all 0.5 mm) is the main reason for the operation of 10 ~ 14 years without major repair or damage.

In addition, the bellows heat exchanger can effectively resist the impact of a water hammer. The shell of the double tube plate heat exchanger is connected with an expansion joint. If it suffers from the impact of water hammer, the expansion joint will be misplaced. This happens to both bellows and straight tube heat exchangers, and the deformation of the shell may cause the tube to twist. It is because the bellows have more expansion margin, the elastic margin of strain is large when undergoing deformation, that is to say, the ability to resist instability is strong in this case. But in any case, in the process of installation to avoid the occurrence of water hammer, can be taken through the use of Angle sitting valve, delay switch and other measures.

Advantages of stainless steel bellow shell heat exchanger

  • High heat transfer efficiency

The special crest and trough design of the bellows makes the fluid flow because of the continuous mutation of the inside and outside section of the tube to form a strong turbulence. Even in the case of a very small flow rate, the fluid can form a strong disturbance inside and outside the tube, which greatly improves the heat transfer coefficient of the heat exchange tube. The heat transfer coefficient is 2~3 times higher than that of the traditional tube heat exchanger.

  • No scaling and blocking

The medium inside and outside the bellows is always in a highly turbulent state, which makes the solid particles in the medium-difficult to deposit scale; On the other hand, affected by the temperature difference of the medium will produce a trace of axial expansion deformation, the curvature will change frequently, the dirt and heat exchange tube will produce a large pull force, even if there is scale calm will therefore break off automatically, so that the heat exchanger has always maintained a lasting, better heat transfer performance.

  • Automatic compensation

The special structure and shape of bellows can effectively reduce the thermal stress under the condition of being heated without adding expansion joints, thus simplifying the structure of products and improving the reliability of products.

  • Long service life

The axial expansion ability is enhanced, which effectively reduces the temperature difference stress and can adapt to the large temperature difference and pressure change, so there will be no leakage caused by pipe mouth rupture. The connection between the baffle plate and the bellows extends the service life of the heat exchanger.

 

How the Nitrogen affect 316LN stainless steel?

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316LN is the Nitrogen addition version based on 316L steel (0.06% ~ 0.08%), so that it has the same characteristics as 316L,  has been used in the manufacture of high-temperature structural components in fast breeder reactor (FBRS). Reducing the carbon content greatly reduces the susceptibility to stress corrosion cracking due to welding in subsequent corrosive environments. The creep, low cycle fatigue and creep-fatigue interaction are the most important considerations for FBRS components. The high-temperature strength of 316L stainless steel can be improved to 316 stainless steel by alloying 0.06% ~ 0.08% N. The influence of nitrogen content higher than 0.08% on mechanical properties of 316L stainless steel at high temperature will be discussed in this paper.

 

Chemical composition of 316LN stainless steel

Furnace N C Mn Cr Mo Ni Si S P Fe
Standards 0.06-0.22 0.02-0.03 1.6-2.0 17-18 2.3-2.5 12.0-12.5 ≤0.5 ≤0.01 ≤0.03 –
1 0.07 0.027 1,7 17.53 2.49 12.2 0.22 0.0055 0.013 –
2 0.11 0.033 1.78 17.63 2.51 12.27 0.21 0.0055 0.015 –
3 0.14 0.025 1.74 17.57 2.53 12.15 0.20 0.0041 0.017 –
4 0.22 0.028 1.70 17.57 2.54 12.36 0.20 0.0055 0.018 –

Thses four batches of 316LN stainless steel with a nitrogen content of 0.07%, 0.11%, 0.14% and 0.22%, and carbon content of 0.03%, were tested to study the effects of nitrogen on tensile, creep, low-cycle fatigue and creep-fatigue properties of 316LN stainless steel. The aim of this experiment is to find the optimum nitrogen content to obtain the best combination of tensile, creep and low cycle fatigue properties. The experimental results show that nitrogen can improve the tensile strength, creep and fatigue strength of austenitic stainless steels. The reasons for the increase in strength include solution enhancement, reduced stacking fault energy (SFE), precipitation hardening, formation of composites (interstitial solutes), atomic segregation, and ordered hardening. Due to their different electron exchange properties, the dissolved nitrogen in austenitic stainless steel has a larger expansion volume than carbon.

In addition to the elastic interaction between nitrogen and dislocation, the electrostatic interstitial dislocation interaction also influences the strength. Dislocation nuclei are characterized by the lack of free electrons, which means they have a positive charge. The nitrogen atoms in austenitic stainless steels are negatively charged due to the position of free electrons near the nitrogen atoms and the electrostatic interaction between the dislocations and the nitrogen atoms.

The effective binding energy between the nitrogen atom and the dislocation increases with the increase of the nitrogen content in Austenitic steel, but the correlation is not obvious for carbon. In Austenitic steels, interstitial nitrogen interacts with substituent elements and tends to form interstitial substituent atomic compositions. The compound binds easily to elements to the left of Fe in the periodic table, such as Mn, Cr, Ti and V. There is a strong correlation between the properties of interatomic bonding (that is, orientation versus unorientation) and the proximity of adjacent atoms in a multicomponent alloy system. Bonding between metal atoms facilitates short-range ordering, which is the bonding of atoms of different elements. Interatomic polarization facilitates the exchange of covalent electrons, the bonding between atoms of the same element. Carbon promotes the aggregation of substitution atoms in the iron-based solid solution, while nitrogen facilitates short-range ordering.

In general, the yield strength(YS) and ultimate tensile strength(UTS) of 316L stainless steel are significantly improved by the alloying of 0.07% ~ 0.22% nitrogen. The increase in strength was observed in all tests in the temperature range of 300 ~ 1123K. Dynamic strain aging was observed within a limited temperature range. The temperature range of dynamic strain aging (DSA) decreases with the increase of nitrogen content.