Steel component material for Boeing 737 aircraft.(HY-industry technical centre)

Introduction for Boeing 737 materials

The main component material of the Boeing 737 is aluminum alloy, but it is often exposed to steel materials other than aluminum alloy during aircraft maintenance or when preparing maintenance plans, such as AISI301 CRES (corrosion-resistant steel), A-286 CRES, Alloy such as 17-7PH (precipitation hardening). There are approximately 12-16% of these steel alloys on the Boeing 737. The high strength and modulus of elasticity are the main advantages of these materials. These advantages are very useful for the design of space-constrained areas, such as some landing gear. component. At the same time, many aircraft components require materials with good corrosion resistance and excellent high and low temperature comprehensive mechanical properties, such as compressor shafts, turbine shafts, etc. in the engine, and some steel alloys can meet this requirement. When we choose steel materials in structural repair, we also consider the factors such as temperature, strength, corrosion resistance, fatigue properties and workability.


austenitic stainless steel Austenitic stainless steel is a stainless steel with a stable austenite structure at normal temperature and was introduced in Germany in 1913. This stainless steel is mainly composed of chromium and nickel. The chromium element provides corrosion resistance, oxidation resistance and high temperature properties. The presence of nickel forms an austenitic structure, providing high toughness and ductility. Austenitic stainless steel is non-magnetic and has low strength and cannot be strengthened by heat treatment but can be strengthened by cold working. Austenitic stainless steel is generally used on aircraft in areas where the environmental requirements are too high to use aluminum alloy. Adding different kinds of trace elements to austenitic stainless steel can change its overall performance. For example, adding sulfur and selenium can increase the cutting performance; reducing carbon content or adding niobium and titanium can reduce the intergranular corrosion tendency and increase the high temperature of the material. And welding properties; the addition of molybdenum can increase corrosion resistance and improve its resistance to Confucianism.


martensitic stainless steel
Martensitic stainless steel is a stainless steel that can adjust its mechanical properties by heat treatment and has a stable martensite structure. The chromium content is generally between 12 and 18%. The higher the chromium content, the higher the carbon content required for the steel to ensure the formation of martensite during heat treatment. Martensitic stainless steels have high strength and corrosion resistance, but their corrosion resistance decreases as the carbon content increases. The commonly used model for martensitic stainless steel is the 400 series, but this material is relatively less used on Boeing 737 aircraft.


precipitation hardening stainless steel
Precipitation hardening stainless steel refers to a kind of high-strength stainless steel which is added with different types and quantity of strengthening elements on the basis of chemical composition of stainless steel, which can increase the strength of steel and maintain sufficient toughness by precipitation of different types and quantities of compounds. The microscopic metallographic structure may be austenite or martensite. The development of this type of stainless steel has been the case for nearly three decades. The first successful discovery was the 17-4PH type stainless steel, which increased the hardness of the steel due to the precipitation of 4% copper contained in the heat treatment. Precipitation hardening stainless steel can generally be divided into three categories according to the tissue morphology:

  • Martensitic precipitation hardening stainless steel. After the solution treatment, the metallurgy is present in a stable martensite structure when it is cooled to room temperature, and it is strengthened by aging treatment in a solid solution state to produce a precipitated phase. Has good weldability, corrosion resistance, strength and toughness. It is mainly used to manufacture high-strength and corrosion-resistant bearing members working at temperatures below 400 °C. It is used in aircraft for compressor discs, casings, ring parts and large casings.

  • Semi-austenitic precipitation hardening stainless steel. After the solution treatment, the metallurgy exists in the unstable austenite structure when it is cooled to room temperature. After a series of heat, cold or mechanical deformation treatment, the austenite transforms into martensite and then strengthens by ageing. . Has good corrosion resistance. Since such steel is an austenite structure after solution treatment, it is usually processed, formed or welded in this state. This type of steel is mainly used for corrosion-resistant structural members with temperatures below 400 °C in the aerospace industry, such as aircraft various pipes, pipe joints, fasteners, etc.

  • Austenitic precipitation hardening stainless steel. Such materials are organized into a stable austenitic structure, and the heat treatment does not change the structure of the structure, so it can only be strengthened by the addition of precipitation strengthening elements by aging treatment. It is actually an iron-based superalloy, which is used to manufacture aircraft engine turbines, blades, fuselages, fasteners, etc.

2. Discrimination of brand of Boeing 737
We encounter a variety of precipitation hardened steel during aircraft maintenance or repair, such as:

  • 17-4PH is a martensitic precipitation hardening stainless steel. 17-4PH has high strength, hardness, good cutting performance and corrosion resistance. Its corrosion resistance is similar to that of general austenitic stainless steel but better than general martensitic stainless steel. It can be soldered without preheating and without local annealing after soldering.

  • 15-5PH is a martensitic precipitation hardening stainless steel. 15-5PH is developed on the basis of 17-4PH steel. The chromium and copper content in steel is lower than that of the latter, and the nickel content is slightly increased. This material has high strength, good processability, excellent mechanical properties, and its corrosion resistance is comparable to that of 17-4PH steel. Its outstanding advantages are high transverse toughness and good forgeability.

  • 17-7PH is a semi-austenitic precipitation hardening stainless steel developed on the basis of 18-8CrNi, also known as controlled phase change stainless steel. After solution treatment, it is unstable austenite structure, has good plasticity, toughness and processability, and has good medium temperature mechanical properties. Corrosion resistance is superior to general martensitic stainless steel. Inert gas protection should be used during welding to prevent oxidation of aluminum in the electrode. In order to obtain good welding efficiency, the weldment after solution annealing is preferably first solution treated and then adjusted and aged. 17-7PH has better plasticity than 15-5PH and is easy to form.

  • 15-7PH belongs to semi-austenitic precipitation hardening stainless steel. It has both the process characteristics of austenitic stainless steel and the high strength of martensitic stainless steel. The welding performance is the same as that of ordinary stainless steel, and arc welding, electric resistance welding and gas shielded welding can be used, and gas shielded welding is the best.

  • A-286 is a nickel-chromium austenitic precipitation hardening stainless steel, and is also one of the earliest iron-based superalloys. It has high yield strength and long-lasting, creep strength below 650 ° C, and has good processing plasticity and satisfactory welding performance. It is suitable for manufacturing high-temperature bearing components of aero-engines that work for a long time below 650 °C.

  • Inconel alloy is austenitic precipitation hardening stainless steel. It has high strength at 700 ° C and high oxidation resistance at 1000 ° C. It can be widely used in various high-demand applications such as turbine engines. It has stable chemical properties at low temperatures and can be aged hardened. Good welding performance. In addition, the commonly used Inconel 625 and 718 alloys have excellent resistance to stress corrosion cracking and pitting, both in high temperature and low temperature environments. Inconel 718 alloy is particularly excellent in oxidation resistance at high temperatures.

  • Monel alloy is a nickel-copper corrosion-resistant alloy, and its precipitation hardening alloy has excellent mechanical properties such as high strength, corrosion resistance and non-magnetic properties. Monel series products are developed with advanced technology and equipment, and have long service life in various acid and alkali media. Therefore, they are widely used in advanced industries such as aerospace to manufacture important parts and fastening. Pieces and so on.


other matters needing attention
15-5PH precipitation hardened stainless steel is currently being used to replace alloy steel bars and castings with strengths of 200 ksi or less. The processing cost of 15-5PH stainless steel is reduced and the corrosion resistance is improved. When sheet metal is required, 15-5PH stainless steel with a thickness of 0.063 inches and above is required because it is not possible to roll a 15-5PH stainless steel sheet below 0.063 inch thickness.

Both the 17-4PH and 17-7PH alloy steels contain delta ferrite, which causes instability of the transverse mechanical properties of the material, but has less effect on this series of stainless steels of smaller thickness. When repairing, it is possible to use a sheet of heat treated to a thickness of 150-170 ksi up to 0.090 inches and a sheet of heat treated to a thickness of 180-200 ksi up to 0.063 inches. Do not exceed the thickness allowed for each heat treatment state, otherwise the material will be affected. Mechanical behavior.

The Boeing 737 series now uses a hydraulic tube of 21Cr-6Ni-9Mn austenitic stainless steel. It can also be used for 17-4PH precipitation hardening stainless steel with a heat treatment condition of 150ksi or 180ksi.

Heated to 220-240 ksi 4330M alloy steel and heat treated to 275-300ksi 4340M alloy steel, these improved alloy steel is also called vacuum remelting steel, which is mainly named according to its process. This process improves the plasticity, fatigue properties and fracture toughness of steel and is widely used on aircraft landing gear, slides and other components.

Fifth ,Conclusion
The key bearing components of the aircraft require high-strength stainless steel with high strength, high toughness, chemical resistance and stress corrosion. There are good reasons to believe that with the continuous development of various steel alloys, it will surely get more on large aircraft. Better application. Understanding the performance of various steel alloys will help us to apply in the actual maintenance process, which will help us better protect the components and structure of the aircraft.


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