Created in the early 1960s, INCONEL alloy 625 is a real workhorse among nickel superalloys. It’s tough, light and versatile, with outstanding chemical and mechanical properties that make it suitable for a whole range of applications. No wonder it’s been an industry favourite for almost six decades.
Alloy 625 was originally developed for use in steam piping – a critical application requiring a high degree of strength, stability and resistance to extreme temperatures. Since then, ongoing research and development has revealed – and enhanced – this alloy’s range of characteristics. For example, it is exceptionally resistant to salt water, making it ideal for use in marine and offshore applications. Its corrosion properties and resistance to pressure also make it a staple choice in the automobile, nuclear and chemical processing industries.
The automobile industry has a particular need for metals that are light, strong and resistant. In this post, we’ll take a closer look at INCONEL alloy 625’s properties and how these are put to good use in vehicle manufacture.
INCONEL alloy 625 is a nickel-chromium-molybdenum alloy with small amounts of iron and niobium. With a minimum of 58% nickel, it particularly benefits from this element’s strength, toughness and corrosion resistance. Resistance is further enhanced with a significant level of chromium: 20–23%. Chromium is highly resistant to corrosion and particularly to oxidising media.
At 8–10%, molybdenum adds both strength and resistance to pitting and crevice corrosion. Molybdenum also contributes extra resistance to non-oxidising media. 5% iron further strengthens the alloy, while a small percentage of niobium helps to prevent intergranular cracking after welding. The outcome is a light, strong, highly workable alloy with great corrosion properties in oxidising and non-oxidising environments.
Strength and resistance
Because of the effect molybdenum and niobium have on alloy 625’s atomic matrix, it does not require heat treatment to increase its strength. It can simply be used in its annealed condition. Alloy 625 is strong across the board, with outstanding tensile strength and corrosion-fatigue strength. It’s particularly resistant to chloride-ion stress-corrosion cracking.
Corrosion resistance is another standout quality of alloy 625. It’s popular for offshore and marine use because it sustains virtually no corrosion from seawater or freshwater. It shows similar resistance to alkaline media and neutral salts.
Harsh and extreme high-temperature environments pose no problem to this superalloy. It also performs very well in low-temperature corrosive conditions due to the addition of chromium and molybdenum.
The combination of power and performance with smaller engine displacement and lower emissions is at the heart of today’s automobile industry. This places particular demands on the metal components used in building a car. Exhaust temperatures are higher than ever, and the push for sustainability means that the vehicle’s service life needs to be as long as possible. This means that, depending on the specific function of a given section of the automobile, each metal component needs the right combination of strength and resistance. Individual concerns could be creep, wear, corrosion, low- or high-cycle fatigue, or extreme temperatures.
The particular qualities of INCONEL alloy 625 make it highly suitable for specific elements of car and other vehicle design, including exhaust end pipes, airbag rupture discs, decoupling elements and lambda sensors.
Exhaust end pipes
Exhaust end pipes have a particularly hard job. They are constantly exposed to corrosive and harsh substances such as rainwater and salt, so they have to have excellent corrosion resistance. They also risk being dented, scraped, or hit by small objects such as stones. However, as the only visible part of the exhaust system and an important feature on the car itself, they need to look immaculate.
INCONEL alloy 625 is valuable here for its resistance to salt and fresh water damage, its lightness and strength, and its corrosion properties. It is also capable of sustaining high exhaust temperatures without failing.
The decoupling elements are another important part of a car’s exhaust system. These dampen vibrations, compensating for heat expansion and reducing potential wear. Tough, durable decoupling elements that do their job properly will increase the service life of the entire exhaust system.
Here, too, INCONEL alloy 625’s properties make it an excellent choice. Because it remains stable at high temperatures and is resistant to wet corrosion – a real risk given the exhaust’s exposure to road salt in winter – decoupling elements made with this alloy will perform well for a long time, extending the life of the exhaust and saving time and money on repairs.
The lambda sensor is also known as the oxygen sensor. Part of the exhaust system, it measures the residual oxygen content in the exhaust gas. Optimising the air/fuel mixture enables the correct amount of fuel to be sent to the engine cylinders so that it works properly and efficiently.
Lambda sensors – modern cars usually have two – are also an important sustainability and safety measure. Placed on either side of the catalytic converter, they are able to monitor whether it is working as it should to filter out harmful exhaust gases. This is a crucial feature and needs to have a long operative life, even exceeding that of the catalytic converter itself. Alloy 625 is generally used to make the lambda sensor’s contact springs.
Airbag rupture discs
Thanks to massive advances in vehicle safety, airbags are now a standard feature in cars. Airbags save lives when they function correctly, but they can be very dangerous if they inflate at the wrong time. The airbag mechanism is controlled by a disc that is designed to rupture at a specific pressure, ensuring that the airbag is deployed exactly when it is needed.
Alloy 625 is an optimal choice for manufacturing these discs because of its resistance to extreme temperatures. This ensures that the airbag won’t inflate accidentally in case of a vehicle fire, potentially causing injuries to driver, passenger and/or rescue personnel.
If you would like to find out more about the applications of alloy 625 in the car industry and elsewhere, just get in touch. Our team will be happy to advise.