High Temp Alloys

A boiler furnace, which forms an integral part of a central heating system, is essentially an enclosed space that creates provisions for the combustion of fuel in order to generate steam in a boiler. Hence, a boiler furnace is also referred to as a steam generating furnace. Typically, a furnace may run on natural gas or electricity. However, those furnaces that use boiling water are referred to as boiler furnace. Keeping in consideration with history, the ASME or American Society of Mechanical Engineers had developed certain standards and regulation codes in order to prevent the occurrence of accidents and to optimise the output of these boiler furnaces. This meant that many manufacturers had to use alloys which would not only resist corrosion but would also have excellent mechanical strength and be able to resist giving in to creep at extreme temperatures.

Which is why durable materials like carbon steel, mild steel, stainless steel are often strong contenders used in its manufacture. What makes carbon steel furnace boilers very attractive to many clients is its life expectancy, which given proper maintenance and use could last up to 20 years. Besides, this the cost of a carbon steel boiler is lower on account of the less alloying costs in comparison to most stainless steel grades. The maximum operating temperatures of carbon steel is around 399°C. Hence, using this alloy for applications where the temperature exceeds 399°C, does not make sense.

In comparison, there are several stainless steel grades that offer good high temperature strength. For instance, grade 309 is an austenitic stainless steel alloy that has been designed for use in elevated temperature applications or grade 310, another austenitic alloy that is known for its heat resistance properties, especially under Mildly Cyclic Conditions to 1100°C. Similarly, other austenitic stainless steel alloys that exhibit excellent mechanical properties despite being exposed to extremely high temperatures include grade 321 and grade 347. What makes austenitic alloys well suited to be used at elevated temperatures is that their chemistry could be controlled in order to tailor them for the application it is to be used for. In instances such as being used as furnace boilers, the carbon content of grades 309, 310, 321 and 347 could be increased. This manipulation in their chemistry leads to impressive creep resistance behaviour, without any compromise in its mechanical properties. Generally, metals expand on heating and often continuous heating in combination with wrong forming practices could lead to the metal cracking or getting damaged. Furnace boilers manufactured under ASTM specifications usually have to conform to its chemistry, mechanical property requirements in addition to its testing methods. The guidelines set up by the ASTM specifications often result in a safe and durable furnace boilers. Not forgetting the corrosion resistance behaviour of these alloys. The higher contents of nickel, chromium as well as molybdenum in the alloy of grades 309, 310, 321 and 347 is what grants them superior resistance to oxidative and reductive conditions.

Since these alloys are frequently exposed to steam, regular steel grades would be susceptible to rusting and hence would crack and break down soon after getting decayed. Unlike regular steel, the alloys which contain a higher content of chromium such as stainless steel grades 309, 310, 321 and 347 form a self-repairing passivated oxide layer which prevents the metals from getting oxidized. Just like furnace boilers have high temperature requirements, jet engine exhaust systems have high temperature requirements. However, unlike furnace boilers, jet engines have the need for alloys that have a higher tolerance to heat as compared to stainless steels. For instance, titanium would be an alloy which would be able to fulfil the needs for high temperature. Apart from the various titanium grades, superalloys tend to have superior creep resistance properties, while offering the highest mechanical properties.

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Furthermore, due to the strong mechanical properties of superalloys such as Inconel 718 or Alloy C-276, the width of walls of the jet engine exhaust systems could be reduced, thus leading to weight savings and reduced cost as well. In addition to these alloy grades, Hastelloy X and Incoloy 800H/ ht could be used at elevated temperatures higher than 1000° F, without getting oxidized and rusted, which is another primary concern for this application.